A calorimeter for the nondestructive assay of tritium-contaminated samples

Science Research Laboratory, Inc. has developed a cryogenic method for accurately assaying tritium within solid materials such as metals. The apparatus has been thermally tested and delivered to the National Institute of Standards and Technology, Gaithersburg, MD, where it will be further refined and calibrated. The current design has a demonstrated thermal sensitivity of 5 μW, which corresponds to a tritium activity of 150 mCi (6 GBq). Minor modifications should improve the sensitivity by a factor of ten (10). The absolute accuracy is expected to be 0.5%. The apparatus is based on a mechanical refrigeration system that offers a base temperature of 7.5 kelvin. Existing non-calorimetric tritium assay methods are sensitive to surface contamination or require that the tritium be removed from the sample. In comparison, calorimetry is sensitive to contamination throughout the sample and requires little sample preparation so long as the sample can be cooled to cryogenic temperatures. In addition to the assay of tritium, the method may be applicable to other radioisotopes including ³³P and ¹²⁵ I