Stephen Croft

• Professor at Lancaster University

This study introduces novel semi-empirical calculations to obtain vacancy transfer probabilities from the K shell for elements in the atomic range 16 ≤ Z ≤ 92. These calculations are derived from experimental data extracted from the scientific literature and previously tabulated by the research team. The results are compared with theoretical, semi-...

The main objective of this paper is to calculate semi-theoretical values for the L-shell X-ray intensity ratios I_Li \/I_Lα (i= β and γ) across a broad range of elements in the atomic number range 36 ≤ Z ≤ 92 and photon excitation energies spanning from 1.916 keV to 200 keV. These values are derived from theoretical calculations and subsequently re...

Detecting shielded special nuclear material, such as nuclear explosives, is a difficult challenge pursued by non-proliferation, anti-terrorism, and nuclear security programs worldwide. Interrogation with intense fast-neutron pulses is a promising method to characterize concealed nuclear material rapidly but is limited by suitable source availabilit...

Ground-level neutron monitors (NMs) are essential tools for monitoring space weather events, including the detection and alerting of ground-level enhancement (GLE) events. This study presents findings from a neutron monitoring survey using two compact N50L neutron slab-based subsystems (N50L detectors encased in lead bricks, used as an analogue to...

The main objective of this study is to obtain three-dimensional empirical K-shell x-ray fluorescence cross-section ( σKα,σKβ and σKtot) values for a wide range of elements 16≤Z≤92 for photons with energies from 5.46 keV to 123.6 keV, using more than 3300 experimental data values published between 1985 and 2023 by numerous researchers. These data va...

In the present work, we offer a collection of documented values for vacancy transfer probabilities 〖 (η〗_XY, X=K,Li; Y=L,M,N,Li,Mj,Np,Op; i=1,2,3; j=1,2,3,4,5; p=1,4,5) sourced from published technical literature spanning 1993 to 2023 for elements in the atomic range 16 ≤ Z ≤ 92. We found 1200 experimental vacancy transfer probability values from 6...

The significance of theoretical, experimental, and analytical methods in calculating the intensity ratios of L-shell transitions for diverse elements lies in their widespread applications across various domains, including physical chemistry and medical research. In the present paper, empirical values for intensity ratios were computed through polyn...

The main purpose of this paper is to collect from various sources over 3300 experimental values of K-shell fluorescence cross sections (σKβ1,3, σKβ2,4, σKα1, σKα2, σKβ1, σKβ2, σKβ´1, σKβ´2, σKα, σKβ and σKtot) of elements with atomic number in the range by photo-ionization in multiple excitation energy ranging from 5.46 to 661.6 keV, which are pres...

We report on a new ground-level neutron monitor design for studying cosmic rays and fluxes of solar energetic particles at the Earth’s surface. The first-of-its-kind instrument, named the NM-2023 after the year it was standardised and following convention, will be installed at a United Kingdom Meteorological Office observatory (expected completion...

In this study, a comprehensive dataset of X-ray emission intensity ratios has been compiled, including I_Lβ⁄I_Lα , I_Lγ⁄I_Lα , I_Ll⁄I_Lα , I_Lγ⁄I_Lβ , I_Ll⁄I_Lγ , I_Ll⁄I_Lβ , I_Lγ5⁄I_Lα , I_Lγ44'⁄I_Lα , I_Lη⁄I_Lα , and I_Lγ1⁄I_Lα , extracted from literature spanning the years 1971 to 2023, and encompassing 83 research papers. Over this timeframe, a...

This paper provides a template of expected uncertainties and correlations for measurements of total neutron cross-section observables by transmission. Measurements with time-of-flight and mono-energetic neutron sources are covered. The information required for evaluations in the resonance region and high energy region is detailed, along with the te...

The covariance committee of CSEWG (Cross Section Evaluation Working Group) established templates of expected measurement uncertainties for neutron-induced total, (n,γ), neutron-induced charged-particle, and (n,xn) reaction cross sections as well as prompt fission neutron spectra, average prompt and total fission neutron multiplicities, and fission...

In this paper, we provide templates of measurement uncertainty sources expected to appear for average prompt- and total-fission neutron multiplicities, ν ¯ p $ \overline\nu_p $ and ν ¯ t $ \overline\nu_t $ , for the following measurement types: absolute manganese-bath experiments for ν ¯ t $ \overline\nu_t $ , absolute and ratio liquid-scintillator...

This paper provides a template of expected uncertainties and correlations for measurements of neutron-induced capture and charged-particle production cross sections. Measurements performed in-beam include total absorption spectroscopy, total energy detection, γ-ray spectroscopy, and direct charged-particle detection. Offline measurements include ac...

Passive Gamma Emission Tomography (PGET) has been developed by the International Atomic Energy Agency to directly image the spatial distribution of individual fuel pins in a spent nuclear fuel assembly and determine potential diversion. The analysis and interpretation of PGET measurements rely on the availability of comprehensive datasets. Experime...

Passive Gamma Emission Tomography (PGET) has been developed by the International Atomic Energy Agency as a way to directly image the spatial distribution of individual fuel pins in a spent nuclear fuel assembly and so determine potential diversion. Constructing the analysis and interpretation of PGET measurements rely on the availability of compreh...

Most of the Pu separated from irradiated commercial nuclear fuel is stored as PuO 2. The primary quantitative nondestructive measurement technique used to verify the amount of Pu in storage containers is passive neutron correlation counting. An important physical property of the oxide material is the ratio, α, of the rate of (α, n) neutrons produce...

A collection of 1118 experimental Kβ⁄Kα X-ray intensity ratios induced by photon impact was published recently by Daoudi et al. (2020). There were errors in some tabulated values. These errors are corrected here, as well as a substantial update including all experimental Kβ⁄Kα X-ray intensity ratios obtained from various sources for elements rangin...

Neutron coincidence counting (NCC) based on shift-register logic (SRL) is extensively used by international nuclear safeguards inspectorates to verify special nuclear material inventories, particularly Pu. An alternative autocorrelation pulse counting analysis technique is founded on Feynman sampling and makes use of the well-known Feynman-Y statis...

Results from a preliminary accelerator-based experiment conducted at the Harwell Tandem Van de Graaff generator to estimate the thick target integrated over angle yield of 6.129 MeV γ-rays from α-particles in the MeV range stopping in natural carbon are briefly outlined. The results should be useful in planning future detailed experiments to genera...

A widely applied technical measure of nuclear safeguards and nonproliferation for the nondestructive verification and mass-assay of items containing separated plutonium is passive neutron multiplicity counting. The primary source of time correlated passive neutrons is the spontaneous fission of Pu, where the ²⁴⁰Pu is usually the dominant isotope wi...

Interest in safeguards verification measurements using passive thermal neutron counting to assay ²³⁵U content in large 30B UF6 canisters has grown in recent years. The prohibitively high cost and impracticality of using reference 30B calibration cylinders extensively will likely make accurate simulations of increasing interest. Accuracy of the simu...

Californium-252 is used as a neutron calibration source for passive neutron correlation counting. Source age and isotopic information are needed to make decay corrections to the neutron emission rate due to the influence of ²⁵⁰Cf. Gamma-ray signatures present in the spectrum from spontaneous fission products and odd-numbered Cf isotopes can be used...

Verification of the fissile (and fertile) content in fresh nuclear fuel assemblies is conducted by the IAEA to enforce the Nuclear Non-Proliferation Treaty using the UNCL (Uranium Neutron Collar – Light Water Reactor Fuel). The UNCL uses an uncorrelated AmLi neutron source to interrogate the fuel, producing a signal of coincident fission neutrons (...

The declared inventory of ²³⁵U in fresh, low enriched uranium, nuclear fuel assemblies is routinely verified by nuclear safeguards inspectorates using the UNCL (Uranium Neutron Collar – Light Water Reactor Fuel) nondestructive assay instrument. The trend in modern fuels is towards higher initial enrichment, which in turn requires a larger number of...

Several recent papers address improved uncertainty quantification (UQ) for measurements used in nuclear safeguards. This paper reviews progress and presents new results for bottom-up (first principles) and top-down (empirical) UQ for safeguards, where the main quantitative measure of uncertainty is the total measurement error standard deviation (SD...

The evaluated O16(n,α) cross sections in the ENDF/B-VIII.0 nuclear data library remain uncertain because of systematic discrepancies in the measured data. In the energy region below the first excited state, R-matrix analyses rely heavily on the inverse reaction, and in particular, they rely on the measured C13(α,n)O16 cross sections reported by Bai...

The capabilities of current safeguards neutron instrumentation can be expanded by acquiring and analyzing data in new ways. A new List Mode Collar (LMCL) was created by retrofitting a traditional safeguards neutron coincidence counting system, the Neutron Coincidence Collar, with newly developed preamplifiers on each of its 18 ³He tubes in active i...

As a by-product of another measurement, ratios of the single-escape (SE) and double-escape (DE) efficiencies relative to the full-energy-peak efficiency (FE) have been measured for two HPGe detectors for 60Co. For a 2.5-cm-thick 95 cm3 crystal the results were SE/FE = 0.00048±0.00020 and 0.00325±0.00024 for 1173 and 1332 keV gamma-rays respectively...

Natural, depleted, and low enriched uranium hexafluoride is manufactured and transported on an industrial scale to support commercial nuclear power plant operation. One-way material production and flow is verified by nuclear safeguards inspectorates is by neutron counting. The dominant neutron source term is (α,n) production due to ²³⁴U which gets...

This paper presents an analysis of the neutron yield and energy distribution of neutron source terms generated from the interaction of α-particles with 17,18O nuclei in fissile actinide compounds. When compared with measured data, the neutron yield and energy distribution data calculated for several isotopic configurations are used to validate the...

Double pulsing in neutron coincidence counters is the result of a pulse processing chain-³He proportional counter timing incompatibility known to nondestructive assay system designers. It is not currently widely acknowledged, or accounted for, in the user community. However, it is gaining attention as list mode data acquisition and analysis becomes...

Compact, bright neutron sources are opening up several emerging applications including detection of nuclear materials for national security applications. At Los Alamos National Laboratory, we have used a short-pulse laser to accelerate deuterons in the relativistic transparency regime. These deuterons impinge on a beryllium converter to generate ne...

High-order angular correlations of fast neutrons emitted by spontaneous fission in californium-252 ( ²⁵² Cf) have been measured with a 2D array of 15 EJ-309 organic liquid scintillation detectors. These are compared with the results of Geant4 simulations using both an uncorrelated, average fission model and correlated fission models, which have als...

Cf neutron sources are particularly convenient, and therefore widely used, to characterize and calibrate many kinds of neutron detectors for both scientific and technological applications. An emerging interest is the use of ²⁵²Cf to calibrate neutron correlation counters for the verification and assay of Pu-items as part of the international effort...

Passive neutron multiplicity counting can be used to determine the absolute emission rate of ²⁵²Cf spontaneous fission sources. Such sources can then be used to characterize and calibrate instruments used to assay Pu for materials control and accountancy, nuclear safeguards, and nuclear security applications. An important contribution determining t...

Understanding and proper treatment of correlated information is very important in measurement science because it is a common problem. Covariance, if unrecognized or ignored without justification, can lead to errors. Correlations can be introduced in many ways. Measurement procedures, assumptions, algorithms, operators, nuclear data, model mismatch,...

In response to the letter "Pension problems" (September p22) in which Neil L Hancox describes his long-running issues with claiming his pension from the now-defunct AEA Technology – the privatized commercial arm of the UK's Atomic Energy Research Establishment (UKAEA).

Bulk uranium items are often measured using active neutron interrogation systems to take advantage of the relatively high penetrability of neutrons, providing the ability to measure uranium mass in large, dense configurations. Because the emitted neutrons from ²³⁵U and ²³⁸U in such configurations are, for all practical purposes, indistinguishable,...

Neutron coincidence counting is a well-established technique used for the nondestructive quantification of special nuclear material during international safeguards inspections. The neutron counters are commonly designed with an annular body, centered about an inner well or cavity into which a measurement item is placed, and the moderating annulus i...

Augmenting or interchanging current international safeguards shift register−based data acquisition systems with list mode data acquisition systems gives nondestructive assay (NDA) systems greater versatility. Neutron list mode data analysis offers comparable analytical results to the more widely used shift register analysis in nuclear material quan...

A model quantifying detector cross-talk and the misidentification of events in fast neutron coincidence distributions is described. This is demonstrated for two experimental arrangements comprising rings of 8 and 15 organic liquid scintillation detectors. Correction terms developed as part of this model are tested with 252Cf and a relationship is d...

In the standard practice of neutron multiplicity counting , the first three sampled factorial moments of the event triggered neutron count distribution are used to quantify the three main neutron source terms: the spontaneous fissile material effective mass, the relative (α,n) production and the induced fission source responsible for multiplication...

Over the past few decades, neutron multiplicity counting has played an integral role in Special Nuclear Material (SNM) characterization pertaining to nuclear safeguards. Current neutron multiplicity analysis techniques use singles, doubles, and triples count rates because a methodology to extract and dead time correct higher order count rates (i.e....

3He gas has been used in various scientific and security applications for decades, but it is now in short supply. Alternatives to 3He detectors are currently being integrated and tested in neutron coincidence counter designs, of a type which are widely used in nuclear safeguards for nuclear materials assay. A boron-coated-straw-based design, simila...

Neutron multiplicity counting using shift-register calculus is an established technique in the science of international nuclear safeguards for the identification, verification, and assay of special nuclear materials. Typically passive counting is used for Pu and mixed Pu-U items and active methods are used for U materials. Three measured counting r...

Lightly encapsulated 252Cf sources are commonly used to characterize and calibrate neutron detectors for safeguards applications without much attention being paid to what it means for the encapsulation to be neutronically “light”. In this work we quantify the impact of encapsulation on both the neutron spectrum and neutron intensity. We find that a...

Neutron time correlation analysis is one of the main technical nuclear safeguards techniques used to verify declarations of, or to independently assay, special nuclear materials. Quantitative information is generally extracted from the neutron-event pulse train, collected from moderated assemblies of ³He proportional counters, in the form of correl...

We extend the familiar B?hnel point-model equations, which are routinely used to interpret neutron coincidence counting rates, by including the contribution of delayed neutrons. After developing the necessary equations we use them to show, by providing some numerical results, what the quantitative impact of neglecting delayed neutrons is across the...

This paper sets up a formalism that is sufficiently general to describe the effects of photofission, photonuclear, (n, xn), (n, n′xγ), and (n, xγ) reactions on the neutron-gamma Feynman-alpha variance-to-mean ratios. Such a formalism is obtained using the Chapman-Kolmogorov (master) forward equation for the above-mentioned set of nuclear reactions....

The SAMMY code system is mainly used in nuclear data evaluations for incident neutrons in the resolved resonance region (RRR), however, built-in capabilities also allow the code to describe the resonance structure produced by other incident particles, including charged particles. (α,n) data provide fundamental information that underpins nuclear mod...

Under the policies proposed by recent International Atomic Energy Agency (IAEA) circulars and policy papers, implementation of safeguards exists when any purified aqueous uranium solution or uranium oxides suitable for isotopic enrichment or fuel fabrication exists. Under IAEA Policy Paper 18, the starting point for nuclear material under safeguard...

Emerging approaches to short-pulse laser-driven neutron production offer a possible gateway to compact, low cost, and intense broad spectrum sources for a wide variety of applications. They are based on energetic ions, driven by an intense short-pulse laser, interacting with a converter material to produce neutrons via breakup and nuclear reactions...

This paper illustrates methods to improve uncertainty quantification (UQ) for non-destructive assay (NDA) measurements used in nuclear nonproliferation. First, it is shown that current bottom-up UQ applied to calibration data is not always adequate, for three main reasons: (1) Because there are errors in both the predictors and the response, calibr...

This paper describes the development of quantitative nuclear data uncertainty analysis tools for the systematic evaluation of complex, often multi-physics problems that rely on a vast array of diverse nuclear data. These tools can be applied to assess total uncertainty from data and are also being designed to quantify contributions of individual nu...

The widely encountered case of plutonium oxide is analyzed to evaluate uncertainties and uncertainty contributions from nuclear data because it exercises a broad range of data: • plutonium and oxygen neutron reaction cross sections, • neutron-induced fission data for plutonium isotopes, • 240Pu spontaneous fission neutron spectrum, • P(ν) for 240Pu...

This paper sets up a formalism which is sufficiently general to describe the effects of photofission, photonuclear, (n,xn), (n,n'xg) and (n,xg) reactions on the neutron-gamma Feynman-alpha variance-to-mean ratios. Such a formalism is obtained by using the Chapman-Kolmogorov (Master) forward equation for the above-mentioned set of nuclear reactions....

We present a methodology to propagate nuclear data covariance information in neutron source calculations from (α,n) reactions. The approach is applied to estimate the uncertainty in the neutron generation rates for uranium oxide fuel types due to uncertainties on 1) 17,18 O(α,n) reaction cross-sections and 2) uranium and oxygen stopping power cross...

Recent IAEA circulars and policy papers have sought to implement safeguards when any purified aqueous uranium solution or uranium oxides suitable for isotopic enrichment or fuel fabrication exists. Under the revised policy, IAEA Policy Paper 18, the starting point for nuclear material under safeguards was reinterpreted, suggesting that purified ura...

A short-pulse laser-driven deuteron beam is generated in the relativistic transparency regime and aimed at a beryllium converter to generate neutrons at the TRIDENT laser facility. These prompt neutrons have been used for active interrogation to detect nuclear materials, the first such demonstration of a laser-driven neutron source. During the expe...

Safeguarding nuclear material is an important and challenging task for the international community. One particular safeguards technique commonly used for uranium assay is active neutron correlation counting. This technique involves irradiating unused uranium with (α, n) neutrons from an Am-Li source and recording the resultant neutron pulse signal...

In neutron coincidence counting using the shift register autocorrelation technique, a predelay is inserted before the opening of the (R+A)-gate. Operationally the purpose of the predelay is to ensure that the (R+A)- and A-gates have matched effectiveness, otherwise a bias will result when the difference between the gates is used to calculate the ac...

The declared linear density of 238U and 235U in fresh low enriched uranium light water reactor fuel assemblies can be verified for nuclear safeguards purposes using a neutron coincidence counter collar in passive and active mode, respectively. The active mode calibration of the Uranium Neutron Collar - Light water reactor fuel (UNCL) instrument is...

International safeguards inspectorates (e.g., International Atomic Energy Agency {IAEA}, or Euratom) rely heavily on neutron assay techniques, and in particular, on coincidence counters for the verification of declared nuclear materials under safeguards and for monitoring purposes. While ³He was readily available, the reliability, safety, ease of u...

Hybrid K-edge densitometry (HKED) is a non-destructive analytical assay technique used to provide rapid determination of actinide concentration in tank solutions. Of special interest for HKED is the estimation, along with associated uncertainties, of the ratio of the flouresence yeilds, ω K, of uranium and plutonium. Limited experimental data for ω...

The F(alpha,n) reaction is the focus of on-going research in part because it is an important source of neutrons in the nuclear fuel cycle which can be exploited to assay nuclear materials, especially uranium in the form of UF6 [1,2]. At the present time there remains some considerable uncertainty (of the order of +/- 20%) in the thick target integr...

In the field of nuclear safeguards, passive neutron multiplicity counting (PNMC) is a method typically employed in non-destructive assay (NDA) of special nuclear material (SNM) for nonproliferation, verification and accountability purposes. PNMC is generally performed using a well-type thermal neutron counter and relies on the detection of correlat...

Various methods of autocorrelation neutron analysis may be used to extract information about a measurement item containing spontaneously fissioning material. The two predominant approaches being the time correlation analysis (that make use of a coincidence gate) methods of multiplicity shift register logic and Feynman sampling. The common feature i...

Nondestructive assay (NDA) of special nuclear material (SNM) is used in nonproliferation applications, including identification of SNM at border crossings, and quantifying SNM at safeguarded facilities. No assay method is complete without "error bars," which provide one widely used way to express confidence in assay results. NDA specialists typical...

Correlated neutron counting using multiplicity shift register logic extracts the first three factorial moments from the detected neutron pulse train. The descriptive properties of the measurement item (mass, the ratio of (α,n) to spontaneous fission neutron production, and leakage self-multiplication) are related to the observed singles (S), double...

Recently two papers on how to perform passive neutron auto correlation analysis on time gated histograms formed from pulse train data, generically called time correlation analysis (TCA), have appeared in this journal Dubi et al. Nuclear Instruments and Methods in Physics Research Section A. Accelerators, Spectrometers, Detectors and Associated Equi...

In the measurement field of international nuclear safeguards, passive neutron coincidence counting is used to quantify the spontaneous fission rate of certain special nuclear materials. The shift register autocorrelation analysis method is the most commonly used approach. However, the Feynman-Y technique, which is more commonly applied in reactor n...

Measuring the double isotope atomic abundance ratio as function of time of a homogenized stock of plutonium using mass spectroscopy provides a means to estimate the half-life of 241Pu, denoted here as t1/2,241. After a logarithmic transformation, estimating t1/2,241 along with a justifiable associated uncertainty is reduced to the analysis of a lin...

Neutron emissions from fissioning nuclear material are temporally correlated. The detection of these correlated neutrons is frequently used to quantify plutonium (Pu) and other fissile materials for international nuclear safeguards and related activities. However, detector dead time affects the observed rates of correlated neutrons in a non-trivial...

Fissioning material emits neutrons in temporally correlated bursts corresponding to the length of the fission chains. Measuring the degree of correlation can be used to find certain properties of the material. In most neutron multiplicity applications, the average rate of neutron detection (called singles), and the detection rates for correlated do...