Evaluating Testing, Costs, and Benefits of Advanced Spectroscopic Portals for Screening Cargo at Ports of Entry Interim Report (Abbreviated Version) (2009) / Chapter Skim
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2 Background on Radiation Detection
2 Background on Radiation Detection
Pages 14-24
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From page 14... ...
16 The relative intensities of multiple gamma rays from a single radionuclide can also be used to help identify it. 17 A radiological weapon uses radioactive material to cause harm based on the radiation the material emits.
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From page 15... ...
Real spectra are necessarily more complicated, due to the existence of alternative physical mechanisms for absorption and scattering of gamma rays in the detector and, to a lesser extent, to imperfections in the way different types of detectors and individual detectors of the same type operate. The most important difference between a real spectrum and the above-described "notional" spectrum is the presence of a broad continuum of gamma rays caused by Compton scattering.
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From page 16... ...
High-Z materials are more effective at attenuating gamma rays, especially low-energy gamma rays, whereas low-Z materials, notably materials containing hydrogen atoms, enhance the absorption of neutrons. 20 High-Z materials close to a source that emits beta particles, such as strontium-90, will enhance the production of Bremsstrahlung, a continuum spectrum of photons (gamma rays)
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From page 17... ...
Sodium Iodide Detectors Sodium iodide detectors consist of a NaI crystal containing approximately 0.1% thallium, coupled optically to a photomultiplier tube. 22 They are scintillation detectors: gamma rays interact with the detector to produce low-energy photons in the energy range of visible light, a phenomenon called scintillation.
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From page 18... ...
measured with a NaI detector. Low detector resolution poses a challenge to analysis algorithms necessary to process the data and obtain meaningful conclusions, especially when there is a large statistical uncertainty in the data (i.e., few counts)
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From page 19... ...
In an ionization detector, the gamma-ray energy is converted directly into electrons, which form the signal proportional to the energy deposited by the gamma ray. 25 Figure 2.3 shows the HPGe spectrum of the same NORM source whose measurement with an NaI detector was shown above (Figure 2.1)
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From page 20... ...
(Note that the presence of uranium-235, whose strongest gamma-ray is buried in the continuum with a NaI detector, is clearly revealed in the HPGe spectrum.) Although the higher energy resolution of HPGe detectors is essential in many laboratory measurements and would be desirable for detecting nuclear and radiological materials, especially under difficult conditions (e.g., masking)
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From page 21... ...
The challenge in template matching is the nearly limitless set of combinations of sources and attenuating materials and thicknesses, along with background radiation. Although software implementing algorithms for gamma-ray spectral analysis has been the subject of intense development in the national laboratories, and several vendors of spectrometers provide such software, there are in fact few commercial products available for radionuclide identification using gamma spectroscopy.
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From page 22... ...
But PVT detectors have very poor energy resolution; they cannot distinguish one gamma ray energy from another, except over broad energy ranges, so they have very limited ability to characterize the source of those gamma rays. The PVT detectors in the RPMs at most ports of entry have been equipped with crude energy resolution in the form of energy windowing: The gamma-ray events are binned into four large energy windows.
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From page 23... ...
The ASP 31 As noted above, gamma-ray energy resolution in a HPGe detector is far superior to that in a NaI detector, but the cost of HPGe crystals is much higher than sodium iodide crystals. The cost and the difficulty of growing large HPGe crystals resulted in the HPGe ASP having a much smaller detector volume than the others.
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From page 24... ...
Although there is evidence that the spectral analysis programs work remarkably well under challenging circumstances, the two vendors' algorithms appear to yield somewhat different results, and it is not clear at this point that either is optimal.
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