CALIBRATION OF NEUTRON MOISTURE PROBES BY TRANSFER THROUGH LABORATORY MEDIA

Abstract

Neutron probes require calibration and subsequent verification of calibration. A decade of calibration analysis data based on calibration transfer standards were examined for variance and suitability of method. Thirteen probes were read periodically, over a decade, in drums of hydrogenous media (water, alum, and urea) and the individual probe shields. Supplemental studies examined effects of temperature, minor change of position of probe in shield, and stability of the hydrogenous media over time. Nuclear counting variability can be made a nonlimiting portion of total counting variability in laboratory calibration checking by taking at least 100,000 counts per sample. Changes in probe count rate not attributable to nuclear decay, changes in positioning of probe in shield, and temperature resulted in errors of less than 0.003 volumetric water fraction. Count rate changes with identifiable source, such as changing a detector tube, produced 17 errors that necessitated recalibration during this period of time. The median error in these count rate changes was 0.0052 volumetric water fraction. Temperature of three probes studied was found to change count rates by an average of -1.15 counts per minute per °C. Changes as small as 1 mm in the position of a probe in its shield were detectable in the shield count rate. In a program where physical integrity was checked frequently, these changes were evidently minimized and did not produce known calibration changes, even though several instances of probes not being centered in the shield were noted. In the calibration drum containing prilled urea, the count rate was found to slowly increase over the decade, evidently because of slow compaction of the media. The change in count rate was found to be significant only after 10 years of drift. The transfer method of calibration and periodic checking appeared satisfactory over the course of a decade. With careful probe handling and frequent inspection for physical integrity, inadvertent changes in calibration attributable to temperature probe positioning in shield, and stability of media in calibration were negligible. Changes in calibration in some probes were noted and corrected by recalibration. Periodic checking appears essential in maintaining the integrity of calibration of neutron probes. Drums of hydrogenous media were satisfactory for this purpose and were easy to use.