Upscaling ground-based backpack gamma-ray spectrometry to spatial resolution of UAV-based gamma-ray spectrometry for system validation

Advances in the development of gamma-ray spectrometers have resulted in devices that are ideal for use in conjunction with the increasingly reliable systems of autonomously flying uncrewed aerial vehicles (UAVs) that have recently become available on the market. Airborne gamma-ray spectrometry (GRS) measurements have many different applications. Here, the technique is applied to a former uranium mining and processing site, which is characterized by relatively low specific activities and, hence, low count rates, requiring relatively large detectors and correspondingly big size UAVs. The future acceptance of the use of such UAV-based GRS systems for radionuclide mapping depends on their ability to measure absolute specific activities of natural radionuclides such as U-238 in near-surface soil that are consistent with the results of established and proven ground-based systems. To determine absolute specific activities on the ground, the gamma radiation data from airborne detectors must be corrected for attenuation caused by the flight altitude above ground. In recent years, mathematical procedures for altitude correction have been developed, that are specifically tailored to the working range of several tens of meters typical for UAVs. However, very limited experimental validation of these theoretical approaches is available. A very large dataset consisting of about 3000 UAV-based and 19,000 backpack-based measurements was collected at a low-grade uranium ore dump in Yangiabad, Uzbekistan. We applied different geostatistical interpolation methods to compare the data from both survey techniques by upscaling backpack data to airborne data. Compared to backpack systems, UAV-based systems have lower spatial resolution, so measurements average over larger areal units (or in geostatistical terminology: “spatial support”). Taking into account the change in spatial support, we illustrate that (1) the UAV-based measurements show good agreement with the upscaled backpack measurements and that (2) UAV surveys provide good delineation of contrasts of the relatively smooth U-238 specific activity distribution typical for former uranium mining and processing sites. We are able to show that the resolution of UAV-based systems is sufficient to map extended uranium waste facilities

Search for a sterile neutrino with the STEREO experiment : shielding optimisation and energy calibration

La recherche de neutrinos stériles et légers est, à l’heure actuelle, l’un des enjeux majeurs de laphysique des neutrinos. Une indication de leur existence résulte de l’anomalie des antineutrinosde réacteur, qui découle du déficit de 6% entre les taux prédits et les taux observéspar les expériences à courte distance de réacteurs. Ce déficit peut être interprété comme uneoscillation à courte distance des neutrinos. L’objectif de l’expérience STEREO, situé auprès duréacteur de recherche de l’Institut Laue Langevin (ILL), à Grenoble, France, est d’étudier cetteoscillation. La cible du détecteur de neutrinos est placée entre 8,9 et 11,1m du coeur compactdu réacteur d’ILL. Le détecteur consiste d’environ 2t d’un scintillateur liquide, dopé avec duGd. Le volume actif est séparé dans le sens de la longueur en six cellules. Les antineutrinos sontdétectés par la désintégration bêta inverse, où ils interagissent avec un proton libre (ion H+) etproduisent un positron et un neutron. Les deux particules sont détectées dans le scintillateurpar une coïncidence retardée où le positron crée un signal prompt et le neutron est capturéaprès un temps de modération. La lumière produite par le scintillateur est mesurée par lesphotomultiplicateurs (PM). Le détecteur est complété par un“gamma catcher” qui entoure la cible et par un veto à muons.Ce manuscrit présente des études concernant la préparation et la mise en exploitation del’expérience STEREO. La conception du blindage magnétique des PM a été menée sur la basede simulations par éléments finis afin d’examiner différentes options, d’étudier en détail lesperformances de l’option retenue ainsi que de déterminer la qualité nécessaire des matériauxutilisés. Sur la base de ces études, la collaboration a retenu un plan de blindage en deuxcouches: une couche de fer doux à l’extérieur, couvrant le détecteur et le veto à muons, et unecouche de mu-métal autour de la cible. Ce blindage réduit les champs magnétiques externes à laposition des PM de la cible à moins de 60μT pour toutes les configurations connues de champsexternes. Ceci réduit à moins de 2% une variation de l’amplification des PM induite par deschangements des champs magnétiques.D’autre part, des études du bruit de fond sur le site de STEREO ont été menées. Unecartographie du bruit de fond du rayonnement gamma a été effectuée avec des détecteurs augermanium et un scintillateur NaI, afin de valider l’efficacité du blindage installé. Uneestimation du taux de bruit de fond est présentée et comparée au taux mesuré avec STEREO.Dans l’état actuel de l’analyse des données, le bruit de fond de coïncidences fortuites est inférieurau bruit de fond corrélé induit par les muons cosmiques. Après une première phase d’exploitationde STEREO, un “doigt de gant” en fin de vie situé à l’avant de STEREO a dû être retiré.Un bouchon était adapté à l’extrémité de ce doigt de gant afin de réduire le bruit du fondpour STEREO. Ce dispositif n’ayant pas pu être réinstallé à la suite l’enlèvement du doigt degant, un nouveau blindage a été proposé par l’ILL. Une série des simulations neutroniques etphotoniques (MCNP) a été effectué pour étudier l’effet de ce changement sur le bruit de fondautour de STEREO et pour décider si le blindage proposé était suffisant. Les deux scénariosavant et après l’enlèvement ont été comparés et selon cette simulation, la situation du bruit defond devrait être améliorée.Enfin une procédure a été proposée et appliquée pour analyser les données de calibration del’échelle d’énergie de STEREO. La procédure a été élaborée pour être applicable pour toutes lessources de calibration disponibles et pour minimiser les incertitudes systématiques. Le résultatpeut être utilisé pour ajuster les paramètres de la simulation Geant4 du détecteur développée parla collaboration, par comparaison avec des données mesurées et après pour déterminer l’échellede l’énergie avec la précision requise de < 2%.Light sterile neutrinos are currently a topic actively discussed in neutrino physics. Oneindication of their possible existence and their participation in neutrino oscillations is the ReactorAntineutrino Anomaly, which states a deficit of about 6% between predicted and observedantineutrino fluxes in short baseline reactor neutrino experiments. The STEREO experimentaddresses this anomaly by searching for neutrino oscillations at baselines of 8.9-11.1m from thecompact core of the research reactor of the Institut Laue Langevin (ILL), Grenoble, France. Forthis purpose a Gd-loaded liquid scintillator detector was designed with an active target massof about 2 t. The target volume is subdivided in six optically separated cells along the line ofpropagation of the neutrinos. The electron antineutrinos emitted from the reactor are detectedvia the inverse beta decay on hydrogen nuclei, where a positron and a neutron are created. Thesetwo particles are detected in the scintillator in delayed coincidence, with the prompt signal fromthe positron and a delayed signal from neutron capture. The scintillation light created in theprocesses is read out by photomultiplier tubes (PMTs) on top of the detector cells. The detectoris completed by a gamma catcher and a muon veto.This manuscript covers parts of the preparation and the commissioning of the STEREOexperiment. As basis for the design process of the magnetic shielding for STEREO’s PMTsa series of finite element simulations was performed. The studies of different general layoutsand required material qualities as well as of details of the final design are summarised. Underconsideration of these studies the collaboration opted for a shielding design, a double layer setupwith an outer soft iron and inner mumetal layer, which has the required shielding efficiency toreduce the magnetic field at the position of the detector PMTs below 60 μT for all known externalmagnetic field configurations. This limits the maximum PMT gain change due to variations ofthe external magnetic fields to < 2%.Furthermore different studies have been performed concerning the on-site background situation.A mapping of the-ray background was conducted with high purity germanium detectorsand a NaI scintillator detector, in order to validate the efficiency of the installed shielding. Thefocus lied on the characterisation of the count rate in the neutron capture energy window. Anestimation of the background rate is presented and compared to the rate obtained in STEREO.At the current state of the analysis the background of accidental coincidences in STEREO is aminor contribution compared to the muon induced correlated background. In addition a seriesof MCNP simulations was performed to determine the impact of a beamtube removal in thevicinity of STEREO on the overall reactor-related background situation. The beamtube wasclosed by a dedicated shielding, optimised for background reduction for STEREO, which couldnot be reinstalled after removal of the tube. A new shielding at the end of the former beamtubewas proposed by the ILL. Its shielding effect was studied with MCNP and compared tothe previous configuration in order to assess whether the new shielding suffices or needs to beimproved. According to these simulations the background situation is expected to improve.Finally a procedure is proposed and applied for the analysis of the energy calibration ofthe STEREO detector. The procedure is designed to be applicable to all available calibrationsources and to minimise systematic uncertainties. It can be used to adjust parameters in theexisting Geant4-based simulation of the detector, developed by the collaboration, by comparisonto measured data and later to determine the energy scale with the required precision of < 2%

Recherche de neutrino stérile par l'expérience STEREO : optimisation du blindage et calibration de l'échelle d'énergie

Light sterile neutrinos are currently a topic actively discussed in neutrino physics. Oneindication of their possible existence and their participation in neutrino oscillations is the ReactorAntineutrino Anomaly, which states a deficit of about 6% between predicted and observedantineutrino fluxes in short baseline reactor neutrino experiments. The STEREO experimentaddresses this anomaly by searching for neutrino oscillations at baselines of 8.9-11.1m from thecompact core of the research reactor of the Institut Laue Langevin (ILL), Grenoble, France. Forthis purpose a Gd-loaded liquid scintillator detector was designed with an active target massof about 2 t. The target volume is subdivided in six optically separated cells along the line ofpropagation of the neutrinos. The electron antineutrinos emitted from the reactor are detectedvia the inverse beta decay on hydrogen nuclei, where a positron and a neutron are created. Thesetwo particles are detected in the scintillator in delayed coincidence, with the prompt signal fromthe positron and a delayed signal from neutron capture. The scintillation light created in theprocesses is read out by photomultiplier tubes (PMTs) on top of the detector cells. The detectoris completed by a gamma catcher and a muon veto.This manuscript covers parts of the preparation and the commissioning of the STEREOexperiment. As basis for the design process of the magnetic shielding for STEREO’s PMTsa series of finite element simulations was performed. The studies of different general layoutsand required material qualities as well as of details of the final design are summarised. Underconsideration of these studies the collaboration opted for a shielding design, a double layer setupwith an outer soft iron and inner mumetal layer, which has the required shielding efficiency toreduce the magnetic field at the position of the detector PMTs below 60 μT for all known externalmagnetic field configurations. This limits the maximum PMT gain change due to variations ofthe external magnetic fields to < 2%.Furthermore different studies have been performed concerning the on-site background situation.A mapping of the-ray background was conducted with high purity germanium detectorsand a NaI scintillator detector, in order to validate the efficiency of the installed shielding. Thefocus lied on the characterisation of the count rate in the neutron capture energy window. Anestimation of the background rate is presented and compared to the rate obtained in STEREO.At the current state of the analysis the background of accidental coincidences in STEREO is aminor contribution compared to the muon induced correlated background. In addition a seriesof MCNP simulations was performed to determine the impact of a beamtube removal in thevicinity of STEREO on the overall reactor-related background situation. The beamtube wasclosed by a dedicated shielding, optimised for background reduction for STEREO, which couldnot be reinstalled after removal of the tube. A new shielding at the end of the former beamtubewas proposed by the ILL. Its shielding effect was studied with MCNP and compared tothe previous configuration in order to assess whether the new shielding suffices or needs to beimproved. According to these simulations the background situation is expected to improve.Finally a procedure is proposed and applied for the analysis of the energy calibration ofthe STEREO detector. The procedure is designed to be applicable to all available calibrationsources and to minimise systematic uncertainties. It can be used to adjust parameters in theexisting Geant4-based simulation of the detector, developed by the collaboration, by comparisonto measured data and later to determine the energy scale with the required precision of < 2%.La recherche de neutrinos stériles et légers est, à l’heure actuelle, l’un des enjeux majeurs de laphysique des neutrinos. Une indication de leur existence résulte de l’anomalie des antineutrinosde réacteur, qui découle du déficit de 6% entre les taux prédits et les taux observéspar les expériences à courte distance de réacteurs. Ce déficit peut être interprété comme uneoscillation à courte distance des neutrinos. L’objectif de l’expérience STEREO, situé auprès duréacteur de recherche de l’Institut Laue Langevin (ILL), à Grenoble, France, est d’étudier cetteoscillation. La cible du détecteur de neutrinos est placée entre 8,9 et 11,1m du coeur compactdu réacteur d’ILL. Le détecteur consiste d’environ 2t d’un scintillateur liquide, dopé avec duGd. Le volume actif est séparé dans le sens de la longueur en six cellules. Les antineutrinos sontdétectés par la désintégration bêta inverse, où ils interagissent avec un proton libre (ion H+) etproduisent un positron et un neutron. Les deux particules sont détectées dans le scintillateurpar une coïncidence retardée où le positron crée un signal prompt et le neutron est capturéaprès un temps de modération. La lumière produite par le scintillateur est mesurée par lesphotomultiplicateurs (PM). Le détecteur est complété par un“gamma catcher” qui entoure la cible et par un veto à muons.Ce manuscrit présente des études concernant la préparation et la mise en exploitation del’expérience STEREO. La conception du blindage magnétique des PM a été menée sur la basede simulations par éléments finis afin d’examiner différentes options, d’étudier en détail lesperformances de l’option retenue ainsi que de déterminer la qualité nécessaire des matériauxutilisés. Sur la base de ces études, la collaboration a retenu un plan de blindage en deuxcouches: une couche de fer doux à l’extérieur, couvrant le détecteur et le veto à muons, et unecouche de mu-métal autour de la cible. Ce blindage réduit les champs magnétiques externes à laposition des PM de la cible à moins de 60μT pour toutes les configurations connues de champsexternes. Ceci réduit à moins de 2% une variation de l’amplification des PM induite par deschangements des champs magnétiques.D’autre part, des études du bruit de fond sur le site de STEREO ont été menées. Unecartographie du bruit de fond du rayonnement gamma a été effectuée avec des détecteurs augermanium et un scintillateur NaI, afin de valider l’efficacité du blindage installé. Uneestimation du taux de bruit de fond est présentée et comparée au taux mesuré avec STEREO.Dans l’état actuel de l’analyse des données, le bruit de fond de coïncidences fortuites est inférieurau bruit de fond corrélé induit par les muons cosmiques. Après une première phase d’exploitationde STEREO, un “doigt de gant” en fin de vie situé à l’avant de STEREO a dû être retiré.Un bouchon était adapté à l’extrémité de ce doigt de gant afin de réduire le bruit du fondpour STEREO. Ce dispositif n’ayant pas pu être réinstallé à la suite l’enlèvement du doigt degant, un nouveau blindage a été proposé par l’ILL. Une série des simulations neutroniques etphotoniques (MCNP) a été effectué pour étudier l’effet de ce changement sur le bruit de fondautour de STEREO et pour décider si le blindage proposé était suffisant. Les deux scénariosavant et après l’enlèvement ont été comparés et selon cette simulation, la situation du bruit defond devrait être améliorée.Enfin une procédure a été proposée et appliquée pour analyser les données de calibration del’échelle d’énergie de STEREO. La procédure a été élaborée pour être applicable pour toutes lessources de calibration disponibles et pour minimiser les incertitudes systématiques. Le résultatpeut être utilisé pour ajuster les paramètres de la simulation Geant4 du détecteur développée parla collaboration, par comparaison avec des données mesurées et après pour déterminer l’échellede l’énergie avec la précision requise de < 2%

Development of a UAV-Based Gamma Spectrometry System for Natural Radionuclides and Field Tests at Central Asian Uranium Legacy Sites

Uranium mining and processing had been widespread in Central Asia since the mid-1940s. However, with the establishment of the newly independent states in the 1990s, many of the former uranium mining and processing facilities and their associated wastes (dumps and tailings) were abandoned and have since posed a threat to the environment. The fact that the sites were left behind without proper remediation for a long time has led to the uncontrolled spread of radioactive and toxic contaminants in the environment due to landslides or flooding. Knowledge of the exact location of some waste facilities was lost as a result of social disruptions during the 1990s. In order to assess radiological risks and plan and implement adequate, sustainable, and environmental remediation measures, the radiological situation at the uranium legacy sites must be repeatedly mapped with the best possible accuracy in terms of both sensitivity and spatial resolution. In this paper, we present the experimental use of an unmanned aerial vehicle (UAV) equipped with gamma spectrometry systems as a novel tool for mapping, assessing, and monitoring radioactivity at sites affected by uranium mining and processing and other activities related to enhanced natural radioactivity. Special emphasis is put on the practical conditions of using UAV-based gamma spectrometry in an international context focusing on low- and medium-income countries. Challenges and opportunities of this technology are discussed, and its reliability and robustness under field conditions are critically reviewed. The most promising future application of the technology appears to be the radiological monitoring, institutional control, and quality assurance of legacy sites during and after environmental remediation. One-off administrative and logistical challenges of the technology are outweighed by the significant amount of time and cost saved once a UAV-based gamma spectrometry survey system is set up

Shape transition in the neutron-rich Y nuclei and its evolution across the isotopic chain

The evolution of the deformation across the Y isotopic chain, in the vicinity of N = 60 boundary, has been studied using gamma spectroscopy methods. The nuclei of interest have been produced by neutron induced fission of 235U and 241Pu targets, during two experimental campaigns named EXILL and FIPPS at the Institute Laue-Langevin in Grenoble. The emitted gamma rays have been collected by HPGe and LaBr3 detectors providing the identification of the high-spin levels in the 94Y and 96Y isotopes up to 6 MeV excitation energy, as well as information about the half-lives of the in-band states in the 98Y nucleus. The obtained results shed new light on the onset of deformation in the neutron-rich Y isotopes and, in general, in the A ≈ 100 region

Search for light sterile neutrinos with the STEREO experiment

The stereo experiment is searching for a non-standard oscillation in the propagation of anti-neutrinos produced by a nuclear reactor which could be the sign for the existence of a sterile state of eV mass and the origin of the reactor anti-neutrino anomaly. In this paper, results from 66 days of reactor turned on and 138 days of reactor turned off are reported excluding large amplitude oscillations. A special focus is put on the data analysis and studies of correlated backgrounds. In particular the origin of the correlated background measured in reactor turned off periods is discussed. This background mainly originates from neutrons produced by cosmic radiation

Search for light sterile neutrinos with the STEREO experiment

International audienceThe stereo experiment is searching for a non-standard oscillation in the propagation of anti-neutrinos produced by a nuclear reactor which could be the sign for the existence of a sterile state of eV mass and the origin of the reactor anti-neutrino anomaly. In this paper, results from 66 days of reactor turned on and 138 days of reactor turned off are reported excluding large amplitude oscillations. A special focus is put on the data analysis and studies of correlated backgrounds. In particular the origin of the correlated background measured in reactor turned off periods is discussed. This background mainly originates from neutrons produced by cosmic radiation

First antineutrino energy spectrum from 235^{235}U fissions with the STEREO detector at ILL

International audienceThis article reports the measurement of the 235U-induced antineutrino spectrum shape by the Stereo experiment. 43 000 antineutrinos have been detected at about 10 m from the highly enriched core of the ILL reactor during 118 full days equivalent at nominal power. The measured inverse beta decay spectrum is unfolded to provide a pure 235U spectrum in antineutrino energy. A careful study of the unfolding procedure, including a cross-validation by an independent framework, has shown that no major biases are introduced by the method. A significant local distortion is found with respect to predictions around E ν ≃ 5.3 MeV. A Gaussian fit of this local excess leads to an amplitude of A = 12.1 ± 3.4% (3.5σ)

Improved STEREO simulation with a new gamma ray spectrum of excited gadolinium isotopes using FIFRELIN

International audienceThe STEREO experiment measures the electron antineutrino spectrum emitted in a research reactor using the inverse beta decay reaction on H nuclei in a gadolinium loaded liquid scintillator. The detection is based on a signal coincidence of a prompt positron and a delayed neutron capture event. The simulated response of the neutron capture on gadolinium is crucial for the comparison with data, in particular in the case of the detection efficiency. Among all stable isotopes,$^{155}$Gd and$^{157}$Gd have the highest cross sections for thermal neutron capture. The excited nuclei after the neutron capture emit gamma rays with a total energy of about 8MeV. The complex level schemes of$^{156}$Gd and$^{158}$Gd are a challenge for the modeling and prediction of the deexcitation spectrum, especially for compact detectors where gamma rays can escape the active volume. With a new description of the Gd (n, $\gamma$ ) cascades obtained using the FIFRELIN code, the agreement between simulation and measurements with a neutron calibration source was significantly improved in the STEREO experiment. A database of ten millions of deexcitation cascades for each isotope has been generated and is now available for the user

Improved Sterile Neutrino Constraints from the STEREO Experiment with 179 Days of Reactor-On Data

The STEREO experiment is a very short baseline reactor antineutrino experiment. It is designed to test the hypothesis of light sterile neutrinos being the cause of a deficit of the observed antineutrino interaction rate at short baselines with respect to the predicted rate, known as the Reactor Antineutrino Anomaly. The STEREO experiment measures the antineutrino energy spectrum in six identical detector cells covering baselines between 9 and 11 m from the compact core of the ILL research reactor. In this article, results from 179 days of reactor turned on and 235 days of reactor turned off are reported in unprecedented detail. The current results include improvements in the description of the optical model of the detector, the gamma-cascade after neutron captures by gadolinium, the treatment of backgrounds, and the statistical method of the oscillation analysis. Using a direct comparison between antineutrino interaction rates of all cells, independent of any flux prediction, we find the data compatible with the null oscillation hypothesis. The best-fit point of the Reactor Antineutrino Anomaly is rejected at more than 99.9% C.L