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.

During the Manhattan project a simple formula was developed by Bethe and Feynman in 1943 to estimate the yield of a fission-only nuclear explosion of a uniformly-dense bare-sphere of supercritical fissile material. We have not found any evidence that Bethe and Feynman knew of the first yield formula obtained by Frisch and Peierls contained within their famous March 1940 memorandum. Similarly, we have not found any technical documents that compare the Bethe-Feynman formula to the earlier works of Frisch and Peierls, or Serber. After adjusting for differences in the labeling of critical radii, we find that earlier formulas only differ by a scaling factor.

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