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Description
The relaxation times of protons and deuterons in a frozen-spin hydrogen-deuteride (HD) target are more than 1 year under the normal experimental conditions (T ~ 0.1 K and B ~ 1 T). These targets have been used successfully for photoproduction experiments both at CLAS in Jefferson Lab (JLab) and at LEGS in Brookhaven Lab. In order to explore its performance under electron beams, a series of polarization measurements with 9.7 MeV and sub-nanoAmp electron beam currents were conducted at the newly commissioned Upgraded Injector Test Facility (UITF) at JLab. Since the deposited energy on a target is almost independent of beam energy, the UITF beam can be used to simulate the GeV level performance. During the eHD tests, the 2 frozen-spin targets, with initial proton-polarizations of 40% and 34%, were used in an in-beam dilution refrigerator which held the HD target at ~0.1 K and ~ 1 T, and proton polarizations were tracked with a field swept NMR polarimeter while exposed to beams under varying conditions of current, dose, beam duty factor and temperature. At a fixed accumulated dose, the spin-relaxation rates drop with current, suggesting depolarization by the charge cloud of the beam. After an accumulated dose of ∼2 µC/cm2, beam-off spin-relaxation rates drop from their super long initial values to the order of weeks (with beam-on T1 values typically an order of magnitude shorter), reflecting a buildup of paramagnetic charge centers within the HD lattice. The accumulated polarization loss was approximately proportional to dose in both targets, dropping to 1/e of their initial values after ∼6 µC/cm2. Thermal equilibrium polarizations of targets not in the frozen-spin state (with intentionally short T1) have been used to deduce the in situ temperature of solid HD while under electron bombardment. A model for depolarization by beam-associated paramagnetic impurities largely accounts for the data, and suggests that improvements in heat removal could lead to significant increases in the in-beam T1.
| Category | Polarized Targets |
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