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

1 MINISTRY OF EDUCATION AND TRAINING MINISTRY OF SCIENCE AND TECHNOLOGY VIETNAM ATOMIC ENERGY INSTITUTE RESEARCH AND DEVELOPMENT OF CYCLIC NEUTRON ACTIVATION ANALYSIS AT DALAT RESEARC

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MINISTRY OF EDUCATION

AND TRAINING

MINISTRY OF SCIENCE 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

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Công trình được hoàn 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 sẽ được bảo vệ trước Hội đồng cấp viện chấm luận án tiến sĩ họp tại: ………

……… Vào hồi ……… giờ … 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

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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

77mSe (T1/2 = 17.45 sec.), 46mSc (T1/2 = 18.75 sec.), 179mHf (T1/2 = 18.68 sec.) và 110Ag (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 20F (T1/2 = 11.03 sec.), 19O (T1/2 = 26.9 sec.),

207mPb (T1/2 = 0.8 sec.), 28Al (T1/2 = 2.24 min.), 52V (T1/2 = 3.75 min.), 51Ti (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

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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) multi-element 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]

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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 k0-CNAA 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 CNAA is a fast technique, reliable and highly sensitive results which allow determination of trace elements for biomedical nd

k0-biological research

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Chapter 1 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 Nuclear reaction Nuclide ,

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 77mSe, 110Ag, 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

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CNAA system in Dalat research reactor The function of analyzing cumulative spectrum is considered a new feature of k0-IAEA software, version k0-IAEA V.8 is used for the purpose of applying k0-CNAA on k0-IAEA software

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Chapter 2 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:

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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 self-shielding 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

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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:

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 k0- CNAA method was formulated as follows:

2.4 Dead-time and pile-up

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Dependency counting rate on dead-time of the GMX-4076 detector was investigated and was shown in Figure 2.4 The results indicate that the sample was measured at less than 10% dead-time, the counting rate does not change much, which means the count is not significantly lost when measured at low dead-time However, when measured at dead-time greater than 10%, the counting rate decreases linearly with deadtime The loss of counts is expressed as characteristic factor (a) and high dead-time cause loss of counts (DT − DT0) From this comment, we have:

In which, A0 is the actual activity; ADT is the activity recorded with the dead-time; a × (DT-DT_0) denotes the lost of counts

DT is the dead-time for each sample measurement, DT0 is the maximum dead-time value of the measuring system when there

is no need to correct

Figure 2.4: Dependency counting rate on dead-time of the

GMX-4076 detector

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 concentration, measurement uncertainty and detection limit

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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

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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)

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Chapter 4 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