International Topical Meeting on Nuclear Research Applications and Utilization of Accelerators

4-8 May 2009, Vienna

Active Neutron Interrogation Approach to Detect Special Nuclear Material in Containers

R.E. Mayer^1,2, A. Tartaglione^1,2,3, and F. Di Lorenzo^1,2,3,4

¹Comisíon Nacional de Energía Atómica, Bariloche, Argentina
²Instituto Balseiro, Universidad Nacional de Cuyo, Argentina
³Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina
⁴Universidad de Buenos Aires, Argentina

Corresponding Author: mayer@cab.cnea.gov.ar

Cargo interrogation in search for special nuclear materials (SNM) like highly-enriched uranium (HEU) or Pu-239 is a first priority issue of international borders security. In this experimental work we present a thermal pulsed neutron based approach which combined with time–of–flight (TOF), demonstrates capability to detect small quantities of SNM shielded with moderate thicknesses of high or low Z materials providing, in addition, a manner to know the approximate position of the searched material. As many efforts are currently under way to exploit fast neutron penetration through cargo material, this work probes into the applicability of the complementary use of slow neutrons, taking advantage of the higher reaction cross sections, aimed at the usual cases of cargo with low neutron moderation capacity. If the surrounding merchandise were a highly moderating medium, the alternate fast neutron beam should be allowed to impinge on the object and undergo moderation in it, at the expense of loosing TOF information.

The actual work employed a 25 MeV electron linac with a refrigerated lead target and a polyethylene neutron moderator as the pulsed source, although the technique is not restricted to that combination which, it must be said, is not the most favourable one. A wide area neutron detection moderating array (shielded from thermal background) was devoted to the detection of fission fast neutrons.

Results are presented concerning the detection of an irradiated volume of SNM comprising some 11 grams of isotope U-235 (in aluminum matrix), although when hidden in a moderating surrounding, the whole 27 grams sample can be taken into account. The sample was detected, placed behind 3 mm steel wall and was also hidden under lead 5 cm thick and within a moderating environment provided by high density polyethylene 5 cm thick. As to position sensitivity, a 100 cm movement of the U-235 sample along the irradiation axis shifted the TOF neutron spectrum 300 μs. This movement is close to the approximate distance difference between a lateral positioning and a central one, within a standard shipment container.

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