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Calibration and Characteristics of Two Neutron Moisture Probes¹

ABSTRACT

The effect of the size of cylindrical containers filled with soil on the calibration of two types of neutron probes was investigated for moisture contents up to 0.25 volume fraction. Neutron count rates were appreciably affected by the value of both diameter and height up to 3 feet. Differences between 3 and 4 feet were samll.

The response of the end-source probe was linear within the range investigated (0 to 0.40 volume fraction). The data also show that the end-source probe design as compared with the centered-source design is more efficient and, therefore, requires a smaller radioactive source for equal precision.

Once an accurate calibration with soil is available, verifying calibration of probes may be done with NaCl or H₃BO₃ solutions, limited to probes of identical design but differing efficiencies or source strengths. Relations found between soil moisture content and solution concentration are given.

A discussion of the range of measurement of soil moisture content with neutron probes is presented, comparing various literature data.

¹ Joint contribution from Soil and Water Conservation Research Division, ARS, USDA and Department of Irrigation, University of California, Davis.

² Chief Soil Scientist, U. S. Water Conservation Laboratory, Southwest Branch, SWCRD, ARS, USDA, Tempe, Ariz., Assistant Professor of Irrigation, Dept. of Irrigation, University of California, Davis, and Laboratory Technician II, Dept. of Irrigation, University of California.

Received for publication November 22, 1960. Accepted for publication March 13, 1961.