Development of a neutron probe for soil humidity measurements using 6 LiI(Eu) detector

A prototype of soil moisture probe was build using a crystal of 6LiI(Eu) as a thermal neutron detector. Light pulses are produced by the exoergic nuclear reaction 6Li (n,α)3He and transmitted through the light guide to a photomultiplier tube on the soil surface. Liquid light guides have several advantages when compared with bundle of glass fibers. First, liquid guides do not suffer from packing fraction losses spaces between fibers that cause reduced coupling efficiency. Second, repeated handling of liquid light guides does not result in the breakage typical of glass bundles, which reduces efficiency over time. Third, liquid guides have excellent UV transmission properties with a cut off the near infrared spectrum yielding an optimum transmission for visible applications. The major advantage of this prototype is the elimination of the electromagnetic interference inside of the soil. Tests were carried out aiming the improvement of electronic and technical viability aspects of neutrons probes. The soil moisture probe calibration curve was carried out in a drum of 60 cm diameter and 42 cm height. This drum was completely filled with an air dry soil. Counts in the center of the drum with the dry and saturated soils make possible to obtain the curves of the soil water content versus the normalized counts for two thermal neutron detectors. The medium value of the counts, the standard deviation and the number of counts were obtained for 6LiI(Eu) and 3He detectors, respectively for water, air dry, and saturated soil. From those measurements, a linear calibration curve was obtained for each of detectors. (author)