Correlation Measurements with 252Cf to Characterize Fissile Material

Measurements using 252Cf as a timed source of neutrons and gammas have in recent years undergone significant maturation. These methods use 252Cf as an observable source of spontaneous fission neutrons and gammas in conjunction with one or more neutron- and/or gamma-sensitive detectors to measure the time-distribution of correlated detector counts following (a) an observed 252Cf-fission event and/or (b) a counting event in another detector. Detection of 252Cf spontaneous fission is frequently achieved via use of a small ionization chamber in which the 252Cf is contained--in this case the timing of source emission events is random. However, one application subsequently described uses a neutron-absorbent ''shutter'' to modulate 252Cf emissions to produce a neutron source with deterministic timing. Other applications, frequently termed noise-analysis measurements, transform the time-distributions to the frequency domain. Collectively, these correlation methods use 252Cf to ''excite'' the fissile material and the response of the material is measured by an array of detectors and analyzed using standard time-correlation and/or frequency-analysis techniques. In recent years numerous advances have been made in the application of these methods to in-situ, or field measurements directed at characterizing various configurations of fissile material in operational facilities