Photodisintegration ofHe3

The 90° photodisintegration cross sections for ${\mathrm{He}}^3$ were measured for incident photon energies between 8.5 and 21.5 MeV by detecting proton-deuteron coincidences in CsI(Tl) scintillators immersed in 1 atm of gas. The source of x rays was the bremsstrahlung beam of the University of Illinois 22-MeV betatron. The simultaneous output pulses from the coincidences were displayed on a dual-beam oscilloscope and photographed. The particle energies were determined by pulse-height measurement, and electrons were rejected by pulse-shape analysis. The peak cross section for the ${\mathrm{He}}^3(\gamma , p)d$ reaction, which is very sensitive to the nuclear size, occurs at 10.8±0.3 MeV and equals 0.092±0.004 mb/sr. Gaussian ground-state wave functions for ${\mathrm{He}}^3$ fail to predict the shape of the cross-section curve. The modified exponential wave function of Gunn and Irving gives a good fit for a root-mean-square nuclear radius of 1.94 F (corresponding to a charge radius of 2.1 F). The ${\mathrm{He}}^3$ charge form factor calculated with this wave function agrees very well with the electron scattering measurements of Collard and Hofstadter for small momentum transfers $q$ and deviates by 20% only at $q^2=5\mathrm{}$ ${\mathrm{F}}^{-2\mathrm{}}$. Also, some information was acquired about the competing three-body photodisintegration process by detecting proton-proton coincidences in the cases where the angle between the two outgoing protons was near either 90 or 180°.