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Low specific activity β-emitters and low energy β-emitters in the gas phase may be measured by gas counting in a low background system. High specific activity β-emitters can often be standardized in a 4 π β-counter which itself is part of a low background counting system with anticoincidence shielding. Low level samples of β- and β– γ emitters may be of high or low specific activity. Quantitative separation of the nuclides is frequently uncertain however. The ratio of the two components can be determined from repeated measurement during the decay of the parent or by subsequent measurement on a chemically separated sample of the daughter nuclide. A standardization method applicable to both nuclides is usually chosen and the total activity of the mixture is determined. Parent–daughter mixtures are the most common example of this type of problem (e.g.: Zr 95–Nb 95 Ca 47–Sc 47 ). Equally difficult are electron capture nuclides with several partially converted γ-rays in coincidence. The γ - and X-ray efficiencies are low when the counter gas pressure is low.
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One method of measuring this nuclide uses a 4 π G–M β-counter with the source sandwiched between foils to absorb the Auger electrons.
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For example, Cu 64 emits negatrons, positrons, X-rays and Auger electrons from the electron capture process and a small percentage of γ-rays following electron capture. Several efficiencies need to be estimated if the nuclide has more than one mode of disintegration. The situation is further complicated if the β-energies are low and the disintegration scheme includes some coincident γ-rays, for example: Ru 103 and Ru 105. The detection efficiency for each radiation must be determined and the percentage of disintegrations feeding the isomeric level must be well known. Nuclides whose decay scheme includes an isomeric transition are difficult to standardize because the delayed γ-rays, internal conversion electrons and characteristic X-rays may be recorded. Allen, in Alpha-, Beta- and Gamma-Ray Spectroscopy, 1968 Delayed states