What happens to neutrons when they are stopped by a material such as lead? If neutrons are stopped by a dense material, they are shedding their energy somehow.  Does this mean the lead is becoming a heavier isotope (neutron capture), does the neutron embed itself into the material and turn to hydrogen?  Is there a probabalistic distribution of reactions?
I would like to know for lead, tungesten, and water.  I don't know what to look up but surely for water there are outcomes like deuterium, tritium, hydrogen gas in the water?  If there are tables of these reactions, that would be a great resource.
 A: A neutron in a block of material can do 3 things each time it passes near a nucleus:


*

*Nothing

*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.
