April-May'05 Polarized Target run: summary and sample results

An HD target (#T4), was condensed on Dec. 10/04 and polarized on Dec.12/04. Following H₂ calibration and empty cell measurements, HD target T4 was loaded into the LEGS in-beam cryostat (QIBC) on April 5^th/05 using the Juelich Transfer Cryostat. The target polarizations at this point were,

P(H) = 61.0 % ,
P(D) = 16.5 % .

The polarization relaxation times (T₁) were very long; no attempt was made to measure them at this stage. Polarization was then transferred from H -> D on April 7^th by saturating the forbidden H-D transition with FM modulated RF in the QIBC. The resulting target polarizations were,

P(H) = 30.0 % ,
P(D) = 31.5 % .

The second π⁰ production run with the polarized HD target began on April 8^th and continued until the NSLS Spring shutdown on May 9^th/05.

The QIBC maintained the target very steadily at about 250 mK throughout the run. The QIBC holding field was kept at the deuterium NMR field of 0.89 T. The target polarizations were monitored twice daily using a λ/2 NMR resonance for D and a 7λ/2 resonance for H which occurred at approximately the same field.

Figure 1 above (HD4_spr05_pol_QIBC.pdf) 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 April 26^th/05 using the allowed RF transition, excited by sweeping the magnetic field at a fixed frequency.

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 months , and
T₁(D) = 12 months.

If the NMR data are broken up into two groups (before and after April 26^th), the value of T₁(D) is observed to increase from 9 months to 16 months as the run progressed. Such effects are expected due to the continued decay in the concentration of para-D₂ impurities.

After a month of running on polarized HD the target was transferred to the 2°K PD dewar for further NMR measurements, and then transferred back to the dilution refrigerator on May 10^th.

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.

Sample results:

Missing energy-difference spectra for π⁰ production from polarized deuterium are shown in Figure 2 below (D(g,pi0) ediff 341MeV dUV1_Aug05.pdf) for a beam energy at the peak of the P₃₃ delta resonance for the two helicity states. Recoil neutrons were demanded in coincidence to identify the D(γ,π⁰n) channel. While the Kel-F target cell contains aluminum cooling wires, the contribution of the cell walls and the Al wires are sampled in runs with the HD removed. These represent only a small fraction of the pion yield.