Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 47 (2014)

Background estimation and operation test of the GEM detectors forthe J-PARC E16 experiment
D. Kawama∗1 for the J-PARC E16 collaboration

Iron “gate”
LG

IronpThickness=1
ConcreteFig. 1. Schematic model of the E16 experiment used in thesimulation. The area is surrounded by concrete blocks,
and is filled by air.

The model used in the E16 background simulation∗1RIKEN Nishina Center- 222 -is shown in Fig. 1. Using this model, we estimated theparticle rate at the detector position, which is locatedat approximately 120 cm from the target surroundedby the lead-glass calorimeters. Fig. 2 shows the energy spectra from the beam dump and the target, andreveals that the main contribution is by neutrons ofseveral hundred keV. By integrating these spectra, weestimated the neutron counting rate of the E16 detector to be in the order of 0.1 kHz/mm2 for the beamdump origin and 0.01 kHz/mm2 for the target origin.
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In the proposed J-PARC E161) experiment, we will
use a 30 GeV primary proton beam at the J-PARC
high-momentum beam line. Because the beam intensity will be ∼1 × 1010 per spill and expected particle
counting rate is 5 kHz/mm2 at most, we will use two
types of GEM detector: the GEM tracker2) and theHadron Blind Detector (HBD)3) . These detectors need
to be operated in high-rate background environment
during the experiment. In a test experiment using the1.0 GeV/c pion beam at the J-PARC K1.1BR beamline, several breakdowns were observed with HBD (30cm × 30 cm × 100 µmt LCP GEM in CF4 gas, 3 × 103gain). The breakdowns are considered to be caused byhadronic background such as neutrons or slow hadrons,
because we did not observe such breakdowns with electron beams. In addition, no breakdowns were observedwith the GEM trackers (10 cm × 10 cm × 50µmt PI
GEM in Ar/CO2 gas, 1 × 104 gain). Therefore we estimated the background particles and their energies for
the E16 experiment and performed an operation testof the GEM detector in the high-rate neutron background.
The background particle counting rate was estimated using the Geant4 simulation. The simulation
employed the physics list of “QGSP BERT HP”,4)
which includes high-precision treatment of low-energy
neutrons (Ekin < 20 MeV). The validity of this simulation for estimation of background particle counting
rate was checked by comparing the background calculation with PHITS5) calculation6) at the K1.1BR area;
both agreed within a factor of 2. For example, theneutron rate was 0.01 Hz/mm2 at the broken GEM ofK1.1BR case.

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110 3Fig. 2. Energy distribution of the background particles
from the beam dump (left panel) and the target (rightpanel).

We performed an operation test of the GEM detector at AVF cyclotron room in the RIKEN RI beamfactory. The detector includes triple-stack 30 cm × 30cm × 50 µmt PI GEMs in the CF4 gas. This is thesame configuration as that employed in the E16 experiment. During the operation, a 12 MeV deuteronbeam with an intensity of 10 pµA was used in theroom. The dominant energies of the neutrons were 0.110 MeV according to the Geant4 simulation, and theGEM detector was operated in the room background.
The neutron radiation level monitored during the operation was 87 mSv/h on average, which is in the order of
1 kHz/mm2 . The counting rate of the GEM itself wasapproximately 10 Hz/mm2 . The GEM could be stably
operated for 15 h without breakdowns with Vgem = 510
V, which corresponds to a gain of 2 × 104 . The total
amount of neutrons corresponds to the 2-month operation in the E16 experimental area. From the test
operation, we can confirm that PI GEMs can work
stably in high neutron background. Further study isnecessary to clarify the reason for stable operation.
References
1) S. Yokkaichi et al., in this report
2) Y. Komatsu et al., in this report
3) K. Kanno et al., in this report4) ”Geant4 reference physics list” in the Geant4 homepage,
http://geant4.cern.ch/5) T. Sato et al., J. Nucl. Sci. Technol. 50:9, 913-923 (2013)
6) K. Tanaka, Private communication