Activation Foils and Flux Wires
Whenever it is necessary to measure reactor neutron flux profiles, a section of wire or foil is
inserted directly into the reactor core. The wire or foil remains in the core for the length of time
required for activation to the desired level. The cross-section of the flux wire or foil must be
known to obtain an accurate flux profile. After activation, the flux wire or foil is rapidly
removed from the reactor core and the activity counted.
Activated foils can also discriminate energy levels by placing a cover over the foil to filter out
(absorb) certain energy level neutrons. Cadmium covers are typically used for this purpose. The
cadmium cover effectively filters out all of the thermal neutrons.
Photographic film may be utilized in x-ray work and dosimetry. The film tends to darken when
exposed to radiation. This general darkening of the film is used to determine overall radiation
exposure. Neutron scattering produces individual proton recoil tracks. Counting the tracks yields
the films exposure to fast neutrons. Filters are used to determine the energy and type of
radiation. Some typical filters used are aluminum, copper, cadmium, or lead. These filters
provide varying amounts of shielding for the attenuation of different energies. By comparing the
exposure under the different filters, an approximate spectrum is determined.