APPLICATION OF ATMOSPHERIC NEUTRONS TO SOIL MOISTURE MEASUREMENT


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Abstract

This paper describes the possibility of continuous remote sensing of the moisture content of soil using atmospheric neutrons produced by cosmic radiations near the ground surface. Using polyethylene-moderated BF3 neutron counters at several different underground depths, we measured time variations of the neutron fluxes, to examine their responses to soil moisture changes quantitatively. From the close correlations between neutron fluxes and soil moisture contents, we show that the fluxes of the underground neutrons with energies from the cadmium threshold of 0.025 eV to about 106 eV, measured at a depth of 20 cm, are affected most sensitively by the moisture content of the soil near the same depth. Their fractional change is represented by a regression coefficient of 1% per unit percent of soil moisture change for a range from 33% (∼2.8 pF) to 52% (∼1.9 pF) at the 20-cm depth. This neutron technique promises to be a simple and reliable measurement that depends on the counting statistics of neutrons.

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