Differential cross sections for the reaction He[superscript 3] ([gamma],[rho]) H[superscript 2] from 45 to 150 MeV

Abstract

Differential cross sections for the reaction He 3 (y,p)H2 were measured for photons of 45 to 150 MeV at five angular settings: (theta p , theta d) = (30°, 142°) I (60°, 106°), (90° I 75°), (120°, 47.5°), and (135°, 34.5°) in the laboratory. At ep of 60° and 90°, [equation] was obtained for photon energies down to 40 MeV. A gaseous He3 target at about 4°K and slightly less than one atmosphere pressure was bombarded with 250 MeV bremsstrahlung. The deuteron detection system, which was a telescope of three semiconductor detectors followed by a plastic scintillator, actually defined the solid angle. The proton telescope of four plastic scintillators was large enough to intercept all protons correlated with deuterons in the deuteron telescope. Photographs were made of the pulses from the detectors as displayed on an oscilloscope whenever there was a coincidence between a proton in the. proton telescope and a deuteron in the deuteron telescope. Pulse heights were used for particle identification and energy assignment.. The theta p = 90° cross sections ranged from 12.68 Z 0.45ub/sr to 1.07 ± 0.14ub/sr as photon energy increased. These 90° cross sections join quite smoothly with results of previous experiments. The total cross section as a function of photon energy, obtained by integrating the differential cross sections, decreased from 146 Z 5~b to 14 ~ 2~b as photon energy increased from 45 to 150 MeV. Integrated and bremsstrahlung-weighted cross sections obtained from the data of this experiment combined with data of lower energy experiments and of threebody photodisintegration experiments are 58.9 ~ 4.0 MeV rnb and 2.46 plus/minus .14 mb, respectively. Both values are in reasonable agreement with sum rule calculations, and the integrated cross section is in excellent agreement with a model-independent dispersion theoretic calculation which includes effects of all multipoles of electric interaction. It was not possible to fit the total cross section with the expression derived for electric dipole transitions with the Gunn and Irving wave function for the ground state of He3 for any reasonable value of the size parameter. With the size parameter that fits the low energy data, the calculated high energy cross sections are roughly a factor of two down from the measured cross sections.