II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Performance evaluation of the electron identification system for theJ-PARC E16 experiment
S. Nakasuga∗1,∗2,∗3 for the J-PARC E16 Collaboration
The J-PARC E16 experiment1) is proposed for measuring the spectral modiﬁcation of vector mesons innucleus. Here, we detect electron-positron pairs generated in ϕ meson decays, produced in pA reactions.
It is crucial for a successful measurement to separate
electrons from huge hadronic backgrounds, especiallypions, which are a primary component of those.
For the electron identiﬁcation, we adopt two-stage
detectors comprising hadron blind detectors (HBDs)
and lead-glass electro-magnetic calorimeters (LGs).
The HBD is a gas-type Cherenkov detector with aCF4 radiator.2) Emitted Cherenkov photons are converted into electrons at a CsI photocathode, and theseelectrons are ampliﬁed by a gas-electron multiplier(GEM).3) The LG is a calorimeter sensitive to EM
showers generated by an incident particle in lead-glass.
The high-energy electrons generate large showers compared with pions; therefore, the LG is able to distinguish electrons from pions based on the quantity of
Cherenkov photons emitted by the showers.
Both detectors were developed independently, andtheir performances were evaluated separately.4,5) If
particle detection by one detector aﬀects another detector, the combined performance of the system possibly becomes worse than expected from the individual performances. For example, an incident pion may
produce knock-on electrons in the CF4 radiator of theHBD, one of which may have suﬃcient energy to produce EM showers in the LG, which is installed behindthe HBD. These eﬀects are expected to be small; however, experimental validation is necessary.
We performed a beam test for a total performance
evaluation of the HBD and LG in the commissioning
run of the E16 spectrometer at the high-momentum
beamline at J-PARC. As shown in Fig. 1, we constructed a novel setup, which covered the forward acceptance of the E16 spectrometer. The responses of theHBD and the LG were examined for pions, identiﬁedby the triple coincidence of scintillation counters andtwo gas Cherenkov detectors (GCs) positioned in frontof the HBD. The primary beam intensity was typically1 × 109 protons/spill suitable for the readout systemof this measurement. We adopted one carbon and two
copper targets, whose total thickness was 0.2% interaction length. This was the same conﬁguration as theE16 experiment.
∗1∗2∗3Department of Physics, Graduate School of Science, Kyoto
University
Advanced Science Research Center, Japan Atomic Energy
AgencyRIKEN Nishina CenterFig. 1. Experimental setup for evaluating the overall performance of the electron identification system.

The HBD and LG distinguished electrons from pions by applying a threshold to the number of detected
photons. We measured the threshold dependence ofthree quantities: single rejection power of the HBD(SRHBD ), that of the LG (SRLG ), and total rejection
power of the HBD and the LG (TR). Here, the rejection power was deﬁned as the number of total events
divided by that of misidentiﬁed events. If the correlation between the detectors is negligible, the productof SRHBD and SRLG is expected to be consistent withTR. At the threshold, which will be used in the future experiment, the values of SRHBD and SRLG weredetermined to be 49 ± 44 and 3.6 ± 0.1, respectively;
therefore, the product of these values was 178 ± 16.
Compared with the product, the measured value of TR
was 174 ± 29. These two values were consistent within
their statistical uncertainties; therefore, we concluded
that the HBD and LG independently work for pions.
In the coming beamtime in February and June
2021, we plan to study and evaluate the performance
minutely with the E16 spectrometer, in terms of ratecapability and the response depending on an incident
angle and momentum of incoming particles.
References
1) S. Yokkaichi, in this report.
2) W. Anderson et al., Nucl. Instrum. Methods Phys. Res.
A 646, 20–24 (2011).
3) F. Sauli, Nucl. Instrum. Methods Phys. Res. A 386,531–534 (1997).
4) K. Kanno et al., Nucl. Instrum. Methods Phys. Res. A
819, 35–58 (2016).
5) S. Ashikaga, Master’s thesis, Kyoto University (2018).

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