October - November '04 Polarized Target Run Summary
(update: Mar. 7/05)
Figure 1
An HD target, condensed on Aug. 3/04 and polarized on Aug.6/04
was loaded into the LEGS in-beam cryostat (QIBC) on Oct 29th using the new Jülich Transfer Cryostat. Data taking with the polarized target continued through Nov. 14/04, at which point the target was removed from the QIBC for empty target running which ran through to Nov. 22/04 (the start of the NSLS winter shutdown).
Thermal equilibrium NMR traces at 4°K for H and D are shown in an accompanying figure H and D NMR TE Lines.pdf. Peak signal heights are on the order of 10-5 volts-out/volts-in. The polarization average over the target is an integral of these curves. Frozen-spin NMR signals taken in a 4°K Production dewar (PD) on Oct. 28/04 are shown in H and D NMR frozen-spin.pdf where peak heights are on the order of 0.1 volts-out/volts-in. The polarizations measured just prior to the transfer of the target to the QIBC for data taking were:
P(H) = 57.2 +/-1.5% , and
P(D) = 15.0 +/-0.7%.
The polarization relaxation (decay) times, T1, are functions of the HD temperature, magnetic holding field and the concentrations of ortho-H2 and para-D2 impurities. The latter concentrations are in turn functions of the length of time (age) the HD has been frozen. By Oct. 28th the HD had been aged for 86 days and the resulting decay times in the 4°K dewar, with fields between 0.1-1.6 Tesla during H and D NMR measurements, were T1(H) = 2.8 days and T1 (D) = 0.6 days. The target was transferred to the QIBC on Oct. 29th, by which time the polarizations had dropped to 53% for H and 7% for D. (Polarization corrections for circuit non-linearities are underway, but are expected to be small.)
The QIBC maintained the target very steadily at 250 mK throughout the run. The QIBC holding field was kept at the deuterium NMR field of 0.886 T so as to be able to monitor the deuteron polarization with frequent NMR traces. Hydrogen was monitored less frequently. An accompanying plot p & d pol in QIBCb.pdf (Figure 1 above) shows the time dependence of the target polarization, with time-zero marking the point when the target entered the QIBC. With both H and D polarized, the individual contributions from H and from D nuclei can be separated from two data sets in which the polarization of either the H or the D is reversed. For this purpose the polarization of the H nuclei was flipped on Nov. 7/04 using the allowed RF transition. This transition was excited by sweeping the magnetic field at a fixed frequency. A rather wide field scan was used which actually included part of the forbidden H(D transition. Although the RF power used was not expected to be sufficient to drive the forbidden transition, the deuterium polarization in fact increased by about 14% (relative). This is likely due to the higher Q associated with the NMR coil in the QIBC.
The in-beam polarization relaxation times in the QIBC at 250 mK are very long and difficult to measure. Fits to the NMR data give approximately:
T1 (H) = 9-12 months , and
T1 (D) = 11-12 months.
These values have about 15% uncertainty. The initial values of Oct. 29/04 for the H polarization were taken at a lower RF power (-50 dBm) than the other measurements (-40 dBm). If these are excluded, the fitted T1 (H) is consistent with 10-12 months throughout the run.
After 17 days of running on polarized HD the target was transferred to the 2-4oK PD dewar for further NMR measurements. Since the target had accumulated 17 days additional aging in the solid state, the ortho-H2 and para-D2 concentrations had dropped so as to increase the relaxation times to T1 (H) = 8.5 days and T1 (D) = 6.2 days.
Two tagger and laser settings were used in this run which covered the γ-ray energy ranges from 422-242 MeV (300/333 nm laser lines) and 262-191 MeV (515 nm laser). Gamma-ray polarizations were randomly flipped (by changing the laser polarization) among seven states:
Right circular;
Left circular;
Linear at 0°;
Linear at +90°;
Linear at +45°;
Linear at -45°;
Unpolarized bremsstrahlung (laser-off).
About two-thirds of the running time was spent in the circularly polarized states. From these we obtain measurements of the differential cross section (σ), the beam asymmetry (Σ) and the two double-polarization asymmetries E and G.
|
