Specific γ-Activity, the Branching Ratio and Half-life of Potassium-40

Abstract

THE specific γ-activity of ordinary potassium, in the form of ‘AnalaR’ potassium chloride, has been determined by the following procedure: (1) A source of potassium-42 (obtained by the irradiation of ‘Specpure’ potassium carbonate in the Harwell pile) was calibrated against a standard radium needle using a small carbon-walled ionization chamber. (2) The response was measured to a solution containing 1,745 gm. potassium (as ‘AnalaR’ potassium chloride), using a large differential high-pressure ionization chamber apparatus. (A description of this apparatus will be given elsewhere.) (3) A measured aliquot of a solution of the calibrated potassium-42 was added to the potassium chloride solution and the additional response observed as in (2). (4) The half-life of the potassium-42 was measured with the large ionization chamber apparatus and found to be (± probable error) 12.516 ± 0.007 hr. Analysis was by the method of least squares and each point on the decay curve was weighted on an experimental basis. It was necessary to correct for the presence of a small amount of unidentified long-lived activity with an apparent half-life of about 177 days. Sodium lines were reported in the spectrographic analysis, although the percentage contamination was not given; the half-life result will therefore tend to be slightly greater than that of absolutely pure potassium-42, because of the presence of some sodium-24 of half-life 15.0 hr.