Criteria of applicability for autoradiography of tritium

Autoradiography is an effective tool for the imaging of radionuclide distributions in various samples. In sophisticated applications with special preparation and development of sample-emulsion combinations and subsequent grain counts it can be highly quantitative, but it requires carefully controlled conditions and a variety of counter-checks, for example through scintillation spectroscopy. Less refined applications use X-ray films as detectors, and their seeming simplicity tends to invite artefacts and misinterpretations. Particular care needs to be taken, if one deals, or presumes to deal, with the low-energy beta-emitter tritium. Because of the short electron ranges the film must be in intimate contact with the sample, which tends to produce chemographic artefacts; without added spectroscopic measurements it is impossible to discriminate the spurious signals from a blackening of the film due to tritium. Recent statements concerning autoradiographic tritium measurements in tree samples have created considerable public concern and have demonstrated the pitfalls of uncritical use. This paper presents order-of-magnitude criteria for the detection threshold in the autoradiography of tritium; they can serve as an exclusion principle for some of the more extravagant misinterpretations.