Nuclear Instruments and Methods in Physics Research A 462 (2001) 603–606

Letter to the Editor

Detection of single electrons emitted by internal photocathodeswith the Gas Electron Multiplier (GEM)

Archana Sharmaa,b,*aPE Division, CERN, CH1211, Geneva 23, Switzerland

bGSI Darmstadt, Germany

Received 3 December 1999; accepted 25 September 2000

Abstract

Detection of single photoelectrons from internal photocathodes using a Gas Electron Multiplier is investigated withthe help of a simulation. Results indicate that full efficiency may be obtained with a two-step amplificationprocess. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Gem; Single electron; Simulation; Photoelectron; Gaseous detector

Detecting single photoelectrons emitted from aninternal photocathode in gaseous counters withgood efficiency has been the subject of intensestudy in recent years [1–3]. With the Gas ElectronMultiplier (GEM) [4,5] high gains (�700) havebeen measured in noble gases [6,7], opening upthe possibility of making a gas photomultiplierin combination with a solid photocathode. Furtherenhancement of gain may be obtained byusing several GEMs in cascade [8]. Use of noblegases has the advantage that they can be obtainedwith high purity, therefore are photocathodefriendly.To detect single electrons with a good efficiency,

it is imperative that transparency of the GEM ishigh, implying a low drift field [9]. On the otherhand, suppression of photon feedback arising

from large avalanches in GEM channels requiresthat the optical transparency of the GEM be small.Combining high diffusion of pure argon, and adrift field just large enough to start multiplication,full efficiency may be obtained by a two-stepmultiplication as demonstrated below.Fig. 1 shows the principle of operation: elec-

trons released from the cathode are amplified in aparallel plate mode in region 1, a fraction of theelectrons is then collected into and multiplied inthe GEM channel shown as region 2. All or part ofthe electrons then continue into the transferregion, where they can be further amplified withanother GEM or detected. Properties of thisstructure have been studied with a 3D modelmade using the MAXWELL [10] simulator. Themodel comprises a GEM with staggered rows ofholes with a diameter of 70 mm at the metal edge,and a pitch of 140 mm. A drift cathode was placedat a distance of 1mm, and a collection electrode ata distance of 250 mm from the bottom surface of

*Corresponding author. Tel.: +41-22-767-3958; fax: +41-

22-767-9330.

E-mail address: archana.sharma@cern.ch (A. Sharma).

0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 3 1 0 - 3

the GEM. Only a mirror symmetric basic volumewas modeled, and the electric fields with differentvoltage settings were imported into Garfield [11].As discussed above, pure argon was taken as theoperational gas and electrons were generated onthe cathode surface.

The region surrounding the points equidistantfrom the centers of three adjacent holes is theworst from where loss of electrons takes place athigh drift fields. Working at values of drift fieldjust high enough for multiplication (12–16 kV/cm),and small voltages across GEM (300–350V) onefinds that the avalanche spread by collisionaldiffusion in argon is sufficient to have, in mostcases, electrons generated in an avalanche startingfacing this worst point, enter a neighboringhole. These issues have been investigated in thefollowing.We start by generating avalanches from elec-

trons distributed along a line on the cathodecorresponding to that drawn from the center of ahole to the center of the triangle between threeadjacent holes. With an overall gain of the order offew tens, in the total system (drift+GEM), there isa nonzero average number, hNi, of electronstransmitted through the GEM in the whole rangeof the co-ordinate, see Fig. 2, line 1. Whendiffusion is switched off in the program, one getsthe expected low values for transparency, seecurve 2, and large variation in hNi. Curve 3shows the average number of electrons for a highergain of several tens. One can see that the numberof collected electrons increases slightly at the edge,due to the higher fields, and then this numberFig. 1. Principle of operation of a GEM.

Fig. 2. The average gain as a function of the radial co-ordinate from the center of a hole.

A. Sharma / Nuclear Instruments and Methods in Physics Research A 462 (2001) 603–606604

drops since field lines end up on the metal surface.Moving further along this line, the slight increaseis due to the fact that more electrons findtheir way through adjacent holes. When gain is

high, these variations are compensated by theavalanche statistics and diffusion. Nevertheless,one needs to increase the overall electric field inthe system, raising the probability of photonfeedback. Therefore, a compromise has to befound between gain variation and total efficiencyfor single-electron transfer, this is obviously anexperimental issue.To investigate efficiency, 1000 electrons were

generated randomly on the cathode, and thepath of each electron created in the avalancheswas followed: to the top Cu surface, kapton,bottom surface or if they were transferred throughthe GEM hole. Fig. 3 shows the distributionof the number of electrons reaching the lowercollection electrode, also known as visible oreffective gain. The inset shows the channel 0count; the inefficiency in this case being 1.7%.Fig. 4 shows the efficiency as a function of hNi,computed only for electrons starting from theworst point in the drift volume. It is seen that for

Fig. 3. Distribution of number of electrons, N transferred through the channel.

Fig. 4. Efficiency as a function of hNi.

A. Sharma / Nuclear Instruments and Methods in Physics Research A 462 (2001) 603–606 605

low visible gains, the efficiency is approximately70–80%, while once hNi reaches around 50, theefficiency is 98%, and then does not vary withgain. Therefore, given a minimum effective gain,an almost fully efficient photodetection is pre-dicted using a noble gas.

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A. Sharma / Nuclear Instruments and Methods in Physics Research A 462 (2001) 603–606606