G. Garty

The GEM photomultiplier operated with noble gas mixtures

We present the results of detailed investigations of the Gas Electron Multiplier (GEM)-based photomultiplier, consisting of a solid CsI photocathode coupled to a cascade of GEM elements. The detector is lled with non-ageing mixtures based on noble gases: Ar, Ne, Ar#Ne, Ar#Xe, Ar#CH 4 and Ar#N 2 . Very high gas gains, reaching 106, and rather fast anode pulses, of a width of 10 ns, were observed in some mixtures. Various phenomena and physical processes, found to a!ect the device operation, are discussed here: additional gain due to secondary scintillation; mixtures with enhanced ionization e

The performance of a novel ion-counting nanodosimeter

ciency; improvement of pulse-height resolution due to avalanche connement in the GEM holes; avalanche extension outside the GEM holes; gain limitation due to ion feedback and charging-up of GEM electrodes; photoelectron backscattering. ( 2000 Elsevier Science B.V. All rights reserved.

Evaluation of lesion clustering in irradiated plasmid DNA.

Abstract We present the performance of a novel device conceived for measuring minute energy deposits in a low-density gas, capable of operating in various radiation fields, including in an accelerator environment. The ion-counting nanodosimeter provides a precise measurement of the ionization distribution deposited within a small wall-less gas volume, modeling nanometer-scales of condensed matter, e.g. the DNA molecule. We describe the instrument and its data acquisition system. The results of systematic studies with low-energy alpha particles, protons and carbon ions are compared to model simulations; they demonstrate the capabilities and indicate the limitations of this novel technique.

FIRST RESULTS ON THE GEM OPERATED AT LOW GAS PRESSURES

Purpose: To measure the yield of DNA strand breaks and clustered lesions in plasmid DNA irradiated with protons, helium nuclei, and γ-rays. Materials and methods: Plasmid DNA was irradiated with 1.03, 19.3 and 249 MeV protons (linear energy transferu200a=u200a25.5, 2.7, and 0.39u2009keV μmu200a–u200a1 respectively), 26 MeV helium nuclei (25.5u2009keV μm) and γ-rays (137Cs or 60Co) in phosphate buffer containing 2u2009mM or 200u2009mM glycerol. Single-and double-strand breaks (SSB and DSB) were measured by gel electrophoresis, and clustered lesions containing base lesions were quantified by converting them into irreparable DSB in transformed bacteria. Results: For protons, SSB yield decreased with increasing LET (linear energy transfer). The yield of DSB and all clustered lesions seemed to reach a minimum around 3u2009keV μmu200a–u200a1. There was a higher yield of SSB, DSB and total clustered lesions for protons compared to helium nuclei at 25.5u2009keV μmu200a–u200a1. A difference in the yields between 137Cs and 60Co γ-rays was also observed, especially for SSB. Conclusion: In this work we have demonstrated the complex LET dependence of clustered-lesion yields, governed by interplay of the radical recombination and change in track structure. As expected, there was also a significant difference in clustered lesion yields between various radiation fields, having the same or similar LET values, but differing in nanometric track structure.

On the efficient electron transfer through GEM

Abstract We report on the properties of the Gaseous Electron Multiplier (GEM) operated at 10–40xa0Torr isobutane and methane. We found stable operation at gains of a few thousand, fast response and effective photon-feedback reduction. The transmission of single electrons through the GEM apertures was studied. Ion-induced feedback, from a wire chamber following the GEM, was found to limit the total two-stage multiplication at high GEM gains. A stable double-GEM operation with reduced ion-feedback was demonstrated. Some applications are discussed.

Further studies of the GEM photomultiplier

The absolute electron transfer efficiency of a gas electron multiplier (GEM) was systematically measured in several gas types and pressures and over a broad range of electric-field configurations, using a single-electron pulse-counting method. A complete understanding of the role played by the relevant variables was obtained; particularly, the critical part of electron transport in the gap preceding the GEM was demonstrated. A small electron multiplication in this gap was shown to result in a full detection efficiency of single-electron events, under proper gas diffusion and multiplication conditions. The relevance to single electron and single photon detection is discussed. The experimental results are in good agreement with simulation calculations.

Single photoelectron detection with a low-pressure gas electron multiplier coupled to a CsI photocathode

We present the results of our further investigation of the Gas Electron Mutiplier (GEM) photomultiplier consisting of a solid CsI photocathode coupled to a cascade of GEMs. It operates with mixtures of pure noble gases or with noble gases having small additives of nitrogen or methane. The GEM photomultiplier (GPM) reaches multiplication factors of 105}106, allowing for detection of single photons. We discuss the limitations of the GPM, imposed by ion feedback and charging-up of the GEM elements, and some phenomena related to the avalanche development in such devices. ( 2000 Elsevier Science B.V. All rights reserved.

Ion-counting nanodosimetry: a new method for assessing radiation damage to DNA

We have operated the Gas Electron Multiplier (GEM) coupled to a CsI photocathode for the detection of single UV photons. Gains of up to 10 000 were observed with a single GEM in pure hydrocarbons in the range 40}400 Torr. Using a double-GEM structure, we have detected single photoelectrons on a strip electrode. We have studied the single-electron detection e

The Absolute Detection Efficiency of Vacuum Electron Multipliers, to keV Protons and Ar + Ions.

ciency of the GEM, a crucial parameter in its operation in single-photon detection applications, both experimentally and by simulation. ( 1999 Elsevier Science B.V. All rights reserved.

ON THE HIGH GAIN OPERATION OF LOW-PRESSURE MICRODOT GAS AVALANCHE CHAMBERS

A novel nanodosimeter is described, based on ion counting. It provides precise model-evaluation of radiation-induced ionization patterns in small condensed-matter volumes of nanometric size. The nanodosimeter consists of a millimetric, low-pressure, wall-less gas cell, serving as an expanded model of a nanometric condensed-matter volume. The method can also be employed for the assessment of radiation damage to advanced nanoelectronics.