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The neutron yields resulting from photoneutron reactions have been measured as a function of betatron energy from threshold to 23 Mev for 22 elements. These yield curves have been analyzed by the photon difference method to give the total cross section 0 = (cr&, "+0&,»+2cr&, 2") as a function of photon energy.Whenever possible, this cross section has been resolved into its components using threshold considerations and subsidiary data. Some general conclusions based on the new results are presented regarding the systematics of (y, n) reactions. Based on part on a thesis presented for the Ph. D. degree at the University of Saskatchewan by R. Montalbetti. f On leave of absence from Departamento de Fisica, Universidade de Sao Paulo, Sao Paulo, Brazil. 'Other neutron emitting cross section are energetically forbidden in the energy region covered in this experiment. s J. Goldernberg and L. Katz, LPhys. Rev. 89, 1300 (1953)g discuss the validity of such subtractions. ' G.

A method is presented for the computation of photonuclear cross sections from their X-ray activation or yield curves. It is based on the photon differences between successive Schiff representations of bremsstrahlung spectra and assumes that the activation curves have smooth first and second derivatives. Artificially prepared activation curves have been satisfactorily solved by this method, and the published photonuclear activation curves determined in this laboratory have been reanalyzed. The resulting cross section curves are in essential agreement with those originally determined; however the new values are believed to be more reliable. New constants for two proposed relationships between (γ, n) "resonance" peak energies and atomic mass number have been determined. Appended to the paper are tables of the Schiff bremsstrahlung spectra for maximum photon energies between 8 and 28 Mev. and also of special functions for cross section computations between the same energies.

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