Coincidence Method of Standardization for the Pure Electron Capture Nuclide Caesium-131

Caesium-131 decays by electron capture to the ground state of xenon-131 without gamma emission. Capture from the K shell predominates, but about 12% of the disintegrations occur by L capture. Vacancies in the L shell are also frequently produced when K shell vacancies are filled. This is the basis of the K-L coincidence method, described by Allen, for measuring the disintegration rate. In the present work, the theory of Allen's method is amended soasto distinguish between the number of L shell vacancies formed simultaneously with the K X-rays and the number associated with K Auger electron emission. A correction is also applied for M shell capture. The method is adapted for use with a 4π β-γ (proportional counter/Nal crystal) coincidence equipment. The output of the Nal crystal system is gated on the full energy pulses from the xenon K X-rays. A correction is made for the escape of iodine X-rays from the crystal in the detection of xenon Kβ X-rays. In the proportional counter spectrum, the pulses resulting from the filling of L vacancies are distinct from those arising from the filling of K vacancies (K events), and a narrow gate is set to accept the former. The expression for the disintegration rate allows for the small number of K events which count within the gate. L events are deleted from the observed counting rate by summation out of the gate whenever simultaneous K events are detected in the proportional counter. A correction is therefore made, in the equation for the disintegration rate, for the pulses which are produced above the gate by K events (usually K Auger electrons). Because the number of L vacancies in coincidence with K Auger electrons is not well known, the accuracy of the method is improved when the K Auger electrons are detected with high efficiency, by using a thin source in 4π geometry. It is not strictly accurate to assume that all the remaining L shell vacancies are detected with equal efficiency, because the L vacancies produced in association with K X-rays are in LIII and LII sub-shells, whereas those formed by capture of L shell electrons are mainly in the LI sub-shell. The different fluorescence yields for the various L sub-shells lead to an additional correction term. (author)