ALPHA AND GAMMA SPECTROSCOPY METHODS FOR THORON PROGENY IMPLANTED IN GLASSES AND OTHER MATERIALS

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1 ALPHA AND GAMMA SPECTROSCOPY METHODS FOR THORON PROGENY IMPLANTED IN GLASSES AND OTHER MATERIALS C. Cosma 1 and I. Chereji 2 1 University of Babes-Bolyai, Faculty of Physics, 3400-Cluj-Napoca, Romania 2 Atomic Physics Institute, 3400 Cluj-Napoca, Romania Many retrospective studies regarding the long time exposure to radon ( 222 Rn) are based on the accumulation of 210 Pb (T 1/2 =22.3y) on the surface of glass objects or other materials and measuring the specific alpha activity of 210 Po (T 1/2 =138 d, E α =5.3 MeV) resulted by implanting of radon daughters, due to the alpha recoil. In the case of thoron ( 22o Rn) this method is not applicable because there is not a long life time isotope to allow a signifficant accumulation of any descendant. The longest life time of the thoron descendants is 212 Pb with a live time of T 1/2 = 10.6 hours. But a study involving thoron daughters implanted at the surface of glasses or other materials (plastic, wood, paper, polyethylene, Plexiglas, Cu or Al foils) can be useful in the simulation of radon progeny implantation. This simulation is possible due to the equilibrium of implantation, which is obtained in a short time (about 50 h) compared to about 100 y in the case of 222 Rn. An alpha spectrometry method was recently applied to measure the thoron contribution in the case of integrating track-etch detectors used for thoron monitoring or in the studies regarding radon and thoron progeny deposition. The purpose of this work is to obtain the alpha and gamma spectra (especially alpha) for thoron daughters implanted at the surface of different materials and to show the possibility of this method to simulate the radon progeny implantation and thus to find some parameters used in the Jacobi model. For measurements we used a PIPS detector of Canberra type (900 mm 2 ) with 25 kev energy resolution and a NaI(Tl) scintillator for gamma counting. The samples were obtained by exposure of our materials in a thoron vessel ( 0.3 L) containing 200 g Th(NO 3 ) 2 5H 2 O. The first results shoved that these alpha spectra can be obtained and that the width of alpha peaks depends on the materials. The constant of implantation estimated for all used samples varies between 10 mbq/m 2 :Bq/m 3 and 62 mbq/m 2 :Bq/m 3 for the case of equilibrium. Key words: thoron progeny, deposition, alpha spectra, implantation, energy peak, peak width INTRODUCTION In order to estimate the risk of indoor radon in producing lung cancer, a lot of epidemiological studies were developed, using underground miners data. Extrapolating these results for the general public, based on the linear no-threshold relationship between radon exposure and lung cancer risk, is now a problem widely discussed by the scientific world (Lubin et al., 1998). In the United States, for example, the BEIR IV committee estimated that about 15,400 or 21,800 lung cancer death per year can be attributed to radon among eversmokers and neversmokers. Considering the results of the latest epidemiological studies, the BEIR VI committee, based on the uncertainties of these analyses, extended this interval to 3,000-32,000 deaths per year. However, the committee admits that it could not exclude the possibility of a threshold relationship between exposure and lung cancer risk at very low levels of radon exposure. To improve the results of the latest epidemiological case-control studies, and therefore to diminish the influence of the multiple parameters involved in the estimation in radon relative risk, an alternative would be a correct assessment of the past radon exposure to each case-control included in these studies. 1007

2 This parameter is very important and the diminishing of its uncertainties can be done using dose reconstruction of past radon exposure, idea proposed by Lively and Ney (1987) and Samuelsson (1988). Many retrospective studies regarding past radon exposure are based on the accumulation of 210 Pb on the surface of glass objects or other materials, and measuring the specific alpha-activity of 210 Po (T 1/2 =138 d, E =5.3MeV) resulting from radon daughters implanting due to alpha recoil. Because 210 Pb has a lifetime of 22.3 years, the glass acts as a memory for the airborne radon activity over several decades. Another way to measure the specific surface activity of implanted radon daughters is the betaactivity measurement of 210 Bi (parent of 210 Po) used by Palfavi et al. (1995). The measurement of implanted 210 Po or 210 Bi surface activity can be correlated to the average past exposure using the Jacobi room model (Jacobi, 1972; Cornelis et al. 1992; Meesen et al. 1995; Cauwels and Poffijn, 1997). In the case of thoron exposure ( 220 Rn) this method is not applicable because there is not a long life time daughter to allow a significantant accumulation of any descendents. The longest life time of thoron progeny that is 210 Pb, (T 1/2 =10.6 hours). But a study involving thoron daughters implanted on surface of glasses or other materials (plexiglass, paper, wood, copper, aluminium or other metals) can be useful for the simulation of radon progeny implantation and for the study of aerosol influence on these phenomenon. The desorption constant of the implanted atoms could be assessed by following the intensity modification of alpha or gamma peaks over time. This simulation is possible due to the equilibrium of implantation, which is obtained in a short time (about 50 hours) as compared to 100 years in the case of radon daughters. The alpha spectroscopy method for the plate-out of radon and thoron progeny used by Bigu (1985). Many studies characterizing the adsorption and desorption of thoron daughters on aerosols have also been made lately (Porstendorfer and Reineking, 1992; Bondietti et al., 1987; Cheng et al., 1994). The purpose of this work is to obtain alpha and gamma spectra (especially alpha) for thoron daughters implanted on surfaces of different materials and to find some characteristics of the implantation process. EXPERIMENTAL METHODS The obtaining of alpha spectra and the alpha counting were made using a PIPS alpha-detector of Canberra type (900 mm 2 ) with 25 kev energy resolution operated under vacuum conditions and connected to a pulse height analyzer type (ICA-70-Hungary). For gamma measurements we used a NaI (Tl) scintillator detector connected to the same pulse analyzer. For alpha energy calibration we used the internal standard pulses generated by the alpha spectrometer, and for gamma energy calibration we used a thorium source of Th (NO 3 ) 2 5H 2 O. This thorium salt, in adequate quantity and simulating the same geometry as the samples with implanted thoron daughters, was directly placed on the NaI (Tl) detector. Comparing the calculated radioactivity of this salt to the activity of implanted sample below the 2610 kev photopeak of 208 Tl, the equilibrium quantity of thoron progeny implanted in the samples could be found. 1008

3 In the case of samples measured by alpha spectrometry, the detection efficiency was calculated taking into consideration the average solid angle under which the detecting surface (900 mm 2 ) is seen from sample surface (100 mm 2 ). The sample is placed on the detector axis, at 5 mm. The efficiency calculated in this way was 35.5%. SAMPLES EXPOSURE Two hundred grams of Th(NO 3 ) 2 5H 2 O powder were put into a metallic box of 8.5 cm diameter and 5.0 cm height, Figure 1. This substance was purchased over 25 years ago, therefore the equilibrium between 232 Th and 234 Ra is almost reached (95%). A thin plastic sieve was placed on the thorium nitrate. The thickness of Th(NO 3 ) 2 5H 2 O layer was 2.9 cm and that of the layer of air above was 2.1 cm. Taking into account the very short live time of thoron (1 minute), its equilibrium concentration is quickly reached in V 2 (164 cm 3 ) after closing the box. We considered this time of maximum of 30 minutes, which also allows the diffusion of the thoron generated from V 2 =164cm 3 into V 1.. The amount of thoron in volume V 1 was determined in a separate experiment by isolating the thorium nitrate in a 0.5 L plastic bottle hermetically closed, the thickness of thorium nitrate being the same as in the box from Figure 1. The equilibrium concentration of thoron above the thorium nitrate in the bottle was measured with the LUK 3A device (Plch 1997) and the RnTh+ mode. Used in this mode, this device can measure both the radon concentration and the thoron concentration when their values are higher than 1 kbq/m 3. In our particular case we used the Janet syringe (annex of this device) to extract a determined quantity of air out of the plastic bottle. The extraction was done using this syringe (150 cm 3 ) fitted with a very thin medical needle and pricking the cork and lateral sides of the bottle. After determining the equilibrium concentration, the equilibrium activity of thoron was calculated multiplying this concentration by the volume of the bottle. The average of five extractions leads to a value of Bq. Supposing that the same equilibrium quantity is to be found in the canister from Figure 1, the thoron concentration in volume V 1 is A 0 =4.88 MBq/m 3. This value will be used to determine the implantation efficiency under equilibrium conditions. Considering the life time of the descendant with the longest life 212 Pb (T 1/2 =10.65h), the implantation equilibrium could be reached in two days (48 hours). The determinations show that after this interval 95.6% of the equilibrium is reached. The samples exposed to implantation (glass, paper, polyethylene, aluminum etc.) are squares of 1-2 mm thickness and 1 cm 2 area, placed in the can from Figure 1, above the plastic sieve. In each case the implantation was measured on the upper side face. Before measuring the implanted activities the samples were cleaned by wiping with a soft material and ethanol. 1009

4 RESULTS AND DISCUSSIONS Figure 2 represents the gamma spectrum obtained for the paper sample. The 2614 kev peak of 208 Tl and also the 238 kev peak of 212 Pb are well determined in the spectrum. In calculations the first photopeak was used, because in the second case, at the energy of 238 kev, when comparing to the thorium nitrate sample, additional contributions of 228 Th and 224 Ra appear. Figure 3 shows the alpha spectrum obtained for the glass sample. The only spectrum peaks are those at the energy of 6.05 MeV and 8.78 MeV. The first peak is obtained from alpha disintegration of 212 Bi (36%) and the second from 212 Po disintegration generated by the beta disintegration of 212 Bi (64%). The 6.5 MeV, 7.5 MeV and 8.5 MeV peaks from this spectrum are peaks generated by the alpha spectrometer standard impulse generator, in order to be used at energy calibration. If the energy level is used for the energies E 1 5 MeV and E 2 7 MeV, the areas of these two peaks are obtained, and those areas can be well determined from N 1 and N 2, the number of impulses recorded for the two thresholds E 1 and E 2. A 1 = N 1 -N 2 A 2 = N 2 (1) We can now calculate the weighting factors at the 212-Bi disintegration: g(6.05 MeV) =A 1 /(A 1 +A 2 )=(N 1 -N 2 )/N 1 g(8.78 MeV) =A 1 /(A 1 +A 2 )=N 2 /N 1 (2) As this spectrum shows, the peaks are relatively narrow, especially the 8.78 MeV peak. The width of the peaks depends much on the degree of the surface roughness as it is presented in Figure 4, where it the alpha spectrum for the Cu sample is represented, which was previously chemically cleaned using a HNO 3 solution. The width of the lines is in this case four times greater than that in the case of the glass sample. With mechanically polished Cu samples the lines are thinner. A similar spectrum with that of the glass sample was obtained in the case of polyethylene, the widths of the lines being very alike, Figure 5. Table 1 presents the implantations data obtained from 8 types of samples. As one can see from this table, the weighting factor (64%) for beta decay of 212 Bi was found. The average value for our results is (63.25±0.95)% as obtained from the third column of this table. The width E of the line depends on the nature and degree of material processing. The narrowest lines were recorded when working with glass and Plexiglas, materials with a very glossy surface. Taking for glass and Plexiglas an average width of 50 kev of the 8.78 MeV line and considering that the alpha detector protective cover introduces a supplementary width of the line, from 25 kev to 35 kev, we can calculate that the excess width of 15 kev is the result of a depth implantation of about 100 nm for 212 Pb. Compared with other obtained data this value is almost 50% greater (Semkov 1988; Cornelis 1990). 1010

5 The excessive line width for the Cu sample treated with HNO 3 (221keV) suggests that the implantation of thoron and radon progeny could be used in the study of surface roughness. When mechanically polished, the same Cu probe gives approximately just half of the initial width, a value that can be also found for the Al sample. Another remark refers to glossy paper, with moderate values of the line width. The last two columns in Table 1 represent the implantation factor at equilibrium, obtained by alpha and gamma spectrometry. This factor is defined as the implanted activity on surface unit (Bq/m 2 ) divided by thoron concentration in the chamber (Bq/m 3 ). The values determined by the two methods (alpha and gamma) are almost the same. Higher values of the implantation factor are seen in the case of glass and paper and over 6 times smaller in the case of Al. The increasing of the surface roughness leads to increase of the implantation factor as shown by the comparison of the lrows 5 and 6 from in this table. Comparing the value found for glass in the case of equilibrium for thoron ( 212 Pb), (65 Bq/m 2 )/(kbq/m 3 ) with the one found experimentally for radon ( 210 Pb), about 25 (Bq/m2 2 )/(kbq/m 3 ), a major difference is observed. It may be explained by important losses due to glass corrosion and diffusion of 210 Pb in the glass surface as remarked by Cauwels and Poffijn (1997) and Lorinc et al. (1997) For glass, exposure was repeated placing the sample at four different heights: 2 mm, 6 mm, 10 mm, 16 mm from the thorium level in the box. The implantation result is observed in Figure 6. The quantity values of implanted 212 Pb decreases, which may reflect the gravitational deposition of aerosols. The following experiment was made for the glass sample, consisting of 5 consecutive measurements. The first measurement was made after the sample was removed from the container, without touching the implanted surface (a). After that, the sample was carefully cleaned with a soft material and remeasured (b). The sample was again removed from the spectrometer chamber and cleaned with ethanol 95%, then re-measured (c). The operation (c) was repeated (d). This sample was measured again the next day, after 19 hours (e). The results are gathered in Table 2. In each case three measurements of 100 s were made over the whole spectrum. The first four operations (a-d) took 0.5 h total. In the first case (a) there is a great contribution of the aerosols gathered on a surface, which can be easily removed (b). A cleaning operation with ethanol (c) removes a part of the 212 Pb and 212 Bi atoms implanted even on the surface, but by repeating this operation the quantity implanted is hardly influenced. Calculating the life time of the implanted progeny, from the values (d) and (c) a value of T=10.55h is obtained, very close to the value of the life time constant of 212 Pb. All our samples in Table 1 underwent the operations (a) and (b) before being measured. If we look in Figure 7 and we compare the schema from this figure with those of radon progeny implantation ( Cornelis et al.,1992; Cauwels and Poffijn,1997 ) one can see that in the thoron case this mechanism is much simpler. Only the alpha decay of 216 Po can be cause implantation in the case of thoron progeny whereas in the case of radon progeny there are two possibilities namely 218 Po and 214 Po. 1011

6 Also in the case of thoron progeny attachment to aerosols may be probably neglected due to short live time of 216 Po ( 0.5 s ). Porstendorfer and Reineking showed that the time of attachment is much longer of about s. The deposited part of thoron progeny is also possible to be studied because the cleaning can be done at different times after the sample is extracted from container and after different cleaning operations. CONCLUSION These measurements proved that alpha and gamma spectra of implanted thoron progeny can be obtained. The implantation efficiencydepends strongly of the material type and surface properties. Between the implantation efficiency and roughness of surface there is a direct relation. In the case of a copper surface cleaning with HNO 3 solution before thoron exposure resulted an increasing the width of the peak to twice that comparedwith the same sample mechanically polished. Thus, this method of implantation of 220 Rn and 220 Rn progeny could be used in studies of different surfaces. Because the implantation mechanism is much more simple in the case of thoron daughters these kinds of experiments are adequate for the simulation of radon progeny implantation and therefore will assist in obtaining realiste values for the parameters for thejacobi room model. REFERENCES [1] Bendietti EA, Papastefanou C, Rongorajon C,. Aerodinamic size associations of natural radioactivity with ambient aerosols. In: Radon and Its Decay Products: Occurrence, Properties and Health Effects; Hopke PK editor. A9C Symposium Ser 331, Washington, DC: American Chemical Society, 1987; 331: [2] Bigu I. Radon and Thoron daughters deposition velocity and unattached fraction under laboratory controlled and in underground mines. J Aerosol Sci 1985; 16: [3] Cauwels P, Poffijn A. The use an empirical correlation between surface activity and integrated radon exposure in a retrospective radon measurement. In: Jozef Sobol editor. IRPA Regional Syposium on Radiation Protection in Neighbouring Countries of Central Europe, 2-6 June, 1997, Praha, Czech Republic, 1997; Vol1: [4] Cheng YS, Yu CC, Tu KV. Intercomparison of activity size distributions of thoron progeny by alpha and gamma counting methods. Health Phys 1994; 66: [5] Cornelis J, Landsheer C, Van Trier A, Vanmarcke H, Poffijn A. Experiments on glass adsorbed Polonium-210. Appl Radiat Isot 1992; 43; [6] Cornelis J, Vanmarke H, Landsheere C, Poffijn A. Modeling radon progeny absorbed in glass. Health Phys 1993; 65: [7] Jacobi W. Activity and potential alpha energy of 222Rn and 220Rn daughters in different air atmospheres. Health Phys 1972; 22: [8] Lively RS, Ney EP. Surface radioactivity resulting from the deposition of 222Rn daughter products. Health Phys 1987; 52:

7 [9] Lorinc P. Studies on deposition of radon daughters on glass. In: Jozef Sabol editor. IRPA Regional Symposium on Radiation Protection in Neighbouring Countries of Central Europe, September 1997, Prague 1997; Vol1: [10] Lubin S. On the discrepancybetwwenepidemiologic studies in individuals and Cohen,s ecologic regression. Health Physics 1998; 75:4-10. [11] Meesen G, Poffijn A, Uyttenhove J, Buysse J. Study of a passive detector for retrospective radon measurements. Radiat Measurements 1995; 25: [12] Palfalvi J, Feher I, Lornic M. Studies on retrospective assessment of radon exposure. Radiat Meas 1995; 25: Porstendorfer J, Reineking A. Indoor behaviour and characteristics of radon progeny. Radiat Prot Dosim 1992; 45: [13] Plch J. Manual for opperating LUK 3A, Prague,1997, pp.97 [14] Porstendorfer J, Reikening A. Indoor behavior and characteristics of radon progeny. Radiat. Prot. Dosim. 1992; 45: [15] Semkow TM. Recoil-emanation theory applied to radon release from mineral grains. Geochim Cosmochim Acta 1990; 54: [16] Samuelsson C. Retrospective Determination of Radon in Houses. Nature 1988; 334:

8 Table 1: Some characteristics of thoron progeny implanted in different materials. Nr Sample g(8.78mev) E material % (kev) N 1 f i (α) f i (γ) imp/100s Bq/m 2 : Bq/m 3 Bq/m 2 : Bq/m 3 1 Glass Polyetilen e 3 Paper Plexiglas Cu Cu HNO 3 7 Al PdAg Table 2: The influence of surface cleaning on the implantation parameters Sample (a) (b) (c) (d) (e) Delay time 0 10min. 20min. 30min. 19hours Intensity (counts/sec)

9 Metalic can h Sample Plastic sieve Thoron (V 1 ) 2,1 cm Th(NO 3 ) 2 5H 2 O 2,9 cm (V 2 ) Figure 1: The container for the irradiation of samples with thoron 1015

10 Figure 2: Gamma spectrum for paper sample irradiated with thoron 1016

11 Figure 3: The alpha spectrum obtained for the glass sample 1017

12 Figure 4: Alpha spectrum of a copper sample cleaned with HNO

13 Figure 5: Alpha spectrum for a polyethylene sample 1019

14 12 10 Intensity (counts/sec) Height (mm) Figure 6: Influence of the gravitational deposition on the deposited and implanted 212 Pb. 1020

15 (attached) (unattached) d) (deposited ) (implanted)) 220 Rn source (g) λ Rn α 8.78 MeV 216 Po X 216 Po λ d u 216 Po 212 Po (1-p 1 )λ 1 p 1 λ 1 λ d u λ 1 (0.15s) a 2 λ 1 (1-a 2 )λ 1 β 64% 212 Pb X' 212 Pb λ d u 212 Pb 212 Pb λ 2 λ 2 λ Bi X" 212 Bi λ d u 212 Bi 212 Bi α 36% 6.05 MeV Figure 7: The schematic decay of thoron as a daughter product from a thoron source (g) 1021

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