Coincidence Events Research Articles

A simplified approach to counting statistics with an imperfect pileup rejector

A simplified theoretical model is developed to predict counting statistics for a stationary Poisson process passing through a spectrometer with pulse-pileup rejection. The model is applicable to digital counters used for spectrometry as well as set-ups utilising analogue electronics for pulse shaping and pileup rejection. In comparison with an existing model for a perfect pileup rejector, the new model addresses the common imperfection of having a finite time resolution of the fast channel, allowing quasi-coincident signals to pass through the pile-up rejector. From the Laplace transform of a simplified interval-density distribution, approximate expressions are derived for the throughput factor and the variance of the number of counted events. The results are compared with computer simulations of a cascade of extending dead time and subsequent pileup rejection. In addition, a rigorous throughput factor is derived from probabilistic reasoning, as well as an effective throughput factor for singular and coincident events.

Effects of Extreme Climatic Events on the Autumn Phenology in Northern China Are Related to Vegetation Types and Background Climates

The increased intensity and frequency of extreme climate events (ECEs) have significantly impacted vegetation phenology, further profoundly affecting the structure and functioning of terrestrial ecosystems. However, the mechanisms by which ECEs affect the end of the growing season (EOS), a crucial phenological phase, remain unclear. In this study, we first evaluated the temporal variations in the EOS anomalies in Northern China (NC) based on the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) from 2001 to 2018. We then used event coincidence analysis (ECA) to assess the susceptibility of EOS to four ECEs (i.e., extreme heat, extreme cold, extreme wet and extreme dry events). Finally, we examined the dependence of the response of EOS to ECEs on background climate conditions. Our results indicated a slight decrease in the proportion of areas experiencing extreme heat and dry events (1.10% and 0.66% per year, respectively) and a slight increase in the proportion of areas experiencing extreme wet events (0.77% per year) during the preseason period. Additionally, EOS exhibited a delaying trend at a rate of 0.25 days/a during the study period. The susceptibility of EOS to ECEs was closely related to local hydrothermal conditions, with higher susceptibility to extreme dry and extreme hot events in drier and warmer areas and higher susceptibility to extreme cold and extreme wet events in wetter regions. Grasslands, in contrast to forests, were more sensitive to extreme dry, hot and cold events due to their weaker resistance to water deficits and cold stress. This study sheds light on how phenology responds to ECEs across various ecosystems and hydrothermal conditions. Our results could also provide a valuable guide for ecosystem management in arid regions.

BINGO innovative assembly for background reduction in bolometric [formula omitted] experiments

BINGO is a project aiming to set the grounds for large-scale bolometric neutrinoless double-beta-decay experiments capable of investigating the effective Majorana neutrino mass at a few meV level. It focuses on developing innovative technologies (a detector assembly, cryogenic photodetectors and active veto) to achieve a very low background index, of the order of 10−5 counts/(keV kg yr) in the region of interest. The BINGO demonstrator, called MINI-BINGO, is designed to investigate the promising double-beta-decay isotopes 100Mo and 130Te and it will be composed of Li2MoO4 and TeO2 crystals coupled to bolometric light detectors and surrounded by a Bi4Ge3O12-based veto. This will allow us to reject a significant background in bolometers caused by surface contamination from α-active radionuclides by means of light yield selection and to mitigate other sources of background, such as surface contamination from β-active radionuclides, external γ radioactivity, and pile-up due to random coincidence of background events. This paper describes an R&D program towards the BINGO goals, particularly focusing on the development of an innovative assembly designed to reduce the passive materials within the line of sight of the detectors, which is expected to be a dominant source of background in next-generation bolometric experiments. We present the performance of two prototype modules – housing four cubic (4.5-cm side) Li2MoO4 crystals in total – operated in the Canfranc underground laboratory in Spain within a facility developed for the CROSS double-beta-decay experiment.

Concurrent droughts across Major River Basins of the World modulated by El Niño–Southern Oscillation

Droughts are among the costliest natural disasters with far-reaching impacts. Focusing on the influence of El Niño–Southern Oscillation (ENSO) on worldwide droughts, this paper presents a formulation of the complex network of concurrent droughts across the Major River Basins (MRBs) of the World. Specifically, concurrent droughts are measured by event synchronization and interconnections of concurrent droughts are determined by significance test. Event coincidence analysis is incorporated into interconnections to quantify the effects of El Niño and La Niña events. A case study is devised for the 3-monthly standardized precipitation index (SPI) of Global Drought and Flood Catalogue during the period from 1950 to 2016. The results highlight that owing to the synchronization of droughts, there exist nine communities of the MRBs, each covering MRBs across different continents. While the correlation between SPI and ENSO exhibits mixed patterns for MRBs grouped by continent, there exist distinct patterns of correlation for MRBs grouped by community. Overall, 28.88% of concurrent droughts tend to be modulated by El Niño events, especially over MRBs in South-West Pacific, South Africa, northern South America and South Asia. In the meantime, 23.51% of concurrent droughts tend to be modulated by La Niña events, especially over MRBs in the United States, southwestern South America, northern China and Central Asia. The modulation of El Niño and La Niña is confirmed by composite analysis of SPI and detailed analysis of the Yangtze River Basin. Overall, the complex network provides useful information for drought forecasting and disaster mitigation.

On the Association of Substorm Identification Methods

AbstractSubstorms are a rapid release of energy that is redistributed throughout the magnetosphere‐ionosphere system, resulting in many observable signals, such as enhancements in the aurora, energetic particle injections, and ground magnetic field perturbations. Numerous substorm identification techniques and onset lists based on each of these signals have been provided in the literature, but often with no cross‐calibration. Since the signals produced are not necessarily unique to substorms and may not be sufficiently similar to be identified for each and every substorm, individual event lists may miss or misidentify substorms, hindering our understanding and the development and validation of substorm models. To gauge the scale of this problem, we use metrics derived from contingency tables to quantify the association between lists of substorms derived from SuperMAG SML/SMU indices, midlatitude magnetometer data, particle injections, and auroral enhancements. Overall, although some degree of pairwise association is found between the lists, even lists generated by applying conceptually similar gradient‐based identification to ground magnetometer data achieve an association with less than 50% event coincidence. We discuss possible explanations of the levels of association seen from our results, as well as their implications for substorm analyses.

Positronium lifetime validation measurements using a long-axial field-of-view positron emission tomography scanner

BackgroundPositron emission tomography (PET) traditionally uses coincident annihilation photons emitted from a positron interacting with an electron to localize cancer within the body. The formation of positronium (Ps), a bonded electron-positron pair, has not been utilized in clinical applications of PET due to the need to detect either the emission of a prompt gamma ray or the decay of higher-order coincident events. Assessment of the lifetime of the formed Ps, however, can potentially yield additional diagnostic information of the surrounding tissue because Ps properties vary due to void size and molecular composition. To assess the feasibility of measuring Ps lifetimes with a PET scanner, experiments were performed in a Biograph Vision Quadra (Siemens Healthineers). Quadra is a long-axial field-of-view (LA-FOV) PET scanner capable of producing list-mode data from single interaction events.ResultsOrtho-Ps (o-Ps) lifetimes were measured for quartz-glass and polycarbonate samples using a 22Na\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{22}\ ext{Na}$$\\end{document} positron source. Results produced o-Ps lifetimes of 1.538 ± 0.036 ns for the quartz glass and 1.927 ± 0.042 ns for the polycarbonate. Both o-Ps lifetimes were determined using a double-exponential fit to the time-difference distribution between the emission of a prompt gamma ray and the annihilation of the correlated positron. The measured values match within a single standard deviation of previously published results. The quartz-glass samples were additional measured with 82Rb\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${^{82}}\ ext{Rb}$$\\end{document}, 68\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{68}$$\\end{document}Ga and 124I\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{124}\ ext{I}$$\\end{document} to validate the lifetime using clinically available sources. A double-exponential fit was initially chosen as a similar methodology to previously published works, however, an exponentially-modified Gaussian distribution fit to each lifetime more-accurately models the data. A Bayesian method was used to estimate the variables of the fit and o-Ps lifetime results are reported using this methodology for the three clinical isotopes: 1.59 ± 0.03 ns for 82Rb\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{82}\ ext{Rb}$$\\end{document}, 1.58 ± 0.07 ns for 68Ga\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{68}\ ext{Ga}$$\\end{document} and 1.62 ± 0.01 ns for 124I\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{124}\ ext{I}$$\\end{document}. The impact of scatter and attenuation on the o-Ps lifetime was also assessed by analyzing a water-filled uniform cylinder (20 ϕ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\phi$$\\end{document}×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 30 cm3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document}) with an added 82Rb\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{82}\ ext{Rb}$$\\end{document} solution. Lifetimes were extracted for various regions of the cylinder and while there is a shape difference in the lifetime due to scatter, the extracted o-Ps lifetime of the water, 1.815 ± 0.013 ns, agrees with previously published results.ConclusionOverall, the methodology presented in this manuscript demonstrates the repeatability of Ps lifetime measurements with clinically available isotopes in a commercially-available LA-FOV PET scanner. This validation work lays the foundation for future in-vivo patient scans with Quadra.

Hydrothermal Conditions Modulate the Impact of Climate Extremes on Vegetation Growth in the Northern Hemisphere

ABSTRACTAimClimate extremes are becoming more frequent under global warming, with substantial repercussions for vegetation growth. The degree to which climate extremes increase the risk of high‐impact events on vegetation growth is of high concern.LocationNorthern Hemisphere (north of 30°N).Time PeriodFrom 2001 to 2022.Major Taxa StudiedPlants.MethodsWe utilised solar‐induced chlorophyll fluorescence (SIF) and the normalised difference vegetation index (NDVI) as proxies for vegetation growth. We performed event coincidence and sensitivity analyses to attribute satellite‐derived vegetation growth extremes to diverse climate extremes (extreme heat, cold, wet and drought) in the Northern Hemisphere.ResultsOur results showed that extreme heat and cold were the main climatic extremes that induced positive and negative vegetation growth extremes north of 30°N, respectively, mainly in cold and humid ecosystems (boreal and temperate forests). Water‐related extreme events accounted for less than one‐third of vegetation extremes. The contribution of drought to positive vegetation growth extreme events (approximately 17%), mainly in cold and humid ecosystems, was even slightly higher than that of extreme wet (approximately 12%), which predominantly impacted relatively warm and arid ecosystems (croplands and temperate grasslands). We further identified potential climatic thresholds that could reverse vegetation growth responses to climate extremes (temperature is 12.5°C and climatic water deficit is ‐60 mm, approximately). We also showed that the past two decades of warming and precipitation changes did not induce a shift in the main climatic drivers of vegetation extremes across northern ecosystems.Main ConclusionsOur results emphasise the crucial role of background hydrothermal conditions in the attribution of vegetation growth extremes to diverse climate extremes across northern ecosystems and have substantial implications for predicting how Northern Hemisphere vegetation will respond to increasing climate extremes in the future.

A deep neural network for positioning and inter-crystal scatter identification in multiplexed PET detectors: a simulation study

Objective. High-resolution positron emission tomography (PET) relies on the accurate positioning of annihilation photons impinging the crystal array. However, conventional positioning algorithms in light-sharing PET detectors are often limited due to edge effects and/or the absence of additional information for identifying and correcting scattering within the crystal array (known as inter-crystal scattering). This study explores the feasibility of deep neural network (DNN) techniques for more precise event positioning in finely segmented and highly multiplexed PET detectors with light-sharing. Approach. Initially, a Geant4 Application for Tomographic Emission (GATE) simulation was used to study the spatial and statistical properties of inter-crystal scatter (ICS) events in finely segmented LYSO PET detectors. Next, a DNN for crystal localisation was designed, trained and tested with light distributions of photoelectric (P) and Compton + photoelectric (CP) events simulated using optical GATE and an analytical method to speed up data generation. Using the statistical properties of ICS events, an energy-guided positioning algorithm was then built into the DNN. The positioning algorithm enables selection of the unique or first crystal of interaction in P and CP events, respectively. Performance of the DNN was compared with Anger logic using light distributions from simulated 511 keV point sources placed at different locations around a single PET detector module. Main results. The fraction of events forward and backward scattered in the LYSO detector was 0.54 and 0.46, respectively, whereas naïve application of the Klein–Nishina formulation predicts 70% forward scatter. Despite coarse photodetector data due to signal multiplexing, the DNN demonstrated a crystal classification accuracy of 90% for P events and 82% for CP events. For crystal positioning, the DNN outperformed Anger logic by at least 34% and 14% for P and CP events, respectively. Further improvement is somewhat constrained by the physics—specifically, the ratio of backward to forward scattering of gamma rays within the crystal array being close to 1. This prevents selecting the first crystal of interaction in CP events with a high degree of certainty. Significance. Light sharing and multiplexed PET detectors are common in high-resolution PET, yet their traditional positioning algorithms often underperform due to edge effects and/or the difficulty in correcting ICS events. Our study indicates that DNN-based event positioning has the potential to enhance 2D coincidence event positioning accuracy by nearly a factor of 3 compared to Anger logic. However, further improvements are difficult to foresee without additional information such as event timing.

Spatial synchrony, temporal clustering and dominant driver of streamflow droughts in Peninsular India

Understanding the spatio-temporal evolution of streamflow droughts and their relationship with potential causative processes is critical for effective drought management. This study assesses spatial synchrony and temporal clustering of streamflow droughts in six major river basins of Peninsular India. The importance of baseflow, rainfall deficits, soil moisture deficits and high temperatures in triggering streamflow droughts is also investigated to identify the dominant driver during the period 1981–2015. Spatial synchrony of streamflow droughts is investigated using multivariate Ripley’s K function and temporal clustering is evaluated using univariate Ripley’s K at various timescales. The interactions of streamflow droughts with potential causative processes are investigated using event coincidence analysis. At regional scale, streamflow droughts in peninsular catchments show strong spatial synchrony even at longer timescales. However, at basin scale, droughts in the catchments show strong spatial synchrony only at smaller timescales, behave independently of each other and achieve asynchrony with time, especially at longer timescales. Streamflow droughts show the strongest temporal clustering at smaller timescales and the strength of clustering decreases after a 3 year timescale. Rainfall deficits primarily control streamflow droughts in Peninsular India at a range of lags, except just before the onset of drought events where baseflow dominates. In addition, trigger coincidence rates of baseflow are lower than rainfall deficits but higher than soil moisture deficits and high temperatures at longer time lags.

Identification of sinkhole‐prone zones by successive coincidence deficit index analysis

AbstractHydrological data‐driven models require the time series of several hydrological events with different time resolutions. The interpretation of any time series event is generally difficult without some sort of filtering or converting it to a single index value. The simultaneous analysis of two or more hydrological events over a definite time span may be more informative about the region of interest. For this purpose, a new index, referred to as the successive coincidence deficit index (SCDI), was introduced to identify sinkhole‐prone regions using the persistent water deficit concept. In this study, monthly integrated multi‐satellite retrievals for GPM based precipitation (P) and gravity recovery and climate experiment‐based groundwater storage (GWS) datasets over Konya Closed Basin (KCB) in Türkiye were used to analyse the sinkhole occurrence. The main finding of this study is that SCDI distribution with high index values, concentrated on the southwestern part of KCB, is in line with the sinkholes occurred mainly after 2010. The proposed SCDI could also serve as a kind of drought index, which enables practitioners to quantify the relationship between drought and sinkhole occurrence. Moreover, the event coincidence analysis was utilized to detect deficiency in GWS over the KCB, which was associated with a rate of 0.8 for P deficiency, and this rate reaches up to 0.9 in the sinkhole region analysed in this study. As a conclusion, the proposed methodology can detect sinkhole‐prone regions to construct risk maps for stakeholders, policymakers, and end users.

Spatiotemporal dependence of compound drought–heatwave and fire activity in China

Climate strongly influences fire activity, and the alignment of multiple weather and climate extremes, such as co-occurrence of hot and dry, leads to more severe fires. Recognizing the knowledge gap on the combination of compound drought–heatwave events and fire activities, we applied an event coincidence analysis to investigate their spatiotemporal dependence in China during 2003–2020 based on remote sensing active fire data and 2 206-site meteorological observations. It is found that the spatial extent of fire activities and their simultaneous occurrence with compound drought–heatwaves both expanded. The geographical hotspots of frequent fire activities following compound drought–heatwave events were predominantly situated in southwestern and southern China. There was a significant upward trend of fire activities following compound drought–heatwave events, with the ratio of compound drought–heatwave events being followed by fire activities reached 0.70 ± 0.06 in 2013–2020, indicating a growth of 19% compared to the period of 2003–2012. Our findings contribute to a better understanding of the spatiotemporal dependence of compound drought–heatwave and fire activity and provide insights into their mitigation strategies.

More accurate suppression of compton continuum based on list mode data acquisition technique

Compton suppression is a beneficial technique employed in gamma-spectroscopy to decrease the Compton continuum and enhance the detection efficiency for various radionuclide identification/measurement applications. However, for cascade gamma-emitters radionuclides the conventional approach of Compton suppression (i.e., surrounding HPGe with NaI(Tl)/plastic detectors) removes all coincident events and reduces the detection efficiency. In this study, to address this challenge, a new approach is proposed that uses serial geometry (i.e., NaI(Tl) detector + sample + HPGe detector), more accurate coincidence time window, and the recorded gamma-ray energy in the detectors. The approach is experimentally tested and compared with the conventional approach. The results showed that by the proposed approach for 60Co, a cascade gamma-emitter nuclide, the suppression factor (SF) is improved and only 8% of photo-peak events, compared to 25% in the conventional approach, are lost by the suppression procedure. This decrease in loss of the photo-peak events in the proposed Compton suppression approach improves the accuracy of the activity measurement of the cascade-emitter radionuclides.

A dedicated in-beam PET system with a modular dual-head for radiotherapy imaging in HIMM

This paper presents a heavy-ion in-beam PET, referred to as ibPET with a specifically custom designed data acquisition system, which was constructed and tested at the Heavy Ion Medical Machine (HIMM) beam-line, HICTC in Wuwei. The data acquisition system of ibPET has undergone several upgrades during the years, And its data processing capability now exceeds one million events per second per channel. To achieve optimal performance of the ibPET under existing conditions, we tested coincident events and noise equivalent count (NEC) under different time coincidence windows.The results show that the signal-to-noise ratio of the reconstructed image data reaches its peak when the time window is around 3 ns, indicating that the ibPET performance is optimal at this point. In order to determine the size of the time window, we measured the system's time resolution FWHM to be 1.63 ns, and ultimately chose a time window size of twice the time resolution FWHM, which is 3.25 ns. Meanwhile, we demonstrate the capability of our ibPET to monitor the dose range with various irradiation. The results demonstrate that the system exhibits the expected exponential decay in counting rate after irradiation, and accurately provides penetration depth positions at different energy levels.

Concurrent and dynamical interdependency of compound precipitation and wind speed extremes over India

Compound precipitation and wind speed extremes have the potential to cause significant socio-economic losses. Therefore, this study aims to investigate the association of compound precipitation and wind speed extremes in observed period (1981–2020) and future periods (2021–2060 (F1) and 2061–2100 (F2)) over India. In this study, we have used IMD precipitation and ERA5 windspeed gridded (0.250 × 0.250) data for the observed period. For future analysis, we have used CMIP6-based EC-Earth3 GCM (SSP2–4.5 and SSP5–8.5) and CORDEX CMIP5 RCM (RCP-4.5 and RCP-8.5) climate models for precipitation and wind speed outputs. We have analyzed the characteristics of precipitation and wind speed extremes using three approaches: empirical method, tail dependence analysis, and event coincidence analysis. The empirical analysis revealed that the proportion of the area with >240 Compound Precipitation and Wind speed Extremes (CPWE) on the same day is 2.8% in observed period. However, it increased to 22.22% in future period F2, particularly in the SSP5–8.5 scenario of the CMIP6 case, and this increase is observed in regions like Am, eastern parts of Aw and Cwa, Bwk, and Dsb. The tail dependence analysis revealed that the area with tail dependence >0.5 constitutes approximately 0.72% in the observed period across the study area. However, under climate change scenarios, this percentage varies between 2.45% to 3.66% and notably, the increase is observed in regions such as Am, Bwk, and Dsb. The results of precursor coincidence rate (pcr) shows a high value of 3.52% in the entire study area for the observed period. The future analysis reveals an increase of areas in between 3.18% to 5.14% for different climate change scenarios. Consequently, the trigger coincidence rate (tcr) results also revealed the occurrence of high tcr in the range of 0.52% to 5.79% across the study area within the temporal window. However, the tcr range increases to 2.17% to 23.32% during the monsoon season during the observed period with respect to the days in the temporal window. The overall results showed that the high values of tail dependence, pcr and tcr are concentrated in the Am, coastal areas of Aw and Cwa, as well as in the Bwk and Dsb regions. This will lead to increase the extreme precipitation driven by extreme wind speed in those regions consequently, the risk of flash floods, landslides and natural hazards may rise in the future. Overall, this study provides valuable insights into the spatial distribution of compound precipitation and wind speed extremes across the study area.

On the timescale of drought indices for monitoring streamflow drought considering catchment hydrological regimes

Abstract. There is a wide variety of drought indices, yet a consensus on suitable indices and temporal scales for monitoring streamflow drought remains elusive across diverse hydrological settings. Considering the growing interest in spatially distributed indices for ungauged areas, this study addresses the following questions: (i) What temporal scales of precipitation-based indices are most suitable to assess streamflow drought in catchments with different hydrological regimes? (ii) Do soil moisture indices outperform meteorological indices as proxies for streamflow drought? (iii) Are snow indices more effective than meteorological indices for assessing streamflow drought in snow-influenced catchments? To answer these questions, we examined 100 near-natural catchments in Chile with four hydrological regimes, using the standardised precipitation index (SPI), standardised precipitation evapotranspiration index (SPEI), empirical standardised soil moisture index (ESSMI), and standardised snow water equivalent index (SWEI), aggregated across various temporal scales. Cross-correlation and event coincidence analysis were applied between these indices and the standardised streamflow index at a temporal scale of 1 month (SSI-1), as representative of streamflow drought events. Our results underscore that there is not a single drought index and temporal scale best suited to characterise all streamflow droughts in Chile, and their suitability largely depends on catchment memory. Specifically, in snowmelt-driven catchments characterised by a slow streamflow response to precipitation, the SPI at accumulation periods of 12–24 months serves as the best proxy for characterising streamflow droughts, with median correlation and coincidence rates of approximately 0.70–0.75 and 0.58–0.75, respectively. In contrast, the SPI at a 3-month accumulation period is the best proxy over faster-response rainfall-driven catchments, with median coincidence rates of around 0.55. Despite soil moisture and snowpack being key variables that modulate the propagation of meteorological deficits into hydrological ones, meteorological indices are better proxies for streamflow drought. Finally, to exclude the influence of non-drought periods, we recommend using the event coincidence analysis, a method that helps assessing the suitability of meteorological, soil moisture, and/or snow drought indices as proxies for streamflow drought events.

Are longer and more intense heatwaves more prone to extreme precipitation?

Simultaneous/sequential occurrence of multiple extreme climate events has the tendency to inflict greater damage compared to isolated extreme climate events. The exploration of simultaneous or continuous extreme events holds profound implications for disaster risk prevention. Therefore, we identified the compound heatwaves and extreme precipitation events (CHP, extreme precipitation events following heatwaves) in China based on ERA5 data. Then, the individual extreme events and compound events were undertaken through event coincidence analysis. Furthermore, the atmospheric background of individual extreme events and compound extreme events was further investigated. The results show that the CHP events with an increasing trend mainly occurred in northern China and northeastern the Qinghai-Tibet Plateau (QTP). Notably, after the year 2000, the time interval of extreme precipitation in CHP events was significantly shortened. In July and August, the trigger coincidence rate of extreme precipitations by heatwaves occurred higher and extreme precipitation events following heatwaves accounted for 40%. Moreover, Extreme precipitation is more likely to occur after a longer duration and higher intensity heatwave in the QTP. Convective effective potential energy (CAPE) and relative humidity manifest a considerable increase during the heatwave duration, greatly augmenting the risk of extreme precipitation events. In addition, the frequency of CHP events is higher in northern and northeastern China during El Niño years. While it predominantly concentrates in the northeast region in La Niña years.

Gamma-ray imaging of Np-237 metal using an organic glass imager

Neutron and gamma-ray imaging systems are deployed within the field of nuclear safeguards for the detection and localization of special nuclear materials and other materials of interest. 237Np is one of these materials of interest due its presence in spent nuclear fuel and potential for use in nuclear weapons when purified. Here, for the first time, a 6 kg neptunium sphere (98.8 wt% 237Np) was measured using a dual-particle imager, from the University of Michigan, consisting of organic glass and inorganic scintillators. The novel composition of organic glass scintillator was recently developed at Sandia National Labs and has been used in particle imaging systems due to its time resolution and particle discrimination capabilities. Gamma-ray energy spectra from single and coincident events were extracted and the sequencing of Compton scatter and photoelectric absorption gamma-ray events was used to generate images using simple backprojection. The emissions of interest in this work are the 312 keV and 416 keV gamma rays from 233Pa, a daughter isotope from the neptunium decay series. The results of this work show that there is close agreement between the true source location in angular space and the converged location from the gamma ray images created using the system. The gamma spectroscopy from single and coincident events also identified the characteristic emission from the daughter isotope and could be used to assist with the identification of 237Np. Furthermore, successful localization of the source with 5 s of data demonstrates the practical application of the imaging system for imaging and detection of material in weapons-useable quantities.

Evaluating the capability of detecting recoil-electron tracks using an electron-tracking Compton camera with a silicon-on-insulator pixel sensor

Sub-MeV gamma-ray line observations are crucial for the understanding of high-energy astrophysical processes. In this energy range, the presence of a significant amount of background poses a serious problem. Therefore, it is essential to reduce this background to achieve high-sensitivity observations. An electron-tracking Compton camera can efficiently mitigate the impact of background on scientific results thanks to its ability to fully resolve the Compton process. In addition if we utilize a semiconductor detector as the sensor, the electron-tracking Compton camera can achieve high-energy resolution for the detection of gamma-ray lines. To operate the electron-tracking Compton camera effectively, the sensor needs to coincidence events for each detection signal and resolve the complex structure of recoiled-electron tracks of a few hundred μm. We focused on the silicon-on-insulator pixel sensor XRPIX2b developed for X-ray observations, which satisfies requirements. We developed a prototype electron-tracking Compton camera using XRPIX2b and carried out a quantitative evaluation of its detection capability for estimating the recoil directions of electrons at various angles. As a result, we successfully detected short recoil-electron tracks with a length of 100μm corresponded to scattering angles of 30°-40° for 511 keV gamma rays entering the sensor. In term of angular resolution measure and scatter plane deviation of a reconstructed image, the evaluations showed results of 15° and 40° (full width at half maximum), respectively, in cases where 511 keV gamma rays were scattered at 90° and recoil electrons tracked the surface of the sensor detection plane. We confirmed that these results were consistent with simulation results, within approximately a 15% margin.

Low-energy radiative backgrounds in CCD-based dark-matter detectors

The reach of sub-GeV dark-matter detectors is at present severely affected by low-energy events from various origins. We present the theoretical methods to compute the single- and few-electron events that arise from secondary radiation emitted by high-energy particles as they pass through detector materials and perform a detailed simulation to quantify them at (Skipper) CCD-based experiments, focusing on the SENSEI data collected at Fermilab near the MINOS cavern. The simulations account for the generation of secondaries from Cherenkov and luminescent recombination radiation; photo-absorption in the bulk, backside layer, pitch adapter, and epoxy; the photon reflection and refraction at interfaces; thin-film interference; the roughness of the interfaces; the dynamics of charges produced in the highly doped CCD-backside-layers; and the partial charge collection on the CCD backside. We consider several systematic uncertainties, notably those stemming from the backside modeling, which we estimate with a “fiducial” and an “extreme” charge-diffusion model, with the former model being preferred due to better agreement with partial-charge collection data. We find that Cherenkov photons constitute about 30% of the observed single-electron events for both diffusion models; radiative recombination contributes negligibly to the event rate for the fiducial model, although it can dominate over Cherenkov for the extreme model. We also estimate the fraction of 2-electron events that arise from 1-electron event coincidences in the same pixel, finding that the entire 2-electron rate can be explained by coincidences of radiative events and spurious charge. Accounting for both radiative and non-radiative backgrounds, we project the sensitivity of future Skipper-CCD-based experiments to different dark-matter models. For light-mediator models with dark-matter masses of 1, 5, and 10 MeV, we find that future experiments with 10-kg-year exposures and successful background mitigation could have a sensitivity that is larger by 9, 3, and 2 orders of magnitude, respectively, when compared to an experiment without background improvements.

Baseflow significantly contributes to river floods in Peninsular India

Extreme rainfall prior to a flood event is often a necessary condition for its occurrence; however, rainfall alone is not always an indicator of flood severity. Antecedent wetness condition of a catchment is another important factor which strongly influences the flood magnitudes. The key role of soil moisture in driving floods is widely recognized; however, antecedent conditions of deeper saturated zone may contribute to river floods. Here, we assess how closely the flood magnitudes are associated to extreme rainfall, soil moisture and baseflow in 70 catchments of Peninsular India for the period 1979–2018. Annual flood magnitudes have declined across most of the catchments. Effect of flow regulations is also assessed to understand the impact of human interventions on flood characteristics. Reservoir regulation has positive effect by reducing the flood peak and volume, whereas the duration of flood events has increased after the construction of dams. Baseflow exhibits similar patterns of trends as floods, whereas trends in rainfall and soil moisture extremes are weakly correlated with trends in flood magnitudes. Baseflow is found to be more strongly influencing the flood magnitudes than soil moisture at various time lags. Further analysis with event coincidence analysis confirms that baseflow has stronger triggering effect on river floods in Peninsular India.