Development of polarized La nuclear target for T-violation search

In compound nuclear states formed by resonant absorption of a low-energy neutron to a lanthanum-139 nucleus, the parity non-conservation effect (P-violation) is amplified by a factor of 10$^6$ via mixing process of S and P wave. The time reversal asymmetry (T-violation) is also expected to be amplified in the same process. The NOPTREX collaboration has a plan of the T-violation search using a polarized lanthanum-139 and a polarized neutron as the first choice. A single crystal of LaAlO₃ doped with Neodymium ions as paramagnetic centers is a good candidate as target material since the crystal with neodymium concentration of 0.03 mol% has shown about 20% polarization at 1.5 K in 2.3 T by using the dynamic nuclear polarization (DNP) technique. The T-violation experiment requires to maintain high polarization of lanthanum in a low magnetic field (~ 0.1 T). 

Our previous studies with the 0.03mol% crystal have estimated longer relaxation time than 1 h at 0.1 K in 0.1 T, so that the DNP tests of various crystals with different neodymium concentrations are necessary. Thus, we have focused on following three issues, one is to establish the way of crystal growth for preparation, the second is to construct the new DNP system for the neodymium optimization, and the third is a heat simulation for determining a crystal dimension, which is necessary for a design of target cryogenics.

We prepared two kinds of crystals with 0.05mol% and 0.01mol% based on the floating zone method at Tohoku University and evaluated them with the simplified DNP system at Yamagata University. The 0.01mol% crystal showed an unsaturated polarization of about 20% in 1.5 K and 2.3 T. We also prepared the DNP system at RCNP, which has the 1.5 K cryogenics capable to continuous operation for a long time. This system allows us to measure a saturated polarization and a long relaxation time.

In this talk, we report the present status and future prospects based on the results of the DNP tests and test operation of the new DNP system.