Developments of Extreme μSR Conditions
at the RIKEN-RAL Muon Facility
Advanced Meson Science Laboratory, RIKEN Nishina Center,
2-1 Hirosawa, Wako, Saitama, 351-0198, Japan.
Isao Watanabe
RIKEN has constructed the RIKEN-RAL Muon Facility (RIKEN-RAL) at the
Rutherford-Appleton Laboratory (RAL) [1]. Since the muon has a self-polarized spinwith S=1/2, the muon can be used for material sciences to sense electronic states ofmaterials. The worldwide strongest double-pulsed muon beams with the wide rage ofthe momentum from 25 to 120 MeV/c are available for material sciences at theRIKEN-RAL. Muon-spin relaxation measurements (μSR) using the spin-polarized
muon beam have been carried out at the RIKEN-RAL from 1994 in collaboration withmore than 50 groups in the world. Recently, requirements to use extreme sample
conditions which are the high pressure and the low temperature with the tiny size ofsample are growing up in order to progress advanced μSR studies of molecularmaterials. In our presentation, we will report developments of these extreme sample
conditions especially for the double pulsed muon-beam available at the RIKEN-RAL
Muon Facility.
Figure 1 shows the newly developed high-pressure cell for the RIKEN-RAL Muon
Facility [2]. The high-pressure μSR setup for the pulsed muon beam has never beendeveloped in the past in any muon facilities. Thus, the developed system for theRIKEN-RAL is the “only-one” in the world. We have collaborated with thehigh-pressure group of RAL and carefully designed keeping safety conditions of the useof the high pressure system at RAL. The big advantage of the gas-pressurized system isto change the gas pressure continuously and keep the desired pressure from the basetemperature up to the room temperature. The high-pressure cell is pressurized by gas
helium up to about 3 kbar using a high-pressure pump in the first stage and is finally
pumped up to about 7 kbar using a gas intensifier. The pressure in the cell is held by anormal Bridgman clamp and piston, which are located at the rear of the cell. The
material of the cell is CuBe. The diameter of the sample space is 18 mm and the depth isabout 8 mm. The sample volume is about 2 cc. The is colled from the room temperaure
down to 1.5 K. Tyhis high-pressure system has been started to be in use from this year
and some high-pressure experiments on molecular materilas are now in progress [3].
Figure 2 show the existing fy-path setup for tiny sample. In usual cases, more than200 mg of sample is needed to achieve the measureable singnal from a sample [4]. Thisis a strong restriction in the case of use of the pulsed muon beam at the RIKEN-RAL.
The existing fly-path set up make measurements using tiny samples less than 200 mg
possible down to 5 K in conjunction with a small gas-flow cryostat. In some cases, wecoud try to use a tiny sample with aroung 50 mg in weight using this existing setup.
However, recent requirements from the study of advanced molecular is to use tiny
samples at low temperatures below 5 K. Thus, we are now preparing the similar fly-path
setup for a 3He cryostat which can be cool a sample down to 0.28 K. The design of theflt-path setup for the the 3He cryostat has been completed and the machining is now
going on. We expect to test this new setup before the spring time in 2009 and to use
with real molecular samples from the summer time in 2009.

Fig.1: High-pressure cell developed for theRIKEN-RAL Muon Facility.

Fig.2: Fly-path setup for tiny size samples.

REFERENCES[1] K. Nagamine et al., Hyperfine Interact. 87 (1994) 1091.
[2] I. Watanabe et al., Physica B (in press).
[3] Y. Ishii et al., in preparation.
[4] I. Watanabe et al., J. Phys. Soc. Jpn. (in press).