Publications
2003
-
(2003) Australasian Physical and Engineering Sciences in Medicine. 26, 4, p. 149-155 Abstract
There is a growing interest in the study of interactions of ionizing radiation with condensed matter at the nanometer level. The motivation for this research is the hypothesis that the number of ionizations occurring within short segments of DNA-size subvolumes is a major factor determining the biological effectiveness of ionizing radiation. A novel dosimetry technique, called nanodosimetry, measures the spatial distribution of individual ionizations in an irradiated low-pressure gas model of DNA. The measurement of nanodosimetric event size spectra may enable improved characterization of radiation quality, with applications in proton and charged-particle therapy, radiation protection, and space research. We describe an ion-counting nanodosimeter developed for measuring radiation-induced ionization clusters in small, wall-less low-pressure gas volumes, simulating short DNA segments. It measures individual radiation-induced ions, deposited in 1 Torr propane within a tissue-equivalent cylindrical volume of 2-4 nm diameter and up to 100 nm length. We present first ionization cluster size distributions obtained with 13.6 MeV protons, 4.25 MeV alpha particles and 24.8 MeV carbon nuclei in propane; they correspond to a wide LET range of 4-500 keV/μm. We are currently developing plasmid-based assays to characterize the local clustering of DNA damage with biological methods. First results demonstrate that there is increasing complexity of DNA damage with increasing LET. Systematic comparison of biological and nanodosimetric data will help us to validate biophysical models predicting radiation quality based on nanodosimetric spectra. Possible applications for charged particle radiation therapy planning are discussed.
-
(2003) Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 513, 1-2 SPEC. ISS., p. 250-255 Abstract
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.
-
(2003) 2003 IEEE Nuclear Science Symposium. Conference Record. Vol. 1. p. 525-529 Abstract
The Micro-Hole & Strip-Plate gas electron multiplier (MHSP) 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-backflow fraction to the top of the first GEM could be reduced down to ∼2% 2D-imaging performed using signals induced by avalanche ions on a Wedge-and-Strip readout cathode, placed at close proximity of the anode strips yielded position resolutions of the order of 200 to 250μm FWHM for 5.9-keV x-rays.
-
(2003) 2003 IEEE Nuclear Science Symposium. Conference Record. Vol. 1. p. 518-524 Abstract
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.
-
(2003) Journal of Urology. 170, 6 I, p. 2258-2262 Abstract
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.
-
(2003) Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 513, 3, p. 473-483 Abstract
A 100 × 100 mm2 2D imaging detector, based on a triple-GEM gaseous multiplier, striped x - y readout anode and discrete delay-line readout, is presented. The fast (2.1 ns tap-1) delay-line circuit was designed to match the anode-charge signal profile, namely its rise-time and length. The detector's imaging capability was systematically studied in Ar/CO2 (70/30) with 5.9 keV X-rays; x - y resolution of σ = 0.05 and 0.1 mm for top and bottom anode strips, respectively, and integral non-linearity of ∼0.15% are demonstrated.
-
(2003) IEEE Transactions on Nuclear Science. 50, 4, p. 847-854 Abstract
We describe the preparation of a sealed gas-filled photomultiplier (GPMT) for the visible spectral range and present the properties of the first prototypes. They consist of a 50 mm diameter semitransparent bialkali (K-Cs-Sb) photocathode coupled to a 30 mm × 30 mm Kapton multi-gas electron multiplier (GEM). High gain of 2 × 104 in two-GEM mode and a quantum efficiency of 13% at 405 nm have been reached at 700 Torr of Ar/CH 4 (95:5). The detector structure and experimental setup are described; results are presented on the GPMT gain, ion-feedback and its suppression, stability, and other critical parameters in various gas mixtures. Hot sealing techniques with In/Sn and In/Bi solders are discussed.
-
(2003) Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 504, 1-3, p. 364-368 Abstract
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.
-
(2003) Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 504, 1-3, p. 93-98 Abstract
We present the current status of our research on GEM-based gaseous photomultipliers. Detectors combining multi-GEM electron multipliers with semi-transparent and reflective photocathodes are discussed. We present recent progress in extending the sensitivity of these detectors into the visible range. We demonstrate the long-term stability of an argon-sealed bi-alkali photo-diode and provide preliminary results of a gas-sealed Kapton-GEM detector with a bi-alkali photocathode. The problem of ion-induced secondary electron emission is addressed.
-
(2003) Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 504, 1-3, p. 369-373 Abstract
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.
-
(2003) Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 502, 1, p. 195-199 Abstract
We discuss recent progress in gaseous photomultipliers (GPMTs) comprising UV-to-visible spectral range photocathodes (PCs) coupled to multiple Gas Electron Multipliers (GEM). The PCs may be either semitransparent or reflective ones directly deposited on the first-GEM surface. These detectors provide high gain, even in noble gases, are sensitive to single photons, have nanosecond time resolution, and offer good localization. The operation of CsI-based GPMTs in CF4 opens new applications in Cherenkov detectors, where both the radiator and the photosensor operate in the same gas. The latest results on sealed visible-light detectors, combining bialkali PCs and Kapton-made GEMs are presented.
-
(2003) Nuclear Instruments & Methods In Physics Research Section A-Accelerators Spectrometers Detectors And Associated Equipment. 497, 2-3, p. 305-313 Abstract
A study of a dual-GEM detector coupled to a strip readout anode is described. The effects of the induction electric field and GEM-to-anode gap are presented, for an operation in atmospheric pressure Ar/CO2 (70/30) and Ar/CH4 (95/5). Visible gain and anode signal pulse-shapes, measured with 5.9 keV X-rays are presented for 1-6mm wide induction gaps and for induction fields ranging up to 6 kVcm-1. The spatial distribution of the anode charge is provided for induction gaps of 2-12 mm. The results are useful for matching the detector parameters to the position recording circuit requirements.