Energy dependence of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>p</mml:mi><mml:mo>+</mml:mo><mml:mmultiscripts><mml:mi>Th</mml:mi><mml:mprescripts/><mml:none/><mml:mn>232</mml:mn></mml:mmultiscripts></mml:mrow></mml:math> fission mass distributions: Mass-asymmetric standard I and standard II modes, and multichance fission
A. C. Berriman, D. J. Hinde, D. Y. Jeung, M. Dasgupta, Hiromitsu Haba, T. Tanaka, K. Banerjee, Tathagata Banerjee, L. T. Bezzina, J. Buete, K. J. Cook, S. Parker-Steele, C. Sengupta, C. Simenel, E. C. Simpson, M. A. Stoyer, B. M. A. Swinton-Bland, E. Williams
Abstract
Background: The predominant mass-asymmetric fission of actinide nuclides occurs mainly through the so-called standard I and standard II modes. Though understood to be caused by shape-dependent shell structures encountered between the fission barrier deformation and scission, the most relevant shell gaps are still not firmly established. The standard I mode had been associated with the spherical doubly magic $^{132}\mathrm{Sn}$, and thus the $Z=50$ proton shell, but recently it has been proposed that standard I and standard II are associated with quadrupole and octupole deformed gaps at $Z=52$ and 56, respectively.Purpose: We investigate how the relative probabilities of the standard I and standard II modes vary with excitation energy near threshold, probing where the two modes bifurcate.Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer have been used to measure fission mass distributions for the $p{+}^{232}\text{Th}$ reaction (forming $^{233}\mathrm{Pa}$) at closely spaced bombarding energy intervals from 6.5 to 28 MeV.Results: A model-independent analysis of the energy dependence of the shape of the mass-asymmetric peak shows a strong dependence of the standard I and standard II relative probability on excitation energy near threshold.Conclusions: The results are consistent with the standard II mode having a lower fission barrier than standard I in $^{233}\mathrm{Pa}$, with the latter increasing continually in relative probability above its barrier energy. It is concluded that multichance fission, in particular last chance fission, plays a strong role in determining the observed energy dependence of all fission modes.