Validation of reference materials for uranium radiochronometry in the frame of nuclear forensic investigations

Abstract

The results of a joint effort by expert nuclear forensic laboratories in the area of age dating of uranium, i.e. the elapsed time since the last chemical purification of the material are presented and discussed. Completely separated uranium materials of known production date were distributed among the laboratories, and the samples were dated according to routine laboratory procedures by the measurement of the (230)Th/(234)U ratio. The measurement results were in good agreement with the known production date showing that the concept for preparing uranium age dating reference material based on complete separation is valid. Detailed knowledge of the laboratory procedures used for uranium age dating allows the identification of possible improvements in the current protocols and the development of improved practice in the future. The availability of age dating reference materials as well as the evolvement of the age dating best-practice protocol will increase the relevance and applicability of age dating as part of the tool-kit available for nuclear forensic investigations.

Nuclear Forensic Science: Analysis of Nuclear Material Out of Regulatory Control

“…These methods are capable of the highest-precision analyses for most uranium and plutonium radiochronometry systems. Because of their higher sensitivity relative to counting methods for most radionuclides, mass spectrometry methods can be used to analyze samples that are comparatively much smaller or younger and therefore have very low total amounts of decay products (e.g., Wallenius & Mayer 2000, Wallenius et al 2002, Eppich et al 2013, Gaffney et al 2015, Kayzar & Williams 2016, Varga et al 2015. Isotope dilution methods are used for quantification of parent and progeny isotope abundances by TIMS or ICP-MS. For ICP-MS and TIMS, the analyte must be purified from sample matrix, whereas SIMS analysis utilizes bulk sample.…”

Identification of uranium signatures relevant for nuclear safeguards and forensics

“…The Th/U separation factor is relatively low; therefore the 230 Th/ 234 U chronometer cannot be used to estimate the date of this ion exchange separation step [17]. However, for most elements, high separation factors (above 50) were observed, identifying this stage as an effective purification and pre-concentration step.…”

Improved sample utilization in thermal ionization mass spectrometry isotope ratio measurements: refined development of porous ion emitters for nuclear forensic applications

“…The relatively rapid ingrowth of 230 Th has enabled this chronometer to be successfully applied for the determination of sample with ages ranging from a few years to several hundred thousand years old [2,14,15]. The accuracy of model-ages determined using the 230 Th/ 234 U chronometer rely heavily on the following assumptions: 1) the parent isotope has been completely separated form daughter products and any residual impurities, 2) the system remains closed after purification, and 3) the measured isotope ratios values are accurate.…”

“…The initial trials detailed in this paper were conducted using uranium isotopic standards; chronometric investigations are not the intended purpose of such samples. The 230 Th/ 234 U chronometer is highly sensitive to initial purification/purity of material, a decontamination factor of greater than 10 7 is required to eliminate positive bias caused by residual 230 Th in the material [14]. Future trials would greatly benefit from the implementation of certified uranium chronometric standards, which are highly purified and have precise, well-documented purification dates.…”

“…These two isotopes, 234 U and 230 Th, represent the longest-lived intermediate decay products of the 4n + 2 decay series of 238 U, illustrated in Figure 5.1, also known as the radium series. Due to the relatively rapid ingrowth of 230 Th, this chronometer has been successfully applied for the determination of sample ages ranging from a few years to several hundred thousand years old [2,14,15].…”