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 reactions 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 cold products after the termination of evaporation cascades of a different numbers of stages, Experimental values for the reduced neutron widths of highly excited compound nuclei were obtained from the measured cross section ratios, Evaporation reaction cross sections were well described in the framework of the statistical model taking into account shell effects according to Ignatyuk. Our present, and also earlier, investigations showed that calculations making use of one single set of the model parameters (the scaling factor to the liquid-drop fission barrier of Cohen, Plasil and Swiatecki, C = 0.6-0.7 and the ratio of the level density parameters, (a) over tilde(f)/(a) over tilde(v) = 1.0) reproduces 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 suitable for a high accuracy estimation of this value. By fitting two sets of the experimental data - the absolute cross section values and cross section ratios - we were able to independently estimate the values 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 before 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.