Constructing a 22 Na Radionuclide Tracer Data Set and a Semi-Empirical Model

Transcription

1 Tracer Data Set CTBTO Science and Technology 2015 Conference Royal Military College of Canada Supported by Health Canada / 23

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4 Generate the first global map of 22 Na surface concentrations First 22 Na data that has been corrected for coincidence summation Examine production and transport - can 22 Na replace 10 Be for some atmospheric studies? Relate results to atmospheric dynamics and Geomagnetic Vertical Cut-off Rigidity (R c ) using a semi-empirical model Is it possible to provide information on which air masses contributed to a sample? Can 22 Na and 7 Be be used as a radiochronometer? Results will be useful as validation data for meteorological models Understanding atmospheric phenomena such as Stratosphere-Troposphere Exchange (STE) Solar and climate change studies 4 / 23

5 Cosmogenic Nuclide Production - A Review Produced by Galactic Cosmic Rays (GCR) spallation with atmospheric gases (primarily N, O, but also Ar) Produced constantly, but rate of production changes with solar cycle, altitude, and latitude 67 % of production occurs in stratosphere with the remainder in the troposphere Production is greater in polar areas, but also more variable (up to 70 % seasonally) Cosmogenic nuclides of interest are: 22 Na (u 1/2 = 2.6 a) And 7 Be (u 1/2 = d) 10 Be (u 1/2 = a) 5 / 23

6 Sunspots, Activity and Cosmic Rays 7 Be [µbq/m 3 ] Cosmic ray intensity [Counts/hour]/100 Sunspot number Year Monthly 7 Be activity at Grindsjön monitoring station, solar activity and cosmic-ray intensity. These three factors are well known to be (anti-)correlated Year 6 / 23

7 Data This project relies on 6 years of International Monitoring System (IMS) data ( ) from all operating worldwide Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring network. 22 Na is rarely measured even with ultra-high sensitivity samplers. Lower detector efficiency at 1274 kev than most other naturals (e.g. 7 Be, Pb, etc.). Low production rate (1000x less than 7 Be) Nuclear cross-section Genesis or production material 7 / 23

8 Data This project relies on 6 years of IMS data ( ) from all operating worldwide CTBT monitoring network. 22 Na is rarely measured even with ultra-high sensitivity samplers. Lower detector efficiency at 1274 kev than most other naturals (e.g. 7 Be, Pb, etc.). Low production rate (1000x less than 7 Be) Nuclear cross-section Genesis or production material How to measure nalyse aerosol filters for 22 Na? 7 / 23

9 There are roughly 50 samplers operational in study period. The original raw spectrometer data was collected and reprocessed to extract 22 Na concentrations. A spectral summation technique developed to improve the 22 Na signal. A highly automated gamma software suite used to analyze the resulting new raw data. A generalized coincidence summation correction will be applied to correct and improve analysis. A visualization suite will be developed to analyze the processed data from a database. Two main phenomena will be examined - geomagnetic effects tmospheric transport effects. 8 / 23

10 Geomagnetic Effects The Earth s geomagnetic field strongly influences production - cosmic rays are deflected by the field and must have potentials greater than R c to interact. Production greatest at the poles and minimal at the equator. How much does the variable production rate influence ground-based observations? 9 / 23

11 Atmospheric Dynamics The major atmospheric circulation cells are shown. A couple important characteristics: The cells operate at different altitudes. The locations of downward transport (30 ). 10 / 23

12 This project seeks to relate three phenomena with a semi-empirical model: 11 / 23

13 This project seeks to relate three phenomena with a semi-empirical model: 1. The ground level air concentration of 22 Na observed worldwide. 11 / 23

14 This project seeks to relate three phenomena with a semi-empirical model: 1. The ground level air concentration of 22 Na observed worldwide. 2. Geomagnetic Vertical Cut-off Rigidity (R c ) and its effect on production. 11 / 23

15 This project seeks to relate three phenomena with a semi-empirical model: 1. The ground level air concentration of 22 Na observed worldwide. 2. Geomagnetic Vertical Cut-off Rigidity (R c ) and its effect on production. 3. The general motion of the major atmospheric transport cells. 11 / 23

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17 Global Environmental Activity Concentration Distribution of 22 Na and 7 Be from Summation Spectra Histogram of 7 Be Act. Conc. Fit µ = 4.76 x = 1.91 Data µ = 5.86 x = Histogram of 22 Na Act. Conc. Fit µ = 0.55 x = 2.57 Data µ = 0.87 x = Act. Conc. (mbq m 3 ) Act. Conc. (µbq m 3 ) Normalised coincidence corrected activity concentration histograms of 22 Na and 7 Be(n=768). The geometric mean, μ, and geometric median, x, are given for analysed data and a Maximum Likelihood Estimation (MLE) fit. 13 / 23

18 ...From Canadian NDC Entire 7 Be distribution (n=3215) analysed from the Health Canada (HC) National Data Centre (NDC) (non-summation spectra), with similar results Histogram of 7 Be Act. Conc. Fit µ = 3.71 x = 2.15 Data µ = 4.86 x = Act. Conc. (mbq m 3 ) Recall μ u u u u =5.86 and μ u u u =4.76 mbq m 3 for the summation spectra. 14 / 23

19 Normalized Relative Flux and 22 Na Data Now to examine atmospheric transport and shielding. Using a radioactive dose model for aircrew at 11 km as an proxy to an actual production rate. Combining the vertical R c of the monitoring sites deceleration parameter (measure of cosmic ray flux) into a relative scaling factor. 15 / 23

20 Normalized Relative Flux and 22 Na Data Now to examine atmospheric transport and shielding. Using a radioactive dose model for aircrew at 11 km as an proxy to an actual production rate. Combining the vertical R c of the monitoring sites deceleration parameter (measure of cosmic ray flux) into a relative scaling factor. Possible to remove the effects of variable production rate due to magnetic shielding. In this case, scaled to equivalent equatorial production. 15 / 23

21 Global Median Scaled and Raw Activity Concentration Summation Data Act. Conc. mbq/m 3 Act. Conc. mbq/m Quarterly 7 Be Raw Summation Data Hadley Ferrell Polar Phi (right) Quarterly 7 Be Scaled Summation Data Hadley Ferrell Polar Phi (right) Polar more variable as expected. Phi (MV) Phi (MV) Act. Conc. (µbq/m 3 ) Act. Conc. (µbq/m 3 ) Quarterly 22 Na Raw Summation Data Hadley Ferrell Polar Phi (right) Quarterly 22 Na Scaled Summation Data Hadley Ferrell Polar 0.6 Phi (right) Cosmic-ray flux varies by about 2, but no observable impact. Implies atmosphere well-mixed. Phi (MV) Phi (MV) 16 / 23

22 Global circulation model results simulating cross-tropopause transport of 7 Be using several weather models. How does the summation data behave? Peaks at 30 where vertical motion of air is toward surface. Some measurements from EML monitoring (US). 17 / 23

23 Latitude Ian Hoffman Na Radionuclide Na Act. Conc. (µbq m 3 ) Be Act. Conc. (mbq m 3 ) 16 Latitude Data matches simulated circulation model fairly well tail at high latitudes in northern hemisphere present, enhanced concentrations at / 23

24 s The first global 22 Na data set has been created for over a half-decade period (over 700 observations). Summation technique was successful at recovering 22 Na signals. 22 Na can now be used as a potential tracer for atmospheric and environmental studies. Variations in production rate do not appear to significantly impact surface concentrations. Bulk motion of the atmosphere appears to be the dominant factor for ground based observations. 19 / 23

25 Acknowledgements I would like to thank the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) for acceptance of this project ccess to IMS data for civil and scientific research through Virtual Data Exploitation Centre (vdec). 20 / 23

26 The End Questions and Comments 21 / 23

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28 Selected II [7] D. Lal, P. K. Malhotra, and B. Peters. On the production of radioisotopes in the atmosphere by cosmic radiation and their application to meteorology. Journal of Atmospheric and Terrestrial Physics, 12(4): , [8] Hongyu Liu, David B. Considine, Larry W. Horowitz, James H. Crawford, Susan E. Strahan, Megan R. Damon, Jose M. Rodriguez, Xiaojing Xu, Claire Carouge, and Robert M. Yantosca. Using beryllium-7 to assess cross-tropopause transport in global models. J. Geophys. Res., in revision [9] J. Masarik and J. Beer. Simulation of particle fluxes and cosmogenic nuclide production in the Earth s atmosphere. J. Geophys. Res., 104 (D10): , May [10] J. Masarik, K. J. Kim, and R. C. Reedy. Numerical simulations of in situ production of terrestrial cosmogenic nuclides. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 259(1): , [11] M. Takada, B. J. Lewis, M. Boudreau, H. Al Anid, and L. G. I. Bennett. ling of aircrew radiation exposure from galactic cosmic rays and solar particle events. Radiation Protection Dosimetry, 124(4): , doi: /rpd/ncm214. URL 23 / 23