Gamma-ray emissions from significant quantities of HEU, DU, and Pu. The HEU and HEU largely overlap aside from emissions above 500 keV. It is important to acknowledge that the concentration of , if present at all, may vary considerably from its modeled value of 200 ppt. Note that the logarithmic flux scales and energy scales differ between materials.
Simulated detector response functions for HPGe, NaI, and PVT spectrometers, each a 7.5 cm right cylinder, exposed to a point source at 25 cm emitting monoenergetic 1001 keV gamma rays. The ratio of the peak area (900–1100 keV in NaI) to the total area is defined to be the peak-to-total efficiency, a measure that varies widely with detector type and size.
Detector response functions for the sensors of Table IV to the SNM fluxes shown in Fig. 1.
Comparison of background detector response functions from PVT, NaI, and HPGe detectors of various sizes consistent with the results of Table V. Data courtesy of Walt Hensley, Pacific Northwest National Laboratory.
Variation in background radiation as measured by a NaI detector driving through urban sections of Seattle, WA.
Schematic of count distributions for a source located in a static background (top), in the presence of shielding and/or background suppression (middle), and in the presence of nuisance sources (bottom). The vertical lines denote potential threshold settings.
Schematic example of ROC curves that quantify the tradeoffs between detection objectives (PD) and operational constraints (PFA).
Example ROC curves for two detection scenarios: portal monitoring and in-transit monitoring. The ROC curves could exhibit different behaviors, and the target PFA ranges (gray shaded boxes) may differ by orders of magnitude.
Count observations made by a moving detector in an urban environment (red trace extending from 0 to 5000 counts). Note the existence of the high-count tail in this distribution. The blue curve centered on 1100 counts is a Poisson distribution with the same mean observed count value. The orange curve centered on 2200 counts is the sum of the static background distribution and a source that induces a count rate equivalent to the static background mean.
Time series of ratios comparing the low- and high-energy components of observed detector response functions. Energies noted in the legend correspond to different cutoffs between high and low energies. Compare the variability here to that observed in Fig. 5.
Example background spectra obtained over different acquisition times with a NaI detector.
Empirical spectrum of a shipment destined for a hospital and surrounded by substantial quantities of lead. Shown for comparison is a spectrum obtained from a sealed source without any attenuating material. Both spectra are normalized to a sum of unity for shape comparison.
Schematic of various directional sensor methods that range from simple mechanical collimation to electronic collimation.
Image acquired by commercial coded aperture imager over 300 s (shown in color) overlaid onto a photograph (shown in black and white). The 20 mCi source was 70 m from the instrument. Reproduced from Woodring et al. (Ref. 98) with permission from Elsevier.
Confirmed incidents involving HEU or Pu.
Emissions from significant quantities of SNM in a cylindrical geometry surrounded by 1 cm of aluminum.
Attenuation lengths required to reduce monoenergetic fluxes by 0.001 in important attenuating materials, as derived from Berger and Hubbell (Ref. 28). Densities are assumed to be those for the pure elemental form. The attenuation length for a range of low-, hydrogenous materials, which are typical of cargo in the stream of commerce, is similar to those listed here for water.
Intrinsic peak efficiencies for various sensors.
Comparison of minimum detectable quantity and a crude benefit/cost ratio for representative sensor types consistent with Fig. 3, assuming a point source emitting 1001 keV gamma-rays located 100 cm from front face of each detector. These calculations assume measurement times of 1 and 100 s. The units are arbitrary.
Prominent gamma-ray emissions from naturally occurring radioisotopes (from Ref. 44).
Gamma-ray emissions from common medical isotopes (top) and commercial sources (bottom) (from Ref. 44).
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