Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Measurement of photoelectron yield in a hadron blind detector for
the J-PARC E16 experimentK. Kanno∗1,∗2 for the J-PARC E16 collaboration
A hadron blind detector (HBD) has been developed
for the J-PARC E16 experiment.1) The E16 experiment aims to detect the in-medium modiﬁcation of a ϕmeson in a nucleus via the ϕ → e+ e− decay. The HBD
identiﬁes the positrons and electrons by converting theemitted Čerenkov photons in CF4 into photoelectrons
with a CsI photocathode. The converted photoelectrons are ampliﬁed by a triple gas electron multiplier(GEM)2) stack to obtain a signal on readout pads. TheCsI photocathode is evaporated on the surface of thetop GEM of the stack. We perform the measurementin the momentum region (up to 4 GeV/c) where only
positrons and electrons can emit Čerenkov photons inthe HBD. Although almost all of the ionization electrons emitted from charged particles are removed byapplying reversed drift-ﬁeld before the triple-GEM section, a huge amount of such charged particles would
contaminate the HBD signal. Therefore, the detection
yield of the photoelectrons is the most signiﬁcant value
required to discriminate the Čerenkov photons at thetrigger level.
The detection yield of the photoelectrons depends on
the quantum eﬃciency of the CsI photocathode andphotoelectron collection eﬃciency. The collection eﬃciency is deﬁned as the ratio of the number of photoelectrons that are collected and subsequently ampliﬁed by the GEMs to the number of photoelectrons
produced at the CsI photocathode. The collection eﬃciency is, therefore, supposed to depend on the electricﬁeld at the surface of the CsI photocathode, whosestrength is determined by the size and pitch of theGEM holes.

and pitch sizes of 55 m and 140 m, respectively,
while type-B has the pitch size of 110 m with thesame hole size. The length of the Čerenkov radiator is50 cm, and the size of the photocathode is 60 cm × 60cm, which is divided into four parts with size 30 cm ×30 cm, as shown in Fig. 1. For the prototype, one 30cm × 30 cm type-A photocathode and one 30 cm × 30
cm type-B photocathode were prepared.
Using the prototype HBD, we performed a beam testwith a 1.0 GeV/c positron beam at the Research Center for Electron Photon Science, Tohoku University.
Figure 2 shows the obtained charge distribution, where
we observed ∼7.6 mean photoelectrons with type-Aand ∼10.7 with type-B.
Because we conﬁrmed that these photocathodes havealmost the same quantum eﬃciency, the diﬀerence inthe detection yield of the photoelectrons is attributed
to the diﬀerence in the photoelectron collection eﬃciency. Based on this result, which fulﬁlls the experimental requirement with type-B GEMs, we have decided to adopt type-B GEMs as the E16 HBD. This is
the ﬁrst result of the HBD made in Japan with large(30 cm × 30 cm) GEMs.

Fig. 2. Charge distribution of two diﬀerent photocathodes.

References
Fig. 1. Photograph (left) and schematic view (right) of theHBD prototype.

1) S. Yokkaichi et al.: in this report
2) F. Sauli: Nucl. Instr. and Meth. A 386 (1997) 531

To evaluate the pitch dependence of the detection
yield of the photoelectrons, we produced a prototype
HBD with two types of GEMs: type-A has the hole
∗1
∗2

RIKEN Nishina Center
Department of Physics, Graduate School of Science, The
University of Tokyo- 226 -