Development of RICH detector for Secondary Beam Particles at J-PARC
High-momentum Beamline
1M. Naruki1 , R. Honda2 , S. Kajikawa3 , K. Shirotori4 , Y. Kimura5 , R. Tatsumi5 , T. Toda5 , M. Tokuda5
Dept. of Physics, Kyoto University, Sakyo-ku, Kitashirakawa Oiwake-cho, Kyoto, Kyoto 606-8502, Japan
2
IPNS, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan
3
Dept. of Physics, Tohoku University, 6-3, Aoba, Aramaki, Aoba-ku, Sendai 980-8578 Japan
4Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan
5Dept. of Physics, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043, JapanThe high-momentum beamline (high-p beamline) has been newly constructed at the J-PARC Hadron Facility
on May 2020. It delivers primary protons up to 30 GeV, in addition, will be also utilized as a secondary beamline
in near future. It enables us to extend hadron spectroscopy into heavy systems, like Ξ, Ω and/or charm baryons.
It should be noted that the maximum beam momentum of existing beamlines is 2 GeV/c, smaller by an orderof magnitude compared with that at the high-p line. The secondary beam particles like pions and kaonsare, unfortunately, not separated in the present high-p design. Therefore, it is a key to implement particle
identiﬁcation detector for beam particles in the momentum range of 5 to 20 GeV/c for future studies.
We are developing a RICH-type detector for the beam particle identiﬁcation. Since the intensity of pions ishigher than that of kaons by three orders of magnitude, beam kaons should be actively identiﬁed instead of pionveto technique. A prototype detector of the RICH has been designed with the silica aerogel whose refractive
index is 1.02, developed at Chiba university. Cherenkov photons are once reﬂected at a spherical mirror andcollected into MPPC arrays, as shown in Fig. 2 (left). The expected number of photons is 9.3 p.e. with the 8 cm

Figure 1: conceptual design of RICH detector
aerogel radiator. The separation power is characterized by an angle resolution of Cherenkov ring, depends on
a position resolution of MPPC, the radiator length and a spherical aberration. The performance of prototype
detector was examined at the LEPS beamline using electrons produced in γA conversions with a lead photon
converter. The number of MPPC arrays of the prototype detector is half compared with that of the real device.
The observed number of photons, N, depending on the the radiator length is shown in Fig. 2 (middle) [1]. Thenumbers are well reproduced with the function;∫∫ L
R(λ)Q(λ)
N = 2πα ∗ acceptance ∗ sin2 θc ∗dλe−(L−x)/d dx
λ20
where λ is wave length, R(λ) is a reﬂectance of the mirror, Q(λ) is a quantum eﬃciency of MPPC, L is
the length of the radiator, d is the transmission length of the aerogel and θc is the Cherenkov angle. Thetransmission length is evaluated to be 4.8 cm from the ﬁt. The number of photon of the real device is estimatedto be 10 p.e. The measured angle resolution is shown in Fig. 2 (right). Based on the result, the expected
Cherenkov angle resolution is obtained to be 1.5 mrad for 8.5 GeV/c beam kaons. It enable us to identify kaons
with 50% eﬃciency under the 3% contamination of π in the beam momentum range of 5 to 8.5 GeV/c. Theﬁnal speciﬁcation of the RICH detector has possibly determined due to theses basic performance. Before theconstruction, we plan to study the high rate capability of read-out modules and establish a selection scheme of
the Cherenkov ring corresponds to beam kaons.
The authors are deeply grateful to staﬀ members of LEPS. We would also like to express our gratitude toH. Noumi and A. Sakaguchi for the support and encouragement.

Figure 2: (left) Measured photon numbers (vertical axis) for corresponding depth of radiator (horizontal axis).
The red line shows the ﬁt result with the function parameterized with the transmission length (see text). (right)
Measured Cherenkov angle resolution (black points). The estimated angle resolution for the real device is plottedwith red points.

References[1] S. Kajikawa, Master thesis, Tohoku University (2020)