Which type neutron is easier to be absorbed by the unstable nuclei. A faster one, or a slower one?

It depends on the nuclei.  Uranium 235 will undergo fission from a thermal (slow) neutron but Uranium 238 will undergo fission from a fast neutron.

Edited January 23, 20214 yr by Bufofrog

8 minutes ago, Aidan Liang said:

Which type neutron is easier to be absorbed by the unstable nuclei. A faster one, or a slower one?

Fast neutrons (with higher kinetic energy) can destroy nuclei i.e. split it to clusters.

https://en.m.wikipedia.org/wiki/Cluster_decay

Search net for "fast neutrons induced cluster decay"

 

Is there a reason why neither of you said anything about absorbing neutrons (i.e. what the OP asked about)?

Anyway, I think the answer is that slow neutrons are generally more likely to be absorbed into the nucleus.

e.g

https://en.wikipedia.org/wiki/Neutron_capture#/media/File:Neutroncrosssectionboron.png

 

In general, the cross-section for absorption drops with energy. There are some resonances which go against this trend. See, for example fig 1here

https://www.researchgate.net/figure/Neutron-absorption-cross-sections-for-several-isotopes-as-a-function-of-incident-neutron_fig5_235915631

 

(edit: x-post)

On 1/23/2021 at 1:08 PM, John Cuthber said:

her of you said anything about absorbing

Im going to believe in the theory that fast neutrons would:

1. Destroy the nucleus

2. Too fast that the nucleus cannot absorb it

Therefore slower neutrons are easier to be absorbed (generally).

However, there are exceptions for different elements

8 minutes ago, Aidan Liang said:

Im going to believe in the theory that fast neutrons would:

1. Destroy the nucleus

They won't.  A neutron will collide elasticity with a nucleus and not destroy or otherwise 'damage' the nucleus.  Don't go on belief, you should go on facts.

Edit:  As Sensi said in rare cases some emissions from the nucleus is possible.  That is not typical.

Edited January 24, 20214 yr by Bufofrog

4 hours ago, Bufofrog said:

They won't.  A neutron will collide elasticity with a nucleus and not destroy or otherwise 'damage' the nucleus.  Don't go on belief, you should go on facts.

Edit:  As Sensi said in rare cases some emissions from the nucleus is possible.  That is not typical.

right thanks!