Absolute measurement of sulfur-35

In measuring absolute value of radioactivity of radioisotope which emits soft β ray, such as ³⁵S and ¹⁴C, error caused mainly by self-absorption of β ray in source. In present paper, preparation method of ³⁵S source with less self-absorption and estimation of amount of self-absorption in source were discussed. The most suitable procedures for source preparation of ³⁵S were as follows; (1) sample solution, sulfuric acid in dilute hydrochloric acid solution, was dropped on thin vinyle film and dried up on a desiccant. (2) this source was dissolved with redistilled water and again dried up in ammoniac atmosphere. ⁹⁵Nb, which emit β ray with the same maximum and mean energy as that of ³⁵S and emit prompt γ ray, was taken. The crystal sizes of ⁹⁵Nb source were compared that of ³⁵S sources under the view of microscope. Then the amounts of self-absorption of β ray in these ⁹⁵Nb sources were derived from the ratio of 4πβ counting rate and 4πβ-γ coincidence counting rate. The amount of self-absorption, which was obtained by using above mentioned method, was about 5% for ordinary source and less than 2% for good source. (author)