Influence of collective rotational degrees of freedom in medium energy fission

Abstract

In the present work rms angular momenta have been deduced for the fission fragments corresponding to 131Tem'g and 133Tem'g in 232Th(~zr f) and 238U(e40MeV, f) systems from the radiochemically determined independent isomeric yield ratios and statistical model based analysis. For 131Te and 133Te the rms angular momenta deduced are 5.9-t-1.0 and 7.9___ 1.2 ta respectively in z3aTh(e4oMev, f) and 7.2+_0.6 and 8.0+0.8 ra respectively in z3SU(~ 4oraev,j~-c~ Comparison of the present data with the literature data for these fragments in the same compound nuclei 236U* and 24Zpu* at lower excitation energies shows increase in the fragment angular momentum with increasing excitation energy and angular momenta of the fissioning nuclei. Fragment angular momentum deduced theoretically for asymmetric and deformed fragments on the basis of thermal equilibration of the collective rotational degrees e.g., rigid rotation, wriggling, tilting, bending and twisting modes considering the effect of multichance fission, are in good agreement with the experimental observations.

Effect of entrance channel parameters on fission fragment angular momentum in medium energy fission

“…Variance due to the contribution of tilting mode towards total fragment angular momentum is zero up to second order as shown by several authors [6,20]. Variance due to the contribution of tilting mode towards total fragment angular momentum is zero up to second order as shown by several authors [6,20].…”

“…As a result of statistical equilibration the wriggling, bending and twisting modes give rise to thermal fluctuations in fragment angular momenta at certain characteristic collective temperature T. Each of these degrees have characteristic variances as prescribed by several authors [6,7,[29][30][31]. For the wriggling mode the variance (o-~v) for a single fragment is given by [6,7] where I~, ( = Iy, ) is the moment of inertia of fragment for rotation around the x ( = y) axis for ellipsoidal shape while /__, is the same for rotation around the z or fission axis. For the wriggling mode the variance (o-~v) for a single fragment is given by [6,7] where I~, ( = Iy, ) is the moment of inertia of fragment for rotation around the x ( = y) axis for ellipsoidal shape while /__, is the same for rotation around the z or fission axis.…”

“…and (2n, y ) process as for different fragments populating 132Im" g states, were evaluated based on the calculated [22] decay widths for neutron and gamma emission and probable relative yields of respective bution. In the spin distribution calculation optical model transmission coefficients for neutrons were used and the spin cut off factor for neutron and gamma emission were evaluated appropriately [6,23] at each stage (~4.5_1.5h). The final ratio of population of spin states corresponding to m-and g-states for various B-values, averaged over various de-excitation sequences were compared to the experimentally obtained ratios of yields of m-and g-states.…”

“…Over and above all these contributions the entrance channel angular momentum as [6] contributes through rigid rotation an average angular momentum of…”

“…In medium energy fission as the fissioning nucleus has relatively higher angular momentum and excitation energy [5], statistical excitation of various collective rotational degrees e.g. wriggling, tilting, bending and twisting as well as rigid rotation [5][6][7] contribute to the fission fragment angular momentum. Thus determination of fragment angular momentum in medium energy fission might provide information about the effect of entrance channel param-eters viz.…”