Radioisotopes decay counting

Beta particle absorption Compound radioisotopic decay Radioisotopic decay Introduction to Geiger detectors Introduction to counting statistics... 

Experiments performed to date with cyclotrons have used positive ions obtained from carbon dioxide and a gas ion source. This is an advantage in the sense that it permits standard pretreatment practices developed over the last 30 years at decay counting laboratories to be routinely employed up to the point of measurement (29). On the other hand, beam currents using gas ion sources are characteristically significantly less than those from the solid samples currently used with electrostatic type accelerators. In addition, memory effects, which make comparisons of a standard to an unknown difficult, have been reported. The first cyclotrons used for radioisotope measurements had previously been used extensively for nuclear physics experiments and the production of high energy ions. Because of these experiments, some cyclotron systems have apparently been contaminated. For the 88-in. Berkeley cyclotron, the construction of an external ion source was designed to attempt to overcome this problem. Unfortunately, the efficiency of the beam transport system in the external ion source introduced other problems (30). 

The section Radioactive Methods in volume 9 of the Treatise on Analytical Chemistry (Kolthoff and Elving 1971) discusses radioactive decay, radiation detection, tracer techniques, and activation analysis. It has a brief but informative chapter on radiochemical separations. A more recent text. Nuclear and Radiochemistry Fundamentals and Applications (Lieser 2001), discusses radioelements, decay, counting instruments, nuclear reactions, radioisotope production, and activation analysis in detail. It includes a brief chapter on the chemistry of radionuclides and a few pages on the properties of the actinides and transactinides. 

Accelerator mass spectrometry (AMS) is an ultrasensitive analytical method for radioactivity analysis. AMS offers 10 -10 -fold increases in sensitivity over LSC or other decay counting methods so that levels as low as 0.0001 DPM can be detected (Brown et al., 2005, 2006). AMS has been applied to mass balance determination, pharmacokinetic studies of total radioactivity, and measurement of chemically modified DNA and proteins in humans after the administration of a low radioisotope dose (approximately lOnCi/person for mass balance and drug metabolism studies) (Buchholz et al., 1999 Garner, 2000 Garner et al., 2002 Liberman et al., 2004 White and Brown, 2004). In addition, off-line HPLC-AMS has been explored for metabolite profiling after... 

A typical procedure for Na assay is One gram proplnt samples sealed in polyethylene vials are irradiated for at least seven hrs in the thermal column. In order to minimize any thermal neutron flux gradient, the samples are rotated uniaxially at 60 rpm. Following irradiation, the samples are allowed to decay for approx 15 hrs to permit all short-lived radioisotopes to decay to insignificance. The 1.369 MeV 7-photopeak of each sample is then counted for 10 min with a 7.6cm x 7.6cm Na iodide scintillation detector coupled to a 400-channel pulse-height analyzer... 

The decay of an individual atom (a disintegration ) occurs at random, but that of a population of atoms occurs in a predictable manner. The radioactivity decays exponentially, having a characteristic half-life (/1/2)- This is the time taken for the radioactivity to fall from a given value to half that value (Fig. 35.1). The values of different radioisotopes range from fractions of a second to more than 1019 years (see also Table 35.2). If t /2 is very short, as with I50 (q/2 2 min), then it is generally impractical to use the isotope in experiments because you would need to account for the decay during the experiment and counting period. 

The results of radiation measurements are, in most cases, expressed as the number of counts recorded in a scaler. These counts indicate that particles have interacted with a detector and produced a pulse that has been recorded. The particles, in turn, have been produced either by the decay of a radioisotope or as a result of a nuclear reaction. In either case, the emission of the particle is statistical in nature and follows the Poisson distribution. However, as indicated in Sec. 2.9, if the average of the number of counts involved is more than about 20, the Poisson approaches the Gaussian distribution. For this reason, the... 

A = decay constant of the radioisotope produced After irradiation, the activity A t) is counted and the flux is determined from Eq. 16.34. Depending on the foil used (reaction involved), information about the neutron energy spectrum may also be obtained. Information about the neutron spectrum < (E) is necessary for the determination of the neutron dose equivalent H. 

Elementary Practical Radiochemistry (Ladd and Lee 1964) contains 20 brief experiments that illustrate detection techniques such as measurement of ingrowth and decay, as well as ion exchange, extraction, and coprecipitation. The text Radioisotope Laboratory Techniques (Faires and Boswell 1981) primarily addresses nuclear physics, radionuclide production, and counting techniques. It briefly mentions laboratory apparatus but omits discussion of separation techniques. 

Fe beta particles are counted with a proportional detector or its gamma rays are analyzed with a Ge detector and spectrometer. The sample is then measured for Fe content with a thin Ge detector and spectrometer or xenon-filled X-ray proportional detector with a thin (e.g., 140 mg cm ) beryllium absorber. The Fe count rate is adjusted for background, the Fe contribution, self-absorption in the plated sample, and the chemical yield, and converted to the disintegration rate. The activity of both radioisotopes is corrected for radioactive decay from the sampling date. 

SECTIONS 21.4 AND 21.5 The SI unit for the activity of a radioactive source is the becquerel (Bq), defined as one nuclear disintegration per second. A related unit, the curie (Ci), corresponds to 3.7 X 10 disintegrations per second. Nuclear decay is a first-order process. The decay rate (activity) is therefore proportional to the number of radioactive nuclei. The half-life of a radionuclide, which is a constant, is the time needed for one-half of the nuclei to decay. Some radioisotopes can be used to date objects C, for example, is used to date organic objects. Geiger counters and scintillation counters count the emissions from radioactive samples. The ease of detection of radioisotopes also permits their use as radiolracers to follow elements through reactions. 

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