Evaluation of 7Be and 133Xe atmospheric radioactivity time series measured at four CTBTO radionuclide stations

doi:10.1016/j.apradiso.2017.11.002

Applied Radiation and Isotopes

Volume 132, February 2018, Pages 24-28

https://doi.org/10.1016/j.apradiso.2017.11.002 Get rights and content

Highlights

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Time series analysis of Be7 and Xe133 atmospheric data and meteorological parameters.
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Noise, harmonics, and cross-correlation patterns have been studied in each station.
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The study aimed at highlighting differences and similarities among the radionuclides.

Abstract

Time series analysis of ⁷Be and ¹³³Xe atmospheric data and meteorological parameters measured at four CTBTO/IMS radionuclide stations have shown great variability in terms of noise structures, harmonic content, cross-correlation patterns and local Hurst exponent behaviour. This study focused on highlighting and characterising both differences and similarities expected for these radionuclides due to their different physical and chemical characteristics along with their different source terms. Detrended crosscorrelation analysis of different meteorological parameters influencing the residuals time series has shown that for Canadian and American stations the parameters mostly affecting radionuclide concentrations in the air seem to be wind direction and atmospheric pressure.

Introduction

Radionuclides found in trace amounts in the atmosphere have been widely used to characterise advection and transport phenomena. Although the ⁷Be and ¹³³Xe radionuclides (half-lives of 53.1 d and 5.2 d, respectively) have different production mechanisms as well as different physical and chemical characteristics, they have been frequently applied in atmospheric studies. The ⁷Be is a cosmogenic radionuclide produced in the lower stratosphere and the upper troposphere during nuclear spallation reactions of cosmic-ray particles with oxygen and nitrogen nuclei (Yoshimori, 2005). Its half-life is long enough to be advected for large distances and be detected in ground-level air with activity concentrations depending, among others, on the latitude and the time season (Lee et al., 2015). Even though beryllium is in the atmosphere in a particulate form (attached to aerosols), hence subject to dry and wet deposition, due to its suitable physical characteristics and region of its production, it has been used to study tropopause folding and to distinguish air masses of different origin, including transport of stratospheric air to troposphere (Liu et al., 2016). It may be also coupled with detections of ²¹⁰Pb which, being an intermediate product of the ²³⁸U decay chain, can be linked to air which is being advected from continental regions (Lee et al., 2007). The ¹³³Xe is a noble gas which has been produced during atmospheric and underground nuclear weapons tests, as well as by reprocessing nuclear facilities (RNF), nuclear power plants (NPP) and isotope production facilities (IPF) during their operational releases. Xenon is chemically inert and is not subject to dry or wet deposition. Due to its suitable characteristics it has been chosen as a possible signature of nuclear weapons tests. It is interesting to mention that in addition to ¹³³Xe, three other isotopes of Xenon are used to identify nuclear explosions (^131mXe, ^133mXe, ¹³⁵Xe). Xenon is being monitored worldwide by the International Monitoring System (IMS), which has been set up by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) in order to monitor over the compliance with the CTBT when it goes into force. The monitoring process is difficult because of low concentrations expected to be released by a low-yield underground nuclear test, the high dilution that characterise atmospheric transport, and also due to the presence of legitimate sources of ¹³³Xe emissions from RNF, NPP and IPF, unevenly distributed in space, and also emitting with both continuous or batch releases, which constitute an undesired background signal.

A comparative study of activity concentrations of ⁷Be and ¹³³Xe in the ground-level air as obtained by the CTBTO/IMS radionuclide network has been carried out with the aim to contribute to a better localisation of undeclared nuclear activity sites which may occur in the future. In order to characterise the site response of different IMS stations, time series of both ¹³³Xe and ⁷Be activity concentrations have been analysed, along with meteorological parameters. To achieve that, spectral analysis was first performed to quantify time series harmonic content. Then, correlation analysis between radionuclides and meteorological residuals has been carried out with the aim to establish which environmental conditions characterise radionuclide occurrence at a given station. In particular, detrended crosscorrelation analysis (DCCA) is used to establish the temporal scale where this correlation is significant. The obtained results are representative of local conditions at the measuring site. Characterisation of the complete transport process is needed, including study of emission time series. For the special case of CTBT monitoring though, analysis of really low activity concentrations being involved, establishing if detected radionuclide activity concentrations are more or less correlated to local meteorological conditions could help improve station-specific categorisation schemes (Plastino et al., 2010). The adopted methodology of analysis is explained in detail in Section 2.

Section snippets

Methodology and data analysis

If the time series data have not been excessively affected by missing data, the generalised Lomb Scargle periodogram (Lomb, 1976) can be evaluated in order to extract periodic components and quantify their percentage weight with respect to the total time series. Once peaks corresponding to periodicities have been removed, residual time series can be investigated. This is done evaluating the local Hurst exponent $H_{t}$ (Ihlen, 2012), related to noise type, and comparing time series outliers

Results and discussion

Results of the comparative analysis of beryllium and xenon time series will be presented and discussed. The harmonic and residual percentages are listed in Table 1, along with the periodicity that contributes the most. Having fully isolated the harmonic components, the residuals time series can be analysed. The local Hurst exponent obtained for ⁷Be and ¹³³Xe residuals time series is shown in Fig. 1.

The Hurst exponent H_t for beryllium shows small fluctuations and is almost always between the

Conclusions

A new methodology of evaluation of ⁷Be and ¹³³Xe concentrations in the atmosphere based on time series analysis, applied on atmospheric activity time series (measured at four CTBTO/IMS radionuclide stations) and to meteorological parameters have shown great variability in terms of noise structures, harmonic content, cross-correlation patterns and local Hurst exponent behaviour. Due to the very different physical and chemical characteristics of the two radionuclides, along with their different

Acknowledgements

Sampling data have been kindly provided through the virtual Data Exploitation Centre (vDEC), within the framework of an agreement between the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) and the Department of Mathematics and Physics - Roma Tre University.

All views expressed are under the sole responsibility of the Authors and not of the Provisional Technical Secretariat CTBTO. The authors would also like to thank the anonymous reviewers for their valuable comments and suggestions

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View full text

Evaluation of 7Be and 133Xe atmospheric radioactivity time series measured at four CTBTO radionuclide stations

Highlights

Abstract

Introduction

Section snippets

Methodology and data analysis

Results and discussion

Conclusions

Acknowledgements

Phys. A

J. Environ. Radioact.

Adv. Space Res.

Phys. A: Stat. Mech. Appl.

Ten years of development of equipment for measurement of atmospheric radioactive xenon for the verification of the CTBT

Pure Appl. Geophys.

Long-range correlation properties of coding and noncoding DNA sequences: genbank analysis

Phys. Rev. E

Detrended fluctuation analysis: a scale-free view on neuronal oscillations

Front. Physiol.

Evaluation of ⁷Be and ¹³³Xe atmospheric radioactivity time series measured at four CTBTO radionuclide stations