A coincidence method of thorium measurement

Transcription

1 A coincidence method of thorium measurement Nevenka Antovic a*, Perko Vukotic a and Nikola Svrkota b a Faculty of Natural Sciences and Mathematics, University of Montenegro, Cetinjski put b.b., Podgorica, Montenegro. b Center for Ecotoxicological Research, Put Radomira Ivanovica 2, Podgorica, Montenegro. Abstract. Literature survey showed that the majority of thorium measurements were performed using alpha or gamma spectrometry, although inductively coupled plasma mass spectrometry, X-spectroscopy and neutron activation analysis were also used for various thorium measurements. Coincidence methods developed using different spectrometers and coincidence schemes were applied for thorium measurements as well. We are developing a coincidence method for thorium and its decay products measurement by a multidetector γ-ray 4πspectrometer. This method, based on coincidence counting of 583 and 2615 photons from cascade transitions that follow β - -decay of 208 Tl, is presented here. The method, developed on the six-crystal spectrometer PRIPYAT-2M, which allows measurements of large volume samples and which is practically insensitive to the shape of analyzed sample, has relatively high detection efficiency and a good sensitivity. In the case of thorium measurement by the 583 photopeak, a minimum detectable activity in a spectrometric double coincidences mode of counting, for s, is estimated to be 1.7 Bq/kg, and in a radiometric double coincidences mode of counting, for 100 s Bq/kg. For the same measurement times, in the case of thorium measurement by the 2615 photopeak, in spectrometric and radiometric double coincidences mode of counting, a minimum detectable activity is estimated to be 2.2 Bq/kg and 2.4 Bq/kg, respectively. A determination of thorium activity in various samples from environment is one of possible applications of the method. Therefore, using this method, double coincidences mode of counting and the 583 photopeak, thorium activity was determined in samples from the Montenegrin environment (including red mud samples from the Aluminium Mill nearby Podgorica the capital of Montenegro), and results are also presented here. KEYWORDS: thorium; multidetector spectrometer; double coincidences. 1. Introduction As it is known, 232 Th activity measurements are of considerable ecological interest. The radioactive series of 232 Th contains 10 daughter radionuclides ( 228 Ra, 228 Ac, 228 Th, 224 Ra, 220 Rn, 216 Po, 212 Pb, 212 Bi, 208 Tl, 212 Po), undergoes a decay chain (7 α-, 5 β-, and following γ-decays) and ends with a stable isotope of lead Pb [1]. Although for various thorium measurements inductively coupled plasma mass spectrometry, X- spectroscopy and neutron activation analysis were also used, a literature survey showed that the majority of thorium measurements were performed using alpha or gamma spectrometry (for example [2-3]). Various coincidence methods developed using different spectrometers and coincidence schemes have been used for thorium measurement as well [4-6]. We are also developing a coincidence method for thorium and its decay products measurement. This method is mainly based on counting of double coincidences of γ-ray cascade transitions which follow β - -decay of 208 Tl by a multidetector 4π γ-coincidence spectrometer. Applying this method to the sixcrystal spectrometer PRIPYAT-2M thorium activity was measured by the photopeak in the spectrum of double coincidences, i.e., detecting simultaneously the and γ-rays in cascade with it [7], as well as, after an additional modification of the spectrometer electronics and software, by the photopeak in the spectrum of double coincidences. * Presenting author, nenaa@rc.pmf.cg.ac.yu 1

2 2. Experimental 2.1 Spectrometer PRIPYAT-2M An outer view of the spectrometer PRIPYAT-2M [8] with 6 NaI(Tl) detectors (each has a diameter of 15 cm and height of 10 cm), outer dimensions of 250 cm x 145 cm x 186 cm and mass of kg, is presented in Fig.1. The front of each detector is positioned on a side of cube with an edge of 17.5 cm, closing a solid angle of 0.7x4π sr, and forming a relatively large detection chamber. The system contains modular pulse-processing electronics in the CAMAC standard, as well as iron and lead passive shielding thickness up to 15 cm, and has an energy resolution of 10.5% for the 137 Cs line. The resolution time for coincidences is 40 ns, and the multiplicity of coincidences ranges from 2 to 6. Figure 1: An outer view of the PRIPYAT-2M spectrometer The spectrometer software (PRIP) has wide service possibilities. It includes the following procedures: calibration, data acquisition, data processing, presentation and archiving, automatic control of all operations. All registered events are stored in the memory of an on-line computer, i.e., information about each registered event arrives at the computer in the form of the number (and multiplicity) of the detectors registered the event, as well as the signal amplitudes (photon energies). Namely, signals from the detectors are amplified, and their time and amplitude analysis is performed by the hodoscope and 6 ADCs (256-channels each), respectively. When a coincidence event of the given multiplicity takes place, the numbers of detectors which have registered the coincidence are identified, as well as energies of detected photons. This software enables three different modes of pulse counting: (1) integral mode [1-6], when all pulses coming from the detectors are counted; (2) coincidence mode, when separate γ-spectra of noncoincident [1-1] and of coincident pulses [2-6] (with multiplicity from 2 to 6) are produced simultaneously, - which has already used for radon measurements [9]; (3) coincidence mode - when γ- spectra of non-coincident [1-1] and of coincident pulses (with multiplicity: [2-2], [3-3], [4-4], [5-5], or [6-6]) are also produced simultaneously. 2

3 Each of the counting modes gives six spectra from single detectors, as well as their sum spectrum which will be used in both - photopeaks analysis, and calculation of activities or specific activities. These spectra can be processed in a spectrometric mode, using standard procedures for calculation and analysis, or in a radiometric mode in the case of known isotopes composition, using the matrix method and the data acquired during measurements of standards of appropriate isotopes with different densities and masses. Total the PRIPYAT-2M spectrometer background count rates in the energy region from 300 to 3000 are 35.6 counts/s in the integral spectrum [1-6], 29.5 counts/s in the non-coincidence spectrum [1-1], and 3.1 counts/s in the double coincidences spectrum [2-2]. Total background count rates in the same energy region in the other coincidence modes of counting ([3-3], [4-4], [5-5], and [6-6]) are: 1.29 counts/s, 0.56 counts/s, 0.38 counts/s, and 0.19 counts/s, respectively. Sum background spectra in different modes of counting obtained for 500 s real time (live time: s - in the integral [1-6] mode of counting, and s - in the non-coincidence and coincidence modes of counting) are shown in Fig. 2. Figure 2: Sum background spectra in different modes of counting As it is mentioned previously, 232 Th activity measurement using the PRIPAT-2M spectrometer is based on counting of and photons in the double coincidences mode of counting. This mode is preferred because it is an optimum variant (the highest sensitivity of the spectrometer, described in [10]), and in the energy region of the and the photopeak background count rates are 0.91 counts/s and 0.16 counts/s, respectively. 2.2 Coincidence method of thorium measurement Although coincident gamma quanta appear in decays of many 232 Th daughter radionuclides, a careful examination of their decay schemes showed that intensive gamma-cascades appear only in the decays of 228 Ac, 212 Bi and 208 Tl [1]. Also, there are only a few gamma-cascades with transitions which have energies and intensities appropriate for easy detection by a coincidence arrangement with sodium iodide detectors and the spectrometer PRIPYAT-2M. All of them follow β - -decay of 208 Tl (to 208 Pb) 3

4 (Fig. 3). The most intensive γ-rays of 208 Pb (I > 1%) are given in Table 1, and the strongest, among two-fold cascades, are (I = 84.5%) (I = 99%), and (I = 12.4%) (I = 99%). It is important to point out that the last γ-ray - emitted from the excited level to the ground state of 208 Pb, is more energetic than any in the series. Figure 3: γ-decay of 208 Pb Table 1: The most intensive γ-rays of 208 Pb. E γ () E level () I (%) In view of the aforesaid, for development of the coincidence method for thorium and its decay products measurement using the spectrometer PRIPYAT-2M, a coincidence registration of the 583 and 2615 γ-rays of 208 Pb have been selected. This is because in many of gamma-cascades which follow β - -decay of 208 Tl one transition is 583, and in all of them the last γ-ray has energy Therefore, if thorium in a probe is in radioactive equilibrium with its descendants (no losses via 220 Rn emanation), using the spectrometer PRIPYAT-2M and the energy region from 300 to 3000, the thorium activity can be determined by the 583 and 2615 photopeak in the spectrum of double coincidences, i.e. detecting simultaneously the 583 and 2615 and γ-rays in cascade with them, and taking into account that in the 232 Th chain only 212 Bi decays in two ways % α- decay to 208 Tl, 64.06% β - -decay to 212 Po [1]. 4

5 The 232 Th probe (height: 10 mm, diameter: 15 mm, mass: 1.41 g, activity: 1000 Bq), where 232 Th and its descendants are in radioactive equilibrium, is positioned in the center of the PRIPYAT-2M spectrometer detection chamber and measured in the double coincidences mode of counting. Sum spectrum obtained for 1000 s (live time: s) is shown in Fig. 4. The 583 photopeak detection efficiency (after subtracting the background and correcting counts in the 583 photopeak for losses due to the pulse summing effect [11]) in this mode of counting is found to be 0.082, while the 2615 photopeak one is found to be Figure 4: Sum double coincidences spectrum of the 232 Th probe Ratios of the count rates in the probe spectrum and under the corresponding energy region in the background spectrum are given in Table 2. Table 2: Ratios of double coincidences count rates in the 232 Th probe spectrum and in corresponding background spectrum in different energy regions. Total ( ) The 583 photopeak region ( ) ( ) The 2615 photopeak region ( ) Determination of the 232 Th activity in environmental samples A determination of thorium activity in various samples from environment is one of possible applications of the method presented here. Therefore, by the 583 photopeak in the PRIPYAT-2M spectrum of double coincidences, thorium activity was determined in five different samples. The results for 1000 s real measuring time are presented in Table 1. The sample 1 (mass: kg) was soil, taken from surface (0 5) cm layer on the Montenegrin Coast - Arza location. This location was selected because some previous investigations showed that sand and pebbles from Montenegrin beaches have a very low content of natural radionuclides [12]. The samples 2 and 3 have been taken from deposit (surface layer) nearby the airport of Podgorica the capital of Montenegro. This location is selected because soil on that place is mixed with burnt rumps of aircraft engine (partly made of the thorium alloy). These samples were hermetically closed in plastic 0.5 l Marinelli beakers, with height of 9.9 cm, and inner diameter of 6.0 cm (as well as the sample 1), and their masses were kg and kg, respectively. 5

6 Two red mud samples from the Aluminium Mill nearby the capital of Montenegro, 1.2 kg each (the samples 4 and 5) were hermetically closed in plastic 1 l Marinelli beakers, with height of 10.9 cm, and inner diameter of 7.2 cm, and also measured (after radioactive equilibrium between 232 Th and its descendants was established) by the spectrometer PRIPYAT-2M in the double coincidences mode of counting. As an illustration, double coincidences spectrum of the red mud sample 4 is shown in Fig. 5. As it was expected thorium activity in these samples was higher than the background one in Montenegro (described in [13]). This is because of higher natural radionuclides content in red bauxite which is used in the manufacturing process [14]. Figure 5: Sum double coincidences spectrum of the red mud sample (4) For a comparison, the same samples were also measured by the coaxial HPGe detector (ORTEC S, efficiency 40%, FWHM 1.72 at 1.33 MeV, FWHM -700 ev at 122 ), and the results (thorium activity determined by the 911 photopeak) showed a good agreement with ones obtained by the spectrometer PRIPYAT-2M (Table 3). Counting times for the samples 1, 2, 3, 4, and 5 were 82688, 49831, 11840, 82438, and s, respectively. A minimum thorium activity detectable by the HPGe detector is obtained using the 3MDA method [15] and is found to be A min = 1.4 Bq/kg (in the case of the red mud sample 4). Table 3: 232 Th activity measured in five samples from the Montenegrin environment. Sample PRIPYAT-2M live measuring time (s) Activity (PRIPYAT-2M) (Bq) Specific activity (PRIPYAT - 2M) (Bq/kg) Specific activity (HPGe) (Bq/kg) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A minimum thorium activity detectable by the spectrometer PRIPYAT-2M in the double coincidences mode of counting is estimated applying an equation derived on base of a confident registration limit L D of pulses from analyzing source [16], already used for an estimation of minimum detectable activity in the coincidence measurement [9]. In the case of thorium activity determination by the 583 photopeak, in the abovementioned experiment (the red mud sample 4) that gives A min = 1.7 Bq/kg, for 1000 s - in the spectrometric double coincidences mode, and A min = 1.3 Bq/kg, for 100 s - in the radiometric mode of counting. If thorium activity is determined by the 2615 photopeak, for the same sample minimum detectable activity is estimated to be 2.2 Bq/kg - for 1000 s measuring time in 6

7 the spectrometric, and 2.4 Bq/kg for 100 s measuring time in the radiometric double coincidences mode of counting. 4. Conclusion It is proved experimentally that the coincidence method for thorium measurement developed on the six-crystal spectrometer PRIPYAT-2M has a good sensitivity, besides a good accuracy and relatively high detection efficiency. The results of thorium activity measurements obtained using this spectrometer in the double coincidences mode of counting and the HPGe one, showed a good agreement, and measurement times in the case of the PRIPYAT-2M spectrometer were significantly shorter. Additionally, approximately the same minimum activity detectable in the PRIPYAT-2M spectrometric mode (for 1000 s) can be obtained in its radiometric mode over 100 s, which is an order of magnitude faster. The advantage of the coincidence method is also associated with the spectrometer PRIPYAT-2M possibility of measuring samples of any shape in particular, large volume samples (up to 5 dm 3 ), without their preliminary preparation and calibration measurements for different sample configurations. Acknowledgement The authors wish to thank S. K. Andrukhovich from the Institute of Physics - Minsk for his help in the modification of the spectrometer PRIPYAT-2M electronics and software. REFERENCES [1] LBNL ISOTOPES PROJECT LUNDS UNIVERSITET (2004). WWW Table of Radioactive Isotopes, Version 2.1, Nuclide Data Search. [2] HIZEM, N., et al., Determination of natural radioactivity in building materials used in Tunisian dwellings by gamma ray spectrometry, Radiation Protection Dosimetry 114, 4 (2005) 533. [3] TZORTZIS, M., TSERTOS, H., Determination of thorium, uranium and potassium elemental concentrations in surface soils in Cyprus, Journal of Environmental Radioactivity 77, 3 (2004) 325. [4] OVERTON, T.R., BURCH, P.R.J., A coincident gamma-ray spectrometer and its application to measurement of low level radioactivity. Physics in Medicine and Biology 11 (1966) 47. [5] BHANDRI, N., et al., Rapid beta gamma coincidence technique for determination of natural radionuclides in marine deposits. Earth and Planetary Science Letters 11, 2 (1971) 121. [6] HASHIMOTO, T., SAKAI, Y., Selective determination of extremely low-levels of the thorium series in environmental samples by a new delayed coincidence method. Journal of Radioanalytical and Nuclear Chemistry 138, 2 (1990), 195. [7] ANTOVIC, N., et al., Thorium measurement by the six-crystal spectrometer PRIPYAT-2M, Radiation Measurements (2008), doi: /j.radmeas [8] ANDRUKHOVICH, S.K., et al., High-sensitive multidetector gamma spectrometers PRIPYAT. Preprint of the Institute of Physics, Belarus Academy of Science, Minsk (1995) (in Russian). [9] ANTOVIC, N., et al., Radon measurement by coincidence gamma-ray spectrometry. Journal of Radioanalytical and Nuclear Chemistry 249, 1 (2001), 159. [10] ANTOVIC, N., et al., Gamma-background and sensitivity of the spectrometer PRIPYAT-2M in different coincidence modes of counting. Bulletin of Montenegrin Academy of Sciences and Arts (in Serbian) (in press). [11] DE CORTE, F., The k 0 -standardization method, University of Gent (1987). [12] VUKOTIC, P., et al., Radioactivity on the Montenegrin Coast, Yugoslavia, Journal of Radioanalytical and Nuclear Chemistry 235, 1-2 (1998), 151. [13] VUKOTIC, P., et al., Background gamma-radiation in Montenegro, Proc. IRPA Regional Symposium on Radiation Protection, Prague (1997), 477. [14] VUKOTIC, P., Doctorate thesis, University of Belgrade, Yugoslavia (1981) (in Serbian). [15] ORTEC, Gamma-vision 32, Gamma-ray spectrum analysis NCA emulator for Microsoft (2003). [16] KUZNJECOV, R. A., Activation analysis, Atomizdat, Moscow (1974) (in Russian). 7