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Molten Salt Reactors (MSRs) are one of six Generation IV reactor designs currently under development around the world. Because of the unique operating conditions of MSRs, which include molten fuel and the continuous removal of gaseous fission products during operation, work was performed to model the production of activation and fission products and analyze the potential impact of emissions on the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Simulations were performed to predict the production of IMS-relevant radionuclides in four MSR designs operating under two scenarios: (1) a sealed reactor with releases only during operational shutdown, and (2) continuous reprocessing or sparging of the fuel salt. From these production estimates the radioxenon and radioiodine signatures were extracted and compared to three current reactor designs (Boiling Water Reactor, Pressurized Water Reactor, High-Power Channel-Type Reactor). In cases where continuous reprocessing of the fuel salt occurred, both the radioxenon and radioiodine signatures were nearly indistinguishable from a nuclear explosion. Estimates were also made of the potential emission rate of radioxenon for three reactor designs and it was found that MSRs have the potential to emit radioxenon isotopes at a rate of 10 15 − 8 × 10 16 Bq/d for 133Xe, which may adversely affect nuclear explosion monitoring, if no abatement is used. An assessment was made of activation products using a candidate fuel salt (FLiBe) mixed with corrosion products for the Thorium Molten Salt Reactor (TMSR-LF1).

The production of $^{3}$H, $^{7}$Be, and $^{22}$Na by interactions of cosmic-ray particles with silicon can produce radioactive backgrounds in detectors used to search for rare events. Through controlled irradiation of silicon CCDs and wafers with a neutron beam that mimics the cosmic-ray neutron spectrum, followed by direct counting, we determined that the production rate from cosmic-ray neutrons at sea level is ($112 \pm 24$) atoms/(kg day) for $^{3}$H, ($8.1 \pm 1.9 $) atoms/(kg day) for $^{7}$Be, and ($43.0 \pm 7.1 $) atoms/(kg day) for $^{22}$Na. Complementing these results with the current best estimates of activation cross sections for cosmic-ray particles other than neutrons, we obtain a total sea-level cosmic-ray production rate of ($124 \pm 24$) atoms/(kg day) for $^{3}$H, ($9.4 \pm 2.0 $) atoms/(kg day) for $^{7}$Be, and ($49.6 \pm 7.3 $) atoms/(kg day) for $^{22}$Na. These measurements will help constrain background estimates and determine the maximum time that silicon-based detectors can remain unshielded during detector fabrication before cosmogenic backgrounds impact the sensitivity of next-generation rare-event searches., Comment: 21 pages, 16 figures, 10 tables. Accepted version of the article published in Physical Review D

A novel approach is proposed to detect underground nuclear explosions (UNEs) through the displacement of natural radon isotopes (222Rn and 220Rn). Following an explosion, it is hypothesized that the disturbance and pressurization of the sub-surface would facilitate the movement of radon from the depth of the UNE towards the surface resulting in increased soil gas activity. The resulting signal may be magnified by a factor of 2.0–4.9 by the decay of radon to its short-lived progeny. Increases in background activity may be useful for identifying locations to perform additional measurements, or as a detectable signal at monitoring stations. To validate this hypothesis, radon detection instrumentation was deployed at the Dry Alluvium Geology (DAG) site of the Source Physics Experiment (SPE) at the Nevada National Security Site (NNSS). Natural fluctuations in the soil gas activity due to barometric pumping, and the lower yield of the chemical explosions (1–50 t) made it difficult to confirm a displacement of radon from the explosions, and further study to validate the proposed hypothesis is recommended.

A spectroscopic stack monitoring system for the measurement of noble gasses discharged from medical isotope production facility and nuclear power plant stacks has been designed and a prototype constructed. The prototype is based on a Marinelli beaker style HPGe measurement operating in a continuous acquisition mode. Continuous acquisition is accomplished with novel software and hardware which allows for unattended acquisition, analysis, and storage of data over multiple workflow definitions. As a direct result of the multiple averaging times and the use of the transistor reset preamplifier, the dynamic range of the system covers nearly 8 orders of magnitude.

Molten salt reactors (MSRs) are gaining support as many countries look for ways to increase power generation and replace aging nuclear energy production facilities. MSRs have inherently safe designs, are scalable in size, can burn transuranic wastes from traditional solid fuel nuclear reactors, can store excess heat in thermal reservoirs for water desalination, and can be used to produce medical isotopes as part of the real-time liquid-fuel recycling process. The ability to remove 135Xe in real time from the fuel improves the power production in an MSR because 135Xe is the most significant neutron-absorbing isotope generated by nuclear fission. Xenon-135, and other radioactive gases, are removed by sparging the fuel with an inert gas while the liquid fuel is recirculated from the reactor inner core through the heat exchangers. Without effective abatement technologies, large amounts of radioactive gas could be released during the sparging process. This work examines the potential impact of radioxenon releases on samplers used by the International Monitoring System (IMS) to detect nuclear explosions. Atmospheric transport simulations from seven hypothetical MSRs on different continents were used to evaluate the holdup time needed before release of radioxenon so IMS samplers would register few detections. Abatement technologies that retain radioxenon isotopes for at least 120 d before their release will be needed to mitigate the impacts from a molten salt breeder reactor used to replace a nuclear power plant. A holdup time of about 150 d is needed to reduce emissions to the average level of current nuclear power plants.

As the world faces a challenging future in maintaining the commercial availability of radioactive isotopes for medical use, new methods of medical isotope production are being pursued. Many of these are small in size and could effectively operate continuously. With the potential for much shorter retention times, a new suite of isotopes may soon be found in the environment. The authors estimate that many more aerosols containing low-level isotopes of gas/volatile origin could be detectable at short range and times, and a few at longer ranges and times as compared to those released in more common nuclear reactor operations.

The acquisition of time-stamped list data provides additional information useful to gamma-spectrometry analysis. A novel technique is described that uses non-linear least-squares fitting and the Levenberg–Marquardt algorithm to simultaneously determine parent-daughter atoms from time sequence measurements of only the daughter radionuclide. This has been demonstrated for the radioactive decay of short-lived radon progeny (214Pb/214Bi, 212Pb/212Bi) described using the Bateman first-order differential equation. The calculated atoms are in excellent agreement with measured atoms, with a difference of 1.3–4.8% for parent atoms and 2.4–10.4% for daughter atoms. Measurements are also reported with reduced uncertainty. The technique has potential to redefine gamma-spectrometry analysis.

An On-site Inspection (OSI) is an important component of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), with the objective of gathering evidence of a possible Treaty violation. This includes the use of gamma-spectrometry equipment to identify 17 particulate radionuclides that are indicative of a nuclear explosion. This research provides an evaluation of high- (germanium), medium- (cadmium zinc telluride) and low-resolution (sodium iodide and lanthanum bromide) detectors using modelled radionuclide signatures from a nuclear explosion and Monte Carlo simulations of a detector response. The complex radionuclide signatures are shown to only be distinguishable using a high-resolution system for 1 week and 1 year post-detonation cases.

A high-sensitivity multidimensional gamma-spectrometer is being developed within the shallow underground laboratory at Pacific Northwest National Laboratory (PNNL, USA). The system consists of two broad energy germanium detectors, inside a low-background shield, fitted with a cosmic veto system. The detector has advanced functionality, including operation in single or combined detector mode, with reductions in the cosmic background of 49.6% and Compton suppression of 6.5%. For selected radionuclides this provides increased peak identification, reductions in uncertainty of 27.6% and MDA improvements of 52.7%. The design uses commercially off-the-shelf components to provide a powerful solution for low-level nuclear measurements.

Pacific Northwest National Laboratory (PNNL) staff developed the Radionuclide Aerosol Sampler Analyzer (RASA) for worldwide aerosol monitoring in the 1990s. Recently, researchers at PNNL and Creare, LLC, have investigated possibilities for how RASA could be improved, based on lessons learned from more than 15 years of continuous operation, including during the Fukushima Daiichi Nuclear Power Plant disaster. Key themes addressed in upgrade possibilities include having a modular approach to additional radionuclide measurements, optimizing the sampling/analyzing times to improve detection location capabilities, and reducing power consumption by using electrostatic collection versus classic filtration collection. These individual efforts have been made in a modular context that might constitute retrofits to the existing RASA, modular components that could improve a manual monitoring approach, or a completely new RASA. Substantial optimization of the detection and location capabilities of an aerosol network is possible and new missions could be addressed by including additional measurements.

A Small Anode Germanium (SAGe) detector was recently installed in the Shallow Underground Laboratory (SUL) at Pacific Northwest National Laboratory (PNNL). The unique electrode structure and the short signal wire length in the detector guarantees superior energy resolution than traditional well detectors. The detector is inside an ultra-low background graded lead shield and is fitted with a CosmicGuard TM cosmic veto background reduction system. In this work, the capabilities of this SAGe well detection system were explored using samples with broad characteristics (a Heisenberg cube and individual samples containing 161Tb and 140Ba). The results demonstrate impeccable sensitivity of the system to resolve low-energy complex spectra and provide trustworthy activity estimates by correcting for cascade summing effects.

The performance of the Radionuclide Aerosol Sampler/Analyzer (RASA) under high-activity conditions has been evaluated to maintain and protect the operations of International Monitoring System (IMS) radionuclide stations. Station measurements following the Fukushima Daichi accident of 11 March 2011 have been combined with laboratory measurements to understand how the quality of the measurements made using the RASA High-Purity Germanium (HPGe) gamma-spectrometer are impacted by increased sample activity and detector dead-time. Detector saturation has been identified to occur at a count rate of 175,000 counts s−1, at which level there are significant impacts to the sample live-time and detector resolution in addition to a system contamination risk. To safeguard against these effects, a dead-time limit of 16.6% is proposed that could be maintained by monitoring the activity collecting on the filter using a Cadmium Zinc Telluride (CZT) detector installed into the particulate collection area of the RASA. This limit would ensure that the IMS technical specifications for measurement time (> 20 h) and detector resolution (2.5 keV at 1332 keV) are met. The CZT would dynamically control the RASA air flow to limit the collection of particulate activity beneath the HPGe dead-time threshold and would also provide measurements useful for CTBT verification purposes and dose assessments.

A Compton-suppressed multidimensional gamma-spectrometer is being developed within the Shallow Underground Laboratory at Pacific Northwest National Laboratory (PNNL, USA). This Advanced Radionuclide Gamma-spectrOmeter system consists of two Broad Energy Germanium detectors, surrounded by a 12-detector sodium iodide Compton suppression shield, situated inside a low-background lead and copper shield, fitted with a cosmic veto background reduction system. The detector has advanced functionality, including operation in single or combined detector mode, coincidence mode and Compton suppressed coincidence mode. Performance testing experiments measuring a neutron irradiated natural uranium sample, have demonstrated order of magnitude improvements in gamma detection sensitivity compared to conventional gamma-spectrometry techniques. Importantly this research demonstrates the feasibility of processing and analysing high-multiplicity datasets and the potential for high-detection sensitivity radionuclide measurements.

There remain technical challenges for an On-site Inspection (OSI) in the high seas environment, which gathers evidence of a violation of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). For terrestrial nuclear explosions, the radionuclide observables are well defined and States Parties have chosen 17 particulate radionuclides that allow discrimination from other nuclear events. However, an underwater nuclear explosion generates induced radionuclides from the neutron activation of seawater, which has the potential to interfere with the measurement of the radionuclide observables using gamma-spectrometry techniques. To understand these effects the inventory of OSI relevant (6.0 × 1016 Bq) and activation (1.6 × 1019 Bq) radionuclides has been calculated for a 1 kT underwater nuclear explosion. The activation products consist predominantly of 38Cl and 24Na, which decay to 5.56% and 0.0007% of their initial activity within 1 and 14 days. Monte Carlo techniques have been used to assess spectral interferences within this timeframe. It is demonstrated that during this period they do not interfere with the measurement of the existing radionuclide observables. Additionally, 24Na has been identified as useful for inspection purposes.

List-mode data acquisition has been utilised in conjunction with a high-efficiency γ–γ coincidence system, allowing both the energetic and temporal information to be retained for each recorded event. Collected data is re-processed multiple times to extract any coincidence information from the γ-spectroscopy system, correct for the time-walk of low-energy events, and remove accidental coincidences from the projected coincidence spectra. The time-walk correction has resulted in a reduction in the width of the coincidence delay gate of 18.4±0.4%, and thus an equivalent removal of ‘background’ coincidences. The correction factors applied to ~5.6% of events up to ~500 keV for a combined 137Cs and 60Co source, and are crucial for accurate coincidence measurements of low-energy events that may otherwise be missed by a standard delay gate. By extracting both the delay gate and a representative ‘background’ region for the coincidences, a coincidence background subtracted spectrum is projected from the coincidence matrix, which effectively removes ~100% of the accidental coincidences (up to 16.6±0.7% of the total coincidence events seen during this work). This accidental-coincidence removal is crucial for accurate characterisation of the events seen in coincidence systems, as without this correction false coincidence signatures may be incorrectly interpreted.

The International Monitoring System of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) uses a global network of radionuclide monitoring stations to detect evidence of a nuclear explosion. The two radionuclide technologies employed—particulate and noble gas (radioxenon) detection—have applications for data fusion to improve detection of a nuclear explosion. Using the hypothetical 0.5 kT nuclear explosive test scenario of the CTBTO 2014 Integrated Field Exercise, the intrinsic relationship between particulate and noble gas signatures has been examined. This study shows that, depending upon the time of the radioxenon release, the particulate progeny can produce the more detectable signature. Thus, as both particulate and noble gas signatures are inherently coupled, the authors recommend that the sample categorization schemes should be linked.

Monte-Carlo simulations are used to evaluate and optimise multiple components of a Compton Suppression System based upon a Broad-energy HPGe primary detector. Several materials for the secondary crystal are evaluated, including NaI(Tl), BGO and LaBr3(Ce). BGO was found to be the most effective across the required energy range, with the sizes of the proposed veto detector then optimised to extract the maximum performance for a given volume of material. Suppression factors are calculated for a range of nuclides (both single and cascade emitters) with improvements of 2 for the Compton Suppression Factors, and 10 for the continuum reduction when compared to the Compton suppression system currently in use. This equates to a reduction in the continuum by up to a factor of ~240 for radionuclides such as 60Co, which is crucial for the detection of low-energy, low-activity γ emitters typically swamped by such a continuum.

Monte-Carlo simulations have been utilised to determine the optimum material and thickness for a γ spectrometer to be used for the assay of radionuclides that emit radiation in the 50–300 keV energy range. Both HPGe and LaBr 3 (Ce) materials were initially considered for use, however the additional background radiation and lack of resolution in the latter drove the selection of HPGe for further optimisation. Multiple thicknesses were considered for the HPGe detector, with the aim of improving the sensitivity of the system by maximising the efficiency for low energy emissions, and reducing the probability of interaction with (and therefore the continuum from) higher energy photons. The minimum amount of material needed to achieve this was found to be 15 mm for a source that is dominated by high energy (>2.614 MeV) photons, and 20–30 mm for a typical reference source (with photons of energy 59.54 keV–2.614 MeV).

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a global network of monitoring stations that perform high-resolution gamma-spectrometry on air filter samples for the identification of 85 radionuclides. At the UK CTBT Radionuclide Laboratory (GBL15), a novel cosmic veto gamma-spectrometer has been developed to improve the sensitivity of station measurements, providing a mean background reduction of 80.8% with mean MDA improvements of 45.6%. The CTBT laboratory requirement for a 140Ba MDA is achievable after 1.5 days counting compared to 5–7 days using conventional systems. The system consists of plastic scintillation plates that detect coincident cosmic-ray interactions within an HPGe gamma-spectrometer using the Canberra LynxTM multi-channel analyser. The detector is remotely configurable using a TCP/IP interface and requires no dedicated coincidence electronics. It would be especially useful in preventing false-positives at remote station locations (e.g. Halley, Antarctica) where sample transfer to certified laboratories is logistically difficult. The improved sensitivity has been demonstrated for a CTBT air filter sample collected after the Fukushima incident.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a global network of monitoring stations that perform high-resolution gamma-spectrometry on air filter samples for the identification of radionuclides indicative of nuclear weapons tests and reactor incidents. These daily measurements have created an invaluable resource for understanding variations in natural background radioactivity, including the contribution of 7Be. Statistical analysis has been performed on 7Be data collected by CTBT stations hosted by the United Kingdom including at British Indian Ocean Territory (RN66), St Helena (RN67) and Tristan da Cunha (RN68) during 2005–2013. The results have been found to follow a lognormal distribution which implies that the 7Be activity is the multiplicative product of many small independent factors, such as meteorology, elevation, local station conditions, sample acquisition and analysis. This has the potential to identify discrepant measurements not attributable to the intrinsic variability of the distribution and indicative of station malfunction. Variations in 7Be activity have been considered on monthly, weekly and daily timescales and characterised using the geometric mean in accordance with the properties of the lognormal probability density function. Seasonal variations have been identified, with summer maxima and winter minima that are attributable to changes in mixing within the stratosphere and troposphere. Such fluctuations have been examined using the Fast Fourier Transform which may indicate variations associated with the 27 day solar cycle.

GEANT4 Monte Carlo simulations have been successfully utilised to characterise a Compton suppressed broad-energy HPGe detector. The detector setup has been fully recreated in the simulation, which has been optimised to consistently reproduce the detector response. The peak efficiencies for both the primary BEGe detector and NaI(Tl) guard detectors agree with the simulated values for multiple test sources within 3 %. Compton suppression has also been simulated, with good agreement seen between the simulated and actual CSF values (

Persisted Memory Mapping File (MMF) techniques have been developed for the processing and analysis of time-stamped list (TLIST) mode data collected from an irradiated salt sample. This is a new technique that is unique to this research and enables additional information to be obtained from a conventional pulse height analysis (PHA) gamma-spectrometry acquisition. A salt sample was irradiated using a DT neutron generator and immediately measured using TLIST acquisition for 1 month to generate a 22 GB data file. MMF techniques were used to process the file without size restrictions at a rate of 388.5 MB minute−1, with instant access to any position within the file, separating the file into a series of time sequenced spectra suitable for PHA analysis. This enabled 14 activation products to be identified at different time intervals (41Ar, 80Br, 80m Br, 47Ca, 34m Cl, 38Cl, 42K, 43K, 56Mn, 22 Na, 24 Na, 85Sr, 87m Sr and 69m Zn), and 6 radionuclides (80Br, 47Ca, 34m Cl, 56Mn, 24Na and 87m Sr) to be definitively confirmed based upon their measured half-life. Using this technique, the detection of 47Ca, and in-growth of its 47Sc progeny, was demonstrated as useful for calculating the time since irradiation.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans all nuclear explosions, including those detonated from an underwater nuclear explosion. To improve the understanding of the radionuclide signatures of such an event, and whether it would be detectable under the verification regime of the CTBT, the 1955 Wigwam underwater nuclear explosive test has been modelled. Inventory calculations and atmospheric transport modelling has been performed to estimate the activity at the radionuclide stations (RN) of the International Monitoring System (IMS). This has utilized reported release values (0.92%) and meteorological data from the event. The research shows that there is a high probability that Wigwam would have been detectable at U.S. IMS stations at Wake Island (RN77) at 8.4 d, Upi, Guam (RN80) at 10.7 d and Sand Point, AK (RN71) at 13.7 d. At these locations, the majority of IMS relevant radionuclides were fission products, such that additional radionuclides from the seawater activation had largely decayed before reaching the stations.

Different configurations of a cosmic veto system have been evaluated for the Radionuclide Aerosol Sampler/Analyzer (RASA). The upgrade has the potential to resolve the issue that U.S. International Monitoring System (IMS) radionuclide stations have difficulty meeting the 140Ba Minimum Detectable Concentration (MDC) certification requirement when the natural background radioactivity is high. It also aims to improve the detection sensitivity of the RASA system for other fission and activation products indicative of a nuclear explosion. This proof of principal utilizes a prototype RASA system located at Pacific Northwest National Laboratory (PNNL), and has demonstrated background reductions by up to 11.3% and improvements in the average MDC by up to 7.7%. Recommendations are made for either a 1 or 3 plate configuration, which improves the 140Ba MDC by 5.6% and 7.2% for a blank filter. Whilst elevated 212Pb activity remains problematic, the cosmic veto system provides a practical solution for sensitivity improvement with little design modification of the RASA. It also has value in improving sensitivity and preventing false-positives at remote U.S. IMS station locations where sample transfer for laboratory analysis is logistically difficult.

A novel method for collecting and processing coincidence data from a Compton Suppressed Low Energy Photon Spectrometer (LEPS) is presented, greatly simplifying the current setup and extending the suppression abilities of the system. Offline analysis is used, eliminating the need to discard coincidence data when vetoing coincident events with fast-timing electronics. Additional coincident events are identified that are usually missed, and which represent interactions in the active NaI(Tl) shield prior to an interaction in the LEPS detector. By suppressing these events, the Compton Suppression factor was improved by 144% for the 661.66 keV decay line in a 137 Cs source. The geometry used for this particular Compton suppression system is highly sensitive to these effects, however similar event profiles are expected in all coincidence systems.

A GEANT4 based Monte Carlo simulation has been successfully utilised to calculate peak efficiency characterisations and cascade summing (true coincidence summing) corrections in two source geometries commonly used for environmental monitoring. The cascade summing corrections are compared with values generated using an existing (validated) system, and found to be in excellent agreement for all radionuclides simulated. The calculated correction factors and peak efficiencies were also tested by analysing well defined sources used in the operation of the International Monitoring System, which undertakes radionuclide monitoring for verification of the Comprehensive Nuclear-Test-Ban Treaty. All abundances of the radionuclides measured matched the values that were previously determined using proprietary software. Using GEANT4 in this way, cascade summing corrections can now be extended to complex detector models and source matrices, such as Compton Suppression systems.

The performance of a radiation shielding system for a hyper-pure germanium detector has been characterised for Terrestrial radiation sources, Cosmic muons, X-ray fluorescence and the Compton scattering of source photons. Several methods to reduce the background seen are quantified, including increasing the inner radius of the Pb cave, and increasing the thickness of the shielding. Substantial improvements in the reduction of fluorescence X-rays are found to be achievable by modifying the liner thicknesses used. Increasing the Sn liner from 1.5 to 2.5 mm will increase the shielding of Pb X-rays from 95 to 99.5 %. Reducing the Cu liner from 1.0 to 0.5 mm maintains a 99.5 % level of shielding for Sn/Cd X-rays, however it greatly reduces the amount of Compton scattering of source photons into the detector (a process that is shown to cause an order of magnitude more events in the background than X-ray fluorescence). Cosmic muons were found to increase the amount of background radiation seen, both through direct interaction and the production of secondary radiation. The Cosmic muon contribution, however, was found to produce a much smaller effect than that caused by Terrestrial radiation and Compton scattered photons/fluorescence from the source. The total level of background radiation entering the detector chamber was found to decrease up to the full 200 mm of Pb shielding simulated.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a network of certified laboratories that perform high-resolution gamma-spectrometry on global air filter samples for the identification of 85 radionuclides. At the UK CTBT Radionuclide Laboratory (GBL15), a novel cosmic veto gamma-spectrometer has been developed to improve the sensitivity of measurements for treaty compliance. The system consists of plastic scintillation plates operated in time-stamp mode to detect coincident cosmic-ray interactions within an HPGe gamma-spectrometer. This provides a mean background reduction of 75.2 % with MDA improvements of 45.6 %. The CTBT requirement for a 140Ba MDA is achievable after 1.5 days counting compared to 5–7 days using conventional systems. The system does not require dedicated coincidence electronics, and remains easily configurable with dual acquisition of unsuppressed and suppressed spectra. Performance has been significantly improved by complete processing of the cosmic-ray spectrum (0–25 MeV) combined with the Canberra Lynx™ multi-channel analyser. The improved sensitivity has been demonstrated for a CTBT air filter sample collected after the Fukushima incident.

PNNL has developed two low-background gamma-ray spectrometers in a new shallow underground laboratory, thereby significantly improving its ability to detect low levels of gamma-ray emitting fission or activation products in airborne particulate in samples from the IMS (International Monitoring System). The combination of cosmic veto panels, dry nitrogen gas to reduce radon and low background shielding results in a reduction of the background count rate by about a factor of 100 compared to detectors operating above ground at our laboratory.

Past nuclear weapon explosive tests provide invaluable information for understanding the radionuclide observables expected during an On-site Inspection (OSI) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). These radioactive signatures are complex and subject to spatial and temporal variability. The Platte underground nuclear explosive test on 14 April 1962 provides extensive environmental monitoring data that can be modelled and used to calculate the maximum time available for detection of the OSI-relevant radionuclides. The 1.6 kT test is especially useful as it released the highest amounts of recorded activity during Operation Nougat at the Nevada Test Site - now known as the Nevada National Security Site (NNSS). It has been estimated that 0.36% of the activity was released, and dispersed in a northerly direction. The deposition ranged from 1 × 10-11 to 1 × 10-9 of the atmospheric release (per m2), and has been used in this paper to evaluate an OSI and the OSI-relevant radionuclides at 1 week to 2 years post-detonation. Radioactive decay reduces the activity of the OSI-relevant radionuclides by 99.7% within 2 years of detonation, such that detection throughout the hypothesized inspection is only achievable close to the explosion where deposition was highest.

Broad-energy HPGe detectors have a useful range of 3 keV to 3 MeV, making them ideal for the assay of environmental samples. Such measurements however, are hindered by variations in the sample matrix, summing effects, and the Compton continuum. Detectors may be characterised by proprietary software in such a situation, however Monte-Carlo modelling is a useful, inexpensive alternative that also provides greater flexibility when determining the detector response and efficiency during a measurement. In the current work, a full GEANT4 model of a broad-energy HPGe detector is presented, and simulations of various samples are compared to experimental data. These are found to be accurate within 3 % at a confidence level of 95 % for energies from 30 to 3,000 keV, with greater variations below 100 keV due to an increased sensitivity to geometrical inaccuracies.

This paper discusses preliminary work into the modelling and processing of coincidence measurements, for which a Monte-Carlo simulation and a post-processing program have both been developed. In the current work, a GEANT4 code is used to simulate a pair of NaI(Tl) scintillators, which are used experimentally to both develop the post-processing program and validate the GEANT4 model. This is found to be accurate to within 9% at a confidence level of 93.0% for energies from 30 to 3,000 keV.

A novel manganese dioxide (MnO2) resin is suitable for the determination and decontamination of lead isotopes (208Pb, 207Pb, 206Pb) in aqueous solutions at trace concentrations. This is desirable due to the toxic nature and high abundance of lead in the environment. Current techniques are both time consuming, expensive and not suitable for low-level lead decontamination. The MnO2 resin has been demonstrated to pre-concentrate with extraction efficiency above 90% for a range of pH values, and with a mean extraction of 92.5% from fresh waters at a flow rate of 100 ml min−1. The lead distribution coefficient is 1.3 × 104 (pH 7) with tolerance to calcium and sodium. Adsorption isotherms have been investigated and the resin shown to follow the Langmuir adsorption isotherm with a saturation point of 41.5 mg per g of MnO2 resin.

Cosmic radiation contributes significantly towards the background radiation measured by a gamma-spectrometer. A novel cosmic veto gamma-spectrometer has been developed that provides a mean background reduction of 54.5%. The system consists of plastic scintillation plates operated in time-stamp mode to detect coincident muon interactions within an HPGe gamma-spectrometer. The instrument is easily configurable and provides improved sensitivity for radionuclides indicative of nuclear weapons tests and reactor incidents, including 140Ba, 95Zr, 99Mo, 141Ce, 147Nd, 131I, 134Cs and 137Cs. This has been demonstrated for Comprehensive Nuclear-Test-Ban Treaty applications to obtain the required 140Ba MDA of 24 mBq within 2 days counting. Analysis of an air filter sample collected during the Fukushima incident indicates improved sensitivity compared to conventional gamma-spectrometers.

Statistical analysis has been performed on the gross alpha- and beta-activity measurements of surface waters collected at the Atomic Weapons Establishment at Aldermaston (UK) during the period January 2002–September 2005. The results have been found to follow a lognormal distribution and this has important applications when considering gross activity exemption thresholds. This implies that the gross activity is the multiplicative product of many small independent factors, such as meteorology, flow conditions and site operations. The influence of meteorological parameters has been investigated using linear regression, and some correlation has been identified between gross beta-activity and parameters indicative of fine weather. Variations in gross activity have been considered on monthly, weekly and daily timescales and characterised using the geometric mean and geometric standard deviation in accordance with the properties of the lognormal probability density function.

A novel technique has been developed for determining the 212 Pb activity of fresh waters. This is of interest to environmental monitoring programmes that utilise gross α-activity methods to screen for anthropogenic radionuclides. The contribution from 212 Pb varies, and is difficult to experimentally measure due to its relatively short half-life ( t ½ = 10.6 h) and low environmental activity ( −1 ). The use of a three-stage technique that encompasses a unique form of pre-concentration, separation and analysis by liquid scintillation counting allows a lower detection limit of 0.006 Bq l −1 with a chemical yield of 92.5 ± 5.6%. The measurement can be obtained within 7 h of sample collection, and is calculated using the radioactive decay of 212 Bi. Other naturally occurring radionuclides may also be extracted using the pre-concentration stage of the technique, with efficiencies above 90% at a range of pH values.

Multidimensional gamma-spectrometry is a powerful technique for making low-level nuclear measurements. The systems typically utilise multiple sodium iodide detectors to measure coincident or anti-coincident gamma-energies. This allows significant background reduction and high selectivity of cascading radionuclides including 22Na. As events are rejected or selected based on their event time, the timing resolution of the multi-channel analyser (MCA) is important. This has been assessed for three compact MCA systems produced by Canberra and Ortec. The Canberra OspreyTM and LynxTM provided the best timing performance, capturing 88.3 and 97.9% of 22Na positron emissions within 200 ns. In comparison only 12.5% of events were measured within 200 ns by the Ortec digiBASE-ETM. However all systems captured the majority of events within 1000 ns, providing multidimensional spectra that allow the evaluation of simultaneous energy combinations.

Compton Suppression techniques have been widely used to reduce the Minimum Detectable Activity of various radionuclides when performing gamma spectroscopy of environmental samples. This is achieved by utilising multiple detectors to reduce the contribution of photons that Compton Scatter out the detector crystal, only partially depositing their energy. Photons that are Compton Scattered out of the primary detector are captured by a surrounding detector, and the corresponding events vetoed from the final dataset using coincidence based fast-timing electronics. The current work presents the use of a Lynx TM data acquisition module from Canberra Industries (USA) to collect data in 'List-Mode', where each event is time stamped for offline analysis. A post-processor developed to analyse such datasets allows the optimisation of the coincidence delay, and then identifies and suppresses events within this time window. This is the same process used in conventional systems with fast-timing electronics, however, in the work presented, data can be re-analysed using multiple time and energy windows. All data is also preserved and recorded (in traditional systems, coincident events are lost as they are vetoed in real time), and the results are achieved with a greatly simplified experimental setup. Monte-Carlo simulations of Compton Suppression systems have been completed to support the optimisation work, and are also presented here.

Sudden increases in the background gamma-radiation dose may occur due to the removal of (222)Rn and (220)Rn progeny from the atmosphere by wet deposition mechanisms. This contribution has been measured using a Geiger-Muller detector at the Atomic Weapons Establishment (Aldermaston, UK) during July 2005-April 2006. The results are approximated by a log-normal distribution and there were nine separate occurrences of the gamma-radiation dose exceeding 125% of the geometric mean value. The increases were associated with periods of heavy rainfall, although no correlation was evident between the dose rate and the amount of rainfall, as increased rainfall dilutes the activity further rather than increasing its atmospheric removal. The events were preceded by periods of fine weather and atmospheric stability that allow for the build-up of (222)Rn and (220)Rn progeny. Similar increases in gamma-radiation dose have been measured at a nearby monitoring station situated approximately 11 miles from Aldermaston. Increases in gamma-radiation dose during heavy rainfall have also been observed throughout the UK, that followed the trajectory of an air mass. All events decreased to typical values within 1-2 h as the water permeated into the ground below and the radioactivity decayed away.

In 2014 the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) undertook an Integrated Field Exercise (IFE14) in Jordan. The exercise consisted of a simulated 0.5-2 kT underground nuclear explosion triggering an On-site Inspection (OSI) to search for evidence of a Treaty violation. This research paper evaluates two of the OSI techniques used during the IFE14, laboratory-based gamma-spectrometry of soil samples and in-situ gamma-spectrometry, both of which were implemented to search for 17 OSI relevant particulate radionuclides indicative of nuclear explosions. The detection sensitivity is evaluated using real IFE and model data. It indicates that higher sensitivity laboratory measurements are the optimum technique during the IFE and within the Treaty/Protocol-specified OSI timeframes.

The Comprehensive Nuclear–Test–Ban Treaty (CTBT) is supported by a network of certified laboratories which must meet certain sensitivity requirements for CTBT relevant radionuclides. At the UK CTBT Radionuclide Laboratory (GBL15), a high-efficiency, dual–detector gamma spectroscopy system has been developed to improve the sensitivity of measurements for treaty compliance, greatly reducing the time required for each sample. Utilising list–mode acquisition, each sample can be counted once, and processed multiple times to further improve sensitivity. For the 8 key radionuclides considered, Minimum Detectable Activities (MDA's) were improved by up to 37% in standard mode (when compared to a typical CTBT detector system), with the acquisition time required to achieve the CTBT sensitivity requirements reduced from 6 days to only 3. When utilising the system in coincidence mode, the MDA for 60 Co in a high–activity source was improved by a factor of 34 when compared to a standard CTBT detector, and a factor of 17 when compared to the dual–detector system operating in standard mode. These MDA improvements will allow the accurate and timely quantification of radionuclides that decay via both singular and cascade γ emission, greatly enhancing the effectiveness of CTBT laboratories.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a network of certified laboratories that perform high-resolution gamma-spectrometry on global air filter samples for the identification of 85 radionuclides. At the UK CTBT Radionuclide Laboratory (GBL15), the use of advanced Compton suppressed systems has been investigated to reduce the Compton continuum and improve detection sensitivity. Samples collected from the Philippines and during the Fukushima incident have been measured, demonstrating Compton continuum reductions of 28–59 % with suppression factors of 0.1–147.0. Detection sensitivity has been improved with typically 40 % lower MDAs, including 140Ba to meet CTBT requirements. True coincidence summing effects have been considered, including the application to remove interferences by the elimination of gamma-rays in cascade. This has been demonstrated for the removal of 134Cs allowing improved 131I measurement.

Under the Comprehensive Nuclear-Test-Ban Treaty an on-site inspection (OSI) may be undertaken to identify signatures from a potential nuclear explosion. This includes the measurement of 17 particulate radionuclides (95Zr, 95Nb, 99Mo, 99mTc, 103Ru, 106Rh, 132Te, 131I, 132I, 134Cs, 137Cs, 140Ba, 140La, 141Ce, 144Ce, 144Pr, 147Nd). This research provides an assessment of the potential to detect these radionuclides during an OSI within 1 week to 2 years after a nuclear explosion at two locations. A model has been developed that simulates the underground detonation of a 1 kT 235U nuclear weapon with 1 % venting. This indicates a requirement to minimise the time since detonation with accurate determination of the test location.

Commercially available Diphonix(®) resin (TrisKem International) was evaluated as a receiving phase for use with the diffusive gradients in thin-films (DGT) passive sampler for measuring uranium. This resin has a high partition coefficient for actinides and is used in the nuclear industry. Other resins used as receiving phases with DGT for measuring uranium have been prone to saturation and significant chemical interferences. The performance of the device was evaluated in the laboratory and in field trials. In laboratory experiments uptake of uranium (all 100% efficiency) by the resin was unaffected by varying pH (4-9), ionic strength (0.01-1.00 M, as NaNO3) and varying aqueous concentrations of Ca(2+) (100-500 mg L(-1)) and HCO3(-) (100-500 mg L(-1)). Due to the high partition coefficient of Diphonex(®), several elution techniques for uranium were evaluated. The optimal eluent mixture was 1M NaOH/1M H2O2, eluting 90% of the uranium from the resin. Uptake of uranium was linear (R(2)=0.99) over time (5 days) in laboratory experiments using artificial freshwater showing no saturation effects of the resin. In field deployments (River Lambourn, UK) the devices quantitatively accumulated uranium for up to 7 days. In both studies uptake of uranium matched that theoretically predicted for the DGT. Similar experiments in seawater did not follow the DGT theoretical uptake and the Diphonix(®) appeared to be capacity limited and also affected by matrix interferences. Isotopes of uranium (U(235)/U(238)) were measured in both environments with a precision and accuracy of 1.6-2.2% and 1.2-1.4%, respectively. This initial study shows the potential of using Diphonix(®)-DGT for monitoring of uranium in the aquatic environment.

The performance of the diffusive gradient in thin film technique (DGT) was evaluated as a tool for the long-term monitoring of water quality, using uranium as a case study. DGTs with a Metsorb™ (TiO2) sorbent were deployed consecutively at two alkaline freshwater sites, the River Enborne and the River Lambourn, UK for seven-day intervals over a five-month deployment period to obtain time weighted average concentrations. Weekly spot samples were taken to determine physical and chemical properties of the river water. Uranium was measured in these spot samples and after extraction from the DGT devices. The accuracy of the DGT device time weighted average concentrations to averaged spot water samples in both rivers was 86% (27 to 205%). The DGT diffusive boundary layer (DBL) (0.037-0.141 cm - River Enborne and 0.062-0.086 cm - River Lambourn) was affected by both water flow and biofouling of the diffusion surface. DBL thicknesses found at both sites were correlated with flow conditions with an R(2) value of 0.614. Correlations were also observed between the DBL thickness and dissolved organic carbon (R(2) = 0.637) in the River Lambourn, indicating the potential presence of a complex zone of chemical interactions at the surface of the DGT. The range of DBL thicknesses found at the River Lambourn site were also attributed to of the development of macro-flora on the active sampling surface, indicating that the DBL thickness cannot be assumed to be water flow dependant only. Up to a 57% under-estimate of uranium DGT concentration was observed compared to spot sample concentrations if the DBL was neglected. This study has shown that the use of DGT can provide valuable information in environmental monitoring schemes as part of a 'tool-box' approach when used alongside conventional spot sampling methods.

Three adsorbents (Chelex-100, manganese dioxide [MnO2] and Metsorb), used as binding layers with the diffusive gradient in thin film (DGT) technique, were evaluated for the measurement of inorganic uranium species in synthetic and natural waters. Uranium (U) was found to be quantitatively accumulated in solution (10-100 mu g L-1) by all three adsorbents (uptake efficiencies of 80-99%) with elution efficiencies of 80% (Chelex-100), 84% (MnO2) and 83% (Metsorb). Consistent uptake occurred over pH (5-9), with only MnO2 affected by pH = 0.97) for all three adsorbents in low ionic strength solution (0.01 M NaNO3). Validation experiments in artificial sea water gave linear mass uptake for Metsorb (R-2 >= 0.9954) up to 12 h and MnO2 (R-2 >= 0.9259) up to 24 h. Chelex-100 demonstrated no linear mass uptake in artificial sea water after 8 h. Possible interferences were investigated with SO42- (0.02-200 mg L-1) having little affect on any of the three DGT binding layers. PO43- additions (5 mu g L-1-5mg L-1) interfered by forming anionic uranyl phosphate complexes that Chelex-100 was unable to accumulate, or by directly competing with the uranyl species for binding sites, as with MnO2 and the Metsorb. HCO3- (0.1-500 mg L-1) additions formed anionic species which interfered with the performance of the Chelex-100 and the MnO2, and the Ca2+ (0.1-500 mg L-1) had the affect of forming labile calcium uranyl species which aided uptake of U by all three resins. DGT field deployments in sea water (Southampton Water, UK) gave a linear mass uptake of U over time with Metsorb and MnO2 (4 days). Field deployments in fresh water (River Lambourn, UK) gave linear uptake for up to 7 and 4 days for Metsorb and MnO2 respectively. Field deployment of the Metsorb-DGT samplers with various diffusive layer thicknesses (0.015-0.175 cm) allowed accurate measurements of the diffusive boundary layer (DBL) and allowed DBL corrected concentrations to be determined. This DBL-corrected U concentration was half that determined when the effect of the DBL was not considered. The ability of the DGT devices to measure U isotopic ratios with no isotopic fractionation was shown by all three resins, thereby proving the usefulness of the technique for environmental monitoring purposes. Refereed/Peer-reviewed

Best values for the standard (III–IV) oxidation potentials for each member of the lanthanide series and for each member of the actinide series are obtained from a variety of sources, namely, from the direct electrode measurements reported in the literature, from linear plots of electron-transfer absorption band energy for various Ln(IV) and An(IV) complexes versus the respective (III–IV) potentials, from linear plots of f → d absorption band energy for various Ln(III) and An(III) complexes vs. the respective (III–IV) potentials, and from theoretical correlations using Jorgensen's refined-electron-spin-pairing-energy theory. Two major results of this work are; (1) the M(III–IV) potentials for M ≐ Cm, Cf, Pr, and Tb are all within −3·2±0·3 V, and (2) the indication that the unknown oxidation states Cf(IV), Es(IV), Pm(IV), Sm(IV), Fm(IV), and possibly Md(IV), listed in the order of decreasing stability, may be stabilized in various media. Altogether, the results are in excellent accord with the actinide hypothesis.