Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Hadron blind detector using a ﬁnely segmented pad readout†
K. Kanno∗1,∗2 for the J-PARC E16 Collaboration

†
∗1
∗2

Condensed from the article in Nucl. Instr. and Meth. A
819, 20 (2016)
Department of Physics, Graduate School of Science, The
University of Tokyo
RIKEN Nishina Center- 219 -

700

0.9
0.8

w/ CSA
500

0.7

400

0.6
0.5

300

0.4
0.3

200
w/ CSA

0.2

100
0
0

w/o CSA
1

2

3

4

5

0.1
6

7

8

Electron detection efficiency

1
w/o CSA

600

Pion rejection factor

We developed a hadron blind detector (HBD) using
a ﬁnely segmented pad readout for the J-PARC E16
experiment.1) The HBD is a Cherenkov detector that
does not require mirrors or transparent windows.2) Our
HBD is used to identify electrons and to reject pions
in the momentum region of 0.4–3.0 GeV/c. The ﬁnely
segmented pad readout enabled us to perform an advanced analysis using the number of signal-detecting
pads in addition to the usual analysis with the total
amount of collected charge. With this advanced analysis method, the pion rejection factor is increased while
keeping the electron detection eﬃciency.
Our HBD consists of a 50-cm-long CF4 radiator, amesh electrode, a CsI photocathode, three layers of
GEM foils and a pad readout. The mesh and threelayers of GEM foils are stacked together onto the readout pad. The CsI photocathode is evaporated on the
top surface of the top GEM foil to detect Cherenkov
photons and convert them into photoelectrons. An incident electron emits Cherenkov photons, whereas anincident pion does not in the momentum region of interest. Thus, a pion can induce a signal only throughthe ionization electrons. However, this signal is greatly
suppressed by controlling the electric ﬁeld of the gap
between the mesh and the top GEM foil such that theionization electrons produced in the gap are swept into
the mesh. The pad readout consists of 25 square pads.
The size of each pad is 10×10 mm2 . This size is smaller
than that of the circular image of the Cherenkov photons, which is 34 mm in diameter in our HBD. Thus, anincident electron is expected to induce multiple signaldetecting pads, whereas an incident pion is expected toleave no signal-detecting pad. The pion rejection factor is expected to be improved by applying a thresholdon the size of a charge cluster. This analysis method
is called cluster size analysis.
To measure the responses of the detector to electrons and pions, we performed a beam test at J-PARC
K1.1BR with a negatively charged secondary beam at
1.0 GeV/c containing approximately 20% electrons. A
signal-detecting pad was deﬁned as a pad with the induced charge larger than that of the pedestal by 7σ,
corresponding to 0.22 e. The cluster size is deﬁned as
the number of neighboring signal-detecting pads. Twopads which share at least a corner are considered asneighbors. Our HBD was operated under a reverse
bias of 5 V/mm. The mean primary charge for incident electrons and pions is 7.3 e and 0.7 e, respectively.

0

Charge threshold (e)Fig. 1. Electron detection eﬃciency and pion rejection factor with and without the cluster analysis as a functionof the threshold on the primary charge. The notation“CSA” represents the cluster size analysis. The open
and closed symbols represent the pion rejection factor
and electron detection eﬃciency, respectively.

This mean primary charge for incident electrons wasconsistent with the expected result calculated basedon the performance of each detector element including the quantum eﬃciency of the CsI photocathode.
Thus, the primary charge induced by an incident electron was considered to be due to photoelectrons. The
mean cluster size for incident electrons and pions was
4.3 pads and 0.5 pads, respectively. An electron had alarger cluster size than a pion.
We calculated the electron detection eﬃciency andthe pion rejection factor with and without the cluster
size analysis as shown in Fig. 1. We achieved an electron detection eﬃciency of 83% with a pion rejection
factor of 120 when we applied a threshold of 2.5 e on
the primary charge and required a cluster size of threeor more. The pion rejection factor with the cluster size
analysis was improved by a factor of approximately
ﬁve.
Based on this result, we constructed a production
model of the HBD for the J-PARC E16 experimentwith hexagonal pads with a side length of 10 mm. The
pion rejection factor of the HBD was evaluated as 160
with a simulation, which satisﬁes the requirement of
the experiment.
References
1) S. Yokkaichi et al.: In this report.
2) W. Anderson et al., Nucl. Instr. and Meth. A 646, 35(2011).