Physical description of nuclear materials identification system (NMIS) signatures

This paper describes all time and frequency analysis parameters measured with a new correlation processor (capability up to 1 GHz sampling rates and up to five input data channels) for three input channels: (1) the ²⁵²Cf source ionization chamber: (2) a detection channel; and (3) a second detection channel. An intuitive and physical description of the various measured quantities is given as well as a brief mathematical description and a brief description of how the data are acquired. If the full five-channel capability is used, the number of measured quantities increases in number but not in type. The parameters provided by this new processor can be divided into two general classes: time analysis signatures and their related frequency analysis signatures. The time analysis signatures include the number of time m pulses occurs in a time interval, that is triggered randomly, upon a detection event, or upon a source fission event triggered. From the number of pulses in a time interval, the moments, factorial moments, and Feynmann variance can be obtained. Recent implementations of third- and fourth-order time and frequency analysis signatures in this processor are also briefly described. Thus, this processor used with a timed source of input neutrons contains all of the information from a pulsed neutron measurement, one and two detector Rossi-α measurements, multiplicity measurements, and third- and fourth-order correlation functions. This processor, although originally designed for active measurements with a ²⁵²Cf interrogating source, has been successfully used passively (without ²⁵²Cf source) for systems with inherent neutron sources such as fissile systems of plutonium. Data from active measurements with an 18.75 kg highly enriched uranium (93.2 wt%, ²³⁵U) metal casting for storage are presented to illustrate some of the various time and frequency analysis parameters. This processor, which is a five-channel time correlation analyzer with time channel widths as small as 1 ns, was developed at the Oak Ridge National Laboratory and the Oak Ridge Y-12 Plant where it is in use for verification and confirmation of weapons components and other fissile materials in storage and for receipts from other Department of Energy and Department of Defense Facilities.