Gamma-ray multiplicities in evaporation residues formed in bombardments ofNd150byNe20

The first three moments of the multiplicity distribution were deduced for the $\gamma$-ray cascades in the evaporation residues produced in bombardments of ${}_{}{}^{150}{}_{}{}^{}\mathrm{Nd}$ with ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ at four energies from 127.7 to 172.4 MeV by means of measurements of the first and higher order coincidence rates between a Ge(Li) spectrometer and eight NaI(T1) detectors. The cross sections for the production of ${}_{}{}^{160-165}{}_{}{}^{}\mathrm{Yb}$ and ${}_{}{}^{156-161}{}_{}{}^{}\mathrm{Er}$ were also measured. The average multiplicity $〈M_J〉$ and the width of the multiplicity distribution ${\sigma }_{M_J}$ for cascades via a state of spin $J$ were found to be independent of $J$. For a given product, $〈M〉$ and ${\sigma }_M$ increase with bombardment energy but for fixed energy they decrease with increasing number of emitted neutrons. The observed values of $〈M〉$ and ${\sigma }_M$ range from 11 to 32 and 6 to 9, respectively. The multiplicities $〈M〉$ in the (${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne},xn\alpha$) products are considerably lower than those for (${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne},xn$) but the widths ${\sigma }_M$ are the same for a given value of $x$. The skewness of the multiplicity distribution is negative, approaching zero as the number of emitted neutrons increases. Angular momentum distributions were deduced (a) for each product prior to $\gamma$ decay and (b) for the initial compound nuclei which lead to each product. In each case these were found to overlap extensively for several neighboring neutron numbers. The $J$ distribution in ${}_{}{}^{163}{}_{}{}^{}\mathrm{Yb}$ extends up to $J\simeq 85$ prior to $\gamma$ decay. At the higher energies $\alpha$ emission plays an important role in the deexcitation process by removing on the average an angular momentum of $\sim 10\hslash$. The present results suggest that $\alpha$ emission follows neutron evaporation. The $J$ distributions obtained are compared with the predictions of the Bass model for fusing collisions. For the lower energies excellent agreement is observed. For the higher energies the Bass model prediction exceeds the measured $\mathrm{xn}$ and $\mathrm{xn}\alpha$ cross sections at all $J$, indicating that other exit channels account for an appreciable fraction of the fusion cross section.