I am familiar with one source of neutrons in
nuclear density meters.

"Another variant is to use a strong neutron source like 241Americium to produce Neutron radiation and then measure the energy of returning neutron scattering. As hydrogen characteristically slows down neutrons, the sensor can calculate the density of hydrogen - and find pockets of underground water, humidity up to a depth of several meters, moisture content, or asphalt content. Neutron sources can also be used to assess the performance of Separator (oil production) in the same way. Gas, oil, water and sand all have different concentrations of hydrogen atoms which reflect different amounts of slow neutons. Using a head which contains an Am241Be source and a slow neutron detector, by scanning it up and down a separator it is possible to determine the interface levels within the separator."

-Wiki