Abstract We have conducted the first study of the aging behavior of a MICROMEGAS detector with GEM preamplification. Because a MICROMEGAS is constructed with minimal insulation and because the electric field is parallel to the amplification gap, superior radiation hardness was previously reported in an Ar-Isobutane gas mixture in a stand-alone device. The MICROMEGAS is expected to tolerate an even larger accumulated charge in a cleaner gas mixture, such as Ar-CO 2 . However, using a MICROMEGAS as a stand-alone device in this mixture could increase discharge probability and lead to faster degradation in detector performance. We show that this problem can be circumvented by employing GEM preamplification. Although it has been previously shown that a GEM ages when operated at a large gain, sharing the gain between a MICROMEGAS and the GEM creates a high-gain, very radiation-hard gas detector. We find that a MICROMEGAS + GEM combination exhibits no deterioration in performance after a total charge accumulation of 23 mC/mm 2 

I Shipsey

J May

J Miyamoto

S Kane

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  1
  An aging study of a MICROMEGAS with GEM                  
preamplification 
S. Kane, J. May, J. Miyamoto*, I. Shipsey 
Dept of Physics, Purdue University, W. Lafayette, Indiana, USA 
 
Elsevier use only: Received date here; revised date here; accepted date here 
Abstract 
We have conducted the first study of the aging behavior of a MICROMEGAS detector with GEM preamplification. Because a 
MICROMEGAS is constructed with minimal insulation and because the electric field is parallel to the amplification gap, 
superior radiation hardness was previously reported in an Ar-Isobutane gas mixture in a stand-alone device. The 
MICROMEGAS is expected to tolerate an even larger accumulated charge in a cleaner gas mixture, such as Ar-CO2. 
However, using a MICROMEGAS as a stand-alone device in this mixture could increase discharge probability and lead to 
faster degradation in detector performance. We show that this problem can be circumvented by employing GEM 
preamplification.  Although it has been previously shown that a GEM ages when operated at a large gain, sharing the gain 
between a MICROMEGAS and the GEM creates a high-gain, very radiation-hard gas detector. We find that a 
MICROMEGAS + GEM combination exhibits no deterioration in performance after a total charge accumulation of 23 
mC/mm2. 
  © 2001 Elsevier Science. All rights reserved 
Keywords: Type your keywords here, separated by semicolons ;  MICROMEGAS; GEM; aging
1. Introduction 
A MICROMEGAS detector is a robust 
micropatterned gaseous detector with minimal 
insulator between the anode and the cathode [1]. The 
detector is being used by the COMPASS experiment 
at CERN [2].  In a MICROMEGAS, a thin metallic 
mesh is placed above the anode plane. Small 
insulating spacers create a narrow gap (100 µm) 
between the mesh and the anode plane. The gap 
defines the amplification region of the detector.  The 
electric field lines in the amplification gap are similar 
to a parallel-plate gas chamber. However, because the 
MICROMEGAS gap is small, it does not require 
large bias to achieve reasonable gas gain, whereas in 
a parallel-plate gas chamber, the bias voltage required 
is prohibitively high because the amplification gap 
used is typically a few millimeters.   
  2
The MICROMEGAS in this aging study employs 
a new micromesh which was fabricated by Kapton-
based photolithography [3]. The final micromesh 
resembles a one-sided GEM but with most insulating 
Kapton etched away, except for small 80 µm 
diameter pillar supports. Fig. 1 shows a photograph 
of a support pillar. The pillars are located every 1 
mm, and the dead area of the detector is therefore 
insignificant [4]. As very little insulating material is 
present in the detector, there is minimal charge build-
up on the mesh. This is one of the reasons for the 
MICROMEGAS rate capability, which can reach as 
high as 107 Hz/mm2 [4]. 
 
 
 
Fig. 1 A photograph of one of the supporting pillars. Its diameter 
is about 80 µm. The electrons go through the many small holes to 
enter the amplification region.  
2. MICROMEGAS with GEM preamplification 
The GEM (gas electron multiplier) is a Kapton-
based micropatterned device that amplifies electrons 
and has high electron transparency so that amplified 
charge is transferred efficiently to a subsequent 
device [5] for further amplification or charge 
collection. We previously performed aging studies of 
both double GEM and triple GEM detectors in Ar-
CO2 gas [6-7]. The studies were performed in an 
ultra-clean environment where the gas quality was 
monitored by gas chromatography. No significant 
aging effect was observed in the triple GEM study, 
while there was a sign of moderate aging in the 
double GEM study. The aging of the double GEM 
was confirmed by optical inspection, which revealed 
dark deposits in the irradiated detector very near the 
GEM holes. This phenomenon appears analogous to 
the dark deposits seen on the thin anode strips of the 
MSGC (Microstrip Gas Chamber) after sustained 
irradiation [8]. In an MSGC, many electric field lines 
are concentrated and terminate on the anode. In a 
GEM, the lower electrode collects some electrons 
while the majority of electrons leave the GEM. The 
termination of field lines on the lower GEM 
electrode, combined with the electric field strength 
and the charge density in the vicinity of the lower 
electrode, presumably creates the conditions 
necessary for polymerization. In the triple GEM, it is 
possible to operate with lower amplification from 
each GEM while maintaining the same overall gas 
gain of a double GEM. For this reason, no aging was 
observed in a triple GEM. It is important to note that 
in both the double GEM and triple GEM aging 
experiments, the charge-collecting anodes did not 
exhibit aging. This is presumably due to a lower 
electric field strength and lower charge density at the 
anodes compared to the low electrode of the second 
GEM in the double GEM study. 
 A prior study suggested that the gain and aging 
behavior of a GEM and a MICROMEGAS may 
differ. In a MICROMEGAS, there is no analog of the 
lower electrode of the GEM, i.e. no structure in the 
mesh that intercepts avalanche electrons. Thus the 
gas gain achievable for the same gap and the same 
voltage is higher in a MICROMEGAS. This 
structural difference may make a MICROMEGAS 
less susceptible to aging than a GEM: in the former, 
the avalanche electrons are collected only by the 
anode strips or pixels, and these are analogous to the 
charge collection electrodes in the double and triple 
GEM aging studies, which did not exhibit aging in 
our previous experiments.  
One effect of GEM preamplifcation on 
MICROMEGAS performance is a reduction of the 
spark rate in the combined device compared to a 
standalone MICROMEGAS. This has been studied in 
[9].  In this work, we examine the radiation hardness 
of the MICROMEGAS+GEM.   
2.1. Detector structure 
The GEM foil has an area of 10 cm x 10 cm with 
140 µm pitch holes in 50 µm thick Kapton. The GEM 
is placed above the micromesh of the 
MICROMEGAS. The detector structure is drawn in 
Fig 2.  
 
  3
 
 
 
 
 
 
 
  
 
 
 
 
Fig 2. MICROMEGAS+GEM detector.  
Primary electrons created in the drift region 
experience amplification twice. Some electrons may 
be created below the GEM; these electrons 
experience amplification only once.  Our previous 
study showed that roughly the same gain on both 
devices gives the maximum rate capability of the 
whole detector [9].  If the total gas gain is shared 
roughly equally by two or more devices, the electron 
diffusion effect relaxes discharge conditions by 
avoiding the Raether condition, leading to stable 
operation. 
3. Aging test setup in Ar+CO2 gas mixture 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3  Aging test setup. The main detector was irradiated at a high 
rate, but the monitor wire chamber was irradiated at low rates. 
A schematic drawing of the aging test setup is 
shown in Fig. 3. The aging apparatus has been 
described in detail in [8]. An X-ray generator is the 
source of photons: one beam irradiates the detector 
under test while a second beam from the same target 
irradiates a single-wire proportional counter, which is 
used to monitor the effects of pressure and 
temperature on the gas gain. The monitor counter is 
also sensitive to any change in the intensity of the X-
ray generator beam, and because it shares the same 
gas system with the detector under test, it is also 
sensitive to any change in the gas mixture’s 
composition. The total charge accumulated by the 
MICROMEGAS+GEM is calculated by integrating 
the signal current over 600 hours. The integrated 
current was measured on the grounded anode strips. 
The anode current reflects the total number of 
electrons collected by the anodes.  Gas gain was 
about 3,000, and the irradiation rate was about 17 
nA/mm2.  Pulse height spectra were taken every hour 
on both the MICROMEGAS+GEM chamber (at the 
micromesh) and the monitor wire chamber. The 
micromesh was biased through an Ortec 142PC  
charge-sensitive preamplifier. The preamplifier has a 
rise time of a few hundred nanoseconds; a large part 
of the signal is therefore formed by slowly moving 
ions traveling in the amplificaiton gap.  However, the 
distance the ions have to cover to induce signals is 
only 50 to 100 µm, and the slowly moving ions are 
quickly removed, minimizing the space charge effect.  
The GEM was biased through a resistor chain. At the 
end of the chain, an ammeter was connected to 
measure any changes in the current flowing in the 
chain and GEM. Both temperature and pressure in the 
room were measured as well.   
4.  Aging result 
Initially, the MICROMEGAS and the GEM were 
biased at Vmesh= 380 V and Vgem= 400 V, but during 
the initial stage of irradiation, we observed a sharp 
gain drop as shown in Fig 4. This was probably 
because the micromesh was adjusting to high ion 
current.  Due to its rapid onset, this gain drop clearly 
Anode Plane
Micromesh and pillars
GEM foilDrift plane
3 mm 
3 mm 50 µm gap 
Low rate irradiation 
Cr X-ray gun 
MICROMEGAS+
GEM chamber 
Gas 
Monitor wire 
chamber  
High rate irradiation
Gas sampling for purity test 
  4
has nothing to do with traditional aging. This fact 
was confirmed by the optical inspection of both 
devices after the experiment.  After about 5 mC/mm2 
of charge accumulation, the bias voltages of the 
MICROMEGAS and GEM were increased by 5 V 
and 10 V, respectively, and the radiation current was 
restored to the original level. The gain drop was not 
observed in the monitor wire chamber because it was 
irradiated at a low rate.  In Fig. 4, after the gain was 
restored, both the MICROMEGAS+GEM chamber 
and the monitor wire chamber exhibited highly 
correlated fluctuations in gain. These changes were 
due to variations in the atmospheric pressure in the 
laboratory, with which they are inversely correlated.  
 
 
 
 
 
 
 
 
 
 
 
 
Fig 4. Pulse heights of MICROMEGAS+GEM and the monitor 
wire chamber.  The changes in the pressure in the laboratory are 
also shown. 
 
 
 
 
 
 
 
 
 
 
 
Fig 5. Signal current, pulse height of the MICROMEGAS+GEM, 
and the monitor wire chamber as a function of accumulated charge. 
 
The signal current induced by the irradiation 
measured on the anode (electron current) responded 
to the pressure changes qualitatively in the same way 
as the pulse height. However, Fig. 5 shows that the 
drop in current is much smaller than the drop in pulse 
height. Moreover, after the additional voltage was 
added to the MICROMEGAS and the GEM, the 
electron current exceeded its original value whereas 
the pulse height did not return to the original value.  
This large discrepancy can be attributed to the 
functioning of the preamplifier, which, when 
connected to the micromesh (cathode), records the 
summed effect of charge movements in different 
locations.  For example, the ions exiting the mesh and 
traveling to the drift region have some cancellation 
effect with respect to the ions arriving on the mesh 
from the avalanche region of the MICROMEGAS. 
Thus, even if the total gain increases, the pulse height 
recorded by the preamplifier does not grow as much 
as the number of avalanche electrons. The electron 
current measured on the anode is insensitive to this 
effect, and all electrons in the avalanche are recorded 
regardless of their origins. 
Optical inspection at the end of the aging test 
revealed no visible deposit on either the 
MICROMEGAS or GEM.  The amount of charge 
accumulated was similar to that in our previous 
double GEM and triple GEM studies. The 
MICROMEGAS+GEM outperformed double GEMs 
and had comparable radiation hardness to a triple 
GEM, but with one less preamplification device in 
the chamber. 
5. Summary 
The first study of aging in a MICROMEGAS 
+GEM chamber has been performed. An early sharp 
gain drop was observed, and the voltage on both the 
MICROMEGAS and GEM had to be raised to restore 
the initial gain. This phenomenon does not appear to 
affect overall detector performance as long as 
sufficient voltage is applied on the mesh. The 
MICROMEGAS+GEM accumulated a charge of 23 
mC/mm2 without loss of performance. The aging 
result presented here demonstrates that the 
combination MICROMEGAS+GEM has aging 
  5
performance superior to that of a double GEM; its 
performance is comparable to that of a triple GEM, 
but with one less preamplification device; the 
MICROMEGAS+GEM has a simpler construction 
and uses less material than the triple GEM. 
Acknowledgement 
The GEM foils were kindly provided by F. Sauli 
of CERN, and the MICROMEGAS was provided by 
I. Giomataris of CEA, Saclay. 
References 
[1] I. Giomataris, et al., Nucl. Instr. and Meth. A 376 (1996) 29. 
[2] D. Thers, et al., Nucl. Instr. and Meth. A 469 (2001)133. 
[3] A. Delbart, et al., Nucl. Instr. and Meth. A 461 (2001) 84. 
[4] G. Charpak, et al., Vienna Conference on Instrumentation, 
Vienna, Feb. 2001. 
[5] F. Sauli, et al., Nucl. Instr. and Meth. A 386 (1997) 531.  
[6] S. Kane, et al., IEEE Nuclear Science Symposium, Lyon, 
Oct. 2000. 
[7] S. Kane, et al., Vienna Conference on Instrumentation, 
Vienna, Feb. 2001. 
[8] E. Gerndt, et al.,  Nucl. Instr and Meth. A 422 (1999) 282. 
[9] S. Kane, et al., International Conference on advanced 
technology and particle physics, Como, Oct. 2000. 
[10] S. Kane, et al., IEEE Nuclear Science Symposium, San 
Diego, Nov. 2001. 
 


An aging study of a MICROMEGAS with GEM preamplification

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An aging study of a MICROMEGAS with GEM preamplification

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