Dependence of nuclear stability on mass number

[dt19]

we're doing some nuclear work at college at the mo, and i came across a few sources which stated that nuclei with an even mass number tend to be more stable than nuclei with an odd mass number. so far so good. when i looked up why this was, there was very little information on the internet but from what i could gather it is because neutrons tend to form pairs with other neutrons, and protons with protons, so the atom is more stable and thus requires a higher energy incident neutron to induce fission. (if any of this is wrong, please correct me!)
is it known why the nucleons tend to pair up? is it something to do with the strong nuclear force?

also, my physics teacher asked me this question (which he doesn't know the answer to) : when a neutron is absorbed by a U235 nucleus, U236 is formed. why does this nucleus immediately split, since as it has an even mass number and should therefore be more stable? I've come across a few mentions of the kinetic energy of the neutron being converted to internal energy of the nucleons, but i don't really understand.

help very much appreciated!

 

[Astronuc]

See this page on nuclear structure.

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/shell.html

In general it is true that nuclide with even numbers are more stable, until one looks at the heavier actinides.

In the case of U236, formed when U-235 absorbs a neutron, the excited U-236 nucleus undergoes internal osciallations, and effectively two new and more stable nuclei form. That is why the energy release from fission is ~200 MeV. BTW, there is a change that U-236 will decay by gamma emission and not fission in approximately 18% of absorptions of thermal neutrons.

See also the page on binding energy
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c2

and http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fiscon.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fisfrag.html

 

[sir-pinski]

The stability of a nucleus is determined by the balance between the strong nuclear force, which holds the nucleus together, and the repulsive electrostatic force between protons. The strong nuclear force is a short-range force that only acts within the nucleus, while the electrostatic force is a long-range force that acts between all charged particles.

The number of protons and neutrons in a nucleus, also known as the mass number, plays a crucial role in determining the stability of a nucleus. As you have mentioned, nuclei with an even mass number tend to be more stable than those with an odd mass number. This is due to the fact that the strong nuclear force is slightly more attractive between particles that are paired up, such as two neutrons or two protons, compared to particles that are unpaired. This is because paired particles have a more symmetrical distribution of charge, making the nucleus more stable overall.

This pairing effect is a result of the properties of the strong nuclear force, which is a result of the exchange of particles called mesons between nucleons. The exact reason why the strong nuclear force favors pairing is not fully understood, but it is believed to be due to the spin and isospin properties of the nucleons.

As for the question about why a U235 nucleus immediately splits after absorbing a neutron, it is important to note that the stability of a nucleus is not solely determined by its mass number. The internal energy of the nucleus also plays a role. When a neutron is absorbed by a U235 nucleus, it increases the internal energy of the nucleus, making it more likely to undergo fission. The kinetic energy of the neutron is converted to internal energy of the nucleons, causing the nucleus to become unstable and split.

I hope this helps to clarify the concept of nuclear stability and the role of the mass number in determining it. Keep up the good work with your nuclear studies!