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Source-Detector Geometry Effect on Neutron Probe Calibration¹

ABSTRACT

A laboratory experiment was conducted to develop a method for the nondestructive determination of the midpoint of the anode wire and the center of the sensitive volume of a boron trifluoride neutron detector tube. These points were found to coincide and would be expected to do so for all such detector tubes of symmetrical construction. The measurements on the detector tube were determined from X-ray radiographs by triangulation. This center point coincides with the midpoint of the portion of the detector between the bottom and seam near the top.

Using the center of the sensitive volume as a reference point, the effect of position of the neutron source on the count rate in hydrogenous media was studied. Four hydrogenous media were used: urea, aluminum sulphate, water, and the paraffin shield for the probe (Nuclear-Chicago, model P-19). Materials were contained in 210-liter drums. Second degree curves fit the data of count rate vs. distance between source and reference point. The center of the sensitive volume coincided with the point of greatest count rate. The center was also the position for greatest sensitivity to water content. Results indicate that changes of source position as small as 0.5 cm from the position at original calibration will significantly change the calibration curve. Users can detect such changes by monitoring ratios of readings in two different hydrogenous media in the laboratory. Any such change in source-detector geometry will produce a change in ratio. Positional changes may be the result of either a source movement or replacement of detector tube with one of different dimensions.

¹ Contribution from the Department of Agronomy, Oklahoma Agr. Exp. Sta. as a part of project H-1466. Journal Manuscript no. 2311.

² Graduate Research Assistant and Professor, respectively, Department of Agronomy, Oklahoma State University, Stillwater, OK 74074.

Received for publication July 27, 1971. Accepted for publication October 8, 1971.