Reactions ofAr40withDy160,Dy164, andYb174

Excitation functions for nuclear reactions induced by ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ ions were measured for the reactions ${}_{}{}^{164}{}_{}{}^{}\mathrm{Dy}({}_{}{}^{40}{}_{}{}^{}\mathrm{Ar},xn){}_{}{}^{204-x}{}_{}{}^{}\mathrm{Po}$, ${}_{}{}^{160}{}_{}{}^{}\mathrm{Dy}({}_{}{}^{40}{}_{}{}^{}\mathrm{Ar},xn){}_{}{}^{200-x}{}_{}{}^{}\mathrm{Po}$, ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}({}_{}{}^{40}{}_{}{}^{}\mathrm{Ar},xn){}_{}{}^{214-x}{}_{}{}^{}\mathrm{Ra}$, and ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}({}_{}{}^{40}{}_{}{}^{}\mathrm{Ar},pxn){}_{}{}^{213-x}{}_{}{}^{}\mathrm{Fr}$. For all of the systems studied, the ($\mathrm{Ar},xn$) reactions only make up a small part of the total reaction cross section of ∼2 b; the largest cross sections encountered in each system were (at the peaks of the respective excitation functions) 30 mb for ${}_{}{}^{164}{}_{}{}^{}\mathrm{Dy}(\mathrm{Ar},5n)$, 10 mb for ${}_{}{}^{160}{}_{}{}^{}\mathrm{Dy}(\mathrm{Ar},4n)$, and 5 mb for ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}(\mathrm{Ar},\frac{4}{n}-5n)$. The probabilities $P_{\mathrm{xn}}$ of neutron emission in the compound systems ${}_{}{}^{204}{}_{}{}^{}\mathrm{Po}$ and ${}_{}{}^{200}{}_{}{}^{}\mathrm{Po}$ were found to be very different, with respective maximum probabilities for the emission of four, five, and six neutrons of 0.064, 0.035, and 0.016 for ${}_{}{}^{204}{}_{}{}^{}\mathrm{Po}$, and 0.010, 0.0025, and 0.0003 for ${}_{}{}^{200}{}_{}{}^{}\mathrm{Po}$. Calculations performed with a statistical-model code, which includes angular-momentum effects and fission competition, are able to reproduce the shapes and magnitudes of the experimental excitation functions, although there is a systematic energy difference, ∼10 MeV, between theory and the data. These model-dependent analyses describe in detail how the particle evaporation and fission deexcitation modes vary with angular momentum and excitation energy. The observed large differences in $P_{\mathrm{xn}}$ values for ${}_{}{}^{204}{}_{}{}^{}\mathrm{Po}$ and ${}_{}{}^{200}{}_{}{}^{}\mathrm{Po}$ are seen to arise from the small difference, ∼1.2 MeV, between the values of $S_n-B_f$, the difference of neutron-separation and fission-barrier energies, in each compound system.