A Correction Method for Coincidence Losses in Neutron Activation Analysis with Short-Lived Nuclides

Abstract

Development of instrumental neutron activation analysis (INAA) methods for simultaneous determination of multielement concentrations is reported in this thesis. Emphasis is placed on the use of short-lived nuclides.

A correction method for count losses (coincidence losses) due to both analyser dead-time and pulse pile-up in gamma-ray spectrometry of short-lived nuclides was developed. The method consists of simultaneous measurements of two time variables using a multichannel scaler: the ADC dead-time and the total count rate at the single channel analyser output of the ADC. This correction method was successfully applied to nuclides with half-lives as short as 0.72 s(38mcl) at rapidly varying initial total count rates as high as 60,000 cps.

A pseudo-cyclic INAA (PCINAA) method which can be used to improve the sensitivity of short- and medium-lived nuclides with half-lives longer than approximately 10 s was developed. An automated rapid cyclic transfer system is described for the measurement of very short-lived (half-life < 10 s) activities by cyclic INAA (CINAA). A mathematical formula is proposed and used to correct for coincidence losses in CINAA. This procedure considerably simplifies data manipulation without sacrificing the accuracy.

An epithermal INAA (EINAA) method to reduce interfering activities of thermal neutron activation products is reported. A novel boron shield to cut off thermal neutrons was developed. This shield can be constructed easily in any chemical laboratory and applied advantageously over conventionally used cadmium foils.

Precision and accuracy of the above INAA methods were evaluated by analysing several standard reference materials. These methods were successfully applied to several other matrices of biological and environmental interest. Further improvements are proposed.

Theoretical aspects of the limits in high count rate gamma-ray spectrometry are described.