The "nuclear car wash": a scanner to detect illicit special nuclear material in cargo containers

Abstract

There is an urgent need to improve the reliability of screening cargo containers for illicit nuclear material that may be hidden there for terrorist purposes. A screening system is described for the detection of fissionable material hidden in maritime cargo containers. The system makes use of a low-intensity neutron beam for producing fission and the detection of the abundant high-energy rays emitted in the -decay of short-lived fission products and -delayed neutrons. The abundance of the delayed rays is almost an order of magnitude larger than that of the delayed neutrons normally used to detect fission, and they are emitted on about the same time scale as the delayed neutrons, i.e., 1 min. The energy and temporal distributions of the delayed rays provide a unique signature of fission. Because of their high energy, these delayed rays penetrate low-Z cargoes much more readily than the delayed neutrons. Coupled with their higher abundance, the signal from the delayed rays escaping from the container is predicted to be as much as six decades more intense than the delayed neutron signal, depending upon the type and thickness of the intervening cargo. The rays are detected in a large array of scintillators located along the sides of the container as it is moved through them. Measurements have confirmed the signal strength in somewhat idealized experiments and have also identified one interference when 14.5-MeV neutrons from the D, T reaction are used for the interrogation. The interference can be removed easily by the appropriate choice of the neutron source.Index Terms-Active interrogation, delayed fission product radiation, neutron irradiation, shielded special nuclear material.

High-yield D–T neutron generator

“…Active neutron interrogation can play an important role in these inspection tasks because neutrons penetrate cargo and shielding materials and generate specific signatures. Neutron induced fission in nuclear materials stimulates the emission of β-delayed γ-rays and neutrons that are characteristic signatures of SNM [1]. Another promising neutron-based method for the detection of nuclear materials is Pulsed Neutron Differential Die Away Analysis [2].…”

Production and applications of neutrons using particle accelerators

“…[335] In a parallel approach, development has also taken place to seek methods for measuring β-delayed gamma rays from short-lived fission products and using this as a signature to detect shielded fissionable material. [336][337][338][339] While gamma rays continue to be emitted from fission products for long time periods following fission, at short periods a similar time dependence is observed as with the β-delayed neutrons. An advantage of this approach is that the intensity of the β-delayed gamma-ray signature is stronger than that of the βdelayed neutrons.…”

Accelerator-driven neutron source for cargo screening

“…Special Nuclear Materials (SNM), which may be shielded and deeply buried in cargo, can be detected by probing the cargo container with a beam of fast neutrons and stimulating characteristic fission signatures. In the so called "Nuclear Car Wash" concept [1] a cargo container is scanned with a neutron beam and the presence of SNM is signaled by the detection of β-delayed gammas with energies above ~3 MeV 1 and β-delayed neutrons from fission products [2]. An upper neutron energy limit is set by activation of cargo and other materials leading to the emission of gammas that interfere with the SNM signal.…”