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

RIKEN Accel. Prog. Rep. 49 (2016)

Development of magnetic shield for the MuSEUM experiment
S. Kanda∗1,∗2 for the MuSEUM Collaboration
Muonium is a hydrogen-like atom consisting of apositive muon and an electron. Precision spectroscopyof its ground state hyperfine splitting (HFS) is themost stringent test of bound-state QED theory andthe most precise determination of the muon mass. At
J-PARC, the MuSEUM (Muonium Spectroscopy Experiment Using Microwave) collaboration is planninga new measurement of muonium HFS proceed in twostages; indirect measurement under a 1.7 T high magnetic field and direct measurement under a precise controlled zero magnetic field. The zero-field experimentis a preliminary measurement with existing facility toward the definitive high-field experiment, which requires a new muon beam line under construction. Our
goal is to improve the precision of muonium HFS by
about one order of magnitude in both experiments.
Spectroscopy of muonium HFS is performed by
the measurement of positron angular asymmetry frommuonium decay, which depends on the muonium energy state. The hyperfine transition is induced by microwave and muon spin flips when the transition occurs. Result of the most recent zero-field experiment1)
is 4.463022 (1.4) GHz (300 ppb) where the error is thestatistical uncertainty. In the zero-field measurementof muonium HFS, one of the major sources of systematic uncertainty is the non-uniformity of a magnetic
field in the muonium formation volume. According
to the numerical simulation, requirement for the field
non-uniformity is less than 100 nT peak-to-peak.
To perform the zero-field experiment, we developed muon and positron detectors2) , three-dimensionalmuon stopping distribution imager3) , RF cavity4) ,magnetic field monitor, and magnetic shield of permalloy (high-permeable alloy of nickel and iron). Figure
1 shows the developed magnetic shield and field probewith a moving system. The shield was designed basedon the results of a stray field analysis by the finiteelement method. It consists of a three-layered box
made of permalloy where the layer thickness is 1.5 mm
and interlayer distance is 50 mm. The magnetic fieldstrength is measured by a high-precision fluxgate probe(MTI FM3500) with a resolution of 0.5 nT. Without
shielding, the typical magnetic field strength in the target volume was 100 µT and field non-uniformity was
about 20 µT peak-to-peak.
Figure 2 shows the measurement result of the strayfield inside the magnetic shield. The magnetic probe
was moved on the beam-axis. The target volume corresponds to the probe position ranging from -150 mm
to 80 mm. Typical field strength was suppressed to∗1
∗2

Department of Physics, University of Tokyo
RIKEN Nishina Center- 227 -Fig. 1. Magnetic shield and fluxgate probeabout 150 nT. A shielding factor, i.e., the ratio of thefield strength with magnetic shield Bw and without
shield Bwo was estimated at Bwo /Bw = 700. The field
non-uniformity was about 30 nT peak-to-peak and theperformance specification of the magnetic shield satisfied the requirement.

Fig. 2. Magnetic field distribution

The measured field non-uniformity was acceptable
for the experiment. However, a factor of two higher
shielding factor is expected from the simulation andlaboratory measurement results. The field investigation implied that the shield is magnetized by the environmental magnetic field during assembly. Magnetization of inner layers of the shield should be suppressedby the attenuation current supplied by the degaussing
coils. The degaussing device and method are understudy. The development of every other experimental
component has been completed and the first trial ofzero-field measurement is scheduled in Feb. 2016 atJ-PARC MLF MUSE.
References
1) D.E. Casperson et al., Phys. Lett. B, 59, 397 (1975).
2) S. Kanda et al., JPS Conf. Proc. 8, 025006 (2015).
3) S. Kanda et al., KEK-MSL Prog. Rep. 2014, 46 (2015).
4) K.S. Tanaka., In this report.