Nghiên cứu phát triển phương pháp phân tích kích hoạt neutron lặp vòng trên lò phản ứng hạt nhân đà lạt để xác định các hạt nhân sống ngắn tt tiếng anh

MINISTRY OF EDUCATION MINISTRY OF SCIENCE AND TRAINING AND TECHNOLOGY VIETNAM ATOMIC ENERGY INSTITUTE RESEARCH AND DEVELOPMENT OF CYCLIC NEUTRON ACTIVATION ANALYSIS AT DALAT RESEARCH REACTOR FOR DETERMINATION OF SHORT-LIVED NUCLIDES Author: Ho Van Doanh Supervisor: Dr Ho Manh Dung Ass Prof Dr Nguyen Nhi Dien Major: Nuclear and Atomic Physics Code: 9.44.01.06 SUMMARY OF DOCTORAL DISSERTATION OF PHYSICS DALAT – 2020 Cơng trình hồn thành tại: Viện Nghiên cứu hạt nhân, Viện Năng lượng nguyên tử Việt Nam Người hướng dẫn khoa học: TS Hồ Mạnh Dũng PGS TS Nguyễn Nhị Điền Phản biện 1: …………………………………………… Phản biện 2: …………………………………………… Luận án bảo vệ trước Hội đồng cấp viện chấm luận án tiến sĩ họp tại: ………………………………………………… ………………………………………………………………… Vào hồi ……… … ngày … tháng … năm 2019 Có thể tìm hiểu luận án tại: - Thư viện Quốc gia Việt Nam - Thư viện Trung tâm Đào tạo hạt nhân INTRODUCTION Neutron activation analysis technique in research reactor can determine about 70 elements in periodic table [1] In which several elements need long time for analysis because they were determined based on long-lived nuclides such as 75Se (T1/2 = 120 day), 46Sc (T1/2 = 84 day), 181Hf (T1/2 = 42 day) and 110mAg (T1/2 = 250 day) to achieve required sensitives, the total of irradiation – decay – measurement takes from several days to few weeks This reduces competitiveness of NAA compared to other analytical techniques As for short-lived nuclides, including 77m Se (T1/2 = 17.45 sec.), 18.68 sec.) 46m Sc (T1/2 = 18.75 sec.), 179mHf (T1/2 = 110 Ag (T1/2 = 24.60 sec.) of the same elements as long-lived nuclides, can reduce significantly analytical time, thereby increasing the number of samples in the same time Therefore, the utilization of short-lived nuclides in NAA technique will be more effective and competitive than other analytical techniques Moreover, several elements can be determined by NAA technique based on only one short-lived nuclide such as 207m 20 F (T1/2 = 11.03 sec.), 19 O (T1/2 = 26.9 sec.), Pb (T1/2 = 0.8 sec.), Al (T1/2 = 2.24 min.), 52V (T1/2 = 3.75 min.), 28 51 Ti (T1/2 = 5.76 min.), This extends analytical capabilities compared to the use of long-lived nuclides in NAA technique Therefore, the utilization of NAA for analyzing elements is achieving more and more attention [2] However, some of problems in NAA technique using short-lived nuclides are that irradiation and measurement time Both are limited leading to statistical counting and uncertainty of measurement which are not satisfactory for most samples This problem can be solved by application of cyclic neutron activation analysis (CNAA) CNAA is a method of neutron activation anlysis for determination of element in which a sample is irradiated, decayed, counted, then irradiated again, and this process is repeated for a number of cycles, the spectra from each counting will be summed to give a final total spectrum By this process, the counts of a short-lived nuclide of interest are considerably increased and the analytical sensitivity of elements is significantly improved [3] At least 20 elements which produce short-lived nuclides (half-life less than 100s) by thermal neutron bombardment, and also more than 10 elements which produce nuclides with half-lives of 100-600s can be determined by thermal and epithermal neutron in CNAA CNAA has been widely applied in biological, environmental, geological, and industrial studies, and the most often measured elements include Se, Ag, F, Hf and Sc There are many advantages of CNAA, include: (1) short experimental time by using short-lived nuclides, (2) significant improvements of detection limits and analytical precision and accuracy, (3) multielement determination of short and medium-lived nuclides, (4) determination of the degree of homogeneity of samples However, CNAA method also has disadvantages such as: (1) The experimental procedure is complicated because irradiation – measurement is repeated many times for both analytical and standard samples to calculate concentration by NAA relative method; (2) Dead time and pile-up are serious problems in CNAA and must be corrected [5] Therefore, in this thesis, cyclic neutron activation analysis based on k0 method (k0-CNAA) has been researched and developed along with some corrections to overcome the above disadvantages for determination of some short-lived nuclides, such as: 77mSe, 110Ag, 179mHf, 46mSc, 165mDy, v.v… All experiments in this thesis have been performed by CNAA system at Dalat research reactor k0-IAEA software after upgrading by Dr Menno Blaauw for k0-CNAA has been studied and applied for the first time in Vietnam In addition, some corrections of CNAA system after upgrading related to the k0CNAA were also implemented in this study The results of this thesis show that the k0-based cyclic neutron activation analysis (k0-CNAA) method has been studied to explore the applicability at Dalat research reactor The k0CNAA is a fast technique, reliable and highly sensitive results which allow determination of trace elements for biomedical nd biological research Chapter THE OVERVIEW OF THE RESEARCH SITUATION OF CYCLIC NEUTRON ACTIVATION ANALYSIS METHOD 1.1 The situation of foreign research on CNAA method The demands for fast, reliable analysis techniques along with high sensitivity requirements to identify trace elements and multi-element analysis capabilities, especially related to environmental monitoring and research programs (requiring analysis of a large number of samples) spurred the development of CNAA method In addition to competition for commercial factors compared to other analytical techniques, another reason that is getting more and more attention is that some short-lived nuclides are subject to relate to environmental and biomedical issues [6] About 20 elements with short-lived nuclides with T1/2 < 100s were analyzed by the CNAA method listed in Table 1.1 Table 1.1: The nuclear data for some elements are determined by the CNAA through short-lived nuclides [3] n Ele 10 11 12 13 O F Sc Ge Se Rb Rh Pd Ag In Sb Ce Dy Nuclear reaction O(n,γ)19O F(n,γ)20F 46 Sc(n,γ)46mSc 74 Ge(n,γ)75mGe 76 Se(n,γ)77mSe 85 Rb(n,γ)86mRb 103 Rh(n,γ)104Rh 106 Pd(n,γ)107mPd 109 Ag(n,γ)110Ag 115 In(n,γ) 116mIn 123 Sb(n,γ)124mSb 138 Ce(n,γ)139mGe 164 Dy(n,γ)165mDy Nuclide 18 19 19 20 O F 46m Sc 75m Ge 77m Se 86m Rb 104 Rh 107m Pd 110 Ag 116m In 124m Sb 139m Ce 165m Dy , % 0.002 1.00 1.00 0.365 0.090 0.722 1.00 0.273 0.482 0.957 0.427 0.0025 0.282 T1/2, s 26.9 11.02 18.75 48 17.45 61.2 42.3 20.9 24.6 14.1 93.0 56.4 75.6 E γ, keV 197.14 1633.60 142.53 139.6 161.93 556.17 555.8 214.9 657.76 1293.6 645.86 757.0 515.5 14 15 16 17 18 19 20 Er Yb Pt Hf Ir W Pb 166 Er(n,γ)167mEr Yb(n,γ)177mYb 198 Pt(n,γ)199mPt 178 Hf(n,γ)179mHf 191 Ir(n,γ)192mIr 182 W(n,γ)183mW 206 Pb(n,γ)207mPb 176 167m Er Yb 199m Pt 179m Hf 192m Ir 183m W 207m Pb 177m 0.336 0.127 0.072 0.273 0.373 0.265 0.241 2.27 6.41 13.6 18.68 87 5.65 0.80 207.8 104 319 216 58 107.9 570 1.2 The situation of national research on CNAA method In 2013, through IAEA's technical assistance project (RER/4/028), Dalat research reactor was equipped with a fast pneumatic sample transfer system This system was applied to determine elements through short-lived nuclides However, the accuracy and limit of detection were not good at lower levels Later, research efforts were made to improve sensitivity by applying various technique of cyclic activation, including manual cyclic-NAA, manual pseudo-CNAA, Replicate-NAA and combine Re-NAA with CNAA or PCNAA The survey results show that these techniques are able to determine some elements through short-lived nuclides such 77m Se, 110 Ag, The results of the survey are detailed in the author's own article [6] 1.3 The k0-CNAA method The k0-CNAA method was first studied in 2012 [7] The cyclic activation was performed at the Research Reactor in Portugal The k0-IAEA program was studied using manual spectral processing steps because at that time the program was not upgraded for k0-CNAA In 2016, K0-IAEA software was upgraded for processing data of k0-CNAA at Dalat research reactor by Menno Blaauw Then, experiments for software testing were also conducted at CNAA system in Dalat research reactor The function of analyzing cumulative spectrum is considered a new feature of k0IAEA software, version k0-IAEA V.8 is used for the purpose of applying k0-CNAA on k0-IAEA software Chapter THEORY OF CYCLIC NEUTRON ACTIVATION ANALYSIS 2.1 Theory of neutron activation analysis From the theoretical basis of the process of activation, decay and measurement of radioactivity, the equation for calculating the concentration of an element in neutron activation analysis is formulated as follows: Np /t c M 1 ρ = S D C W NA θγ [Gth φth σ0 + Ge φe I0 (α)] εp In which: ρ is the concentration of the element of interest; NP is counts at the energy of interest; tc is the measurement time, S is the saturation factor during irradiation; D is correction factor for decay; C is the correction factor for the decay during the measurement; W is the sample mass; M is the atomic mass; NA is the Avogadro constant;  is isotope abundance; γ is the gamma ray emission probability; Gth is the self-shielding correction factor of thermal neutron; th is the thermal neutron flux; 0 is the capture cross-section of thermal neutron; Gepi is the selfshielding correction factor of epithermal neutron; e is the epithermal neutron flux; I0(α) is the resonant integral for the peithermal neutron spectrum with 1/E1+α distribution 2.2 Cyclic neutron activation analysis method Cyclic activation analysis (CAA) is a method of activation analysis for elemental analysis in which a sample is irradiated, decayed, counted, then irradiated again, and this process is repeated for a number of cycles, the spectra from each counting being summed to give a final total spectrum By this process, the counts of a short-lived nuclide of interest are considerably increased and the analytical sensitivity of elements is significantly improved From the principle of the method, the basic equation to calculate the concentration of an element by the cyclic neutron activation analysis is established as follow: ρ= Npc /t m M 1 S D C W Fc NA θγ [Gth φth σ0 + Ge φe I0 (α)] εp In which, Fc is cyclic factor, Fc = [ N (1−e−λT ) − e−λT (1−e−NλT ) (1−e−λT ) ], N is number of cycles;  is decay constant; T = ti + td + tc + tw (tw is waiting time between two cycles) 2.3 k0-CNAA method From the equation of the k0-NAA method and the CNAA equation, the basic equation to calculate the elemental concentration in the sample by the k0CNAA method was formulated as follows: Npc /t c ) S D C W Fc a Gth,m f + Ge,m Q 0,m (α) εp,m ρ= Asp,m k 0,m (a) Gth,a f + Ge,a Q 0,a (α) εp,a ( In which, the symbol of a denotes the element of interest, the symbol of m denotes the flux monitor, Asp is the specific activity; k0 is the k0 factor; f is the ratio of thermal neutron flux to epithermal neutron flux; Q0(α) is I0/0; p is the efficiency of the detector 2.4 Dead-time and pile-up 10 Chapter CNAA EXPERIMENT ON DALAT RESEARCH REACTOR To conduct this experiment to verify the k0 cyclic neutron activation analysis method (k0-CNAA), the cyclic neutron activation analysis system at Dalat reactor was presented in details about the structure, operating principles, etc Through the experimental process, the sample was irradiated and measured with the fully automatic cyclic irradiation-measurement function The neutron spectral parameters used in k0-CNAA and the flux variation at two sample irradiation positions of Channel 13-2 and Thermal Column were determined when the reactor was operating at a nominal capacity of 500 kW The GMX-4076 spectrometer connected to the cyclic system has been calibrated to match the actual sample measurement configuration Correction effects of dead-time were also performed The individual spectra obtained by measurements are added to the total spectrum by "spectral accumulation software" The total spectrum was processed through K0-IAEA software with version V.8 to calculate elemental uncertainty and detection limit 12 concentration, measurement Figure 3.1: Illustration diagram of the cyclic neutron activation analysis system Preparation of sample for cyclic activation Some of the standard samples used in this study are NIST-1566b (Oyster Tissue), IAEA-436 (Tuna), NIST-1577b (Beef Liver) and NIST-2711a (Montana Soil II) These standards are from the International Atomic Energy Agency (IAEA) and the National Institute of Standards and Technology (NIST) The samples were weighed with a weight of about 100 ~ 200 mg and placed in a clean PE vial, then put it in a 3.5 ml capsule Typically, the sample weight is 100 mg for geological samples and 200 mg for biological samples To evaluate the k0-CNAA method, a SMELS I standard sample was weighed 28.04 mg In addition, two flux moniors have been prepared to determine the flux before and after irradiation and measurement of the SMELS I Standard sample of NIST-1566b and NIST-2711A have also been prepared for CNAA These samples were used to evaluate the k0-CNAA method for biological and geological 13 sample objects In addition, the NIST-2711a standard sample was also used to assess the effect of dead-time in cyclic neutron activation analysis Irradiation and measurement of the sample using the cyclic activation system The sample is activated at channel 13-2 or Thermal Column by means of a cyclic activation system Thermal neutron flux at channel 13-2 is about 4.0 x 1012 cm-2.s-1 and in Thermal Column is about 1.2 × 1011 cm-2.s-1 when Dalat reactor operates at a nominal capacity of 500 kW The transfer time of the sample from the irradiation position to the detector is about 3.5 seconds The sample was then measured on a digital signal processing spectrometer which is using an HPGe detector k0-CNAA data processing Spectrums obtained from experiments were processed through the upgraded k0-IAEA software for k0-CNAA method Figure 3.2: Cumulative spectra of NIST-1566b (N = 5) 14 Chapter RESULT AND DISCUSSTION This chapter presents the results of the test and evaluation of the cyclic neutron activation analysis method based on the k0 (k0-CNAA) method by analyzing the reference sample of SMELS I (standard sample for testing k0-NAA quality) and some types of biological standard samples In particular, the measurement uncertainty of the k0-CNAA method was also calculated based on NIST-2711A standard sample (Montana II Soil) The contributing components include irradiation time errors, errors in the determination of neutron spectra parameters, errors of efficiency calibration and errors from other parameters during experiments The correction of dead-time effects were verified by comparing the counting rate before and after calibration In addition, the NIST-2711A sample was used to verify the dead-time correction formula because this sample has a complex background so the dead-time up to 62% To evaluate the possibility of the cyclic neutron activation analysis, the element Selenium at different content levels in some biological standard samples was determined by the methods: conventional cyclic (CNAA), pseudo-cyclic (PCNAA), replicate (Re-NAA), and combination of Re-NAA and CNAA or PCNAA methods 4.1 Results of evaluating k0-CNAA at Dalat reactor SMELS I is a standard sample used for evaluation of method k0 Therefore, the SMELS I sample was used in this study to evaluate the analytical results by the k0-CNAA method 15 at Dalat reactor Figure 4.1 shows the ratio of experimental concentration (GTPT, ex) and certified concentration (GTCN, ce) by the k0-CNAA for SMELS I sample In general, the results show that most of the ex of the elements in each cycles were different from the ce less than 10%, except for some cases where the statistical counts were not enough, such as Cs and La in some initial cycles Figure 4.1: Ratios of ex and ce for SMELS I sample Figure 4.3 shows the ex of a number of elements in the NIST-1566b sample determined through the total spectrum of by the k0-CNAA method for 5th cycle The results were compared and evaluated with the ce 9/11 elements have ex/ce ratios less than 15% The ex of Cu and Mg have a difference of about 20% compared to the ce 16 Hình 4.3: ex/ce and its errors for NIST-1566b with N=5 4.2 The correction result of high dead-time effect The results in Figure 4.8 show the relative counting rates of 46m Sc before and after the correction with various dead-time For measurements of dead-time is less than 10%, the counting rate is almost unchanged and approximately up to the corrected value For measurements with dead-time is greater than 10%, the counting rate decreases linearly with increasing dead-time After correction by the proposed experimental formula, the loss of counts were almost replenished and it could be corrected with dead-time up to 80% 17 Figure 4.8: Relative counting rates at the peak of 142.5 keV of 46m Sc before and after correcting dead-time effect Figure 4.11 shows the experimental concentration of some elements in the NIST-2711A standard sample which is determined by the total spectrum of cycles by the k0-CNAA method The results were calibrated for high dead-time effect After correction, the concentration of some elements increased an average of about 20% and got closer to the certified values Most elements have ex/ce ratios less than 10% except for Hf and Ca with a deviation of about 15% Figure indicates all the ex are smaller than the ce, this may be because loss counts have not been replenished completely when the measurement are performed at too high dead-time (over 50%) Even so, the difference between ex and ce is less than 15% for all eight elements This result is still acceptable in measurements of shortlived nuclides with CNAA method at dead-time up to 62% 18 Figrure 4.11: ex/ce of NIST-2711A and its errors 4.3 The determination result of the Selenium in biological samples Figure 4.15 shows a marked improvement in DLs of selenium that were determined by a combination of cumulative NAA and PCNAA DLs were improved by increasing a number of replicates of the same sample and number of cycles As regards PCNAA, DLs decrease significantly at the end of 3rd cycle with a factor of 1.8 times in comparison with the conventional NAA and slightly decrease from 4th to 6th cycle As regards cumulative NAA (first points in vertical axis of lines) DLs improved at the end of 3rd replicate with a factor of 1.6 times The DLs ratios of the combination of cumulative NAA and PCNAA to conventional NAA were 1.9:1, 2.8:1, 3.8:1, 4.5:1 in the same order as a number of replicates and cycles In other words, an improvement by a factor of 2.8 times was obtained from an accumulative gamma-ray spectrum of fresh subsamples at the third cycle This factor can be increased by 19 increasing the number of sub-samples and cycles If necessary, sub-samples repeat cycles to achieve a huge improvement of 4.5 times Hình 4.15: Detection limits of 77mSe in IAEA-436 by the combination of cumulative NAA with pseudo-cyclic NAA The precision and accuracy of the technique have been evaluated by analyzing a number of biological certified reference materials of varied selenium levels The concentration of the selenium in the CRMs was calculated from the result for the selenium standard sample using a relative method of standardisation The averages of three determinations are shown in Table 4.19 The data indicate that the precision of measurement have become better from first through third or fourth cycle and afterward worse Precision of ± to 6% can be easily achieved at third or fourth cycle for all An agreement between measured and certified values was acceptable in the consideration of the deviation of the above mentioned two values within percent with u-score < 1.64 for all 20 Table 4.19: Effect of number of cycles on precision of measurement of selenium Reference Material (Reported value) Tuna Fish, IAEA-436 (4.63 ± 0.48) Oyster Tissue, NIST-1566b (2.06 ± 0.15) Bovine Liver, NIST-1577b (0.73 ± 0.06) Cycle No 6 This work Aver ± SD 4.64 ± 0.62 4.87 ± 0.60 4.32 ± 0.23 4.29 ± 0.17 4.42 ± 0.43 4.51 ± 0.39 2.19 ± 0.29 2.13 ± 0.23 1.96 ± 0.08 1.96 ± 0.10 2.01 ± 0.15 2.04 ± 0.15 0.79 ± 0.13 0.72 ± 0.10 0.81 ± 0.05 0.82 ± 0.04 0.81 ± 0.11 0.79 ± 0.10 21 RSD (%) 13.5 12.3 5.4 4.0 9.8 8.6 13.2 10.8 4.0 5.3 7.6 7.1 16.7 13.8 6.3 4.8 13.3 13.2 u-score 0.01 0.31 -0.58 -0.67 -0.32 -0.20 0.40 0.25 -0.58 -0.55 -0.24 -0.09 0.44 -0.07 1.07 1.20 0.61 0.46 CONCLUSION The cyclic neutron activation analysis method based on standardization k0 (k0-CNAA) has been developed and successfully applied at Dalat research reactor This method is capable of rapid analysis, high reliability and sensitivity, especially for trace elements with indicator properties used in biomedical and biological research The thesis has solved the initial objectives with the main results including: The cyclic neutron activation analysis (CNAA) was built on Dalat research reactor with fully automated experimental process used cyclic activation system and data processing by software k0-IAEA This method allows quick determination of elements Se, Ag, F, Hf, Sc, Dy, Al, V, Ti, Cu, Ca, and Mg via short-lived nuclides with T1/2 < 30 seconds such as 77mSe, 110Ag, 20 28 F, 179m Al, Hf and 52 V, 51 Ti, 46m Sc; nuclides with T1/2 < 10 minutes such as 66 Cu, 49 Ca and 27 Mg Moreover, the k0-CNAA method is also capable of determination a number of mediumlived nuclides such as 38 Cl, 56 Mn, 42 K, 24 Na, 198 Au (T1/2 > 10 minutes) In addition, the measurement uncertainty of the k0CNAA method has been determined Most sources of errors have been quantified during the experimental procedure Software k0-IAEA (after upgrading to k0-CNAA by the author of the software - Dr Menno Blaauw), was first applied, tested and evaluated results at the Dalat research reactor The k0IAEA software has functions of the cumulative spectrum processing in k0-CNAA which are fully automated and able to produce reliable results Experimental results show the deviation 22 between the analytical value and the certification value of most elements is less than 10% The counting loss with high dead-time up to 62% was investigated for correction with an accuracy of less than 15% for most elements in the geological sample The initial results of some complex samples show the CNAA method can be used to analyze complex matrix such as environmental and geological samples Moreover, the results of applying the CNAA method for determination of Selenium at various content levels were performed in the thesis When combining cyclic methods such as replicate of the sample (Re-NAA) with a regular cyclic NAA, the detection limit can be improved about times for the Selenium in biological samples The k0-CNAA method established in the thesis can determine selenium in biomedical samples with concentration lower-ppm, completely meeting as demand the practical requirements The results obtained in the thesis have achieved a number of new points, including the successful development of cyclic neutron activation analysis based on the k0 (k0-CNAA) at Dalat research reactor, and first applied k0-IAEA software for k0CNAA The results of this thesis are ready for use in biomedical and environmental research in Vietnam 23 LIST OF PUBLICATIONS National publications: [1] Van Doanh Ho, Dong Vu Cao, Quang Thien Tran, Ngoc Son Pham, Thi Sy Nguyen, Giang Nguyen, Nhi Dien Nguyen (2014), “A new rapid neutron activation analysis system at Dalat nuclear research reactor”, Journal of Nuclear Science and Technology, Vol 4, No.1, pp 82-91 [2] Van Doanh Ho, Quang Thien Tran, Thi Sy Nguyen, Nhi Dien Nguyen (2014), “Determination of selenium by short-time NAA using 77mSe at the Dalat research reactor”, Journal of Nuclear Science and Technology, Volume 4, No.3, pp 36-42 International publication (ISI): [1] Van Doanh Ho, Manh Dung Ho, Quang Thien Tran, Thi Sy Nguyen, Nhi Dien Nguyen (2016), “Combination and optimization of the cyclic NAA modes at the Dalat research reactor for determination of selenium in biological materials using 77mSe”, Journal of Radioanalytical and Nuclear Chemistry, Volume 309, Issue 1, pp.185-188 [2] Manh Dung Ho, Quang Thien Tran, Van Doanh Ho, Dong Vu Cao, Thi Sy Nguyen (2016), “Quality evaluation of the k0standardized neutron activation analysis at the Dalat research reactor”, Journal of Radioanalytical and Nuclear Chemistry, Volume 309, Issue 1, pp.135-143 [3] Manh Dung Ho, Quang Thien Tran, Van Doanh Ho, Thi Sy Nguyen (2016), “Determination of multi-element composition of Vietnamese marine sediment and tuna fish by k0-standardized 24 neutron activation analysis” Journal of Radioanalytical and Nuclear Chemistry, Volume 309, Issue 1, pp 235-241 [4] Van Doanh Ho, Manh Dung Ho, Thanh Viet Ha, Quang Thien Tran, Dong Vu Cao (2018), “The upgrading of the cyclic neutron activation analysis facility at the Dalat research reactor”, Journal of Radioanalytical and Nuclear Chemistry, Volume 315, Issue 3, pp 703-709 REFERENCES [1] Greenberg R.R., Bode P., E.A.D.N Fernandes (2011), "Neutron activation analysis: A primary method of measurement", Spectrochimica Acta Part B 66, pp.193-241 [2] Molnar G.L., Revay Z., Szentmiklosi L (2004), "New perspectives for very short-lived neutron activation analysis", J Radioanal Nucl Chem 262(1), pp.157-163 [3] Hou X (2000), "Cyclic activation analysis", Encyclopedia of Analytical Chemistry, pp.12447-12459 [5] Spyrou N.M (1981), "Cyclic activation analysis - A review", J Radioanal Nucl Chem 61, pp.211-242 [6] Van Doanh Ho, Manh Dung Ho, Quang Thien Tran, Thi Sy Nguyen, Nhi Dien Nguyen, Combination and optimization of the cyclic NAA modes at the Dalat research reactor for determination of selenium in biological materials using 77mSe, J Radioanal Nucl Chem, 309 (2016) 25 [7] Dung H.M., et al (2012), "Development of k0-based cyclic neutron activation analysis for short-lived radionuclides", J Radioanal Nucl Chem 291, pp.485-492 [8] Lindstrom R.M., Fleming R.F (1995), "Dead time, pileup and accurate gamma-ray spectrometry", Radioactivity & Radiochemistry 6(2), pp.20-27 [9] Pomme S., Fitzgerald R., Keightley J (2015), "Uncertainty of nuclear counting", Metrologia 52, pp.3-17 [10] Spyrou N.M (1981), "Studies on some problems and applications of cyclic activation analysis", J Radioanal Nucl Chem 61, pp.175-182 [11] Spyrou N.M (1982), "Usefulness of thermal and epithermal cyclic activation analysis", J Radioanal Nucl Chem 72(1), pp.155-182 26 ... thermal neutron; th is the thermal neutron flux; 0 is the capture cross-section of thermal neutron; Gepi is the selfshielding correction factor of epithermal neutron; e is the epithermal neutron. .. Chemistry, Volume 309, Issue 1, pp 235-241 [4] Van Doanh Ho, Manh Dung Ho, Thanh Viet Ha, Quang Thien Tran, Dong Vu Cao (2018), “The upgrading of the cyclic neutron activation analysis facility at the... pp.135-143 [3] Manh Dung Ho, Quang Thien Tran, Van Doanh Ho, Thi Sy Nguyen (2016), “Determination of multi-element composition of Vietnamese marine sediment and tuna fish by k0-standardized 24 neutron