Photonuclear reactions inZr90

Abstract

Photonuclear reactions in $^{90}\mathrm{Zr}$ were studied in the excitation region of the giant dipole resonance. Experimental results which include all the most important reaction channels consist of cross sections integrated over angles, and for photoprotons, of additional data on angular distributions. Photoproton cross sections were determined by a least-squares-fit analysis of proton spectra resulting from a series of irradiations with bremsstrahlung of different end-point energies. In addition to the ground state $^{90}\mathrm{Zr}(\ensuremath{\gamma},{p}_{0})^{89}\mathrm{Y}_{\mathrm{g}.\mathrm{s}.}$, photoproton cross-section data for three groups of $^{89}\mathrm{Y}$ excited states are presented. Other results which consist of cross sections for reactions ($\ensuremath{\gamma},{p}_{1}$), ($\ensuremath{\gamma},{n}_{1}$), ($\ensuremath{\gamma}, 2n$), ($\ensuremath{\gamma},\mathrm{np}$), and ($\ensuremath{\gamma},n$) were obtained through different techniques based on the measurement of activation yields. It is shown that 70% of the total photoproton cross section is due to the transition to the ground, the 1.74 MeV, and possibly the 1.51 MeV states of $^{89}\mathrm{Y}$. The photoneutron cross section is dominated by the contribution involving the first excited state and all those states of $^{89}\mathrm{Zr}$ which subsequently decay to it. Experimental data are interpreted in terms of isospin splitting of the giant dipole resonance. In the total photoproton cross section, apart from a broad resonance at 16 MeV similar to the dominant structure of the ($\ensuremath{\gamma},n$) reaction, an additional resonance of approximately the same magnitude appears at 20.5 MeV. The latter resonance is attributed to the ${T}_{>}$ part of the giant dipole resonance. Its integrated cross section of about 8% of the total photonuclear cross section accounts for the main part of the expected ${T}_{>}$ strength. Neutron channels do not seem to contribute significantly to the decay of the ${T}_{>}$ resonance. It is concluded from ($\ensuremath{\gamma},\mathrm{np}$) and ($\ensuremath{\gamma},2n$) reactions that the ${T}_{>}$ resonance is superimposed on a ${T}_{<}$ background extending to higher energies.NUCLEAR REACTIONS $^{90}\mathrm{Zr}(\ensuremath{\gamma},p)$, ($\ensuremath{\gamma},n$), ($\ensuremath{\gamma},np$) and ($\ensuremath{\gamma},2n$), ${E}_{\ensuremath{\gamma}}=12\ensuremath{-}30$ MeV; measured photoproton spectra and bremsstrahlung activation yields; deduced $\ensuremath{\sigma}(E,\ensuremath{\vartheta})$ and $\ensuremath{\sigma}(E)$.