DECAY WIDTHS OF HIGHLY EXCITED RA COMPOUND NUCLEI

Excitation functions were obtained for the xn, pxn and alpha xn decay channels of Ra-216,Ra-218,Ra-220 compound nuclei produced in the react ions Ne-22 + Pt-194,Pt-196,Pt-198 in the excitation energy range of 40 -160 MeV. Due to the employment of three platinum isotope targets, the cross section ratios were measured with a high degree of accuracy for evaporation reaction channels resulting in the formation of fixed col d products after the termination of evaporation cascades of a differen t numbers of stages, Experimental values for the reduced neutron width s of highly excited compound nuclei were obtained from the measured cr oss section ratios, Evaporation reaction cross sections were well desc ribed in the framework of the statistical model taking into account sh ell effects according to Ignatyuk. Our present, and also earlier, inve stigations showed that calculations making use of one single set of th e model parameters (the scaling factor to the liquid-drop fission barr ier of Cohen, Plasil and Swiatecki, C = 0.6-0.7 and the ratio of the l evel density parameters, (a) over tilde(f)/(a) over tilde(v) = 1.0) re produces correctly the cross sections of the evaporation reactions in a wide range of compound nuclei extending from Bi to U. Moreover, the measured cross section ratios are exceptionally sensitive to the value of the parameter (a) over tilde(f)/(a) over tilde(v), i.e. are very s uitable for a high accuracy estimation of this value. By fitting two s ets of the experimental data - the absolute cross section values and c ross section ratios - we were able to independently estimate the value s of two principal parameters of the model, C and (a) over tilde(f)/(a ) over tilde(v). Our calculations showed that a large part of the pre- fission neutrons is evaporated by hot pre-actinide compound nuclei bef ore they reach the saddle point (50 to 100% of the total number of pre -fission neutrons are pre-saddle). (C) 1997 Elsevier Science B.V.