Some elements produce charged particles with characteristic energy in neutron interactions.

 

principles of ndp diagram showing a charged particle interaction

 

When these elements are distributed near a surface, the particle energy spectrum is modulated by the distance the particle traveled through the surface.  NDP uses this information to determine the distribution of the elements as a function of distance to the surface.

table listing neutron depth profiling reaction parameters
An illustration of equipment setup for NDP, with the beam exiting the reactor from the left and illuminating the sample in a vacuum chamber:

illustration of equipment setup for NDP, chamber on the left, sample in the middle and neutron beam on the right

 

A spectrum generated by assessing a sample from the National Institute for Standards Technology:

 

graph showing spectrum generated with counts on the y-coordinate and energy on the x-coordinate