Overview of NWIS Software

Abstract

The Nuclear Weapons Identification System (NWIS) is a system that performs radiation signature measurements on objects such as nuclear weapons components. NWIS consists of a {sup 252}Cf fission source, radiation detectors and associated analog electronics, data acquisition boards, and a computer running Windows NT and the application software. NWIS uses signal processing techniques to produce a radiation signature from the radiation emitted from the object. This signature can be stored and later compared to another signature to determine whether two objects are similar. A library of such signatures can be used to identify objects in closed containers as well as determine attributes such as fissile mass and in some cases enrichment. NWIS uses a {sup 252}Cf source on one side of the object to produce radiation that its detectors measure on the other side of the target (active mode). If the object naturally emits enough radiation, the {sup 252}Cf source is not required (passive mode). The NWIS data acquisition hardware has five detector channels. Each channel receives shaped detector pulses and times those pulses with 1 nanosecond resolution. In active mode measurements one of these channels receives pulses from a detector measuring the {sup 252}Cf source fissions. Thus, for active mode measurements, NWIS has the time of each {sup 252}Cf fission and the subsequent injection of neutrons and gamma rays into the object. The remaining channels receive pulses from the detectors measuring radiation from the object. These detectors record the amount and time of radiation exiting the object. By correlating the radiation events among the source and the other detectors, and among the detectors themselves, a characteristic response of the object to {sup 252}Cf radiation or its own internal radiation is measured. The data acquisition hardware consists of two custom-made boards. The Data Capture and Compression (DCC) board is built around a Gallium Arsine (GaAs) chip designed at ORNL. This chip assigns a time to each pulse received on the five detector channels and passes five compressed streams of time stamp data to the Data Acquisition (DA) board. The DA board performs additional data compression, consolidates the five data streams into one, formats the data, and passes it across the computer's PCI bus into computer memory. The computer processor performs the signal processing required to calculate the signatures and saves the result to disk. Signature analysis software provides the means to manipulate and match signatures in a signature library.