Publications

We report on the first observation of electroluminescence and charge amplification with a Virtual Cathode Chamber (VCC) microstrips plate immersed in liquid xenon. Both were observed in an intense non-uniform electric field in the vicinity of 2-μm narrow anode strips deposited, with a 2 mm pitch, on a semiconductive glass substrate (S8900), with a cathode film on its backside. An initial light yield of ∼ 460 VUV photons per drifting electron was measured, which degraded within tens of minutes stabilizing at (27.0 ± 3.1) photons per electron. The electroluminescence was accompanied by electron multiplication with an estimated charge gain ≲ 5. Further investigations are necessary to understand and mitigate the light yield degradation phenomenon. We expect other substrate materials, including VUV-transparent ones, to provide large stable photon yields, compatible with our model predictions. The VCC configuration has demonstrated great potential in single-phase noble-liquid detectors, particularly for dark-matter searches, neutrino physics and other fields.

Dual-phase liquid-xenon time projection chambers (LXe TPCs) deploying a few tonnes of liquid are presently leading the search for WIMP dark matter. Scaling these detectors to 10-fold larger fiducial masses, while improving their sensitivity to low-mass WIMPs presents difficult challenges in detector design. Several groups are considering a departure from current schemes, towards either single-phase liquid-only TPCs, or dual-phase detectors where the electroluminescence region consists of patterned electrodes. Here, we discuss the possible use of Thick Gaseous Electron Multipliers (THGEMs) coated with a VUV photocathode and immersed in LXe as a building block in such designs. We focus on the transfer efficiencies of ionization electrons and photoelectrons emitted from the photocathode through the electrode holes and show experimentally that efficiencies approaching 100% can be achieved with realistic voltage settings. The observed voltage dependence of the transfer efficiencies is consistent with electron transport simulations once diffusion and charging-up effects are included.

The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.

Cryogenic versions of Resistive WELL (RWELL) and Resistive Plate WELL (RPWELL) detectors have been developed, aimed at stable avalanche multiplication of ionization electrons in the vapor phase of LAr (dual-phase TPC). In the RWELL, a thin resistive DLC layer deposited on top of an insulator is inserted in between the electron multiplier (THGEM) and the readout anode; in the RPWELL, a resistive ferrite plate is directly coupled to the THGEM. Radiation-induced ionization electrons in the liquid are extracted into the gaseous phase. They drift into the THGEM's holes where they undergo charge multiplication. Embedding resistive materials into the multiplier proved to enhance operation stability due to the mitigation of electrical discharges thus allowing operation at higher charge gain compared to standard THGEM (a.k.a. LEM) multipliers. We present the detector concepts and report on the main preliminary results.

Gamma-Beams at the HI gamma S facility in the USA and anticipated at the ELI-NP facility, now constructed in Romania, present unique new opportunities to advance research in nuclear astrophysics; not the least of which is resolving open questions in oxygen formation during stellar helium burning via a precise measurement of the(12)C(alpha, gamma) reaction. Time projection chamber (TPC) detectors operating with low pressure gas (as an active target) are ideally suited for such studies. We review the progress of the current research program and plans for the future at the HI gamma S facility with the optical readout TPC (O-TPC) and the development of an electronic readout TPC for the ELI-NP facility (ELITPC).

Digital and Semi-Digital Hadronic Calorimeters (S)DHCAL were suggested for future Colliders as part of the particle-flow concept. Though studied mostly with RPC-based techniques, investigations have shown that Micro Pattern Gaseous Detector (MPGD)-based sampling elements could outperform in terms of average pad multiplicity or at higher rates. An attractive, industry-produced, robust, particle-tracking detector for large-area coverage, e.g. in (S)DHCAL, could be the novel single-stage Resistive Plate WELL (RPWELL). It is a single-sided THick Gaseous Electron Multiplier (THGEM) coupled to the segmented readout electrode through a sheet of large bulk resistivity. We summarize here the preliminary test-beam results obtained with 6.5 mm thick (incl. electronics) 48×48 cm² RPWELL detectors. Two configurations are considered: a standalone RPWELL detector studied with 150 GeV muons and high-rate pions beams and a RPWELL sampling element investigated within a small-(S)DHCAL prototype consisting of 7 resistive Micro-MEsh Gaseous Structure (MICROMEGAS) sampling elements followed by 5 RPWELL ones. The sampling elements were equipped with a Semi-Digital readout electronics based on the MICROROC chip.

We present for the first time, discharge-free operation at cryogenic conditions of a Resistive-Plate WELL (RPWELL) detector. It is a single-sided Thick Gaseous Electron Multiplier (THGEM) coupled to a readout anode via a plate of high bulk resistivity. The results of single- and double-stage RPWELL detectors operated in stable conditions in Ne/5%CH₄ at 163 K are summarized. The RPWELL comprised a ferric-based (Fe³⁺) ceramic composite ("Fe-ceramic") as the resistive plate, of volume resistivity similar to 10¹¹ Ω·cm at this temperature. Gains of similar to 10⁴ and similar to 10⁵ were reached with the single-stage RPWELL, with 6 keV X-rays and single UV-photons, respectively. The double-stage detector, a THGEM followed by the RPWELL, reached gains similar to 10⁵ and similar to 10⁶ with X-rays and single UV-photons, respectively. The results were obtained with and without a CsI photocathode on the first multiplying element. Potential applications at these cryogenic conditions are discussed.

Charging-up processes affecting gain stability in Thick Gas Electron Multipliers (THGEM) were studied with a dedicated simulation toolkit. Integrated with Garfield++, it provides an effective platform for systematic phenomenological studies of charging-up processes in MPGD detectors. We describe the simulation tool and the fine-tuning of the step-size required for the algorithm convergence, in relation to physical parameters. Simulation results of gain stability over time in THGEM detectors are presented, exploring the role of electrode-thickness and applied voltage on its evolution. The results show that the total amount of irradiated charge through electrode's hole needed for reaching gain stabilization is in the range of tens to hundreds of pC, depending on the detector geometry and operational voltage. These results are in agreement with experimental observations presented previously.

The bubble-assisted Liquid Hole Multiplier (LHM) is a recently-proposed concept for the combined detection of ionization electrons and primary scintillation photons in nobleliquid time projection chambers. The LHM comprises a perforated micro-pattern electrode (e.g. Thick Gas Electron Multiplier - THGEM, or Gas Electron Multiplier - GEM) immersed in the liquid, with a bubble of the noble gas supported underneath. Ionization electrons and scintillation-induced photoelectrons extracted from a cesium iodide photocathode drift through the electrode's holes and induce electroluminescence (EL) signals in the bubble; these are recorded by photon detectors located closely below the electrode. We present recent results in the development of LHMs, comparing the response of different electrodes to ionization and photon-induced electrons.

The XENON1T experiment at the Laboratori Nazionali del Gran Sasso (LNGS) is the first WIMP dark matter detector operating with a liquid xenon target mass above the ton-scale. Out of its 3.2 t liquid xenon inventory, 2.0 t constitute the active target of the dual-phase time projection chamber. The scintillation and ionization signals from particle interactions are detected with low-background photomultipliers. This article describes the XENON1T instrument and its subsystems as well as strategies to achieve an unprecedented low background level. First results on the detector response and the performance of the subsystems are also presented.

Gamma-ray and fast-neutron imaging was performed with a novel liquid xenon (LXe) scintillation detector read out by a Gaseous Photomultiplier (GPM). The 100 mm diameter detector prototype comprised a capillary-filled LXe converter/scintillator, coupled to a triple-THGEM imaging-GPM, with its first electrode coated by a CsI UV-photocathode, operated in Ne/5%CH₄ at cryogenic temperatures. Radiation localization in 2D was derived from scintillation-induced photoelectron avalanches, measured on the GPM's segmented anode. The localization properties of ⁶⁰Co gamma-rays and a mixed fast-neutron/gamma-ray field from an AmBe neutron source were derived from irradiation of a Pb edge absorber. Spatial resolutions of 12± 2 mm and 10± 2 mm (FWHM) were reached with ⁶⁰Co and AmBe sources, respectively. The experimental results are in good agreement with GEANT4 simulations. The calculated ultimate expected resolutions for our application-relevant 4.4 and 15.1 MeV gamma-rays and 1-15 MeV neutrons are 2-4 mm and ∼ 2 mm (FWHM), respectively. These results indicate the potential applicability of the new detector concept to Fast-Neutron Resonance Radiography (FNRR) and Dual-Discrete-Energy Gamma Radiography (DDEGR) of large objects.

The phenomenon of avalanche-gain variations over time, particularly in Micro Pattern Gaseous Detectors (MPGD) incorporating insulator materials, have been generally attributed to electric-field modifications resulting from "charging-up" effects of the insulator. A robust methodology for characterization of gain-transients in such detectors is presented. It comprises three guidelines: detector initialization, long gain-stabilization monitoring and imposing transients by applying abrupt changes in operation conditions. Using THWELL and RPWELL detectors, we validated the proposed methodology by assessing a charging-up/charging-down model describing the governing processes of gain stabilization. The results provide a deeper insight into these processes, reflected by different transients upon abrupt variations of detector gain or the irradiation rate. This methodology provides a handle for future investigations of the involved physics phenomena in MPGD detectors comprising insulating components.

We describe progress made with a liquid-Xe (LXe) detector coupled to a gaseous photomultiplier (GPM), for combined imaging and spectroscopy of fast neutrons and gamma-rays in the MeV range. The purpose of this detector is to enable the detection of hidden explosives and fissile materials in cargo and containers. The expected position resolution is about 2 m and 3.5 mm for fast neutrons and gamma-rays, respectively. Experimental results obtained using an ²⁴¹Am source yielded energy and time resolutions of 11% and 1.2 ns RMS, respectively. Initial results obtained with the position-sensitive GPM are presented.

DARk matter Wimp search with liquid xenoN (DARWIN²) will be an experiment for the direct detection of dark matter using a multi-ton liquid xenon time projection chamber at its core. Its primary goal will be to explore the experimentally accessible parameter space for Weakly Interacting Massive Particles (WIMPs) in a wide mass-range, until neutrino interactions with the target become an irreducible background. The prompt scintillation light and the charge signals induced by particle interactions in the xenon will be observed by VUV sensitive, ultra-low background photosensors. Besides its excellent sensitivity to WIMPs above a mass of 5 GeV/c², such a detector with its large mass, low-energy threshold and ultra-low background level will also be sensitive to other rare interactions. It will search for solar axions, galactic axion-like particles and the neutrinoless double-beta decay of ¹³⁶Xe, as well as measure the low-energy solar neutrino flux with

In-beam evaluation of a fully-equipped medium-size 30 × 30 cm² Resistive Plate WELL (RPWELL) detector is presented. It consists here of a single element gas-avalanche multiplier with Semitron ESD225 resistive plate, 1 cm² readout pads and APV25/SRS electronics. Similarly to previous results with small detector prototypes, stable operation at high detection efficiency (> 98%) and low average pad multiplicity (~ 1.2) were recorded with 150 GeV muon and high-rate pion beams, in Ne/(5%CH₄), Ar/(5%CH₄) and Ar/(7%CO₂). This is an important step towards the realization of robust detectors suitable for applications requiring large-area coverage; among them Digital Hadron Calorimetry.

Bubble formation in liquid xenon underneath a Thick Gaseous Electron Multiplier (THGEM) electrode immersed in liquid xenon was observed with a CCD camera. With voltage across the THGEM, the appearance of bubbles was correlated with that of electroluminescence signals induced by ionization electrons from alpha-particle tracks. This confirms recent indirect evidence that the observed photons are due to electroluminescence within a xenon vapor layer trapped under the electrode. The bubbles seem to emerge spontaneously due to heat flow from 300 K into the liquid, or in a controlled manner by locally boiling the liquid with resistive wires. Controlled bubble formation resulted in energy resolution of σ/E 7.5% for ∼ 6000 ionization electrons. The phenomenon could pave ways towards the conception of large-volume 'local dual-phase' noble-liquid TPCs.

In this work we discuss the mechanism behind the large electroluminescence signals observed at relatively low electric fields in the holes of a Thick Gas Electron Multiplier (THGEM) electrode immersed in liquid xenon. We present strong evidence that the scintillation light is generated in xenon bubbles trapped below the THGEM holes. The process is shown to be remarkably stable over months of operation, providing - under specific thermodynamic conditions - energy resolution similar to that of present dual-phase liquid xenon experiments. The observed mechanism may serve as the basis for the development of Liquid Hole Multipliers (LHMs), capable of producing local charge-induced electroluminescence signals in large-volume single-phase noble-liquid detectors for dark matter and neutrino physics experiments.

Two-phase Cryogenic Avalanche Detectors (CRADs) with THGEM multipliers, optically read out with Geiger-mode APDs (GAPDs), were proposed as potential technique for charge recording in rare-event experiments. In this work we report on the degradation of the GAPD performance at cryogenic temperatures revealed in the course of the study of two-phase CRAD in Ar, with combined THGEM/GAPD-matrix multiplier; the GAPDs recorded secondary scintillation photons from the THGEM holes in the Near Infrared. The degradation effect, namely the loss of the GAPD pulse amplitude, depended on the incident X-ray photon flux. The critical counting rate of photoelectrons produced at the 4.4 mm² GAPD, degrading its performance at 87 K, was estimated as ∼ 10⁴ s^-1. This effect was shown to result from the considerable increase of the pixel quenching resistor of this CPTA-made GAPD type. Though not affecting low-rate rare-event experiments, the observed effect may impose some limitations on the performance of CRADs with GAPD-based optical readout at higher-rate applications.

Two-phase Cryogenic Avalanche Detectors (CRADs) with GEM and THGEM multipliers have become an emerging potential technique for charge recording in rare-event experiments. In this work we present the performance of two-phase CRADs operated in Ar and Ar+N₂. Detectors with sensitive area of 10 × 10 cm², reaching a litre-scale active volume, yielded gains of the order of 1000 with a double-THGEM multiplier. Higher gains, of about 5000, have been attained in two-phase Ar CRADs with a hybrid triple-stage multiplier, comprising of a double-THGEM followed by a GEM. The performance of two-phase CRADs in Ar doped with N₂ (0.1-0.6%) yielded faster signals and similar gains compared to the operation in two-phase Ar. The applicability to rare-event experiments is discussed.

Thick Gas Electron Multipliers (THGEMs) have the potential of constituting thin, robust sampling elements in Digital Hadron Calorimetry (DHCAL) at future colliders. We report on recent beam studies of new single- and double-THGEM-like structures: the multiplier is a Segmented Resistive WELL (SRWELL) - a single-faced THGEM in contact with a segmented resistive layer inductively coupled to readout pads. Several 10 x 10 cm² configurations with a total thickness of 5-6 mm (excluding electronics) with 1 cm² pads were investigated with muons and pions. The pads were coupled to a scalable readout system APV chip, APV-SRS (Raymond et al. [22]). Detection efficiencies in the 98% range were recorded with an average pad-multiplicity of ∼ 1.1. The resistive anode resulted in efficient discharge clamping, with potential drops of a few volts; the discharge probabilities were similar to 10^-7 for muons and similar to 10^-6 for pions, at rates of a few kHz/cm² and for detectors in the double-stage configuration. Further optimization work and research on larger detectors are underway. 

The second Jπ=2⁺ state of C12, predicted over 50 years ago as an excitation of the Hoyle state, has been unambiguously identified using the C12(γ,α₀)Be8 reaction. The alpha particles produced by the photodisintegration of C12 were detected using an optical time projection chamber. Data were collected at beam energies between 9.1 and 10.7 MeV using the intense nearly monoenergetic gamma-ray beams at the HIγS facility. The measured angular distributions determine the cross section and the E1-E2 relative phases as a function of energy leading to an unambiguous identification of the second 2⁺ state in C12 at 10.03(11) MeV, with a total width of 800(130) keV and a ground state gamma-decay width of 60(10) meV; B(E2:22+→01+)=0.73(13)e2 fm4 [or 0.45(8) W.u.]. The Hoyle state and its rotational 2⁺ state that are more extended than the ground state of C12 presents a challenge and constraints for models attempting to reveal the nature of three alpha-particle states in C12. Specifically, it challenges the ab initio lattice effective field theory calculations that predict similar rms radii for the ground state and the Hoyle state.

Radiation-induced proportional-electroluminescence UV signals, emitted from the holes of a Thick Gas Electron Multiplier (THGEM) electrode immersed in liquid xenon, were recorded with a PMT for the first time. Significant photon yields were observed with gamma photons and alpha particles using a 0.4 mm thick electrode with 0.3 mm diameter holes; at 2 kV across the THGEM the photon yield was estimated to be ∼ 600 UV photons/electron over 4π. This may pave the way towards the realization of novel single-phase noble-liquid radiation detectors incorporating liquid hole-multipliers (LHM); their concept is presented.

We present the results of first studies of the Resistive Plate WELL (RPWELL): a single-faced THGEM coupled to a copper anode via a resistive layer of high bulk resistivity. We explored various materials of different bulk resistivity (10⁹ - 10¹² Ωcm) and thickness (0.4 - 4 mm). Our most successful prototype, with a 0.6 mm resistive plate of ∼ 10⁹ Ωcm, achieved gains of up to 10⁵ with 8 keV x-ray in Ne/5%CH₄; a minor 30% gain drop occurred with a rate increase from 10 to 10⁴ Hz/mm2. The detector displayed a full "discharge-free" operation - even when exposed to high primary ionization events. We present the RPWELL detector concept and compare its performance to that of other previously explored THGEM configurations - in terms of gain, its curves, dependence on rate, and the response to high ionization. The robust Resistive Plate WELL concept is a step forward in the Micro-Pattern Gas-Detector family, with numerous potential applications.

THGEM investigations in Ne mixtures focused so far atmospheric pressures. However, the applications of GPMs operating at cryogenic temperatures result in THGEM operation in neon mixtures with gas densities higher than those at atmospheric pressure at room temperature. In addition, X-ray and charged particle detection would benefit from higher gas densities. In this work we studied the room temperature operation of Single and Double-THGEM detectors in Ne/CF₄ gas mixtures with pressures up to 3bar. Measurements with soft X-rays obtained gains of 7 × 10⁴ and 3 × 10⁴ with a Double-THGEM, in 1bar and 3bar of Ne/CF₄ (95:5), respectively. For Ne/CF₄ (90:10), gains of 1 × 10⁵ and 4 × 10⁴, were obtained with the Double-THGEM. It is shown that the maximum achievable gain is not significantly affected by the increase of gas pressure. Energy resolution for 5.9keV X-rays was of the order of 23%. In this work, we also report on the effect of our getter-based gas purification system and the consequent gas impurity on the THGEM's performance. We show that without purification the operation voltages of the THGEM detector gradually increase with time, requiring higher voltages to reach a given gain, while the maximum achieved gain did not change.

The properties of UV-photon imaging detectors consisting of CsI-coated THGEM electron multipliers are summarized. New results related to detection of Cherenkov light (RICH) and scintillation photons in noble-liquid are presented.

The very high momentum particle identification detector proposed for the ALICE upgrade is a focusing RICH using a C₄F₁₀ gaseous radiator. For the detection of Cherenkov photons, one of the options currently under investigation is to use a CsI-coated triple thick GEM with metallic or resistive electrodes. We will present results from the laboratory studies as well as preliminary results of beam tests of a RICH detector prototype consisting of a CaF₂ radiator coupled to a 10×10 cm² CsI-coated triple thick GEM equipped with a pad readout and GASSIPLEX-based front-end electronics. With such a prototype the detection of Cherenkov photons simultaneously with minimum ionizing particles has been achieved for the first time in a stable operation mode.

Operation results are presented of a UV-sensitive gaseous photomultiplier (GPM) coupled through a MgF₂ window to a liquid-xenon scintillator. It consisted of a reflective CsI photocathode deposited on top of a THick Gaseous Electron Multiplier (THGEM); further multiplication stages were either a second THGEM or a Parallel Ionization Multiplier (PIM) followed by a MICROMEsh GAseous Structure (MICROMEGAS). The GPM operated in gas-flow mode with non-condensable gas mixtures. Gains of 10⁴ were measured with a CsI-coated double-THGEM detector in Ne/CH₄(95:5), Ne/CF ₄(95:5) and Ne/CH₄/CF₄ (90:5:5), with soft X-rays at 173 K. Scintillation signals induced by alpha particles in liquid xenon were measured here for the first time with a double-THGEM GPM in He/CH₄(92.5:7.5) and a triple-structure THGEM/PIM/MICROMEGAS GPM in Ne/CH₄(90:10) with a fast-current preamplifier.

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1

Background PSA blood test and other present screening tools fail to provide the required sensitivity and specificity and, at early stages, lack correlation with tumor grade, volume, and location. Thus alternative approaches are highly desired. We present and assess a novel method for PCa detection, grading, volume evaluation and tumor location, based on non-invasive zinc concentration mapping in the gland by means of a dedicated rectal probe. Methods Zinc-concentration values measured in histologically examined tissue fragments from needle biopsy of 598 patients were analyzed. They were used to generate computer simulated zinc-concentration maps, further analyzed with image-processing tools. The tumor detection performances versus Gleason grade were assessed. Results A significant increase of zinc depletion with increasing Gleason pattern (grade) classification was established. Tumor detection performance in zinc-concentration maps progressively improves with the cancer's first component score. Reliable information on the location, size and Gleason-grade combination of the lesion can be extracted for clinically relevant volumes. Conclusions Zinc depletion in the prostate peripheral zone is the basis for a novel, non-invasive PCa detection, localization, volume evaluation and grading method. Its realization and application as a pre-biopsy and pre-treatment examination, or a follow-up tool, relies on the development of a dedicated transrectal probe. It should have significant impact on biopsy effectiveness, point at a possible extraprostatic extension and provide critical data for focal treatment. The information on tumor grade and distribution may have an important impact on disease management. Prostate 70: 1701-1708, 2010.

The operation principle and preliminary results of a novel gas-avalanche patterned hole electron multiplier, the Thick-COBRA (THCOBRA), are presented. This micro-hole structure is derived from the THGEM and MHSP. Sub-millimeter diameter holes are mechanically drilled in a thin G10 plate, Cu-clad on both faces; on one of the faces the Cu is etched to produce additional anode strips winding between circular cathode strips. Primary avalanches occurring within the holes are followed by additional ones at the anode-strips vicinity. Gains in excess of 5^*104were reached with 22.1 keV x-rays in Ar, Ne and Ar-10%CH4, with 12.2 % FWHM energy resolution in Ar-10%CH4. Higher gains were measured with single photoelectrons. This robust multiplier may have numerous potential applications.

The operation of single-, double- and triple-THGEM UV-detectors with reflective CsI photocathodes (CsI-THGEM) in Ne/CH₄ and Ne/CF ₄ mixtures was investigated in view of their potential applications in RICH. The studies were carried out with UV, x-rays and β-electrons and focused on the maximum achievable gain, discharge probability, cathode excitation effects and long-term gain stability. Comparative studies under similar conditions were made in CH₄, CF₄ and Ne/CF ₄, with a MWPC coupled to a reflective CsI photocathode (CsI-MWPC). It was found that at counting rates ≤ 10 Hz/mm² the maximum achievable gain of CsI-THGEMs is determined by the Raether limit; at counting rates > 10 Hz/mm² it dropped with rate. In all cases investigated the attainable CsI-THGEM gain was significantly higher than that of the CsI-MWPC, under similar conditions. Furthermore, the CsI-THGEM UV-detector suffered fewer cathode-excitation induced effects as compared to CsI-MWPC and had better stability at high counting rates.

We briefly present the principal properties of THGEM detectors, in standard and in noble gases (Xe, Kr, Ar and Ne), and shortly discuss their potential applications in accelerator- and non-accelerator-based experiments. UV-photon imaging in RICH, tracking of ionizing particles, large-volume TPCs for rare-events, sampling elements in digital calorimetry, ionization and scintillation recording from noble-liquid detectors, X-ray and neutron radiography etc.

The dependence of the avalanche charge gain in Thick Gas Electron Multipliers (THGEM) on the purity of Ne, Ar and Xe filling gases was investigated. The gain, measured with alpha-particles in standard conditions (atmospheric pressure, room temperature), was found to considerably drop in gases purified by non-evaporable getters. On the other hand, small N(2) admixtures to noble gases resulted in high reachable gains. The results are of general relevance in the operation of gas-avalanche detectors in noble gases, particularly that of two-phase cryogenic detectors for rare events.

We summarize the development of visible-sensitive gaseous photomultipliers, combining a semitransparent bi-alkali photocathode with a state-of-the-art cascaded electron multiplier. The latter has high photoelectron collection efficiency and a record ion blocking capability. We describe in details the system and methods of photocathode production and characterization, their coupling with the electron multiplier and the gaseous-photomultiplier operation and characterization in a continuous mode. We present results on the properties of laboratory-produced K₂CsSb, Cs₃Sb and Na₂KSb photocathodes and report on their stability and QE in gas; K₂CsSb photocathodes yielded QE values in Ar/CH₄(95/5) above 30% at wavelengths of 360-400 nm. The novel gaseous photomultiplier yielded stable operation at gains of 10⁵, in continuous operation mode, in 700 Torr of this gas; its sensitivity to single photons was demonstrated. Other properties are described. The successful detection of visible light with this gas-photomultiplier pave ways towards further development of large-area sealed imaging detectors, of flat geometry, insensitive to magnetic fields, which might have significant impact on light detection in numerous fields.

The operation of Thick Gaseous Electron Multipliers (THGEM) in Ne and Ne/CH₄ mixtures, features high multiplication factors at relatively low operation potentials, in both single- and double-THGEM configurations. We present some systematic data measured with UV-photons and soft x-rays, in various Ne mixtures. It includes gain dependence on hole diameter and gas purity, photoelectron extraction efficiency from CsI photocathodes into the gas, long-term gain stability and pulse rise-time. Position resolution of a 100 × 100 mm² X-ray imaging detector is presented. Possible applications are discussed.

We briefly review the concept and properties of the THick Gaseous Electron Multiplier (THGEM); it is a robust, high-gain gaseous electron multiplier, manufactured economically by standard printed-circuit drilling and etching technology. Its operation and structure resemble that of gaseous electron multiplier's (GEM's) but with 5-20-fold expanded dimensions. The millimeter-scale hole-size results in good electron transport and in large avalanche-multiplication factors, e.g. reaching 10⁷ in double-THGEM cascaded single-photoelectron detectors. The multiplier's material, parameters and shape can be application-tailored; it can operate practically in any counting gas, including noble gases, over a pressure range spanning from 1 mbar to several bars; its operation at cryogenic (LAr) conditions was recently demonstrated. The high gain, sub-millimeter spatial resolution, high counting-rate capability, good timing properties and the possibility of industrial production capability of large-area robust detectors, pave ways towards a broad spectrum of potential applications; some are discussed here in brief.

We shortly describe recent progress in photon detectors combining bi-alkali photocathodes and cascaded patterned gas-avalanche electron multipliers. It permitted the development and the first feasibility demonstration of high-gain gaseous photomultipliers sensitive in the visible spectral range, operated in continuous-mode with single-photon sensitivity.

We present the operation of the recently introduced Photon Assisted Cascaded Electron Multiplier (PACEM) in xenon at high pressure. The PACEM is a multi step electron multiplier where the VUV electroluminescence produced in the electron avalanches is used for signal propagation: the VUV electroluminescence produced in the first element of the cascade induces the emission of photoelectrons from a CsI photocathode placed on the top-surface of the second element. These photoelectrons are further multiplied, via charge avalanche. A metallic mesh electrode placed between the first and the second elements of the cascade completely blocks the charge transfer between them. Optical gains of 10³ were achieved in xenon at atmospheric pressure, dropping to 25 at 3.3 bar, for applied voltages of 700 and 1100 V, respectively. Taking into account the subsequent charge multiplication, total gains are higher than those obtained with a triple GEM and double THGEM.

The properties of the THick Gas Electron Multiplier (THGEM) operated with soft X-rays in Kr at room temperature, are presented. Charge gains ranging from 10⁴ to 3 × 10² and from 4 × 10⁴ to 10³ were reached with single- and double-THGEM detectors in a pressure range of 0.5-3.0 bar, respectively, for operation voltages in the range of 800-2600 V. The gain was limited by photon- and ion-feedback effects, and by electron field-emission from the Cu-electrodes at THGEM operation voltages above 2000 V. Energy resolutions in the range of 21-34% FWHM were measured with 5.9 keV X-rays. Other characteristics, including pulse-shape and electric fields in THGEM structures, are discussed.

The operation of the recently introduced Photon Assisted Cascaded Electron Multiplier (PACEM) in CF₄ is investigated. The PACEM uses the VUV scintillation produced in the electron avalanches of the first multiplier of the cascade to transfer the signal to the subsequent ones. The VUV scintillation induces the emission of a large number of photoelectrons from a CsI photocathode placed on the top-surface of the second multiplier. The photoelectrons are further multiplied in the subsequent stages of the cascade, resulting in efficient signal amplification. A mesh, set at a fixed voltage, is placed between the first and the second multi-pliers to block the charge transfer between them, thus suppressing all the ion backflow (IBF) heading to the first cascade element. The PACEM electrically isolates the first multiplier of the cascade and only the ions produced in the electron avalanches of the first element may flow back into the drift region. Operating in CF₄, absolute IBFs as low as ∼1 ion per primary electron are achieved for drift fields of 0.1 kV/cm, while for fields of 0.5 kV/cm the absolute IBF is ∼10 ions/primary electron. This corresponds to an IBF of 10^-4 at gains of 10⁴, for Time Projection Chambers (TPC) operating conditions, and IBFs of ∼10^-5 at gains of 10⁶ for Gaseous Photomultipliers (GPM) operating conditions.

A tracking ion counting nanodosimeter was employed to acquire spatial ionization patterns produced by charged particles in propane gas at 1.3 mbar. Data were taken at the James M. Slater MD Proton Treatment and Research Center with 250 MeV, 17 MeV, 5 MeV and 1.5 MeV proton beams, 4.8 MeV alpha particles and electrons from a Sr-90/Y-90 source. For each particle type, measurable quantities used for track structure reconstruction included the number of ionizations and their location within a wall-less, cylindrical sensitive volume measured with a resolution of about 5 tissue-equivalent nanometers, and primary particle coordinates. Measured ionization frequency distributions as a function of distance from particle track were compared with results of a dedicated Monte Carlo track structure code.

We review latest progress in gaseous photomultipliers (GPMs) combining solid photocathodes and various types of novel electron multipliers. Cascaded gaseous electron multipliers (GEMs) coated with CsI photocathodes can efficiently replace UV-sensitive wire chambers for single-photon recording in Cherenkov and other detectors. Other hole-multipliers with patterned electrodes (Micro-Hole and Strip Plates) and improved ion-blocking properties are discussed; these permit reducing considerably photon- and ion-induced secondary effects. Photon detectors with other electron-multiplier techniques are briefly described, among them GPMs are based on Micromegas, capillary plates, Thick-GEMs and resistive Thick-GEMs. The two latter techniques, robust and economically produced, are particularly suited for large-area GPM applications, e.g. in RICH. Cascaded hole-multipliers with very high ion-blocking performance permitted the development and the first demonstration of visible-sensitive GPMs operated in continuous mode, with bialkali photocathodes and single-photon sensitivity. Recent progress is described in GPMs operating at cryogenic temperatures for rare-event noble-liquid detectors and medical imaging.

We present the results of our recent studies of a Thick Gaseous Electron Multiplier (THGEM)-based detector, operated in Ar, Xe and Ar:Xe (95:5) at various gas pressures. Avalanche-multiplication properties and energy resolution were investigated with soft x-rays for different detector configurations and parameters. Gains above 10⁴ were reached in a double-THGEM detector, at atmospheric pressure, in all gases, in almost all the tested conditions; in Ar:Xe (95:5) similar gains were reached at pressures up to 2 bar. The energy resolution dependence on the gas, pressure, hole geometry and electric fields was studied in detail, yielding in some configurations values below 20% FWHM with 5.9 keV x-rays.

We report on recent measurements with Thick GEM-like (THGEM)-based imaging detectors. The THGEM is a robust gaseous electron multiplier similar to GEM but with larger dimensions. It has high electron multiplication, of 10⁵ and 10⁷ in single- and double-THGEM structure, respectively, fast signals and ∼ 10 MHz / mm² counting rate capability. It can be produced in any shape and over large area. In view of many possible applications of THGEM-based imaging detectors, in particle physics and beyond, we have recently studied the localization properties of a 2D 10 × 10 cm² detector. The results of these studies are presented.

The operation of the recently introduced Photon Assisted Cascaded Electron Multiplier (PACEM) in CF₄ is investigated. The PACEM uses the VUV scintillation produced in the electron avalanches of the first multiplier of the cascade to transfer the signal to the subsequent ones. The VUV scintillation induces the emission of a large number of photoelectrons from a CsI photocathode placed on the top-surface of the second multiplier. The photoelectrons are further multiplied in the subsequent stages of the cascade, resulting in efficient signal amplification. A mesh, set at a fixed voltage, is placed between the first and the second multipliers to block the charge transfer between them, thus suppressing all the ion backflow (IBF) heading to the first cascade element. The PACEM electrically isolates the first multiplier of the cascade and only the ions produced in the electron avalanches of the first element may flow back into the drift region. Operating in CF₄, absolute IBFs as low as ∼1 ion per primary electron are achieved for drift fields of 0.1 kV/cm, while for fields of 0.5 kV/cm the absolute IBF is ∼10 ions/primary electron. This corresponds to an IBF of 10^-4 at gains of 10⁴, for TPC operating conditions, and IBFs of ∼10^-5 at gains of 10⁶ for GPM operating conditions.

We present the Photon-Assisted Cascaded Electron Multiplier (PACEM) as a potential alternative for ion back-flow suppression in gaseous cascade electron multipliers. Using a Micro Hole and Strip Plate-Gas Electron Multiplier (MHSP-GEM) configuration, the number of ions flowing back to the scintillation region is about 1.5 ions per primary electron at an optical gain of 6.5 and a drift field of 0.1 kV/cm, and about 10 ions per primary electron at an optical gain of 10 and a drift field of 0.5 kV/cm. These allow reaching ion back-flow values close to 10^-4 and 10^-5 at typical operation conditions of TPCs and GPMs, respectively.

The performance of a new VUV photosensor, based on an micro-hole and strip plate (MHSP) electron multiplier with a CsI photocathode deposited on its top electrode, is described. This photosensor presents gains above 10⁴ when operating in an Ar-5%Xe gas mixture at 1 atm. Although the gain of a single MHSP is not sufficient for efficient detection of single photons, this simple UV photosensor may be useful for the detection of higher light levels, such as primary- and secondary scintillation in noble gases.

A new idea on electrostatic deviation and capture of back-drifting avalanche-ions in cascaded gaseous hole-multipliers is presented. It involves a flipped reversed-bias Micro-Hole & Strip Plate (F-R-MHSP) element, the strips of which are facing the drift region of the multiplier. The ions, originating from successive multiplication stages, are efficiently deviated and captured by such electrode. Experimental results are provided comparing the ion-blocking capability of the F-R-MHSP to that of the reversed-bias Micro-Hole & Strip Plate (R-MHSP) and the Gas Electron Multiplier (GEM). Best ion-blocking results in cascaded hole-multipliers were reached with a detector having the F- R-MHSP as the first multiplication element. A three-element F-R-MHSP/GEM/MHSP cascaded multiplier operated in atmospheric-pressure Ar/CH4 (95/5), at total gain of similar to 10(5), yielded ion back-flow fractions of 3 . 10(-4) and 1.5 . 10(-4), at drift fields of 0.5 and 0.2 kV/cm, respectively. We describe the F-R-MHSP concept and the relevance of the obtained ion back-flow fractions to various applications; further ideas are also discussed.

We present and discuss in details a concept for the reduction of ion back-flow in GEM-based cascaded gaseous electron multipliers, by incorporating Micro-Hole & Strip Plate (MHSP) elements operating in reversed-bias mode (R-MHSP). About an order of magnitude reduction in ion back-flow is achieved by diverting back-drifting ions from their original path. A R-MHSP/2GEM/MHSP cascaded multiplier operated at total gain of ∼ 1.5 * 10⁵ yielded ion back-flow fractions of 1.5*10^-3 and 4*10 ^-4, at drift fields of 0.5 and 0.1 kV/cm, respectively. A 2R-MHSP/MHSP cascaded multiplier operated at a total gain of ∼ 105, yielded an ion back-flow fraction of 3*10^-3. We discuss the concept for trapping back-flowing ions in these cascaded multipliers and the relevance to gaseous photomultiplier and TPC applications; directions for future developments are outlined.

A new imaging method that combines high-efficiency fast-neutron detection with sub-ns time resolution is presented. This is achieved by exploiting the high neutron detection efficiency of a thick scintillator and the fast timing capability and flexibility of light-pulse detection with a dedicated image intensifier. The neutron converter is a plastic scintillator slab or, alternatively, a scintillating fibre screen. The scintillator is optically coupled to a pulse counting image intensifier which measures the 2-dimensional position coordinates and the Time-Of-Flight (TOF) of each detected neutron with an intrinsic time resolution of less than 1 ns. Large-area imaging devices with high count rate capability can be obtained by lateral segmentation of the optical readout channels.

Over the past two years, we have developed and tested an efficient, large-area, sub-mm spatialresolution, fast-neutron imaging system with time-of-flight spectroscopic capability. The detector is based on a 30 mm thick scintillating fiber screen viewed by a time-gated optical readout, described in another contribution to this conference. In order to analyze key parameters affecting detector performance, Monte-Carlo simulations using the GEANT 3.21 code were performed. To characterize the intrinsic spatial resolution of the scintillating fiber screen, a neutron transmission image of a steel mask containing a series of slits with various widths, pitch and thicknesses was simulated. The point spread function of the scintillating fiber screen was determined by exposure to an infinitesimally narrow neutron beam, incident perpendicular to the surface, calculating the spatial distribution of the energy deposited by the protons in the screen fibers. The energy distribution of the (n,p) protons produced in the screen and the amount of scintillation light subsequently created were also calculated. All the above simulations were performed for 3 neutron energies (2, 7.5 and 14 MeV). For the detector tests performed at the PTB cyclotron, the neutron beam-line geometry was simulated as accurately as possible, in order to calculate the contribution of neutron scatter as well as gamma rays and to enable a comparison with the experimental results.

Nanodosimetric spectra, measured in a well-defined ionisation sensitive volume of an ion-counting gaseous nanodosemeter, may have a valuable predictive value of radiation damage to DNA. In such devices, the distributions of radiation-induced ions are measured after their drift in gas. The sensitive-volume size, corresponding to a DNA segment length, can be tuned by selecting an appropriate time window for ion counting; the method's accuracy depends on the velocity distribution of the drifting ions. The results of ion-drift measurements in an ion-counting nanodosemeter were used for the precise calculation of its sensitive volume length. Monte Carlo simulations of nanodosimetric spectra, performed with the obtained data, are in good agreement with experimental data. The method's limitations, arising from the spread of drift velocities, are discussed.

An ion-counting nanodosemeter (ND) yielding the distribution of radiation-induced ions in a low-pressure gas within a millimetric, wall-less sensitive volume (SV) was equipped with a silicon microstrip telescope that tracks the primary particles, allowing correlation of nanodosimetric data with particle position relative to the SV. The performance of this tracking ND was tested with a broad 250 MeV proton beam at Loma Linda University Medical Center. The high-resolution tracking capability made it possible to map the ion registration efficiency distribution within the SV, for which only calculated data were available before. It was shown that tracking information combined with nanodosimetric data can map the ionisation pattern of track segments within 150 nm-equivalent long SVs with a longitudinal resolution of ∼5 tissue-equivalent nanometers. Data acquired in this work were compared with results of Monte Carlo track structure simulations. The good agreement between 'tracking nanodosimetry' data acquired with the new system and simulated data supports the application of ion-counting nanodosimetry in experimental track-structure studies.

We studied electron amplification and light emission from avalanches in oxygen-containing gas mixtures. The mixtures investigated in this work included, among others, CO₂ and N₂O mixed with Triethylamine (TEA) or N₂. Double-Step Parallel Gap (DSPG) multipliers and THick Gas Electron Multipliers (THGEM) were investigated. High light yields were measured from CO₂+N₂ and CO₂+TEA, though with different emission spectra. We observed the characteristic wave-length emission of N ₂ and of TEA and used a polymer wave-length shifter to convert TEA UV-light into the visible spectrum. The results of these measurements indicate the applicability of optical recording of ionizing tracks in a TPC target-detector designed to study the cross-sections of the ¹⁶O(γ,α)¹²C reaction, a central problem in nuclear astrophysics.

One of our two methods for fast-neutron imaging with spectrometric capability is presented here. It is a neutron-counting technique based on a hydrogenous neutron converter coupled to Gaseous Electron Multipliers (GEM). The principles of the detection techniques and the optimization of the converter, electron amplification and the readout are described. Evaluation of the properties is derived from a experiment in a pulsed neutron beam of spectral distribution between 2 and 10 MeV.

We present a review on the research carried out with the Micro-Hole & Strip plate (MHSP) gaseous electron multiplier. In this new device charge multiplication occurs in two stages: radiation-induced electrons are multiplied in small holes and the resulting avalanche is further multiplied on anode strips patterned on the multiplier's bottom electrode. This structure provides fast multiplication process, has a high total gain even in noble gas mixtures and provides good localization properties. Detectors based on this principle have shown gains above 10⁴ and energy resolution of about 14% for 5.9-keV X-rays; large gains were obtained at high gas pressures as well, allowing for efficient detection of higher-energy X-rays. The properties of UV-photon detectors comprising an MHSP coupled to semitransparent CsI photocathodes or with reflective ones deposited on the multiplier's top surface were investigated. The operation mechanism of these detectors and their most significant results are presented.

We report on the properties of thick GEM-like (THGEM) electron multipliers made of 0.4 mm thick double-sided Cu-clad G-10 plates, perforated with a dense hexagonal array of 0.3 mm diameter drilled holes. Photon detectors comprising THGEMs coupled to semi-transparent CsI photocathodes or reflective ones deposited on the THGEM surface were studied with Ar/CO2 (70:30), Ar/CH4 (95:5), CH4 and CF4. Gains of ∼105 or exceeding 106 were reached with single- or double-THGEM, respectively; the signals have 5-10 ns rise times. The electric field configurations at the THGEM electrodes result in an efficient extraction of photoelectrons and their focusing into the holes; this occurs already at rather low gains, below 100. These detectors, with single-photon sensitivity and with expected sub-millimeter localization, can operate at MHz/mm2 rates. We discuss their prospects for large-area UV-photon imaging for RICH.

A new mathematical method of measuring electron emission induced by low energy ions from solids is described and used to calculate secondary electron emission according to the recorded pulse-height spectra of ions and ultraviolet (UV) photons. Using the UV single secondary electron spectra, we predict the shape of many secondary electron distributions under consideration of detection efficiency of MCP detector. These calculated distributions allow us to characterize the secondary electrons yield, and to give a secondary electron distribution for measured data. It seems rather feasible to determine secondary electron yield emitted by low energy ions at very low ion fluxes.

In this work we report on absolute quantum photoyield (QPY) measurements in the 140-220 nm (8.85-5.63 eV) photon energy range of nanometer thick diamond films. The QPY value at a photon wavelength of 140 nm increases with film deposition time from 2.5% (carbonized silicon substrate) to 14% for a nominal film thickness of 70 nm, followed by a decrease to 12% and stabilization for continuous films of thicknesses above ∼150-200 nm. Prior to QPY measurements the film's surface was conditioned by exposure to microwave hydrogen plasma, to induce negative electron affinity. The surface properties, phase composition and microstructure of the films were examined by electron spectroscopic and microscopic methods. Different factors are considered to explain the enhancement of QPY of the nano-metric thick films.

We present the performance of a Micro-Hole and Strip Plate (MHSP) electron multiplier in argon-xenon mixtures, at pressures of 1-7 bar. This microstructure can operate at high pressures without significant reduction of the maximum achievable gain. Absolute gains of 1-4 × 10³ were reached in Ar/50mbar Xe over this pressure range; the maximum gain is imposed by the discharge limit, dropping at higher pressures. Energy resolutions between 14% and 16% were reached for 6 keV X-rays; they do not degrade significantly with increasing pressure. Better performances are expected by improved manufacturing of the MHSP.

In gaseous photomultipliers, avalanche generated ions back-flowing to the photocathode can drastically limit the detector operation and lifetime. This is especially the case for photocathodes with low electron emission threshold, where impinging ions induce ion feedback effects by secondary electron emission. We present ways of reducing ion-backflow to the photocathode, and thus suppress ion-feedback effects in multi-stage Gas Electron Multiplier detectors. We studied the effect of the various electric fields on the ion transport in the detector and present our results on active ion gating with a dedicated gating electrode.

Gas Electron Multipliers with a reflective photocathode deposited on the surface of the first multiplying element are very attractive devices for photon detection and imaging over large area at moderate cost. They combine production and operation simplicity, high sensitivity to single photons, fast time response and accurate localization. In this work we present in detail the mechanisms governing the operation of these photon detectors. The results of electron extraction, transfer, multiplication and detection processes in this multi-element structure are presented and analyzed. We discuss the role of important elements and parameters influencing the detector's operation and performance: the GEM geometry, the choice of the different electric fields and the gas mixture.

We studied gain and position resolution of gaseous UV-photon detectors combining single- and cascaded- glass capillary-plate multipliers and CsI photocathodes. Two modes of operation were investigated: a conventional one, where the main amplification occurs within capillary holes and a parallel-plate amplification mode, where the main amplification occurs between the capillary plate and the readout anode. Results of these studies demonstrate that in the parallel-plate amplification mode one can reach both high gains (>10 ⁵) and good position resolutions (~100μm) even with a single-element multiplier. This offers a compact amplification structure, which can be used in many applications.

We report on the performance of a micro-hole and strip plate, fabricated with standard gas electron multiplier-production procedures, presenting 40-μm hole-diameter and 30-μm wide anode strips. Multiplication factors of 5 × 10⁴ were reached in an Ar/Xe (95/5) atmosphere at about 1bar; the energy resolution is of the order of 14% (FWHM) for 5.9-keV X-rays.

We summarize our recent advances in gaseous photomultipliers (GPMTs) for the UV and visible spectral range. They combine photocathodes and advanced multi-stage Gas Electron Multipliers (GEMs). Principles of operation and properties are discussed, with emphasis on time and localization resolutions, ion-feedback suppression, novel electron multipliers and sealed photon detectors for visible light.

We present results from our recent investigations on detectors comprising cascaded gas electron multipliers (GEM) and cascaded GEMs with micro-hole and strip (MHSP) electrodes as a final amplification stage. We discuss the factors governing the operation of these fast radiation-imaging detectors, which have single-charge sensitivity. The issue of ion-backflow and ion-induced secondary effects is discussed in some detail, presenting ways for its suppression. Applications are presented in the fields of photon imaging in the UV-to-visible spectral range as well as x-ray and neutron imaging.

A 100 × 100 mm² 2D imaging detector, based on a triple-GEM gaseous multiplier, striped x - y readout anode and discrete delay-line readout, is presented. The fast (2.1 ns tap^-1) delay-line circuit was designed to match the anode-charge signal profile, namely its rise-time and length. The detector's imaging capability was systematically studied in Ar/CO₂ (70/30) with 5.9 keV X-rays; x - y resolution of σ = 0.05 and 0.1 mm for top and bottom anode strips, respectively, and integral non-linearity of ∼0.15% are demonstrated.

We report on the performance of a new gaseous electron multiplier: the Micro-Hole & Strip Plate (MHSP). It consists of two independent charge-amplification stages in a single, double-sided micro-structured electrode, deposited on a thin insulating substrate. Charge gains in excess of 10³ were obtained in a MHSP operated with soft X-rays in Ar/CO₂ (70/30). We present the results of a systematic study of the MHSP properties and those of a double-stage GEM+MHSP multiplier. Applications to gaseous photomultipliers are discussed.

We discuss a new thermal-neutron imaging detector that combines a neutron converter coupled to a position-sensitive gaseous photodetector. The neutron converter consists of a thin (0.5mm) layer of boron-rich liquid scintillator. It is viewed by an atmospheric-pressure, gas-avalanche photomultiplier with a bialkali photocathode. Scintillation-induced photoelectrons are multiplied by a cascade of Gas Electron Multipliers (GEM). The multi-GEM supplies the pulse-height, time and position information for each converted neutron. Such a fast, large-area detector can operate at high radiation flux.

We present the performance of a sealed gaseous photomultiplier consisting of a cascade of 3 or 4 Gas Electron Multiplier (GEM) elements coupled to a semitransparent CsI photocathode, in Ar/CH₄ (95/5). A few-month stability study of the photocathode in a sealed mode is presented. Increasing the number of GEMs in cascade substantially reduces the ageing of the detector under strong irradiation. The ion feedback to the photocathode has probably a minor effect on the ageing rate.

The transport of low-energy (

We describe the operation principle and properties of a CsI-coated GEM photodetector. This type of detector performs photon detection with reflective photocathodes, which are easy to produce and have high quantum efficiency. In the proposed configuration, the detector is practically free of avalanche-induced photon feedback effects. The influence of the GEM voltage and the electric fields close to the CsI-GEM electrode on the photon detection efficiency is studied. The conditions for obtaining full extraction of photoelectrons from the photocathode and their transfer through the GEM apertures are presented.

We report on the improvements in the position sensitive readout of a xenon-filled gas scintillation proportional counter. Using an imaging optic for UV-light in the region of 170 nm, the position resolution could be improved by more than 30%. In addition, we have obtained first encouraging results for the use of the recently developed gas electron multiplier together with a CsI-photocathode as a large area UV-detector system.

The effect of moderate heating (200-300°C) in vacuum on the photoemission from air-exposed hydrogen-terminated chemical vapor deposited diamond films was studied in the photon spectral range of 140-210 nm (8.9-5.9 eV). A three- to fivefold enhancement was observed, stable in high vacuum and in some high purity gases, but unstable in air. The surfaces were also examined by x-ray induced photoelectron spectroscopy and ultraviolet induced photoelectron spectroscopy before and after the heating process and upon exposure to air and to oxygen. The results provide good evidence that the strong dipole originating from H₂O molecules absorbed on the diamond surface is responsible for the observed effect. A simple model is presented for quantitative estimation of the effect.

A sealed gas photon detector with a GEM (Gas Electron Multiplier) cascade coupled to a semitransparent CsI photocathode has been studied. High gains, reaching 10⁶, and fast anode signals, of a width of 15 ns, were obtained with this GEM-Photomultiplier in Ar/CH₄ (95/5), in a photon counting mode. The life-time of this first prototype detector under high photon flux and large anode currents is provided.

We report on the absolute single-electron transfer efficiency of a Gas Electron Multiplier (GEM). It is shown that the electron transfer and thus the detection efficiency, depend not only on the GEM geometry and gain but mostly on the electric field and electron diffusion in the gas volume preceding the GEM. We have demonstrated that conditions can be found, including pre-amplification of the single electrons in the gap preceding the GEM, in which full detection efficiency is obtained. The experimental electron transfer efficiency results are confirmed by Monte Carlo simulations.

Modern gas avalanche detectors are instruments of choice for detecting single charges deposited in gas or emitted from thin solid radiation converters. We discuss principal factors governing the operation of gas avalanche photomultipliers, combining solid photocathodes with advanced micro-pattern gaseous multipliers and summarize the properties of UV photocathodes and film-protected photocathodes for the visible spectral range. We review recent progress and applications of single-charge counting detectors and discuss in some detail their application to nanodosimetry and its relevance to studies of radiation damage to DNA.

Modern gas avalanche detectors are instruments of choice for detecting single charges deposited in gas or emitted from thin solid radiation converters. We discuss principal factors governing the operation of gas avalanche photomultipliers, combining solid photocathodes with advanced micro-pattern gaseous multipliers and summarize the properties of UV photocathodes and film-protected photocathodes for the visible spectral range. We review recent progress and applications of single-charge counting detectors and discuss in some detail their application to nanodosimetry and its relevance to studies of radiation damage to DNA.

Gas avalanche detectors are instruments of choice for radiation detection and localization in numerous fields of basic and applied research. Recent advances in detection techniques, involving multiplication and detection of single or a few charges deposited in gas media, or emitted from solid converters into gas, are described. The properties of radiation converters and associated advanced gas multipliers are discussed, with an accent on the recently introduced gas avalanche imaging photomultipliers. Applications in the fields of radiation damage studies to DNA, digital mammography and early detection of cancer tumors are presented.

Thin alkali halide films are currently used as transmissive UV-photocathodes and as protecting layers for visible photocathodes. The surface morphology of 20 and 75 nm thick evaporated CsI, NaI and CsBr films was investigated by means of a scanning electron microscope, to which the samples were transferred, under vacuum, with practically no contact with air. It is shown that the film continuity, in particular that of NaI, is strongly affected by short exposure to moisture. CsI, which is the less hygroscopic material among the three, exhibits the most continuous structure.

We report on our latest results of K-Cs-Sb visible photocathodes, coated with thin CsI and CsBr protective films, for applications within gas avalanche photomultipliers. Data on the sensitivity to exposure to oxygen and moisture is presented, as well as on the stability under gas multiplication and aging under gas avalanche. There are good indications that the protected photocathodes can withstand long-term multiplication mode, with low-level impurity gases. Possible applications are discussed.

The absolute quantum photoyield (QPY) of polycrystalline diamond films in the range of 140-210 nm is reported, for undoped and B-doped films, as a function of deposition conditions and postgrowth surface treatment. B-doping, as well as geometrical structure, crystalline size and quality of the deposited films did not affect the photoemission properties, whereas exposure to microwave (MW) hydrogen plasma significantly improved the QPY to more than 12% at 140 nm, compared to 4-6% measured for untreated films deposited by hot filament chemical vapor deposition (HFCVD) at temperatures of ≥900 °C. Undoped diamond films deposited by the MW plasma chemical vapor deposition (MWCVD) at the same temperature showed a QPY of ~12% at 140 nm without additional hydrogen plasma treatment. A reduction in deposition temperature in the HF reactor resulted in an increase of QPY up to 11%. We have observed a decrease of the QPY in time, down to QPY values of 5-6%, for samples exposed to ambient air. The decrease occurred on time scales of hours to weeks, for non-hydrogenated and postgrowth hydrogenated films. The high QPY values were regenerated by repeating the hydrogenation process. The degradation of hydrogen-terminated films was found to be related to the adsorption of small amounts of oxygen, as detected by Auger electron spectroscopy. The observed oxygen adsorption at room temperature is in contrast to that in previous studies, claiming the stability of hydrogen-terminated diamond surfaces.

We report about a nuclear track imaging system which is designed to study in detail the ionization topology of charged particle tracks in a low-pressure gas. The detection method is based on a time projection chamber (TPC) filled with low-pressure triethylamine (TEA). Ionization electrons produced by energetic charged particles are three-dimensionally imaged by recording light from electron avalanches with an intensified CCD system. The detector permits to investigate the spatial ionization distributions of particle tracks in gas, of equivalent length and resolution in tissue of 4 μm and 40 nm (RMS), respectively. We explain the relevance of this technique for dosimetry, describe the experimental method and the basic operation parameters. First results of the chamber response to protons and alpha particles are presented.

Ultraviolet-photon absorption constants were measured for thin CsI, KI, RbI, NaI and CsBr evaporated films, in the 140-200 nm spectral range. The constant measured for CsI is consistent with literature data; the absorption data for the other materials are given here for the first time. The absorption lengths for all materials investigated vary between 10 and 40 nm. Post-evaporation thermal treatment of the films, known to enhance the photoemission properties of CsI, NaI and CsBr films, had no effect on their photoabsorption properties.

Several methods for producing nanocrystalline diamond thin films with electron emission thresholds in the 2-5 volt/micron range are developed. The films are grown in Ar-C₆₀-H₂, Ar-CH₄, Ar-CH₄-H₂, Ar-CH₄-N₂, and N₂-CH₄ microwave plasmas, and with grain sizes ranging from approximately 5 to 100 nm and differs in morphology and electronic properties. Photoemission yields and STM images indicate that topographically enhanced electric fields may play a role in the low thresholds observed for some of these films; however, for other films, the required field enhancement is much too large to be explained in terms of local surface topography on a diamond surface, but the observed low emission thresholds appear to be associated with enhanced interband state density.

We report on the measurements of the electron drift velocity and longitudinal diffusion in n-pentane gas mixtures and on the effect of the drift velocity on the timing properties of Thin Gap Chambers. Gas mixtures of n-pentane-CO₂, n-pentane-CO₂-CH₄, and n-pentane-CO₂-CF₄ were investigated. An increase of the drift velocity has been observed with mixtures containing CF₄, while still maintaining the very high, saturated gain, typical for these chambers. The overall improvement of TGC timing properties is such that 99% detection efficiency can be reached within a 20 ns gate. A simulation of the chamber timing properties using the measured drift velocities, reproduces well the measured data and can be used to predict chamber performance as a function of geometry and gas mixture.

The optical and photoemission properties of hydrogen-free DLC films prepared under conditions which result in different types of carbon bonding (previously determined from electron energy loss spectroscopy) are reported. The films were prepared using a mass selected ion beam deposition system covering the C⁺ energy range 10 eV-2 keV, mostly on Si held at room temperature. Previous measurements have shown that the sp³ content of these films varied from 0 to > 80%. The optical constants (n, k, absorption coefficient (α)) of these films were measured by ellipsometry. The energy gaps were derived from Tauc plots (E_g) and from α=10⁴ cm^-1 values (E₀₄). The energy gaps were found to vary with the sp³ content from E

Wire-chamber gaseous electron multipliers coupled to CsI photocathodes, provide interesting means for UV-photon imaging over a very large area. We report on the measurement of the CsI resistivity, important for the understanding of rate-dependent effects in CsI-based detectors. Photon counting at high rates is also closely connected to long-term photocathode stability. We present results on ageing of CsI photocathodes under high photon flux, with one atmosphere CH₄ and under a detector gain of 10⁵, for several substrate materials. We compare the ageing results with that of CsI photocathodes coated with thin NaF protective films.

We applied the electron counting method, in low-density gas, for ultra-soft X-ray spectroscopy. The method is based on the spatial expansion of photon-induced ionization electron clusters, and the recording of pulse trails formed by individual electron avalanches in a fast multiplication element. A detector, operating at room temperature with low gas pressure, was characterized with Particle-Induced X-ray Emission (PIXE) and with electron-induced X-ray emission in a Scanning Electron Microscope (SEM). High detection efficiency for soft X-rays is obtained due to the very thin polymer window between the detector and the radiation source. The low-density detector is blind to Bremsstrahlung and charged-particle background. We present the results of the spectral analysis of characteristic X-rays emitted from low-Z elements, in the energy range 100-1500 eV. The recorded X-ray spectra, using both the information of the number of counted electrons and the length of the electron pulse trail, are unfolded with the help of a computer simulation of the detector response to X-ray photons. This detailed simulation of the electron detection and counting process provides an efficient means for a quantitative spectral analysis and permits the precise reconstruction of complex energy spectra.

An approach, which permits to overcome certain fundamental limitations iu spatial resolution in nanodosimetry is been developed. It is based on counting the number of radiation induced single ions, extracted from a wall-less sensitive region, filled with gas. The extracted ions are acceleratecl in vacuum into an electron multiplier.Such development requires the knowledge of the ion diffusion within the dectector. As the gases of interest for nanodosimetry have multi-atomic molecules, the transport picture is rather complicated and involves a variety of mechanisms, resulting in series of ion species produced in the track, and their subsequent transformations during the diffusion process. The diffusion parameters, which should actually take into account all these mechanisms, are not available for most of the gases of interest.A special setup for measuring ion transport parameters has been built, in which first measurements for propane were performed. Calculations, based on the obtained data, show that in an ion counting nanodosimeter a closed wall-less sensitive volume of about 0.2 nm to 1nm across can be formed. Due to the absence of electron avalanche (like in electron counting devices), the choice of working gas is not restricted. This permits the use of various gases forsimulation of the chemical composition of living cell fine subsystems.

We report on photoemission properties of visible K-Cs-Sb reflective photocathodes coated with thin CsBr protective films. We present the absolute quantum efficiency and its stability under prolonged contact with oxygen. It is found that K-Cs-Sb photocathodes coated with 280 Å thick CsBr films have 5% quantum efficiency at 312 nm and can withstand an exposure to 150 Torr of oxygen for half an hour. We discuss the possible role of good lattice constants matching between the photocathodes and the protecting film.

Microdot avalanche chambers (MDOT) equipped with thin semitransparent Cr photocathodes, were characterized with UV photons at low gas pressure. Gains superior to 104 were reached with gas multiplication at the dots. In a mode where preamplification in the gas volume precedes the additional dot multiplication, gains superior to 106 were measured at 30-60 torr of propane. The fast amplification mechanism results in narrow high amplitude pulses with 2-3 ns rise time, visible with no further electronic amplification means. We present here our preliminary results and briefly discuss potential applications.

A model for calculating the electron inelastic mean free path and stopping power in insulators in the 50 eV-10 keV energy range is presented. Both valence and core electron contributions have been considered. The valence part has been estimated following the dielectric theory modified to include the energy gap; the core contribution has been evaluated on the basis of the classical binary encounter theory. Inelastic mean free path and stopping power calculations based on this model have been performed for several alkali halides: LiF, NaCl, KCl and CsI. They are compared to existing experimental data and Penn model's predictions for the mean free path and to Bethe's values for the stopping power; a fair agreement is found for incident electron energies higher than 100-200 eV.

Long-term stability of air-sensitive solid photocathodes operating in gas could be attained by coating their surface with a thin solid dielectric film. The protective film should have good electron transmission properties. We present data on LiF, NaF, CsF, MgF₂, BaF₂, SiO₂ and Al₂O₃ films. The electron escape length and the film's protection properties were studied on relatively stable photocathodes, namely CsI, CuI and Al. We observed a reduction of the electron affinity due to adsorption of polarized water molecules on alkali fluorides. We report on the stability of CsI photocathodes coated with thin films, under exposure to air and under gas multiplication conditions. The possible protection of visible photocathodes is discussed.

High accuracy imaging of thermal neutrons has been demonstrated with detectors having solid converters coated with secondary electron emitters and coupled to gaseous electron multipliers. The present work concentrates on study of the neutron detection efficiency of detectors equipped with composite CsI coated ⁶Li neutron converters. Monte Carlo simulations of the neutron conversion and the charged particle and electron emission processes permit to optimise the converter parameters. The experimental study of the detection efficiency is described in detail and the results are compared with the theoretical predictions.

CsI films are known to be efficient photoconvertors, intensively investigated for UV-photon imaging devices. The article reviews the production and characterization techniques of CsI photocathodes and their photoemission properties in vacuum and gas media, in charge collection and multiplication modes. The important roles of surface phenomena, gas and electric fields are described. The stability of the films is discussed.

Calculations of electron inelastic mean free paths and stopping powers for several alkali halides (KF, KCI, KBr, and KI) and metal oxides (BeO, MgO, SiO₂, and Al₂O₃) have been performed in the 50 eV to 10 keV energy range. The complex dielectric formalism, improved to include the energy gap, was used for estimating the valence part of the transport characteristics, whereas the part related to electron-core interactions was evaluated according to Gryzinski's theory. An extended comparison of these calculations with the available experimental data as well as with other theoretical predictions is presented. Trends of the energy dependence of the inelastic mean free path and stopping power in alkali halides are studied. The role of the plasmon deexcitation process as a source for low-energy electrons in secondary electron emission spectra is discussed. The presented data can be used in Monte-Carlo simulations of electron transport in the considered materials.

A new method is presented for deriving the Fano factor, F, and the mean energy per ion pair, W_i, in counting gases. It is based on the technique of individual counting of single ionization electrons induced in low-pressure gas samples by soft x-ray photons. A correlation of the experimental data with a detailed simulation of the electron deposition and counting process permits the extraction of the Fano factor and the mean energy per ion pair values. We present data of F and W_i for C₂H₆ and Ar/C₂H₆ over the energy range of 100-1500 eV. The energy dependence of these parameters reflects the atomic level structure of the gases. We discuss in detail the accuracy of this technique and its advantages and limitations. Ways are proposed for improving the technique and for broadening the energy range.

We report on measurements of low-mass electron pairs in 450 GeV p-Be, p-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of 5.0±0.7(stat)±2.0(syst) in the invariant mass range 0.2

A new class of radiation imaging detectors is described. They combine thin solid converters and gaseous electron multipliers. CsI or CsI-coated films provide an efficient means for UV, X-ray and thermal neutron conversion. The latter is followed by the emission of single photoelectrons or multiple secondary electrons in the eV range. The surface conversion and the electron multiplication close to the radiation impact location guarantee an accurate localization, fast response and low occupancy, regardless of the angle of incidence. A low gas pressure permits the operation at very intense radiation flux, with a low sensitivity to ionizing background. The article briefly reviews the new technique and its potential applications.

A novel, fast, hadron-blind imaging TRD is proposed. Traditonal radiators are followed by novel, particle-blind photon imaging detectors, combining thin Csl photon convertors and fast gaseous electron multipliers. The expected performance of the TRD, based on multiple radiator/detector elements, is simulated, using experimental and theoretical data of the photon detector characteristics. Such TRDs can provide 100 GeV electron detection efficiency above 90% with hadronic background rejection factors of 10² - 10⁴. The nanosecond timing, very low occupancy and the high rate capability make these devices very competitive tools for ultrarelativistic electron identification under critical conditions expected in future particle physics experiments.

We discuss the idea of a vacuum or gaseous photomultiplier with a CsI-coated wire photocathode. It benefits from an electric field enhanced quantum efficiency. We present a method for protecting air-sensitive photocathodes with thin dielectric film coating. Results of CsI coated with thin LiF and MgF2 films are shown. The protection of other UV and visible photocathodes is discussed.

We have developed a high resolution thermal neutron imaging detector, combining a composite solid neutron convertor and a low-pressure gaseous avalanche electron multiplier. Neutron-induced charged particles from a primary convertor element induce multiple low-energy secondary electrons from a second electron-emissive film. These are multiplied inthe gas upon their emission. A localization resolution of the order of 0.5 mm (fwhm) was measured with detectors equipped with Gd and Li convertors coated with thin CsI films. These detectors are characterized by good imaging properties (also in divergent neutron beams), fast time resolution, low sensitivity to gamma background and the capability of operating at very high neutron flux. Possible applications are discussed.

We present recent results of the study of surface properties and quantum efficiency (QE) of CsI photocathodes prepared on various substrates. Microanalysis methods provide laterally resolved surface morphology and chemical composition of the photoemissive film. Integral measurements of the QE of CsI were done with a monochromator system and a RICH device. It was shown that CsI films deposited on large area Ni- or Ni-Au-coated printed circuit electrodes have a uniform crystalline structure and an average QE close to that reached on polished stainless steel. The films have a good stability in air over periods of Ih. On a microscopic scale of 3-30 mu m, the films exhibit nonuniform emission properties correlated with variations in the chemical composition.

A new class of X-ray imaging gaseous detectors was developed, based on photon conversion in a thin film and the detection of the emitted secondary electrons with a low-pressure avalanche wire chamber. Their main features are parallax-free imaging with a submillimeter spatial resolution over a broad range of X-ray energies, a nanosecond response and a counting rate capability above 1 MHz/mm². The imaging characteristics of a large area secondary electron emission (SEE) detector equipped with CsI, Ag and Ta photoconvertors, in the photon energy range of 6-60 keV are summarized. In localization capability this detector can successfully compete with commercial image intensifier systems and medical film-screen combinations.

We report on a large volume, high-pressure, TPC-like xenon gas scintillation counter for X-ray spectroscopy in the range 60-511 keV. The UV scintillation light is detected with a low-pressure two stage proportional counter using a CsI photocathode. The detector has excellent energy and time resolutions. The location of the absorbed quanta can be accurately determined in all three dimensions. Furthermore the 3D topological information of the deposited primary charges provides an efficient means for the suppression of the Compton background present in many close ion-atom collision experiments.

The conversion of X-ray photons in a thin solid film and the detection of the subsequent secondary electrons in a low-pressure avalanche wire chamber is the basis for a new type of fast, high resolution parallax-free X-ray imaging detectors. The combination of the high emission yield of low energy secondary electrons from alkali halides with the particular properties of low-pressure avalanche electron multipliers results in a fast detection process, free of space charge effects even at high radiation flux. We summarize recent experimental and theoretical results connected with these secondary emission gaseous detectors which have a broad field of potential applications. Simulations of a real-size ultrafast transition radiation detector based on this principle indicate its outstanding particle identification properties, of prime importance for the very demanding environment of future colliders.

We describe the two RICH detectors of the CERES electron pair spectrometer at the CERN SPS which are used for electron identification and, in conjunction with a novel silicon drift detector, for tracking in pp, pA and AA collisions. The RICH detectors are operated at a high γ_th congruent-to = 32 (CH₄ at 1 atm.) and are thus rather insensitive to hadrons. The UV-detectors are multistep counters with a multiwire chamber as the last stage. They are operated at gains of 2-4 x 10⁵ using a mixture of He + 6% C₂H₆ (or CH₄)+TMAE. The two UV-detectors are equipped with 53800 (48400) square pads. The front end electronics consists of modules with 256 (121) channels, based on a 64-channel charge-sensitive preamplifier VLSI chip. The total readout time is 280 (1600) mus per event. Subsets of the pad data are used as input to a fast trigger processor selecting events with at least two separated electron rings. The trigger achieved an enrichment factor of approximately 100 in proton-induced interactions, and a factor of ∼ 3 in ³²S Au collisions. The RICH detectors perform very close to their design values. We observe clean Cherenkov rings with an average number of 11.2 (12.5) photons/ring. This is consistent with the calculated value N₀ = 131 (75) cm^-1 within the systematical error of about 10%. The spatial resolutions of 1.0 and 0.8 mrad (rms) for individual photons are dominated by chromatic aberration and in very good agreement with theoretical expectations.

The CERES experiment (CErenkov Ring Electron Spectrometer) studies the production of low mass e⁺e^- pairs in proton-proton, proton-nucleus and nucleus-nucleus interactions at the CERN SPS. The CERES spectrometer, has a novel design based on two Ring Imaging Cherenkov (RICH) counters, and it operates close to its design specifications. Data were recorded with 200 GeV u sulfur beam and 450 GeV proton beam. The analysis is in progress. We have extracted first e^+--pairs samples for p+Be, p+Au and S+Au collisions. In addition other physics topics were addressed. Inclusive photon spectra were measured in S+Au interactions. No excess over known hadronic sources was found within our present systematic error of 11%. Results on high p_i charged pion spectra are presented up to 4 GeV c. We also studied the production of e⁺e^--pairs m the strong electromagnetic fields of very peripheral S+Pt collisions. The data are well described by a first-order perturbative QED-calculation.

We have for the first time identified e⁺e^--pairs produced by strongly varying electromagnetic fields in distant SPt collisions at 200 GeV/u projectile energy at the CERN-SPS. The differential cross section for the mass range 10 MeV/c² ≤ m_ee ≤ 100 MeV/c² and the polar angle range 141 mrad ≤ θ_e ≤ 260 mrad agrees well with a lowest-order perturbation QED-calculation.

A two-stage multiplication mechanism has been observed in microstrip gas chambers (MSGC) operated in a pressure range of 10-50 Torr of isobutane. The resulting high gain (> 10⁴) of single electrons photo-produced on a CsI photocathode is attributed to a preamplification in the gas gap followed by anode strip multiplication. The large and fast rise of the induced pulse in this mode leads to an efficient single electron detection at relatively low charge gain. The reduced positive ion feedback preserves radiation convertors coupled to such electron multipliers from sputtering damage. MSGCs operated in this mode are expected to have a subnanosecond time resolution and very high rate capability. Some potential applications are briefly discussed.

Radiobiological data reveal that radiation action on the living cell is at the nanometre level. In this paper a new approach is proposed for the measurement of the radial radiation induced ionisation distribution across the particle track at this level. It is based on coupling a parallel-plate ionisation chamber to a cylindrical proportional electron multiplier. A detector of this type, filled with propane at pressures of 227 and 454 Pa, was used for the measurement of the radial ionisation distribution induced by a particles. The displacement of the detector with respect to the α particle beam provided a tissue-equivalent ionisation distribution at the nanometre level. The data obtained with this novel technique are presented and the secondary effects due to radiation induced electron emission from the detector walls are discussed. Further developments are also discussed.

A silicon drift detector of circular geometry giving unambiguously the radial and azimuthal coordinates of particle's interaction point for events with high multiplicity is part of the experimental set up of the NA45 experiment at CERN SPS. The paper reviews the characteristics of the detector and of its assembly among the other detectors of the experiment. The first experimental results showing the performance of the detector in term of resolution and its effectiveness in reconstructing the position of the event are reported.

We present the result of a systematic investigation of the quantum efficiency of reflective CsI photocathodes in the spectral range 140-220 nm. Powder and crystal forms of CsI were used for evaporation of bulk and porous photocathodes. The measurements were made without gaseous amplification in vacuum and in methane. The results of radiation resistance measurements at doses reaching 4500 Gy are also reported.

Secondary emission detectors based on a thin solid photoconvertor coupled to a low-pressure gaseous electron multiplier provide subnanosecond, parallax-free, high accuracy X-ray imaging. The application of CsI photoconvertors was studied over a broad photon energy range. At normal incidence, detector quantum efficiencies of 5-0.6% and localization accuracies of 200-500 μm (FWHM) were recorded with respective photon energies of 6-60 keV and CsI layers 200-4000 nm thick. At grazing incidence an improvement of the order of 1/sin θ{symbol} was demonstrated in localization accuracy and detection efficiency. Some preliminary indications on the CsI stability are reported and radiographic images of small objects are presented. Possible future applications of these devices are discussed.

A model for electron transport and emission in CsI is proposed. It is based on theoretically calculated microscopic cross sections for electron interaction with the nuclear and the electronic components of the solid. A Monte Carlo program based on this model was developed to simulate secondary electron emission induced by x rays and electrons in the energy range of 1 to 10 keV. The calculated secondary emission yields agree with existing experimental data. The model provides all necessary characteristics for the design of radiation detectors based on secondary electron emission. It can be expanded to higher incident energies and other alkali halides.

A large-area low-pressure multistep counter designed for the detection and timing of X-rays with energies below 1 keV is described. The dependence of energy resolution on gas pressure was investigated and found to improve at lower pressures. At 7.6 Torr the FWHM resolution was 58% for 279 eV carbon K X-rays. The time resolution is dependent on the gas composition, pressure and detector thickness. At 200 Torr of ArCH₄ (50%-50%), two thirds of the detected carbon X-rays occur within 12 ns for a detector thickness of 4 mm.

We propose new techniques of X-ray spectroscopy and imaging, based on the use of low-pressure multistep gaseous electron multipliers. Ultrasoft X-rays are detected by counting single-electron clusters induced in the gas. X-ray induced UV-photons in gas scintillation chambers are read out with wire chambers coupled to CsI photocathodes. X-rays converted in foil-electrodes are imaged by fast multistep avalanche electron multipliers. We discuss the advantages of the various techniques and present experimental results and Monte Carlo simulations.

UV-photons from an Xe-filled gas scintillation counter are detected with a CsI photocathode coupled to a double stage, low-pressure wire chamber. The high quantum efficiency, (9 pct), of the UV-detector yields a high detection efficiency of both primary and secondary scintillation photons. An energy resolution of 4.1 pct (FWHM) was recorded with 60 keV X-rays, inducing secondary scintillation in 5 bar of Xe; the wire chamber operated at 20 Torr of CH4. The two-dimensional planar localization of UV-photons, combined with the drift time measurement of primary electrons, provides a three-dimensional, multihit, imaging capability of X-ray photon interactions, with a resolution of 2-3 mm (FWHM). The stability of the CsI photocathode under different operation conditions and its sensitivity to exposure to air are discussed.

A Gas Scintillation Counter (GSC) with high background rejection capability, fast timing characteristics and good energy resolution has been developed for quasimolecular X-ray spectroscopy in inelastic heavy ion - atom collisions. The detector presented in this paper consists of a high pressure Xe-filled absorption cell and a secondary scintillation parallel gap. Highly efficient detection of the primary light provides a time resolution of 3 ns (FWHM). The energy resolution is 4.3% (FWHM) for 60 keV X-rays. A rejection efficiency of 92% for Compton background in the energy range between 35 and 160 keV is obtained by applying the K - fluorescence gating method and pulse-shape analysis, to the secondary scintillation pulse.

A beam test of a high-density projection chamber with optical readout is presented. The device consists of a prototype of the DELPHI HPC calorimeter on which a parallel-plate, light-emitting structure was installed, replacing the original multiwire proportional chamber readout system. It produces detailed images of the energy deposited by electromagnetic showers; hadronic interactions are easily discriminated from these. The computerized readout system gives full quantitative information on the events. showing good energy and position resolutions.

We have made a quantitative systematic study of the light emitted through the electron avalanche process in low-pressure gases. The mixtures under investigation were argon/(CH₄ or C₂H₆)/(TMAE or TEA). For a given avalanche charge gain, we measured comparable light yields for mixtures containing TEA and TMAE; typical values are of 0.1-1 photons per electrons in the avalanche. We found a linear increase of the light intensity with the amplification gap width. Light production by electrons drifting in these gas mixtures without charge multiplication was also demonstrated.

The incredibly high particle multiplicities, resulting from collisions of relativistic nuclei with heavy targets, create needs for new detection devices capable of efficiently dealing with such complex events. Optical methods such as scintillating fibres and gaseous imaging detectors may provide solutions to various tracking and particle identification problems in this field.

A novel two-dimensional readout scheme for gas detectors is presented that uses small metal pads with 2.54-mm pitch as an anode. The pads are read out via 128-channel VLSI low-noise preamplifier/multiplexer chips. These chips are mounted on 2.8-cm*2.8-cm modules, which are directly plugged onto the detector backplane, delay-chained with jumpers, and read out sequentially. The readout has been successfully tested with a low-pressure, two-step, TMAE-filled, ultraviolet-sensitive ring imaging Cerenkov counter. A single-electron efficiency of >90% was observed at moderate chamber gains (

Transverse-energy distributions have been measured in the pseudo-rapidity region -0.1 >η_lab

UV photons from a Cherenkov radiator are multiplied in a multistep avalanche chamber operating in a gated mode at low gas pressure (40 Torr). The gas mixture is C₂H₆-argon ( 80 20) and TMAE at 34°C. Visible light emitted from single photoelectron avalanches is detected by a CCD camera coupled to an image intensifier system. The detector was tested with 5 GeV c electrons, using a CH₄ radiator gas at 1 atm. Cherenkov rings essentially free of particle background and of secondary photon feedback were obtained in this mode of operation with a mean number n ≅ 11.5 (N₀ ≅ 76 cm^-1). We present this new method and discuss its performance.

We have developed a low-pressure multistep, TMAE- filled, UV sensitive detector for Cerenkov ring imaging (RICH) for ultrarelativistic heavy ion experiments (HELIOS-CERN). In the present work we describe the detector structure and its basic properties and we present here the experimental setup and the detector performance in tests with high energy electrons using three modes of readout: 3 wire coordinates and flash ADC's Pad readout - optical recording of the avalanches. Single photons can be localized with an accuracy of σ⋍a 2 mm, mainly due to single photoelectron diffusion and to chromatic dispersion in the radiator gas. The experimental No values, obtained by the three methods, are close to those expected, taking into account the various losses due to the test conditions.

We describe a large-area timing and position-sensitive multistep gaseous detection heavy ions. It consists of a preamplification stage operating as a parallel plate avalanche chamber directly coupled to a multiwire proportional chamber. The detector operates at a pressure range of 1-4 Torr, with very thin window foils. The principle of operation and the performance of the detector when used with various ions are presented. It has a high gain and good time resolution (better than 180 ps fwhm) and localization properties (better than 0.2 mm (fwhm)). Its efficiency for low energy high mass ions was tested with ¹⁶⁰Gd ions and found to be 93% (equal to the electrode transparency) down to kinetic energies of 1.3 MeV.

MWPCs operated at gas pressures around 1 Torr are shown to have excellent timing and position response for heavily ionizing particles. The avalanche growth process at such conditions is of a double-step nature and is characterized by fast collection times of electrons and positive ions. A substantial spatial distribution of the avalanche (1-2 mm fwhm) leads to an excellent position resolution even when interpolating between neighbouring wires. Very thin (100-200 φg/cm² of polypropylene), fast timing and position-sensitive detectors are providing time resolutions of the order of 100-130 ps (fwhm), and position resolutions, with delay-line read-out, of the order of 100 φm (fwhm). Various detectors of this type, with sizes reaching 80 cm², operating at counting rates up to 10⁵ c/s·mm², are used as routine tools for timing and imaging of heavily ionizing particles.

With a large preamplifying gap of 2 cm, combined with a localization multiwire chamber, it is shown that measurement of pulse height provides the depth of the atom which absorbs a monoenergetic low-energy X quantum to a great accuracy. The preliminary results show that for 8 keV X-rays, accuracies of a fraction of a millimetre are achieved. No error is introduced by the limited efficiency of the absorbing gap, thus permitting the use of low-cost argon mixture fillings. Large-surface parallax-free gaseous detectors of planar structure can be built for monoenergetic diverging sources.

Some outstanding properties of low-pressure MWPCs open new prospects in the domain of heavy as well as light particle detection. In the present mode of operation, the mechanism of avalanche formation is divided into two steps: it combines an amplification process in a high, constant, electric field region with a second one, occurring in the vicinity of the proportional wires. Large gains associated with fast charge-collection times are reached at pressures as low as a fraction of a Torr, leading to excellent timing properties and to a high counting-rate capability. Transmission timing detectors, having a total thickness in the 100 μg/mm² range, have been designed for heavy-ion detection and offer time resolutions of the order of 100 ps (fwhm) at rates as high as 10⁵ c/cm² s. The properties of this type of detectors are summarized and localization and multitrack detection capabilities are discussed. Present and future heavy-ion detection systems based on this simple, efficient and low cost technique are described. Possible applications in nuclear and particle physics are also discussed.