MINISTRY OF EDUCATION VIETNAM ACADEMY
AND TRAINING OF SCIENCE AND TECHNOLOGY
GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY
…………***…………
HOANG HUU DUC
STUDY ON THE TRANSPORTATION OF SOME NATURAL
AND MANUFACTURING RADIATION UNITS FROM SOIL TO
VEGETABLES AND TEA BY MODERN NUCLEAR
TECHNIQUES
Major: Atomic and nuclear physics
Code: 9440106
SUMMARY OF SUBSTANCES SCIENCE DOCTORAL THESIS
Hanoi - 2021
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ABSTRACT
1. The urgency of the thesis
Human beings are inevitably exposed to radiation from naturally occurring radioactive materials including 7Be, 40K, 238U, ... and man- made fallout from nuclear acidentes and nuclear weapons testing or
by everyday life activities such as cesium or iodine which can be
entered into terrestrial environment and absorbed by plant. These
radioisope is then able to be go into human body through the food
chain and inducing bad affect to human health.
In order to evaluate the transfer of radionuclides from soil to
plant, the transfer factor (TF), which is defined as ratio of the
concentration of radionuclide in dried plant and soil where plant is
grown (Bq/kg dried soil or plant) is widely used.
In past 50 years, IAEA has supported member states in study of
soil-plant transfer of the radionuclides and many experimental data
have been collected. However, only some soil-plant combination in
some geographical areas in the world is investigated.
In Vietnam, there were several researches about this topic in recente
years but it is insufficient.
In this thesis, soil-plant transfer of natural occurred radioactive
isotopes 238U, 232Th and 40K were selected to study.
2. Aim of the thesis
- Development of a new method for determining concentration of the
radionuclides in enviromental samples by mean of gamma spectroscopy.
- Establishing a data set of the soil-plant transfer factor (TF) of radionuclides 238U, 232Th and 40K for some vegetables in Truong Sa island and that of 238U, 232Th, 40K and 137Cs for tea tree from Luong
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My farm, Hoa Binh province, Vietnam and evaluate the influence of
natural radionuclide concentration to the soil-plant transfer process of artificial radionuclide 137Cs.
3. Main research content of thesis
- General overview of the environmental radioactivity, soi-plant
transfer process of the radionuclides and research objectives together
with areas of interest.
- Development of a new method for calculation of concentration
of the radionuclide in environmetal sample by using gamma
spectroscopy with HPGe detector and based on the internal
efficiency curve and one absolute efficiency point.
- Methods and techniques used in gamma spectrum analysis and some
advanced corrections to improve accuracy of gamma spectrum analysis.
- Data analysis and calculation the soi-plant TF for some of the
selected plant. Evaluation of the mutual influence between the
radionuclides in their soil-plant transfer process.
CHAPTER 1. OVERVIEW OF THE ENVIRONMENTAL
RADIOACTIVITY AND SOIL-PLANT TRANSFER
1.1. Natural radioisotopes
1.1.1. Single natural radioisotopes
Single natural radioisotopes include: (1) products of nuclear reaction
between cosmic rays and elements in atmosphere and (2) primordial
ones having long-enough half-life which is not belonging to three natural decay series 238U, 235U and 232Th.
1.1.2. Natural decay series
As mentioned, there are three natural decay series which are so
called uranium, thorium and actinium respectively. Uranium series is
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started by the 238U and ended by the stable isotope 206Pb. Thorium series is started by 232Th and ended by stable isotope 208Pb while that for actinium are 235U and 207Pb respectively.
1.2. Artificial radionuclides
Up to now, there are about 350 artificial radionuclides discovered in the environment. However, there are mainly 137Cs, 90Sr, 14C, 3H and isotopes of Pu existed in the environment now.
1.3. Characteristics and distribution of the radionuclides in the
terrestrial environment
1.3.1. Individual characteristics of radionuclides in relation with
enviroment pollution
Main characteristics to evalulate the impact of the radionuclides to
environment are their half-life, activity and kind of the radiation emitted.
1.3.2. Physical and chemical characteristics and distribution of 238U, 232Th, 40K and 137Cs in terrestrial environment
The change in the status of radioactive isotopes in soil depends on
the physical, chemical properties of the soil, the soil biota and other
environmental factors. Cs can be strongly absorbed in clay contained
soil, especially in mica-like type. For the case of uranium, its mobility,
bioavailability and toxicity are governed by factors as chemical form,
soil dynamics. While, in most terrestrial environment, Th exist in oxidation state 4+ and 6+. K+ is a main nutrient of the plant and it apprears in almost all soil kind. Many studies show that isotopes of U,
Th and K distribute quite eventually in soid layers, while Cs is mainly
concentrated in the soil layer 0 - 30 cm.
1.4. The absorption of radionuclides by plant
For terrestrial ecosystem, absorption of the mineral as well as the
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radionuclides by plant is mainly through their root.
1.4.1. Transfer factor (TF)
Transfer fator TF is defined as: TF = Cthv/Cd (1.3)
Where, Cthv is the acivity of radionuclides in plant (Bq/kg dried
plant) and Cd is that in soil (Bq/kg dried soil).
1.4.2. The absorption 238U by the plant
The absorption uranium by the plant is usually stronger than that for
thorium and polonium and it has tendency to accumulate in leaves,
especially in the plant with leaves for consuming. As in report of IUR
(International Union of Radioecology) (1994), TF for uranium in some
grass in Mediterranean area is in range of 0,0023 to 0,23. Many studies
show a positive correlation between U with P and Ca in leafy vegetables.
1.4.3. The absorption 232Th by the plant
Thorium is considered to be immobile in the soil and it sticks
steadily in the soid hence it is difficult for plant to absorb although
Thorium is high in soil. The average TF for thorium in thyme in area
around a thorium mine is 0,011 and it varies from 0,0011 to 0,11.
1.4.4. The absorption 232Th by the plant
The absorption 40K is not the same for the different plants. TF for vegetation at some areas in Atlantic is between 0,9 and 13.8. Generally, TF of 40K varies in wide range and this value depends on plants and concentration of 40K in the soil.
1.4.5. The absorption 137Cs by the plant
The process of transporting Cs from soil to plants is a complex
process and involves many parameters. Clay and organic matter play a
significant role in the retention or mobilization of caesium within the
soil as seen in the previous section and will obviously affect the
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availability of caesium in the soil and therefore it’s transfer into vegetation. In 1989, IUR reported that TF for grass grown in clay (loam, pH 6), sand (pH 5) and peat is 1,1.10-1; 2,4.10-1 and 5,3.10-1 respectively, showing that clay and organic matter affect TF. The TF value for clay is 1,1.10-1, while grass grown on organic soils (peat) has a much higher TF coefficient of 5,3.10-1.
Many studies show that there is a competition in absorption 137Cs and 40K of the plant. In order to evaluate this competition, discrimination factor was proposed:
DF = (CCs-thv/CK-thv)/(CCs-đ/CK-đ) (1.4)
Here CCs-thv, CK-thv are concentration of Cs and K in the plant; CCs-đ,
CK-đ are concentration of Cs and K in the soil.
The case of DF <1 means absorption K by plant is higher than Cs.
As can be seen, TF varies in wide range and it depends on many
factors. Evaluation the soil-plant transfer needs to base on four main
factor as: (1) Concentration of the radionuclides in the soil; (2) Chemical
and physical properties of the soil; (3) kind of plant itself; (4) Nutrient
status of the soil.
CHAPTER 2. EXPERIMENTAL TECHNIQUE
AND METHOD
2.1. Gamma spectroscopy method in analysis concentration of
the radionuclides in environmental samples.
2.1.1. Direct comparison method
AT, the activity of radioisotope of interest-T in the analyzed
sample is then calculated with the following formula:
AT = (nj/njref)ATref (2.1)
Where nj and njref are the count rates of the gamma-ray emitted
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by radionuclide T with energy Ej in the analyzed sample and the
reference material, respectively.
2.1.2. The method of building full-energy peak efficiency curve
(FEPEC)
The ε(E) curve can be built by experiment or simulation by
following:
ε(Ej) = εref(Ej).C(Ej) (2.2a)
The following equation gives us AT:
AT=(nj/Iγj)/ε(Ej) (2.2b)
Where: C(Ej) = Cs(Ej)/Cref(Ej) is the relative selfabsorption correction
factor; Cs(Ej) và Cref(Ej) are self-absorption correction factors for
efficiencies of the analyzed sample and the reference material,
respectively; Iγj is the emission probability which is well-known, εref(Ej)
is the absolute efficiency of detector system at energy Ej.
2.2. The method used intrinsic (relative) efficiency curve and an
absolute efficiency value
Assuming multi-gamma-ray emitter T in analyzed sample emits
several measurable gamma-lines so that from the same gamma
spectrum, one can easily calculate a series of FT(Ej) = nj/Iγj ratios
(namely n1/Iγ1, n2/Iγ2, n3/Iγ3 …). By least-squares fitting dataset FT(Ej)
to a suitable mathematical function, the fitted FT(E) function is
obtained. Then, Eq. (2.2b) is re-written as: FT(E) = ε(E).AT (2.3)
Then, AT can be determined by knowing only one value of fullenergy peak efficiency at a certain energy E*: AT = FT(E)/ε* (2.4) Where 𝜀∗ = 𝜀(𝐸∗), E* should be within the energy range of nuclide T. In this method, the 1460,83 keV gamma-ray of 40K in the reference material RGK-1 was used to determined ε* according to Eq. (2.2a).
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Assuming that the analyzed sample contains another radionuclide
(named Y) beside T. Let us call FY(E) the intrinsic efficiency function
constructed by using gamma-lines from Y. Similarly, activity AY can
be estimated from Eq. (2.4) as the case of AT. Moreover, FY(E) must
be proportional to FT(E). Then, one has: we have:
FY(E) = (AY/AT).FT(E) = R.FT(E) (2.5)
Where R = AY/AT is constant for a given sample, which can be
derived from the measurement. It means that from Eq. (2.5), the activity
AY can also be estimated from AT and R by the following equation:
AY = R.AT = (FY(E)/FT(E)).AT (2.6)
In summary, the activity of radioisotopes in the sample analyzed is
determined by this method using the following steps:
Step 1: Determine the internal magnetic efficiency curve of
radioisotope T.
Step 2: Determine the absolute efficiency, eref at the energy peak
1460.83 keV using the RGK-1 standard sample.
Step 3: Calculate the relative self-absorption coefficient C, at the
energy level 1460.83 keV. This step can be omitted in certain cases (see
also section 2.3.2.2).
Step 4: Determine the activity of radioactive isotopes in the
environmental sample determined using the formulas (2.4) - (2.6).
2.3. Data analysis and advanced correction to improve precision
of analysis method
2.3.1. Gamma spectrum analysis
In this thesis, ROOT toolkit, CERN was used to analyse the
gamma spectrum.
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2.3.1.1. Background subtraction
In the ROOT toolkit, in order to substract the spectrum
background, zero-area digital filter algorithm is applied. Detail of
this algorithm could be found in the thesis.
2.3.1.2. Method for automatic photo-peak search
In order to search a peak in the gamma spectrum, besides the
popular used method, a special algorithm was implemented into ROOT,
which is so-called “quantum mechanics” algorithm. It was proven that
this new algorithm is very effective in searching a low statistic peak.
2.3.1.3. Peak deconvolution
For deconvolution superimposed peaks in gamma spectrum,
algorithms GOLD and Richardson-Lucy deconvolution is applied in
ROOT toolkit.
2.3.2. Some advanced corrections for improving accuracy of the
analysis Radioisotopes in environmental samples usually have low activity. As a result, environmental samples are usually large in size and are
usually placed close to the detector during measurement. Therefore,
the analysis results are influenced by errors due to true summing
coincidence and self-absorption effect.
In this thesis, in order to evaluate the uncertainty by true summing
coincidence, we based on the internal efficiency curve, while seft-
absorption correction could be ignored due to the advantage of our-
own developed method which based on internal efficiency curve and one absolute efficiency point at 1460.81 keV of 40K by which self- absorption is negligible.
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2.4. Uncertainty evaluation
This section introduces confidence limits of results, formulas for
error propagation, and fitting error.
2.5. Calculation errors in determination of radionuclide
concentration in environmental samples using gamma
spectroscopy toghether with our developed method
This section presents the formulas for error of fitting the internal
efficiency curve F(E), error in determination the efficiency ε(1460,83
keV) and errors in calculation activity AT.
2.6. Research object and sampling method
2.6.1. Research object
2.6.2. Research location
In this thesis, spinach and cabbage grew 05 floating islands belonging
to Truong Sa island and tea tree from Luong My Farm (Hoa Binh)
were selected to study.
2.6.3. Sampling
Sample collection is carried out as regulation of the laboratory
(Military Insititute for Chemistry and Environment). For tea tree at
Luong My farm, soil samples were collected by the depth layers
which serves for evaluation the depth distribution of the radionuclide
of interest.
2.6.4. Sample processing and analysis
Collected samples after processing were measured by gamma
spectroscopy system with HPGe detector at Centre of Nuclear
Physis, Institute of Physics and Military Insititute for Chemistry and
Environment.
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2.6.5. Data analysis
Data processing methods in the thesis include: calculating the
sample mean, standard deviation, standard error, estimating
confidence interval for expectation, comparing two average values,
comparing UWL, LWL, UCL, LCL, correlation coefficient Spearman.
CHAPTER 3. VALIDATION OF THE NEW DEVELOPED
METHOD
3.1. Experiment for verifying the method used internal efficiency
curve and one absolute efficiency point
3.1.1. Experimental arrangement
Sample for validation experiment is prepared from reference
materials of IAEA, which are: Sample-01 (RGTh-1); Sample-02
(mixture of RGU-1, RGTh-1 and RGK-1) and Sample-Ref that is
used to determine efficiency at 1480,63 keV. These samples were
measured by high-resolution HPGe gamma-ray spectroscopy system
(symbol GEM20P4-70, produced by Ortec-AMETEK) at the Center
for Nuclear Physics, Institute of Physics.
3.1.2. Data analysis and results
3.1.2.1. Determination of the absolute efficiency and self-absorption correction for 1460.83 keV characteristic gamma ray of 40K
The result was found to be εref (1460,83 keV) = (0,532 ± 0,003)%
(Fig 3.6).
The relative self-absorption correction factors C(1460.83 keV) =
1.001 ± 0.001 was found between Sample-1 and Sample-Ref, while
C(1460.83 keV) = 1.003 ± 0.001 was for (Sample-2, Sample-Ref)
pair (Fig 3.7)
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3.1.2.2. Gamma-rays of
226Ra and 228Th decay series for
constructing FT(E)
Within the scope of this thesis, The following mathematical form
is proposed to describe FT(E) functions:
𝐹𝑇(𝐸) = 𝐸𝑥𝑝(𝑝0 + 𝑝1 ln(𝐸) + 𝑝2 ln(𝐸)2 + 𝑝3 ln(𝐸)3)
(3.3)
3.1.2.3. Calculate activity of radionuclides in Sample-1 Activity of 228Th
The FTh228(E) was obtained by fitting to the nj/Iγj ratios at 238.832 keV of 212Pb, 1620.74 keV of 212Bi and 583.191, 860.564, 2614.533 keV of 208Tl (Fig 3.1). The result ATh228 = APb212 = ABi212 = ATl208∕ is determined by the Eq. (2.4) and shonw in theTable 3.2 Activity of 228Ra and 226Ra
The four measurable gamma-rays of 228Ac were 338.32, 794.947, 911.204, 968.971 keV. It is easily seen that 1460.83 keV is far away
out of this energy range. In this case, Eq. (2.5) was used and the
FRa228(E) is proposed as following:
𝐹𝑅𝑎228(𝐸) = 𝑝4𝐸𝑥𝑝(15,1 − 3,9 ln(𝐸) + 0,393 ln(𝐸)2 − 0,0166ln(𝐸)3)
(3.4)
The parameters p4 (Fig 3.2) is equal to R in Eq. (2.5), hence from
Eq. (2.6), ARa228 is determined as: ARa228=p4ATh228 (3.5)
FRa226(E) (Fig 3.3) and ARa226 in Sample-1 would be determined
exactly in the same way as FRa228(E) và ARa228. The results for ARa226 and
ARa228 in Sample-1 are listed in Table 2.
Table 3.2. Results for Sample-1. All the results were expressed with
95% confidence intervals
Radionuclide
228Th = 212Pb = 212Bi = 208Tl/p 228Ra = 228Ac 226Ra = 214Pb = 214Bi
Activity (this work) (Bq/kg) 3236 ± 76 3268 ± 77 76,00 ± 1,78
Activity (IAEA) (Bq/kg) 3250 ± 88 3250 ± 88 78,00 ± 6
Deviation (%) - 0,43 0,55 - 1,6
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Compared to data of IAEA, our results gave a good agreement with
the errors is less than 2%.
Fig. 3.1. The intrinsic efficiency curve of 228Th in Sample-1
Fig. 3.2. The intrinsic efficiency curve of 228Ra in Sample-1
Fig. 3.3. The intrinsic efficiency curve of 226Ra in Sample-1
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3.1.2.4. Calculate the activity of radionuclides in Sample-2
In the gamma-spectrum of Sample-2, only 226Ra had enough number of photo-peaks which had high statistic for building FT(E)
(Fig. 3.4). The activities ATh228 and ARa228 were determined through
FRa226(E) by using Eq. 2.6. Table 3.3 listed the results in Sample-2.
The results were in good agreement with data of IAEA.
Table 3.3. Results for Sample-2. All the results were expressed
with 95% confidence intervals.
Radionuclide
Activity (this work) (Bq/kg)
Activity (IAEA) (Bq/kg)
Deviation (%)
216,2 ± 8,4
218,5 ± 6,0
- 1,05
228Th = 212Pb = 212Bi = 208Tl/p 228Ra = 228Ac 226Ra = 214Pb = 214Bi
218,5 ± 6,0 329,3 ± 2,0
- 0,14 0,15
218,2 ± 8,5 329,8 ± 6,4
Fig. 3.4. The intrinsic efficiency curve of 226Ra in Sample-2
3.1.3. Monte-Carlo simulation evaluates the influence of density and
chemical composition on fullenergy peak efficiency and relative self-
absorption correction factor at the energy of 1460.83 keV
A Geant4 code, was implemented for studying the variation of -
fullenergy peak efficiency on the chemical composition of
environmental samples and the density of RGK-1 reference material.
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The full-energy peak efficiency at 1460.83 keV for various environmental samples is within ± 1% of standard RGK-1 (Fig. 3.5).
Fig. 3.5. The full-energy peak efficiency at 1460.83 keV for various environmental samples In practical work, if samples having wide density range are analyzed, one can consider εref(1460,83 keV) in 3% region ( ≈ 0,8- 1,7 g/cm3) unchanged and use value εref(1460,83 keV) = (0,532 ± 0,003) for radioactivity calculation, as long as all samples have the
same shape with Sample-Ref (Fig. 3.6).
Fig. 3.6. The full-energy peak efficiency at 1460.83 keV, εref varies with the density of RGK-1
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Similarly, for Soil and Cellulose, there also exists a 3% region, in
which the relative absorption correction factor can be considered
unchanged and equal to 1 (see two dash lines in Fig. 3.7). The results
for Soil and Cellulose were obtained by using LabSOCS.
Fig. 3.7. The dependence of C on density for 7.6 cm × 3 cm cylindrical samples
3.1.4. Conclusion about method used internal efficiency curve and
one absolute efficiency point
The obtained validation results show that new developed method
is a good alternative one for determination of the concentration of
radionuclides in the environment samples with gamma spectroscopy.
3.1.5. Apply the updated method for soil samples
For a control check of the new method (described in Section 2.2)
we used five soil samples collected in Savannakhet, Laos.
3.1.5.1. Apply the updated method
In principle, a FT(E) function ought to be constructed for each
sample. But, these five soil samples can be considered to have the
same density and chemical components. Therefore, it is possible to
use one FT(E) function for all five samples. The gamma-spectrum of
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sample M125 was chosen to construct FT(E) function because it has
high statistics peaks. The function FRa226(E) for the sample M125 is
determined as follows (Fig. 3.8):
𝐹𝑅𝑎226(𝐸) = 𝐸𝑥𝑝(9,252 − 3,733. ln(𝐸) + 0,4014 ln(𝐸)2 − 0,01846 ln(𝐸)3)
(3.9)
Fig. 3.8. The intrinsic efficiency curve of 226Ra in sample M125 The results are listed in Tables 3.6 to 3.10.
3.1.5.2. Apply the FEPEC method
The full-energy peak efficiency curve was derived from gamma-
spectrum of RGU-1 (Fig 3.9). The results are listed in Tables 3.6 to 3.10.
Table 3.6. Results for M125. All the results were expressed with 95%
confidence intervals
Activity (Bq/kg)
Radionuclides
228Th = 212Pb = 212Bi = 208Tl/ 228Ra = 228Ac 226Ra = 214Pb = 214Bi
Updated method 25,2 ± 1,0 25,6 ± 1,0 23,8 ± 0,5
FEPEC method 25,6 ± 1,4 27,4 ± 1,6 23,6 ± 0,8
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Table 3.7. Results for M106. All the results were expressed with 95%
confidence intervals
Activity (Bq/kg)
Radionuclides
228Th = 212Pb = 212Bi = 208Tl/ 228Ra = 228Ac 226Ra = 214Pb = 214Bi
Updated method 11,9 ± 1,0 12,0 ± 1,1 14,8 ± 0,7
FEPEC method 12,3 ± 1,0 12,8 ± 1,1 14,7 ± 0,7
Table 3.8. Results for M108. All the results were expressed with 95%
confidence intervals
Activity (Bq/kg)
Radionuclides
228Th = 212Pb = 212Bi = 208Tl/ 228Ra = 228Ac 226Ra = 214Pb = 214Bi
Updated method 7,2 ± 0,7 12,2 ± 0,8 13,2 ± 0,5
FEPEC method 7,4 ± 0,7 13,0 ± 0,8 13,2 ± 0,5
Table 3.9. Results for M122. All the results were expressed with 95%
confidence intervals
Activity (Bq/kg)
Radionuclides
228Th = 212Pb = 212Bi = 208Tl/ 228Ra = 228Ac 226Ra = 214Pb = 214Bi
Updated method 29,7 ± 2,0 32,9 ± 2,3 27,7 ± 1,2
FEPEC method 30,5 ± 2,0 35,0 ± 2,3 27,6 ± 1,1
Table 3.10. Results for M134. All the results were expressed with
95% confidence intervals
Activity (Bq/kg)
Radionuclides
228Th = 212Pb = 212Bi = 208Tl/ 228Ra = 228Ac 226Ra = 214Pb = 214Bi
Updated method 18,0 ± 1,4 18,8 ± 1,6 18,7 ± 0,8
FEPEC method 18,5 ± 1,5 20,0 ± 1,6 18,6 ± 0,8
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Fig 3.9. The full-energy peak efficiency curve was derived from gamma-spectrum of RGU-1 for high-resolution HPGe gamma-ray spectroscopy system at Center for Nuclear physics
The results obtained from the two methods are almost similar. It
can be concluded that the method used internal efficiency curve and
one absolute efficiency point are reliable alternatives.
3.2. Study and evaluation the transfer of radionuclide from soil
to vegetable in Truong Sa island
These samples were measured by high-resolution HPGe gamma-
ray spectroscopy system (symbol PGC1518, produced by Canbbera)
at the Military Insititute for Chemistry and Environment. From the
statistical analysis data, evaluation the soil-vegetable transfer of
radionuclides in Truong Sa island will only focus on two main
factors: (1) concentration of radionuclides in the soil and (2) kind of
vegetable. In this study, full-energy peak efficiency curve method
was used.
3.2.1. Transfer factor for the 238U
Concentration of 238U in soils for growing spinach and cabbage is not very different from each other. However, concentration of 238U in
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the spinach is 3 to 4 times higher than that in the cabbage. It means
that ability for absorption of the radionuclide in different plants is
very different. TF(U/Spinach) is in range of (1,31 - 3,33) and
TF(U/Cabbage) is in range (0,39 - 0,90).
By considering the correlation coefficient Spearman, we see that when concentration of 238U in the soil increase then absorption ability of the vegetable have tendency to decrease.
3.2.2. Transfer factor for 232Th Concentration of 232Th in soil for the cabbage is quite similar to that for the spinach. While average concentration of 232Th in the spinach is 2,5 to 3 times higher than that in the cabbage. The result show that average concentration of 238U and 232Th in the soil on the islands is quite similar. However, these two vegetables absorb 238U better than 232Th. In the spinach, concentration of the 238U is two times higher than concentration of 232Th and this ratio for cabbage is 1,5. This phenomenon was also observed in the experiment
of Mortvedt. TF(Th/Spinach) is in range of (0,52-1,92) and
TF(Th/Cabbage) is in range of (0,38-0,58).
By considering the correlation coefficient Spearman, we see that when concentration of 232Th in the soil increase then absorption ability of the vegetable have tendency to decrease.
3.2.3. Transfer factor of 40K The average concentration of 40K in the spinach is 1,5 to 2 times higher than that in the cabbage. TF(K/Spinach) is varies from 1,07 to
2,18 and TF(K/Cabbage) is in range of 0,64 to 1,26. Contra-variant
relation between C(K) and TF(K/Cabbage), TF(K/Spinach) and
covariate relation between C(K) and C(K/Cabbage), C(C/Spinach) were
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observed. Despite potassium is an essential nutrient for the plant but it
only absorbs a sufficient amount for its growth. As concentration of
potassium in the soil is getting higher the absorption of potassium by the
plant is getting lower.
3.2.4. Summary for research results in Truong Sa islands
232Th.
- Both vegetable spinach and cabbage absorb 238U better than
- Absorption ability of different plants is different (the spinach
absorb radionuclide in the soil better than the cabbage).
- Correlation between concentration of the radionuclides in the
soil and in the plant is covariate and the correlation between
concentration of the radionuclide in the soil and TF is contra-variant.
It means that when concentration of radionuclides in the soil is getting
higher the absorption of radionuclide by the plant is getting lower.
3.3. Study soil-plant transfer of radionuclide in tea tree, Luong My
farm
In this study, new developed method was applied. These samples
were measured by high-resolution HPGe gamma-ray spectroscopy
system (symbol GEM20P4-70, produced by Ortec-AMETEK) at the
Center for Nuclear Physics, Institute of Physics.
We can consider all soil sample have the same density and
chemical component, and all plant sample have the same density and
chemical component. Therefore, only one soil (D1-20) and one plant
sample (R3) with their corresponding gamma spectrum containing
high statistic photo-peak, was used to buil function FT(E) (Fig 3.24
and 3.25).
𝐹𝑅𝑎226(𝐸) = 𝐸𝑥𝑝(11,0204 − 3,2478. ln(𝐸) + 0,1801 ln(𝐸)2
(3.16)
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𝐹𝑅𝑎226(𝐸) = 𝐸𝑥𝑝(15,2961 − 4,48526. ln(𝐸) + 0,270965 ln(𝐸)2
(3.17) Spectroscopy efficiency at 1460,83 keV 𝜀𝑟𝑒𝑓(1460,83𝑘𝑒𝑉) =
0,532 ± 0,003.
Formula 2.4 was used to calculate concentration of 226Ra in the samples D1-20 and R2. Concentration of the other radionuclides in
sample D1-20 and remaining soil samples was calculated based on
FRa226(E) corresponding to D1-20. The concentration of the other
radionuclides in sample R3 and remaining tea trea was calculated based on FRa226 corresponding to R3.
Fig 3.24. Internal efficiency curve for 226Ra in the sample D1-20
Fig 3.25. Internal efficiency curve for 226Ra in the sample R3
Evaluation of the soil-tea trea transfer of the radionuclides in this
area was only based on concentration of the radionuclides in the soil.
3.3.1. Distribution of the radionuclides in the soil Average concentration of 238U, 232Th và 40K in different soil layers are almost the same, while that for the 137Cs is very different between soil layers. At the soil layer from 0 to 10 cm, the average
concentration is 4.78 Bq/kg dried soil but at the layer between 40-50
cm, the average concentration is only 0.20 Bq/kg dried soil (see
figure 3.26 to 3.29).
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Fig 3.26. Distribution of 40K by depth
Fig 3.27. Distribution of 137Cs by depth
Fig 3.28. Distribution of 238U by depth
Fig 3.29. Distribution of 232Th by depth
Studied results also shown that there is not any correlation
between these radionuclides in the soil.
3.3.2. Soil-tea tree trasfer factors of the radionuclides
3.3.2.1. Transfer factor of 40K Concentration of 40K is from 170.50 to 220.40 Bq/kg dried leaves, from 67.61 to 98.30 Bq/kg dried trunk and from 21.79 to 80.60 Bq/kg dried root. The results lead us to conclusion that 40K is mainly is concentrated in the tea leaves. TF(K/leaves) varies from 0,60 to
1.03; TF(K/Truck) is in range of (0,17-0,41); TF(K/Root) is in range
of (0.11 to 0.38). Correlation between the concentration of the 40K in soil and TF(K) is contra-variant and that between C(K) and C(K/Che) is
22
variant. The absorption mechanism for the 40K was explained in subsection 3.2.3.
3.3.2.2. Transfer factor for the 137Cs Concentration of 137Cs is from 0.24 to 0.66 Bq/kg dried leaves, from 0.34 to 0.89 Bq/kg dried trunk and from 0.93 to 1.55 Bq/kg dried root. Accumulation ability of tea tree parts for the 137Cs is in contrast to the 40K. The 137Cs was found the highest in the root in comparison to the trunk and the leaves. TF(Cs/Leaves) is from 0.15
to 0.37; TF(CS/Trunk) is from 0.21 to 0.51; and TF(Cs/Root) is from
0.56 to 0.87.
Similarly, as other radionuclides, tea tree also absorbs the Cs a
sufficient amount only. When concentration of the Cs in the soil
increases, the absorption ability of the tea tree will decrease and this ability in relation with the appearance of the 40K in the soil will be discussed further in subsection 3.3.2.5.
3.3.2.3. Transfer factor for 238U Concentration of 238Cs is from 17.06 to 27.64 Bq/kg dried leaves, from 3.03 to 7.02 Bq/kg dried trunk and from 2.36 to 6.17 Bq/kg dried root. The 238U is mainly accumulated in the leaves with concentration is about 5 times higher than that in the trunk and root. The obtained results are consistent with other results in the world. Uranium migrates towards the outer extremities of trees, such as growing tips, young leaves. TF(U/Leaves) is in 0.50 to 0.87; TF(U/Trunk) is in 0.10 to 0.19; TF(U/Root) is in 0.07 to 0.17. The absorption mechanism for the 238U was explained in subsection 3.2.1.
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3.3.2.4. Transfer factor for the 232Th
Concentration of the 232Th is from 12.70 to 18.89 Bq/kg dried leaves, from 1.54 to 9.67 Bq/kg dried trunk and from 1.49 to 4.22 Bq/kg dried root. As 238U, 232Th is mainly accumulated in the leaves in comparison to the trunk and root. The average concentration of 232Th in the leaves is 4 times and 6 time higher than that in trunk and the root respectively. By comparison the average concentration of the 238U and the 232Th in different parts of the tea tree (table 3.29), we can observe that concentration of 238U is higher than 232Th. Despite concentration of 238U in soil is less than. This tendency is also observed in many other researches-the plants usually absorb uranium
more than thorium and polonium. TF(Th/Leaves) is in the range of
(0.25-0.45); TF(Th/Trunk) is in the range of (0.03-0.23) and
TF(Th/Root) is in the range of (0.04 to 0.10). The absorption mechanism for the 232Th was explained in subsection 3.2.2. Table 3.29. The average concentration of the 238U and the 232Th in the tea tree parts
The average concentration in tea tree (Bq/kg khô)
Radionuclides
Root
Trunk Leaves
The average in whole tree
238U
4,08
4,30
21,47
9,95
232Th
2,58
4,26
15,82
7,55
3.3.2.5. Study the correlation between radionuclides in the soil-tea
tree transfer process
Due to the concentration of the radionuclide in the soil is main
factor to evaluate the soil-tea tree transfer of tea tree then it was
considered as a variable to assert correlation between different
24
radionuclides in tea tree and TF.
The calculated results of the correlation coefficient Spearman rs between 238U, 232Th, 40K and 137Cs in the soil and trea as well as the the TFs show is no correlation between that almost there
concentration of the radionuclides in the soil and the tree. Only one contra-variant correlation between concentration of the 40K in the soil and concentration of the 137Cs in the tea tree with rs = - 0.759 and between concentration of 40K in the soil and TF(Cs) with rs = - 0.774 was observed. When concentration of the 40K in the soil increases then concentration of the 137Cs in the tea tree decreases. In other word, ability for absorption of the 137Cs by the tea tree decreases as concentration of the 40K in the soil increase, although there was not any correlation between these two radionuclides observed in the soil. The results mean ability for absorption of the 137Cs is governed by the 40K. For further consideration, we calculate the coefficient DF(Cs/K)
and the result obtained is smaller than 1, it means that the tea tree absorbs 40K better than the 137Cs. Figure 3.36 and 3.37 describe the correlation between concentration of the 40K in the soil and concentration of the 137Cs in the tea tree, TF(Cs). The correlation between C(K) and C(Cs/Tea
tree), TF(Cs) were well described by first order function, the contra-
variant was determined as follows:
(3.18) 𝐶(𝐶𝑠/𝑇𝑒𝑎𝑡𝑟𝑒𝑒) = −0,0032. 𝐶(𝐾) + 1,4237
𝑇𝐹(𝐶𝑠) = −0,2026. 𝐶(𝐾) + 4,147
(3.19) with the determination coefficient R2 are 0,6323; 0,5992 respectively.
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Fig 3.36. Correlation between the concentration of the 40K in the soil and the 137Cs in the tea tree. Fig 3.37. Correlation between the concentration of the 40K in the soil and TF(Cs)
3.3.3. Result summary
- Radionuclides 238U, 232Th and 40K is distributed uniformly in the
soil layers.
- 137Cs was mainly confined in the first 10 cm of the soil, the concentration of the 137Cs was decreased dramatically from second layer (10-20 cm). Distribution function of the 137Cs concentration in the soil had not maximum point hence 137Cs had been supplemented continuously by descending from adjacent low mountain.
- Radionuclide 238U, 232Th and 40K were mainly accumulated in
the tea leaves. In contrary, 137Cs was mainly found in the root.
- Tea tree is grown for getting the leaves so the ability for
absorption of the 238U is better than that for the 232Th.
- 40K competes and prevents tea tree absorbs 137Cs and this fact was proven by the discrimination coefficient DF Cs/K. At all 28
investigated sites, DF Cs/K was determined smaller than 1.
- Correlation about the concentration of a radionuclide in the soil
and in the tea tree is variant but that with TF is contra-variant. It said
that when the concentration of radionuclides in the soil increases
26
highly then absorption process of the tea tree will be slowed down,
only a sufficient amount is absorbed for its growth.
CONCLUSION
With the planned goal and research contents, the thesis has
achieved the main results as follows:
1. Within the thesis, a new method for calculation concentration
of radionuclides by mean of the gamma spectroscopy has been
developed. The method used the internal efficiency curve and one
absolute efficiency point and the homogeneity between analyzed and
the standard samples is not required.
2. The data about radionuclides transfer between soil and two
kind of vegetable in Truong Sa island were obtained.
3. The data about 238U, 232Th, 40K and 137Cs transfer between soil
40K competes and prevents tea tree absorbs 137Cs and this fact was
and tea tree in LuongMy Tea Farm were obtained.
proven by the discrimination coefficient DF Cs/K < 1.
4. Correlation about the concentration of a radionuclide in the soil
and in the tree is variant but that with TF is contra-variant. It said that
when the concentration of radionuclides in the soil increases highly
then absorption process of the tea tree will be slowed down, only a
sufficient amount is absorbed for its growth.
SUGGESTIONS
- The new method developed in the thesis can only determine the
activities of gamma rays emitting radiation that can be measured
with energy from 238 keV - 3000 keV. In the coming time, there will
27
continue to be research on developing extended methods for the
energy region below 238 keV.
- The thesis only investigates the process of transporting
radioactive isotopes from soil to tea tree, cruciferous vegetables and
spinach. Therefore, there should be further studies on some common
plants and be used as food, drink or medicinal plants in Vietnam. It
aims to model the process of transporting radioactive isotopes from
soil to plants in the food chain and assessing the dose of radiation to
humans.
NEW FINDING OF THE THESIS
- Develop new methods in gamma spectrum analysis: The method
used internal efficiency curve and one absolute efficiency point.
- Distribution of the 238U, 232Th, 40K and 137Cs radionuclides in the
soil
- The data about radionuclides transfer between soil and two kind
of vegetable in Truong Sa island were obtained.
- The data about 238U, 232Th, 40K and 137Cs transfer between soil and
tea tree in LuongMy Tea Farm were obtained.
- A negative correlation has been determined in the process of
transport from soil to tea of isotopes of 40K and 137Cs.
- Data set of the discrimination coefficient DF Cs/K: The results
mean ability for absorption of the 137Cs is governed by the 40K.
28
LIST OF WORKS HAS BEEN PUBLISHED
International publication
1. L.T. Anh, P.V. Cuong, N.C. Tam, N.H. Ha, H.T. Thao, H.H.
Duc, S. Leuangtakoun, L.Q. Viet, B.V. Loat, “A
development for determining the activity of radionuclides in
the environmental sample by HPGe γ-spectroscopy using
only one absolute efficiency value and an intrinsic efficiency
curve”, Nuclear Inst. and Methods in Physics Research, A,
2019, 941, 162305.
National publication
137Cs
L.H. Khiem, Correlation between and
2. H.H. Duc, P.V. Cuong, B.V. Loat, L.T. Anh, N.D. Minh, 40K concentration in soil and tea tree in Luong My farm, Hoa
Binh province, Viet Nam, Communications in Physics, 2019,
Vol 29, No. 4, 527-539.
3. H.H. Duc, N.D. Minh, P.V. Cuong, L.T. Anh, S. Leuangtakoun, B.V. Loat, Transfer of 238U and 232Th from soil to tea tree in Luong My farm, Hoa Binh province, Viet
Nam, VNU Journal of Science: Mathematics – Physics,
2019, Vol. 35, No 4, 106-115.
4. V.T.K. Duyen, D.X. Hoai, H.H. Duc, N.T. Loi, Evaluation
of Radioactivity in the Soil of Spratly Islands, VNU Journal
of Science: Mathematics – Physics, 2015, Vol. 31, No, 1S,
172-177.
5. Hoàng Hữu Đức, Đậu Xuân Hoài, Vũ Thị Kim Duyên,
Nguyễn Thanh Lợi, Xác định hoạt độ phóng xạ trong trầm
tích tại một đơn vị quân đội đóng quân ven biển, Hội nghị
Địa lý toàn quốc lần thứ VIII, 2014.
29
