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The photoneutron cross section for 4He has been measured from threshold up to 47 MeV using monoenergetic photons and a high-pressure gas sample. The results agree with earlier monoenergetic-photon and certain photoneutron time-of-flight (liquid-sample) results at the lower energies, and consequently disagree with the results of other measurements at these energies. At the higher energies, however, t' he present results are essentially in agreement with all previous results.NUCLEAR REACTIONS 4He(y, n); measured 0(E"), 21 to 47 MeV, with monoenergetic photons, 4m neutron detector, high-pressure gas sample; four-nucleon system; isospin mixing; charge asymmetry.

The photoneutron cross sections for H and 'He have been measured from threshold to -25 MeV with monoenergetic photons from the annihilation in flight of fast positrons.These reactions include the two-body breakup of H and the three-body breakup of both H and He; these measurements for H are the first to span the energy region across the peaks of the cross sections. An eAicient BF3-tube-and-paraffin neutron detector and high-pressure gaseous samples were employed in these measurements.The results, when compared with each other and with results for the two-body breakup cross $ection for 'He from the literature, show that: (a) the two-body breakup cross sections for 'H and 'He have nearly the same shape, but the one for He lies lower in magnitude; (b) the threebody breakup cross section for . He lies higher in magnitude, broader in the peak region, and also rises less sharply from threshold than that for 'H; and (c) these differences between the cross sections for the breakup modes largely compensate in their sum, so that the total photon absorption cross sections for H and 'He are nearly the same in both size and shape at energies near and above their peaks. Theoretical results from the literature disagree with the experimental results to a certain extent over the entire photon-energy region for which the photoneutron cross sections were measured. Sum rule predictions also fail to reproduce the experimental results. These discrepancies constitute a challenge to the principle of charge symmetry of the nuclear force, but more complete theoretical calculations are needed to ascertain whether these discrepancies can be ascribed entirely to electromagnetic effects.NUCLEAR REACTIONS H(y, n), H(y, 2n), He(y, n); measured a(E&), threshold to -25 MeV; monoenergetic photons, high-pressure gas samples; two-body breakup, three-body breakup, charge asymmetry.

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