A newly measured target nuclide has a slow-neutron capture cross section of at…

PHYSICS · Multi-part · Fermi University Physics · Nuclear Physics · Nuclear reactions · Nuclear reactions

The Nucleus Resolved, Ch Reaction mechanisms: resonances, the compound nucleus, direct reactions and the optical model

Problem

A newly measured target nuclide has a slow-neutron capture cross section of

240\,\mathrm{b}

at thermal energy (

0.0253\,\mathrm{eV}

) and no known resonance below

5\,\mathrm{eV}

. Its capture cross section measured separately at

1.0\,\mathrm{eV}

44\,\mathrm{b}

. ((a)) Predict, using the

1/v

law, the capture cross section expected at

1.0\,\mathrm{eV}

from the thermal value. ((b)) Compare with the measured

44\,\mathrm{b}

and comment on whether the target obeys the

1/v

law over this range, and why a small deviation of this kind is expected as the energy approaches a resonance above

5\,\mathrm{eV}

. ((c)) The same target, bombarded with

14\,\mathrm{MeV}

neutrons, gives an

(n,\alpha)

reaction whose alpha-particle angular distribution is strongly forward peaked with clear maxima and minima, and whose alpha energy spectrum shows one discrete line rather than a continuous spread. Predict, from these two signatures and the reaction clock, whether this

14\,\mathrm{MeV}

reaction proceeds by a direct or a compound-nucleus mechanism, and justify the choice.

Related theory

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