Measurement of μSR spectra in a scintillating LaF₃ material

A new experimental method to search for T-violating transverse muon polarization ( PT ) in the K⁺ → π ⁰ μ ⁺ ν decay using an electro-magnetic calorimeter has been proposed. The test experiment to find good scintillating materials was performed at the J-PARC Material and Life Science Facility (MLF), and the residual μ ⁺ polarization in a scintillating CeF ₃ material was found to be higher than 90%. It is highly preferable to find other scintillating materials whose performance is nearly equivalent to CeF ₃ to increase effectiveness of the new polarimeter system in various experiments. A muon spin relaxation/rotation ( μ SR) experiment was performed using a 2% Ce-doped LaF ₃ crystal. The time-integrated residual polarization in LaF ₃ within the time region of 10 μ s was obtained to be 83% and 87% for LF=100 and 140 Gauss, respectively, which is high enough to perform the new T-violation experiment at J-PARC. The ZF μ SR spectra in the temperature region from room temperature to 328 K were taken to check the reproducibility of the μ SR spectra reported in the previous experiment, in which the rapid transition of the zero-field μ SR spectra was observed at 323 K. The ZF μ SR spectrum obtained at 328 K is in good agreement with that at room temperature, and an obvious change reported in the previous study [8] was not observed in the present work.