When ${\mathrm{He}}^4$ is bombarded with 28-Mev protons the dominant reaction is elastic scattering, exhibiting a strong peak in the forward direction and a slight rise in the backward direction. The dominant inelastic reaction is the inverse Butler or pickup deuteron process.

The two remaining inelastic reactions observed, ${\mathrm{He}}^4(p, 2p){\mathrm{H}}^3$ and ${\mathrm{He}}^4(p, pn){\mathrm{He}}^3$, have cross sections of 8.9±1.0 and 4.8±1.3 mb, respectively. These are an order of magnitude or more smaller than the cross sections of the dominant processes.

Of the remaining possible reactions—($p, \mathrm{pd}$), ($p, n2p$), ($p, p\gamma$)—only the last was energetically possible at the beam energy used. It was not observed. This indicates a lack of evidence for an excited level that decays principally by $\gamma$-ray emission; it also indicates that the inverse photodisintegration process, suggested by Flowers and Mandl, has too small a cross section to contribute in this energy region.

The rest-frame momentum spectrum of protons from the ${\mathrm{He}}^4(p, 2p){\mathrm{H}}^3$ reaction shows no evidence of an excited level in ${\mathrm{He}}^4$ in the energy region explored.

• Received 25 March 1957

DOI:https://doi.org/10.1103/PhysRev.107.1050