Measurement of nuclear spin relaxation time in lanthanum aluminate for development of polarized lanthanum target.

A polarized lanthanum target applicable to long-term beam experiments is being developed for the study of spin-dependent nuclear interactions and the search for the violation of time-reversal symmetry (T-violation) in the spin optics of polarized epithermal neutrons. In addition, a large nuclear polarization away from the thermal equilibrium condition is preferred to flexibly adjust the external magnetic field for maximizing the sensitivity to the T-violation by appropriately controlling the neutron spin rotation, which requires a sufficiently slow relaxation of the nuclear polarization. Therefore, the relaxation time of nuclear polarization ($T_1$) is an essential parameter for our applications. In the present study, a single crystal of lanthanum aluminate doped with neodymium ions is chosen as the target material, assuming the application of the dynamic nuclear polarization technique. We measured the $T_1$ of $^{139}{\rm La}$ and ${}^{27}{\rm Al}$ by observing the nuclear magnetic resonance in the temperature range of $0.1$-$1.5$ K under a magnetic field of $0.5$-$2.5$ T. The results show that $T_1$ has a distribution of up to 100 h, which can be explained by assuming the electron spin-spin reservoir. We estimate $T_1 \sim$ 1 h at $0.1$ K with a magnetic field of $0.1$ T, which is one of possible conditions for the T-violation experiment. Possible improvements to achieve a longer $T_1$ that is desirable for the T-violation experiment are discussed.