Improved neutron activation analysis with short-lived nuclide decay compensation

Abstract

A novel technique in neutron activation analysis has been developed, particularly for short-lived nuclides. The purpose is to optimize the count rate by keeping it relatively constant so that the counting period can be prolonged, in order to increase the counting statistics and consequently, increase the sensitivity and accuracy of the method. The rapid radioactive decay of the short-lived nuclides, resulting in rapid decrease of the count rate, is compensated for by changing the distance between the irradiated sample and the detector and hence the geometry factor and the efficiency of the system during the counting period. After several trials with air- and oil-driven piston systems, the slow approach of the sample holder to the detector was implemented by means of a mechanical device consisting of a slow d.c. motor with variable rotation speed reduction, combined with a rotary-to-linear motion converter. In order to avoid pulse pile-up effects, dead-time losses and background interferences, it is necessary to keep the count rate within certain limits and hence to partially compensate the exponential radioactive decay by a linear approach of the sample to the detector, although the radiation increase on the detector is inversely proportional to the square of the sample–detector distance. For complete decay compensation, the sample motion speed should decrease exponentially. The relatively high background at long sample–detector distances, particularly in trace element analysis, necessitates good shielding. As the system is calibrated by use of standards, repeatable experimental conditions have to be ensured. Promising preliminary results have been obtained. However, automation of the short-lived nuclide analytical system has been improved by the recent installation of a programmable logic controller to facilitate the operation of the system.