Darin Desilets

Spatial and temporal distribution of secondary cosmic-ray nucleon intensities and applications to in situ cosmogenic dating

Cosmogenic nuclide production rates depend critically on the spatio-temporal distribution of cosmic-ray nucleon fluxes. Since the 1950s, measurements of the altitude, latitude and solar modulation dependencies of secondary cosmic-ray fluxes have been obtained by numerous investigators. However, until recently there has been no attempt to thoroughly evaluate the large body of modern cosmic-ray literature, to explain systematic discrepancies between measurements or to put these data into a rigorous theoretical framework appropriate for cosmogenic dating. The most important parameter to be constrained is the dependence of neutron intensity on atmospheric depth. Our analysis shows that effective nucleon attenuation lengths measured with neutron monitors over altitudes 0^5000 m range from 128 to 142 g cm 32 at effective vertical cutoff rigidities of 0.5 and 14.9 GV, respectively. Effective attenuation lengths derived from thermal neutron data are somewhat higher, ranging from 134 to 155 g cm 32 at the same cutoff rigidities and over the same altitudes. We attribute the difference to a combination of two factors: the neutron monitor is more sensitive to the higher end of the nucleon energy spectrum, and the shape of the nucleon energy spectrum shifts towards lower energies with increasing atmospheric depth. We have derived separate scaling models for thermal neutron reactions and spallation reactions based on a comprehensive analysis of cosmic-ray survey data. By assuming that cosmic-ray intensity depends only on atmospheric depth and effective vertical cutoff rigidity, these models can be used to correct production rates for temporal changes in geomagnetic intensity using paleomagnetic records. 6 2002 Elsevier Science B.V. All rights reserved.

Measuring soil moisture content non‐invasively at intermediate spatial scale using cosmic‐ray neutrons

[3] We present a novel non-invasive technique that utilizes the dependence of the low-energy cosmic-ray neutron intensity above the ground surface on the hydrogen content of soil. The cosmic-ray method is based on slowing down and thermalization of cosmic-ray neutrons by hydrogen atoms present in soil. Soil moisture greatly affects the rate at which fast neutrons are moderated, controlling neutron concentration in soils and prescribing their emission into the air. Dry soils have low moderating power and are therefore highly emissive; wet soils are more moderating and therefore less emissive as highly moderated neutrons are more efficiently removed from the system. The change in soil neutron emission is sufficient to produce a clear signal in the neutron intensity above the surface. For soil moisture content varying from zero to 40% volumetrically, the corresponding decrease in cosmic-ray neutron intensity above the surface is 60%, a hundredth of which can easily be measured using a neutron detector.

On scaling cosmogenic nuclide production rates for altitude and latitude using cosmic-ray measurements

Abstract The wide use of cosmogenic nuclides for dating terrestrial landforms has prompted a renewed interest in characterizing the spatial distribution of terrestrial cosmic rays. Cosmic-ray measurements from neutron monitors, nuclear emulsions and cloud chambers have played an important role in developing new models for scaling cosmic-ray neutron intensities and, indirectly, cosmogenic production rates. Unfortunately, current scaling models overlook or misinterpret many of these data. In this paper, we describe factors that must be considered when using neutron measurements to determine scaling formulations for production rates of cosmogenic nuclides. Over the past 50 years, the overwhelming majority of nucleon flux measurements have been taken with neutron monitors. However, in order to use these data for scaling spallation reactions, the following factors must be considered: (1) sensitivity of instruments to muons and to background, (2) instrumental biases in energy sensitivity, (3) solar activity, and (4) the way of ordering cosmic-ray data in the geomagnetic field. Failure to account for these factors can result in discrepancies of as much as 7% in neutron attenuation lengths measured at the same location. This magnitude of deviation can result in an error on the order of 20% in cosmogenic production rates scaled from 4300 m to sea level. The shapes of latitude curves of nucleon flux also depend on these factors to a measurable extent, thereby causing additional uncertainties in cosmogenic production rates. The corrections proposed herein significantly improve our ability to transfer scaling formulations based on neutron measurements to scaling formulations applicable to spallation reactions, and, therefore, constitute an important advance in cosmogenic dating methodology.

Field testing of the universal calibration function for determination of soil moisture with cosmic‐ray neutrons

The semitheoretical universal calibration function (UCF) for estimating soil moisture using cosmic-ray neutron sensors was tested by comparing to field measurements made with the same neutron detector across a range of climates, soil, latitude, altitude, and biomass. There was a strong correlation between neutron intensity and the total amount of hydrogen at each site; however, the relationship differed from that predicted by the UCF. A linear fit to field measurements explained 99% of the observed variation and provides a robust empirical means to estimate soil moisture at other sites. It was concluded that measurement errors, neutron count corrections, and scaling to remove altitudinal and geomagnetic differences were unlikely to explain differences between observations and the UCF. The differences may be attributable to the representation of organic carbon, biomass or detector geometry in the neutron particle code, or to differences in the neutron energy levels being measured by the cosmic-ray sensor and modeled using the particle code. The UCF was derived using simulations of epithermal neutrons; however, lower energy thermal neutrons may also be important. Using neutron transport code, we show the differences in response of thermal and epithermal neutrons to the relative size of the hydrogen pool. Including a thermal neutron component in addition to epithermal neutrons in a modified UCF provided a better match to field measurements; however, thermal neutron measurements are needed to confirm these results. A simpler generalized relationship for estimating soil moisture from neutron counts was also tested with encouraging results for low biomass sites.

Comment on ‘Scaling factors for production rates of in situ produced cosmogenic nuclides: a critical reevaluation’ by Tibor J. Dunai

In a recent paper, Dunai [1] discusses several problems with the altitude and latitude scaling of production rates given by [2^4]. Dunai describes three major £aws in Lals scaling: (1) cosmic-ray measurements are ordered according to geomagnetic latitude calculated from an axial dipole representation of the geomagnetic ¢eld; (2) the high-altitude (atmospheric depth 580^770 g cm32) attenuation length at 413N geomagnetic latitude is assumed to be constant down to sea level ; and (3) the scaling expression is given in terms of elevation rather than atmospheric depth. In an attempt to improve on Lals scaling, Dunai derives a new scaling model following a procedure similar to that of [4], but incorporating some neutron monitor, nuclear emulsion and cloud chamber data unavailable to [4]. In this comment we show that Dunais scaling model is based on several false assumptions. Due to the signi¢cance of these false assumptions, we ¢nd no evidence that Dunais scaling model represents an improvement over Lals scaling model. We also point out that geological factors a¡ecting production of cosmogenic nuclides can be di¤cult to evaluate, and suggest that the 3He data [5] used to con¢rm the Dunai [1] scaling may underestimate production rates.

Using Cosmic-Ray Neutron Probes to Monitor Landscape Scale Soil Water Content in Mixed Land Use Agricultural Systems

With an ever-increasing demand for natural resources and the societal need to understand and predict natural disasters, soil water content (SWC) observations remain a critical variable to monitor in order to optimally allocate resources, establish early warning systems, and improve weather forecasts. However, routine agricultural production practices of soil cultivation, planting, and harvest make the operation and maintenance of direct contact point sensors for long-term monitoring challenging. In this work, we explore the use of the newly established Cosmic-Ray Neutron Probe (CRNP) and method to monitor landscape average SWC in a mixed agricultural land use system in northeast Austria. The calibrated CRNP landscape SWC values compare well against an independent in situ SWC probe network (MAE = 0.0286 m3/m3) given the challenge of continuous in situ monitoring from probes across a heterogeneous agricultural landscape. The ability of the CRNP to provide real-time and accurate landscape SWC measurements makes it an ideal method for establishing long-term monitoring sites in agricultural ecosystems to aid in agricultural water and nutrient management decisions at the small tract of land scale as well as aiding in management decisions at larger scales.

Modeling cosmic ray neutron field measurements

The cosmic-ray neutron method was developed for intermediate-scale soil moisture detection, but may potentially be used for other hydrological applications. The neutron signal of different hydrogen pools is poorly understood and separating them is difficult based on neutron measurements alone. Including neutron transport modeling may accommodate this shortcoming. However, measured and modeled neutrons are not directly comparable. Neither the scale nor energy ranges are equivalent, and the exact neutron energy sensitivity of the detectors is unknown. Here, a methodology to enable comparability of the measured and modeled neutrons is presented. The usual cosmic-ray soil moisture detector measures moderated neutrons by means of a proportional counter surrounded by plastic, making it sensitive to epithermal neutrons. However, that configuration allows for some thermal neutrons to be measured. The thermal contribution can be removed by surrounding the plastic with a layer of cadmium, which absorbs neutrons with energies below 0.5 eV. Likewise, cadmium-shielding of a bare detector allows for estimating the epithermal contribution. First, the cadmium difference method is used to determine the fraction of thermal and epithermal neutrons measured by the bare and plastic-shielded detectors, respectively. The cadmium difference method results in linear correction models for measurements by the two detectors, and has the greatest impact on the neutron intensity measured by the moderated detector at the ground surface. Next, conversion factors are obtained relating measured and modeled neutron intensities. Finally, the methodology is tested by modeling the neutron profiles at an agricultural field site and satisfactory agreement to measurements is found. This article is protected by copyright. All rights reserved.

Scientist water equivalent measured with cosmic rays at 2006 AGU Fall Meeting

Cosmic rays continually bombard Earth, generating fast neutrons in the ground and atmosphere as a by-product of nuclear disintegrations. The presence of this easily measurable “background” radiation suggests the possibility of a novel and harmless method for measuring water content in soil and the water equivalent depth of snow, two critical variables controlling surface processes. A method utilizing cosmic rays has several advantages over soil and snow monitoring tools now available to hydrologists (e.g., microwave remote sensing, invasive electromagnetic methods, snow tubes, and snow pillows). It is passive, requires only moderately expensive equipment, of the order of U.S.

Arctic Climate Systems Analysis

10,000, and can be insensitive to soil salinity depending on the neutron energies selected. Most important, a cosmic ray water sensor is noninvasive and can measure water content or snow water equivalent over a scale desirable for input to hydrologic models (100 meter radius) but unattainable by other instruments. While the ability to measure soil water and snowpack in the field with cosmic rays has been validated, this method has applications to other water-rich systems, including biological ones.

New Capabilities for Arctic Research Through the Use of Unmanned Aircraft

This study began with a challenge from program area managers at Sandia National Laboratories to technical staff in the energy, climate, and infrastructure security areas: apply a systems-level perspective to existing science and technology program areas in order to determine technology gaps, identify new technical capabilities at Sandia that could be applied to these areas, and identify opportunities for innovation. The Arctic was selected as one of these areas for systems level analyses, and this report documents the results. In this study, an emphasis was placed on the arctic atmosphere since Sandia has been active in atmospheric research in the Arctic since 1997. This study begins with a discussion of the challenges and benefits of analyzing the Arctic as a system. It goes on to discuss current and future needs of the defense, scientific, energy, and intelligence communities for more comprehensive data products related to the Arctic; assess the current state of atmospheric measurement resources available for the Arctic; and explain how the capabilities at Sandia National Laboratories can be used to address the identified technological, data, and modeling needs of the defense, scientific, energy, and intelligence communities for Arctic support.