RIKEN Accel. Prog. Rep. 48 (2015)

Ⅲ-2. Atomic & Solid State Physics (Muon)

Development of online muon beam profile monitor for the MuSEUM
experiment
S. Kanda∗1,∗2 for the MuSEUM Collaboration
Muonium is the bound state of a positive muon andan electron. Because neither muon nor electron has
an internal structure, muonium ground state hyperﬁnesplitting (MuHFS) can be the most precise probe forthe test of the bound state QED and for the determination of muon mass via the ratio of magnetic moments ofa muon and a proton. At J-PARC, we plan to performa precision measurement of the MuHFS by microwave
spectroscopy of muonium. Spectroscopy of the energy
states is performed by measurement of positron asymmetry from muonium decays. Our goal is to improve
the precision by an order of magnitude compared withthat of the most recent experiment. In order to achieve
the goal, we utilize J-PARC’s highest-intensity pulsed
muon beam2) , highly segmented positron detector with
Silicon PhotoMultiplier(SiPM)3) , and an online/oﬄine
muon beam proﬁle monitor.
The online muon beam proﬁle monitor analyses themuon beam from the aspect of beam shape and relative
intensity. Requirements for the beam proﬁle monitor
are minimum destruction and high reconﬁgurability ofthe beam. We utilize thin plastic scintillation ﬁberand SiPM. Figure 1 shows the conceptual design ofthe muon beam proﬁle monitor. The detector consistsof an one dimensional array of thin plastic scintillation
ﬁber with a 100 µm diameter. The ﬁbers are boundinto a bundle and connected to a SiPM. For a front-end
electronics, EASIROC front-end chip4) was utilized as
an ASD (ampliﬁer, shaper, and discriminator). The
pulse height is digitized by an external peak holding
ADC.

Figure 2 shows the developed prototype and its
cross-sectional view. The ﬁbers were arrayed on a polyimide ﬁlm of 25 µm thickness and bonded by epoxy
resin. Forty ﬁbers were bound into one band of 4 mmwidth. Two bands were arrayed with a 2 mm gap.
In November 2014, a beam test was performed atFig. 2. Developed prototype of the online muon beam profile monitor

J-PARC MLF MUSE D2 beam line. Figure 3 shows
the measured photon yield as a function of total muon
beam intensity. Saturation in higher intensity region iscaused by the limited number of SiPM pixels (in case ofthis prototype, number of pixels was 667). The beam
intensity was controlled by movable slits and measured
by an oﬄine beam proﬁle monitor5) , which consists ofa gated image intensiﬁer and a cooled CCD. The movable slits only change beam density and keep beam proﬁle unchanged. Based on the result of the beam test,
the design of a full-scale detector is in progress. SiPM
with narrower pixel pitch will be used and process ofﬁber bonding will be optimized for uniform thickness.

Fig. 3. Measured photon yieldFig. 1. Conceptual design of the online muon beam profile
monitor∗1∗2

Department of Physics, University of Tokyo
RIKEN Nishina CenterReferences
1) W. Liu et al.: Phys. Rev. Lett. 82 (1999) 711
2) A. Toyoda et al.: J. Phys. Conf. Ser. 408 (2013) 012073
3) S. Kanda et al.: JPS Conf. Proc. 2 (2014) 010404
4) S. Callier et al.: Physics Procedia 37 (2012) 1569
5) T. U. Ito et al.: NIM A 754 (2014)- 278 -

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