Excitation Functions, Recoil Ranges, and Statistical-Theory Analysis of Reactions Induced inZr90with 25- to 80-MeV Helium Ions

Excitation functions and mean recoil ranges have been measured for radioactive products resulting from the helium-ion bombardment of natural zirconium and targets which were isotopically enriched in ${\mathrm{Zr}}^{90}$. Helium-ion energies varied between 25 and 80 MeV. Products observed were ${\mathrm{Mo}}^{90}$, ${\mathrm{Nb}}^{90}$, ${\mathrm{Zr}}^{89}$, ${\mathrm{Zr}}^{88}$, ${\mathrm{Y}}^{88}$, ${\mathrm{Zr}}^{87}$, ${\mathrm{Y}}^{87}$, ${\mathrm{Zr}}^{86}$, and ${\mathrm{Y}}^{86}$. The excitation function was also measured for the production of ${\mathrm{Y}}^{87}$. The recoil ranges are compared with the range theory of Lindhard, Scharff, and Schiott to determine the energy ranges over which the production of the nuclide is consistent with major contributions from a compound-nucleus mechanism. Excitation functions for products with $A\ge 87$ were compared with the statistical model to investigate the influence of shell structure on level densities. The level-density models used were those due to Newton and to Rosenzweig in addition to the standard Fermi-gas level density. Rosenzweig's model gave the best over-all agreement with the experimental results, with the standard Fermi-gas level density a close second. Several excitation functions were calculated using the $s$-wave approximation to estimate the effect of $\gamma$-ray de-excitation due to angular-momentum restrictions. Threshold position and excitation-function widths were found to agree quite satisfactorily with experimental values when this was done.