In View of “On-Site” Nuclear Forensics and Assay of Fissile Materials in Sealed Packages by High-Resolution γ-Ray Spectrometry

Abstract

Several incidences of nuclear smuggling during the past few decades have raised the demand for the development of a strong “on-site” nuclear forensic infrastructure. High-resolution γ-ray spectrometry (HRGRS) plays an important role in nuclear forensics. However, the existing methodologies, developed primarily for nuclear fuel cycle applications, are relative and rely on the availability of a standard, limiting their use for the absolute assay of special nuclear materials in nonstandard geometry samples with an unknown matrix, which is vital to make a quick “on-site” decision on the severity, potential radiological threat, and intended use of an interdicted package. In this work, a methodology has been developed using HRGRS for quantifying fissile (235U, 239Pu) and other radioisotopes, which is applicable to sealed packages without requiring the knowledge of the sample geometry and the matrices. By combining experiments and Monte Carlo simulations, an iterative methodology has been proposed for “point” to “extended” source absolute efficiency transformation and demonstrated further for the absolute isotopic assay of uranium and plutonium standards, mock-up nuclear forensic samples, and an unknown nuclear material mixture with a nonstandard geometry, compound matrices, and a wide variation in the elemental and isotopic compositions with a view to imitate an “on-site” experience. The present methodology requires an assay time of only a few minutes to an hour and thus promises “on-site” nuclear forensic analysis of suspected flagged packages at borders and ports using high-resolution γ-ray spectrometry. Furthermore, the present methodology is versatile and can also be adopted for wider applications, beyond nuclear forensics.

Interrogating a Mixed Actinide Basket Using High-Resolution γ-Ray Spectrometry: A Nuclear Forensic Perspective on Possible Smuggling Scenarios

“…The point to extended source universal efficiency transfer procedure for the assay of homogeneously mixed samples with the insignificant effect of container material attenuation has been discussed in detail in our previous publication. 18 In brief, a point source detector efficiency curve was generated over the γ-ray energy range of 1−2500 keV using the Geant-4 Monte Carlo simulation for the measurement of a point radioactive source at a sample-todetector distance of 25 cm. The Geant-4 simulated point source efficiency curve was transferred to an extended source absolute efficiency curve for a given sample-to-detector geometry using an iterative self-attenuation correction approach.…”

“…or a homogeneous mixture of more than one element/isotope (for example, nuclear material mixture). 18 In the Pu samples, 241 Am was also present as the decay product of 241 Pu formed homogeneously in the Pu matrix. Due to the homogeneous mixing of all isotopes in the samples in the previous work, a universal absolute efficiency calibration was reasonable for the absolute assay of any γ-ray-emitting radionuclides present.…”

“…In our previous work, an iterative methodology was developed for point to extended source absolute efficiency transformation for random geometry samples with a single radioelement (for example, U, Pu, etc.) or a homogeneous mixture of more than one element/isotope (for example, nuclear material mixture) . In the Pu samples, 241 Am was also present as the decay product of 241 Pu formed homogeneously in the Pu matrix.…”

“…The point source detector efficiencies at sample-to-detector distances (R) of 25 and 130 cm, ε R (E i ), were obtained separately using the Geant-4 Monte Carlo simulation. While the simulated point source efficiency curve at 25 cm for the 30% coaxial HPGe can be found in our earlier publication, 18 the same at 130 cm for the 50% HPGe has been computed separately using the Geant-4 code in the present study and given in the Supporting Information (see Figure S2). To establish the methodology and the concept, good counting statistics on the measured γ-ray spectra were ensured by keeping the acquisition time high (about 5−8 h), in most cases, except the sample containing natural uranium (Mock-up_A), where due to low specific activity of 238 U (1.24 × 10 4 Bq/g), the acquisition of a statistically good γ-ray spectrum needed an overnight counting [counts/second at 258.227 keV = 0.056 ± 0.004; the least abundant (0.000764 ± 0.000021 photons/disintegration 53 ) γ-ray of 234 Pa m (in equilibrium with 238 U) that has been detected].…”

“…We have recently proposed an iterative methodology for absolute efficiency calibration of a coaxial HPGe detector for the assay of plutonium and other γ-emitting radioisotopes in nonstandard geometry voluminous packages with unknown matrix density, where starting with the point source detector efficiency, the extended source absolute efficiency curve for a given sample has been iteratively constructed by varying the sample attenuation. 18 In the present work, the efficiency transfer methodology, developed earlier for the plutonium assay, 18 has been extended further for the assay of multicompartmental mixed actinide systems, to mimic a practical seized sample in a nuclear smuggling scenario where multiple small containers/packets carrying different actinides may be packed together in a single bag (can be artistically visualized as a "mixed actinide basket"). In such scenarios, the additional problems compared to those addressed in the previous methodology need to be addressed, without which it would fail.…”