Photoneutron Cross Sections forZr90,Zr91,Zr92,Zr94, andY89

Abstract

Photoneutron cross sections, including σ[(γ, n)+(γ, pn)] and σ(γ, 2n) for Zr90, Zr91, Zr92, Zr94, and Y89, and σ(γ, 3n) for Zr94, have been measured as a function of photon energy up to 30 MeV with a monoenergetic photon beam from positron annihilation in flight. The photon energy resolution varied from less than 300 keV at the lowest to 400 keV at the highest energies. The average neutron energy was determined simultaneously with the cross-section data. Nuclear information extracted from the data includes giant-resonance parameters, integrated cross sections, level densities, and threshold (and hence mass) values. In particular, it is found that a Lorentz-shaped curve gives an accurate representation of the total photoneutron cross section near the peak of the giant resonance, although in several cases there is important structure about 5 MeV above the giant resonance. The variation with neutron number and atomic weight of these quantities is studied for these nuclei near the N=50 closed neutron shell.