Characteristics of safeguarded nuclear materials
Instrumentation for monitoring nuclear materials

In this paragraph, only non-proliferation-sensitive materials are mentioned.

The actinides thorium, uranium, plutonium, americium, curium and californium are the main alpha and/or gamma and/or neutron emitters –, the latter either resulting from spontaneous fission, or generated when these elements are associated with a light target (lithium, beryllium, boron...).

Alpha particles have a very short path through matter. In silicon (a material commonly used to detect them), a 6 MeV alpha particle (order of magnitude of the particles emitted by these elements) will be stopped by 30 µm of material thickness. For this reason, we will only consider gamma and neutron radiation, which have a longer range...