Feasability of using a graphite slowing-down-time spectrometer in the nondestructive assay of nuclear materials

A slowing-down-time spectrometer (SDTS), constructed for the study of nondestructive assay of fissile nuclear materials, is in its early stages of operation at the Nuclear Engineering Teaching Laboratory of the University of Texas at Austin. The spectrometer is made of a 101×105×122 cm3 graphite rectangular parallelepiped and is based on injecting pulses of 14 MeV neutrons into the pile. The neutron source is a Texas Nuclear Corporation neutron generator that produces neutrons via the D–T reaction. Measurements and calculations have been conducted to study the time behavior of the neutrons and the assay capabilities of this spectrometer. A 3He detector covered with 0.8 mm of Cd was used to perform neutron die-away measurements in the graphite. A 4He detector was used to perform an assay of a fuel pin containing 13% by mass 239Pu. The calculations were made using the MCNP4B code, and a realistic 3-D mock-up of the experiment. Good agreement is found to exist between the measured time spectra and the ones predicted from the calculations. This included the experimental observation of a predicted rapid fall-off in the die-away spectrum due to the existence of the cadmium cover. Therefore, the time–energy coupling that is expected to hold in a SDTS is experimentally verified. In addition, the calculations and the measurements indicate that the interrogation of fissile materials is possible using a graphite SDTS. This is clear in the measured and calculated time signatures for the 239Pu sample, which include structure that reflects the two resonance groups and the 0.3 eV resonance peak observed in the 239Pu fission cross section.