Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 48 (2015)

Simulation of HBD response in the J-PARC E16 experiment basedon test results

A hadron blind detector (HBD) has been developed for the J-PARC E16 experiment.1) The E16 experiment aims to investigate the origin of QCD mass
through the spectral change of a ϕ meson in a nucleus.
The mass spectrum is reconstructed through electronpositron decays. A detector for electron identiﬁcationis required. The HBD is utilized in the E16 experiment for this purpose. The HBD identiﬁes electrons
by converting Čerenkov photons emitted by an incident
electron in a CF4 radiator into photoelectrons using aCsI photocathode. These photoelectrons are ampliﬁed
by gas electron multiplier2) (GEM) foils and are subsequently read out as an electric signal. Other charged
particles apart from the electron do not emit Čerenkov
photons in the momentum region where we perform themeasurement, 0.4–3 GeV/c. With this scheme, theHBD distinguishes electrons from the other charged
particles.
We have constructed a prototype of the HBD and
performed a test experiment using a positron beam of
1.0 GeV/c at Research Center for Electron Photon Science, Tohoku University. We successfully observed 11
photoelectrons per incident positron with the prototype3) when the incidence angle to the beam was 0◦ .
This result was consistent with the expected result calculated based on the performance of each detector element. However, only HBD response to an electron wasobtained in this beam test, and the response to a pionis also required to determine the pion rejection power
for a given electron detection eﬃciency. We estimatedthe HBD response to a pion through simulation.
For this simulation, we used the HBD response toa pion that has already been obtained with anotherprototype. This prototype has a smaller pad readout
and longer gap lengths between GEM foils. Taking
into account of the diﬀerences between the prototype
and another prototype, we performed the simulation
to evaluate the prototype response to a pion. With
this simulation, we estimated an electron detection efﬁciency of 85% with a pion rejection factor of 50 for
1.0 GeV/c electrons and pions at the threshold of 6.5 ewhen the incidence angle was 0◦ . In the actual J-PARC
E16 experiment, charged particles including electronsand pions are emitted from a target and the incidence
angle of these particles to the HBD is approximately 0–45◦ . Furthermore, these charged particles move along
a curved line because of the magnetic ﬁeld in the spectrometer used in the experiment, resulting in a larger∗1∗2

RIKEN Nishina Center
Department of Physics, Graduate School of Science, The
University of Tokyo

Electron detection efficiency

K. Kanno∗1,∗2 for the J-PARC E16 Collaboration
1.210.8
0.6
0.4
0.2001234Momentum [GeV/ c]

5Fig. 1. Electron detection eﬃciency as a function of electron momentum.

incidence angle to the HBD.
To evaluate the electron detection eﬃciency in theoﬄine analysis, we performed another simulation byincluding the possible momentum and incidence angle distribution of electrons from ϕ meson decays.
In the simulation, ϕ mesons were generated by a 30GeV/c proton-induced reaction using the nuclear cascade code JAM.4) These ϕ mesons decay into electronsand positrons, and then electrons move according to
the magnetic ﬁeld. We assumed pion samples had
the same track as the electron samples. The HBD
responses to an electron and a pion were estimatedbased on the result of the beam test and the simulation
mentioned in the previous paragraph. The electron detection eﬃciency as a function of electron momentum
with a pion rejection factor of 100 is shown in Fig. 1.
The eﬃciency decreases in the region of 0.4–1.5 GeV/c.
This fact reﬂects that the signal amplitude of a pionincreases with decreasing momentum. A pion sample
having lower momentum has a larger curvature in amagnetic ﬁeld and consequently has a larger incidence
angle to the HBD, namely, a larger signal amplitude.
The large errors in the high-momentum region are due
to availability of limited statistics. The overall eﬃciency is 60% with a pion rejection factor of 100 at thethreshold of 7.5 e, which meets the requirement of theexperiment.
As the next step, we will measure the response toa pion, which has been simulated, using a pion beamwith GEM foils and the readout pad conﬁguration forthe production type of HBD.
References
1) S. Yokkaichi et al.: in this report
2) F. Sauli: Nucl. Instr. and Meth. A 386, 531 (1997)
3) K. Kanno et al.: RIKEN. APR. 47, 226 (2014)4) Y. Nara et al.: Phys. Rev. C61, 024901 (1999)- 243 -完全版2014_本文.indd 243

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