Differential cross sections for the reaction 4He(γ, n)3He have been measured at laboratory angles of 53°, 68°, 83°, 98°, 128°, and 143° for excitation energies between 22 and 32 MeV by the photoneutron time of flight technique. The photoneutron spectra were used to obtain the angular distribution coefficients in the Legendre series expansion for the differential photoneutron cross sections. Analysis of these coefficients indicates that the photodisintegration process in this nucleus, while dominated by E1 photon absorption followed by channel spin zero (no spin flip) p wave neutron emission, is affected by E1 channel spin one and E2 contributions. In the higher energy region (E > 27 MeV), the amount of E2 absorption observed was ~6% of the total and is dominated by the channel spin one E2 matrix element. The total cross section obtained from the measured differential cross sections yields a value of σ(γ, p)/σ(γ, n) near unity, in agreement with a calculation of this ratio which requires no charge symmetry breaking component of the nuclear force.

The 98' differential photoneutron cross section for 4He has been measured for excitation energies between 22 and 33 MeV with a liquid 'He target at its normal boiling point and for excitation energies between 23 and 37 MeV with a 4He gas target at a pressure of 51.6 bars. The cross-section values obtained with the gas target were approximately a factor of 1.9 greater than those obtained with the liquid target. This apparent dependence of the cross section upon the physical state of the He target may explain the large values for the ratio cr(y, p)/cr(y, n) obtained by comparing photoneutron and photoproton cross sections obtained with targets in different physical states.

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The spectra of deexcitation gamma rays following photodisintegration of 19F and 31P have been measured to study further the reaction channels involved in photodisintegration of nonclosed shell nuclei. Besides (γ,n) and (γ,p) decays to excited states of the residual nuclei, strong (γ,α) channels from 19F to the first excited state of 15N and from 31P to the first two excited states of 27Al were observed.

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