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Proceedings in Radiochemistry

A Supplement to Radiochimica Acta

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Comparison of deep underground neutron flux measured with a helium-3 filled proportional counter and evaluated from element composition or isotopic ratio of 36Cl/Cl in granite rock

T. Kubota / T. Ohta / Y. Mahara / A. Kudo
Published Online: 2011-09-08 | DOI: https://doi.org/10.1524/rcpr.2011.0040


The chlorine isotopic ratio and elemental composition of two granite cores collected in a deep underground tunnel has been measured; neutrons were detected using a helium-3-filled proportional counter placed in a borehole. The thermal neutron flux determined from the chlorine isotopic ratio from accelerator mass spectrometry is (3.8 ± 0.2) × 10−5 and that determined from the granite's elemental composition is (4.4 ± 0.3) × 10−5(cm−2 s−1). The results of in-situ measurements using the proportional counter yielded a thermal neutron flux value of (3.3 ± 0.1) × 10−5(cm−2 s−1). This good agreement among the results shows the validity of the neutron flux evaluation obtained from the elemental composition and/or the isotopic ratio of 36Cl/Cl. Further, it reveals that the neutron flux in the underground environment remains stable for at least one million years, i.e., the period over which chlorine-36 attains equilibrium.

Keywords: Neutron flux; Underground; (Alpha, n) reaction; Spontaneous fission; AMS

About the article

Published Online: 2011-09-08

Published in Print: 2011-09-01

Citation Information: Proceedings in Radiochemistry A Supplement to Radiochimica Acta, Volume 1, Issue 1, Pages 227–232, ISSN (Print) xxxx-xxxx, DOI: https://doi.org/10.1524/rcpr.2011.0040.

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