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We report on the frozen-spin polarized hydrogen--deuteride (HD) targets forphotoproduction experiments at SPring-8/LEPS. Pure HD gas with a small amountof ortho-H2 (~0.1%) was liquefied and solidified by liquid helium. Thetemperature of the produced solid HD was reduced to about 30 mK with a dilutionrefrigerator. A magnetic field (17 T) was applied to the HD to grow thepolarization with the static method. After the aging of the HD at lowtemperatures in the presence of a high-magnetic field strength for 3 months,the polarization froze. Almost all ortho-H2 molecules were converted to para-H2molecules that exhibited weak spin interactions with the HD. If theconcentration of the ortho-H2 was reduced at the beginning of the agingprocess, the aging time can be shortened. We have developed a new nuclearmagnetic resonance (NMR) system to measure the relaxation times (T1) of the 1Hand 2H nuclei with two frequency sweeps at the respective frequencies of 726and 111 MHz, and succeeded in the monitoring of the polarization build-up atdecreasing temperatures from 600 to 30 mK at 17 T. This technique enables us tooptimize the concentration of the ortho-H2 and to efficiently polarize the HDtarget within a shortened aging time.

Monitoring the build-up of hydrogen polarization for polarized Hydrogen-Deuteride (HD) targets with NMR at 17 Tesla