A novel experimental methodology has been proposed for measuring μ + polarization utilizing scintillating materials. A critical requirement for this technique is the selection of scintillator capable of maintaining the μ + spin polarization for a duration spanning several μ + lifetimes. Hitherto, only CeF 3 and 2% Ce-doped LaF 3 (Ce) scintillating crystals have been considered viable candidates. Nevertheless, a significant limitation of these materials is that their scintillating photon yield is approximately 90% lower compared to conventional scintillators such as NaI(Tl) and CsI(Tl). This substantial reduction warrants further investigation to identify alternative materials that possess both a high scintillating photon yield and a long μ + spin relaxation time. To address this limitation, the residual μ + polarization in a TbF 3 crystal at ambient temperature was measured for the first time at the J-PARC Material and Life Science Facility (MLF). TbF 3 adopts the β -YF 3 crystal structure, which is distinctly different from CeF 3 and LaF 3 (Ce). The resultant residual polarization was determined to be 50%, a value found to be largely insensitive to applied external fields. This level of polarization is significantly high to facilitate the measurement of the μ + polarization in a wide range of experiments, thereby establishing β -YF 3 crystals as highly prospective candidates for this new experimental method.
Ide et al. (Sun,) studied this question.