The analysis of (n,α) and (p,α) reactions discussed in this paper shows the existence in heavy nuclei of 4-nucleon clusters, which can be excited and ejected as anα-particle in a pre-compound emission mechanism.
A correct description of this effect can be obtained when these clusters are thought of as single particles with level density given byg/4.
The probability of the existence of these clusters is represented by the parametersϕ, which takes values between 0.8 and 0.1.
An interesting aspect ofϕ is its correlation with the mass numberA. This seems to show that the value ofϕ depends on the collective properties of nuclei more than on the shell model configuration.
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