Nuclear fission mechanism: neutron capture Consider this nuclear fission reaction: <mrow cla

Jaylene Duarte

Jaylene Duarte

Answered question

2022-05-18

Nuclear fission mechanism: neutron capture
Consider this nuclear fission reaction:
235 U + 1 n 236 U  (excited) 92 K r + 141 B a + 3 1 n
I have not understand why a thermal neutron ( 1 n should be captured by the nuclei of 235 U.
What are the conditions under which this neutron is captured?
My reasoning is the following: the energy that the neutron must have to be captured by the 235 U should be equal to the S n (separation energy for a neutron) of 236 U which I found is 6.34 MeV, higher than the energy of the thermal neutron. So my reasoning must be wrong but I cannot understand where I make mistakes...
I looked on the Povh Rith Particles and Nuclei but it does not explain the reaction well. Could anyone give me a reference or explain here the fission reaction ?

Answer & Explanation

Kylan Simon

Kylan Simon

Beginner2022-05-19Added 17 answers

You're misinterpreting the separation energy S n . If you wanted to start with 236 U and end up with 235 U and a neutron at rest, you'd have to add 6 MeV. When the neutron is captured, it's into some nucleon orbital 6 MeV above the 236 U ground state; as the excited nucleus cools that energy gets distributed among all the nucleons. Lighter nuclei throw off the energy of neutron capture by emitting a cascade of photons; in uranium, fission is another available pathway for the nucleons to reduce their interaction energy.
A nucleus with a negative neutron separation energy S n < 0 is beyond the neutron drip line.

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