Interpretation of Counting Data

Abstract

IS an account cf Goseromei't sponsored Rork ^either the I . nor cmv persoa acttsg on behalf of tho Commisston INTRODUCTIONThe general availability of radioisotopes has placed a powerful tool in the hands of investigators in many fields of science, and it is the purpose of this report to aid these workers in the design of meaningful experiments utilizing radioactivity. An understanding of the various factors which may contribute to errors in measurement is essential for the choice of appropriate techniques and measuring devices and for the proper evaluation of experimental data. This report is not intended as a treatise on the measurement of radioactivity, but it is hoped that the presentation of a handy guide to some of the problems involved and the techniques generally employed in avoiding or correcting for them, together with references to the pertinent literature, will provide an adequate orientation and introduction to the subject. GENERAL CONSIDERATIONSAlthough the discussion here will be limited to counting methods of radioactive assay, which are of more general interest, a variety of techniques are available, e.g., photographic, calorimetric, charge collection, colloid deposition, crystal coloring, etc. Different techniques may, of course, involve somewhat different error analyses; however, the problems associated with the determination of the absolute disintegration rate of a radioactive source by counting techniques afford an excellent example of the commonly encountered difficulties. Such a determination involves a knowledge of the decay scheme of the rachoactive nuclide and the efficiency of the detector for the particles or rays emitted by the source. It is thus necessary to know the fraction of the radiation leaving the source which enters the sensitive volume of the detector ("geometry factor") and the intrinsic efficiency of the detector for the radiation. The interactions of radiation with matter alter the simple geometrical relationship between the source and detector by processes of absorption and scattering in the material of the sample itself, its support, any covering material, the space between the sample and the counter, the counter window, and the housing of the detector unit. These effects are complex and depend critically on the size and m.aterial of the sample and its support and on the energy and type of the radiation. Special techniques are required to minimize these effects for the determination of disintegration rates to a few percent accuracy.For many purposes an experiment can be designed so that relative counting rates of samples of a particular nuclide will suffice. It is then only necessary to coxmt under nearly identical conditions to obtain precisions of the order of one to two percent. This criterion, in practice, is relatively simple to meet and, whenever possible, comiparative cotmting is therefore recommended. Of course, if it is necessary to compare the activity of sources which differ in chemical or physical nature or in the energy of their radiations, these differe...

Simple Absolute Measurement Technique for Beta Radioactivity. Application to Naturally Radioactive Rubidium

“…Bothe (2) and Danziger (5) in theoretical studies of beta absorption deduced the main features of the whole phenomenology and their curve for versus Z agrees well with the experimental data of Seliger (24), Glendenin and Solomon (9), and Steinberg (26).…”

“…It is generally agreed--cf. Steinberg (26)-that the back-scattered radiation is softer than the original. For materials of Z belowT 15, the factor by which the absorption coefficient of the back-scattered radiation measured under 2t conditions is increased is about 2.…”