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Nuclear Technology / Volume 168 / Number 2 / November 2009 / Pages 310-316
Neutron Measurements / Special Issue on the 11th International Conference on Radiation Shielding and the 15th Topical Meeting of the Radiation Protection and Shielding Division (Part 2) / Radiation Protection
We propose here a unique, patented shield design that transforms the complex neutron spectrum from a plutonium-beryllium (PuBe) neutron source to nearly the precise neutron signature leaking from a sphere of weapons-grade plutonium (WGPu) material. This will facilitate testing for detection of a significant quantity of weapons plutonium without the expense or risk of testing detector components with real materials. The Monte Carlo (MCNP5) and Deterministic (PENTRAN) computational codes have been used in developing the shield assembly. A nickel composite alloy shield for a PuBe capsule has been designed, built, and laboratory-tested to enable the neutron leakage spectrum from a standard 1-Ci PuBe source (mean energy of 4.6 MeV) to be transformed, through interactions in the shield, into a very close reproduction of the neutron spectrum leaking from a large, subcritical mass of WGPu metal (average neutron energy of 2.1 MeV). Nearly all current calibrations of neutron detectors use 252Cf for generation of a fission neutron spectrum, which decays with a half-life of [approximately]2.7 yr and is very expensive to procure. By converting to this design, PuBe sources relying on 239Pu (T1/2 = 24110 yr) and lasting hundreds of years could then be used to precisely calibrate and test detectors for simulated WGPu neutrons. Alternative custom designs are also possible with further transport-based modeling.