Snow shielding factors for cosmogenic nuclide dating inferred from Monte Carlo neutron transport simulations

Abstract Conventional formulations of changes in cosmogenic nuclide production rates with snow cover are based on a mass-shielding approach, which neglects the role of neutron moderation by hydrogen. This approach can produce erroneous correction factors and add to the uncertainty of the calculated cosmogenic exposure ages. We use a Monte Carlo particle transport model to simulate fluxes of secondary cosmic-ray neutrons near the surface of the Earth and vary surface snow depth to show changes in neutron fluxes above rock or soil surface. To correspond with shielding factors for spallation and low-energy neutron capture, neutron fluxes are partitioned into high-energy, epithermal and thermal components. The results suggest that high-energy neutrons are attenuated by snow cover at a significantly higher rate (shorter attenuation length) than indicated by the commonly-used mass-shielding formulation. As thermal and epithermal neutrons derive from the moderation of high-energy neutrons, the presence of a strong moderator such as hydrogen in snow increases the thermal neutron flux both within the snow layer and above it. This means that low-energy production rates are affected by snow cover in a manner inconsistent with the mass-shielding approach and those formulations cannot be used to compute snow correction factors for nuclides produced by thermal neutrons. Additionally, as above-ground low-energy neutron fluxes vary with snow cover as a result of reduced diffusion from the ground, low-energy neutron fluxes are affected by snow even if the snow is at some distance from the site where measurements are made.

[1]  Samuel Glasstone,et al.  Principles of nuclear reactor engineering , 1956 .

[2]  Darin Desilets,et al.  Spatial and temporal distribution of secondary cosmic-ray nucleon intensities and applications to in situ cosmogenic dating , 2003 .

[3]  S. Roesler,et al.  Calculation of radiation fields in the atmosphere and comparison to experimental data. , 1998, Radiation research.

[4]  Raymond Davis,et al.  CHLORINE‐36 IN NATURE , 1955 .

[5]  J. Staiger,et al.  Atmospheric scaling of cosmogenic nuclide production : Climate effect , 2007 .

[6]  J. Middleton,et al.  Pothole and channel system formation in the McMurdo Dry Valleys of Antarctica: New insights from cosmogenic nuclides , 2012 .

[7]  R. Scott,et al.  Measuring soil moisture content non‐invasively at intermediate spatial scale using cosmic‐ray neutrons , 2008 .

[8]  R. Purves,et al.  Simulation of snow shielding corrections for cosmogenic nuclide surface exposure studies , 2005 .

[9]  R. Finkel,et al.  Calibration of cosmogenic 36Cl production rates from Ca and K spallation in lava flows from Mt. Etna (38°N, Italy) and Payun Matru (36°S, Argentina) , 2011 .

[10]  H. Craig,et al.  Geomorphology and In-Situ Cosmogenic Isotopes , 1994 .

[11]  Raymond L. Murray,et al.  The Elements of Nuclear Reactor Theory , 1953 .

[12]  M. Shea,et al.  Scaling time-integrated in situ cosmogenic nuclide production rates using a continuous geomagnetic model , 2008 .

[13]  D. Lal,et al.  Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models , 1991 .

[14]  R. Reedy,et al.  Production rate systematics of in-situ-produced cosmogenic nuclides in terrestrial rocks: Monte Carlo approach of investigating 35Cl (n, γ) 36Cl , 1994 .

[15]  M. Kurz.  In situ production of terrestrial cosmogenic helium and some applications to geochronology , 1986 .

[16]  Wilmot N. Hess,et al.  COSMIC-RAY NEUTRON ENERGY SPECTRUM , 1959 .

[17]  P. Kubik,et al.  The probable importance of snow and sediment shielding on cosmogenic ages of north-central Colorado Pinedale and pre-Pinedale moraines , 2004 .

[18]  F. Phillips,et al.  Terrestrial in situ cosmogenic nuclides: theory and application , 2001 .

[19]  F. Phillips,et al.  An improved approach to calculating low-energy cosmic-ray neutron fluxes near the land/atmosphere interface , 2001 .

[20]  Nathaniel A. Lifton,et al.  Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications [rapid communication] , 2005 .

[21]  W. J. Shuttleworth,et al.  COSMOS: the COsmic-ray Soil Moisture Observing System , 2012 .

[22]  D. Elmore,et al.  Accelerator mass spectrometry in geologic research , 2003 .

[23]  R. Wieler,et al.  Correction of in situ cosmogenic nuclide production rates for geomagnetic field intensity variations during the past 800,000 years , 2001 .

[24]  R. Wieler,et al.  Production rates of cosmogenic nuclides in boulders , 2003 .

[25]  T. Ferré,et al.  Nature's neutron probe: Land surface hydrology at an elusive scale with cosmic rays , 2010 .

[26]  Jeffrey A. Dunne,et al.  Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces , 1999 .

[27]  C. Grimani,et al.  Galactic cosmic-ray energy spectra and expected solar events at the time of future space missions , 2011 .

[28]  R. Middleton,et al.  Precise cosmogenic 10Be measurements in western North America: Support for a global Younger Dryas cooling event , 1995 .

[29]  Kenneth M. Anderson,et al.  The theoretical basis of ACE, an Age Calculation Engine for cosmogenic nuclides , 2012 .

[30]  T. Dunai.  Influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides , 2001 .

[31]  D. Sugden,et al.  Cosmogenic 10Be and 26Al exposure ages of tors and erratics, Cairngorm Mountains, Scotland : Timescales for the development of a classic landscape of selective linear glacial erosion , 2006 .

[32]  F. Phillips,et al.  Cosmogenic 36Cl accumulation in unstable landforms: 1. Effects of the thermal neutron distribution , 1994 .

[33]  Darin Desilets,et al.  Elevation dependence of cosmogenic 36Cl production in Hawaiian lava flows , 2006 .

[34]  D. Sugden,et al.  Cosmogenic nuclides 10Be and 26Al imply limited Antarctic Ice Sheet thickening and low erosion in the Shackleton Range for >1 m.y. , 2004 .

[35]  R. Reedy,et al.  Terrestrial cosmogenic-nuclide production systematics calculated from numerical simulations , 1995 .