An introduction to radioactivity measurements

Problems

Many comments in preceding chapters made it clear that accurate radio-activity measurements require specialised instruments and attention to numerous details, in particular an adequate knowledge of the decay data of the radionuclides of interest.

Highly accurate radioactivity measurements are rarely of interest outside standard laboratories, but two facts require attention: (a) all work with radioactivity relies ultimately on internationally established activity standards (Section 2.2.1), and (b) laboratories working with radioactivity must have facilities for at least moderately accurate radioactivity measurements since radiation protection authorities will not permit radioactive materials to be used unless their activity is known with sufficient accuracy, often within ±10%.

Since most nuclear decays are signalled by the emission of either an α or a β particle, the measurement of these decay rates could be expected to be straightforward: one counts the emitted particles for a known time and states the result as decays per second or becquerel (see Eq. (2.1)).

To proceed in this way could cause serious errors. For example, there are the β particle decays via excited states de-excited by conversion electrons (Section 3.6.2) which the detector treats like β particles, adding them to their number. Furthermore, to account for all emitted particles, counting must be done in exactly 4π or another accurately known geometry requiring appropriate detector arrangements and the source must be thin enough to permit all particles emitted from within its atoms to escape to be counted. In addition, there are uncertainties due to imperfections of the signal processing equipment (Section 4.5), the possible presence of unwanted radiations (Section 4.6) and ambiguities in interpretation of results (yet to be discused).