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Preliminary study of the applicability of the thin gap method on alpha emitters
Author’s Accepted Manuscript Preliminary study of the applicability of the thin gap method on alpha emitters D. Horváth, G. Bátor, T. Kovács
www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(15)30226-8 http://dx.doi.org/10.1016/j.apradiso.2015.10.034 ARI7181
To appear in: Applied Radiation and Isotopes Received date: 18 August 2015 Revised date: 28 September 2015 Accepted date: 29 October 2015 Cite this article as: D. Horváth, G. Bátor and T. Kovács, Preliminary study of the applicability of the thin gap method on alpha emitters, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2015.10.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preliminary study of the applicability of the thin gap method on alpha emitters D. Horváth, G. Bátor, T. Kovács* Institute of Radiochemistry and Radioecology, University of Pannonia, H-8200 Veszprém, Hungary *Corresponding author. E-mail address: [email protected] Abstract The thin gap method as an in-situ radiotracer technique is widely used. This study investigated the applicability of alpha emitters. PIPS and CsI alpha spectrometers were applied in a thin gap cell. A suitable 210Po source was prepared by spontaneous deposition, Mylar foil was used to simulate water. A maximum intensity decrement of 7% within 25 microns was observed. Even though this method is suitable for the study of surface phenomena, further investigation is necessary e.g. into water and heat sensitivity. Keywords in situ, thin gap, radiotracer method, alpha emitters, 210Po source
Introduction Most of the current techniques for measuring the adsorption of different alpha emitters are based on ex-situ methods. Widely used pieces of equipment are Passivated Implanted Planar Silicon (PIPS) detectors, gas scintillators and liquid scintillation counters. In this case there is no equilibrium between the liquid and solid phases during measurements. Radiotracer techniques are widely used to investigate surface processes at solid-liquid interfaces (Horányi, 1999, Horányi, 2004, Varga et al., 2001, Varga et al., 2007). These methods can be divided into two categories: the in-situ and ex-situ methods. In the case of in-situ techniques adsorption can be measured continuously at the solid-liquid interface without disturbing the equilibrium. All of the in-situ radiotracer methods are based on the thin layer principle of Aniansson (Aniansson, 1951) which states that in the case of proper cell arrangement an increased intensity can be measured which originates from the adsorbed particles. The background noise is low in the case of alpha or soft beta emitters as the range of these forms of radiation is short in liquid phases. Based on this principle there are several techniques, namely the “thin-layer”, the “foil” and the “electrode lowering” or “thin-gap” methods (Horányi, 1999, Horányi, 2004, Varga et al., 2001a, Varga et al., 2007). In our institute over the last two decades two of the above methods, namely the foil and the thin gap, have been applied and developed. The widely used type of the foil method was introduced by Horányi (Kolics and Horányi, 1996). In this arrangement the bottom of an electrochemical cell is made of a thin foil. The adsorption occurred on the surface of the foil within the cell, while the detector was placed beneath the cell thus it did not come into contact with the media. The foil can be made from different materials to fulfil the requirements of the measurements. In most cases a plastic foil is used, and the top of the foil is covered with a suitable material such as vacuum-deposited metal layers, coatings, paints or even metal or oxide powders. The disadvantage of this method is the foil itself. It has to be thin enough to let the radiation pass through it. On the other hand it has to be thick enough to be able to hold the weight of the liquid phase. The advantages of this method are that different solid phases can be used, and the investigated surface is part of an electrochemical system thus the charge and the mass transport can be measured in parallel.
The electrode-lowering method was developed by Kazarinov (Kazarinov, 1966, Kazarinov et al., 1975). In this arrangement during the measurements, the working electrode is set into two different positions as can be seen in Fig. 1. In the raised position the adsorption of labelled species occurs while the distance between the working electrode and the detector is greater than the range of the radiation in the given media. In this position a constant background noise can be detected originating from the solution phase. In the lowered position the working electrode is lowered onto the detector surface thus only a thin layer of solution remains between the two surfaces and the intensity increment originating from the adsorbed species can be detected with low background noise from the so-called “gap”. Since 1987 the original method has been improved by various means. First Krauskopf and Wieckowski (Krauskopf et al., 1987) introduced their new cell arrangement in which the original detector was replaced with a well-polished glass scintillator, and a working electrode with smooth surfaces ( < 2) was used. With this improvement the gap between the electrode and the detector can be minimised in the lowered position, thus the sensitivity of the method can be increased by one order of magnitude. Over the last two decades further developments have been introduced by Varga and his co-workers (Bujak and Varga, 2008, Hirschberg et al., 1998, Hirschberg et al., 1999, Horváth et al., 2013, Varga et al., 2001b). The most important are: the modifications of the cell design, measurements with soft gamma-emitting radionuclides and the detection of adsorption processes on rough surfaces. These in-situ techniques play an important role in the investigation of interfacial processes at solid-liquid interfaces with the use of beta or low-energy gamma emitters but none of them are capable of measuring the sorption processes of alpha emitting radionuclides. However, it is important to understand the adsorption-desorption properties of alpha emitters. In the case of nuclear power plants, the presence of actinides can cause several problems. Through the absorption of neutrons these contaminants can disturb the self-sustaining chain reaction. Measurement of the alpha radiation is complicated, thus with the understanding of its mechanism one cannot only measure but calculate the adsorption rate of alpha emitters on different surfaces. The aim of the present work is to develop an in-situ radiotracer method based on Aniansson’s thin-layer principle which is capable of measuring the adsorption and desorption processes of isotopes with alpha radiation on solid surfaces. Materials and Methods
Based on the above-described principle of Aniansson two different cells were made. One of the cells can be seen in Fig. 2. In this case a CsI(Tl) scintillation crystal with 10mm in diameter and 5 mm in thickness was mounted in the middle of a ceramic holder, and this holder was placed at the bottom of the same glass cell which used for the original thin gap method. The crystal was made and mounted by Gamma Tech Corp., Hungary. For the measurements a 16 dinode-photoelectron multiplier and an ORTEC DSPEC LF-type 8k multichannel analyser were used. The other glass cell can be seen in Fig. 3. In this case the bottom of the cell was a commercial PIPS detector (CANBERRA Industries Inc.) which was connected to a Tennelec TC 256 alpha spectrometer to record the spectra. Analytical grade reagents were bought from Merck. The solutions were diluted with high purity water (Millipore Simplicity). For the calibration of the cells the source was prepared in our laboratory. The main steps of source preparation were the leaching of
210
Po from powdered uranium ore, purification with
spontaneous deposition then with evaporation and leaching and finally again with spontaneous deposition. The glass cell used for evaporation can be seen in Fig. 4. Glass cells were made by Csonka és Fiai Ltd., Hungary. During source preparation the uranium ore was first ground. The powder was measured in an Erlenmeyer flask and different acids were added. The solutions were evaporated to a small volume three times. After that the procedure was repeated with deionised water and the polonium was spontaneously deposited onto stainless steel discs. Results and discussion To calibrate a system a proper isotope is necessary. In this case a pure alpha emitter was needed, with a relatively high activity. A suitable source (optimal activity and geometry) was not available so the source was prepared in our laboratory. The 210Po isotope was leached out from powdered uranium ore with different leaching methods (Kovács et al., 2007, Murray et al., 2007). The prepared samples were measured with conventional PIPS detectors in vacuum chambers and the results can be seen in Fig. 5. Based on the measurements the highest yield can be achieved with concentrated acids for 210
Po, while the highest purity with 1:1 diluted nitric acid. As can be seen from fig 5. in case
of 1:3 diluted hydrochloric acid the ratio of the 210Po and the other alpha emitters (230Th, 238U)
is the highest and the deposited amount of
210
Po is high. Thus the 1:3 diluted hydrochloric
acid was chosen for the leaching. Based on the literature it is important to avoid evaporating the sample to a dry state thus the polonium is volatile. On the other hand if decontamination is the aim this property is applicable to remove the 210Po contamination from different surfaces (Miura et al., 2005). Several samples were prepared with the above-mentioned leaching procedure, but most of them were not pure enough, there was a tailing to lower energies as can be seen in Fig. 6. To achieve greater levels of purification the samples were heated in a specially designed glass cell for 60 minutes at 200, 300 and 400°C. Based on Miura, at this temperature the polonium did not evaporate from stainless steel surfaces, but other impurities may have. During the treatment a continuous argon stream flowed through the cell and on the other side of the glass cell a cryogenic trap was placed, to prevent polonium contamination. After the cell was dismounted the sample was measured with a conventional PIPS detector in a vacuum chamber. At 200 and 300 °C the shape of the spectra did not change. At 400°C the gross intensity remained the same, but the tailing effect was eliminated as can be seen in Fig. 6. The biggest limitation of the measurements is the range of the alpha radiation in dilute aqueous media. To check the applicability of the equipment, this range was determined both empirically and experimentally. To calculate the range of the alpha radiation in the given media, the range of the radiation in air has to be calculated with the Lapp and Andrews formula (Eq. 1): (
)
(1)
During the validation process a 210Po isotope was used. For this isotope E = 5.307 MeV, thus (
⁄
)
(2)
The Bragg-Kleeman rule (Eq. 3) was used to calculate the range in water: √
(3)
In the case of water Awater = 18amu, water = 1 gcm-3, while Aair = 14.4amu, air = 1.2*10-3gcm3
, thus Rwater can be calculated as:
√ For the experimental calibration a
210
(4)
Po source and a PIPS detector were used. To simulate
the absorption of the radiation in dilute aqueous media a Mylar foil (DuPont Teijin Ltd. UK) was used with a density (1.23 g cm-3) and effective atomic mass number close to that of water. During the measurement a Mylar foil with a thickness of 2 microns was used and different numbers of Mylar foil were placed between the detector and the 210Po source. The results can be seen in Fig. 7. From the calculation and the measurements it can be seen that the range of
210
Po in a dilute
aqueous solution is approximately 50 microns. From the measurements it can be seen that the optimal distance between the detector and the sample is a maximum of 20 microns to detect every possibly detectable alpha particle. At 24 microns the absorption of the alpha radiation in the Mylar foil is approximately 8%, but 30% at 36 microns. The radiation of
210
Po cannot
reach the detector as long as the thickness of the Mylar foil is greater than 50 microns. Based on previous measurements in the case of well-polished surfaces the width of the gap is less than 10 microns (Buják and Varga, 2008). The two biggest difficulties of this development are that the detectors are sensitive to water, and the range of the radiation is low, plus it has to pass through the different layers between the sample and the detector. To prove the applicability of this method, a pure 210Po solution of relatively high specific activity was prepared. To prepare the solution the spontaneously deposited polonium was washed down from the stainless steel discs with dilute hydrochloric acid then the solution was evaporated. The newly designed cells were filled with 0.5 ml of this solution then the intensity was measured. After the addition of 0.5 ml of deionised water the measurements were repeated. Based on the measurements it can be stated that the seal of the detector with a 2 micron thick Mylar foil is enough to prevent the detector and solution from coming into contact as well as being thin enough to let the radiation pass through and reach the detector. From the comparison of the two measurements it is foreseeable that if the activity is halved, the measured intensity is also halved, thus the detected radiation is in proportion with the amount of species in the detectable solution layer which is not more than 50 microns based on the calculations and measurements.
Conclusions From the research and developments it can be stated that the developed cell arrangement is consistent with the Aniansson thin layer principle. During measurements the background noise is negligible due to the short range of the alpha radiation, while the intensity increment is high if the distance between the investigated surface and the detector is less than 50 microns. The radiation of alpha emitters can be detected with an energy selective detector and the measured intensity is proportional to the species within 20 microns. Furthermore with the described method a high purity relatively high specific activity which is highly versatile.
210
Po source can be prepared of
References G. Aniansson, The radiotracer measurement of the adsorption of dissolved substances on liquid surfaces and an application to impurities in dodecyl sodium sulfate solutions, Journal of Physical Chemistry 55 (1951) 1286-1299. (DOI: 10.1021/j150491a002) R. Buják, K. Varga, On the applicability of radionuclides emitting low energy X-rays for in situ radiotracer adsorption studies, Journal of Radioanalytical and Nuclear Chemistry 275 (2008) 181-191. (doi: 10.1007/s10967-007-6957-x) G. Hirschberg, Z. Németh, K. Varga, A detailed study of the reliability of some crucial parameters used in the in-situ radiotracer sorption studies, Journal of Electroanalytical Chemistry 456 (1998) 171-191. (doi:10.1016/S0022-0728(98)00207-1) G. Hirschberg, P. Baradlai, K. Varga, G. Myburg, J. Schunk, P. Tilky, P. Stoddart, Accumulation of radioactive corrosion products on steel surfaces of VVER type nuclear reactors. I. 110Ag, Journal of Nuclear Materials 265 (1999) 273-284.( doi:10.1016/S00223115(98)00656-4) G. Horányi, Radiotracer studies of adsorption/sorption phenomena at electrode surfaces, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, (1999) 477492. (ISBN: 9780824760007) G. Horányi, Radiotracer studies of interfaces, in: G. Horányi (Ed.), Interface Science and Technology, vol. 3, Elsevier, Amsterdam, (2004). (ISBN: 9780120884957) D. Horváth, K. Berkesi, K. Varga, L. Péter, T. Kovács, R. Buják, T. Pintér, Development and application of the in situ radiotracer thin gap method for the investigation of corrosion processes. I. Adaptation of the thin gap method for the application of porous surfaces, Electrochimica Acta 109 (2013) 468-474. (doi:10.1016/j.electacta.2013.07.167) T. Kovács, K. Nagy, J. Somlai G. Szeiler 210Po and 210Pb concentration of cigarettes traded in hungary and their estimated dose contribution due to smoking, Radiation Measurements 42 (2007) 1737–1741 (doi:10.1016/j.radmeas.2007.07.006) V.E. Kazarinov, New electrochemical method for the study of adsorption from solutions (in Russian), Elektrokhimiya 2 (1966) 1170.
V.E. Kazarinov, G.J. Tysyachnaya, V.E. Andreev, On the reasons for the discrepancies in the data on methanol adsorption on platinum, Journal of Electroanalytical Chemistry 65 (1975) 391-400. (doi:10.1016/0368-1874(75)85131-8) A. Kolics, G. Horányi Some considerations about the applicability of the in situ radiotracer “foil” method for the study of accumulation processes on compact stainless steel electrodes I. Measurement of -radiation, Electrochimica Acta 41 (1996) 791-802. (doi:10.1016/00134686(95)00323-1) E.K. Krauskopf, K. Chan, A. Wieckowski, The in situ radiochemical characterization of adsorbates at smooth electrode surface, Journal of Physical Chemistry 91 (1987) 2327-2332. (DOI: 10.1021/j100293a024) K. M. Matthews, Chang-Kyu Kim, P. Martin, Determination of
210
Po in environmental
materials: A review of analytical methodology, Applied Radiation and Isotopes 65 (2007) 267-279. (doi:10.1016/j.apradiso.2006.09.005) T. Miura, T. Obara, H. Sekimoto, Removal of Polonium from stainless steel surface contaminated by neutron irradiated lead-bismuth euthectic, Progress in Nuclear Energy 47 (2005) 624-631. (doi:10.1016/j.pnucene.2005.05.065) K. Varga, G. Hirschberg, P. Baradlai, M. Nagy, Combined application of radiochemical and electrochemical methods for the investigation of solid/liquid interfaces, in: E. Matijevic (Ed.), Surface and Colloid Science, vol. 16, Kluwer Academic/Plenum Press, New York, (2001a) 341-393. (doi: 10.1007/978-1-4615-1223-3_3) K. Varga, G. Hirschberg, Z. Németh, G. Myburg, J. Schunk, P. Tilky, Accumulation of radioactive corrosion products on steel surfaces of VVER-type nuclear reactors. II. 60Co, Journal of Nuclear Materials 298 (2001b) 231-238. (doi:10.1016/S0013-4686(01)00665-X) K. Varga, A. Szabó, R. Buják, Radioaktív szennyeződések kialakulása, vizsgálati módszerei és felszámolása atomeroművekben,in: B. Csákvári (Ed.), A kémia újabb eredményei, Akadémiai Kiadó, Budapest, (2006) 129-182. (ISBN: 9789630584630)
Figures: Fig. 1.: The two position of the working electrode during the measurement (Horváth et. al., 2013) Fig. 2.: Cell arrangement with the CsI(Tl) detector(Horváth et. al., 2013) Fig. 3.: Cell arrangement with the PIPS detector Fig. 4.: The glass cell used for the heat treatment of the steel discs for the cleaning of 210Po sources Fig. 5.: The results of the different acid leaching processes. Fig.6.: The spectra of the 210Po source before , and after heating at 400°C for 60 minutes. Fig. 7.: The intensity of the 210Po source in case of different thickness of Mylar foils between the source and the detector
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Highlights A new radiotracer method for the measurement of alpha emitters is introduced. The applicability of the method is verified. A process for the preparation of pure 210Po isotope is introduced.
www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(15)30226-8 http://dx.doi.org/10.1016/j.apradiso.2015.10.034 ARI7181
To appear in: Applied Radiation and Isotopes Received date: 18 August 2015 Revised date: 28 September 2015 Accepted date: 29 October 2015 Cite this article as: D. Horváth, G. Bátor and T. Kovács, Preliminary study of the applicability of the thin gap method on alpha emitters, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2015.10.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preliminary study of the applicability of the thin gap method on alpha emitters D. Horváth, G. Bátor, T. Kovács* Institute of Radiochemistry and Radioecology, University of Pannonia, H-8200 Veszprém, Hungary *Corresponding author. E-mail address: [email protected] Abstract The thin gap method as an in-situ radiotracer technique is widely used. This study investigated the applicability of alpha emitters. PIPS and CsI alpha spectrometers were applied in a thin gap cell. A suitable 210Po source was prepared by spontaneous deposition, Mylar foil was used to simulate water. A maximum intensity decrement of 7% within 25 microns was observed. Even though this method is suitable for the study of surface phenomena, further investigation is necessary e.g. into water and heat sensitivity. Keywords in situ, thin gap, radiotracer method, alpha emitters, 210Po source
Introduction Most of the current techniques for measuring the adsorption of different alpha emitters are based on ex-situ methods. Widely used pieces of equipment are Passivated Implanted Planar Silicon (PIPS) detectors, gas scintillators and liquid scintillation counters. In this case there is no equilibrium between the liquid and solid phases during measurements. Radiotracer techniques are widely used to investigate surface processes at solid-liquid interfaces (Horányi, 1999, Horányi, 2004, Varga et al., 2001, Varga et al., 2007). These methods can be divided into two categories: the in-situ and ex-situ methods. In the case of in-situ techniques adsorption can be measured continuously at the solid-liquid interface without disturbing the equilibrium. All of the in-situ radiotracer methods are based on the thin layer principle of Aniansson (Aniansson, 1951) which states that in the case of proper cell arrangement an increased intensity can be measured which originates from the adsorbed particles. The background noise is low in the case of alpha or soft beta emitters as the range of these forms of radiation is short in liquid phases. Based on this principle there are several techniques, namely the “thin-layer”, the “foil” and the “electrode lowering” or “thin-gap” methods (Horányi, 1999, Horányi, 2004, Varga et al., 2001a, Varga et al., 2007). In our institute over the last two decades two of the above methods, namely the foil and the thin gap, have been applied and developed. The widely used type of the foil method was introduced by Horányi (Kolics and Horányi, 1996). In this arrangement the bottom of an electrochemical cell is made of a thin foil. The adsorption occurred on the surface of the foil within the cell, while the detector was placed beneath the cell thus it did not come into contact with the media. The foil can be made from different materials to fulfil the requirements of the measurements. In most cases a plastic foil is used, and the top of the foil is covered with a suitable material such as vacuum-deposited metal layers, coatings, paints or even metal or oxide powders. The disadvantage of this method is the foil itself. It has to be thin enough to let the radiation pass through it. On the other hand it has to be thick enough to be able to hold the weight of the liquid phase. The advantages of this method are that different solid phases can be used, and the investigated surface is part of an electrochemical system thus the charge and the mass transport can be measured in parallel.
The electrode-lowering method was developed by Kazarinov (Kazarinov, 1966, Kazarinov et al., 1975). In this arrangement during the measurements, the working electrode is set into two different positions as can be seen in Fig. 1. In the raised position the adsorption of labelled species occurs while the distance between the working electrode and the detector is greater than the range of the radiation in the given media. In this position a constant background noise can be detected originating from the solution phase. In the lowered position the working electrode is lowered onto the detector surface thus only a thin layer of solution remains between the two surfaces and the intensity increment originating from the adsorbed species can be detected with low background noise from the so-called “gap”. Since 1987 the original method has been improved by various means. First Krauskopf and Wieckowski (Krauskopf et al., 1987) introduced their new cell arrangement in which the original detector was replaced with a well-polished glass scintillator, and a working electrode with smooth surfaces ( < 2) was used. With this improvement the gap between the electrode and the detector can be minimised in the lowered position, thus the sensitivity of the method can be increased by one order of magnitude. Over the last two decades further developments have been introduced by Varga and his co-workers (Bujak and Varga, 2008, Hirschberg et al., 1998, Hirschberg et al., 1999, Horváth et al., 2013, Varga et al., 2001b). The most important are: the modifications of the cell design, measurements with soft gamma-emitting radionuclides and the detection of adsorption processes on rough surfaces. These in-situ techniques play an important role in the investigation of interfacial processes at solid-liquid interfaces with the use of beta or low-energy gamma emitters but none of them are capable of measuring the sorption processes of alpha emitting radionuclides. However, it is important to understand the adsorption-desorption properties of alpha emitters. In the case of nuclear power plants, the presence of actinides can cause several problems. Through the absorption of neutrons these contaminants can disturb the self-sustaining chain reaction. Measurement of the alpha radiation is complicated, thus with the understanding of its mechanism one cannot only measure but calculate the adsorption rate of alpha emitters on different surfaces. The aim of the present work is to develop an in-situ radiotracer method based on Aniansson’s thin-layer principle which is capable of measuring the adsorption and desorption processes of isotopes with alpha radiation on solid surfaces. Materials and Methods
Based on the above-described principle of Aniansson two different cells were made. One of the cells can be seen in Fig. 2. In this case a CsI(Tl) scintillation crystal with 10mm in diameter and 5 mm in thickness was mounted in the middle of a ceramic holder, and this holder was placed at the bottom of the same glass cell which used for the original thin gap method. The crystal was made and mounted by Gamma Tech Corp., Hungary. For the measurements a 16 dinode-photoelectron multiplier and an ORTEC DSPEC LF-type 8k multichannel analyser were used. The other glass cell can be seen in Fig. 3. In this case the bottom of the cell was a commercial PIPS detector (CANBERRA Industries Inc.) which was connected to a Tennelec TC 256 alpha spectrometer to record the spectra. Analytical grade reagents were bought from Merck. The solutions were diluted with high purity water (Millipore Simplicity). For the calibration of the cells the source was prepared in our laboratory. The main steps of source preparation were the leaching of
210
Po from powdered uranium ore, purification with
spontaneous deposition then with evaporation and leaching and finally again with spontaneous deposition. The glass cell used for evaporation can be seen in Fig. 4. Glass cells were made by Csonka és Fiai Ltd., Hungary. During source preparation the uranium ore was first ground. The powder was measured in an Erlenmeyer flask and different acids were added. The solutions were evaporated to a small volume three times. After that the procedure was repeated with deionised water and the polonium was spontaneously deposited onto stainless steel discs. Results and discussion To calibrate a system a proper isotope is necessary. In this case a pure alpha emitter was needed, with a relatively high activity. A suitable source (optimal activity and geometry) was not available so the source was prepared in our laboratory. The 210Po isotope was leached out from powdered uranium ore with different leaching methods (Kovács et al., 2007, Murray et al., 2007). The prepared samples were measured with conventional PIPS detectors in vacuum chambers and the results can be seen in Fig. 5. Based on the measurements the highest yield can be achieved with concentrated acids for 210
Po, while the highest purity with 1:1 diluted nitric acid. As can be seen from fig 5. in case
of 1:3 diluted hydrochloric acid the ratio of the 210Po and the other alpha emitters (230Th, 238U)
is the highest and the deposited amount of
210
Po is high. Thus the 1:3 diluted hydrochloric
acid was chosen for the leaching. Based on the literature it is important to avoid evaporating the sample to a dry state thus the polonium is volatile. On the other hand if decontamination is the aim this property is applicable to remove the 210Po contamination from different surfaces (Miura et al., 2005). Several samples were prepared with the above-mentioned leaching procedure, but most of them were not pure enough, there was a tailing to lower energies as can be seen in Fig. 6. To achieve greater levels of purification the samples were heated in a specially designed glass cell for 60 minutes at 200, 300 and 400°C. Based on Miura, at this temperature the polonium did not evaporate from stainless steel surfaces, but other impurities may have. During the treatment a continuous argon stream flowed through the cell and on the other side of the glass cell a cryogenic trap was placed, to prevent polonium contamination. After the cell was dismounted the sample was measured with a conventional PIPS detector in a vacuum chamber. At 200 and 300 °C the shape of the spectra did not change. At 400°C the gross intensity remained the same, but the tailing effect was eliminated as can be seen in Fig. 6. The biggest limitation of the measurements is the range of the alpha radiation in dilute aqueous media. To check the applicability of the equipment, this range was determined both empirically and experimentally. To calculate the range of the alpha radiation in the given media, the range of the radiation in air has to be calculated with the Lapp and Andrews formula (Eq. 1): (
)
(1)
During the validation process a 210Po isotope was used. For this isotope E = 5.307 MeV, thus (
⁄
)
(2)
The Bragg-Kleeman rule (Eq. 3) was used to calculate the range in water: √
(3)
In the case of water Awater = 18amu, water = 1 gcm-3, while Aair = 14.4amu, air = 1.2*10-3gcm3
, thus Rwater can be calculated as:
√ For the experimental calibration a
210
(4)
Po source and a PIPS detector were used. To simulate
the absorption of the radiation in dilute aqueous media a Mylar foil (DuPont Teijin Ltd. UK) was used with a density (1.23 g cm-3) and effective atomic mass number close to that of water. During the measurement a Mylar foil with a thickness of 2 microns was used and different numbers of Mylar foil were placed between the detector and the 210Po source. The results can be seen in Fig. 7. From the calculation and the measurements it can be seen that the range of
210
Po in a dilute
aqueous solution is approximately 50 microns. From the measurements it can be seen that the optimal distance between the detector and the sample is a maximum of 20 microns to detect every possibly detectable alpha particle. At 24 microns the absorption of the alpha radiation in the Mylar foil is approximately 8%, but 30% at 36 microns. The radiation of
210
Po cannot
reach the detector as long as the thickness of the Mylar foil is greater than 50 microns. Based on previous measurements in the case of well-polished surfaces the width of the gap is less than 10 microns (Buják and Varga, 2008). The two biggest difficulties of this development are that the detectors are sensitive to water, and the range of the radiation is low, plus it has to pass through the different layers between the sample and the detector. To prove the applicability of this method, a pure 210Po solution of relatively high specific activity was prepared. To prepare the solution the spontaneously deposited polonium was washed down from the stainless steel discs with dilute hydrochloric acid then the solution was evaporated. The newly designed cells were filled with 0.5 ml of this solution then the intensity was measured. After the addition of 0.5 ml of deionised water the measurements were repeated. Based on the measurements it can be stated that the seal of the detector with a 2 micron thick Mylar foil is enough to prevent the detector and solution from coming into contact as well as being thin enough to let the radiation pass through and reach the detector. From the comparison of the two measurements it is foreseeable that if the activity is halved, the measured intensity is also halved, thus the detected radiation is in proportion with the amount of species in the detectable solution layer which is not more than 50 microns based on the calculations and measurements.
Conclusions From the research and developments it can be stated that the developed cell arrangement is consistent with the Aniansson thin layer principle. During measurements the background noise is negligible due to the short range of the alpha radiation, while the intensity increment is high if the distance between the investigated surface and the detector is less than 50 microns. The radiation of alpha emitters can be detected with an energy selective detector and the measured intensity is proportional to the species within 20 microns. Furthermore with the described method a high purity relatively high specific activity which is highly versatile.
210
Po source can be prepared of
References G. Aniansson, The radiotracer measurement of the adsorption of dissolved substances on liquid surfaces and an application to impurities in dodecyl sodium sulfate solutions, Journal of Physical Chemistry 55 (1951) 1286-1299. (DOI: 10.1021/j150491a002) R. Buják, K. Varga, On the applicability of radionuclides emitting low energy X-rays for in situ radiotracer adsorption studies, Journal of Radioanalytical and Nuclear Chemistry 275 (2008) 181-191. (doi: 10.1007/s10967-007-6957-x) G. Hirschberg, Z. Németh, K. Varga, A detailed study of the reliability of some crucial parameters used in the in-situ radiotracer sorption studies, Journal of Electroanalytical Chemistry 456 (1998) 171-191. (doi:10.1016/S0022-0728(98)00207-1) G. Hirschberg, P. Baradlai, K. Varga, G. Myburg, J. Schunk, P. Tilky, P. Stoddart, Accumulation of radioactive corrosion products on steel surfaces of VVER type nuclear reactors. I. 110Ag, Journal of Nuclear Materials 265 (1999) 273-284.( doi:10.1016/S00223115(98)00656-4) G. Horányi, Radiotracer studies of adsorption/sorption phenomena at electrode surfaces, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, (1999) 477492. (ISBN: 9780824760007) G. Horányi, Radiotracer studies of interfaces, in: G. Horányi (Ed.), Interface Science and Technology, vol. 3, Elsevier, Amsterdam, (2004). (ISBN: 9780120884957) D. Horváth, K. Berkesi, K. Varga, L. Péter, T. Kovács, R. Buják, T. Pintér, Development and application of the in situ radiotracer thin gap method for the investigation of corrosion processes. I. Adaptation of the thin gap method for the application of porous surfaces, Electrochimica Acta 109 (2013) 468-474. (doi:10.1016/j.electacta.2013.07.167) T. Kovács, K. Nagy, J. Somlai G. Szeiler 210Po and 210Pb concentration of cigarettes traded in hungary and their estimated dose contribution due to smoking, Radiation Measurements 42 (2007) 1737–1741 (doi:10.1016/j.radmeas.2007.07.006) V.E. Kazarinov, New electrochemical method for the study of adsorption from solutions (in Russian), Elektrokhimiya 2 (1966) 1170.
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Figures: Fig. 1.: The two position of the working electrode during the measurement (Horváth et. al., 2013) Fig. 2.: Cell arrangement with the CsI(Tl) detector(Horváth et. al., 2013) Fig. 3.: Cell arrangement with the PIPS detector Fig. 4.: The glass cell used for the heat treatment of the steel discs for the cleaning of 210Po sources Fig. 5.: The results of the different acid leaching processes. Fig.6.: The spectra of the 210Po source before , and after heating at 400°C for 60 minutes. Fig. 7.: The intensity of the 210Po source in case of different thickness of Mylar foils between the source and the detector
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Highlights A new radiotracer method for the measurement of alpha emitters is introduced. The applicability of the method is verified. A process for the preparation of pure 210Po isotope is introduced.