Cosmic ray muon scattering tomography is one of four techniques currently being investigated at AWE for the detection of special nuclear material (SNM). In order to develop a prototype muon detection system, it is necessary to consider the requirements of the radiation detectors with respect to; coincidence timing for system triggering; tracking of the muon trajectory; and determination of muon energy. The detector requirements for a prototype muon scattering tomography system are presented and a variety of detector types considered and assessed against these requirements.The advantages, disadvantages, potential compromises and compatibility with other complementary detection techniques are discussed. Future plans are outlined for an initial prototype and future, long-term development of a muon scattering tomography system for detection of SNM.is not therefore possible to tum this on or off: or to optimise the energy. Scattering tomography using cosmic ray muons may not provide a complete answer to the problem of illicit trafficking of RN materials, but could well form part of the solution. II. COSMIC RAY MUONSHigh energy cosmic rays -comprising roughly 90% protons, 9% a-particles and the remainder heavier nuclei -strike the upper reaches of the Earth's atmosphere at a rate of about 1000 1m 2 Isec [2]. These undergo deep inelastic collisions with molecules in the Earth's atmosphere to produce cascades of lighter particles. These include short-lived pions, which decay into muons.Muons have a rest energy of 105.7 MeV/c 2 , around 200 times that of an electron, and can be either positively or negatively charged. Although they have a short lifetime of --2.2 micro seconds, their near-relativistic speeds mean they form a significant fraction of the earth's cosmic radiation at sea level. The muon flux at sea level is approximately 10,000 m-2 min-1 [3], which is roughly equivalent to one through the surface of the hand every second. This is, however, dependent on a number of factors, including latitude, longitude, altitude, time in the solar cycle and angle. It is often claimed [4] that the intensity falls otT as cos 2 {) (where {) is the angle from the zenith) though a number of papers suggest that such statements may be rather sweeping [5][6][7]. The energy spectrum of cosmic ray muons at sea level varies over several orders of magnitude, from approximately lOMeV to 10 GeV. The graph below was generated using EXPACS [8] version 2.14, and assumes latitude of 52°. Integrating this curve with respect to energy suggests a total muon flux of 1.56 x 10-2 cm-2 sec-I, which is in agreement with the often quoted value of 10,000 m-2 min-l mentioned previously. Figure 1 also gives a mean muon energy of 4.01 GeV, which is the commonly used mean value.

Active interrogation is a method used to enhance the likelihood of detection of shielded special nuclear material (SNM); an external source of radiation is used to interrogate a target and to stimulate fission within any SNM present. Radiation produced by the fission process can be detected and used to infer the presence of the SNM. The Atomic Weapons Establishment (AWE) and the Naval Research Laboratory (NRL) have carried out a joint experimental study into the use of single pulse, high-intensity sources of bremsstrahlung x-rays and photoneutrons in an active interrogation system. The source was operated in both x-ray-only and mixed x-ray/photoneutron modes, and was used to irradiate a depleted uranium (DU) target which was enclosed by up to of steel shielding. Resulting radiation signatures were measured by a suite of over 80 detectors and the data used to characterise detectable fission signatures as a function of the areal mass of the shielding. This paper describes the work carried out and discusses data collected with proportional counters, NaI(Tl) scintillators and Eljen EJ-309 liquid scintillators. Results with the x-ray-only source demonstrate detection ( ) of the DU target through a minimum of of steel, dropping to when using a mixed x-ray/photoneutron source. The proportional counters demonstrate detection ( ) of the DU target through the maximum steel shielding deployed for both photon and mixed x-ray/photoneutron sources.Index Terms-Active interrogation, bremsstrahlung x-ray, electron accelerators, nuclear security, photoneutron, special nuclear material (SNM).

We irradiated a depleted uranium ( ) target with intense, single 50 ns pulses of bremsstrahlung to study the behavior of , , NaI(Tl), and liquid scintillation detectors in a harsh radiological environment. The target was exposed unshielded, and shielded with borated high-density polyethylene, or steel, and delayed -ray and neutron signatures were measured. We found that a high confidence measurement of the delayed emission could be obtained in this environment and show the results from each detector array, for varying amounts of shielding, in terms of the signal-to-noise ratio vs. time and the relationship between the mean of the signal-to-noise ratio vs. areal mass density.

No abstract

No abstract