Neutron activation analysis absolute method at Puspati triga mark II research reactor for elemental analysis

Abstract

This research aimed to study the capability of the neutron activation analysis absolute method by using PUSPATI TRIGA Mark II research reactor, at the Malaysia Nuclear Agency (NM). Most of the quantitative measurements of neutron activation analysis (NAA) were done through comparative method, which was found to have high degree of errors due to the differences in the matrix. NAA based on absolute method is a more direct analysis of the irradiated samples based on (n, ?) reaction rate without using any standard or comparator. In this study the reaction rate of (n, ?) was adopted based on the Høgdahl conventions as well as Westcott formalism. This technique requires absolute gamma ray measurements and neutron spectrum parameters for the calculation of weights or concentrations of elements present in the sample. The neutron spectrum parameters such as epithermal neutron flux shape factor (a), thermal to epithermal neutron flux ratio (f), and thermal and epithermal neutron flux (fth and fepi) were determined at 40 irradiation positions of the rotary rack (RR) as well as at one location of pneumatic transfer system (PTS). The value of a in 40 RR was found to be in the range of 0.0060 to 0.1170 with an average of 0.0172 and 0.0028 at PTS. The f parameter ranged from 14.74 to 30.26 with an average value of 19.00 at 40 RR, while at PTS the value was 15.00. The results of fth and fepi at 40 RR were found to be in the range from 0.87 × 1012 to 2.55 × 1012 n cm-2 s-1 and from 0.41 × 1011 to 1.37 × 1011 n cm-2 s-1, respectively. The average values of fth and fepi were 2.17 × 1012 n cm-2s-1 and 1.16 × 1011 n cm-2 s-1 respectively, and at PTS with value of 3.89 × 1012 n cm-2 s-1 for fth and 2.59 × 1011 n cm-2s-1 for fepi. In addition, the spectral index parameter ( r ?? ? Tn /T0 ) was determined at 40 RR and PTS based on Westcott formalism. The average value was found as 0.0550 at RR and 0.0493 at PTS. The accuracy and precision of the proposed method were investigated by analyzing CRMs Soil-7, SL-1, IAEA-313, IAEA-312, NBS 1633A, USGS STM-1 and MAG-1 standard samples. The results showed a good agreement with the values reported in certificate with Z-score within 0< |Z|<2. Moreover, CRMs Soil-7, NBS 1633A and MAG-1 were analysed for relative method, and the results were found to be in good agreement with certified values. However, the results were slightly less than the results obtained by absolute method. Finally, the absolute NAA method was applied to determine the elemental concentration of U, Th and rare earth elements in rock samples based on Høgdahl convention except for Eu and Lu which were determined based on Westcott formalism. All samples were also analysed using relative method for comparison with the results obtained by absolute method. The deviations of both methods in most cases were found to be less than 10%.