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In the neutron p-resonant absorption of $^{139}$La, it is known that the Parity Non-Conservation effect (PNC) is enhanced by a factor of 10$^{6}$ compared to the nucleon-nucleon scattering. According to recent experiments with neutron-gamma reactions in $^{139}$La, it is highly possible that violating effects of the Time-reversal symmetry is also amplified with the similar mechanism of the PNC [1]. These results show that $^{139}$La has the potential as a powerful probe for the discovery of unknown CP-violations. The NOPTREX collaboration plans to explore the T-violation effect using polarized the $^{139}$La targets realized by the dynamic nuclear polarization (DNP) as a first attempt. In the T-violation search, the LaAlO$_3$ single crystal doped with Nd$^{3+}$ is a good candidate as a target material because the crystal with the Nd$^{3+}$ concentration of 0.03 mol % has shown the $^{139}$La polarization of about 50 % at less than 0.3 K in 2.3T [2]. To apply it, it is necessary to investigate the spin-lattice relaxation time (T1) of $^{139}$La in the magnetic field of about 0.1 T because an external field for the polarized target is likely to be such a low field for reducing systematic effects due to pseudomagnetic rotation. We have studied the relaxation of $^{139}$La and $^{27}$Al with the 0.03 mol % crystal and concluded that longer T1 is necessary for the T-violation experiments [3]. To overcome the problem, it is key to optimize the Nd$^{3+}$ concentration since a low Nd$^{3+}$ amount not only makes the T1 longer, but also decreases the efficiency of the DNP. Recently, we have grown crystals with precise control of the Nd$^{3+}$ concentration by ourselves and observed the enhancement of the $^{139}$La polarization using these crystals. In the crystals of Nd$^{3+}$ concentration at 0.01 mol %, the $^{139}$La polarization of about 20 % has been confirmed in a 2.335 T magnetic field and 1.3 K temperature, which is almost the same as the polarization achieved under the same condition in the past experiment. Especially, it has been found that the T1 is longer than the one of the 0.03 mol % crystal. In this presentation, we report the present status of the research and development of the polarized $^{139}$La target and discuss the feasibility of the T-violation searches based on the results of the DNP experiments with the Nd$^{3+}$: LaAlO$_3$ crystals.
[1] T. Okudaira et al., Phys. Rev. C 97, 034622 (2018), T. Yamamoto et al., Phys. Rev. C 105, 039901 (2022)
[2] P. Hautle and M. Iinuma, Nucl. Instrum. Meth. Phys. Res. A 440, 638 (2000).
[3] K. Ishizaki, et al., Nucl. Instrum. Meth. Phys. Res. A 1020, 165845 (2021).
| Category | Polarized Targets |
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