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

RIKEN Accel. Prog. Rep. 53

Temperature dependence of the total muonium emission yield fromsilica aerogel using µSR method
K. Ishida,∗1 K. Suzuki,∗2 T. Mibe,∗2 M. Iwasaki,1,∗3 S. Kamal,∗4 and J. Brewer∗4
We are developing thermal-energy muonium (Mu =µ+ e− ) production as a source of the cold muon beam forthe muon g-2/EDM experiment planned at J-PARC.1)
We observed at TRIUMF that the Mu emission ratefrom silica aerogel to vacuum is enhanced by surfaceablation.2,3) However, the present Mu yield measurement performed by tracking the muon decay positronis limited to a region away (∼5 mm) from the surface
owing to the large background from the muoniums decaying in aerogel. To enable measurement of the Mu
emitted in vacuum including the region near the surface, where we are planning to shoot a Mu ionizing
laser, we performed a new measurement using the Mu
spin rotation (MuSR) method in the ARGUS spectrometer at the RIKEN-RAL Muon Facility. In MuSR, theprecession of the muon spin can be detected by asymmetric emission of the muon decay positrons. Since theprecession frequency of Mu is diﬀerent from that of diamagnetic muon under an applied ﬁeld, we can measure
the fraction of muons forming the Mu state.
In the measurement, we added a gold foil to thedownstream of the silica aerogel target. First, we set
the muon-beam momentum so that the muons fully stopin the middle of the aerogel and measured the Mu spin
relaxation intrinsic to the aerogel material. Then, wechose another momentum so that the muon stopping
distribution peaked at the downstream surface of theaerogel. In this half-stopping condition, we observed
an increase in the Mu spin relaxation rate.4) This wasinterpreted as a result of Mu transfer to the gold foil
after emission to vacuum. Once in gold, Mu changes
its state to diamagnetic muon and cannot contribute to
the Mu precession signal anymore.
Here, we report the ﬁrst measurement of the temperature dependence of the Mu emission rate. In a naive
model, where Mu is emitted after diﬀusion in spacesin the aerogel nano-structure, we expect that the Mu
emission rate is proportional to the thermal Mu velocity, v ∝ T 1/2 , where T is the aerogel temperature.
Keys for the experiment are the cooling of the aerogeland the aerogel temperature measurement. Since thethermal conductivity of the silica aerogel is very low,
we relied on cooling by thermal radiation emission. We
designed a sample holder that covers all the faces of theaerogel and mounted it on the cryostat. In a cooling test
conducted prior to the MuSR measurement, we used aK-type thermocouple directly inserted into the aero∗1
∗2
∗3
∗4

RIKEN Nishina Center
IPNS, KEK
Meson Science Laboratory, RIKEN
University of British Columbia

Fig. 1. Temperature dependence of the Mu relaxation ratewith the muons stopped in the middle (full stop) and at
the downstream edge of the silica aerogel (half stop).

gel in addition to the semiconductor thermometers on
the cryostat and the sample holder. The thermocouple
was chosen because of its small size and low heat mass.
While cooling down from room temperature, the aerogel
temperature initially followed that of the sample holder,though with some delay. Subsequently, the aerogel temperature was saturated at approximately 180 K and stabilized, however cold the sample holder was. This wasunderstood as the eﬀect of leakage of thermal radiationfrom the room-temperature surface through an opening
used to insert the thermocouple. The saturation temperature was consistent with that expected with area
fraction of the opening, which was about 10%. In theMuSR measurement, the thermocouple was removed,
and the aerogel sample was completely surrounded bythe cold surface. This ensured that the aerogel temperature reached equilibrium with that of the holder.
A preliminary result of the Mu relaxation rate measurement is shown in Fig. 1. The relaxation rate in theaerogel (full stop) did not change much with the temperature, while the rate in the half-stop condition waslarger and showed some temperature dependence. We
plan a detailed analysis to study whether it agrees withthe diﬀusion model assuming the Mu velocity distribution equilibrated with the temperature.
References

1) M. Abe et al., Prog. Theor. Exp. Phys. 2019, 053C02(2019).
2) P. Bakule et al., Prog. Theor. Exp. Phys. 2013, 103C01(2013).
3) G. A. Beer et al., Prog. Theor. Exp. Phys. 2014, 091C01(2014).
4) K. Ishida et al., RIKEN Accel. Prog. Rep. 52, 181 (2019).

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