Cosmic-ray Neutron Measurements Research Articles

Mapping spatiotemporal soil moisture in highly heterogeneous agricultural landscapes using mobile dual‐spectra cosmic‐ray neutron sensing

AbstractAccurate large‐scale soil moisture (SM) maps are crucial for catchment‐scale hydrological models used for water resource management and warning systems for droughts, floods, and wildfires. SM can be mapped by mobile cosmic‐ray neutron (CRN) systems of moderated detectors at homogeneous landscapes of similar soil and vegetation. In this study, we present a new approach for mobile CRN detection to perform to its full potential, where CRN measurements can also be converted to SM at heterogeneous landscapes. The approach is based solely on thermal and epithermal neutron datasets obtained from mobile dual‐spectra CRN detection, combined with theoretical developments using a particle transport model. For each measurement point, the land cover type is identified using the thermal‐to‐epithermal (T/E) ratio, and the relevant neutron‐count‐to‐soil‐moisture conversion function is estimated from CRN stations located at the main land cover types in the catchment. With this approach, the requirement of collecting 100+ soil samples for each point along the survey route is omitted. We use this T/E‐dependent approach to obtain SM maps from 12 CRN surveys and compare it with a simple approach where only the conversion function from the agricultural site is used. SM by the simple approach is comparable to the estimates of the agricultural stations of a capacitance sensor network, while the estimates of the T/E‐dependent approach also compare well with the heathland and forest stations. With accurate SM estimates for all landcover types, the average error is reduced from 0.089 to 0.038 when comparing CRN SM with space‐borne Soil Moisture Active Passive Mission estimates.

Cosmic ray neutron spectrometry and dosimetry at High Altitude Research Laboratory, Gulmarg, Kashmir, India

Cosmic ray neutrons are produced by the interaction of primary cosmic rays with the air molecules present in the atmosphere. At the ground level, these secondary neutrons are one of the important contributors of natural cosmic ray radiation background dose. With two major peaks around 1 MeV and 100 MeV, the neutron energies can extend up to 10 GeV. At the ground level, considerable presence of thermal neutrons can also be observed due to collisions and moderation of fast neutrons with the soil and other surrounding materials. A study on the evaluation of cosmic ray neutron spectra, and the resulting ambient dose equivalents was carried out at High Altitude Research Laboratory, Gulmarg, India (altitude 2650 m, N34.0484°, E74.3805°) considering the enhanced contribution of cosmic ray background in the region. Extended energy Bonner Sphere Spectrometer (BSS) was used for the continuous measurement of cosmic ray neutrons during the period of October 2018 to June 2019. Neutron spectra and ambient dose equivalents were unfolded from the BSS counts with the Monte Carlo generated response matrix and the FRUIT unfolding code. Along with Bonner sphere spectrometer, Tissue Equivalent Proportional Counter (TEPC) was also used for a limited period for independent measurement of cosmic ray dose equivalents and Linear Energy Transfer (LET) distribution. For comparison, neutron spectra and dose equivalent rates were also calculated using Monte Carlo code FLUKA and EXPACS software program. The ambient dose equivalent rates were found to vary in the range of 24–34 nSv. h−1 during the entire observation period of 9 months. No considerable seasonal variations were observed during the course of the study. Dose equivalent rates, estimated by the TEPC were slightly higher than the neutron dose equivalent rates measured by BSS. Experimentally measured values match reasonably well with theoretically simulated results.

Assimilation of Cosmic‐Ray Neutron Counts for the Estimation of Soil Ice Content on the Eastern Tibetan Plateau

AbstractAccurate observations and simulations of soil moisture phasal forms are crucial in cold region hydrological studies. In the seasonally frozen ground of eastern Tibetan Plateau, water vapor, liquid, and ice coexist in the frost‐susceptible silty‐loam soil during winter. Quantification of soil ice content is thus vital in the investigation and understanding of the region's freezing‐thawing processes. This study focuses on the retrieval of soil ice content utilizing the in situ soil moisture (i.e., liquid phase) and cosmic ray neutron measurements (i.e., total water including liquid and ice), with Observing System Simulation Experiments. To derive the total soil water from neutron counts, different weighting methods (revised, conventional, and uniform) for calibrating the cosmic‐ray neutron probe (CRNP) were intercompared. The comparison showed that the conventional nonlinear method performed the best. Furthermore, to assimilate fast neutrons using the particle filter, the STEMMUS‐FT (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil) model was used as the physically based process model, and the COSMIC model (Cosmic‐ray Soil Moisture Interaction Code) used as the observation operator (i.e., forward neutron simulator). Other than background inputs from disturbed initializations in the STEMMUS‐FT, model uncertainties were predefined to assimilate fast neutrons. We observed that with enough spread of uncertainties, the updated states could mimic the CRNP observation. In all setups, assimilating CRNP measurements could enhance total soil water analyses, which consequently led to the improved detection of soil ice content and therefore the freezing thawing‐process at the field scale.

Comparative study of soil moisture estimations from SMOS satellite mission, GLDAS database, and cosmic-ray neutrons measurements at COSMOS station in Eastern Poland

The paper presents a detailed comparative study of three surface soil moisture datasets retrieved from Cosmic-Ray Soil Moisture Observing System (COSMOS) in situ neutron measurement, CATDS Soil Moisture Ocean Salinity (SMOS) satellite microwave observations, and modelled data of the Global Land Data Assimilation System (GLDAS). Subsurface datasets were also calculated from the in situ and satellite measurements by using an exponential filter and were compared with the GLDAS estimates. For these comparisons, the Triple Collocation (TC) method, Nash–Sutcliffe Efficiency (NSE) coefficients, trend charts, and scatterplots were used.The main goal of this work was to verify the concordance of cosmic-ray neutron measurements with low-resolution soil moisture data from the CATDS SMOS and GLDAS products. The second objective was to determine the possibility of obtaining comparable subsurface soil moisture products from the aforementioned soil moisture data source.The obtained results show that data from the COSMOS sensor, which assesses soil moisture in an area 600m in diameter, agree reasonably well with the CATDS SMOS and GLDAS data having spatial resolution of about 25km. These conclusions suggest that COSMOS Derlo measurements can be particularly useful for validation of low-resolution satellite soil moisture observations as well as modelled values.The results obtained by using the TC method also revealed satisfactory agreement among all studied surface soil moisture data.However, the performed analysis shows some preponderance of SMOS and COSMOS data over GLDAS products. Although GLDAS data show a noticeable smoothing effect, COSMOS and CATDS SMOS more effectively reveal temporal soil moisture changes. Thus, for some applications, the use of CATDS SMOS estimates rather than GLDAS products may be more appropriate.Results retrieved by using an exponential filter are significant and encouraging. In particular, subsurface soil moisture values calculated from CATDS SMOS show stronger correlation with COSMOS Derlo data than those of GLDAS. Furthermore, COSMOS Derlo data also respond more intensively to surface soil moisture changes.

Translating aboveground cosmic-ray neutron intensity to high-frequency soil moisture profiles at sub-kilometer scale

Abstract. Above-ground cosmic-ray neutron measurements provide an opportunity to infer soil moisture at the sub-kilometer scale. Initial efforts to assimilate those measurements have shown promise. This study expands such analysis by investigating (1) how the information from aboveground cosmic-ray neutrons can constrain the soil moisture at distinct depths simulated by a land surface model, and (2) how changes in data availability (in terms of retrieval frequency) impact the dynamics of simulated soil moisture profiles. We employ ensemble data assimilation techniques in a "nearly-identical twin" experiment applied at semi-arid shrubland, rainfed agricultural field, and mixed forest biomes in the USA. The performance of the Noah land surface model is compared with and without assimilation of observations at hourly intervals, as well as every 2 days. Synthetic observations of aboveground cosmic-ray neutrons better constrain the soil moisture simulated by Noah in root-zone soil layers (0–100cm), despite the limited measurement depth of the sensor (estimated to be 12–20cm). The ability of Noah to reproduce a "true" soil moisture profile is remarkably good, regardless of the frequency of observations at the semi-arid site. However, soil moisture profiles are better constrained when assimilating synthetic cosmic-ray neutron observations hourly rather than every 2 days at the cropland and mixed forest sites. This indicates potential benefits for hydrometeorological modeling when soil moisture measurements are available at a relatively high frequency. Moreover, differences in summertime meteorological forcing between the semi-arid site and the other two sites may indicate a possible controlling factor to soil moisture dynamics in addition to differences in soil and vegetation properties.

An assessment of the effect of horizontal soil moisture heterogeneity on the area-average measurement of cosmic-ray neutrons

[1] The cosmic-ray neutron probe measures soil moisture over tens of hectares, thus averaging spatially variable soil moisture fields. A previous paper described how variable soil moisture profiles affect the integrated cosmic-ray neutron signal from which depthaverage soil moisture is computed. Here, we investigate the effect of horizontal heterogeneity on the relationship between neutron counts and average soil moisture. Observations from a distributed sensor network at a site in southern Arizona indicate that the horizontal component of the total variance of the soil moisture field is less variably in time than the vertical component. Using results from neutron particle transport simulations we show that 1-D binary distributions of soil moisture may affect both the mean and variance of neutron counts of a cosmic-ray neutron detector placed arbitrarily in a soil moisture field, potentially giving rise to an underestimate of the footprint average soil moisture. Similar simulations that used 1-D and 2-D Gaussian soil moisture fields indicate consistent mean and variances of a randomly placed detector if the correlation length scales are short (less than � 30 m) and/or the soil moisture field variance is small (<0.032 m 6 m � 6 ). Taken together, these soil moisture observations and neutron transport simulations show that horizontal heterogeneity likely has a small effect on the relationship between mean neutron counts and average soil moisture for soils under natural conditions. Citation: Franz, T. E., M. Zreda, T. P. A. Ferre, and R. Rosolem (2013), An assessment of the effect of horizontal soil moisture heterogeneity on the area-average measurement of cosmic-ray neutrons, Water Resour. Res., 49, doi:10.1002/wrcr.20530.

Measurement of cosmic ray neutrons with Bonner sphere spectrometer and neutron monitor at 79°N

In 2007, a Bonner spheres spectrometer (BSS) was installed in Ny-Ålesund, Spitsbergen, at about 79°N. The spectrometer allows continuous measurement of the spectral fluence rate distribution of secondary neutrons from cosmic radiation in absolute terms. In this way, the system complements a neutron monitor (NM) that was installed in 2005, in Barentsburg, Spitsbergen, at about 78°N. To compare the readings of both systems, the NM response functions to neutrons and protons were calculated by means of the GEANT4 code, in the energy range between 10 meV and 100 GeV, and between 40 MeV and 10 GeV, respectively, using different intra-nuclear cascade (INC) models at energies above 20 MeV. Sample spectral fluence distributions as measured by means of the BSS system for neutrons in November and December 2007 were used and folded with the calculated GEANT4 NM response. The resulting calculated NM count rates were then compared to those actually measured by the NM system and a reasonable agreement between 7% and 43% was obtained, depending on the nuclear models used in the GEANT4 calculations and the assumed 10B enrichment of the NM counters used to detect the neutrons.

Characterization of Radiation Instruments at the Summit of Mt. Fuji

The opportunity of exposure to high-energy radiation up to GeV is increasing as in civilian aircrafts and at particle accelerators. The transport of such energetic particle is still difficult to describe precisely and thus verification by measurement using a well characterized instrument is indispensable for reliable dosimetry. However, no reference calibration field has been established for the high-energy range. We thus propose to use a facility at the summit of Mt. Fuji (3776 m in altitude; N35.36°, E138.73°), the highest place in Japan, for characterization of radiation instruments that are possibly used in high-energy radiation fields. For demonstration of the effectiveness, two moderator-type neutron monitors (NCN1 and WENDI-II) having different energy response functions were employed for cosmic-ray neutron measurements in the summer of 2009. In comparison with numerically simulated values, it was found that the extended energy neutron monitor (WENDI-II) has a good property suitable for measurement of the cosmic-ray neutrons with the broad range of energy. The unexpected response of NCN1 is to be investigated in ongoing studies.

Electrochemical etching processes for the detection of neutrons and radon-decay products

The electrochemical etching, because of its complexity, is of interest when it makes it possible to achieve detection characteristics which are not encountered with the chemical etching. These unique characteristics can be found for example for the personal dosimetry of low-energy neutrons around nuclear reactors and for the detection of both low- and high-energy cosmic-ray neutrons at civil aviation altitudes. In particular sufficiently large signal-to-noise ratios for cosmic ray neutron measurements can be achieved by using stack of polycarbonate- and/or CR-39-detectors, since the electrochemical etching processes make it possible: (a) the rapid scanning of large detector areas, and (b) the counting of coincidence events in paired detectors induced by a-few-microns long tracks. The detection of the radon decay products is hindered by the fact that their concentrations are altered in the vicinity of detector surface during the measurement. Polycarbonate detectors may be useful in solving these problems both because they register radon-decay products far away from the plated-out surface and they can be manufactured with any possible geometry and/or shape. However, it is possible to use several combinations of chemical and electrochemical etching steps which implies the possibility of new applications of track detectors for the registration of neutrons, cosmic rays and radon decay products.

An application of cosmic-ray neutron measurements to the determination of the snow-water equivalent

Application of the water absorption character of cosmic-ray neutrons to the determination of the water equivalent depth of a snow cover is shown. We call this device the cosmic-ray snow gauge. The attenuation curves of the cosmic-ray snow gauge set under or in water or snow are experimentally obtained. Continuous field observations of the snow depths during four snow seasons demonstrate the effectiveness of the present device even for a snow cover of more than 1 m water equivalent, where the relative accuracy is found to be a few percent of the water depth. Possible effects of primary cosmic rays, barometric pressure and water content of soil upon the determination of snow depths are discussed.

Albedo Neutron Source for High-Energy Protons Trapped in the Geomagnetic Field

Absolute intensity measurement of cosmic ray neutrons in atmosphere, obtaining albedo leakage neutron flux near solar minimum

Albedo Neutron Source for High-Energy Protons Trapped in the Geomagnetic Field

An absolute intensity measurement of cosmic-ray neutrons in the atmosphere, in the energy range from thermal to 20 MeV, leads to a value of 0.36\ifmmode\pm\else\textpm\fi{}0.06 neutron ${\mathrm{cm}}^{\ensuremath{-}2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ for the albedo neutron leakage flux at 42\ifmmode^\circ\else\textdegree\fi{} geomagnetic latitude near solar minimum. The significance of this value for evaluating the contribution of the cosmic-ray albedo-neutron-decay source for the high-energy protons in the inner radiation belt is discussed.