J.D. Bierman, J.P. Lestone, J.F. Liang, D.J. Prindle A.A. Sonzogni and R. Vandenbosch

In the previous article we have discussed some of the results of a collaborative experiment with a group from Bombay to measure fission fragment anisotropies. As part of that collaborative effort we measured anisotropies to bombarding energies considerably higher than for the data we discussed in the preceding report. These results are shown by the data points in Fig. 3.9-1. Here we discuss the interpretation of these higher energy anisotropies. We have noted that the anisotropies seem to scale with the mean square angular momentum. To demonstrate this we have arbitrarily taken the ¹²C + ²³⁶U data as a reference line (dashed), and scaled W(180)/W(90) -1 values by < > to get the full and dotted lines for the other systems in the left panel of the figure. This simple scaling seems to adequately account for the difference in anisotropies for the different systems. This is expected to be the case for that fraction of the partial wave distribution where there is a finite fission barrier. The surprise is that the remaining fraction also scales as < >. This scaling is not expected in one model of non-equilibrium fission¹ where the anisotropy of this fraction is assumed to be independent of < >. This latter assumption has also been tested by finding the anisotropy of the non-equilibrium fraction required to account for the ¹²C + ²³⁶U anisotropy and using this anisotropy for the non-equilibrium fraction for the other systems. The results are shown in the right hand panel of the figure, and are seen to be incompatible with the data.

What might be the origin of the scaling of the anisotropies with < > for values for which the fission barrier vanishes? One possible interpretation is that the anisotropy for these partial waves is determined at some shape of the fissioning nucleus in its evolution towards scission which is the same for all entrance channels.

Fig. 3.9-1. In a) the full and dotted curves are obtained by < > scaling. In b) it is assumed that the anisotropy is independent of < > for that fraction of the cross section for which the fission barrier is less than 0.4 MeV.