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NEUTRON PROBE STANDARDS: TRANSPORT SHIELDS OR A LARGE DRUM OF WATER?

The use of standards for the normalization of neutron probe count rates is reviewed. There appear to be two main conventions: the use of an “infinite”, common standard (a large drum of distilled water) and the use of the convenient, H-rich shielding built into the housing of most probes. Counts made in infinite standards are not influenced by surrounding material, are less affected by temperature, and are thus less prone to error. Either type of standard can be used to normalize the count rates of a given probe to correct for instrumental drift. The two conventions appear to have given rise to different approaches to calibration: 1) If probes' own shields are used as standards, each probe (even those of the same type) has to have its own calibration because the shields are not identical, although an intercalibration procedure may be used to transfer the calibration of one probe to another; 2) If a large common (water) standard is used as a routine, the count rates of different probes of the same type can be normalized on the same basis, allowing the possibility of using the same soil calibration. The latter approach was examined using several neutron probes from the same manufacturer. Count rates in the soil and in the water standard were measured to very high precision and the resulting count ratios were compared. The most extreme difference between the ratios was equivalent to a volumetric water content difference of 0.003. This was comparable with the differences reported by other authors between water content values given by different, individually calibrated probes. These results strongly indicate that, following the normalization of count ratios using a common water standard, the same soil calibration can be used for probes of the same design. This can save a large amount of time which would otherwise be spent deriving individual calibrations. The results obtained were markedly better than those obtained through the intercalibration of probes using plastic transfer standards. The benefits of using a large water standard outweigh any disadvantages such as lack of portability.