Printed in: Nucl. Instr. & Meth. A433 (1999) pp 476-481
Single photoelectron detection with a low-pressure gas electron
multiplier coupled to a CsI photocathode
G. Garty*, A. Breskin, R. Chechik, E. Shefer
Weizmann Institute of Science, 76100 Rehovot, Israel
Abstract
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$ciency of the GEM, a crucial parameter in its operation in single-photon detection applications, both
experimentally and by simulation.
1. Introduction
The GEM is a newly developed gaseous electron
multiplier, described in detail in Refs. [1}4]. The
gas electron multiplication, by a factor of up to 10,
takes place within small holes in a dielectric sheet,
under a proper electric "eld across the holes. The
focusing of the primary electrons and the extraction
of the avalanche electrons is controlled by the "elds
E and E on both sides of the GEM (see Fig. 1).
The study of GEM operation at low gas pressure
was proposed for various applications requiring
e$cient detection of single electrons, while avoiding secondary processes [4]. A GEM electrode
inserted between a radiation converter, such as
a photocathode or a gas volume, and a second
ampli"cation element considerably reduces second-
* Corresponding author.
E-mail address: [email protected] (G. Garty)
-1-
ary e!ects, improving the stability and the detection e$ciency of such detectors.
We report here on the study of single-electron
detection e$ciency with a single-GEM coupled to
a multiwire element, operated at low gas pressure.
We also report on the double-GEM structure
coupled to readout strips, operated here for the "rst
time as a single-electron detector.
In our measurements we used two GEM geometries, both manufactured by TECH-ETCH [5] on
a 50 lm thick copper-kapton sandwich:
E A `transparenta GEM geometry, having a hexagonal lattice of double conical apertures, having
an inner diameter of 90 lm and an outer diameter of 110 lm, and a pitch of 140 lm, resulting
in a transparency of 56%.
E An `opaquea GEM geometry, having a square
lattice of apertures, having an inner diameter of
50 lm and an outer diameter of 80 lm, and a pitch
of 200 lm, resulting in a transparency of 5%.
Fig. 1. The experimental setup with one GEM and a multiwire.
(a) Without preampli"cation and (b) with preampli"cation
Both GEMs were operated in a #ow mode with
high-purity hydrocarbons (40}400 Torr of CH ,
C H , i-C H and Ar/C H (10/90)).
Fig. 1 shows the basic experimental setup. A CsI
photocathode, irradiated by an Ar(Hg) UV lamp
through a quartz window, induces single photoelectrons. These electrons are focussed into the
GEM by an electric "eld E , multiplied in the GEM
apertures and transferred via the "eld E to a
multiwire (MW) element.
For the double-GEM study we used a cascade of
two `transparenta GEMs coupled to a printed
board strip-readout electrode, located 0.5 mm from
the second GEM.
The electric "eld con"gurations in the GEM, and
the electron transport properties, were simulated
using MAXWELL's 3D parameter extractor [6]
and GARFIELD [7], respectively.
2. Results and discussion
Fig. 2 shows the gain measured on a single
`transparenta GEM at various pressures of ethane
and methane, under intense UV illumination. It
was obtained by measuring the current collected on
the GEM's lower face with reversed E (see Fig. 1),
and normalizing it to the current i impinging on
the GEM. The latter was obtained by measuring
the current on the GEM's upper face with both
faces at ground potential, and correcting it for the
electron collection e$ciency, with the help of our
Monte Carlo simulations. In the present condi-2-
Fig. 2. Gain curves as function of the potential across the GEM,
for di!erent gases and pressures. The graphs result from currents
recorded on the GEM faces, using a `transparenta GEM, with
electron losses taken into account (see text).
tions, this ine$ciency amounts to 20%, most of it
due to electrons lost, due to di!usion, to the kapton
inside the GEM holes. The "eld E was maintained
at the collection plateau in all measurements. Gains
of up to 20 000 were measured in 200 Torr ethane
before instabilities arose. In pure methane
(400 Torr), the maximal stable gain was &1000,
due to photon feedback. At low-pressure (40 Torr)
gains of a few hundreds to a few thousands were
observed, in di!erent gasses. The measured gain
curves have no collection plateau, pointing at the
poor electron focussing and transmission at low
GEM "elds. The simulations show (Table 1) that
due to the high electron di!usion at low gas pressures, a signi"cant fraction of the electrons are lost
in the holes and on the GEM surfaces, when the
"eld across the GEM is small. At GEM gains above
100 the "eld strength is su$cient to e$ciently focus
the electrons into the holes. Nevertheless, some
electron losses still exist due to the di!usion of
electrons onto the GEM surfaces.
The GEM e$ciency is crucial for applications
where we have to detect only a single or a few
electrons. One way to improve the chances of detecting a single primary electron, is to apply a small
preampli"cation in the gas volume preceding the
GEM. Even a small gain is su$cient to recover
possible losses of primary photoelectrons; due to
electron di!usion in the gas, at least one of the
avalanche-generated secondary electrons will reach
Table 1
Simulation of the electron transport of the `transparenta GEM at 40 Torr Ar/C H (10/90): percentage of electrons collected on di!erent
electrodes at di!erent "eld con"gurations. The `detection e$ciencya corresponds to the total fraction of electrons producing an
avalanche in the GEM and/or MW
PP
gain
1
6
E
V/cm Torr
7.5
7.5
80
80
E
V/cm Torr
GEM
gain
Photocathode
GEM
up
Kapton
GEM
down
MW
`Detection
e7ciencya
33
33
1
500
10%
20%
71%
12%
10%
4%
5%
29%
4%
35%
4%
64%
33
33
1
500
0%
0%
7%
0%
43%
3%
6%
3%
43%
94%
43%
97%
a GEM hole. Even though the electrostatic focussing into the GEM is inferior at high electric "eld,
E , we expect a rise in the detection e$ciency with
increasing preampli"cation up to the point where
the preampli"cation is just compensated by the
defocusing losses.
We measured the relative single-electron detection e$ciency by counting pulses from the
GEM#MW structure, under constant UV illumination. Contrary to measurements based on current distribution on the GEM electrodes [4], where
the gain and transfer e$ciency are inseparable,
measuring single-electron pulses provides a reliable
account of the detection e$ciency at di!erent "eld
con"gurations of the GEM and its surrounding
electrodes. While varying the respective gains on
the preampli"cation gap, GEM and MW, the total
gain of the detector was maintained constant at
10. This eliminated the dependence on the electronics threshold and reduced secondary e!ects. At
this gain we have already observed a saturated
single-electron pulse height spectrum (see Fig. 3a).
As we have no account of the absolute number of
photoelectrons produced, we can only measure the
behavior of the relative detection e$ciency of di!erent con"gurations. An absolute measurement can
be performed using the electron counting technique
[8], namely counting electron swarms of known
size, induced by ultrasoft X-rays in a gas volume
coupled to a GEM.
Fig. 4 shows the results of measurements performed on the two GEM geometries: The `opaquea
GEM at 40 Torr isobutane (Fig. 4a), and the
`transparenta GEM at 40 Torr Ar/C H (10/90)
(Fig. 4b). In the `opaquea GEM, a preampli"cation
Fig. 3. Single electron pulse height spectra: (a) saturated spectrum obtained from a GEM # MW structure at 40 Torr of
i-C H , using the `opaquea GEM. (b) Exponential spectrum
from the double GEM# readout pads in 400 Torr CH , using
two `transparenta GEMs.
-3-
Fig. 4. Relative detection e$ciency of `opaquea (a) and `transparenta (b) GEM: (a) an increase of &40% in counting rate
occurs upon preampli"cation by ;6 preceding the GEM; (b) a
decrease of a factor of &2 occurs upon preampli"cation ;6;
a gain limitation due to ion feedback is seen without preampli"cation at a GEM gain of &200.
of 6 raises the relative electron detection e$ciency
by &40%. Indeed, our measurements have shown
that the relative detection e$ciency does not significantly increase above a preampli"cation gain of 2.
In the `transparenta GEM, on the other hand, the
situation is reversed, and the detection e$ciency
drops drastically, by a factor of 2}3, when adding
a preampli"cation.
Since no intuitive explanation has been found for
this di!erence we have run simulations of the electron transport in the `transparenta GEM, at
40 Torr of Ar/C H (10/90). The radiation conver sion and electron collection gaps were both 1 mm
wide. The simulation does not include avalanche
formation, but only electron transport and di!usion. We assume that under high GEM gain, any
electrons transported into the apertures will pro-
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duce a detectable avalanche. Under GEM gain
1 only electrons reaching the MW will produce
a detectable signal.
The results of the simulation are shown in Table
1, where for each electric "eld con"guration we
provide the fraction of electrons collected in each
region of the detector. The last column provides the
`detection e$ciencya which equals to the total fraction of electrons producing an avalanche in the
GEM and/or the MW. In the case of no preampli"cation, despite the excellent focussing of electric
"eld lines, which was seen from the simulations, the
single-electron transport is not good. Many electrons reaching the GEM arrive, through di!usion,
onto the top GEM face where they are lost. With
increased electric "eld in the GEM, the focussing
"eld near the GEM face increases and these electrons are attracted into the apertures. In the case of
high GEM gain, we are able to detect electrons
reaching the lower face of the GEM, as they, nevertheless, induce an avalanche in the GEM (not included in the simulation), part of which will reach
the MW.
When we add preampli"cation, the focussing is
seen to deteriorate, however, this is more than
compensated for by the avalanche statistics. In the
simulation of a preampli"ed electron, we tracked
six electrons from a distance of 1mm above the
GEM and recorded only the one which got farthest
down. As only one electron, from the preampli"cation avalanche, reaching the MW is su$cient for
detection of the original photoelectron, we are able
to overcome the intrinsic ine$ciency of the GEM
focussing. The addition of a ;6 preampli"cation,
in this case, is seen to lead to an increase of the
`detection e$ciencya by &50%, consistent with
what we measured for the opaque GEM. This is,
however, in disagreement with our measurements
performed for the `transparenta GEM, and should
be further investigated.
A signi"cant gain limiting e!ect was observed in
the GEM operated at low gas pressure [4], due to
ions accelerated within the GEM apertures and
sputtering its upper face, under high electric "eld.
The resulting secondary electrons are then focussed
into the GEM apertures, inducing secondary
avalanches, at a typical delay on the order of
microseconds after the primary avalanche. The
Fig. 5. Charge pulses from a GEM#MW structure at a total
gain of 10 in 40 Torr Ar/C H (10/90). (a) No preampli"cation:
multiple ion feedback is evident. (b) A preampli"cation gain of
6: no ion feedback observed.
Fig. 6. Pulses from the double GEM structure coupled to
printed board readout strips, operated in 400 Torr CH . (a) A
preampli"cation gain of 100 followed by a gain of 100 on each
GEM, with a low (collection) "eld above the readout strips.
(b) A preampli"cation gain of 10 followed by a gain of 500 on
the "rst GEM, a gain of 20 on the second GEM and a gain of
2 above the readout strips, yielding faster and larger pulses.
Pulses recorded using a fast (t (0.5 ns) current ampli"er,
Z "50 ), gain"200.
preampli"cation preceding the GEM, by modifying
the "eld line maps, was seen to greatly decrease
this e!ect by diverting the ions to the photocathode. Fig. 5 depicts charge pulses obtained
from the GEM#MWPC structure at 40 Torr of
Ar/C H (10/90) and a total gain of 10 (GEM gain
200). In (a) multiple ion feedback events are evident, reducing the stability of the detector. When
we add a preampli"cation "eld corresponding
to a gain of 6, (b) the ion feedback is totally
eliminated.
In order to take advantage of the GEM properties, and since a single GEM does not provide
su$cient gain for single-electron detection, we attempted to detect single photoelectrons using
a double-GEM followed by a readout strip electrode, as demonstrated previously by Sauli [3]. We
have operated this structure without or with small
additional multiplication between the last GEM
and the strips. In Fig. 6 we see single-electron pulses
obtained from a double-GEM coupled to the strip
readout electrode, at 400 Torr of methane. Fig. 6a
shows pulses recorded when the strips are operated
as a collection electrode; the pulses are rather slow
and of small amplitude. When adding even a small
ampli"cation across the GEM-strip gap (gain of 2,
shown in (b)), the signals are faster and of higher
amplitude. A typical pulse height spectrum of this
structure is seen in Fig. 3b, with a gain of 100 on
both GEMs and a gain of 2}3 on the readout trips.
3. Conclusions
We have studied single- and double-GEM detector structures, coupled to a CsI photocathode, at
low gas pressures, up to 400 Torr of hydrocarbons.
Stable gains of up to 10 were reached with single
photoelectrons, in a single GEM. We have observed saturated pulse height spectra with the
GEM followed by a multiwire chamber.
We have shown that a double-GEM followed by
a readout strip electrode provides su$cient gain for
the detection of single electrons.
-5-
The accent in this work was put on understanding the single-electron transport from the photocathode into the GEM and their detection
e$ciency. This is of crucial importance for applications in imaging or counting of single photons or
electrons. We have measured the relative singleelectron detection e$ciency for two types of
GEMs, as a function of its gain and of the optional
additional preampli"cation. From our simulations
we expect that the addition of a small preampli"cation gain will dramatically improve the single-electron detection e$ciency, as the di!usion of the
preampli"ed avalanche should compensate for the
electron losses. In reality this was con"rmed only in
the `opaquea GEM where electron focussing is
expected to be, apriori, poor. With the `transparenta GEM, where we expect the initial e$ciency to
be higher, the single-photon detection e$ciency
was seen to drop drastically with the addition of
preampli"cation. This drastic drop is not yet
understood and is still under investigation.
It was observed that preampli"cation in the
gap preceding the GEM, reduces gain limitations
due to ion feedback e!ects, but on the account
of a larger probability of damaging the photocathode.
In the future, we plan to study the absolute
single-electron detection e$ciency, using the electron counting method. In addition, we are studying
other detector/photocathode arrangements, as for
-6-
example depositing the photocathode directly on
the top face of the GEM.
Acknowledgements
We wish to thank F. Sauli for helpful discussions
and R. Veenhof and S. Iranzo for their extended
help with the simulations. The work was partially
supported by the Israel Science Foundation. The
GEM electrodes were generously supplied by
TECH ETCH Inc., for which we are very grateful.
A. Breskin is the W.P. Reuther professor of research in the peaceful use of atomic energy.
References
[1] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531.
[2] F. Sauli, Nucl. Instr. and Meth. A 433 (1999) 464 and
references therein.
[3] A. Bressan, R. De Oliveira, A. Gandi, J.C. Labbe,
L. Ropelewski, F. Sauli, D. Mormann, T. Muller, H.J. Simonis, Nucl. Instr. and Meth. A 425 (1999) 254.
[4] R. Chechik, A. Breskin, G. Garty, J. Mattout, F. Sauli, E.
Shefer, Nucl. Instr. and Meth. A 419 (1998) 423 and references therein.
[5] TECH-ETCH Inc. http://www.tech-etch.com/.
[6] Ansoft Corporation, http://www.ansoft.com.
[7] R. Veenhof, CERN, Gar"eld, http://consult.cern.ch/
writeup/gar"eld/.
[8] A. Pansky, A. Breskin, R. Chechik, G. Garty, E. Klein, Nucl.
Instr. and Meth. A 392 (1997) 465}470.