Implications of the Photonuclear Effect inZr90

The reactions ${\mathrm{Zr}}^{90}(\gamma , n){\mathrm{Zr}}^{89}$ and ${\mathrm{Zr}}^{90}(\gamma , n){\mathrm{Zr}}^{89m}$ have been studied in detail. The threshold for ${\mathrm{Zr}}^{90}(\gamma , n){\mathrm{Zr}}^{89m}$ is 12.37±0.09 Mev; that for ${\mathrm{Zr}}^{90}(\gamma , n){\mathrm{Zr}}^{89}$ is 11.78±0.09 Mev. A discussion is given of the precision of ($\gamma , n$) threshold determinations and of the error sometimes introduced because of large spin differences. The detailed shape of the ($\gamma , n$) cross section is used to obtain data on both the absorption mechanism of gamma rays and the competition between the emission of neutrons and gamma rays by a compound nucleus. At about 12.4 Mev in ${\mathrm{Zr}}^{90}$, less than 60% of the absorption occurs by the $E1\mathrm{}$ process if $E2\mathrm{}$ is the only other absorption process. If both $M1\mathrm{}$ and $E2\mathrm{}$ absorption occur equally, only about 10% of the absorption is $E1\mathrm{}$. The competition between gamma ray and neutron emission could not be obtained quantitatively but the results are consistent with other quite different measurements of this competition.