Publications

Xenon dual-phase time projections chambers (TPCs) have proven to be a successful technology in studying physical phenomena that require low-background conditions. With 40t of liquid xenon (LXe) in the TPC baseline design, DARWIN will have a high sensitivity for the detection of particle dark matter, neutrinoless double beta decay (0 ν β β), and axion-like particles (ALPs). Although cosmic muons are a source of background that cannot be entirely eliminated, they may be greatly diminished by placing the detector deep underground. In this study, we used Monte Carlo simulations to model the cosmogenic background expected for the DARWIN observatory at four underground laboratories: Laboratori Nazionali del Gran Sasso (LNGS), Sanford Underground Research Facility (SURF), Laboratoire Souterrain de Modane (LSM) and SNOLAB. We present here the results of simulations performed to determine the production rate of ¹³⁷ Xe, the most crucial isotope in the search for 0 ν β β of ¹³⁶ Xe. Additionally, we explore the contribution that other muon-induced spallation products, such as other unstable xenon isotopes and tritium, may have on the cosmogenic background.

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.

We detail the sensitivity of the proposed liquid xenon DARWIN observatory to solar neutrinos via elastic electron scattering. We find that DARWIN will have the potential to measure the fluxes of five solar neutrino components: pp, ⁷Be, ¹³N, ¹⁵O and pep. The precision of the ¹³N, ¹⁵O and pep components is hindered by the double-beta decay of ¹³⁶Xe and, thus, would benefit from a depleted target. A high-statistics observation of pp neutrinos would allow us to infer the values of the electroweak mixing angle, sin ²θ_w, and the electron-type neutrino survival probability, P_ee, in the electron recoil energy region from a few keV up to 200 keV for the first time, with relative precision of 5% and 4%, respectively, with 10 live years of data and a 30 tonne fiducial volume. An observation of pp and ⁷Be neutrinos would constrain the neutrino-inferred solar luminosity down to 0.2%. A combination of all flux measurements would distinguish between the high- (GS98) and low-metallicity (AGS09) solar models with 2.1–2.5σ significance, independent of external measurements from other experiments or a measurement of ⁸B neutrinos through coherent elastic neutrino-nucleus scattering in DARWIN. Finally, we demonstrate that with a depleted target DARWIN may be sensitive to the neutrino capture process of ¹³¹Xe.

The bubble-assisted Liquid Hole Multiplier (LHM) is a novel concept for the combined detection of ionization electrons and scintillation photons in noble-liquid time projection chambers. It consists of a perforated electrode immersed in the noble liquid, with heating wires generating a stable vapor bubble underneath. Radiation-induced ionization electrons in the liquid drift into the electrode's holes and cross the liquid-vapor interface into the bubble where they induce electroluminescence (EL). The top surface of the electrode is optionally coated with a CsI photocathode; radiation-induced UV-scintillation photons extract photoelectrons that induce EL in a similar way. EL-photons recorded with an array of photo-sensors, e.g. SiPMs, provide event localization. We present the basic principles of the LHM concept and summarize the results obtained in LXe and LAr.

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 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.

A study of the localization properties of a single-element Resistive Plate WELL (RP-WELL) detector is presented. The detector comprises of a single-sided THick Gaseous Electron Multiplier (THGEM) coupled to a segmented readout anode through a doped silicate-glass plate of 10¹⁰ Ω·cm bulk resistivity. Operated in ambient\nech gas, the detector has been investigated with 150 GeV muons at CERN-SPS. Signals induced through the resistive plate on anode readout strips were recorded with APV25/SRS electronics. The experimental results are compared with that of Monte Carlo simulations. The effects of various physics phenomena on the position resolution are discussed. The measured position resolution in the present configuration is 0.28 mm RMS - compatible with the holes-pattern of the multiplier. Possible ways for improving the detector position resolution are suggested.

A thin single-element THGEM-based, Resistive-Plate WELL (RPWELL) detector was operated with 150 GeV/c muon and pion beams in Ne/(5%CH₄), Ar/(5%CH₄) and Ar/(7%CO₂); signals were recorded with 1 cm² square pads and SRS/APV25 electronics. Detection efficiency values greater than 98% were reached in all the gas mixtures, at average pad multiplicity of 1.2. The use of the 10⁹ Ω cm resistive plate resulted in a completely discharge-free operation also in intense pion beams. The efficiency remained essentially constant at 98–99% up to fluxes of ∼10⁴ Hz/cm², dropping by a few % when approaching 10⁵ Hz/cm². These results pave the way towards cost-effective, robust, efficient, large-scale detectors for a variety of applications in future particle, astroparticle and applied fields. A potential target application is digital hadron calorimetry.

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 perform a low-mass dark matter search using an exposure of 30 kg×yr with the XENON100 detector. By dropping the requirement of a scintillation signal and using only the ionization signal to determine the interaction energy, we lowered the energy threshold for detection to 0.7 keV for nuclear recoils. No dark matter detection can be claimed because a complete background model cannot be constructed without a primary scintillation signal. Instead, we compute an upper limit on the WIMP-nucleon scattering cross section under the assumption that every event passing our selection criteria could be a signal event. Using an energy interval from 0.7 keV to 9.1 keV, we derive a limit on the spin-independent WIMP-nucleon cross section that excludes WIMPs with a mass of 6 GeV/c2 above 1.4×10-41 cm2 at 90% confidence level.

The XENON1T experiment is currently in the commissioning phase at the Laboratori Nazionali del Gran Sasso, Italy. In this article we study the experiment's expected sensitivity to the spin-independent WIMP-nucleon interaction cross section, based on Monte Carlo predictions of the electronic and nuclear recoil backgrounds.The total electronic recoil background in 1 tonne fiducial volume and (1, 12) keV electronic recoil equivalent energy region, before applying any selection to discriminate between electronic and nuclear recoils, is (1.80 ± 0.15) centerdot 10−4 (kgcenterdotdaycenterdotkeV)−1, mainly due to the decay of 222Rn daughters inside the xenon target. The nuclear recoil background in the corresponding nuclear recoil equivalent energy region (4, 50) keV, is composed of (0.6 ± 0.1) (tcenterdoty)−1 from radiogenic neutrons, (1.8 ± 0.3) centerdot 10−2 (tcenterdoty)−1 from coherent scattering of neutrinos, and less than 0.01 (tcenterdoty)−1 from muon-induced neutrons. The sensitivity of XENON1T is calculated with the Profile Likelihood Ratio method, after converting the deposited energy of electronic and nuclear recoils into the scintillation and ionization signals seen in the detector. We take into account the systematic uncertainties on the photon and electron emission model, and on the estimation of the backgrounds, treated as nuisance parameters. The main contribution comes from the relative scintillation efficiency Script Leff, which affects both the signal from WIMPs and the nuclear recoil backgrounds. After a 2 y measurement in 1 t fiducial volume, the sensitivity reaches a minimum cross section of 1.6 centerdot 10−47 cm2 at mχ = 50 GeV/c2.

A novel method utilizing the Fast Neutron Resonance Transmission Radiography is proposed for non-destructive, quantitative determination of the weight percentages of oil and water in cores taken from subterranean or underwater geological formations. The ability of the method to distinguish water from oil stems from the unambiguously-specific energy dependence of the neutron cross-sections for the principal elemental constituents. Monte-Carlo simulations and initial results of experimental investigations indicate that the technique may provide a rapid, accurate and non-destructive method for quantitative evaluation of core fluids in thick intact cores, including those of tight shales for which the use of conventional core analytical approaches appears to be questionable.

We discuss recent advances in the development of cryogenic gaseous photomultipliers (GPM), for possible use in dark matter and other rare-event searches using noble-liquid targets. We present results from a 10 cm diameter GPM coupled to a dual-phase liquid xenon (LXe) TPC, demonstrating - for the first time - the feasibility of recording both primary (''S1'') and secondary (''S2'') scintillation signals. The detector comprised a triple Thick Gas Electron Multiplier (THGEM) structure with cesium iodide photocathode on the first element; it was shown to operate stably at 180 K with gains above 10⁵, providing high single-photon detection efficiency even in the presence of large α particle-induced S2 signals comprising thousands of photoelectrons. S1 scintillation signals were recorded with a time resolution of 1.2 ns (RMS). The energy resolution (σ/E) for S2 electroluminescence of 5.5 MeV α particles was ∼ 9%, which is comparable to that obtained in the XENON100 TPC with PMTs. The results are discussed within the context of potential GPM deployment in future multi-ton noble-liquid detectors.

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.

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.

This work presents the first results dealing with THGEMs coupled to submillimetric multiplication gaps, operated in an atmospheric pressure of Ne/CF₄. The experimental studies done so far with a THGEM coupled to submillimetric multiplication gaps achieved charge-gains of 4 × 10 ⁴ and 1 × 10⁵ in Ne/CF₄ (95:5), for 0.4 mm and 0.8 mm gaps, respectively, values that are one order of magnitude higher than those obtained in single-THGEM configuration and approximately half from those obtained for a cascaded-THGEM configuration. The present studies evaluate the performance operation in terms of the charge-gain characteristics and X-ray energy resolution.

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. 

Gaseous PhotoMultipliers (GPM) are a very promising alternative of vacuum PMTs especially for large-size noble-liquid detectors in the field of Functional Nuclear Medical Imaging and Direct Dark Matter Detection. We present recent characterization results of a Hybrid-GPM made of three Micropattern Gaseous Structures; a Thick Gaseous Electron Multiplier (THGEM), a Parallel Ionization Multiplier (PIM) and a MICROMesh GAseous Structure (MICROMEGAS), operating in Ne/CF ₄ (90:10). Gain values close to 10 ⁷ were recorded in this mixture, with 5.9 keV x-rays at 1100 mbar, both at room temperature and at that of liquid xenon (T=171 K). The results are discussed in term of scintillation detection. While the present multiplier was investigated without photocathode, complementary results of photoextraction from CsI UV-photocathodes are presented in Ne/CH ₄ (95:5) and CH ₄ in cryogenic conditions.

Preliminary results of a photon detector combining a Micromegas-like multiplier coated with a UV-sensitive CsI photocathode are described. The multiplier is made in a CMOS compatible InGrid technology, which allows to postprocess it directly on the surface of an imaging IC. This method is aimed at building light-sensitive imaging detectors where all elements are monolithically integrated. We show that the CsI photocathode deposited in the InGrid mesh does not alter the device performance. Maximum gains of ∼6000 were reached in a single-grid element operated in Ar/CH₄, with a 2% ion backflow fraction returning to the photocathode.

A new concept for avalanche ion back-flow (IBF) reduction in cascaded gaseous detectors is presented. The Thick-COBRA is a Thick-GEM (THGEM) with a patterned electrode, aiming at reducing secondary effects by trapping avalanche ions. The patterned electrode faces the electron drift region, trapping ions flowing back from the cascaded multiplier and those produced within the Thick-COBRA. Total IBF values of about 5% were obtained, about six times below that reached with a THGEM in a triple-THGEM configuration, keeping full single-electron detection efficiency. Experimental and simulation studies of IBF and electron collection/transmission efficiencies are presented; the suitability for application of Thick-COBRA in photon detectors for RICH is discussed.

This article presents a detector system consisting of three components, a CMOS imaging array, a gaseous-detector structure with a Micromegas layout, and a UV-photon sensitive CsI reflective photocathode. All three elements have been monolithically integrated using simple post-processing steps. The Micromegas structure and the CMOS imaging chip are not impacted by the CsI deposition. The detector operated reliably in He/isobutane mixtures and attained charge gains with single photons up to a level of 6×10⁴. The Timepix CMOS array permitted high resolution imaging of single UV-photons. The system has an MTF50 of 0.4 lp/pixel which corresponds to approximately 7 lp/mm.

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.

We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO₂(80%) + N₂(20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HIγS) facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm³. Ionization electrons drift towards a double parallel-grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche-induced photons from N₂ emission are collected, intensified and recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional track images. The event's time projection (third coordinate) and the deposited energy are recorded by photomultipliers and by the TPC charge-signal, respectively. A dedicated VME-based data acquisition system and associated data analysis tools were developed to record and analyze these data. The O-TPC has been tested and calibrated with 3.183 MeV alpha-particles emitted by a ¹⁴⁸Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and ¹²C particles from the dissociation of ¹⁶O and of three alpha-particles from the dissociation of ¹²C have been measured during initial in-beam test experiments performed at the HIγS facility at Duke University. The full detection system and its performance are described and the results of the preliminary in-beam test experiments are reported.

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.

A novel concept of optical signal recording in cryogenic two-phase avalanche detectors, with Geiger-mode Avalanche Photodiodes (G-APD) measuring avalanche-scintillation photons in a thick Gas Electron Multiplier (THGEM), has been studied in view of its potential applications in rare-event experiments. The effective detection of avalanche scintillations in THGEM holes has been demonstrated in two-phase Ar with a bare G-APD without wavelength shifter, i.e. insensitive to VUV emission of Ar. At gas-avalanche gain of 400 and under ±70° viewing-angle, the G-APD yielded 640 photoelectrons (pe) per 60 keV X-ray converted in liquid Ar; this corresponds to 0.7 pe per initial (prior to multiplication) electron. The avalanche-scintillation light yield measured by the G-APD was about 0.7 pe per avalanche electron, extrapolated to 4π acceptance. The avalanche scintillations observed occurred presumably in the near infrared (NIR) where G-APDs may have high sensitivity. The measured scintillation yield is similar to that observed by others in the VUV. Other related topics discussed in this work are the G-APD's single-pixel and quenching resistor characteristics at cryogenic temperatures.

The article deals with the detection efficiency of UV-photon detectors consisting of Thick Gas Electron Multipliers (THGEM) coated with CsI photocathode, operated in atmospheric Ne/CH₄ and Ne/CF₄ mixtures. We report on the photoelectron extraction efficiency from the photocathode into these gas mixtures, and on the photoelectron collection efficiency into the THGEM holes. Full collection efficiency was reached in all gases investigated, in some cases at relatively low multiplication. High total detector gains for UV photons, in excess of 10⁵, were reached at relatively low operation voltages with a single THGEM element. We discuss the photon detection efficiency in the context of possible application to RICH.

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.

Experimental measurements of the extraction efficiency f of the UV-induced photoelectrons emitted from a CsI photocathode into gas mixtures of Ne with CH₄, CF₄, CO₂ and N₂ are presented; they are compared with model-simulation results. Backscattering of low- energy photoelectrons emitted into noble gas is significantly reduced by the admixture of molecular gases, with direct impact on the effective quantum efficiency. Data are provided on the dependence of f on the type and concentration of the molecular gas in the mixtures and on the electric field.

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.

A novel concept for ion blocking in gas-avalanche detectors was developed, comprising cascaded micro-hole electron multipliers with patterned electrodes for ion defocusing. This leads to ion blocking at the 10^{- 4} level, in DC mode, in operation conditions adequate for TPCs and for gaseous photomultipliers. The concept was validated in a cascaded visible-sensitive gas-avalanche photomultiplier operating at atmospheric pressure of Ar / CH₄ (95/5) with a bi-alkali photocathode. While in previous works high gain, in excess of 10⁵, was reached only in a pulse-gated cascaded-GEM gaseous photomultiplier, the present device yielded, for the first time, similar gain in DC mode. We describe shortly the physical processes involved in the charge transport within gaseous photomultipliers and the ion blocking method. We present results of ion back-flow fraction and of electron multiplication in cascaded patterned-electrode gaseous photomultiplier with K-Cs-Sb, Na-K-Sb and Cs-Sb visible-sensitive photocathodes, operated in DC mode.

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 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.

Evaluation and monitoring of the cancer risk from space radiation exposure is a crucial requirement for the success of long-term space missions. One important task in the risk calculation is to properly weigh the various components of space radiation dose according to their assumed contribution to the cancer risk relative to the risk associated with radiation of low ionization density. Currently, quality factors of radiation both on the ground and in space are defined by national and international commissions based on existing radiobiological data and presumed knowledge of the ionization density distribution of the radiation field at a given point of interest. This approach makes the determination of the average quality factor of a given radiation field a rather complex task. In this contribution, we investigate the possibility to define quality factors of space radiation exposure based on nanodosimetric data. The underlying formalism of the determination of quality factors on the basis of nanodosimetric data is described, and quality factors for protons and ions (helium and carbon) of different energies based on simulated nanodosimetric data are presented. The value and limitations of this approach are discussed.

BACKGROUND. The correlation between Zinc concentration in the prostate's peripheral zone to the onset or presence of malignant process needs to be evaluated in detail. METHODS. Zinc concentration was measured in ∼1-4 mm³ segments of fresh needle-biopsy cores, with X-ray fluorescence, and correlated with the histological findings of these tissue segments. RESULTS. Local Zinc concentration is correlated with the presence of cancer (PCa); the higher the Gleason score the greater the Local Zinc depletion. The Zinc value averaged over the entire extracted tissue is specific only to Gleason score 8-9 PCa. The results refer to patients avoiding Zinc-rich supplements since those show elevated prostatic Zinc concentration in identified cancer tissue. A computer simulation analysis of randomly located 0.03-3.3 cm³ lesions, with particular Gleason score and the measured Local Zinc concentration, revealed a potential diagnostic approach definitely superior to PSA, with sensitivity to the tumor grade and with excellent detection capability for Gleason score >6. Further clinical studies have been designed, both on full prostates after radical prostatectomy as well as on biopsy cores at higher resolution, to establish the accuracy of the method for Gleason score = 6. CONCLUSIONS. The PCa diagnostic potential of Local Zinc concentration is confirmed and there is indication that the amount of Zinc depletion could be used as a measure of the Gleason score of the tumor. Local Zinc concentration mapping has the potential to improve patient selection for biopsy, biopsy site selection and local therapy (e.g., Cryotherapy, Brachytherapy) site selection.

A newly developed experimental system for measurements of electron emission induced by low energy ions at below low ion flux (10⁵ ions s ^-1 cm^-2) avoiding the fast emission degradation caused by high ion fluxes (usually applied 10¹⁰-10¹³ ions s ^-1 cm^-2) has been established. The method, based on an event-by-event recording of the emitted cluster of electrons for each impinging ion event, overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the electron yield. This method provides not only the average number (γ) of IIEE under a given ionic impact, but actually higher moments of their distribution, which contains more information on the physical processes involved in the IIEE phenomenon. The secondary electron emission coefficients showed a dependence on the kinetic energy of incident argon ions and protons ranging from 1 keV to 10 keV on hydrogenated diamond film surfaces.

The time resolution of a double-stage Thick-GEM (THGEM) detector was measured with UV-photons and relativistic electrons. The photon detector, with semitransparent- or reflective-photocathode yielded time resolution of about 8-10 ns RMS for single photoelectrons and 0.5-1 ns RMS for few-hundred photoelectrons per photon-pulse. Time resolution of about 10 ns RMS was recorded for relativistic electrons from a ¹⁰⁶Ru source.

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 main goal being the construction of large-area, low-cost position-sensitive micropattern gaseous photomultipliers (GPMs) based on gas electron multipliers (GEM) cascades, we have studied the single-photon imaging capabilities of a 4-GEM GPM with a reflective CsI-photocathode deposited on the top surface of the 1st GEM. The charge readout was performed with a simple/economic Wedge and Strip (W&S) readout electrode, using pulses induced through a resistive electrode. We have demonstrated the feasibility of single-photon imaging GPM with sub-millimeter resolution; single-photon spatial resolutions as small as ∼150 μm (FWHM), i.e. ∼65 μm (RMS), were measured in Ar/5%CH₄ gas mixture at atmospheric pressure.

The imaging properties of cascaded-GEM (Gas Electron Multiplier) and cascaded-GEM/MHSP (Micro-Hole and Strip Plate) gaseous electron multipliers, equipped with 2-D Wedge & Strip (W&S) readout electrodes, are presented. The W&S electrode was capacitively coupled to the electron multiplier through a resistive electrode, to adapt the charge-cloud size to the 1.6 mm readout pitch. The studies were carried out with soft X-rays and single UV-photons in Ar/5% CH(4) gas mixture at atmospheric pressure. Spatial resolutions of similar to 225 mu m and similar to 250 mu m (FWHM) were reached, with 5.9 keV X-rays, in 3-GEM and 2-GEM/MHSP based detectors, respectively. Single photoelectrons, emitted from a CsI photocathode deposited on the top electrode of the first GEM in the cascade, were localized with resolution of similar to 170 mu m (FWHM) in a 4-GEM gaseous photomultiplier.

A newly developed experimental system enables measurements of IIEE at a very low ion flux (10⁵ ions/s cm²) avoiding the fast emission degradation caused by high ion fluxes (usually applied 10¹⁰-10¹³ ions/s cm²). The method overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the IIEE yield. We report on the investigation of the IIEE from hydrogenated undoped and B-doped diamond films as a function of (i) moderate heating in vacuum prior to the measurements, (ii) H⁺ and Ar⁺ energy in the range of 1-10 KeV, and (iii) film thickness and microstructure. An IIEE yield (γ) enhancement was typically detected when the films were heated to 300 °C in vacuum. In the B-doped diamond film heated to 300 °C, γ rose nearly linearly from ∼ 20 to ∼ 100 electrons/ion, for 1 to 10 KeV Ar⁺ ions, respectively. The values of γ obtained with H⁺ showed a more moderate, nonlinear increase from ∼ 8 electrons/ion at 1 KeV up to ∼ 90 electrons/ion at 10 KeV. In heated undoped diamond films of different thicknesses the measured values of γ were similar for all the studied films and somewhat lower than in B-doped film: from ∼ 10-16 to 60-70 electrons per 1 to10 KeV Ar⁺, respectively, and from 14-26 to 50-60 electrons per 1 to10 KeV H⁺, respectively. The experimental results were interpreted using TRIM calculations.

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.

We present our studies of prostatic Zn concentration measurements, carried out in the light of a novel prostate cancer (CAP) diagnosis method proposed by us. The method is based on in vivo prostatic Zn mapping by XRF trans-rectal probe. We report on the extensive clinical studies, intended to assess the validity of the novel proposed diagnostic method. Zn content was measured in vitro in needle-biopsy samples from several hundreds of patients, and was correlated with histological findings and other patient parameters. For this purpose, a technique of absolute Zn content determination in ∼1 mm³ fresh tissue samples by XRF was developed. The experimental details and the main clinical-evaluation results are presented. We further outline the suggested design of the XRF trans-rectal probe for an efficient in vivo detection and mapping of the Zn fluorescence radiation from the prostate through the rectal wall. Laboratory phantom studies, a preliminary design concept and its expected performance are also reported.

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 present a Photon-Assisted Cascaded Electron Multipliers (PACEM) which has a potential for ion back-flow blocking in gaseous radiation detectors: the avalanche from a first multiplication stage propagates to the successive one via its photons, which in turn induce photoelectron emission from a photocathode deposited on the second multiplier stage; the multiplication process may further continue via electron-avalanche propagation. The photonmediated stage allows, by a proper choice of geometry and fields, complete blocking of the ion back-flow into the first element; thus, only ions from the latter will flow back to the drift region. The PACEM concept was validated in a double-MHSP (Micro-Hole & Strip Plate) cascaded multiplier operated in xenon, where the intermediate scintillation stage provided optical gain of ∼ 60. The double-MHSP detector had a total gain above 10⁴ and energy resolution of 18% FWHM for 5.9 keV x-rays.

We present two methods of independently mapping the dimensions of the sensitive volume in an ion-counting nanodosimeter. The first method is based on a calculational approach simulating the extraction of ions from the sensitive volume, and the second method on probing the sensitive volume with 250 MeV protons. Sensitive-volume maps obtained with both methods are compared and systematic errors inherent in both methods are quantified.

We present the results of GEANT3 simulations of a full PET system made of Liquid Xenon (LXe)-TPC γ-camera modules. In such camera both ionization and scintillation signals will be detected to provide the three coordinates and the energy of the converted γ-ray. For that purpose, we will develop advanced ionization detectors operating in liquid xenon as well as fast cryogenic Gaseous Photomultipliers (GPMs) with CsI photocathodes, dedicated to the detection of the LXe scintillation signal. The measurement of the conversion coordinates and the energy will allow to reconstruct the correct Compton sequences and thus to identify the first interaction vertices of both correlated annihilation γ-rays. Moreover, measurement of the Depth of Interaction (DOI) will lead to a parallax-free tomograph. The GEANT3 simulation code of the LXe-TPC PET provided very promising expected performances from a realistic detector: good sensitivity to 511 keV γ-rays (∼93% for a 9cm depth LXe module) and good 3D spatial resolution (250 μm FWHM for first interaction vertex localization); the latter is close to the intrinsic physical limitations of the method.

We present latest results of a micro-hole and strip plate (MHSP) electron multiplier operating in pure xenon at atmospheric pressure. We report on avalanche gain of ∼2×10⁴, stable within ∼3% after an accumulated charge of 4 μC/mm², when irradiated with 0.5 kHz/mm² 5.9 keV photons. No photon-induced secondary effects or discharges have been observed at this gain. An energy resolution of 14% was obtained with 5.9 keV X-rays.

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.

We have analyzed and compared the performance of two novel fast-neutron imaging methods with time-of-flight spectroscopy capability. Key parameters such as detection efficiency, the amount of energy deposited in the converter and the spatial resolution of both detector variants have been simulated by means of neutron and charged-particle transport codes.

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 results from our recent investigations on detectors comprising cascaded gas electron multipliers (GEMs) and cascaded GEMs with microhole and strip plate (MHSP) multiplier as a final amplification stage. We discuss the factors governing the operation of these fast radiation-imaging detectors, which have single-electron 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 ultraviolet-to-visible spectral range as well as X-ray and neutron imaging.

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.

The microhole and strip plate (MHSP) gas electron multiplier was studied as a stand-alone device or in combination with a cascade of gas electron multipliers (GEMs), for X-ray and UV-photon detection. An MHSP operating in Ar/5%Xe yielded gains above 10⁴ and energy resolutions of about 14% FWHM for 5.9-keV X-rays. Gains as high as 10⁷ were reached in a 3-GEM/MHSP gaseous photomultiplier operating in an Ar/5%CH₄; the ion back-flow ratio to the top of the first GEM could be reduced to ∼2%. Two-dimensional (2-D) imaging was performed using signals induced by avalanche ions on a wedge and strip (W&S) readout cathode, placed at close proximity to the anode strips; position resolution of 200-250 μm FWHM for 5.9-keV X-rays was measured.

We report on avalanche-ion back-flow measurements in the novel Micro-Hole and Strip-Plate (MHSP) multiplier and in gaseous photomultipliers comprising Gas Electron Multipliers (GEMs) followed by an MHSP. In a 3-GEMs/MHSP photomultiplier with reflective photocathode, avalanche-ion back-flow fraction of ∼7% and ∼2% were recorded for respective effective gains of 10 ⁷ and 10⁶, in Ar/CH₄ (95/5) at 760Torr. This is about one order of magnitude reduction in ion back-flow compared to the best values measured in 4-GEMs photomultiplier at the same gain. We describe the mode of operation of the MHSP and explain its ion back-flow reduction features.

This paper describes a phantom-based feasibility study for a potential in vivo determination of zinc in prostate, which could bring about improved diagnosis of prostate cancer. An x-ray fluorescence topographic technique was developed, which will permit determination of the Zn content in the prostate through the rectum, namely behind a 2-3 mm thick layer of the rectal wall. The topographic approach, together with a reconstruction method developed here, minimizes the interference of Zn from non-prostatic tissue. The phantom studies show that it will be possible to determine Zn in a prostatic compartment behind a few mm thick layer of tissue using a specially designed transrectal probe. Such a probe is currently under development in our laboratories.

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.

Purpose: In cancer affected prostate cells lose the ability to concentrate zinc, resulting in a substantial decrease in Zn in the prostate. We investigated the possibility of using prostatic zinc combined with prostate specific antigen (PSA) as a novel tool for the reliable diagnosis of prostate cancer. Materials and Methods: Using the x-ray fluorescence method the Zn concentration was determined in vitro in prostate samples extracted by surgery from 28 patients. Clinical records included age, serum PSA, sextant prostate needle biopsy, previous medical therapy, surgical procedure and histological findings. Results: A new relationship was found between Zn in prostate tissue and PSA in blood, which allows improved separation between prostate cancer and benign prostate hyperplasia, and might have a significant impact on the reliable diagnosis of prostate cancer. Conclusions: Zn concentration is not uniform even in the same anatomical region of the prostate, so that a number of measurements at various locations are required for a diagnostic procedure. The most interesting finding in this study is the relationship between Zn concentration and PSA. A combination of these parameters represents a significant improvement on the diagnostic value of each of them separately and provides a powerful tool for more accurate diagnosis. Although the method may be applied in vitro on biopsy samples, our study underlines the importance of developing a facility for in vivo Zn determination in the prostate.

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 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.

The transport of low-energy (

The properties of a 3-GEM (Gas Electron Multiplier) element photomultiplier, with a semitransparent CsI photocathode and CF₄ gas filling, are presented. Compared to other gas mixtures, such as CH₄, Ar/CH₄, Ar/N₂ and He/Ar/N₂, CF₄ has superior performance: the highest gain, approaching 10⁷, the fastest, 8 ns wide signal and the lowest photoelectron backscattering; the latter allows to reach photocathode quantum efficiency values approaching that in vacuum. The time resolution of the multi-GEM photomultiplier for single photoelectrons was measured to be 2 ns. These properties are of high relevance for applications in Cherenkov detectors and in tracking devices.

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.

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.

We report on the efficient operation of a CsI-coated GEM photon detector. We describe its operation mode and the dependence of the single electron detection efficiency on the electric fields. Conditions for obtaining full efficiency of photoelectron extraction and their focusing into the GEM apertures, in 1 atmCH₄, are presented. The quantum efficiency of the CsI-coated GEM is 35% at 150 nm.

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.

Ultrananocrystalline diamond (UNCD) films 0.1-2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH₄-Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high (60-100 μA/tip) emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In addition, scanning tunneling microscopy studies indicate that the emission sites from UNCD films are related to minima or inflection points in the surface topography, and not to surface asperities. These data, in conjunction with tight binding pseudopotential calculations, indicate that grain boundaries play a critical role in the electron emission properties of UNCD films, such that these boundaries: (a) provide a conducting path from the substrate to the diamond-vacuum interface, (b) produce a geometric enhancement in the local electric field via internal structures, rather than surface topography, and (c) produce an enhancement in the local density of states within the bulk diamond band gap.

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.

We report on the photoemission properties of 300 angstroms thick transmissive- and 5000 angstroms thick reflective UV-sensitive CsBr photocathodes. Following post-evaporation heat treatment at 70 °C the absolute quantum efficiency is 35% at 150 nm, with a red boundary cut-off at about 195 nm. Extensive aging studies of CsBr and CsI photocathodes, under high photon flux and under ion bombardment in gas avalanche multiplication mode, were carried out for the first time without exposure to air. The results are compared with the previously published data on CsI aging and the methodology of the aging tests is discussed in details.

Gas avalanche detectors, combining solid photocathodes with fast electron multipliers, provide an attractive solution for photon localization over very large sensitive areas and under high illumination flux. They offer single-photon sensitivity and the possibility of operation under very intense magnetic fields. We discuss the principal factors governing the operation of gas avalanche photomultipliers. We summarize the recent progress made in alkali-halide and CVD-diamond UV-photocathodes, capable of operation under gas multiplication, and novel thin-film protected alkali-antimonide photocathodes, providing, for the first time, the possibility of operating gas photomultipliers in the visible range. Electron multipliers, adequate for these photon detectors, are proposed and some applications are briefly discussed.

The single-ion detection efficiency of two types of electron multipliers is provided for incident energies reaching 10 keV. Absolute efficiencies above 90% were measured for a discrete dynode electron multiplier (DDEM), in response to Ar⁺ ions and protons, above an energy of 5 keV. The efficiency to detect protons is consistently higher than that of Ar⁺ ions, due to a higher secondary electron emission yield. For a channel electron multiplier we have measured a similar maximal detection efficiency for Ar⁺ ions; though strongly varying across the detector surface.

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 on the results of coating K-Cs-Sb visible photocathodes, with thin CsBr and CsI protective films, in view of their possible combination with gas electron multipliers. Absolute quantum efficiency, photoelectron transport through the coating film, the effect of exposure to oxygen and to air and the photocathode behavior under intense photon flux are presented. The coated K-Cs-Sb photocathodes have typically maximum quantum efficiency values of 5-6% at 312 nm. They can withstand exposure to 150 Torr of oxygen for more than half an hour. Appropriate electron multipliers and possible applications are briefly discussed.

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.

We describe a technique for protecting alkali-antimonide visible light photocathodes against deterioration by exposure to impurities, during handling or storage in poor vacuum or gas. The photocathodes are coated with a ∼ 1 μm vacuum-deposited hexatriacontane film, which can be subsequently removed by low-temperature sublimation. We show that Cs₃Sb coated photocathodes can be exposed for several minutes to considerable amounts of oxygen, without deterioration. Their initial photoemission properties are almost fully recovered after film removal.

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.

A new experimental approach for a neutron dosemeter and spectrometer is presented. It is based on an optically read out time projection chamber (TPC). Charged particles entering a low-pressure sensitive volume of about one litre create ionisation electrons along their track, which are detected and localised using an optoelectronic detection system. This allows type, energy, and direction of neutron-induced secondaries to be analysed. It will therefore be suited as an advanced active neutron monitor for the measurement of dosimetric quantities and will also supply spectrometric information. The method as well as the planned structure and its expected performance are presented. Results are reported that were obtained with a first experimental device which was built to study particle induced light and charge production in electron avalanches in various triethylamine-based gas mixtures.

In order to operate gaseous photomultipliers in the visible range it was suggested to protect sensitive photocathodes against contact to air and counting gases by their coating with a thin solid dielectric film. We present data on coating of cesium-antimony photocathodes with alkali-halide (NaI, CsI, CsF, NaF), oxide (SiO) and organic (hexatriacontane, calciumstearate) films. The photoelectron transmission through these films and their protection capability have been studied in detail. Cesium-antinomy photocathodes are shown to withstand exposure to considerable doses of oxygen and dry air when coated with NaI films. This opens ways to their operation in gas media.

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.

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.

The detection of photons over a very broad energy range is a challenging task in numerous fields of basic and applied research. We review here some techniques that have the potential of becoming the leading tools for photon detection in the next decades. The review covers gaseous detectors, hybrid detectors, solid state and superconducting devices.

A technique for sub-keV x-ray spectroscopy over a large area at room temperature is presented. Photon-induced ionization electron trails deposited in a low-pressure gas are efficiently counted by a multi-stage electron multiplier. The analysis of the number of deposited electrons and the electron trail length, combined with a detailed simulation of the detection process, provide a powerful method for resolving x-ray lines down to 70 eV. Detection efficiencies above 40% and an energy resolution of 55% (FWHM) were measured for the Be K (108.5 eV) x-ray line. It is demonstrated that light elements with an abundance below 1% in composite samples can be quantitatively resolved. The experimental technique, analysis method and x-ray spectroscopic results obtained with the particle-induced x-ray emission technique and at a scanning electron microscope are presented.

Radiation-induced low-energy secondary electrons multiplied in gas by a series of wire electrodes provide an efficient means for fast and accurate radiation localization under very high flux. We discuss here the properties of secondary electron emission (SEE) X-ray detectors, operated at atmospheric pressure helium-based mixtures.

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

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.

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.

The photoemission from solid surfaces into gas is important in view of the application of solid photocathodes in fast-RICH devices. The photoemission from CsI into gas has been investigated in He-, Ar-and CH₄-based gas mixtures as a function of the electric field at the photocathode surface. Measurements were made both in laboratory, with a UV source, and in a beam with a RICH detector. The results are interpreted in terms of current models of electron transport in gas. The electron collection efficiency, below gas ionization threshold, is reduced by backscattering. This phenomenon is particularly important in He-based gas mixtures. In CH₄ and Ar/CH₄ mixtures the backscattering is very low. At high electric field, under charge multiplication, a full collection efficiency, similar to that in vacuum, is obtained in all gases investigated. We discuss the parameters governing the choice of the gas mixture in this kind of photon detectors.

We present the first results of the application of a novel digital X-ray imaging detector, based on secondary electron emission from a solid converter, to high-rate crystallographic studies. Results from diffraction and small-angle scattering experiments from several crystallized proteins, performed at the European Synchrotron Radiation Facility, ESRF, Grenoble, are presented and compared with a phosphor-based imaging system. Future developments of this detector system are proposed.

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.

We revise the quantum efficiency of NaI and CuI reflective photocathodes. We have shown that the photoyield from NaI and CuI can be considerably enhanced by a post-evaporation heat treatment, similarly to that observed before for the CsI photocathode. We discuss possible reasons for the heat enhancement.

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 report here on the results obtained by the CERN RD26 collaboration on the production and characterization of large area photocathodes, susceptible to equip fast UV-photon imaging devices. Such detectors are planned for some Ring Imaging Cherenkov (RICH) detector projects, in particular HADES at SIS Darmstadt, BABAR at the SLAC asymmetric B-factory, and ALICE at the LHC (CERN).

We have measured the electron emission from a CsI-coated multiwire cathode, induced by ultraviolet photons and electrons, in vacuum at high electric fields. We found an enhancement in quantum efficiency of a factor of 1.5 at 160 nm, 3 at 185 nm, and 25 above 200 nm, at a field of 500 kV/cm. At the short wavelengths the amplitude of the effect is a linear function of the square root of the field strength. The enhancement of the electron-induced secondary electron emission yield is dependent on the primary electron energy: for energies above 1 keV it varies by a factor of 2 to 10. A simple model of the field enhancement of the photoemission is suggested. Practical applications are discussed.

We performed a systematic investigation of the quantum efficiency of some solid reflective photocathodes in the spectral range 140-240 nm. The measurements were made without gaseous amplification in vacuum and in methane. No significant difference was found among CsI photocathodes prepared by vacuum deposition at different institutes, either from powders or from crystals of different origins, and measured either in vacuum or in methane. Amorphous silicon photocathodes were prepared by the plasma enhanced chemical vapor deposition technique. We present the results for several doping conditions of amorphous silicon and for p-n junctions. Some organometallic photocathodes, containing iron or other transition metals (cerium), were evaporated and measured. Among them decamethylferrocene exhibits the highest quantum efficiency in the range 190-240 nm.

A model of electron transport in alkali halides, below 10 eV, is described. It is based on theoretically calculated microscopic cross sections of electron interactions with lattice phonons. Both acoustic and optical scatterings are taken into account, the former being also treated as a quasielastic process that randomizes the electron motion. Monte Carlo calculations based on the model simulate the UV-induced photoelectron emission from CsI. The calculated quantum efficiency and energy spectra are in good agreement with experimental data, in the photon energy range of 6.3-8.6 eV. The probability for an electron to escape from CsI, NaCl, and KCl is provided as a function of its energy and creation depth. A comparison is made between our approach and other phenomenological models.

The potential of a thermal neutron imaging system based on a composite neutron convertor foil combined with a low-pressure, multistep avalanche chamber is demonstrated. Neutron-induced charged particles from a primary convertor element induce multiple low-energy electrons escaping from a second thin film of high electron-emissive material. We investigated the performance of detectors with Gd- and Li-based primary convertors coated with CsI as a secondary electron emitter. It is shown, that the detector can be operated with high stability at a sufficiently high gain to detect all escaping particles. A localisation resolution of 0.4 mm (fwhm) was obtained. With Li-based convertors a very low γ-ray sensitivity was established. The good imaging performance, free of parallax errors in divergent neutron beams, fast time resolution, low occupation time and high count rate capability, make this instrument an excellent tool for time-resolved neutron scattering experiments and for neutron radiography and tomography.

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.

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.

The quantum efficiency (QE) of four photomultipliers of the R1460 series of Hamamatsu, used in the last years as a reference in our UV-photocathode studies, was determined with the help of a calibrated vacuum UV-photodiode. Large discrepancies were found with respect to the QE curves supplied for each tube by the manufacturer. The correction curves of the photomultipliers are presented and should be used to normalize our previously published QE data of various photoemissive materials. Corrected data of CsI photocathodes are presented here.

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.

The sensitivity to minimum ionizing particles, of CsI-based gaseous detectors currently developed for X-ray and UV-photon imaging in future colliders, was studied in detail. The detection efficiency of minimum ionizing electrons was measured with a thin CsI film coupled to a low-pressure gaseous electron multiplier. The contributions from secondary electrons emitted from the solid and from ionization electrons in the gas, were studied as function of the gas pressure, the CsI thickness and the electron incidence angle. Monte Carlo simulations of the solid and gas contributions are in good agreement with the experimental results. It was found that the secondary emission from the CsI film contributes less than 5% to the detection efficiency of minimum ionizing electrons, over a thickness range of 200 to 2000 nm. We demonstrated that the direct ionization of the gas is dominant, even in He-rich mixtures, unless very low gas pressures are used.

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.

Ionization electrons deposited by soft X-rays in a low pressure (10 Torr) gas medium are efficiently counted by a multistage electron multiplier, providing an accurate measurement of the X-ray photon energy. Energy resolutions of 56-28% FWHM were measured for X-rays of 110-676 eV, recording electrical induced charges or visible photons emitted during the avalanche process. It is demonstrated that a combined analysis of the number of electrons and the electron trail length of an event, provides a powerful and competitive way of resolving ultra-soft X-rays. We present the experimental technique, discuss the advantages and limitations of the Primary Electron Counter, and suggest ways to improve its performances.

The results of a systematic investigation of the properties of CsI and CsI-TMAE photocathodes, in the spectral range of 150-200 nm, are presented. The measurements were carried out in high vacuum and in CH₄ atmosphere. Effects of the photocathode regeneration and the enhancement of the quantum efficiency due to temperature and TMAE adsorption are observed. The stability of the TMAE effect was investigated and is discussed. A comparison is given with different results of other groups.

We present a new X-ray imaging method with a potential application to ultrarelativistic particle identification by Transition Radiation detection. It is based on the conversion of the X-ray photons in a thin layer of CsI and the amplification of secondary emitted electrons in a low-pressure multistep avalanche electron multiplier. The obvious advantages of the solid X-ray convertor are the parallax-free imaging and the ultrafast response. The detector has an X-ray localization resolution accuracy better than 0.2 mm (fwhm), a subnanosecond time reponse and a reduced sensitivity to minimum ionizing particles as compared to Xe-filled detectors. We present the experimental results on the detector performance with X-rays of 6-60 keV and with relativistic electrons. They are accompanied by mathematical modelling and Monte Carlo simulations of Transition Radiation Detectors based on this novel technique.

A large area, low-pressure gas-filled UV-imaging photomultiplier with CsI-photocathode is presented. The double-step electron multiplier with a 10 Torr CH₄ gas-filling enables stable high gain operation. The detection efficiency of photons in the wavelength range λ ∼ 170 nm (Xe scintillation light) is about 10% for 200 to 2000 nm thick photocathodes. We investigated the influence of various substrate materials, the thickness of the CsI-layer, the gas pressure and gas composition on the performance of the photocathode. Furthermore we studied the stability of the photocathode under different operating conditions and its sensitivity to air. Measurements of the timing characteristics of the device yielded an ultimate time resolution of 350 ps (FWHM).

We report on a search for supermassive nuclei in nature with masses up to 10⁷ amu. Such exotic nuclei might consist, for example, of stable strange matter, which comprises a mixture of up, down, and strange quarks, or of relic particles from the early Universe. The experiments are based on Rutherford backscattering of heavy ions, preferably²³⁸U, from various target samples. The measured parameters of a detected particle are its time-of-flight, scattering angle, and specific ionization. From this information the mass of the target nucleus can be inferred. Upper limits for the abundance of strange supermassive nuclei with masses A-4·10² to 10⁷ amu relative to the number of nucleons were found to be in the range 10^-11 to 10^-15. For the narrower mass range A -10³ to 10⁴ amu the limit is 2· 10^-17.

A large area detection system is described which consists of twelve low-pressure multi-wire proportional counters and is used in the search for exotic super-massive nuclei. The experiments are based on Rutherford backscattering of heavy ions, preferably ²⁰⁸Pb or ²³⁸U, from various target samples. The measured parameters of a detected particle are its time-of-flight, scattering angle, and specific ionization. From this information the mass of the target nucleus can be inferred. The present experimental sensitivity for the detection of exotic nuclei with at least twice the mass of the projectile is about 10^-12 relative to the number of nucleons.

UV-photons are detected by a low-pressure photosensitive multistep gaseous detector. Photoelectrons are multiplied in two charge amplification stages. A third, light amplification stage operating in a scintillation mode, provides light yields > 5×10⁷ visible photons per single photoelectron avalanche, in ArC₂H₆-TMAE gas mixtures. We present results on absolute photon yields in various TMAE gas mixtures, at low gas pressures and at low charge gains. We describe the operation mechanism and some basic properties of the gated 3-stage detectors, such as stability of operation at high background rates and localization resolution, particularly at large TMAE concentration and high temperature operation conditions. Further applications are discussed.

A simple method for the determination of the total width of the 9.17 MeV level in ¹⁴N is described. The method is based on the use of a resonant detector which contains nitrogen in its active volume. With the help of the resonant detector the ratio of Γ_γ0/Γ_T was found to be 0.052 ± 0.004. This result together with the data from a conventional resonant absorption experiment yields for the total width of the level a value of 122±8 eV.

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.

Photons emitted by avalanches in gases can be detected wit h an image Miens:tier coupled to it thettitimi camera. We have investigated the emuiut properties of %, anions gas Illklure% in order to increase the light.> kid. tic% cral applications of the imaging chamber are illustrated: For high-granularity tracking of comples dents and of electromagnetic and hadronic shouers: For Cherenkov ring-imaging of charged particles and clixt•omagnoic %lion ers: And for use as a !racking device in a neutrino-tagging experiment.

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.

The application of low-pressure two-step gas detectors to the ring-imaging Cherenkov (RICH) technique is demonstrated. A RICH prototype with a UV-photon detector of 20×20 cm², filled with 53 mbar of C₂H₆ + TMAE at 30°C, was exposed to 3 GeV celectrons. Photoelectrons were localized with a FADC system with a spatial resolution of 2 mm rms. The ring reconstruction technique is presented. The advantages of using this method in high background experiments, such as in relativistic heavy ion collisions, are discussed.

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

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 (

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.

A position sensitive Multistep Avalanche Chamber (MSC), operated with low pressure isobutane (5-20 Torr) was coupled to a solid photocathode. It is shown that gaseous amplification factors above 10⁸ are obtained, thus enabling the efficient detection of single photoelectrons. Imaging of single photoelectrons was performed using the delay-line technique. Position resolution of the order of 200 μm (fwhm) is obtained with a photoelectron detection efficiency close to 100%. Further applications are discussed.