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Two-Body Photodisintegration of 3He at Lund (Feldman, O'Rielly, Berman, Briscoe, Strakovsky) The 3H/3He experiment at SAL helped to establish a new collaboration with the photonuclear group at Lund. Martin Karlsson, a student from Lund, participated in that run at SAL and made many valuable contributions to the experiment. His thesis experiment (which involves the GW group) will be an extension of the two-body photodisintegration of 3He at photon energies down to 12 MeV (below the 17-MeV lower limit of the SAL data). The peak of the photodisintegration cross section is at about 12 MeV, which is below the energy range accessible using the SAL tagger. Moreover, the largest discrepancies in previous data sets have occurred in this lower-energy region (as can be seen in Fig. 1 above). These data would also provide an independent cross-check of the 3He results obtained at SAL, since the data will effectively overlap in the energy range 17-32 MeV. In addition, the potential dependence in the theory is manifested more strongly at the lower energies [San98] (see Fig. 1), so these data will be useful in discriminating between various NN potentials. This experiment used tagged photons in the range Eg = 12-32 MeV and involved not only the same solid-state detector telescopes as used in the SAL experiment, but also a novel Bragg-PPAC detector system which was designed by Russian collaborators [Kot99] and which has been optimized for detection of low-energy deuterons. The Bragg chamber is a gaseous ionization chamber with a Frisch grid – electron collection is carried out along the direction of the incident particles. Since the ionization density distribution follows the energy-loss distribution along the particle track, the charge collected at the anode generates a time-dependent pulse height representing the ionization density as a function of position – the Bragg curve. For particles that stop in the chamber, this curve has a maximum near the end of the range, the Bragg peak. Thus, if a signal is read out using a flash ADC, a complete history of the ionization density is provided, allowing a determination of the energy, the Bragg peak and the range of the incident particle. By running at Lund with the same solid-state detector system as used at SAL, we obtain a consistency check between the two analyses (Lund and SAL) in the energy overlap region (17-32 MeV). Furthermore, by running the solid-state detectors simultaneously with the Bragg chamber, we are able to link the normalization of the Bragg data to the known geometry of the solid-state detectors used both in Lund and at SAL. In this manner, all of the data can ultimately be tied together and provide powerful internal-consistency checks on the absolute cross sections obtained from these two independent experiments. The data were obtained in 2000-01 and early results were presented at a Lund workshop [Kar01]. Preliminary cross sections have been obtained for the data from the solid-state telescopes and are within 5-7% of the SAL data, consistent with statistical uncertainties. A separate Monte-Carlo simulation of the detector geometry is being developed that will give independent information on the acceptance of the solid-state telescopes. Then the final Lund cross sections (obtained virtually independently) can be directly compared with the SAL cross sections for 3He. In addition, data were obtained on the 2H(g,p)n reaction to serve as a further consistency check, as was done in the SAL experiment. These deuterium photodisintegration data are currently being analyzed. Preliminary results for the Lund 3He(g,d)p experiment will be presented at the upcoming LOWq Workshop [Kar03]. |
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