Determination of the activity concentration of plant samples by gammaray spectrometrometry

Radioactivity has always existed in the environment.The radioactive sources in the environment can be divided into two main categories: Natural radioactivity incluce radioactive isotopes from Earths surface or the primordial radioactive isotopes (the radioactive isotopes of decay chains of 40K,238U,232Th ) and cosmogenic radioisotopes which produced as a result of the interaction os cosmic rays with the Earths material; artificial radioactive isotopes (137Cs) which produced by manmade such as medical and industrial uses of radioisotopes,nuclear testing weapon,nuclear accidents,the operation of nuclear power plants and mining....

VIETNAM NATIONAL UNIVERSITY, HANOI

VNU UNIVERSITY OF SCIENCE

FACULTY OF PHYSICS

- -Nguyen Thi Diem

Determination of the activity concentration of plant samples by gamma-ray spectrometrometry

Submitted in partial fulfillment of the requirements for the degree

of Bachelor of Science in Nuclear Technology

(Advanced program)

Supervisors: Bui Văn Loat Assoc.Prof

Vu Thi Kim Duyen,MSc

Ha Noi - 2017

ACKNOWLEDGEMENT

I would like to express my gratitude to my supervisor, Assoc.Prof Bui Van Loat and Master Vu Thi Kim Duyen for their trust in me which encouraged me to know the strength in myself and motivated me to work harder and achieve this success

My sincere thanks are also extended to Center for Technology Environment Treatment where has helps me to finish the practical part of my research

Besides, I would like to thank all teachers, lecturers, researchers and other seniors in Faculty of Physics, particularly Department of Nuclear Technology, VNU University of Science, who always create good conditions for students to study and research

I would like to give special thanks to my family and all my friends who have supported and promoted me in studying and researching They have become my faith and motivation throughout a hard time

Student,

Nguyen Thi Diem

List of Figure

Fig.1.1 Schematic of the exponential decay of activity for 210 Pb 4

Fig.1.2 Radioactive decay chains of 232 Th 6

Fig.1.3 Radioactive decay chains of 238 U 7

Fig.1.4 Radioactive decay chains of 235 U 7

Fig.1.5 The pathway of radionuclides to man 10

Fig.2.1 The diagram of gamma -ray spectrometry system 13

Fig 3.1 Graph of the energy-channel dependence 18

Fig 3.2 The TN1 spectroscopy was measured in counting time t=197058s 19

Fig 3.3 Establishment of efficiency calibration curve 21

Fig.3.4 Comparison of 226 Ra among plant samples in the present work 26

Fig.3.5 Comparison of 232 Th among plant samples in the present work 27

Fig.3.6 Comparison of 40 K among plant samples in the present work 28

List of Table

Table 1.1 Some common cosmogenic nuclides 8

Table 1.2 Some Man-made radionuclides 9

Table 2.1 Sampling stations 12

Table 3.1 The energy-channel dependence 18

Table 3.4 The efficiency at full-absorption peak of gamma radiation 20

Table 3.2 Minimum Detectable Activity of gamma-ray system 21

Table 3.3 The activity value of radioactive isotopes 22

Table 3.4 Activity concentration of analytical samples 23

Table 3.5 Average activity concentration of 226 Ra, 232 Th, 40 K 25

Table 3.6 Compare of activity concentration of 226 Ra, 232 Th, 40 K (in Bq/kg) in some vegetable 26

Contents

CHAPTER 1 2

LITERATURE REVIEWS 3

1.1 Phenomenon and the radioactive decay law 3

1.1.1 The radioactive decay law 3

1.1.2 The radioactive decay chains 4

1.2 Radioactive in nature 5

1.2.1 Primordial Radioisotopes 5

1.2.2 Cosmogenic radioisotopes 8

1.3 Artificial radioisotopes 8

1.4 Transfer of the radioactive isotopes from soil into plant 9

1.5 Characteristic of radioactive concentration in plant samples 11

1.5.1 The original of radionuclides in plant samples 11

CHAPTER 2 12

MATERIALS AND EXPERIMENTAL METHODS 12

2.1 Collecting samples 12

2.1.1 Sampling stations 12

2.1.2 Preparation of samples 12

2.2 Equipment 13

2.3 Analytical Methods 14

2.3.1 Gamma spectrum analysis 14

CHAPTER 3 18

RESULTS AND DISCUSSION 18

3.1 Establishment of energy calibration and efficiency calibration curve 18

3.1.1 Establishment of energy calibration 18

3.1.2 Establishment of efficiency calibration curve 19

3.2 Results of the activity concentration in plant samples 22

3.2.1 Calculation of Radionuclides 22

3.1.2 Results and Discussion 26

Conclusion 30

REFERENCES 31

APPENDICES 33

Determination of the activity concentration of plant samples by gamma-ray

spectrometry

Student: Nguyen Thi Diem

Faculty: Physics

Supervisors: Bui Van Loat

Vu Thi Kim Duyen

Student ID:13000151 Course: QH.2013.T.CQ

Abstract

This study is aimed at the determination of contamination of natural

radionuclides such as 226Ra, 232Th, 40K in plants This present work determined of

activity concentration of radionuclides in plant samples (rice, bean, corn, potato),

which was collected at Ha Noi, Hai Duong-Viet Nam, and Laos The samples were

dried, sealed and kept in a cylindrical container and stored for a period of 30 days

They were counted and quantified using high purity germanium (HPGe) detector to

analyze spectrometer at respective progeny energy then calculated activity

concentation of plant samples in analytical areas Radionuclides in analytical

samples observed include 226Ra, 232Th, 40K The activity concentration of these

radionuclides was found in the following ranges: 0.21 to 2.47; 0.19 to 1.98; 27.66 to

382.87 (Bq/kg) respectively The data is discussed and compared with those given

in another study such as USA, Iran, Southern Serbia

Keywords: Natural radionuclides, activity concentration, HPGe detector,

40

K, 226Ra, 232Th

Introduction

Radioactivity has always existed in the environment The radioactive sources in the environment can be divided into two main categories: Natural radioactivity include radioactive isotopes from Earth’s surface or the primordial radioactive isotopes ( the radioactive isotopes of decay chains of 40K, 238U, 232Th) and

cosmogenic radioisotopes which produced as a result of the interaction of cosmic rays with the Earth’s material; artificial radioactive isotopes (137

Cs) which produced

by man-made such as medical and industrial uses of radioisotopes, nuclear testing weapon, nuclear accidents, the operation of nuclear power plants and mining… The radioisotopes in upper layers of the atmosphere have polluted the Earth The process depends on the meteor, the climate, the geochemist Besides, the other man-made also makes the change of distributing of radioisotopes as by- produced from nuclear fuel cycle and other from mining, fuel enrichment, fuel airborne

particles may be intercepted by plants or return to the top soil These radioisotopes can be transferred to human through the food chain, so it caused danger to their health There are two ways to absorb radioactive into the plant,which is: deposition

on leaves and fruit and deposition onto soil and uptake by plants through the roots Evaluation of the process radioisotopes moves from soil to plants is very

importance Internal exposure and external exposure in the human body may grow

up the probability of induced cancer and various radiation-induced problems in the body of human and may be detrimental to the whole population In particular, this study can estimate of radioisotopes in Ha Noi’s surface and compare it with near areas, such as Hai Duong province and Laos

The content of the thesis includes three chapters:

Chapter 1: Literature Reviews

Chapter 2: Material and Methods

Chapter 3: Results and Discussion

CHAPTER 1 LITERATURE REVIEWS 1.1 Phenomenon and the radioactive decay law

1.1.1 The radioactive decay law

The radioactive atoms in a radioactive substance decay according to a

random process The probability of a nucleus decaying in a time is independent of

time It was noted three years after the discovery of radioactive that decay rate of a

pure radioactive substance decreases in time according to an exponential law which

is called the Radioactive decay law [6] In fact, the radioactive decay law transfers

the nucleus unstable into another nucleus by emitting alpha, beta, gamma ray If no

new nuclide are introduced into a given radioactive substance, this law predicts how

the number of radioactive nuclide which are present at time t decreases with time The number dN, decaying in a time interval dt is proportional to N, and so:

where λ is the decay constant which equals the probability per unit time for decay of

an atom From equation (1.1), so:

where N, represents the original number of nuclide present at t=0 The half-life is

the time requires for one-half of the original nuclide to decay, denoted by the

symbol T1/2 Putting N=No/2, it follows that:

where the mean lifetime is the average time that a nucleus is likely to survive before

it decays and equals 1/λ, the reciprocal of the decay constant The activity, A is the

rate at which decays occur in a sample and can be obtained by differentiating

equation (1.2) From (1.2) we have (1.4) equation:

-λt o

where A o N0is the initial activity at t=0, A is the activity at time t [6]

Fig.1.2 Schematic of the exponential decay of activity for 210 Pb [6]

1.1.2 The radioactive decay chains

N1(t) is the number of nuclide of the original radioactivity (the mother) and

λ1 is its decay constant N2(t) is the number of nuclide of the radioactive product (the daughter) and λ2 is its decay constant The radioactive decay chain was described by two equation:

By integrating equation (1.10) and (1.11) and using the initial condition

N2(0)=0 the following results are obtained [1]:

1 2

10 1 2

1 Singly Occurring Radionuclides

About 20 naturally occurring single primordial radionuclides have been identified Most are radioactive isotopes with half-life are more 1010 and usually around 1015 years [6]

Potassium is commonly distributed in the Earth’s crust 40K has half-life of 1.277 109 yr 40K decays through decay to stable 40Ca 80% of the time The remaining 10.72% of 40K undergoes decay by electron capute to stable 40Ar This latter decay branch also emits a characteristic gamma-ray at 1.461 MeV The mean activity concentration of 40K found in the crustal rock is about 0.62 Bq/g, Soil have lower concentration of 40K activity, with the mean found to be around 440 Bq/kg The concentration of potassium in sea water is also significant, averaging about 11 Bq/l [6]

2 Decay Chain

There are three main decay series These are the natural decay chains heads

by 238U (4,5 billion year half-life), 232Th (14,1 billion year half-life), and 235U (700 million year half-life) respectively These each then decays through complex decay chains of alpha and beta decays and end at the stable 208Pb, 207Pb, 206Pb nuclides respectively [6]

U, 235U and 232Th are the parents of three natural decay series, called the uranium series (238U), the actinium series (235U) and the thorium (232Th) series, respectively Natural uranium is a composite of the isotopes 238U (99,28%), 234U (0,0057%) and 235U (0,72%) The decays chain of 238U includes 8 alpha decays and

6 beta decays respectively The decays chain of 235U includes 7 alpha decays and 4 beta decays respectively Natural thorium has only one primordial isotope that of 232

Th having a natural isotopic ratio of 100% The decays chain of 232Th includes 6 alpha decays and 4 beta decays respectively [7] Besides, 238U, 235U, 232Th include Radon (Rn) which is gas-radiation

Fig.1.2 Radioactive decay chains of 232 Th [18]

Fig.1.3 Radioactive decay chains of 238 U [18]

Fig.1.4 Radioactive decay chains of 235 U [18]

1.2.2 Cosmogenic radioisotopes

Cosmic radiation permeates all of space, the sources, the sources being primarily outside of our solar system The radiation is in many forms, from high- speed heavy particles to high energy photons and muons The upper atmosphere interacts with many of the cosmic radiation, and produces radioactive nuclides Here is a table with some common cosmogenic nuclides: [19]

Table 2.1 Some common cosmogenic nuclides [19]

Activity

Carbon 14 14C 5730 years Cosmic-ray interactions 14

N(n,p)14C

0.22 (Bq/kg)

Hydrogen 3 3H 12.3 years

Cosmic-ray interactions with N and O, spallation from cosmic-rays,

6Li(n, )3

H

1.2 10-3(Bq/kg)

Beryllium 7 7Be 53.28 days Cosmic-ray

interaction with N and O

0.01 (Bq/kg)

1.3 Artificial radioisotopes

The artificial radioactive isotopes were made from the activity of human such as mining, weapon testing, nuclear accidents, the operation of nuclear power plants… Table 1.2 presents a few human produced or enhance nuclides

Table 1.2 Some Man-made radionuclides [19]

of thyroid problems

Iodine 129 129I 1.57 107 years

Fission product produces from weapons testing and fission reactors

Cesium 137 137Cs 30.17 years

Fission product produced from weapons testing and fission reactors

Strontium 90 90Sr 28.79 years

Fission product produced from weapons testing and fission reactors

Pu+

1.4 Transfer of the radioactive isotopes from soil into plant

The main radioactive isotopes released into the environment by human activities such as nuclear weapon testing or detonation; the nuclear fuel cycle, including the mining and production of nuclear materials for use nuclear power plants or nuclear bombs; accidental release of radioactive material from nuclear

power plants [8] After radioactivity released into the environment, radioactive nuclides formed clouds that moved across the global world and settled down as radioactive fallout Radioactive fallout contaminated the entire environment Besides, Radioactivity concentrated in water then transferred radioactive isotopes into soil and plant system These radioactive elements are concentrated mostly in the surface layers of soil The low mobility of radioactive elements in soil holds them in the root zone Plants assimilate the radioactive substances with others necessary for their growth, then dangerous isotopes may get into animal tissues and finally as food into organisms of human beings [9] In here, This radioactive contamination formed internal dose through ingestion or external exposure then it pervaded into the human body, so it caused danger to human’s health

This model is “ Major pathways of radionuclides to man in the event of an uncontrolled release of radioactivity."

Fig.1.5 The pathway of radionuclides to man [10]

1.5 Characteristic of radioactive concentration in plant samples

1.5.1 The original of radionuclides in plant samples

Naturally radioactive isotopes exist widely spread in the earth’s environment such as soil, water, air, or human body.Gamma radiations emitted from naturally occurring radioactive materials such as uranium-238 (238U), thorium-232 (232Th) and potassium-40 (40K) are generally known as terrestrial background radiation [11] Besides, Radioactive isotopes which produced as a result of the interaction of cosmic rays with the Earth’s materials such as 14

C called cosmogenic radioactive isotopes After production in the upper atmosphere, 14C combined with oxygen to form carbon dioxide, CO2 Plants absorbed 14CO2 through photosynthesis [2]

Nowadays, Artificial radioactivity was formed by human-made from medicine radiation (X-ray), accident nuclear reactor, mining, fuel enrichment… The process of forming radioactive elements (natural or artificial) takes place in the crust of the earth where the radioactive decay of the original radioactive nucleus occurs in the soil or radioactive fallout into the air [3]

Since the growing of plant system related to environmental conditions (soil, water, air), Plants contain an amount of radioactivity (natural radioactivity or artificial radioactivity) Plants contacts directly with waste radioactivity in air Besides, it is absorbed radioactivity contamination from soils and water through root system Nowadays, In some places, soil was contaminated by artificial radioactivity then vegetables were grown in this soil, so it became food chain of human [3]

1.5.2 The radioactive isotope of plant samples

Vegetables and fruits have amount of radioactive element Its concentration depends on different factors such as radioactive soil system, water, air pollution, the level of radioactive uptake of each species plants The results of scientific research use to analysis the component, concentration, the property of radioactive elements

in plant samples: fruit, vegetable, plants contain heavy metal or elements emit gamma radiation [3,9] The vegetable, fruits, pea, rice, the plant has grown up from sugar, coffee, flour… in Earth’s surface, containing natural radioactivity, such as

232Th, 238U, 210Pb, 226Ra, 228Ra Eating vegetables can seriously damage health because of the consumption amount of natural radioactive elements equal to half-life long 14,5µSv [3,14]

CHAPTER 2 MATERIALS AND EXPERIMENTAL METHODS 2.1 Collecting samples

2.1.1 Sampling stations

Samples were collected and set the characteristic symbol It follows that by table 2.1:

Table 2.1 Sampling stations

Symbol Sample Location of study

M1 Ordinary rice An Phu- Hoa Phu- Ung Hoa- Ha Noi - VN

M2 Sticky rice An Phu- Hoa Phu- Ung Hoa- Ha Noi -VN

K1 Potato Co Phap- Cong Hoa- Nam Sach- Hai Duong - VN K2 Potato An Phu- Hoa Phu- Ung Hoa- Ha Noi - VN

M4 Corn An Phu- Hoa Phu- Ung Hoa- Ha Noi - VN

M5 Bean An Phu- Hoa Phu- Ung Hoa- Ha Noi - VN

G3 Sticky rice Co Phap- Cong Hoa- Nam Sach- Hai Duong - VN G2 Ordinary rice Co Phap- Cong Hoa- Nam Sach- Hai Duong - VN

2.1.2 Preparation of samples

Samples were cleaned by water for removing sands and/ or soil, dried in air, and weighed for determining the corresponding fresh mass in kilogram After that, they were oven-dried at approximately 105 C until a constant weight was reached The dried samples were then crushed A portion of each dried sample was taken at random, weighed, sealed, and kept in a cylindrical plastic container in high of 3cm, the geometrical dimensions of the samples was kept identically [12] The prepared samples were stored for a period more than 30 days The activity concentration of each sample was measured using an HPGe Detector to analyze samples

2.2 Equipment

-Low level gamma spectrometer (CANBERRA) using the ultrapure semiconductor detector (HPGe) with the relative efficiencies of 15%, the energy resolution of 1332 keV,the peak of 60Co of 1.66 keV connecting to the lead box for reducing gamma radiation background and meeting environment standards to below 0.9 pulse/seconds in the energy region from 100 keV to 3000 KeV [4]

- FH40-F2 dose rate meter with the measurement range from 0.1 Sv/h to 0.99 mSv/h in the energy range from 45 keV to 1.3 MeV [4]

- An analytical balance is 0.1 mg accuracy [4]

Fig.2.1 The diagram of gamma -ray spectrometry system [2]

2.3 Analytical Methods

2.3.1 Gamma spectrum analysis

The main purpose of the gamma spectrum analysis is to determine energy and area of the peak of spectrometry as the basic for element identification and determination of radioactivity The gamma spectrum recorded consist of a number

of peak on a background The best important peak is photoelectric peak This peak

is the result of interaction of gamma radiation with the detector’s material through photoelectric effect The result of the interaction process is that the full energy of gamma radiation is released in the volume of the detector

In the gamma radiation, the peak position corresponding to the energy of gamma ray and activity are determined by the area of the peak With gamma ray greater than 1022keV appear backscatter peaks about 200:300 keV, 511 keV peak, (Eγ 511 keV) peak, (Eγ 1022 keV) peak The large size detector sill appears additional peak of the total of two gamma radiation cascade This peak of energy make gamma spectrometry become complex and some case can interfere with each other

The background in gamma spectrum of the contribution of the Compton scattering takes place in the detector and in the natural radiation from the detector’s material, the soil, all around the detector and from cosmic ray In many cases, The background has a great influence on gamma spectrum quality, so shielding is very necessary Normally people use lead as a shielding material to limit the natural radiation background

In the experiment, The energy of gamma radiation corresponding to photoelectric peak is determined by energy calibration Radioactivity is determined based on the peak which subtracted background at full-absorption peak of the characteristic radiation The result of the measurement area of spectral peaks impact

on the result of determination the half-life of radioactive isotopes To determine of spectral peaks, there are is two methods: number method and joint method

Nowadays, most of the amplitude analysis of spectrum are done with the help of computer programs This program was set based on private computer and it can both record and dispose of spectroscopy Spectrum analysis can be partially automated once process after it had been performed such as setting parameters,

energy calibration, resolution, efficiency However, In many cases, it is necessary

to have direct interventions such as to detect spectral irregularities, to decide which spectral region or spectral to be handled, for overlap that need special treatment From these reason that the spectrum analysis programs are separate steps with many options for the flexible program, compatible with all the most of the request in setting to record and analyze gamma spectrometry The spectrum analysis is which uses computer programs can identify and hand all most peak with good quality.The collecting data provide complete information about gamma spectrum such as position, energy, resolution, the background counts corresponding with error, information about dead time, the parameter of energy calibration, efficiency calibration… In automated gamma-ray processing programs that contain radioactive isotopes, it is possible to directly identify and calculate the activity of radioactive isotopes from gamma spectrum [2,3]

2.3.2 Determination of radioactivity:

In the case, daughter nuclide was made up of the excited state, it releases energy in the form of characteristic radiation to return a lower excited state or basic state

The probability of nuclide at excited state has high energy Ei which transfer gamma into low energy state Ej depends on the quantum state of the first and last state

The Count rate of characteristic gamma radiation of energy Eγ decay from the sample in the unit time:

where: nγ is count rate of characteristic gamma ray

H is radioactivity of sample

Iγ is the intensity of energy Eγ

With certain gamma ray energy, Iγ known, when we determine the number

of gamma ray of energy Eγ which decays in the unit time, we will know the activity

of radioactivity H We determine nγ based on the area at full-absorption peaks