Analysis of gamma radiation from a radon source: Indications of a solar influence
Highlights
► The decay of radon is influenced by factors other than the geological and atmospheric environment. ► Radon decay variations exhibit oscillations previously found in decay data from several other nuclides. ► Some of these oscillations seem to be attributable to solar rotation. ► These oscillations are stronger in nighttime data than in daytime data.
Introduction
Radon (222Rn) is a radioactive inert gas formed from the disintegration of 226Ra as part of the 238U decay chain. It occurs with varying concentrations in the geological environment, and is widely used for tracking temporally varying geological processes [1]. The nature of the physical processes driving the temporal patterns observed in 222Rn time series is not clear, particularly the extent to which environmental parameters, such as temperature, atmospheric pressure, and humidity, can influence the 222Rn level [2], [3]. To help understand these processes, Steinitz et al. have carried out experiments that track the variation of alpha and gamma radiation arising from a source of 222Rn in a closed volume of air [4]. In this article, we analyze a sequence of 28,733 measurements of gamma radiation acquired in an experiment (EXP #3 in [4]) over the time interval 28 January 2007 to 10 May 2010. The experimental setup is described briefly in Appendix A, and environmental drivers of radon production are discussed in Appendix B.
In Section 2, we present the gross features of the data sequence, which exhibits a strong dependence on time of day and time of year. We also show similar displays of the laboratory temperature and of the solar elevation, finding a stronger correlation of the gamma-detector measurements with the latter than with the former. In Section 3 we present an analysis of the measurements in terms of frequency and time of day, finding that most of the modulation is associated with nighttime measurements. For this reason, we also present separate power-spectrum analyses of the daytime and nighttime measurements. In Section 4, we compare these results with those of our previous time-series analyses of BNL and PTB data [5], [6], [7], and we comment on a recent study of the decay rate of 137Cs experiment carried out at the Grans Sasso Laboratory [8]. Our results are discussed in Section 5.
Section snippets
Overview of the sequence of gamma-detector measurements
The mean daily count rate of photons detected by the gamma detector is 2.3610e+05. These counts are due primarily to the beta decay of 214Pb and of 214Bi, but there are other sources of gamma rays in the 222Rn decay chain, and one cannot rule out that some counts may be due to Compton scattering of ambient photons by beta-decay electrons.
Fig. 1 shows a plot of the daily count, normalized to mean value unity, as a function of time. Since the early values are anomalous, we exclude them from
Power spectrum analyses
We have carried out a power-spectrum analysis of the gamma-detector measurements when broken down by hour of day. For the frequency band 0–5 year−1, the result is shown in Fig. 11. The strongest feature corresponds to an annual oscillation centered on mid-day. The next strongest feature (frequency 2 year−1) is a semi-annual oscillation. This is mainly a night-time feature, but it has a peak at 17 h.
Fig. 12 covers the frequency band 5 –10 year−1. The oscillations in this band all occur at nighttime.
Comparison with BNL and PTB data
It is interesting to compare the results of our analysis of the GSI data with the results of similar analyses of the BNL (Brookhaven National Laboratory) data [10] and PTB (Physikalisch-Technische Bundenanstalt) [11] data. We have previously shown the results of simple power-spectrum analyses of these two datasets [5], [6], [7]. However, to facilitate visual comparison with our present analysis of GSI data, we now show instead the results of time–frequency analyses.
We show in Fig. 18, Fig. 19
Discussion
The patterns we see in the GSI radon measurements are so striking as to be quite incompatible with the textbook picture that nuclear decay rates are all constant. The first question to be addressed is the extent to which the variations have an environmental or experimental origin. Fig. 6, Fig. 7, Fig. 8, Fig. 9 show that the gross variations of the gamma-detector measurements are quite similar to those of the ambient temperature. However the annual waveform of the gamma-detector measurements
Acknowledgement
The work of EF was supported in part by US DOE contract No. DE-AC02-76ER071428.
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