by uranium, radium and thorium. These elements give an average dose of. 0,5 mGy/year. Also, the internal irradiation, a component of the natural irradiation,.
EFFICIENCY CALIBRATION IN GAMMA SPECTROMETRY BY USING 232 Th SERIES RADIONUCLIDES*
1
L. DARABAN1, D. IANCU2, D. NITA3, LAURA DARABAN3 University Babeş-Bolyai, Faculty of Physics, Kogalniceanu Str. 1, 400084 Cluj-Napoca, Romania 2 C.N.C.A.N. Bucharest, Romania 3 University Babes-Bolyai, Faculty of Engineering and Environmental Science, Cluj-Napoca, Romania Received November 15, 2012
In this paper a new method for preparation of any shape radioactive sources with a desired activity is described, by using ThO2 in secular equilibrium for calibration in efficiency of a HPGe detector. By knowing the absolute specific activity of ThO2 and the disintegration probability of the γ- emitters from the 232Th series, the efficiency curve of the spectrometer was well determined for different shape of samples. We also used this method for preparation of calibration samples in Marinelli backers, by mixing a determined quantity of ThO2 with artificial soil for environmental radioactivity measurements in different types of soils from the forest and uranium mines zones, in which radionuclides from the uranium family were identified and the total activity of the samples was determined. Key words:
232
Th series, gamma spectrometry, calibration, efficiency curve, Marinelli beaker, soil gamma activity.
1. INTRODUCTION It is an often used practice, when determining the activity of gamma emitters with HPGe detectors, to compare measurements of an unknown source with those of a calibrated source of equivalent geometry and density. The total efficiency is defined as the ratio of the number of pulses recoded in the spectrum and the number of photons emitted by the source. When a standard source of 152Eu is prepared, after the neutron activation also the impurity of 154Eu is obtained in the sample. As shown in [1], the standardization of a 152Eu source is not an easy task, a measurement in a coincidence system 4πβ−γ is required. *
Paper presented at the First East European Radon Symposium – FERAS 2012, September 2–5, 2012, Cluj-Napoca, Romania. Rom. Journ. Phys., Vol. 58, Supplement, P. S99–S107, Bucharest, 2013
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An efficiency calibration method was developed in our laboratory to measure the radioactivity of volume samples with HPGe detectors. These semiconductor detectors have a high resolution, determining the energies of the gamma radiation emitted by the natural radioisotopes from the soil and building materials, especially by uranium, radium and thorium. These elements give an average dose of 0,5 mGy/year. Also, the internal irradiation, a component of the natural irradiation, is caused by the inhalation of air and ingestion of water and aliments (in the human body). 2. EXPERIMENTAL A gamma spectrometer with a HPGe semiconductor detector type 1519 Canberra was used for the measurements, operated under a high voltage of 3.5 kV, with relative efficiency of 15%, equipped with a multichannel analyzer with 4096 channels working with the aquisition software Genie 2000. The detector is surrounded by a Pb shield type NZ 138, designed to prevent gamma photons from the environment to reach the detector, so that the radionuclides activity from the soil at the natural level can be measured. This shielding system was manufactured in our laboratory at the UBB Cluj-Napoca.
Fig. 1 – View of a Pb shield NZ 138 of the HPGe detector.
For natural samples with a very low activity it is difficult to separate the recorded signal from the background radiation, especially in a laboratory environment. In these conditions, measurements can be performed only using a Pb shielding (Figure 1).
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Efficiency calibration in gamma spectrometry by using 232Th series radionuclides
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2.1. METHODOLOGY OF THE CALIBRATION IN ENERGY AND EFFICIENCY OF THE EQUIPMENT The efficiency calibration of the analyzer channels using standard sources is a procedure periodically performed for the HPGe detector, as in [2] establishing the connection between the energy of the gamma radiation and the number of the channel. After the identification of the energy using standard sources, the efficiency value is calculated taking into account the probability of disintegration for each energy. This data is necessary for the calibration of the detector in efficiency, as in the formula: ε=N/(εg·t·Λ·p)
(1)
where: ε = is the detector efficiency N = is the peak area in number of counts at that energy Λ = is the absolute activity of the standard [Bq] t = is the measurement time p = is the probability of disintegration of the radionuclide εg = is the geometric efficiency. By applying the formula (1), the peak areas N for each peak energy can be deduced and by taking into account the scheme factor (p), the detector efficiency can be calculated [3-5]. In the beginning we used a standard point-source of known activity of 3.33 kBq of 226Ra, which emits many energy lines, as in the spectrum presented in Figure 3, where 226Ra is in secular equilibrium with its descendents [6]. The efficiency is calculated again with the same formula and the obtained results are presented in Table 1 and Figure 2. Table 1 Detector efficiency calculation using a source of 226Ra Energy (keV) 243.7 296.8 353.4 609.8 769 934.7 1120.9 1239.6 1378.4
Peak area 171868 364903 589516 363348 21707 12566 62058 17966 14570
Scheme factor (p) 0.074 0.19 0.37 0.46 0.049 0.03 0.15 0.057 0.04
Detector efficiency (%) 19.5 16.2 13.4 6.6 3.7 3.5 3.5 2.6 3.1
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Figure 2 - Characteristic spectrum of 226Ra and its descendants with 60Co.
Fig. 3 – Detector efficiency curve using a source of 226Ra.
4
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Efficiency calibration in gamma spectrometry by using 232Th series radionuclides
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In Figure 3 it can be noticed that the shape of the efficiency curve is similar with those from the literature for this type of detector. 2.2. EFFICIENCY CALIBRATION OF VOLUM SAMPLES WITH ThO2 IN SECULAR EQUILIBRIUM In order to measure the radioactivity of the soil (and other samples with low activity) it is important to use samples in high volumes. In our laboratory we conceived and manufactured some Marinelli beakers adapted to the shape of the shielding in the laboratory. In order to measure the radioactivity of the soil (and other samples with low activity), it is important to use samples in high volumes. In our laboratory we conceived and executed some Marinelli beakers adapted to the shape of the shielding in the laboratory. Further, a sample containing oxide of Th older than 30 years [7-10] was used as thorium progeny implantation for alpha spectrometry and also as a calibration source in efficiency as in [11-14], where the absolute activity per gram of ThO2 was of 3565 ± 15 Bq/g. In this procedure the geometrical factor was included in equation by using a beaker with the same geometry as the sample. The main gamma lines of 232Th at secular equilibrium with its daughter radionuclides are presented in Table 2, it can be seen that 212Bi decays by both emitting an alpha particle (36 %) and beta particle (64 %). Table 2 The Energy (keV) 209.4 270.3 328 338.4 463.0 911.1 968.9 1459.2 1587.9 510.7 583.1 860.5 2614.5 727.17 238.6
232
Th series parameters
Radionuclide 228 Ac 228 Ac 228 Ac 228 Ac 228 Ac 228 Ac 228 Ac 228 Ac 228 Ac 208 Tl 208 Tl 208 Tl 208 Tl 212 Bi 212 Pb
Intensity (%) 4.55 3.77 3.36 12.01 4.64 29.0 17.46 0.92 3.71 22.5 86.0 12.0 100 11.8 43.1
In order to measure the radioactivity of the soil (and other samples with low activity), it is important to use samples in high volume. In our laboratory we manufactured some Marinelli beakers adapted to the shape of the shielding in the laboratory.
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The calibration of Marinelli beakers can be made locally in each laboratory by using mixtures of soil-radionuclides for the calibration using old Th (in equilibrium with its daughter nuclei), in the shape of ThO2 or Th(NO3)4 [3], as in Table 3, in a mixture with a matrix of soil of CaCO3, SiO2, Al2O3 or other compounds with the density similar to that of the samples that are about to be analized, following the calibration procedures described in [10, 11]. In order to calibrate the Marinelli beaker with ThO2 it is necessary to prepare a standard mixture (artificial soil) in a beaker containing SiO2 with 2 % Al2O3, with a total mass of 260 g, to which we added a quantity of 218 g CaSO4· 2H2O (gypsum). The half-life of ThO2 of an order to tenth of years was enough in order to establish a secular equilibrium. The Marinelli beaker was filled with a mixture of a total mass of 541 g, from which 0.32 g of ThO2 (Carlo Erba, Italy) and added it to the matrix of 1239.04 Bq activity was homogeneously dispersed in the entire volume of the sample. In order to obtain an uniform dispersion of ThO2 in the artificial soil is necessary to mix them using a special procedure described in [11]. The measurement time was about 1 hour and the result is presented in Figure 4.
Fig. 4 – Characteristic gamma spectrum for the artificial ( SiO2 · Al2O3 ·CaSO4) soil, impurified with 0.32 g of ThO2 with an absolute activity of Λ =1239 Bq.
Efficiency calibration in gamma spectrometry by using 232Th series radionuclides
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Table 3 Data from the calibration spectrum using ThO2 in Marinelli beaker pi (Scheme factor)
εi (detector efficiency)
(%)
(%)
(%)
-
47718
0.6
43.1
89
Pb-212
336.5
7388
2
12
50
Ac-228
581.8
12407
1
86
20
Tl-208
725
2732
5
11
19
Bi-212
910
8420
2
29
23
Ac-228
966.4
6057
2
17.5
28
Ac-228
Energy
Ni (peak area)
keV
Counts/hour
237.4
Statistic error for peak area calculation
Radionuclide
The slope of efficiency with Marinelli beacker is done in Figure 5.
Detector effciency (%)
Efficiency with Marinelli beacker 100 90 80 70 60 50 40 30 20 10 0 0
200
400
600
800
1000
1200
Energy (keV)
Fig. 5 – The detector efficiency for large sample in Marinelli beacker determined with ThO2.
3. APPLICATIONS FOR THE MEASUREMENT OF SOIL SAMPLES A sample of soil is characterized by defining its structure, the agricultural state and the vegetative bed [12-14]. After the calibraton of detector was measured the gamma spectrum of a sample of mosses (Sphagnum sp.) with soil from Baita river, an area with uranium ore. From the Figure 6 spectra results the presence of
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the following nuclides from uranium family: contaminated sample.
226
Ra,
214
8
Pb and
214
Bi in a
Fig. 6 – Characteristic gamma spectrum of a sample of mosses (Sphagnum sp.) with soil from Baita river, with a total activity of 544 Bq (without the contribution of 40K and the presence of 137Cs), measured for 1 hour.
If the annual average of the γ-global activity for the samples of uncultivated soil was about 311.5 Bq/kg and the maxium value was of 390.9 Bq/kg recorded on, the values are situated in the range of variation of the natural background in the area [13-14]. The activities of the radionuclides from natural samples show values in this range. 4. CONCLUSIONS In these experiments a shielding made of low level background Pb was used and the measurements were performed with a HPGe gamma spectrometer in order to aquire the gamma spectra for the measured samples. In the beginning an energy calibration of the spectrometer was made and then a calibration in efficiency using standard samples with 226Ra small samples and other type Marinelli beakers containing ThO2 in secular equilibrium was also studied. The characteristic gamma spectrum of the pure ThO2 was then obtained,
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Efficiency calibration in gamma spectrometry by using 232Th series radionuclides
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by knowing the scheme factors and the descendent radionuclides. The calibration in efficiency was also made for samples of artificial soil containg ThO2. This method was then applied to a sample of soil from the area of mines with uranium, obtaining a spectrum in which it was possible to identify radionuclides from the uranium series. The total gamma activity of a contaminated sample of 390.9 Bq/kg was determined, measured for natural radionuclides. The obtained results were placed under the level of warning, fiting into the limits of variation of the natural background in the area, measured also on natural soil samples from the Faget forest. Acknowledgements. This paper was realised with the support of POSDRU/89/1.5/5/60/89 project funded by the European Social Found and Romanian Government.
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