The applicability of relative and k0 standardization methods in instrumental neutron activation analysis to environmental samples at ITU TRIGA Mark II research reactor

Neutron activation analysis (NAA) is an analytical method used to determine element contents of the samples in various matrices. Instrumental neutron activation analysis (INAA), a non-destructive method, requires no chemical treatments prior to or after the irradiation and it is the most applied NAA method worldwide. Standardization methods in NAA are sorted into absolute, relative and comparator standardization. Although the absolute merhod is experimentally simple, the uncertainty of some physical parameters for many (n,γ) reactions and radionuclides are large, leading to a relatively large uncertainty of the element amounts. In relative method a standard with known mass of element to be determined is coirradiated together with a sample, and both of them are measured under the same experimental conditions. Although the relative method provides high accuracy results, it suffers in multi-element analysis from several inconveniences, such as the irradiation and the measurement of standards. The single-comparator method introduces k-factors, which are experimentally determined by coirradiation of a standard and a single-comparator together. Then, the amount of an element in a sample is determined by coirradiation of the sample and the single-comparator. Its obvious advantage is the experimental simplicity in case of k-factors are priorly determined. On the other hand, a serious disadvantage is the rigidity of k-factors, which are strictly bound to local experimental conditions. If an experimentally determined k-factors of the single-comparator method are normalized for the experimental conditions of irradiation and counting, k0-factor is defined. The k0 method requires the determination of neutron spectrum parameters, thermal neutron flux (φt), epithermal flux distribution parameter (α), thermal-to-epithermal neutron flux ratio (f), comparator factor (Fc,Au), and the detector calibrations prior to analysis. Among the advantages of the k0 method is the possibility of multi-element analysis of large series of similar samples using commercially accessible or free programs by saving time. In this study, the relative and k0 standardization were carried out for the element analyses of environmental samples considering the idea of applicability of INAA at ITU TRIGA Mark II research reactor for scientific and commercial purposes. In this context, calibrations of HPGe detectors, calibration of neutron spectrum, element analyses of certified reference materials and proficiency test materials were done and quality control of results were evaluated. Besides, a six months research, which was supported by TUBITAK 2214-A International Research Fellowship Program (for Doctorate Students), were done in Nuclear Physics Institute of the Czech Academy of Sciences (CAS NPI) on the soil and plant samples taken from the various regions of Istanbul to contribute the studies carried out in ITU TRIGA Mark II research reactor. Irradiations were done in ITU TRIGA Mark II research reactor central irradiation channel at 250 kW power for 6 hours and 207 kW power for 4 hours. Several MS Excel programs were prepared to determine the element concentrations and evaluate the quality of the results with relative method. The aim was to make faster analyses in routine studies and to minimize the data entry errors. k0 standardization was carried out by k0-IAEA (version 7.16) and Kayzero for Windows (version 3.05) programs. Besides, Kragten method was used for determination of neutron spectrum parameters and their uncertainties. In relative standardization As, Ce, Co, Cr, Cs, Fe, K, La, Na, Rb, Sb, Sc, Th, U and Zn were determined in soil samples. According to the results, the major uncertainty contribution is due to statistical counting. In k0 standardization, the bare triple-monitor method, using Au and Zr monitors, was applied to determine neutron spectrum parameters and their uncertainties with k0-IAEA, Kayzero for Windows and Kragten method. The neutron spectrum parameters calculated with k0-IAEA, Kayzero for Windows and Kragten method were found to be in agreement. Taking into consideration low f values (˂20), it can be said that the neutron spectrum was poorly thermalized and negative α values were calculated. The uncertainty range of α value was from 43% to 1700%. In k0 standardization Ag, As, Au Ba, Br, Cd, Ce, Co, Cr, Cs, Eu, Fe, Hf, K, La, Na, Rb, Sb, Sc, Sm, Sr, Tb, Th, U, W, Yb, Zn and Zr were determined in soil and plant samples. The performance of the method was evaluated by analysing NIST standard reference materials (SRMs) 2711, 1633b and 1547. The relative expanded uncertainties (k=2) calculated with Kayzero for Windows are in the range of 7–33 % for NIST SRM 2711 and 7-25 % for NIST SRM 1547, and with k0-IAEA the above ranges are 17–90 % and 18-60 %, respectively. When comparing the results obtained by k0-IAEA and Kayzero for Windows, it was determined that the k0-IAEA results are associated with higher uncertainties compared to those obtained by Kayzero for Windows due to the different approaches used in their calculation. In k0-IAEA program, the uncertainties of neutron spectrum parameters, as well as other uncertainty components, are taken into account to calculate the uncertainty of element concentrations. However, in Kayzero for Windows the uncertainty of element concentrations are calculated by adding a 3.5 % systematic uncertainty to the counting statistics. Since the relative uncertainties of neutron spectrum parameters, especially for the α value, calculated with k0-IAEA program are high (>100%), this fact explains the high uncertainties of the element concentrations determined. The results obtained from this study showed that, the investigated elements were determined with high accuracy by relative standardization. However, 13 more elements were determined by k0 standardization. Taking into consideration the irradiation and measurement conditions involved, plant samples that caused problems according to their matrix properties in relative standardization, could be analysed by k0 standardization method. As INAA is not used in routine analyses in Turkey, this study is important for being the first extensive application of the method in environmental samples and being the very first application of the k0 standardization method by k0-IAEA and Kayzero for Windows program, which are used for routine analyses in NAA laboratories worldwide, at ITU TRIGA Mark II research reactor. It is anticipated that the future studies with alternative neutron flux monitors, SRMs of different matrices, and suitable HPGe detectors would improve the quality of k0-INAA results. Thus, the method can be applied to samples in various matrices for the determination of more elements and used for commercial purposes besides the scientific researches.