# What happens if a neutron flies towards a nucleus?

Rutherford experiment shows that alpha-particles when they fly towards metal foil sometimes (in minority of cases) can bounce. An explanation proposed was that atoms in fact have positively charged nuclea and majority of space is covered by fields of negative charge caused by electrons. Indeed, these fields must have much smaller charge density so that they (almost) don't affect alpha particles.

However, according to this explanation a neutron when it flies towards a nucleus should not bounce because of electromagnetic force. This is because neutron is an uncharged particle. Gravity is too weak to have any significant effect between neutron and nucleus so they don't merge. Weak forcd probably also is too weak. Therefore we can only consider strong force.

So, when a neutron flies towards a nucleus with high speed, what happens? Does strong force come into effect? Or does it just pass through a nucleus?

• When neutrons travel inside a material, they will undergo scattering (elastic and inelastic) as well as other reactions, while interacting with the nuclei via the strong, nuclear force. Given a beam of neutron with intensity I0, when traveling through matter it will interact with the nuclei with a probability given by the total cross section σT ....can see-ocw.mit.edu/courses/nuclear-engineering/… – drvrm Sep 4 '18 at 11:11
• Did you know this happens in an atomic bomb? The chain reaction consists of free neutrons being absorbed by nuclei, which then fission and emit further neutrons. – Mitchell Porter Sep 4 '18 at 11:16
• @MitchellPorter, so, they are absorbed making some unstable (more and more unstable with each next neutron) atoms which then almost simultaneously decays causing their massive emission? – user168013 Sep 4 '18 at 11:35
• Both the strong and weak force can cause the neutron to interact with the neucleus. Also despite being neutral, the neutron has a nonzero magnetic moment though it's quite small and is more a consequence of being composed of quarks. – Triatticus Sep 4 '18 at 14:23
• Some on-site searches that might help a bit: physics.stackexchange.com/search?q=neutron+scattering and physics.stackexchange.com/search?q=neutron+capture. – dmckee Sep 5 '18 at 14:20

The place to go for neutron scattering data is The Evaluated Nuclear Data Files site, hosted in the US at Brookhaven National Laboratory. There one can get data for a wide range of neutron scattering possibilities, including the cross section vs energy.

Since fission was mentioned in the comments, lets look at U-235. Entering that target nucleus, asking for all neutron reactions, and requesting the cross section (sig = $\sigma$) looks like:

One gets back a long list of possible reactions starting with:

Line one is the total neutron cross section vs energy. Line 2 is the elastic scattering component, and line 3 is the inelastic component. Line 7 is the fission cross section. Things like line 8 are an inelastic neutron scattering through a particular level. Further down one find the (n,p), (n,$\alpha$), and other similar reactions.

Selecting a few of the boxes and hitting 'Plot' up above results in:

So, that is how you find out what neutrons will do.

• The ENDFs are the source for details, but I'm not sure they are a good source for someone who needs a "big picture" outline of neutron-matter interactions. Not that it is an easy subject to give a summary of, of course. – dmckee Sep 5 '18 at 14:16
• @dmckee - of course, but it is probably the fastest way to find "all" the known interactions. Plus I figure (assuming the Q isn't closed) that I can point to this answer in the future, given the number of past questions that were answerable with a look at ENDF (or ENSDF)... – Jon Custer Sep 5 '18 at 14:20