NIST:铀粒子年龄测定、聚集和模型年龄最佳估计值(2025) 24页

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Cite this: DOI: 10.1039/d5an00249d
Received 4th March 2025,
Accepted 7th June 2025
DOI: 10.1039/d5an00249d
rsc.li/analys t
Uranium particle age dating, aggregation, and
model age best estimators
Evan E. Groopman, *
a
Todd L. Williamson,
a
Timothy R. Pope,
b
Michael G. Bronikowski,
c
Spencer M. Scott
c
and Matthew S. Wellons
c
We present important aspects of uranium particle age dating by Large-Geometry Secondary Ion Mass
Spectrometry (LG-SIMS) that can introduce bias and increase model age uncertainties, especially for
small, young, and/or low-enriched particles. This metrology is important for applications related to
International Nuclear Safeguards. We explore inuential factors related to model age estimation, including
the eects of evolving surface chemistry on inter-element measurements of particles (e.g., Th and U),
detector background, and aggregation methods using simulated and actual particle samples. We intro-
duce a new model age estimator, called mid68, that supplements 95% condence intervals, providing a
best estimate and uncertainty about the most likely age. The mid68 estimator can be calculated using
the Feldman and Cousins method or Bayesian methods and provides a value with a symmetric uncertainty
that can be used for calculations and approximate aggregation of processed model age values when the
raw data and correction factors are not available. For particles yielding low
230
Th counts amidst nonzero
detector background, their underlying model age probability distributions are asymmetric, so the mid68
estimator provides additional robust information regarding the underlying model age likelihood. This
study provides a comprehensive and timely examination of critical aspects of uranium particle age dating
as more laboratories establish particle chronometry capabilities.
Introduction
Environmental sampling of nuclear facilities has been a
routine Nuclear Safeguards practice for the International
Atomic Energy Agency (IAEA) for the last several decades.
13
Environmental uranium can be chemically and isotopically
analyzed at the bulk (>nanogram) and particle (
levels to infer the operational history of nuclear facilities.
Similar types of analyses have been performed on interdicted
nuclear materials for nuclear forensic purposes, e.g., Kristo
4
and Moody et al.
5
The IAEA Department of Safeguards has
identified age determination of U and Pu relevant to the
origin of nuclear materials to be a top priority R&D need.
6
For decades, bulk analytical techniques have been used to
determine dates for the purification or manufacture of nuclear
material using the decay of radioisotopes, such as U and its
decay products, e.g.,
234
U
230
Th
226
Ra and
235
U
231
Pa
227
Ac.
Model ages can be constructed by comparing the ratio of
decay products to parent radioisotopes using their character-
istic decay rates, under the assumption that the chronometers
were initially reset from material processing, i.e., only parent
isotopes were present at the time of material production.
Incomplete purification of decay products would bias these
analyses, resulting in artificially older model ages. Recently,
the National Institute of Standards and Technology (NIST)
extended the application of age dating using the
234
U
230
Th
chronometer (t
1/2
= 245.6 ka) to the regime of individual U
microparticles using Large-Geometry Secondary Ion Mass
Spectrometry (LG-SIMS).
7
Challenges associated with the ana-
lyses of trace isotopes in atom-limited microparticles have
long been acknowledged, highlighting the need for continued
metrology and reference material development.
715
Several important factors impact the accuracy and precision
of U particle age dating analyses by LG-SIMS.
7,10
One set of
factors regards intrinsic sample attributes, such as the enrich-
ment level, particle mass, and material age. Generally, the
higher the
234
U enrichment (which is often correlated with
235
U enrichment), the larger the sample mass, and the older
the material, the more
234
U and
230
Th atoms will be available
to analyze, which increases the relative precision of a measure-
ment. Other factors are instrumentation and analysis protocol-
related, including the ion yield and instrument transmission,
detector background rate, measurement duty cycle per isotope,
Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d5an00249d
a
Materials Measurement Science Division, National Institute of Standards and
Technology, Gaithersburg, MD, 20899, USA. E-mail: evan.groopman@nist.gov
b
Pacific Northwest National Laboratory, Richland, WA, 99354, USA
c
Savannah River National Laboratory, Aiken, SC, 29808, USA
This journal is © The R oy al Society of Chemistry 2025 Analys t
Open Access Article. Published on 09 June 2025. Downloaded on 6/13/2025 12:21:53 PM.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
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