Angular Momentum Associated with Slow-Neutron Resonances

Abstract

Available data on the statistical weight factor $g$ for $l=0\mathrm{}$ neutron resonances are summarized and the reliability of each result is discussed. There are reliable measurements for seventeen resonances which lie between 0 and 50 kev. In many cases, the results were obtained in two or more independent experiments. In sixteen cases, the compound nucleus has the spin $J=I+\frac{1}{2}$, while in only one case the spin is $J=I-\frac{1}{2}$. No data are available for weak resonances. Despite the limited data, it is strongly indicated that the compound nucleus is formed preferentially in the $I+\frac{1}{2}$ spin state. At present there is no basis for deciding whether this result arises from a difference in the level spacing $D$, or a difference in the average neutron width ${{\overline{\Gamma }}_n}^0$ for the two possible spin states. Regardless of the source of this effect it is apparent that the frequent assumption—that the strength function $\frac{{{\overline{\Gamma }}_n}^0}{D}$ is the same for the two spin states—may not be correct.