Beryllium-7 activity at Comprehensive Nuclear-Test-Ban Treaty stations hosted by the United Kingdom

Abstract

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) is supported by a global network of monitoring stations that perform high-resolution gamma-spectrometry on air filter samples for the identification of radionuclides indicative of nuclear weapons tests and reactor incidents. These daily measurements have created an invaluable resource for understanding variations in natural background radioactivity, including the contribution of 7Be. Statistical analysis has been performed on 7Be data collected by CTBT stations hosted by the United Kingdom including at British Indian Ocean Territory (RN66), St Helena (RN67) and Tristan da Cunha (RN68) during 2005–2013. The results have been found to follow a lognormal distribution which implies that the 7Be activity is the multiplicative product of many small independent factors, such as meteorology, elevation, local station conditions, sample acquisition and analysis. This has the potential to identify discrepant measurements not attributable to the intrinsic variability of the distribution and indicative of station malfunction. Variations in 7Be activity have been considered on monthly, weekly and daily timescales and characterised using the geometric mean in accordance with the properties of the lognormal probability density function. Seasonal variations have been identified, with summer maxima and winter minima that are attributable to changes in mixing within the stratosphere and troposphere. Such fluctuations have been examined using the Fast Fourier Transform which may indicate variations associated with the 27 day solar cycle.