Thermal neutrons and fission

Question

You need slow neutrons because if the neutrons are too "quick" then they scatter of the atoms instead of being captured by them. You can imagine a big lump of playdoh and a much smaller ball of playdoh. If you shoot the small ball with high velocity at the big lump then the ball is scattered - like two billiard balls. But when you put them slowly together by hand they stick nicely together.

If the small playdoh is enough energetic, it can destroy into parts the big lump right? So maybe with a single neutron with high energy and high speed we can split a uranium atom into small parts instead of scattering it? Would it be possible?

score 2 · Accepted Answer · answered May 24 '20 at 15:24

2

If you want to make nuclear fission you want the uranium-235 nucleus to capture the neutron to become uranium-236 which then decays into krypton and barium plus 3 neutrons (which fuel the chain reaction)

$$ n + ^{235}\mbox{U} \to ^{236}\mbox{U}^* \to ^{92}\mbox{Kr} + ^{144}\mbox{Ba} + 3\ n $$

If you shoot a very energetic neutron towards a uranium nucleus, if you have a reaction (which may not even be the case as the cross-section may decrease with the energy), you can produce subproducts of the fission that may not sustain a chain reaction.

If you go to extreme energies you will not have the interaction between the neutron and the nucleus, but rather only the interactions of the neutron with just a nucleon of the uranium nucleus, or the interaction between the quarks themselves.

In many of these cases you do not even convert mass into energy, but rather you use energy to produce mass, as new particles can be produced (deep-inelastic scattering).

answered May 24 '20 at 15:24

Davide Dal Bosco

2,302

1

The cross section for 235U fission vs energy is here: https://physics.stackexchange.com/questions/549774/thermal-neutrons-in-fission/553986#553986 – Jon Custer May 24 '20 at 15:58
@JonCuster So it definitely decreases with the energy! – Davide Dal Bosco May 24 '20 at 16:21
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Indeed. But note that isn’t the case for 238U. – Jon Custer May 24 '20 at 17:31
So it is true that with an energetic neutron toward a uranium nucleus I can split it in small parts, but this "reaction" maybe is useless in term of energy. – Salmon May 24 '20 at 17:51

Thermal neutrons and fission

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