Low level counting

Many nuclear processes occui one after the other within a very short time of the order of picoseconds or less - for instance a. or fi decay followed by y-ray emission or emission of a cascade of y rays. The events are practically coincident, and for many purposes it is of interest to know whether two particles or photons are emitted practically at the same time or not. For detection and measurement of coincident events two detectors and a coincidence circuit are used. The detectors are chosen according to the coincidences to be measured, e.g. ot-y, fi y, y-y, X-y, y5-e or other types of coincidences, and the coincidence circuit records only events occurring within a given short time interval. Scintillation counters and semiconductor detectors are commonly used for these measurements. 

On the other hand, by application of an anticoincidence circuit only those events are recorded that are not in coincidence with others. 

Coincidence studies are very useful for detailed investigation of decay schemes. For that purpose, in both detectors the pulse heights are determined simultaneously, giving the energies of the coincident particles or photons, respectively. 

An application of anticoincidence circuits is the anti-Compton spectrometer. The Compton continuum in y spectra can be reduced relative to the photopeaks by placing the Ge detector inside a second detector, usually a scintillation detector, connected in anticoincidence, so that only pulses in the central detector that are not coincident with those in the outer detector are recorded. Anti-Compton spectrometers are very useful for measurement of y rays of very high energy. 

If samples of very low activity are to be measured, the contribution of the background to the counting rate and hence the error of the measurement are relatively high. The influence of the background can be reduced by intensiflcation of the detector shielding and by coincidence or anticoincidence circuits. 

---

Rosholt NJ (1967) Open system model for uranium-series dating of Pleistocene samples. In Radioactive Dating and Methods of low-level Counting. 1. A. E. A. Proc Ser Publ, SM-87/50, p 299-311 Rosholt JN, Antal PS (1962) Evaluation of the Pa /U-Th °/U method for dating Pleistocene carbonate rocks. US Geol Survey Prof Paper 450-E 108-lll... 

Figure 3. Concept for the ultralow-level counter. Low-level counting facilities could be improved by using pulsed lasers to ionize daughter atoms selectively (when neutral) by using a RIS scheme. Shown here is the electronics logic that could be used to further reduce background for more details, see Ref. 8.

Principal characteristics of small sample liquid scintillation counting (lsc), gas proportional low-level counting (11c) and atom counting by accelerator mass spectrometry (AMS) are summarized in Table 1, and systems we have used are shown in figure 1. The most important differences (apart from cost and availability)... 

Olsson, I., Stenberg, A., Radioactive Dating and Methods of Low Level Counting, IAEA Symposium, Monaco, March 2-10, 1967, 69-78. 

Haynes, C. V., Bone Organic Matter and Radiocarbon Dating, In Radioactive Dating and Methods of Low-Level Counting, pp. 163-168, Vienna, International Atomic Energy Agency, 1967. 

Regarding microbiological results, for pure steam and water for injection, it is expected that they be essentially sterile. Because sampling frequently is performed in nonsterile areas and is not tmly aseptic, occasional low-level counts due to sampling errors may occur. Agency policy is that less than 5 CFUs/100 ml is an acceptable action limit. 

E.J. Zeller, P.W. Levy and P.L. Mattern, Proc. Symp. Radioactive Dating and Method of Low Level Counting , IAEA, 1967, 53. 

Bien, G., Suess, H., Radioactive Dating Methods Low Level Counting,... 

A new detergent, Triton-101, in association with p-xylene is used for the suspension of 10 ml. of water with a Y value of approximately 0.5 nCi/liter. The application of a new instrumental technique with three photomultipliers decreases further the Y value of both mixtures. A Teflon cylinder with a volume of 250-300 ml. is used as sample container for low level counting with a Y value of approximately 0.2 nCi/liter. Selected results of samples collected during 1967 are reported, and the radiation dose to the population of the United States from tritium is estimated to be approximately 0.2 mrem./year. 

The selection of an appropriate technique for low level counting has been discussed by Moghissi et al. (9). If M ml. of water with a specific activity of Y nCi/liter are counted with an efficiency of E c.p.m./d.p.m. for t minutes, the number of counts obtained are as follows ... 

The first attempt to develop an efficient scintillation method for low level counting consisted of evaluating the dioxane-based solution and studying the many factors influencing the efficiency and the background (9). 

Figure 1. Large volume cell for low level counting of tritium by liquid...

One of the areas in which the skills of radiochemists are used is the area of low-level chemistry and low-level counting. Areas as diverse as the detection of solar neutrinos or the study of environmental radioactivity involve low-level techniques. For example, despite concentration of the radiotracers of interest during sampling procedures in environmental studies, quite often one is left with a sample containing... 

Once the low-level radioactive sample has been collected and any chemical procedures performed prior to counting, it is ready for counting. Because of the extremely small disintegration rates encountered, special techniques, called low-level counting, must be used to assay the sample. We shall survey some of the highlights of this area, which has been the subject of many articles and monographs (Knoll, 2000). 

Low-level counting of y-ray emitters using solid scintillation counters is an extensively used technique. The most important aspect of low-level solid scintillation counting is to decrease the counter background. Typical contributions to a solid scintillation counter s background rate from various sources are shown in Table 19.3. 

Figure 19.8 Various measures of detection limits for low-level counting. (From Currie, 1968.)...

For example, if the background under a photopeak of interest in a 7-ray spectrum was 100 cpm, then ctb = VTOO = 10 cpm. According to the recipes given above, one would need to detect a photopeak area of 23 cpm to say that a nuclide was present and would need to detect at least 141 cpm to measure the amount of nuclide present. One would need a count rate of 47 cpm to ensure, before making the measurement, that the nuclide in question could be detected. The relationship of these limits to other measures used to describe low-level counting is shown in Figure 19.8. 

The radiation sources are supported directly by three laboratories a health physics laboratory, a radiochemistry-low level counting laboratory, and a dosimetry laboratory. 

---