Activation Foils and Flux Wires:

When it is must to measure reactor neutron flux profiles, a section of wire or foil is inserted straightly into the reactor core.  The foil or wire remains in the core for the length of time needed for activation to the desired level.  A cross-section of the flux wire or foil must be known to acquire an accurate flux profile. After activation, the flux wire or foil is rapidly erased from the reactor core and the activity counted.

Activated foils could also discriminate energy levels through placing a cover over the foil to filter out (absorb) certain energy level neutrons.  A Cadmium covers are classically used for this aim.  The cadmium cover efficiently filters out all of the thermal neutrons.