GAMMA RAY DETECTION IN CARGO

Jackson Choate

Cargo Scanning•Thousands of cargo containers enter ports each day•Every minute inspecting a cargo container keeps it from shipping•Advances in gamma ray, high energy x-ray, and neutron scanning are reducing inspection times

Gamma-Ray Radiography

•Gamma ray scanners can penetrate denser materials than conventional x-rays•Cobalt-60 produces 1.25 MeV gamma rays capable of penetrating 15 – 18 cm of steel•Provides an “x-ray” of cargo containers •Gamma ray detectors can also pick up photon emissions from radioactive sources, but some may be benign, i.e. Potassium-40

High Energy X-Ray Radiography

•High Energy X-rays can be produced through Bremsstrahlung process•Electrons used are accelerated up to 6 MeV, producing x-rays capable of penetrating 30 – 40 cm of steel•Penetrate deeper than radioactive gamma rays, but are more expensive to use•Deliver about 1000 times more radiation to stowaways

Neutron Interrogation•Inelastic neutron scattering produces gamma rays with distinctive energies•Neutrons generated by pulsed fusions of deuterium and tritium•Gamma ray energies can be compared to those of known materials•Does not provide an image, but photon spectrum provides more information about what materials are present

High Energy Photon Detection•To create an image, photons must be correlated to their origin via a collimator made of a dense material•Collimated photons then hit a scintillator, emitting a lower energy photon•The lower energy photon can then be detected via a photomultiplier

Neutron Detection

• Neutrons can be detected through scintillation as well, but are more commonly detected through absorption reactions

• Gas proportional detectors use gases with high neutron absorption cross-sections, such as 3He or BF3 (using 10B), to emit ionized particles

n + 3He → 3H + p + e-

n + 10B → 7Li + α + 2e-

• However, these detectors can only count the number of neutron interactions, not their energies

Conclusion

High energy particles and radiation can be used to minimize the amount of time it takes to inspect cargo

Gamma rays and high energy x-rays provide a thorough image of target

Neutrons provide a photon spectrum that can be compared to known materials

Sources

Clements, Elizabeth. "High-energy X-rays search containers." Symmetry Aug 2010: n. pag. Web. Oct 2010.

Gilmore G, Hemingway J. Practical Gamma-Ray Spectrometry. John Wiley & Sons, Chichester: 1995.

Jones,J. L.; Haskell, K. J.; Hoggan, J. M.; Norman, D. R. (June 2002) (PDF). ARACOR Eagle-Matched Operations and Neutron Detector Performance Tests. Idaho National Engineering and Environmental Laboratory. http://www.inl.gov/technicalpublications/Documents/3310854.pdf. Retrieved Oct. 2010.