Counting techniques radioactive errors

One common characteristic of many advanced scientific techniques, as indicated in Table 2, is that they are applied at the measurement frontier, where the net signal (S) is comparable to the residual background or blank (B) effect. The problem is compounded because (a) one or a few measurements are generally relied upon to estimate the blank—especially when samples are costly or difficult to obtain, and (b) the uncertainty associated with the observed blank is assumed normal and random and calculated either from counting statistics or replication with just a few degrees of freedom. (The disastrous consequences which may follow such naive faith in the stability of the blank are nowhere better illustrated than in trace chemical analysis, where S B is often the rule [10].) For radioactivity (or mass spectrometric) counting techniques it can be shown that the smallest detectable non-Poisson random error component is approximately 6, where ... 

Tracer techniques comprise all methods in which microamounts (traces) of radionuclides or labelled compounds are added to a system, in order to pursue (trace) the fate, transport or chemical reaction of a certain element or compound in that system. Radioactive tracers are preferabley used, because they can be detected and measured in very low concentrations and with high sensitivity, as is evident from Table 13.1. With a measuring period of 10 min and an overall counting efficiency of 20%, lOBq can be determined with a statistical error of about 3%. 

Like any other experimental method, determination of blood flow by radioactive microspheres is subject to many sources of error, including individual variation among the sample population, the counting accuracy of the total microspheres in the tissue sample by the gamma counter, and the effect of arteriovenous shunting or the migration of the microspheres. Despite all the limitations of this method, the microsphere technique of blood flow determination has become the most powerful method available today and has been used to evaluate the accuracy of other techniques of blood flow measurement. 

The use of stable isotopes can provide richmetabolic information that differs from that of radioisotopes. For example, quantihcation of radioisotopes is in terms of energy emitted from the loss of an electron from the nucleus (i.e. isotopic decay), where the energy emitted is directly proportional to the number of radioactive atoms ( C, H, S) present per sample or molecule weight (i.e. specific activity). Often, specific activity of a whole molecule (e.g. glucose, fatty acids) is determined, and this has three particular disadvantages. First, the concentration of the molecule must be determined and the compormd must be isolated for radioactive counting. Both analytical techniques can introduce error in measurement. Second, with or H tracers, total molecule specific activity is usually measured. 

---