Asymmetry of mass and charge division in spontaneous fission |
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Authors: | P P Chakraborty D N Sharma M R Iyer A K Ganguly |
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Institution: | (1) Health Physics Division, Bhabha Atomic Research Centre, 400 085 Bombay |
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Abstract: | The order-disorder model has been used to explain asymmetry of mass and charge division and related phenomena in fission.
According to this model the fission process involves two steps consisting of charge polarisation into two ‘impending fragment
clusters’ with beta stable neutron numbers and subsequent distribution of the balance neutrons between the two. Mode of elemental
division of the fissioning nuclei is attributed to the charge polarisation in the first step. Theory of reaction rate has
been applied to the system.
The frequency term is obtained by applying the conditional stochastic process under charge polarisation constraint and the
energy-dependent term is given by the condition of minimum in free energy of the system. Using this, the relative probability
of polarisation into given charge pair is arrived at.
The model uses stable neutron numbers for the charges as the only input. No explicit assumption or quantification on the preference
of formation of shell closure species in fission is necessary. The statistics developed on the principle of equala priori probability of all charge polarisation is used. The shell effects come into play only in deciding a stable neutron number
for the charges. The total isotopic yield distribution for a number of fission reactions shows asymmetry in the actinide region
which reduces with increasing mass/charge of the fissioning nuclide and bunching of the higherz peaks. Although the mass yields obtained therefrom for a number of fission reactions agree with experimental results, the
peaks of the distributions are slightly shifted away from the symmetric point and the distributions are somewhat narrower.
Charge distribution parameters obtained from these results are also presented. |
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Keywords: | Spontaneous fission order-disorder model charge polarisation stable neutron number total isotopic yield asymmetry of mass and charge division charge distribution |
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