Journal of Radioanalytical and Nuclear Chemistry, Vol. 267, No.1 (2006) 195–203

Determination of thoron and radon ratio by liquid scintillation spectrometry H. Yoshikawa,1* T. Nakanishi,1 H. Nakahara2 1 Kanazawa

2 Tokyo

University, Kakuma-cho, Kanazawa, Ishikawa 920-1192, Japan Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan (Received June 20, 2005)

A portable liquid scintillation counter was applied for the analysis of alpha-ray energy spectrum to determine the ratio of 220Rn/222Rn in fumarolic gas in the field. A surface-polished vial was developed, by which a Gaussian distribution could be approximated for the alpha-ray energy spectra and the peak areas of the nuclides could be estimated independently, because of the wide FWHM in the liquid scientillation pulse. A fumarolic gas sample was collected in Mt. Kamiyama (Hakoneyama geothermal field in Japan) having low 220Rn/222Rn ratio of 2.20±0.13.

Introduction We are interested in the 220Rn/222Rn ratio in fumarolic gas. This ratio varies as a result of the 226Ra– 222Rn and 224Ra–220Rn reactions requiring different time periods to reach equilibrium. The rate to attain a radio-equilibrium between 224Ra and 220Rn is faster than that between 226Ra and 222Rn. The reasons for this change in the 220Rn/222Rn ratio is due to changes in the underground structure, an altered speed of the carrier gas, and a difference in the gas path, such as the variation in the distance from the source. These phenomena alter the period of stay of the fumarolic gas. It is difficult to obtain information regarding such variables as the period of stay from the chemical and physical analysis of other chemical compositions. However, using two radioactive isotopes with different half-lives makes it easier to obtain this sort of information. The presence of an acidic gas in the fumarolic gas is problematic. The major components of the volcanic gas are steam which makes up more than 90% of its volume, and the acidic gases SO2 and CO2 which are common constituents of volcanic gases. An acidic gas hinders the general radioactivity analysis of radon, e.g., by the filtration method and by the electrical method. Thus, in this study we employed the liquid scintillation counting technique. Radon can be extracted from the fumarolic gas using a liquid scintillator based on a solution of 0.1 g 1,4-bis-2-(5-phenyloxazolyl)-benzene (POPOP) and 4 g 2,5-diphenyl oxazole (PPO) in 1 dm3 toluene and because radon is easily absorbed in an organic solvent, this method may be used to analyze samples in situ. For the 220Rn detection in the field a portable LSC is necessary because 220Rn has a short half-life (T1/2 = 55 s). Such an equipment was developed by SATO in 1982.1 However, we have developed previously an * E-mail: [email protected] Address for correspondence: JAEA, 4-33 Muramatsu, Tokai-mura, Ibraki 319–1194, Japan 0236–5731/USD 20.00 © 2006 Akadémiai Kiadó, Budapest

advanced type of LSC that applies an integral counting technique with three independent amplifiers.2 This integral counting method is simple and accurate when it is used to determine the levels of 222Rn and of 220Rn, if the 220Rn/222Rn ratio is higher than 10. 220Rn decay curve, shown in Fig. 1, illustrates that this method can be applied succesfully in such a case as the Kurino hot spring gas. However, for the hot spring gas found at Owakidani, in Hakoneyama geothermal field (Mt. Kamiyama), the 220Rn/222Rn ratio is in the range of 1 to 4. This is a value at which it is difficult to observe the attenuation of the 220Rn decay curve because of the short half-life of the isotope.3 To demonstrate the difficulty, the 220Rn decay curve observed at this site is also shown in Fig. 1. In this case, the data fit the decay and growth curves for 222Rn and its progeny quite well, but the first data points were affected by the activity of 220Rn. Therefore, using this LSC/integral counting method actually it increases the error and reduces the accuracy of measurement for a sample with a small 220Rn/222Rn ratio. To overcome such problems, in this study, we have developed a new, on-site alpha-ray spectrum evaluation method to measure and observe the activity of 220Rn. Experimental Apparatus The fluorescence radiated from the liquid scintillator is transmitted by one photomultiplier tube (PMT) which is set in a transverse direction to the vial.4 We decided to use only one PMT to secure the system’s portability. A multi-channel analyzer (MCA) is connected to the LSC for data collection and calculation. Preparation of a radioactive standard source for alphaspectroscopy An alpha-ray spectrum from a 210Po source is available to check the shape of the alpha-ray peak, 210Po has a single alpha-ray peak of 5.305 MeV (100%).

Akadémiai Kiadó, Budapest Springer, Dordrecht

H. YOSHIKAWA et al.: DETERMINATION OF THORON AND RADON RATIO BY LIQUID SCINTILLATION SPECTROMETRY

The source was produced by the neutron activation of bismuth nitrate in a nuclear reactor: 209Bi

(n, ) 210Bi

210Po

Chemical separation of the 210Po was carried out for about 2 months of decay after irradiation. The irradiated bismuth nitrate was dissolved in 5 ml conc. HNO3 and diluted with 10 ml distilled water. To avoid contamination with other radioactive elements, a treatment of the Bi and Po solution, by dissolution and precipitation as a sulfide, was performed several times. From the diluted Bi and Po solution 210Po was extracted by 10% di(2-ethylhexyl)phosphoric acid (HDEHP) in toluene solution, contacted with 1M HNO2. The recovery of 210Po was about 70%, a value calculated from an LSC alpha-ray counting. A number of quenching standard solutions of 210Po were prepared by additions of specified quantities of a standard solution of 210Po in tetra-chloromethane toluene. The other alphaspectrum sources were the 220Rn and 222Rn solutions prepared by bubbling air containing 220Rn or 222Rn into toluene using a syringe. The air containing 220Rn was collected from the upper gaseous layer of a bottle of thorium nitrate reagent, and that containing 222Rn was from a container that stored the 226Ra solution. The 220Rn samples were used immediately after preparation,

to obtaine the spectrum of the short half-life nuclide, The sample of the 220Rn/222Rn progeny was used 3 hours after preparation, to allow the nuclides to reach their radioactive equilibrium. 220Rn.

Investigation of the light emission The alpha-ray spectroscopy was performed by using a multi-channel analyzer (MCA, Canberra Series 35+) connected to the portable LSC. In order to calculate the absolute decay rate obtained by LSC alpha-ray spectoscopy, the light-emission of the fluorescence of the sample vial is an important factor in counting with just one PMT. The result of the alpha-ray spectroscopy of 210Po performed by the portable LSC with and without slit (see later) is shown in Fig. 2. Though, theoretically, a single peak of Gaussian distribution should be obtained, we can see that a small shoulder is present on the low energy side of the main 210Po peak, at 5.305 MeV. Apparently, an unusual light emission occurred in the components of this equipment. Since we intend to use this instrument for field measurements of the alpha-rays of 220Rn and 222Rn we need to obtain a peak in form of a Gaussian distribution to be able to distinguish the alpha-ray spectra of these nuclides.

Fig. 1. Observed activity counts in the field; M, N Kurino Hot Spting in Kyushu Island; , O Hakoneyama in Honshu Island. Open symbols are calculated values from observed data by fit for t = 55.6 s; closed symbols are observed activity counts

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Fig. 2. Pulse-height spectra for 210Po

In the sample in which 220Rn and 222Rn coexist, the analysis of 216,218Po is important. These nuclides are the progenies of 220Rn and 222Rn, respectively, and have an alpha-spectrum peak on the low energy side of those of 220Rn and 222Rn. The alpha-spectra of 220Rn, 216Po, 222Rn, and 218Po were taken immediately after sampling in the field. The overlapping of their alpha-peaks must be corrected for the analysis each of the radioactive nuclides separately. However, the resolution of the LSC method is not very good. Some authors reported it as 5.8% for one PMT, but it is generally about 10%. It is difficult to measure the energy of the alpha-rays of 222Rn, 218Po, 220Rn, and 216Po as perfectly separate individual peaks, as they are closely grouped being 5.48 MeV, 6.00 MeV, 6.28 MeV and 6.77 MeV energies. Furthermore, it is difficult to evaluate the individual nuclide peak areas unless the form of each peak has a Gaussian distribution. Since to observe the Gaussian spectrum of a single alpha-peak, the light radiated by the sample must be uniformly collected by the photomultiplier and, therefore, the specific geometric configuration between the surface of sample vial, the reflecting mirror, and the PMT becomes important. In this study, the effects on the light collection of such factors as the installation of the slit, the shape and material of the sample vial, and the vial position with respect to the PMT surface are discussed.

Examination of the light collection To evaluate the effect of the light collection on the form of the single alpha-ray spectrum of 210Po, first the effect of the slit, was investigated. A black, circular slit, whose diameter was 1/2 of the diameter of the PMT, was placed on the central axis of the PMT between the PMT and the vial. The spectrum measured before (1) and after (2) introducing the slit is shown in Fig. 2. The two spectra are similar in shape, both exhibiting a low energy shoulder, although spectrum (2) has a lower intensity because of loss of light through the slit. Although spectrum (2) is narrower, there is a shift in intensity to the low energy side that complicates the analysis. When examining the light collection, it is instructive to consider the meniscus of the liquid in the vial. When the vial is observed from the PMT side, the meniscus between the LS and the air space will form a reflecting mirror. A strong light comes by collection of all lights originated from the vial for the low width meniscus mirror, even if the LS shines uniformly. This effect is seen as a peak shift to the high energy side of the energy spectrum, forming double peaks as combined with the original spectrum. If the light both by the vial and the meniscus is reflected, the energy spectrum becomes Gaussian though the shape of the peak broadens.

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Material and vial processing In our consideration about the formation of the abnormal light collection, the material and shape of the vial were also taken into account. In this experiment, various vials were used to check the light collection (Fig. 3). These vials were made of quartz [(2), (4)], polyethylene [(1), (2)], and glass [(1), (2), (3)], the results are presented in Table 1. The shape of the alpha-

ray energy spectrum improved in the order of (3), (2), and (1). In some cases, it also has a shoulder peak on the low energy side. The spectrum (4) could not be analyzed because of the large shift to the low energy side. That of (3) took the form of a single peak when a specially made drum type vial was used, since this type of vial did not produce a meniscus. However, we believed that the usual commercial available vial could be improved both in its operability in the field and in some other respect.

Fig. 3. Vial types

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H. YOSHIKAWA et al.: DETERMINATION OF THORON AND RADON RATIO BY LIQUID SCINTILLATION SPECTROMETRY Table 1. Effect of the vial Type of the vial 22 ml std. vial

Material

Surface

High density polyethylene

Original Covered by a tracing paper Painted white color at the back side for the light reflecting Chemical etching Sand blasted Covered by aluminum foil to reflect Sand blasted and setting reflection plate at the back side Original Covered by a tracing Original

Glass

Quartz

Small vial

Glass

High density polyethylene

Original Sand blasted Sand blasted and setting reflection plate at the back side Original

Small flask, 1.5 ml

Quartz

Original

Drum type vial

Glass

Original Covered by aluminum foil to reflect Covered by a tracing paper

To scatter the light from the inside of the vial, a tracing paper was rolled around the outside surface. This manipulation had improved the Gaussian distribution of the spectrum definitely. In addition, the outside surface of an ordinary 22 cm3 glass vial and a mini vial was frosted by sandpaper. The alpha-ray energy spectrum of 210Po taken by using a frosted-glass vial is shown in Fig. 4. For both types of vial, the frosting changed the spectrum to a Gaussian, having a good resolution. However, considering the volume of the liquid scintillator used in the solvent extraction of Rn gas in the field, the 22 ml opaque glass vial would be a better choice for that. The surface was carefully uniformly polished by sandpaper, because the vial is only about 1 mm thick. Various type of sandpaper was used to change the degree of polishing; and the process was divided into three steps. However, in no cases could we see a difference in the ratio of the FWHM/peak channel. Therefore, it can be confirmed that the observed alphaspectrum of 210Po was similar to a Gaussian when the surface of the vial was obscured by polishing. Results Peak analysis The alpha-ray energy spectrum was analyzed with the multi-channel analyzer to read the net counts of the peak areas corresponding to each nuclide. It is difficult to calculate the peak areas for the alpha-decay nuclides independently, which have approximately the same energy, because of their large FWHM when measured by liquid scintillation pulse. The alpha-ray energy of 220Rn is 6.28 MeV; other nuclides with a similar energy

Shape of the spectra FWHM/peak ch., % Form 29.6 With shoulder at low energy side 31.7 Gaussian distribution 31.1 With shoulder at low energy side 34.2 20.2 17.8 17.2

With skirt at high energy side Gaussian distribution With skirt at high energy side With skirt at high energy side

35.0 36.2 32.7

With shoulder at low energy side Gaussian distribution With skirt at high energy side

34.0 18.2 18.4

With shoulder at low energy side Gaussian distribution Gaussian distribution

28.5

Gaussian distribution

– 24.7 24.4 12.4

With shoulder at low energy side

Gaussian distribution

are 216Po (6.77 MeV), a progeny of 220Rn, 218Po (6.00 MeV), a progeny of 222Rn, and 222Rn (5.48 MeV). The time dependence of the change in the alpha-ray energy spectrum is usually measured to correct the interference between nuclides having different halflives. For 220Rn analysis the correction of the effects of these nuclides is especially important. One such case is that of 216Po with a half-life of 0.15 seconds. The activity of 216Po quickly reached a radioactive equilibrium with 220Rn, but this is lost during the activity measurement of 220Rn during the time period of the half-life of 220Rn. By counting both the 220Rn and 216Po activity, the detected counts become the double of the activity of 220Rn which gives a better statistical accuracy. Other such cases include 222Rn which has a half-life of 3 days, and the 218Po which is a progeny of 222Rn and has a half-life of 3.11 minutes. A time of 33 minutes is required to reach a radioactive equilibrium between 222Rn and 218Po. The half-lives of both 222Rn and 218Po are sufficiently longer than the half-life of 220Rn to create a background interference in the 220Rn measurement, considering that the decay of 222Rn, and the buildup and decay of 218Po pass during the 220Rn measurement, because 222Rn coexists with 220Rn in the sample. Furthermore, a quick sampling, extraction, and measurement are required to detect 220Rn in the field. Next, we estimated the overlap of their alpha-peaks in the 220Rn region. To find the 220Rn alpha-peak height by this portable LSC, an analysis region from the 220Rn peak position to 5 on the high energy side was used. The extension of the overlap of the alpha-ray spectrum of each nuclide in this region was determined independently, and is presented in Table 2. 199

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Table 2. Overlap factor (in percents) for each peak counts Set up ROI area Over the peak channel of 216Po Over the channel between 216Po and 220Rn Over the peak channel of 220Rn

220Rn+216Po

34 50 67

218Po

8 18 32

222Rn

0 1 4

Fig. 4. Pulse-height spectra for 210Po using frosted glass vial

As quickly as possible after the preparation of the gas sample, the counting efficiency was obtained by summing the 220Rn and 216Po activity for this analytical region. We found the counting efficiency to be 67% of the summated spectrum of 220Rn and 216Po in this region. This percentage equaled 1.34 counts for each 2 counts of the 220Rn and 216Po. Further, it was needed to find the mixture rate of the 222Rn and the 220Rn

200

218Po

generated in the initial counting for 220Rn, since the measurement of 220Rn in a natural sample which includes 220Rn and 222Rn takes 10 minutes in the field. There are few other progeny after 218Po of the uranium series in the post-10 minutes sample, as 214Pb, a progeny of 218Po, which has a half-life of 27 minutes. Then, the overlapping rate of the 222Rn and 218Po was examined, and confirmed that the contamination rate of

H. YOSHIKAWA et al.: DETERMINATION OF THORON AND RADON RATIO BY LIQUID SCINTILLATION SPECTROMETRY

the 222Rn was 4% and that of the 218Po was 32% in the spectral analysis region of 220Rn. The activities of both the 222Rn and 218Po could be estimated from the results of the 222Rn analysis performed more than 3 hours later in the next step. After 3 hours and 10 minutes, the detection for the 222Rn analysis is easy, because 222Rn and its other progenies reach radio-equilibrium, and because 220Rn and its progenies which interfere in the 222Rn measurement, decayed sufficiently in that time. The activities detected were 3 alpha-radiations (from 222Rn, 218Po, and 214Po) and 2 beta-radiations (from 214Pb, and 214Bi) of those found of 222Rn to 210Pb in the uranium series. To analyze this activity, the integral counting method was used to convert it in the absolute decay rate. Finally, the activity of 220Rn was calculated by measuring the counts in the superposited 220Rn+216Po analysis region, corrected for the counts gained by 222Rn and 218Po. These were estimated in turn, from the results of the 222Rn measurement, using the overlapping rate of each of the nuclides. Correction of quenching The variation of volume and composition of the chemical species in the fumarolic gas in different sampling locations, produce various quenching effects in the LSC measurement. The analytical region in the spectrum ought to be adjusted to account for the degree of quenching in each sample, because the peak position in the alpha-ray energy spectrum and the standard deviation of the peak change are depending upon the amount of quenching. In this study, the quenching evaluation of the spectrum was carried out by the generally used external standard method as a quenching compensation for the LSC. The change in the spectrum using a sample containing blown-in 220Rn and 222Rn was examined; in this sample, a chemical reagent changed the degree of quenching. The degree of quenching, which matched the external standard ratio for a commercial LSC (radiation source: Ra) was obtained as a function of the gradient of the energy-channel calibration line, of the change of the intersection of the line, and of the variation of the standard deviation of the alpha-ray energy peak. The external standard ratio of an unknown sample was also measured in the laboratory after sampling, and the level of 220Rn was calculated by the spectrum obtained in the field for the analytical region, corrected by the function of the external standard ratio.

Wilkinson) in the portable personal computer, and the counting results were stored directly on a floppy in a list mode without a calculation at ten second time intervals. The decay counts of 220Rn taken over 55 seconds were analyzed later in the laboratory. The 222Rn was determined by analyzing the spectrum after activities of 220Rn and 216Po were decayed. Alternatively, 220Rn was determined and the result was converted to the absolute decay rate; for this procedure, the integral counting method with 3 and 2 radiations was used after the 222Rn and its progeny nuclides had reached radioactive equilibrium at the laboratory. In addition, the external standard ratio was measured to correct the quenching in the sample in the laboratory. The field determination of 220Rn was carried out to investigate whether the level of 220Rn activity in the fumarolic gas, degassing a gas of small 220Rn/222Rn ratio, could be quantitatively determined by comparing the results with those measured by the integral counting method. For the Owakidani hot spring gas in Hakoneyama, which has a small 220Rn/222Rn ratio (1– 3.5) the conventional method and our new spectral method were compared. The conventional method features the use of integral counting to convert the measured counts to the absolute decay rate directly after the sampling. In this way, it is difficult to observe the decay curve of 220Rn in the sample Owakidani hot spring gas (Fig. 6). The lower curve of this figure is a decay curve of 220Rn and 216Po, calculated by the integral method. The experimental curve agrees with the buildup curve of the 222Rn progeny, and shows that the effect of the 220Rn counts can be observed only in the first part of the counting period. On the other hand, in the upper curve in this figure, which shows the results obtained by our new method, we can see that the decay curve of 220Rn and its progeny 216Po decreases within 55 seconds (Table 3). Thus, we have proven that this spectral method is effective in the case of a sample gas whose 220Rn/222Rn ratio is small enough.

Application to a field sample and discussion The setup of the field equipment is shown in Fig. 5. The output signal from the amplifier of the portable LSC(OKEN-S-1287) was connected to the multichannel analyzer (MCA/PC98B(labo:Co), 50 MHz

Fig. 5. Diagram of the counting system in the field

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Fig. 6. Comparison of the observed counts gained by the integral counting method and this method

Table 3. Comparison of the activity ratios 220Rn/222Rn gained by the integral counting method and by this method in the field 222Rn,

Bq/l Integral counting method 96.2 ± 3.7 74.0 ± 3.7 73.6 ± 1.2 120.6 ± 1.0 125.5 ± 0.5 126.8 ± 1.0 117.9 ± 0.9 87.8 ± 0.4 91.5 ± 1.2 84.2 ± 1.2 90.5 ± 0.7 76.3 ± 1.0 103.0 ± 0.7 101.0 ± 0.6 93.5 ± 1.0 88.3 ± 0.5 Mean: 96.9 ± 0.38

220Rn/222Rn

Integral counting method 1.1 ± 0.7 1.1 ± 0.7 3.5 ± 2.0 2.0 ± 1.5 3.5 ± 2.0

Mean: 2.2 ± 0.7 Error: 32%

This method 1.35 ± 0.18 2.90 ± 0.30 2.15 ± 0.41 3.30 ± 0.34 1.34 ± 0.17

Mean: 2.20 ± 0.13 Error: 6%

Sampling site: Owakidani in Hakon-yama geothermal area (Bozu-jigoku). Sample: hot spring gas.

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Conclusions To measure 220Rn/222Rn in fumarolic gas in the field, an -ray energy spectroscope with a liquid scintillation counter has been developed. The form of each peak in the energy spectrum taken must have a Gaussian distribution to obtain the absolute decay rate. The problems in this endeavor are to collect the light with the measuring equipments, and to arrange the apparatus so as to condense it uniformly. To overcome these difficulties, the geometric relationship between the vial and the surface of the photomultiplier (PMT) was examined. The experimental work performed included the installation of slits with various widths, changing the shape and processing the surface of the sample vial, and altering the vial position with respect to PMT. The results showed that, using a surface-polished vial, the form of the peak of the alpha-ray spectrum approximated the Gaussian distribution. Then, an experiment in the field using the same vial was carried out. To analyze the pulse-heights, a portable liquid scintillation counter and a laptop computer were combined. Using this new method, the ratio of 220Rn to 222Rn (220Rn/222Rn) in the fumarolic gas can be determined on-site.

H. YOSHIKAWA et al.: DETERMINATION OF THORON AND RADON RATIO BY LIQUID SCINTILLATION SPECTROMETRY

* The authors thank Dr. K. SUEKI for the preparation of the calculation program for the automatic -ray spectrum detection; the stuffs and students of Tokyo Metropolitan University for their kind help in the gas sampling at the field.

References 1. J. SATO, H. TAKAHASHI, K. SATO, Intern. J. Appl. Radiation Isotopes, 32 (1981) 592. 2. H. YOSHIKAWA, M. YANAGA, K. ENDO, H. NAKAHARA, Health Phys., 51 (1986) 343. 3. H. YOSHIKAWA, K. ENDO, H. NAKAHARA, Geochemistry of Gaseous Elements and Compounds, Theophrastus Publications, S. A., Greece, 1990, p. 149. 4. D. L. HORROCKS, Rev. Sci. Instr., 35 (1964) 334.

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