Absolute Measurements of the Activity of Beta Emitters

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Abstract
This is an account of counting methods used to determine the absolute disintegration rate of radio-isotopes which emit one β particle per disintegration. Two main methods of measurement are discussed. Their limitations are stated and the corrections necessary for accurate work are explained. A third method is outlined. The β-γ coincidence method is suitable for assay of isotopes which disintegrate by the emission of a β particle and one or more γ rays. Defined solid angle methods are especially suitable for pure β emitters. Two equipments are described, one using a windowless counter of a demountable type. A 4π solid angle counter with an internal source is suggested, particularly for the assay of positron emitters. Liquid counters used as sub-standards are described and methods of ensuring reproducible results are given. A resumé is made of the present state of agreement, between various observers, upon absolute standards of activity. The bases of measurement are stated for British Standards of activity of Na24, P32, and Co60 for the year 1949.

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