# Periodicity in Signals of Long-Term Gamma Background Measurements in the Chornobyl Exclusion Zone

A. D. Skorbun1, O. A. Kuchmagra1, B. M. Sploshnoy2, A. O. Doroshenko1

1Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine,
36a, Kirova st., Chornobyl, 07270, Ukraine
2Laboratory of Metrology of the SSE “Chornobyl Integrated Plant”,
72, Kirova st., Chornobyl, 07270, Ukraine

DOI: doi.org/10.31717/2311-8253.19.2.6

### Abstract

An automated radiation monitoring system (ARMS), which contains dozens of detectors, located throughout the 30-km zone, is functioning in the Exclusion Zone near the Chornobyl Nuclear Power Plant (ChNPP). Indications of detectors are regularly recorded and stored in the database. In the regular sets of measurements of the gamma background dose rate, it is revealed both explicit seasonal (winter-summer), and latent, shorter-period variations in the intensity of the gamma background, among which especially noticeable are diary changes. To prove that these diary changes are not related to the meteorological conditions or the equipment operation features, signals from several detectors for the period of May 1–15, 2018 have been analyzed. It is concluded that observed periodic signal changes are not equipment effects. For the purpose of checking the randomness of detected periodic changes in the intensity of signals of the gamma background, data from six observing points have been analyzed. Observing this effect simultaneously in many independent observation posts undeniably proves its reality. It is proved that the detection units and the ARMS as a whole are not sensitive to external influence factors, such as heating of the
detector, the parameters of the power supply network ~220 V, and to the electromagnetic and electrostatic fields.
The effect of daily changes in the signal intensity is quite large sometimes, visible by the naked eye, and the magnitude of the oscillations in the examples considered is approximately ±10%. It significantly exceeds the noise component of the registered signal.
Comparison of data from many observation posts leads to the conclusion that the observed daily periodicity is not an accident. The nature of such intensity changes remains unclear. We emphasize that unlike the work of [8], in which daily and other oscillations of radon decay rates are directly associated with processes on the Sun, on the basis of our data, such a conclusion can not be made, primarily because the fixed maximum and minimum of signal values are observed at approximately 17 and 05 hours, respectively. And not at noon and midnight, when the Sun’s influence has maximum or minimum, respectively.
On the plots of the analyzed signals and in the pictures of their wavelet coefficients, one can see an obvious decrease in the intensity of modulation and interruptions in the regularity of the daily flow, which also can not be associated with the peculiarities of the equipment or the meteorological conditions. The detected periodic changes outside the noise signal of the gamma background should be taken into account when calculating the uncertainties of the results obtained during the implementation of radiation monitoring and prediction the radiation situation in the Exclusion Zone.

Keywords: gamma background, monitoring, periodic changes.

### References

1. Skorbun A. D., Spirin A. I., Sploshnoy B. M. (2018). Analiz dolgovremennih izmereniy gamma-fona v Chernobilskoy zone otchuzhdeniya [Analysis of long-term gamma-background measurements in the Chornobyl exclusion zone]. Proceedings of the XXV Annual Scientific Conference of the Institute for Nuclear Research of the National Academy of Sciences of Ukraine (April 16–20, 2018, Kyiv, Ukraine). Kyiv: Institute for Nuclear Research, pp. 78–79. (in Ukr.)

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Published
2019-08-31

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