Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 48 (2015)

Property of LCP-GEM in pure dimethyl ether at low pressures†

†∗1
∗2
∗3
∗4Condensed from the article in JINST 9 C01002 (2014)
RIKEN Nishina Center
Department of Physics, Tokyo University of Science
NASA/GSFC
Department of Mechanical Engineering, Kogakuin University

Ratio(this experiment / Magboltz)

We developed a photoelectric X-ray polarimeter
onboard X-ray advanced concepts testbed (XACT)
sounding rocket, the ﬁrst dedicated NASA rocket mission for high sensitivity observation of cosmic X-raypolarization.1,2) The polarimeter uses a time projection chamber technique to obtain the distribution ofphotoelectron emissions from which the polarizationof the incident X-rays can be measured. A key deviceof the polarimeter is a 100-µm-thick gas electron multiplier (GEM) foil with a copper-clad liquid crystal polymer insulator (LCP-GEM),3,4) which ampliﬁes the signal that keeps the track image of photoelectrons. The
required gain of the LCP-GEM for the polarimeter is
3,000 without discharge. Fine photoelectron track images are essential for a highly sensitive measurement of
X-ray polarization. By extending the track length using a low-pressure gas, high-resolution photoelectron
track images can be obtained. On the other hand,
the low-pressure gas decreases the X-ray detection efﬁciency of the polarimeter. We anticipate that theoptimum gas pressure of pure dimethyl ether (DME)
is 50−150 Torr by considering the trade-oﬀ between
the detected count rate and the modulation factor.1)
However, LCP-GEMs have never been operated below
190 Torr in DME gas. Under such low gas pressures,
discharge is one of the most signiﬁcant risks to the successful operation of GEMs. To explore this unknown
regime, we performed a systematic investigation of thegain properties of a 100-µm-thick LCP-GEM in DME
at low pressures for the ﬁrst time.
We developed a prototype gas-chamber detector,
which possesses the same geometry as the ﬂight modelof XACT. By irradiating the chamber with 6.4 keV
X-rays, we obtained the spectrum from the readout
pad and ﬁtted it with a Gaussian model. An energy
resolution of approximately 20% at the FWHM wasachieved. The highest gain under stable operation at190 Torr was 2 × 104 at dVGEM = 560 V, while thatat 20 Torr was approximately 300 at dVGEM = 470 V.
Above 50 Torr, the highest gain exceeded 3000, whichmeets our requirements for the XACT polarimeter.
The gain curves could be reproduced by exponentialfunctions; however, a change of slope with pressure
was observed. In addition, we determined the real
GEM gain derived from the sum of charge amounts
induced in the readout pad and GEM anode using theα / P [ /cm/Torr]

Y. Takeuchi,∗1,∗2 T. Tamagawa,∗1,∗2 T. Kitaguchi,∗1 S. Yamada,∗1 W. Iwakiri,∗1 F. Asami,∗1,∗2A.Yoshikawa,∗1,∗2 K. Kaneko,∗1,∗2 T. Enoto,∗1,∗3 A. Hayato,∗1 T.Kohmura,∗4 and the GEMS/XACT team

10Magboltz
10 Torr
190110 Torr
0 Torr7050 Torr
0 Torr4030 Torr
20 Torrline fitted
itte byb exponencial
exponencialfunction
nction

11.2
10.8
0.6
0500

1000

1500

EGEM / P [V/cm/Torr]

2000

2500

3000

Fig. 1. Top: Comparison between the measured α and theα simulated by Magboltz7) with 1σ errorbars.Bottom:
Ratio of α obtained in this experiment to that obtainedwith Magboltz calculation.

same data set. The real gain represents the ampliﬁcation degree of electron drifting in the GEM hole,while the eﬀective gain is the real gain multiplied by
the ampliﬁed electron collection eﬃciency of the readout pad.5) The real gain is approximately twice theeﬀective one because the charge amount of the GEM
anode is almost the same as that of the readout pad.6)
The highest gain at 190 Torr is 4 × 104 , while thatat 20 Torr is around 600. These real gain curves alsoshow deviation from an exponential function.
To comprehensively characterize the gain variations
with diﬀerent gas pressures and GEM voltages, we derived the ﬁrst Townsend coeﬃcient, α. Fig. 1 shows
the observed α as a function of EGEM /P superposedwith the results of Magboltz calculations.7) EGEM is
the electric ﬁeld applied to the GEM. The data points
can be roughly reproduced by an exponential function,although they show deviation from it in the higher
EGEM /P range. This is because the DME ions gain
suﬃcient kinetic energy from such a strong electric ﬁeld
to ionize the DME gas and emit additional electrons
that are ampliﬁed in the GEM hole. The measured αvalues were approximately 80% of ones derived fromMagboltz calculations, which suggests that the length
over which the number of electrons were ampliﬁed in
the GEM hole was diﬀerent from the GEM thickness
but close to 80 µm.
References
1) K. Gendreau et al.: Proc. SPIE, 8443 84434V-1 (2012)2) J.K. Black et al.: NIM, A581 755 (2007)
3) T. Tamagawa et al.: NIM, A560 418 (2006)
4) T. Tamagawa et al.: NIM, A608 390 (2009)5) J. Benlloch et al.: NIM, A419 410 (1998)
6) Y. Takeuchi et al.: JINST, 7 C03042 (2012)7) S.F. Biagi: NIM, A421 234 (1999)- 179 -

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