Signals and interferences in the nuclear car wash J.A. Church a, * , D.R. Slaughter a , S. Asztalos a , P. Biltoft a , M.-A. Descalle a , J. Hall a , T. Luu a , D. Manatt a , J. Mauger a , E.B. Norman a , D. Petersen b , S. Prussin b a Lawrence Livermore National Laboratory,7000 East Ave. L-186, Livermore, CA 94550, USA b University of California, Berkeley, Berkeley, CA 94720, USA Available online 1 May 2007 Abstract The screening of sea-going cargo containers for highly enriched uranium (HEU) and other fissile material is a challengin This is due in part to the cargo itself, which acts as an attenuator to any radiation that might signal its presence. In the nu b-delayed high-energy c-rays following neutron-induced fission are utilized as this signal. The delayed c-rays above 3 MeV are highly penetrating and have energies above natural background radiation. In addition, the half-lives of most fission products emi at these energies are less than 160 s, making it feasible to construct decay curves on a time scale which preserves the flo through the port. A particular goal of the project is to understand the rate of false alarms. To this end, experiments are un investigate possible interferences, and to understand variations in the overall c-ray background. The experiments and pre are discussed. Work performed under the auspices of the DOE by the UC LLNL W7405Eng4,UCRL-PROC-224803. Published by Elsevier B.V. PACS: 25.85.Ec; 23.40.s; 29.40.Mc; 89.20.Bb Keywords: Active neutron interrogation; n-Induced fission; b-Delayed radiation; Plastic scintillators; SNM 1. Introduction In the fight againstterrorism atour nation’sports, active cargo container interrogation can confirm the pres- ence of fissile material in the assessment of a possible threat. At Lawrence Livermore National Laboratory, one such system ‘the nuclear car wash’ [1–4] is currently under- going feasibility testing. The system employs active neutron interrogation, using neutrons to induce fission, and then taking subsequent b-delayed high-energy c-rays as the sig- nal that fissile material has been stowed in the cargo. In some cases, the decay of delayed neutrons is used in con- junction with that of the delayed c-rays as this signature. One goalfor the system is to limit the scanning time per containerto 1 min while retaining95% detection and 0.1% false alarm rates. A system prototype has been set up to facilitate tests with hydrogenous and metallic cargos. Recent experimen designed to test the effects of incident neutron attenuati examine the c-ray background, and measure the severity several possible signal interferences, show that the meth is successful, and has the potential to play a major role in reducing the false alarm rates in any combined-method scanningapparatusthat may be implemented atthe seaports. 2. b-delayed radiation signals fissile material The nuclear car wash uses b-delayed radiation to signa the presence of fissile material in cargo. It has been show that after thermal-neutron induced fission, b-delayed c-rayswith E > 3 MeV are a unique signature for 235 U and 239 Pu [5].Many of these fission products have half- lives ranging 1 s < t 1/2 < 30 s and also have high Q-values to b-decay [6]. This indicates thathere is a chance for 0168-583X/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nimb.2007.04.264 * Corresponding author. Tel.: +1 925 423 8339. E-mail address: church4@llnl.gov (J.A. Church). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 261 (2007) 351–355 NIM B Beam Interactions with Materials & Atoms