Photoneutron cross sections, including 0. [(y, n) +(y, Pn)l, o(y, 2n), and cr(y, 3n) for Ba and a[(y, n) +(y, Pn)] for N, were measured with monoenergetic photons from threshold to 29 MeV.The partial cross sections were determined by neutron multiplicity counting, and the average neutron energies for both singleand double-photoneutron events were determined simultaneously with the cross-section data by the ring-ratio technique. The N data, when combined with data from other laboratories, appears to show that the (y, Pn) process dominates the decay of the giant resonance in this nucleus. The giant-resonance parameters for Ba~38 are nearly the same as those for Pr, which has the same (magic) neutron number.
INTRODUCTIONPhotoneutron cross sections for Ba'" and N' were measured as part of a continuing survey to examine the influence of the characteristics of nuclei on the giant resonance. One of the key elements in the survey is "Ba'" which has a closed neutron shell (N= 82} and which differs from»Pr' ' (measured previously at this laboratory') by three protons. The question to be resolved is whether the giant-resonance parameters for these two nuclei are the same, since the giant-resonance decay is dominated by neutron emission.The Ba'" sample was in the form of Ba(NO, }, .It therefore was necessary to measure first the photoneutron cross section for N'~i n order to obtain the Ba'" cross section. (The oxygen contribution was determined from previous measure-ments. ''} Although the N' nucleus is interesting in its own right (it is self-conjugate, odd-odd, and light}, its photoexcitation and subsequent decay are very complex, involve several reaction channels, and require a much more comprehensive study than was done in the present experiment. The present contribution, however, determines several quantities of interest vital to that study, and gives an over-all view of the N' giant resonance.
