Neutron Interactions: Collisions & Absorption

[Morga]

At a macroscopic level it appears that when two objects collide, they physically interact and the atoms touch. However the charges of the electrons actually repel one another and they don't physically touch. Correct?

A neutron however has no charge so what happens when a neutron collides with an atom or another neutron?

Does a collision occur because of mass or just repulsion from charge?

Are neutrons just absorbed when they collide with light stable elements?

Thanks in advance.

 

[malawi_glenn]

Neutrons interacts via the strong nuclear force. The nuclear force is attractive on distances less than approx 1.5fm and very repulsive at distances less than approx 0.2fm; That why in, for instance neutron - neutron collisions, neutrons don't physically "touch" each other. But that also depends on energy, if the energy is so high that the repulsion of the strong force at very short distances can be overcomed, then the quarks of the particles will start to interact with each other. And in bound systems, you also have the Pauli principle.

 

[genneth]

Remember that our current model of particle physics doesn't actually assign a size to particles -- they're not actually little spheres. They are only points, surrounded by a hazy cloud of virtual particles (i.e. potential interactions). If two particles approach each other fast enough, fairly interesting consequences of the interaction of their clouds can occur. Further, in most realistic physical situations, the boundary conditions to the entire quantum system will result in discrete energy levels, and so in some sense discrete positions. In this case, for fermions, no two particles could be in the same state, which can also manifest as a macroscopic force.

 

[Astronuc]

A neutron however has no charge so what happens when a neutron collides with an atom or another neutron?

Neutrons will not interact with the electrons due to the lack of charge. Neutrons will either be absorbed by a nuclear or decay with a half-life of about 10.23 minutes. In the meantime, free neutrons will through the space occupied by electrons and occasionally collide with a nucleus and be scattered. Now some collisions excite the nucleus of the atom, and with some collisions, other nucleons may be knocked out, but that dependents on the energy of the neutron.

Does a collision occur because of mass or just repulsion from charge?

Collisions occur because a particle (which has mass) exists. Charged particles interact via their charge (electric field) with other charges and the charges repel/or attract without collision (in the sense that billiard balls collide - but as genneth mentioned neutrons are not really hard little spheres). Nevertheless, momentum and kinetic energy are part of the interaction.

Are neutrons just absorbed when they collide with light stable elements?

As mentioned, neutrons may be absorbed, they may scatter from a nucleus, or if energy is sufficiently high, they may knock out another nucleon, either proton or neutron. Eventually a neutron will decay spontaneously into a proton, electron and electron-associated antineutrino, if it isn't absorbed by a nucleus.

 

[Meir Achuz]

"Neutrons will not interact with the electrons due to the lack of charge. "
There is an e-n interaction due to the magnetic moment of the neutron.

 

[Astronuc]

I should have said 'not significantly interact', as in ionization.