Differential cross sections for the coherent photoproduction of ${\pi }^0$ mesons from ${\mathrm{He}}^3\left[{\mathrm{He}}^3\right(\gamma ,{\pi }^0\left){\mathrm{He}}^3\right]$ were measured for recoil angles of 20°, 30°, 40°, and 50° and were found to be 2-8 μb/sr, 5-25 μb/sr, 17-56 μb/sr, and 17-84 μb/sr, respectively. A gaseous ${\mathrm{He}}^3$ target at 20.4°K was irradiated by 340-MeV bremsstrahlung, and ${\mathrm{He}}^3$ recoils were observed with silicon semiconductor detectors. At least one $\gamma$ ray from the pion decay was detected with a photon shower counter. A coincidence triggered an oscilloscope on which all pulses were displayed and photographed. The experimental cross sections are compared with those obtained by various impulse-approximation calculations. Although the calculations depend on the manner of specifying the energy and momentum dependence of the photon-nucleon amplitudes in the ${\mathrm{He}}^3$ nucleus, a calculation assuming that the three nucleons were initially at rest accounts for most of the measurements.

• Received 3 January 1969

DOI:https://doi.org/10.1103/PhysRev.183.873