TY - JOUR
T1 - Spatial and temporal distribution of secondary cosmic-ray nucleon intensities and applications to in situ cosmogenic dating
AU - Desilets, Darin
AU - Zreda, Marek
N1 - Funding Information:
We thank D. Lal and J. Masarik for their helpful comments on the manuscript. We also acknowledge the invaluable assistance provided by V. Radhakrishnan and his staff at Raman Research Insitute for our neutron measurements in India. This material is based upon work supported by the National Science Foundation under Grants EAR-0001191, EAR-0126209 and ATM-0081403 and by Packard Fellowship in Science and Engineering 95-1832. [RV]
PY - 2003/1/30
Y1 - 2003/1/30
N2 - 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-2 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-2 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.
AB - 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-2 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-2 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.
KW - Cosmic rays
KW - Cosmogenic
KW - Neutron monitor
KW - Production rates
KW - Scaling
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U2 - 10.1016/S0012-821X(02)01088-9
DO - 10.1016/S0012-821X(02)01088-9
M3 - Article
AN - SCOPUS:0037472682
VL - 206
SP - 21
EP - 42
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
IS - 1-2
ER -