Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 49 (2016)

Study of the eﬀect of radiation damage on the quantum eﬃciency ofa CsI photocathodeK. Kanno∗1,∗2 for the J-PARC E16 Collaboration∗1∗2

RIKEN Nishina Center
Department of Physics, Graduate School of Science, The
University of Tokyo- 196 -

Quantum efficiency (%)

5045

Before irradiation

40

After irradiation

35302520
15105Rel.
err.

We measured the eﬀect of radiation damage on thequantum eﬃciency of a CsI photocathode. This study
is of great importance for the performance of the JPARC E161) hadron blind detector (HBD), which uses
a CsI photocathode to detect Cherenkov photons produced by an incident electron in the CF4 radiator. In
the E16 experiment, the HBD is required to identify
electrons with high eﬃciency in the high-rate neutronenvironment (<10 MeV, 3 kHz/cm2 ). The radiation
damage of a CsI photocathode caused by these neutrons will degrade the sensitivity of the photocathode
to Cherenkov photons. This degradation of a photocathode decreases the electron detection eﬃciency of
the HBD.
We performed a test experiment at the RIKEN AVF
cyclotron to estimate the eﬀect of the radiation damage
on a CsI photocathode caused by a high-rate neutronenvironment. The radiation damage was evaluated bycomparing the quantum eﬃciency before and after theexperiment. In the experimental area, low-energy neutrons were produced by stopping an α-particle beamat 12.5 MeV/u from the cyclotron at a beam dump. ACsI photocathode contained in a prototype HBD wasirradiated with these low-energy neutrons. It should
be noted that the CsI photocathode was not directly
irradiated with the α-particle beam. The photocathode was not exposed to air at all, because the watercontaminant caused the CsI photocathode to change
its composition and degraded the quantum eﬃciency.2)
The prototype was operated under CF4 gas ﬂow, andthe water contaminant was monitored and maintained
below 15 ppm, which ensured that the photocathode
did not deteriorate at all owing to the water contaminant. The CsI photocathode was evaporated by Hamamatsu Photonics K.K. and the thickness was 300 nm.
The neutron rate evaluated with a 3 He gas-ﬁlled neutron monitor in the area and a GEANT4 simulation
was approximately 17 kHz/cm2 (<10 MeV) at the position the prototype was located, which was higher
than that of the E16 experiment by a factor of approximately six. The photocathode was irradiated for
35 h in total, and this duration corresponded to approximately nine days in the E16 experiment in termsof the total amount of neutrons.
The result is shown in Fig. 1. The top panel represents the quantum eﬃciency of the CsI photocathodebefore and after the irradiation. The bottom panel
represents the systematic errors in the relative value of0.2
0.15
0.10.050120 130 140 150 160 170 180 190 200

Wavelength (nm)1098Energy (eV)7Fig. 1. Quantum eﬃciency of the photocathode (top panel)and its systematic error (bottom panel) as a functionof wavelength and energy.

quantum eﬃciency. This systematic error arises fromthe calibration error of our quantum eﬃciency measurement system. Thus, the enhancement of the quantum eﬃciency after the irradiation over the wavelengthof 120–200 nm is signiﬁcant. Although we detected nochanges in the CsI photocathode by visual inspection,
this enhancement might be caused by the change in thesurface structure of the photocathode by high-rate neutrons. The surface structure of a photocathode aﬀects
the photoelectron escape probability from the photocathode.3) Even though further studies are required tounderstand this enhancement, this result ensures thatthe quantum eﬃciency does not decrease for at leastnine days in the E16 experiment.
References
1) S. Yokkaichi et al.: in this report.
2) A. Breskin: Nucl. Instr. and Meth. A 371, 116 (1996).
3) C. Lu, K. T. McDonald: Nucl. Instr. and Meth. A 343,
135 (1994).