Nondestructive Quantitative Analysis of Difficult-to-Measure Radionuclides 107Pd and 99Tc

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

The total fissile content in seized nuclear materials is of immense importance and needs to be estimated with reasonable accuracy as a part of nuclear forensics for early decision-making in legal proceedings. High-resolution γ-ray spectrometry (HRGRS), because of its nondestructive nature, is a powerful tool for the assay of such samples to reach a quick "on-site" decision on the severity, intended use, and associated radiological threat. If the seized package contains fissile isotopes of more than one actinide in a multicompartmental heterogeneous mixture, analogous to the most likely scenario of a "smuggled mixed actinide basket", its "on-site" quantification can be extremely challenging. This makes up an increasing share of the absolute HRGRS in nuclear forensics and demands for fundamentally new approaches. In the present work, the challenges associated with varying attenuation experienced by γ-rays of different actinides at different subcontainments of the heterogeneous sample matrix have been addressed by an iterative efficiency transfer approach from "point" to "extended" source for individual actinides and demonstrated for the assay of four mock-up samples and a legacy packet, mimicking seized packages containing nuclear materials. An absolute isotopic inventory of the fissile and other radioisotopes has been obtained within <10% along with the assay of total U and Pu within <3% of the expected values with measurement uncertainty <10% for the majority. The present approach has a good potential for "on-site" nuclear forensics in nuclear smuggling scenarios and also can be adapted easily for a wide variety of other applications.

Effect of sample density in prompt γ-ray analysis

A high-accuracy analytical method is broadly required to obtain reliable research results. Thus, prompt γ-ray analysis (PGA), one of the most accurate non-destructive analytical methods, has been employed in various fields. However, the measurement accuracy of PGA is also known to degrade in hydrogenous samples. The degradation is caused by variation in the measurement sensitivity (counts per milligram) following the change in neutron energy due to scattering with hydrogen nucleus. Number of scatterings is well known to depend on the hydrogen content in a sample. However, considering multiple scatterings, hydrogen density, which has not been taken into account as yet, may also lead to the accuracy degradation. Here, we show the effect of the hydrogen density in PGA by evaluating the measurement sensitivity of samples with the same hydrogen content and different densities. We find that the measurement sensitivity varies by more than 30% depending on the hydrogen density even at the same hydrogen content. The variation is a particularly serious problem for PGA requiring a few percent accuracy in most cases. Additionally, although the variation is apparently observed in hydrogenous samples, the similar phenomenon can occur in other nuclides with a large scattering cross section; it may affect nuclear cross-section measurements using neutrons in such fields as astrophysics and nuclear energy.