Operation of Gas Electron Multiplier (GEM) with Propane Gas at Low Pressure

L. De Nardo1,2, F. Dal Corso2, M. Pegoraro2

1 Dipartimento di Fisica dell'Università di Padova, Padova, Italy. 2 INFN, Sezione di Padova, Padova, Italy.

INTRODUCTION

In microdosimetry the effects of ionizing radiation on biological targets can be experimentally investigated by measuring the statistical distribution of energy-deposition events at a microscopic level. The reference instrument to carry on this investigation is the Tissue-Equivalent Proportional Counter (TEPC). A traditional TEPC is a proportional counter with a central anode wire operated with a tissue-equivalent (TE) gas mixture. A few attempts to develop a different kind of TEPC, based on a Gas Electron Multiplier (GEM) or Thick Gas Electron Multiplier (THGEM) are also reported in the literature, both for radiotherapy and radioprotection application [1-3]. The GEM is a two-side copper-clad Kapton foil, perforated with a high density of holes of some tens of micrometer size [4]. We have also designed and constructed a multi-element mini-TEPC based on a GEM (GEM-TEPC) for hadrontherapy application. The counting gases commonly used in microdosimetry are methane-based or propane-based TE gases (TE-CH4 and TE-C3H8) [5], at low pressure, to simulate a microscopic tissue site. Pure C3H8 gas is also employed in microdosimetry, especially for space application, due to more stable behavior on long-time operation and to higher gas gain with respect to TE-C3H8. Due to the advantages provided by the use of C3H8, we have investigated the possibility to employ this gas in our counter, by measuring the effective gas gain of GEM of standard geometry at 22 and 28 kPa of C3H8 gas.

MATERIALS AND METHODS

The GEM-TEPC has been designed and constructed with 16 cylindrical sensitive volumes of 2 mm height and diameter. The elements of the counter are a cathode, made of A-150 plastic, a GEM foil, with a sensitive area of 5×5 cm2, and a readout printed circuit board (PCB), with 16 circular pads of 2 mm diameter, disposed in a 4×4 matrix (pitch of 4 mm). Figure 1 provides a schematic view of the setup. The mass per area of the sensitive volumes diameter is 0.08 mg/cm2 and 0.10 mg/cm2, respectively at 22 and 28 kPa of C3H8 gas at 20°C.

The GEM structure used in this work is the so called standard GEM, that is a Kapton foil of 50 µm thickness, with a 5 µm-thick copper coating on each side, perforated on a triangular pattern with a high density of holes with a double conical shape. The diameter of the holes is 70 µm

in the copper surfaces, 50 µm in the Kapton. The distance between the centers of the holes is 140 µm. The GEM foil is glued between two fiberglass frames, each one of 0.5 mm thickness. The GEM and the PCB were realized at the CERN printed circuits workshop. The drift region (space between cathode and GEM) and the induction region (space between GEM and PCB) have an height of 2 mm and 0.5 mm, respectively and are filled with C3H8 gas. A 244Cm alpha source has been inserted inside a cavity on the cathode, to let alpha particles entering the detector through a 0.5 mm diameter, 2.6 mm long collimator, in correspondence of one of the collecting pad of the PCB. The charge produced on the readout pad was fed into a charge preamplifier and pulses were then directly sent to a CAEN DT5724 Digitizer equipped with DPP-PHA Firmware (Digital Pulse Processing for the Pulse Height Analysis) controlled by a PC. This provides a digital solution equivalent to shaping amplifier plus peak sensing ADC.

The effective gas gain G is defined as the amount of charge collected on the readout pad divided by the primary charge released inside the sensitive volume of the counter:

WEe

QG

/ ; (1)

where Q is the total charge collected on the readout pad, ΔE is the energy loss of alpha particle in the drift region of the counter, W is the average energy for ion-electron pair creation (28±0.5 eV [6]), e is the electron charge. ΔE has been calculated by using the ICRU 49 data tables [7]. The most probable value of ΔE is (80.5± 3.2) keV and (107.0 ± 4.3) keV, respectively at 22 and 28 kPa.

Q has been calculated through the equation Q=C∙Vpeak, where C is a calibration factor, determined by feeding known voltage pulses into the test input of the preamplifier and Vpeak is the voltage value of the peak of the measured alpha particle spectrum.

Fig. 1. Schematic view of the experimental setup.

EFFECTIVE GAS GAIN MEASUREMENTS

G was measured at 22 and 28 kPa of C3H8 as a function of ∆VGEM, ED and EI, where ∆VGEM is the voltage difference across GEM while ED and EI are the drift and induction field, respectively. G measured as a function of ∆VGEM, for fixed value of ED and EI (ED=0.45 and 0.6 kV/cm and EI=3.0 and 4.0 kV/cm, respectively at 22 and 28 kPa), is plotted in Figure 2, pointing out the exponential rise at both pressures. Maximum gain of about 5∙103 were obtained in stable condition at both pressures. This value does not correspond to the maximum safe gain because measurements were stopped before observing any discharge, to prevent damage to the GEM.

Fig. 2. Effective gain measured at 22 and 28 kPa.

The dependence of G on the drift field strength, ED, was measured for fixed value of ∆VGEM and EI. The electric field in the drift region should be strong enough to limit recombination between ions and electrons, but it should not be too strong because this causes some field lines to terminate on the GEM electrode, reducing the fraction of the drifting electrons collected in the holes (electrical transparency) [8]. The electrical transparency of the GEM depends on the value of ED and ∆VGEM and its optical transparency, ratio of open to total area. It has been reported that the two effects, recombination and transparency, cause an optimum gain to be attain between two values of ED, a plateau [8].

Figure 3 shows the results obtained with ∆VGEM=430V and EI=4 kV/cm. G increases with increasing ED until about 0.65 kV/cm and then reach a plateau. Measurements were stopped before observing a decrease of G after the plateau.

The effect of the induction field EI on G was measured for fixed value of ∆VGEM and ED. Figure 4 shows the results obtained with ∆VGEM=430V and ED=0.6 kV/cm. G increases almost linearly with EI, as expected in absence of

parallel plate multiplication process in the induction region [8]. Similar results were obtained at 22 kPa.

Fig. 3. Relative gain vs. ED (∆VGEM=430V, EI=4 kV/cm).

Fig. 4. Relative gain vs. EI (∆VGEM=430V, ED=0.6 kV/cm).

FUNDINGS

This work is supported and funded by the V commission of INFN, within the framework of the MITRA research project.

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