3 Dynamics of Very Light Nuclei 3 . 1 Polarized - Beam and Polarized - Target Reactions 3 . 1 . 1 A Polarized Solid 3 He Target for Neutron - Transmission Experi - ments

In the past year we have completed measurements of polarized neutron–polarized He scattering in the few MeV region. Measurements of the total cross-section differences ∆σT and ∆σL have been obtained for neutrons with incident energies 1-10 MeV. The results are sensitive to the excited state structure of the alpha particle and may be compared to phase-shift predictions based on a number of analyses of the n-He system. The ∆σT results are in agreement with a recent R-matrix analysis of A = 4 scattering and reaction data, and lend support to the He level scheme derived from that analysis. The ∆σL data is currently being analyzed. The experiments were performed with the largest polarized solid 3He target ever used in a nuclear physics experiment [Kei95]. The target, which contains 0.4 moles of He, is polarized to 38% in the TUNL spin-spin polarized target cryostat by cooling the sample to 12 mK in a 7 Tesla magnetic field. Such a target is suitable for nuclear physics measurements which produce beam heating of a few microwatts and are insensitive to the large magnetic field. The density of the condensed phases of He corresponds to approximately 100 MPa of room temperature gas. In a neutron transmission experiment, the effectiveness of the target can be described by a figure of merit (Ptx3) 2 where Pt is the target polarization and x3 the target thickness in atoms/cm2. The number of transmitted neutrons that must be counted to achieve a given precision is inversely proportional to (Ptx3) . If we compare the TUNL condensed He target (Pt = 38%, x3 = 4.3× 10 atoms/cm) with a state of the art He gas target (Pt = 55%, x3 = 1.9× 10 atoms/cm), the figure of merit of the solid target is larger by a factor of 250. For example, our neutron transmission measurement would have required over 150 days to complete with a gas target. The solid He samples were grown directly from liquid He at 1.1 K. The target cell is constructed of beryllium copper alloy for strength and high thermal conductivity. To improve the thermal linkage to the solid He, the sample space is filled with 3 mm silver powder packed to 19% of the density of solid silver. The target cell is shown in Figure 3.1– 1. The density of the solid in the target was determined by the melting and freezing