All atoms in the body are capable of undergoing nuclear reactions when exposed to neutrons. When an atom captures a neutron, that atom is transformed to another nuclear state of the same element. The new atom can be radioactive. It will then decay and emit a radioactive signal which may be measured by special detectors.

 

Neutron activation example
Neutrons interact with
common table salt (sodium chloride)

 

Two types of activation techniques

 

Only in-vivo neutron activation analysis (IVNAA) provides non-destructive testing of total body levels of the major body elements: calcium, sodium, chlorine, phosphorus, nitrogen, hydrogen, oxygen, and carbon.

Some partial body techniques exist as well. These methods measure
specific elements or tissues in the body. For example, cadmium, mercury, iron, iodine, aluminum, boron, lithium, and silicon may be measured. Target organs include the kidneys, liver, brain, lungs, heart, and thyroid.

Types of IVNAA

Delayed Gamma Neutron Activation Analysis (DGNAA)
Neutron activation influences certain elements, such as calcium, to produce gammas for a period of time. The gammas are measured by the whole body counter.

Prompt Gamma Neutron Activation Analysis (PGNAA)
Neutron activation stimulates some elements, such as nitrogen, to produce very short-lived gammas which must be immediately detected and counted.