Photoneutron Cross Sections inMg24,Mg25,Zr90,Zr91

The variation of $\sigma (\gamma , n)$ with $E_{\gamma }$ from threshold to 24 Mev has been measured by detecting neutrons produced from betatron bremsstrahlung irradiation of separated isotope samples in oxide form. Despite the large threshold differences (9.3 Mev for the Mg isotopes and 5.0 Mev for the Zr isotopes) the peaks of the giant dipole resonances are very close for each pair of adjacent isotopes: ${\mathrm{Mg}}^{24}$, 19.5 Mev; ${\mathrm{Mg}}^{25}$, 20.3 Mev; ${\mathrm{Zr}}^{90}$, 16.4 Mev; ${\mathrm{Zr}}^{91}$, 16.3 Mev. The half-widths of the Zr resonances are unusually small, 4.1 and 5.4 Mev, compared to 8 Mev for nearby nuclei observed previously. This is apparently a "magic number" effect.