A neutron in a block of material can do 3 things each time it passes near a nucleus:
- Elastic scattering - transfers some of its kinetic energy to the nucleus
- Inelastic scattering - the nucleus+neutron turn into several particles, usually liberating several neutrons (fission)
- Absorption - transmutes the nucleus into a heavier isotope
Variants 3 and 4 are similar, so I just call them "nuclear reaction".
The outcome for every encounter is random. Combining a great number of encounters, these are the possible outcomes:
- Nothing (if the sheet of material is too thin) - the neutron flies onward
- Some scattering (if the probability of absorption is low) - the neutron flies out with less kinetic energy and different direction; the material is heated a little
- Endless scattering (if the probability of absorption is very low, and the block of material is very big) - the neutron's kinetic energy becomes comparable with thermal energy; it wanders among the atoms until it beta-decays
- Nuclear reaction (if the probability of absorption is significant)
Note that option 3 leads to a proton (positively-charged ion of hydrogen) and a beta particle (an electron with much kinetic energy, which immediately flies away).
If a nuclear reaction occurs, the result has many possibilities:
- A stable heavier nucleus (the case for hydrogen and oxygen, as well as heavier elements)
- A radioactive heavier nucleus (the usual case for heavy elements)
- Several smaller nuclei and/or neutrons (the usual case for uranium-235)
It may be possible to estimate the probability for various outcomes using measured data for neutron cross-sections (see e.g. here): if the absorption cross-section is big when compared to scattering cross-section, the probable outcome is absorption. However, this is complicated by dependence of the cross-section on the neutron's energy.