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

RIKEN Accel. Prog. Rep. 50 (2017)

Beamline tune for muonium emission from silica aerogel
towards ultra-slow muon project at RIKEN-RAL
S. Okada,∗1 S. Aikawa,∗2 G. Beer,∗3 J. Brewer,∗4 K. Ishida,∗2 M. Iwasaki,∗2 S. Kanda,∗5 H. Kawai,∗6R. Kitamura,∗5 Y. Ma,∗2 G. Marshall,∗4 T. Mibe,∗7 Y. Oishi,∗8 A. Olin,∗4 M. Otani,∗7 N. Saito,∗7 M. Sato,∗9M. Tabata,∗6 and T. Tsukihana∗10(a)Three-axis
Helmholtz coilμ+ beam

Asymmetry [%]degraders, where degrader thicknesses for “full-stop”
and “half-stop” conditions are indicated. The aerogeltarget has a density of 28 mg/cm3 and thickness of
7.8 mm which corresponds approximately to the fullstop range. The obtained curve was consistent witha momentum bite of 2% assuming a Gaussian distribution at RIKEN-RAL.6) The absolute values of muonium asymmetry amplitudes are also comparable withwhat we assessed with a Monte Carlo simulation.
Now we have established the method of degradertuning and are ready for a laser ionization study.

BY2(b) Preliminary10(0.18 mTesla)-1degraders(Al foils)
Asymmetry amplitude at time 0 [%]A sharp low-energy muon beam may be realized byaccelerating several-eV positive muons, so-called ultraslow muons, generated by the laser ionization of ther1,2)
vacuum.1,2)
mal muonium atoms in vacuum. This technology
is the focus of attention in material science using μSR
techniques and particle physics, e.g., measurement of
the muon anomalous magnetic moment, g-2, and elecJ-PARC.3)3).
tric dipole moment at J-PARC
Continued research on muonium emission into vacuum from several stopping materials revealed that silica aerogel with a laser-ablated surface is a good candidate.4) To demonstrate the actual ultra-slow muon
production with silica aerogel, a new experimental
setup has been prepared at RIKEN-RAL, as previouslyreported.5) Here, we report the results of a commissioning run conducted in September 2016.
Positive muons stopping in the surface region of theaerogel target contribute to muonium emission intovacuum. To maximize this for the beam at RIKENRAL6)6), we adjusted the thickness of a degrader so thatRAL,
the number of muons stopping in the aerogel is approximately half of the full stop, while the remainder
passed through mostly to regions beyond those of interest for muonium decay in vacuum. Since aerogelis the only material capable of muonium production
installed in the setup, we can assess the amount ofmuonium produced by measuring asymmetry amplitudes at time zero of the muonium precession signal
with a frequency of 13.9 GHz/T, well separated fromthe frequency of muons, 136 MHz/T.
Figure 1(a) shows our experimental setup. A threeaxis Helmholtz coil system is used for the cancellationof stray magnetic ﬁelds, and for applying a transverse
ﬁeld of 0.18 mT to rotate the spin of muonium. Figure1(b) shows a typical muonium spin rotation asymmetry distribution measured by a set of μSR counters.
The ﬁt result shows a rotation cycle of 400 ns being consistent with our expectation for muonium with
0.18 mT. Figure 1(c) shows the measured range curve
as a function of the total thickness of aluminum-foil-2(c) Preliminary

5678Time [μs]Full stop

1Half stop
0.50050

100Degrader thickness [mg/cm 2]

Fig. 1. (a) Photograph of the experimental setup at thePort 3 beamline of the RIKEN-RAL muon facility. (b)A typical muonium spin rotation asymmetry distribution, where the muon exponential decay time dependence has been removed. (c) Range curve obtained bymeasuring the asymmetry of muonium produced at asilica aerogel target as a function of degrader thickness.

References
1) K. Nagamine et al., Phys. Rev. Lett. 74, 4811 (1995).
2) P. Bakule et al., NIM B 266, 355 (2008).
3) M. Aoki et al., Conceptual Design Report for The Measurement of the Muon Anomalous Magnetic Momentg−2 and Electric Dipole Moment at J-PARC, (2011).
4) G. A. Beer et al., PTEP 2014, 091C01 (2014).
5) S. Okada et al., RIKEN Accel. Prog. Rep. 49, 228(2015).
6) T. Matsuzaki et al., NIM A 465, 365 (2001).

∗1

AMO Physics Laboratory, RIKEN
RIKEN Nishina Center∗3 Department of Physics and Astronomy, UVic
∗4 Science Division, TRIUMF∗5 Department of Physics, The University of Tokyo∗6 Department of Physics, Chiba University∗7 IPNS, KEK∗8 IMSS, KEK∗9 Condensed Molecular Materials Laboratory, RIKEN∗10 RIKEN Center for Advanced Photonics∗2- 243 -