Yoshimori, M

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29th International Cosmic Ray Conference Pune (2005)  2, 365-368  Temporal Variations in Surface Concentrations of Radionuclides as Viewed from Altitude Distributions  M. Yoshimori Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan Presenter: M. Yoshimori (yosimori@rikkyo.ne.jp), jap-yoshimori-M-abs-3-sh35-poster  We measured temporal variations in surface concentrations of two terrestrial radionuclides of 210Pb (half life 22.3 years) and 226Ra (half life 1600 years) in Tokyo. 210Pb is a decay daughter of 222Rn (half life 3.8 days) and emits a gamma-ray line at 46.50 keV and 226Ra is a parent nucleus of 222Rn and emits a gamma-ray line at 185.90 keV. The present results indicate that the surface concentration of 210Pb is seasonally variable with enhancements in spring and autumn, while 222Rn exhibits a constant surface concentration. We discuss the difference in the temporal variations between 210Pb and 226Ra from a viewpoint on their altitude distributions.    1. Introduction  There are numerous terrestrial radioactive nuclides on the Earth’s surface. For example, uranium-series nuclides (210Pb, 214Pb, 214Bi, 222Rn and 226Ra), thorium-series ones (208Tl, 212Pb and 212Bi), actinium-series one (235U) and 40K are well known. These terrestrial radioactive nuclides are emitted from the Earths crust into the atmosphere and are expected to be distributed in the lower troposphere. The surface concentration of 210Pb were measured along with 7Be [1], [2], [3], [4], [5], and the atmospheric transport process and the residence time were simulated [6], [7], [8], [9]. Although a correlation between the surface concentrations of 7Be and 210Pb were studied, the surface concentration of 210Pb depends on location and season. The concentrations of 210Pb from the continents were reported to be much higher than that from the oceans because the 222Rn is a continent source. On the other hand, the surface concentration and altitude distribution of 222Rn was not enough reported.   In this paper we describe the measurements of the surface concentrations of two terrestrial radionuclides of 210Pb and 226Ra. These two radionuclides are both uranium-series ones and related with the decay processes of 226Ra ? 222Rn ? 218Po? 214Bi (214Pb) ? 210Pb. The experiment started in Tokyo (35N/139E) in January 2002 for 210Pb and in June 2003 for 222Rn, and their temporal variations were studied. We discuss their altitude distributions from the measured temporal variations.   2. Measurement  We have continuously measured the surface concentrations of 210Pb and 226Ra using a high-volume air sampler. The air-flow rate of the air sampler is 1 m3 per min and natural radionuclide-borne aerosol particles are collected on a glass fiber filter (25 cm x 20 cm). The radionuclides are sampled for one week (totally collected air volume is 104 m3 per week) and a gamma-ray lines emitted from radionuclides are measured with a high-resolution Ge spectrometer. The measurement and method are given in detail [5].   A typical gamma-ray spectrum of the collected radionuclides is shown in Figure 1. Numerous gamma-ray lines of cosmogenic and terrestrial radionuclides are detected.  Although the intensities of gamma-ray lines of 210Pb (46.50 keV) at 180 channel and 226Ra (185.90 keV) at 740 channel are low, these lines are clearly discriminated from the background. The expanded spectrum of the 210Pb and 226Ra line region below 1000 channel (250 keV) is shown in Figure 2.  366      M. Yoshimori                 Figure 1.  A typical gamma-ray spectrum of the collected radionuclides.              Figure 2.   210Pb (46.50 keV) and 226Ra (185.90 keV) lines.    The temporal variations in the count rate of 210Pb line measured in January 2002 to June 2005 are plotted in Figure 3. The count rates vary with time and exhibit the enhancements in spring and autumn every year. The count rate of 1 count/min corresponds to the 210Pb radioactivity of 0.83 mBq/m3. The temporal variations in the  210Pb count rate is similar to those in the 7Be [10]. The temporal variations in the count rate of 226Ra line in June 2003 to June 2005 are shown in Figure 4. The count rate is almost constant without seasonal variations and the temporal variations are quite different from those of 210Pb. The count rate of 1 count/min corresponds to the 226Ra radioactivity of 0.75 mBq/m3.   Temporal Variations in Surface Concentrations…                                              367               Figure 3. Temporal variations in count rate of 210Pb line at 46.50 keV.                  Figure 4.  Temporal variations in count rate of 226Ra line at 185.90 keV.  3. Discussion  We discuss the difference in temporal variations between 210Pb and 226Ra from a view point of the altitude distribution. Air mass motions that affect the surface concentration vary with the altitude. If the terrestrial radionuclides have similar altitude distributions, their temporal variations in the surface concentrations are expected to be similar. In general, the terrestrial radionuclides are thought to be distributed in the lower troposphere. It indicates that their surface concentrations could not vary with time. However, the present measurement shows that the surface count rate of 210Pb is enhanced in spring and autumn though that of  368      M. Yoshimori  226Ra is constant with time. 210Pb exhibits similar temporal variations to 7Be that are mostly produced at high altitudes (lower stratosphere and higher troposphere). The present result may suggest that the distribution of  210Pb expands to higher altitudes.   Here we consider the possibility that 210Pb moves up to higher altitudes. 210Pb is the decay daughter of 222Rn. The Earth’s crust contains the long-lived 238U that decays over 226Ra to 222Rn. 222Rn emanates out of the Earth’s crust into the atmosphere. Since 222Rn is a water-insolvent noble gas, part of 222Rn could move up to the upper atmosphere without attachment to an aerosol particle and decay into 210Pb.  Since 210Pb is metallic, it is attached to a small particle at high altitudes and falls to the surface without decay (half life of 22.3 years). There is the measurement that supports the high 210Pb concentration at high altitudes. It indicated that the 210Pb concentration does not decrease with altitude but is nearly constant (about one third of the surface concentration) above 1 km [10]. On the other hand, metallic 226Ra in the Earth’s crust escapes directly into the atmosphere and is attached to the aerosol particle. Therefore, the distribution of 226Ra should be restricted in the lower troposphere.    The enhancement of surface 7Be concentration in spring and autumn is thought to be caused by the air mass mixing between the stratosphere and troposphere or the efficient vertical transport in the troposphere [11], [12], [13]. If we assume such atmospheric motions, the measured enhancement of the surface count rate of 210Pb in spring and autumn could be explained from the assumption of distribution of 210Pb at high altitudes.   4. Acknowledgements  This work has been supported by a Grant-in-Aid for Scientific Research (No. 16654043) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.   References  [1]  C.R. Olsen et al., J. Geophys. Res. 90, 10,487 (1985). [2]  J.F. Todd et al., J. Geophys. Res. 95, 11,106 (1989). [3]  J.E. Dibb et al., J. Geophys. Res. 97, 16,709 (1992). [4]  M. Baskaran et al., J. Geophys. Res. 98, 20-,555 (1993). [5]  M. Yoshimori et al., Adv. Space .Res.  32 (No.12), 2691 (2003). [6]  W.C. Graustein and k.K. Turekin, J. Geophys. Res. 91, 14,355 (1986). [7]  S. Rehfeld and M. Heimann, J. Geophys. Res. 100, 26,141 (1995). [8]  D.M. Koch et al., J. Geophys. Res. 101, 18,651 (1996). [9]  H. Liu et al., J. Geophys. Res. 106, 12,109 (2001). [10] L. Kownacka, Nukleonika 47, 79 (2002). [12]  H.W. Feely et al., J. Environ. Radioactivity 9, 223 (1989). [13]  D.M. Koch et al., J. Geophys. Res. 18,651 (1996). 

Temporal Variations in Surface Concentrations of Radionuclides as Viewed from Altitude Distributions

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