WIS99 / 43 / Jul-DPP
Single Electron Detection with GEM Rachel Chechik1, Amos Breskin and Guy Garty Department of Particle Physics, The Weizmann Institute of Science 76100 Rehovot, Israel Abstract Sensitivity to single electrons is requested in detectors based on the electron counting technique, measuring very low radiation-deposited energies. It is also required in novel large-area gaseous photomultipliers, employing solid photocathodes and gaseous electron multipliers. The Gas Electron Multiplier (GEM) has been considered for these applications, and we report on its properties when operated at pressures of 10-400 torr of pure hydrocarbons and Ar/hydrocarbon mixtures. Despite the small dimensions of the GEM apertures, and the low gas density, charge gains exceeding 3000 are routinely measured for single electrons. The rapid multiplication process results in fast single-electron pulses, measuring down to 10ns at the base. We discuss the operation of the GEM coupled to a CsI photocathode and demonstrate its capacity of blocking photon feedback. We report on the presence of ion-induced feedback effects and propose ways of eliminating it. The relative detection efficiency for single photoelectrons, under various GEM operation conditions, is studied both experimentally and by Monte-Carlo simulations.
__________________________________________________________________________________ 1. Introduction Various applications require the reliable and efficient detection of single electrons. Detectors based on the single-electron counting technique are used to measure very small amounts of energy deposited by radiation in a small gas volume; the method has been applied for soft x-ray spectroscopy [1] and is currently used in nanodosimetry for the estimation of potential radiation damage to the DNA molecule [2]. Large-area gaseous photomultipliers, comprising a solid photoconverter coupled to a gaseous electron multiplier, are developed for singlephoton detection and x-ray imaging in synchrotron radiation diffraction studies [3], particle physics [4] and in medicine. Some of these detectors are required to operate under very low gas-pressures, others under very high repetition rates. The common mandatory requirements in all these applications are the full efficiency for single-electron detection and the absence of feedback signals, which may falsify the results. The detector, therefore, must have a high gain (preferably above 105) and very fast response (pulses of a few ns at full-width). The GEM [5], being a micropattern-amplifying element in which the avalanche develops within a very small aperture volume (typically 80µm in diameter and 100µm long), is inherently very fast. State-of-the GEMs yield gains exceeding 103 or even 104 per single electrode [6], and this can be further increased by cascading several GEMs [7], or a GEM with other multipliers. It is therefore a promising tool for the applications mentioned above. In particular, the GEMs can be produced with a rather low optical transparency, down to a few percents. However, under appropriate electric fields arrangement, providing strong electron focusing into the GEM apertures, they can manifest high electron transparency. This feature is very attractive for their 1
use in gaseous photomultipliers as a de-coupling element between the photoconvertor and the main multiplying element. The photocathodes, when fully exposed to the avalanche photons and ions, suffer from sever feedback phenomena and ion-sputtering damage. The possibility of reducing photon- and ioninduced effects with a GEM element is a very important feature of the GEM, studied by us. We summarize here the main results from our study of the GEM as a single-electron detector. We refer the reader to other, more extended publications on this subject [8, 9]. 2. Experimental arrangement The basic experimental arrangement is depicted in figure 1. Single photoelectrons, induced from a semitransparent CsI photocathode by UV photons (pulsed H2 or continuos Ar(Hg) lamps), are focused into the GEM apertures by the electric field E1. This field is normally set at the photoelectron collection plateau (Fig. 1a) but in another mode of operation the photoelectrons are slightly preamplified in the gap between the photocathode and the GEM (Fig. 1b). Following the multiplication in the apertures, the charge is extracted from the GEM by the electric filed E2 and is further multiplied by a multi-wire (MW) electrode. In some cases, the MW was replaced by a second GEM coupled to a stripped readout electrode, or by a second GEM followed by a MW electrode. Currents and signals were recorded from various electrodes of the GEM and the MW. Most of the GEMs under study [10] had apertures arranged in a hexagonal array of 140µm pitch, and aperture diameter 90µm/110µm (Kapton/metal). In some cases a more opaque GEM was used, with a rectangular array of 200µm pitch and 50µm/80µm (Kapton/metal) aperture
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diameter. Pure hydrocarbon gases, CH4, C2H6 and iC4H10 and a mixture Ar/C2H6 (10/90) were used, in the pressure range of 10-400 torr.
Fig. 1. The experimental set-up with one GEM and a MW.
3. Results and discussion Gain: The GEM gain was measured by the current-recording method described in details in references 9 and 10. As there is no possibility of separation between charge multiplication and charge collection into the apertures, the measured current is a product of the two, thus representing an effective gain. Gains of a few hundreds (up to 103) were recorded at gas pressures below 100 torr and of a few thousands at pressures of 200-400 torr, the highest value being 2*104 at 200 torr of C2H6. The data are presented in references 9 and 10. Rise time: Single-electron pulses, recorded on the MW anode following the GEM, have rise-time of 4 ns at 40 i-C4H10, identical to that provided by the MW alone. This gives an upper-limit estimate to the rise-time being 2ns. For more data see ref. 9, 10. Photon feedback screening: As pointed out above, the possibility to prevent avalanche photons from hitting the photocathode and initiating feedback pulses, is one of the important advantages of the GEM. This is demonstrated in figure 2: In Methane, which is a poor photon quencher, and with a CsI photocathode exposed to the MW-avalanche photons, a sever gain limitation is observed at 8*104 (top). With an opaque GEM (effective gain=1) inserted at a 3 mm distance between the CsI and the MW, a six times higher total gain can be achieved, with very little residual photon feedback (bottom). Nevertheless, some photon feedback due to the GEM avalanche itself is observed if the GEM gain is increased to high values. Positive-ions: The electric field on the GEM metal surface, particularly at the aperture rim, is rather high. This is responsible for a new type of positiveion-induced feedback process, visible in any configuration in which a high-gain GEM is followed by another high-gain avalanche element. In this case the ions from the last avalanche are focused back into the GEM aperture, where they are strongly accelerated, and then sputter the GEM top electrode,
Figure 2. Photon feedback with a CsI photocathode in 40 torr Methane. Top: a MW exhibits an intense photon feedback and the gain is limited to 8x104; bottom: a MW+GEM (gain=1) exhibits a reduced photon feedback and a gain increase to 5×106.
inducing feedback pulses due to secondary-electron emission. This is the same effect that limits the gain in Microstrip Gas Chambers (MSGC), and was discussed at length in the literature [11]. The effect is usually hardly seen with GEMs operated with standard gases at atmospheric pressure, but is very pronounced at low gas-pressures or with noble gases (see discussion in ref 12 and 13), where the high reduced-electric field or high operation-voltages are responsible for very energetic positive ions. Figure 3(a) demonstrates the intense ion-induced feedback (total gain 105) created by such positive-ion feedback at 40 torr i-C4H10, with GEM gain 100. Obviously, as described in reference [8], the ion feedback and the total gain limitation are increasing with increased GEM gain, due to the increased acceleration field. A possible solution is to succeed the highgain GEM with a low-gain one, which will act as a buffer between the two high-gain elements. This is demonstrated in figure 3(b), showing feedback-free signals at total gain of 7x106, obtained with GEM1 at gain 300, GEM2 at gain 10 and the MW providing the final amplification (i-C4H10, 40 torr). For more details see [8]. Preamplification in the gap preceding the GEM: Increasing the field E1 (fig. 1(b)) to provide some small preamplification M (M=2-6) to the photoelectrons, prior to their focusing into the GEM apertures, has several consequences. a) It improves the quantum efficiency (QE) of the photocathode by reducing the back-scattering of photoelectrons; this is
particularly important in noble gases and some of their mixtures (see detailed discussion in [14]). b) It
total gain. For this purpose the single-electron pulses from the MW anode were shaped and processed with
a)
b)
Fig. 4. Charge pulses from a GEM+MW, total gain 105 , 40 Torr Ar/C2H6 (10/90). a) No preamplification: multiple ion-induced feedback is evident. b) A preamplification gain of 6: no ioninduced feedback is observed.
Fig. 3. (a) Ion-induced feedback signals are dominating and limiting the total gain of a GEM+MW, for GEM gain=100 and high gain on the MW; (b) Ion-induced feedback is eliminated and a much higher total gain, of 7x106, is obtained in DOUBLE-GEM+MW, for GEM1 gain=300 and GEM2 gain=10. i-C4H10 , 40 torr.
modifies the field-line configuration in this gap, directing part of the positive ions from the GEM back to the photocathodes. This may increase the photocathode damage by ion sputtering but it dramatically reduces positive-ion feedback, as seen in figure 4 (40 torr Ar/C2H6 (10/90), total gain 105). c) It modifies the rise-time of the signal. d) It may have a strong effect on the single-electron detection efficiency due to two competing effects: on one hand the electron statistics is changed and there is a higher probability to detect at least one out of the M electrons, as compared to the original single photoelectron. On the other hand the increase of E1 deteriorates the effectiveness of electron focusing into the GEM apertures, under a given GEM gain. The competition between these two effects depends strongly on the GEM aperture geometry, on the gain and on the type of gas and its pressure (electron diffusion). Single electron detection efficiency: The relative single-electron detection efficiency was studied, in the geometry shown in figure 1, as function of the preamplification field and the GEM gain, by following the counting rate of single-electron pulses under constant photocathode illumination and fixed
a charge amplifier and a multichannel analyzer. The results are shown in figure 5. For the more opaque GEM (see paragraph 2), operated at 40 torr i-C4H10, an increase by about 40% in counting rate was measured when E1 was increased to provide a preamplification by M=6. For the more transparent GEM, operated at 40 torr Ar/C2H6 (10/90), the effect is reversed and the counting rate dropped by about a factor 3 as soon as the preamplification reached a value of M=4. These results, which should be considered as preliminary, probably reflect the twofold consequence discussed above. In the opaque GEM (5% optical transparency), without preamplification, even the best focusing field-line arrangement is insufficient to provide full singleelectron detection efficiency, because electron diffusion deviates them from following the field lines, and a fraction of the electrons misses the GEM apertures. The improved primary statistics, provided by the preamplification, therefore helps to increase the efficiency. For the more transparent GEM (38% optical transparency), with much less metallic surface, in spite of the somewhat higher electron diffusion in this gas, the efficiency is probably at its maximum with low E1. The opposite effect, of reduced focusing, occurs for high E1 values. One should notice that the results reported here refer only to the relative single-electron detection efficiency, for which more details may be found in reference 9. Measurements of the absolute detection efficiency are underway.
4. Summary The GEM operated at low gas-pressures, of 10-400 torr pure or mixed hydrocarbons, was shown to provide fast pulses (
