Chapter IV Experimental Technique 43-76
4.2 Experimental Procedure 54
4.2.11 Experimental Measurements 70
The following measurements were carried out in the work:
Elemental concentrations,
Quality control,
Detection limits,
Total uncertainty budget,
Correlation coefficient,
Attenuation effects,
Natural activity,
Concentration of radionuclides,
External and internal dose rate, and
Radium equivalent activity.
4.2.11.1 Elemental Concentration
For concentration calculation, the relative standardization approach was applied using the formula,
71
(
)
( )
(
)
(
4.6)Here, sam indicates sample and std indicates standard, Nc is net peak area or net counting, tc is counting time, D (= decay factor) = (= decay constant) =
12
2 ln
t , with t½ being the half-life,
td is decay time and C is counting factor =( ) and W = mass of irradiated element in g.
In this equation the ratio of irradiation factors (1- ) was ignored because of the same irradiation time for both sample and standard. The activation formula for relative standardization approach was formulated in the Excel sheet.
4.2.11.2 Quality Control
QC was performed by determining elemental concentration levels in the standard reference materials IAEA SL-1 and NIST Coal Fly Ash (1633b) compared to IAEA-Soil- 7. It determines the reliability of the analysis by comparing the measured values with certified ones. Considering Soil-7 as the principal standard, the ratios of obtained concentration to the certified concentration have been calculated for different elements of SL-1 and 1633b. This ratio is almost known as the quality control (QC) of that specific element.
4.2.11.3 Detection Limit
The ability of a given NAA procedure to determine the minimum amount of an element reliably is presented by the detection limit. The detection limit depends on the irradiation, the decay and the counting conditions. It also depends on the interference situation including such points as the ambient background, Compton continuum from higher energy gamma rays, as well as the gamma ray spectrum interference from such
72
factors as the blank from pre-irradiation treatment and from packing materials. However, in reality, the NAA detection limit depends on:
The amount of material to be irradiated and to be counted,
The neutron flux,
The duration of the irradiation time,
The total induced radioactivity that can be measured,
The duration of the counting time,
The total turn-around time,
The detector size, counting geometry and background shielding.
The detection limit can be calculated based the on following criteria
Currie’s Formula and
3 (3 sigma) criteria
a. Currie’s Formula
A number of formulae for calculating MDA exist. The method proposed by Currie [8] is generally the most commonly used one for radioactivity measurements. Eqn. (4.7) gives the proposed formula for radioactivity measurements. This method was used for calculating MDA values in the present work.
LD = k2 + 2.LC (4.7) Where, LD = Detection Limit
LC = Critical Limit k = Confidence level
Substituting in the definition for LC gives:
LD = k2 + 2. K. o (4.8) Where, σo = Standard deviation when net signal is zero.
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Given that σo is the standard deviation of the net counts when the net counts are equal to zero, we get,
o = (G2 B2)
Where, G= Standard deviation of the gross signal and B = Standard deviation of the background with the net counts being zero, the gross and back.
b.
3
(3 Sigma) Criteria
In the present experiment the detection limit was calculated based on the 3 criteria [9]
using the following formula:
Detection Limit Concentration of the sample (4.9) Where, BG is the background under a -ray peak.
The measured detection limits for all the elements will be quoted in Chapter-5.
4.2.11.4 Total Uncertainty Budget
Uncertainty of the measurement is defined as "a parameter associated with the results of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measured ones. Its quantification is of utmost importance in all types of measurement. Uncertainty of the samples was calculated according to the 1993 ISO Guide and was added to the uncertainty in the measurement [10].
4.2.11.5 Natural Activity
The net count of the sample is obtained by subtracting a linear background distribution of the pulse height spectra from the corresponding peak energy area. From the sample net counts activity of the sample were calculated using the formula,
(4.10)
Here, A is Activity of the sample in Bq/kg, CPS is the net peak counts per seconds, and CPS = CPS for sample - CPS for background value, Eff is the peak detection efficiency
74
of the gamma energy, IA is absolute intensity of the gamma ray energy and W is sample net weight in kg.
Gamma ray intensities were taken from the literature [11]. The peak detection efficiencies were calculated from the full energy peak detection efficiency curve constructed using Al2O3 based 226Ra standard as shown in section 4.2.7.1.
Uncertainty for Natural Sample
The uncertainty of the natural sample was calculated from the counting statistics of the samples. It is done by the formula.
U
n=
(
4.11) Where, Cs is counting statistics andCs=
, CNTs is counting per second, and A is activity of the sample.4.2.11.6 Dose Rate
There are two types of dose rate: external dose rate and internal dose rate.
External Dose Rate
The dose rate is defined as the amount of radiation energy absorbed per unit time (unit of radiation dose rate is Gy.h-1). It is calculated by using the formula which is given by the United National Scientific Committee on the Effects of Atomic Radiation report to estimate the absorption of gamma radiation dose rate in the outdoor at one meter above soil surface. The formula is,
Dex= {(0.472) CU-238+ (0.662) CTh-232+(0.0432) CK-40} Gy.h-1 (4.12)
Here, Dex is the external dose rate, CU-238, CTh-232 and CK-40 are average activity concentrations of 238U, 232Th and 40K respectively.
75
Internal Dose Rate
The internal dose rate is assumed to be 1.2 times higher than the external dose rate. The formula is,
Din= Dex× 1.2 (nGy.h-1) (4.13)
For food sample internal dose rate is very important because it may cause more harm to the people than the other radiation [12].
4.2.11.7 Radium Equivalent Activity
The radium equivalent (Raeq) is a radiation hazard index used for the evaluation of the radiation hazards of the gamma rays due to the NORM radionuclides [12]. It is given by the relation,
Raeq = CRa+ 1.43CTh+ 0.077CK (4.14)
Where CRa, CTh and CK are the activity concentrations of 226Ra, 232Th and 40K respectively.
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References
[1] Zulquarnain, M.A., Haque, M.M., Salam, M.A., Islam, M.S., Saha, P.K., Sarder, M.A., Haque, A., Soner, M.A.M., Uddin, M.A., Rahman, M.M., Kamal, I., Islam, M.N., Hossain, S.M., “Experience with the operation, maintenance and utilization of the 3MW research reactor of Bangladesh”, Int. J. Nuclear Energy Science and Technology, 4(4), 299-312, (2009).
[2] http://www. aere.org.bd
[3] Kamal, H., “Assessment of radiological contamination due to shipbreaking using HPGe gamma ray spectrometry system”, M.Sc.Thesis, Chittagong University (2004) [4] http://www. Canberra.com/products/detectors/germanium-detector.asp
[5] Canberra, “Co-axial Ge Detector System Introduction Manual”, (1979).
[6] Kapoor, S.S., Ramamurthy,V.S., “Nuclear Radiation Detectors”, Wiley Eastern Limited, New Delhi (1986).
[7] Hossain, I., Sharip, N., Viswanathan, K. K., “Efficiency and resolution of HPGe and NaI(Tl) detectors using gamma ray spectroscopy”, Scientific Research and Essays, 7(1), 86-89, (2012).
[8] Knoll, “Radiation Detection and Measurement”, 2nd Edition, John Wiley & Sons, New York, (1989).
[9] Felice, P. D., “ISO standards on determination of the detection limit and decision threshold for ionising radiation measurements”, ICRM Gamma Spectrometry Working Group Workshop, (2009).
[10] Balla, M., Moln, Z., “Uncertainty budget and validation of NAA using reference materials”, Journal of Radio-analytical and Nuclear Chemistry, 259 (3), 395-400, (2004).
[11] Guidebook, “Measurement of Radionuclides in Food and the Environment”, 139- 144, International Atomic Energy Agency, Vienna, (1989).
[12] Abdulaziz, A., El-Taher, A., “A Study on Transfer Factors of Radionuclides from Soil to Plant”, Life Sci.,J., 10(2), 532-539, (2013).
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Chapter V
Results and Discussion
5.1 Introduction
The aim of the study is to investigate the nutritional and toxic elements and also to estimate the natural radioactivity concentrations in the locally available baby foods. For the investigation in total 15 samples that include powder milks and cereals were collected from different places of Savar area in Dhaka. Two types of experiments were performed for the determination of toxic and essential elements and also for radioactivity concentration measurements in the collected baby foods. The NAA technique was applied for the determination of toxic and essential elements. The natural radioactivity concentrations were determined using the digital HPGe gamma spectrometry system.
5.2 Toxic and Essential Elements using NAA Technique
For NAA experiments, all the samples together with SRMs and CRMs were prepared for irradiation followed by INST Laboratory protocols and then irradiated by the thermal neutron flux of 3 MW TRIGA MARK-ІІ Research Reactor at INST, Atomic Energy Establishment (AERE), Savar, Dhaka. Two independent irradiations were performed for determination of elements based on short and long-lived radionuclides produced via short and long irradiation. After irradiation, gamma rays of the irradiated samples were counted two times using two independent sets of gamma ray spectrometry systems: one is CANBERRA HPGe detector (25% relative efficiency) coupled with computer based digital gamma spectrometer of ORTEC Dspec JrTM with Maestro-32 acquisition software; another one is also the CANBERRA HPGe detector (40% relative efficiency) coupled with computer based digital gamma spectrometer of CANBERRA DSA with Genie-2000 acquisition software.
For detecting short lived elements like Al, Ca, Mn, Mg, V etc. each of the samples, SRM and CRM were irradiated individually for 1 minute in the Rabbit Irradiation Channel of the reactor with the thermal power of 250 kW [1]. Whereas, for detecting long lived elements like As, Cr, Co, Fe, Hf, K, La, Na, Sb, Th, U, Zn etc. all the samples, SRMs
78
and CRMs were irradiated simultaneously in the same irradiation channel for 7 minutes with the thermal power of 2.4 MW [1].
In case of short-lived irradiation scheme, two gamma ray countings were performed: (i) first counting was performed for all the irradiated samples, CRMs and SRM for 300s with the decay interval of 8 min at 10 cm distance from the HPGe detector to maintain the detector dead time less than 10% for the determination of Al, Ca, Mg, Ti, V, etc.; (ii) 2nd counting was performed for 600 s with the decay interval of 2h at the surface of the detector for the determination of Mn, Na, K, etc.
Long irradiated samples, SRMs and CRMs were also counted two times: (i) 1st counting was performed during 1h by allowing 2-5 days decay for the determination of As, Hf, K, La, Na, Sb, etc.; (ii) 2nd counting was performed after 3 weeks decay for the period of 2h for the determination of Cr, Co, Fe, Sc, Th, U, Zn, etc.
The gamma ray spectra of a sample in all counting schemes mentioned above are shown in Figs. 5.1-5.3.
Fig. 5.1 Gamma ray spectrum of short irradiated Cereal-2 sample (counting duration: 300 s, decay time: 8 min)
Cl (1642)
Al (1778)
Na (2754)
Ca (3082)
79
Fig. 5.2 Gamma ray spectrum of short irradiated Cereal-2 sample (counting duration: 600 s, decay time: 2h)
Fig. 5.3 Gamma ray spectrum of long irradiated Cereal-2 sample (counting duration: 1h, decay time: 3 d)
Br(554)
Na (2753) Na (1368)
Cl (1642) Zn (1114)
80
Gamma peak analysis was performed using the software Hypermet PC and also checked manually. The quantification of elements was performed based on the comparative standardization approach.
The nuclear data used to identify gamma peak and concentration calculation are summarized in Table 5.1.
Table 5.1 Nuclear data for identified elements from NAA technique [2]
Serial
No. Elements Target
isotope Nuclear
reaction Half life Gamma energy (keV)
1 Al 27Al 27Al(n,γ)28Al 2.31min 1778.9
2 Br 31Br 31Br(n,γ)32Br 35.87 hr 554.3
3 Ca 48Ca 48Ca(n,γ)49Ca 8.8 min 3083
4 Cs 134Cs 134Cs(n,γ)135Cs 2.07 y 795.8
5 K 41K 41K(n,γ)42K 12.52 h 1525
6 Mg 26Mg 26Mg(n,γ)27Mg 9.45 min 1014
7 Mn 55 Mn 55Mn(n,γ)56Mn 2.58 hr 1810.7
8 Na 23Na 23Na(n,γ)24Na 15 h 1369
9 Zn 64Zn 64Zn(n,γ)65Zn 245 d 1115.4
5.2.1 Quality Control (QC)
Three SRMs/CRMs were used for concentration calculation based on relative standardization approach as well as quality control analysis. The QC was performed by determining elemental concentrations in NIST-1633b and IAEA-SL-1.1 relative to IAEA-Soil-7 and comparing their measured values with the certified ones. The
81
concentration ratios of obtained to certified values of various elements in INST-1633b and IAEA-SL-1.1 were plotted in Fig. 5.4.
Fig. 5.4 Ratio of obtained concentration vs. certified concentration for SL-1.1 and 1633b As can be observed from the figure above that the deviations were neither completely positively biased, nor negatively biased. In most of the cases the deviations lie within 10% and in a few cases within 15% that indicates the reliability of the analysis. Poor counting statistics was responsible for the higher deviation. This QC chart gives the reliability to analyze the unknown samples with adequate precision and accuracy.
5.2.2 Detection Limit
Detection limit is defined as the minimum measurable quantity of any element which can be determined in the present experimental system. The procedure of the detection limit calculation has been discussed in the previous chapter. Since, all the interested elements cannot be identified in any one sample, it is essential to calculate the detection limit [3].
The calculated detection limits of various elements are quoted in Table 5.2. An example of the calculations for Sodium has been given in Appendix A.
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
Al Br Ca Cs K Mg Mn Na Zn
Concentration ratio of obtained to certified value
Name of the elements
Quality control
1633b SL-1.1
82
Table 5.2 Nuclear data table for the detection limit of identified elements of NAA
Element Detection limit (mg/kg)
Element Detection limit
(mg/kg) Element Detection limit (mg/kg)
Al 36.58 Dy 0.56 Rb 0.23
As 0.42 Eu 6.95 Sb 0.09
Ba 6.14 Fe 472 Sc 0.09
Br 0.46 Hf 0.96 Sm 1.29
Ca 767 K 1112 Ti 264
Ce 5.25 La 0.61 Th 5.25
Co 0.71 Mg 0.06 U 0.51
Cs 0.46 Mn 3.62 V 3.51
Cr 6.95 Na 13.5 Zn 22.57
5.2.3 Estimation of Uncertainty Budget
Uncertainty of the measurement is defined as “a parameter associated with the results of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measured one. Its quantification is of utmost importance in all types of measurement. Uncertainty of the samples was calculated according to the 1993 ISO Guide and was added to the uncertainty in the measurement [4]. Finally, the uncertainties associated with the present experiment (Table 5.3) for different samples are discussed below.
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Table 5.3 Origin and typical magnitude of uncertainties in NAA Uncertainty
component
Origin Typical relative standard uncertainty
U1
Sample and comparator preparation
U1a Mass determination of a sample 0.05 % U1b Mass determination of a comparator 0.2 % U1c Mass changes of samples due to moisture
uptake during weighing
0.1 %
U1d Blank variation and the necessary correction
(due to analytical content in the irradiation vial)
0.5 %
U2 Irradiation
U2a Irradiation geometry differences 0.10%
U3
Spectrometry measurement
U3a Counting statistics Depends on spectrum
U3b Counting geometry difference 3 % U3c Piles pile-up losses (random coincidences) 0.4 %
U3d Peak integration method 0.3 %
U1 = {(0.05)2 + (0.2)2 + (0.1)2 +(0.5)2}% =0.55%
U2 = 0.1%
U3 = { (U3a)2sam + (U3a)2std + (U3b)2 + (U3c)2 + (U3d)2sam + (U3d)2std } The combined uncertainty,
Uc = { U12 +U22+ U32}% (5.1) This uncertainty is converted to absolute uncertainty as follows:
Uabs = (Uc/100) * sample concentration This uncertainty is added to the main result.
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5.2.4 Concentrations of Identified Elements
Elemental concentrations of 9 elements e.g., Al, Br, Ca, Cs, Mg, Mn, Na, K, Zn were determined qualitatively and quantitatively in 15 baby foods collected from Savar locality. The obtained concentrations of each element in the 15 baby foods with given values quoted by the producers are presented in Tables 5.4-5.6. The international permissible limits of identified elements in the present experiments are presented in Table 5.7. The concentrations of identified elements are presented in the Table B1. A typical concentration calculation of Na is given in Table C1.
Table 5.4 Measured elemental concentrations in baby foods along with their given values (mg/kg)
Sample
Name Al Br Ca
Given value
Obtained
value Ratio of given to obtained
Given value
Obtained
value Ratio of given to obtained
Given
value Obtained
value Ratio of given to obtained
Anchor NG BDL Not found NG 47.9 Not found 9500 9420 1.008
Cereal-1 NG 42.8 Not found NG 17.7 Not found 6100 7050 0.865 Cereal-2 NG 40.9 Not found NG 10.7 Not found 4000 4890 0.818
Cereal-
3(Fruit) NG 41.4 Not found NG 12.4 Not found 4350 5110 0.851 Cereal-
3(Vegetable) NG 43.9 Not found NG 8.4 Not found NG 5260 Not found Cereal-4 NG 43.0 Not found NG 13.1 Not found 5400 6660 0.811
Cereal-
Kitory NG 43.5 Not found NG 4.5 Not found NG 4900 Not found
Dano NG 58.9 Not found NG 17.3 Not found 8800 10020 0.878
Diploma NG 54.3 Not found NG 36.3 Not found NG 9400
Junior
Horlicks NG 48.7 Not found NG 9.3 Not found 8100 9190 0.881
Lactogen-1 NG 34.7 Not found NG 7.2 Not found 3330 4220 0.789 Lactogen-2 NG 42.7 Not found NG 13.2 Not found 5500 7470 0.736
Marks NG 55.0 Not found NG 25.1 Not found NG 10190 Not found
Nido NG 62.9 Not found NG 33.5 Not found 8600 10390 0.828
Red Cow NG 45.6 Not found NG 21.8 Not found 9300 8980 1.036
NG means value ‘Not Given’ by the producer and BDL mean ‘Below Detection Limit’
85
Table 5.5 Measured elemental concentrations in baby foods along with their given values (mg/kg)
Sample Name
Cs K Mg
Given value
Obtained value
Ratio of given to obtained
Given value
Obtained value
Ratio of given to obtained
Given value
Obtained value
Ratio of given to obtained
Anchor NG BDL Not found 12000 15570 0.771 800 BDL Not found
Cereal-1 NG BDL Not found 5000 7950 0.629 NG 38.9 Not found
Cereal-2 NG 0.388 Not found 480 6170 0.078 NG 41.2 Not found
Cereal-
3(Fruit) NG BDL Not found 5500 6590 0.835 NG 33.6 Not found
Cereal-3 (Vegetable)
NG BDL Not found NG 4900 Not found NG 73.5 Not found
Cereal-4 NG BDL Not found 5600 5340 1.049 NG 28.0 Not found
Cereal-
Kitory NG 3.95 Not found NG BDL Not found NG 54.4 Not found
Dano NG BDL Not found 12000 13060 0.919 850 2.1 Not found
Diploma NG BDL Not found NG 14800 Not found NG 3.4 Not found
Junior
Horlicks NG 5.55 Not found NG 8000 Not found 418 27.1 15.410
Lactogen-1 NG BDL Not found 5780 8850 0.653 500 9.6 Not found
Lactogen-2 NG BDL Not found 6450 BDL Not found 530 0.54 97.985
Marks NG 5.36 Not found NG 13390 Not found NG 2.2 Not found
Nido NG BDL Not found NG 17470 Not found NG 2.8 Not found
Red Cow NG BDL Not found NG 14790 Not found NG 2.7 Not found
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Table 5.6 Measured elemental concentrations in baby foods along with their given values (mg/kg)
Sample Name
Mn Na Zn
Given
value Obtained value
Ratio of given to obtained
Given
value Obtained value
Ratio of given to obtained
Given
value Obtained value
Ratio of given to obtained
Anchor NG BDL Not found 2800 4120 0.680 30 29.4 1.019
Cereal-1 NG 5.1 Not found 2750 2609 1.054 30 39.3 0.764
Cereal-2 NG 5.8 Not found 1100 1140 0.965 30 26.3 1.142
Cereal-
3(Fruit) NG 5.2 Not found 1100 1000 1.100 30 27.8 1.081
Cereal-3
(Vegetable) NG 11.6 Not found NG 1120 Not found NG 24.3 Not found
Cereal-4 NG 4.3 Not found 3000 2320 1.293 NG 39.3 Not found
Cereal-
Kitory NG 8.0 Not found NG 3830 Not found NG 33.1 Not found
Dano NG BDL Not found 3000 2660 1.128 26 38.2 0.681
Diploma NG BDL Not found NG 2390 Not found NG 32.5 Not found
Junior
Horlicks NG 4.5 Not found NG 3110 Not found NG 40.9 Not found
Lactogen-1 76 BDL Not found 1350 1630 0.828 51 43.0 1.186
Lactogen-2 46 BDL Not found 2280 2570 0.887 52.6 74.5 0.706
Marks NG BDL Not found NG 230 Not found NG 40 Not found
Nido NG BDL Not found 3350 3260 1.028 45 82.5 0.545
Red Cow NG BDL Not found NG 3030 Not found NG 25.6 Not found
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Table 5.7 Data table for the international permissible limit of identified elements of the present experiment (using NAA technique)
Sl. No. Name of the elements
Recommended permissible limit for
foodstuff Recommended daily intake for
children
1 Al 1mg/kg TWI [5] infant<1mg/kg body
weight/day) [5]
2 Br Undetected Undetected
3 Ca (2075–3825) ppm powder milk
[6]
(2475–3600) ppm infant formula [6]
1000 mg/day [8]
4 Cs - -
5 Cu (0.11–2.37) ppm infant formula [6]
(0.10–1.40) ppm Powder milk [6] (340-440) µg/day (for children) [9]
6 K (2610-51340) mg/kg [5] 2300 mg/day (adult women)
[10]
3100 mg/day (adult women) [10]
7 Mg (250–825) ppm infant formula [6]
225–875 ppm Powder milk [6] 250-350 mg/day [11]
8 Mn 0.50µg/kg PTWI [7]
(0.01–0.07) ppm infant formula (0.02–0.09) ppm powder milk [6], 0.6 mg/kg
(1.2-1.5) mg/day (for children) [9] (1.8-2) mg/day for female [9]
2.3 mg/day for male [9]
9 Na (2610-51340) mg/kg [4] (1-3.8) mg/day for adult [8]
10 Zn 4.69–11.34 ppm infant formula [5]
2.22–12.80 ppm [5] 20 mg/day(for adults) [6]
(4-5) mg/day (for children) [12]
As can be observed from Tables 5.4-5.6, the given values of mineral content in baby foods quoted by the producers are somewhat lower than the present experimental values in most of the cases. Each of the elemental status in baby food and their international permissible limits are described individually below.