LIST OF TABLES
ABSTRACTThis report describes all time and frequency analysis parameters measured with the new Nuclear Weapons Identification System (NWIS) processor with three input channels: (1) the 252Cf source ionization chamber; (2) a detection channel; and (3) a second detection channel for active measurements. An intuitive and physical description of the various functions 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 functions increases in number but not in type. The parameters provided by this new NWIS processor can be divided into two general classes: time analysis signatures including multiplicities and frequency analysis signatures. Data from measurements with an 18.75 kg highly enriched uranium (93.2 wt %, ' "U) metal casting for storage are presented to illustrate the various time and frequency analysis parameters.The Nuclear Weapons Identification System @&VIS) was developed primarily for identification of nuclear weapons/components that contain highly enriched uranium (HEU) (Ref. 1). For active measurements (i. e. where the nuclear weapons/components are "stimulated"), it utilizes a 252Cf source in an ionization chamber to define the time of spontaneous fission of 252Cf (614000 fission per sec per pg). The neutron source is usually located adjacent to the weapons/ components/ containers with a pair of deteczcis on the opposite side of the weapons/components/ containers. Neutrons from the source enter the component inducing fission that produces additional neutron and gamma rays, some of which are detected. The time behavior of the detected events after Cf fission in a plastic scintillator sensitive to neutrons and gamma rays without distinction is sketched in Fig. 1.1, The directly transmitted gamma rays arrive at the detector slightly delayed in time from the 252Cf fission. Because all gamma rays travel with the speed of light, the dispersion of this part of the signal in time gives the combined time resolution for detecting the 252Cf spontaneous fission in the ionization and the gamma ray in the detector. This directly transmitted gamma ray component can be used for active gamma ray imaging much like an x-ray image but using gamma rays fiom spontaneous fission of 252Cf as the source. After the transmitted gamma rays, the neutrons that travel directly from the source through the material without collision arrive at the detector. These have a time dispersion because of the variation in the energy of neutrons from 252Cf fission. The next particles to arrive are the scattered neutrons, and finally both neutrons and gamma rays from induced fission. This figure is a simplification in that the various groups of neutrons overlap in real systems.
252NWIS with a 252Cf-source basically an active neutron interrogative method that can be used for HEU and Pu systems. However, for systems like Pu with a large inherent neutron source from the spontaneous fission of 2JoPu, NWIS can be employed f...
