Dipole and Quadrupole Transition Probabilities in Neutron-Capture Gamma Radiation

Abstract

An analysis of the intensities of neutron-capture $\ensuremath{\gamma}$ rays in even-charge nuclei shows that at high energies the emission probability of $E1$ radiation is greater than that of any other multipole order. This conclusion is supported by additional evidence from odd-charge nuclei. In three nuclei (${\mathrm{Mg}}^{25}$, ${\mathrm{Si}}^{29}$, and ${\mathrm{S}}^{33}$) a direct comparison shows that (at the same energy) the emission probability of $E1$ is 200 times greater than that of $M1$ radiation. The rate of emission of $E2$ radiation has been compared directly with $E1$ radiation in only one instance, viz., ${\mathrm{Mg}}^{25}$, where (at 7 Mev) it was found to be lower by a factor of 2000. Further evidence is adduced to show that this ratio is not exceptional and that the rate of emission of $E2$ radiation (at 7 Mev) is less than that of $M1$ radiation. The absolute rates of emission for $E1$ and $M1$ $\ensuremath{\gamma}$ rays are evaluated in those instances where the radiation width of the capturing state is known. When corrected for the level spacing near the initial state (and for the nuclear radius, in the case of $E1$ radiation), the rates of emission are remarkably constant; they are independent of the nuclear charge and mass over a range where the level spacing may vary by a factor of ${10}^{4}$ or more. The emission rates of $E1$ and $M1$ radiation are generally ten times lower than those predicted by the formula of Weisskopf, which is based on the independent-particle model. The emission rates do not exceed those expected from that formula in the case of the exceptionally strong $M1$ ground-state $\ensuremath{\gamma}$ rays from ${\mathrm{F}}^{20}$ and ${\mathrm{Al}}^{28}$. It is shown that the identification of the spins and parities of excited states in many nuclei can be made on the basis of intensity measurements. Finally, the influence of closed shells on the $\ensuremath{\gamma}$-ray spectra is discussed.