C. Steven Whisnant

Measuring the relative concentration of H2 and D2 in HD gas with gas chromatography

Frozen-spin polarized hydrogen deuteride (HD) targets are being prepared for photonuclear experiments at Jefferson Lab. The targets are frozen HD gas. To polarize and achieve the required spin-lattice relaxation times, the targets are made from high purity HD gas in which the residual H(2) and D(2) concentrations are known. To determine these residual concentrations, a gas chromatograph is used. The separation of the gas components is done in a column cooled to ≈120 K by immersing it in a mixture of liquid nitrogen and isopentane and using neon as the carrier gas. The different hydrogen isotopes have different transit times through the column and their arrival is registered by a thermal conductivity detector. The peaks in the chromatograms are fit using an exponentially modified Gaussian line shape to extract the areas of these peaks. The ratios of areas are corrected for differences in thermal conductivity to give the relative concentrations. Here, H(2) and D(2) concentrations on the order of a few percent have been measured with uncertainties of less than ± 5%. The linear response of the system is confirmed to within about ± 6.3%.

Brute Force with a Gentle Touch: Vibration Isolation Techniques Used to Increase HD Target Polarization

The performance of statically polarized high‐field/low‐temperature targets is a strong function of the base temperature during polarization. At the Laser‐Electron Gamma Source (LEGS) facility, highly polarized Hydrogen Deuteride targets are created in a dilution refrigerator/15 tesla superconducting magnet system, and converted to a frozen spin state. This allows them to retain polarization when placed in a beam at a lower field (0.7 T) and higher temperature (1.3 K). An increase in temperature from the 0 T state to the 15 T state of the refrigerator suggested eddy currents were primarily responsible for heating of the cold finger. Vibration‐isolation techniques have been developed to reduce the level of eddy currents due to vibration inside the polarizing field. These techniques reduced the amplitude of vibration due to the pumping system by two orders of magnitude and lowered the cold finger temperature with field energized from ∼ 17 mK to ∼ 12 mK. The potential gain in polarization is substantial.

Status of Frozen‐spin Polarized HD Targets for Spin Experiments

The first experiments have been carried out with polarized HD targets at the Laser‐Electron‐Gamma‐Source (LEGS) facility. By holding targets at low temperature and high field (17 mK and 15 Tesla) in a Dilution Refrigerator (DF) for six weeks a frozen‐spin state was reached, with equilibrium polarizations for protons and deuterons of 70% and 17%, respectively. Multiple measurements of the relaxation times and multiple transfers of the targets reduced these values so that experimental runs were carried out with polarizations of 30% and 6%, respectively. The relaxation times for protons and deuterons were observed to be 13 days and 36 days, respectively, in the beam line cryostat at 1.3 K and 0.7 Tesla magnetic field. For the future runs significantly higher D polarizations are possible by transfer of spin from the proton to the deuteron using an rf forbidden adiabatic fast passage. Higher polarizations and longer relaxation times are expected from ongoing development.