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

RIKEN Accel. Prog. Rep. 48 (2015)

Development of gas system for MuSEUM experiment
K. S. Tanaka∗1,∗2 on behalf of J-PARC MuSEUM Collaboration
We are planning to measure the energy of groundstate hyperﬁne structure (HFS) of muonium at JPARC/MLF. Muonium is a hydrogen-like bound statethat consists only leptons, and its HFS is a good probefor testing the QED theory. The latest experiment at
LAMPF obtained the following value:1)
ex∆HF SM= 4.463302765(53) GHz (12 ppb) .

∗2RF cavity

CCD camera(1)

The total uncertainties were determined using the statistical uncertainties. We will achieve an accuracy
more than 10 times greater than that of the latest experiment by using the H-line at J-PARC.
Muons polarized in the revese direction of momentum enter the bore of a large superconducting solenoid
magnet from the J-PARC/MLF muon beamline. A RF
cavity is located at the center of the magnet containing pure Kr gas. Muons stop by collisions in the gas,and polarized muoniums are formed by the electron
capture process.
In a magnetic ﬁeld the ground state splits into four
substates. ν12 and ν34 are obtained by the microwave
magnetic resonance technique. High-momentum decay positrons are emitted preferentially in the direction of the muon spin. By driving the transitions with
an applied microwave magnetic ﬁeld perpendicular tothe static magnetic ﬁeld, the muon spin can be reversed and the angular distribution of high-momentum
positrons changes from predominantly upstream todownstream with respect to the beam direction. The
cavity was designed to be resonant simultaneously in
the TM110 mode at the ν12 transition frequency andin the TM210 mode at the ν34 frequency2) . The muonium HFS (∆ν) is obtained by summing ν12 and ν34 .
A Gas chamber surrounds the RF cavity to seal
in the Kr gas. The chamber consists of only aluminum and its upstream foil is thin enough (100 µm)
for muons to pass through. We performed gas introduction tests in the chamber. The gas pressure ismonitored at the 0.02 % level by a silicon pressuretransducer. The gas in this system is sampled regularly by small cylinders. Since spin-exchange collisions occur between muonium and paramagnetic contaminant particular oxygen, we intend to determine
the purity of a gas at the parts per million level using Q-mass. We studied the performance of the gas
panel without gas sampling and precise gas pressure
monitoring on November 2014 (Fig. 1). In this test,
we measured muonium distributions under several gas
pressures (Fig. 2). The gas pressure in the chamber
maintain a steady at sub Torr level during this measurement.
∗1

Gas panel

Gas chamber

Fig. 1. Experimental setup at 2014A beamtest. The gaschamber is mounted on the support rail. The gas panel
is behind the gas chamber.

0.1 atm

0.3 atm

0.7 atm

1.0 atm

0.5 atm

Fig. 2. Muonium distributions at the center of the chamber
for diﬀerent gas pressures.

References
1) W. Liu et al.: Phys. Rev. Lett. 82, 711 (1999)
2) K. S. Tanaka on behalf of MuSEUM Collaboration K.
S. Tanaka et al.:, JPS Conf. Proc. 2, 010405 (2014)

RIKEN Nishina Center
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