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