Fissile Material Detection by Differential Die Away Analysis

Abstract

Detection and interdiction of Special Nuclear Material (SNM) in transportation is one of the most critical security issues facing the United States. Active inspection by inducing fission in fissile nuclear materials, such as 235U and 239Pu, provides several strong and unique signatures that make the detection of concealed nuclear materials technically very feasible. Differential Die‐Away Analysis (DDAA) is a very efficient, active neutron‐based technique that uses the abundant prompt fission neutrons signature. It benefits from high penetrability of the probing and signature neutrons, high fission cross section, high detection sensitivity, ease of deployment and relatively low cost. DDAA can use any neutron source or energy as long as it can be suitably pulsed. The neutron generator produces pulses of neutrons that are directed into a cargo. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. If SNM is present, the thermalized neutrons create a new source of (fission) neutrons with a distinctive time profile. An efficient laboratory system was designed, fabricated and tested under a US Government DHS DNDO contract. It was shown that a small uranium sample can be detected in a large variety of cargo types and configurations within practical measurement times using commercial compact (d,T) sources. Using stronger sources and wider detector distribution will further cut inspection time. The system can validate or clear alarms from a primary inspection system such as an automated x‐ray system.