Why does the cross section show anomalous peaks over arange of energies known as resonance region in neutron radiative capture process?
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Do you know what a nuclear energy level diagram looks like, and what it means? – Jon Custer Oct 02 '16 at 18:01
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i know what it looks like but i may not be very good at interpreting it. – Sabeeka Nazeer Oct 02 '16 at 18:20
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Yes, they are a bit complex at first. The point is, it will show you the appropriate reactions and energy levels in the nucleus that result in the larger-than-expected cross section. – Jon Custer Oct 02 '16 at 22:14
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I would appreciate if you could view this post as an attempt at an answer, as I wrote it to try to learn more about the subject.
The probability of neutron radiative capture is represented by the radiative capture cross section $\sigma_\gamma$. The cross-section can be divided into three regions according to the incident neutron energy.
- 1/v Region
- Resonance Region
- Fast Neutrons Region
Obviously, (but of great importance) to the outcome of the interaction, is the fact that the time spent in the vicinity of the nucleus is inversely proportional to the relative velocity between the neutron and nucleus. A compound nucleus has a lifetime inversely proportional to its total width. Radiative captures are associated with narrow resonance widths and scattering is associated with wider resonances.
Two caveats I should mention in relation to the general statements made above. Account must be taken of the reaction cross sections involved in thermal neutron absorption. Also, as neutron moderation reduces the initial high energy of the neutron, consideration must be given to the appearance of absorption peaks at certain neutron energies that are particular to a specific nuclide.
A consquence of the nuclear thermal energy is that, with increasing temperature, we find that Doppler broadening improves the probability of finding a reasonance peak. The increase in the capability of uranium-238's to absorb neutrons at higher temperatures (without fissioning) is a vital negative feedback mechanism utilised in the safety mechanisms of nuclear reactors.
The largest reaction cross-sections are usually found at distinctive neutron energies that result in states of the compound nucleus which have relatively long lifetimes. The compound nuclei associated with these particular energies are known as nuclear resonances and their formation is typical within the resonance region. The resonance widths can, in general be correlated with increasing energies. However, as the energy level increases, the resonance widths become comparable to the widths between resonances, and distinctive resonances cannot be distinguished. The narrowest resonances are usually compound states of heavy nuclei (such as fissionable nuclei).
The nucleus will occasionally emit a $\gamma $ ray (radiative capture), as the particular decay mode of the compound nucleus is independent of the method of creation of the compound nucleus. Scattering can also occur, as a neutron is ejected. The "objective" of the compound nucleus is to achieve stablisation.
The compound nucleus will only emit a neutron only after that neutron acquires sufficient energy, following its collision of with another with greater nuclear binding energy. There is a delay involved as the excitation energy of the compound nucleus is split between several nucleons. The larger the compound nucleus leads to the fact that the average time before neutron emission is significantly longer in duration than in those cases where a relatively small number of nucleons are present. Again, this emission delay is directly linked to the number of nucleons amongst which the excitation energy is distributed.
So radiative capture is relatively unimportant in light nuclei, but in heavy nuclei, it becomes increasingly important.