Detecting Fissionable Material in Cargo Containers Via High-Energy G-Rays Emitted by Fission

Detecting fissionable material in cargo containers via high-energy -rays emitted by fission products

Eric B. Norman Lawrence Livermore National Laboratory Livermore, CA 94551 U. S. A.

Abstract

We are developing a technique to detect fissionable material hidden inside sea- going cargo containers. Our method involves active neutron interrogation followed by searches for high-energy gamma rays emitted after the beta decays of short-lived fission products. The latest results from our laboratory tests are shown along with ideas about how such a system could be employed in shipping ports.

Approximately 90% of the world’s trade moves via sea-going containers. This cargo is an attractive vehicle for smuggling illicit material. More than 6,000,000 containers enter the U.S. annually. The successful delivery of one weapon of mass destruction in a container could be catastrophic. Thus, we are concentrating on developing a technique to rapidly identify fissionable material hidden in a fully-loaded cargo container.

One of the major challenges in this area is the large volume and mass of material in each container. Standard sea-going containers are 2.5 x 2.5 x 6-12 meters in size and can contain up to 27 metric tons of material. Furthermore, the cargo is non-homogeneous. Thus, passive screening techniques based on the detection gamma rays emitted by the decays of 235U or 239Pu need to be supplemented by active techniques in order to provide very high reliability of detection.

Methods based on the use of neutron (or photon) interrogation followed by the detection of beta-delayed neutrons emitted by fission products are being developed at places such as Los Alamos National Laboratory [1]. However in hydrogenous cargo scenarios, these low-energy beta delayed neutrons will be severely attenuated, thus diminishing the potential signal. Table 1 lists the most intense gamma rays above either 3 or 4 MeV emitted by fission products produced via thermal-neutron induced fission of 235U and 239Pu. For 235U the total intensity of gamma rays above 3 MeV is 0.127 per fission, while for 239Pu this number is 0.065 per fission [2,3]. These intensities are approximately a factor of 10 larger than those of the beta-delayed neutrons emitted from the fission products from these isotopes. We have therefore decided to base our technique on the much more abundant and more penetrating high-energy gamma rays emitted by a number of short-lived fission fragments.

Initial small scale tests conducted at Lawrence Berkeley National Laboratory confirmed that following the thermal neutron-induced fission of 235U and 239Pu, many low-intensity gamma rays are emitted above 3 MeV. Furthermore, if one simply integrates all of the events observed Table 1. High-energy gamma-ray yields from thermal neutron-induced fission of (a) 235U and (b) 239Pu. The data used to calculate the yields shown here were taken from the fission product yields of England and Rider [2] and the decay data from Firestone and Ekstrom [3].

(a) (b)

Nuclide Half-life > 4 MeV > 3 MeV Nuclide Half-life > 4 MeV > 3 MeV (sec) gammas gammas (sec) gammas gammas per per per per fission fission fission fission 85Se 39. 0.0 0.0012 87Br 55. 0.0015 0.0025 86Br 55. 0.0013 0.0013 88Br 16. 0.0013 0.0020 87Br 55. 0.0045 0.0073 90-mRb 258. 0.00038 0.0021 88Br 16. 0.0045 0.0072 90Rb 156. 0.0025 0.0046 89Br 4.4 0.0016 0.0021 91Rb 58. 0.0020 0.0063 89Kr 189. 0.00064 0.0029 92Rb 4.5 0.0045 0.0049 90-mRb 258. 0.00063 0.0036 93Rb 5.9 0.00031 0.0029 90Rb 156. 0.0089 0.016 95Sr 25. 0.0003 0.0017 91Kr 8.6 0.000047 0.0020 97Y 3.8 0.0 0.013 91Rb 58. 0.0052 0.017 98Y 0.59 0.0024 0.0055 92Rb 4.5 0.011 0.012 106Tc 36. 0.0 0.0066 93Rb 5.9 0.00078 0.0073 140Cs 64. 0.0 0.0026 94Rb 2.7 0.00022 0.0015 141Cs 25. 0.0 0.0014 95Rb 0.38 0.000027 0.0011 142Cs 1.8 0.00037 0.0022 95Sr 25. 0.00052 0.0031 Total, Varying 0.017 0.065 97Y 3.8 0.0 0.017 including 98-mY 0.59 0.003 0.007 activities 136Te 17.5 0.0 0.0020 not shown 136I 83. 0.0005 0.0011 138I 6.5 0.00043 0.0010 above 3 MeV, and plots the results as a function of 140Cs 63. 0.0 0.0038 time after the end of bombardment, the effective “half- 141Cs 25. 0.0 0.0017 life” is observed to be on the order of 20 seconds [4,5]. 142Cs 1.8 0.00054 0.0014 Neutron irradiations of most common non-fissionable Total, Varying0458010.0458 0.127 materials showed low intensity or non-existent gamma including activities rays above 3 MeV and typically much longer half- not lives. The combination of a wedge-shaped energy shown spectrum (decreasing monotonically in intensity with increasing energy from 3 – 7 MeV) and this 25 second half life is a unique signature of fission. One potential interfering isotope that was identified in these initial tests was 16N that can be 16 19 produced via the O(n,p) reaction (Eth = 10.2 MeV) or the F(n,) reaction (Eth = 1.6 MeV). The half-life of 16N is 7.1 seconds and its decay produces gamma rays at 6.1 and 7.1 MeV [3]. Thus to avoid (or at least substantially reduce) the possibility of 16N interference, one should interrogate with neutrons whose energy is below 10 MeV. In the system developed at Lawrence Livermore National Laboratory, after initial tests   

    

HEU U O   3 8   61 cm 376.5 g   m c 1 6 15 cm

 

max En = 7 MeV 30 s on, 100 s off

Figure 1. Schematic representation of the cargo screening test facility at LLNL. The target of highly-enriched uranium (HEU) can be placed at a variety of heights (Rf) inside simulated cargos. Neutrons are produced in the basement using 4 MeV deuterons on a thick deuterium gas target. Following the neutron irradiation, gamma rays are detected in the array of plastic scintillators shown on the left. conducted with 14 MeV neutrons [6], we have chosen to utilize neutrons of maximum energy around 7 MeV produced by bombarding a thick deuterium gas target with 4 MeV deuterons. These neutrons are produced in a basement and are directed upwards toward the cargo in which a target of highly-enriched 235U (HEU) can be placed. We typically irradiate for 30 seconds, turn the neutrons off, and then count for 100 seconds using an array of large plastic scintillation detectors that are located on opposing sides of the cargo. Figure 2. Integrated number of gamma rays observed between 3 and 4 MeV following a single 30-second long neutron irradiation as a function of time from the end of bombardment. The various data sets shown were (starting from the top) taken with: HEU + 30 cm wood; bare HEU; HEU + 60 cm wood; HEU + 90 cm wood; wood only. The presence of a short-lived component due to the decays of fission fragments is clearly seen in all the data taken with HEU.

Some early data taken with neutrons produced by bombarding a thick carbon target with 4 MeV deuterons are shown in Figure 2. The presence of a short-lived component due to the decays of fission fragments is clearly seen in all the data taken with HEU. Note that each of the data sets shown in this figure was taken following a single 30-second long irradiation. More recent tests done with 7 MeV neutrons have shown conclusive signals even through 4 feet of wood. Other cargo scenarios are now being investigated. Figure 3 illustrates one of the concepts now under consideration for how this type of active neutron-based interrogation might be implemented in a port environment. Hidden WMD

Detecto

r Neutrons arrays Neutron (hidden generator )

Figure 3. Artist’s conception of the ”nuclear car wash”. A collimated beam of neutrons emerges from a below-ground accelerator facility and irradiates the cargo container. After the beam is turned off, the two arrays of scintillation detectors are used to detect high-energy beta-delayed gamma rays emitted by short-lived fission products.

Acknowledgments The results presented in this paper represent the work of a large number of scientists, engineers, and technicians in the “nuclear car wash” team at LLNL. This work was performed under the auspices of the U. S. Department of Energy by University of California, Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

References [1] R. C. Little, these proceedings. [2] T. R. England and B. F. Rider, ENDF-349, Los Alamos National Lab. Report, LA-UR-94- 3106 (1994). [3] R. B. Firestone and L. P. Ekstrom, WWW Table of Radioactive Isotopes (2004); http://ie.lbl.gov/toi. [4] E. B. Norman et al., Nucl. Instrum. & Meth. A 521 (2004) 608. [5] E. B. Norman et al., Nucl. Instrum. & Meth. A 534 (2004) 577. Active[6] D. R. neutron Slaughter et interrogation al., Nucl. Instrum. & Meth. B 241 (2005) 777. “The nuclear car wash”

Cargo Neutrons