Variation in γ-ray count rates as a monitor of precipitation rates, radon concentrations, and tectonic activity

Abstract

Precise and reproducible correlations between environmental γ-ray count rates (GRR) with rain precipitation rates (RPR) and snow precipitation rates (SPR) have been measured to be 0.18±0.02 and 4.3±0.04 (counts/s)/(mm/h), respectively. The GRR, observed with two 12.3 cm3 NaI detectors, also exhibit a diurnal cycle (period of 1.01±0.03 day, maximum around sunrise and location dependent amplitude) and a seasonal cycle (smaller amplitude and maximum in mid-February). Increased GRR during precipitation have been observed at ground level, but the effects of the diurnal cycle and other noise are reduced by more than 20 at 15 m above ground. An improved signal-to-noise ratio, response time of less than one hour and precise correlations between the peaks in GRR and precipitation rates enable real-time determinations of RPR and SPR to better than 1 mm/h. The relative insensitivity of these correlations to other meteorological conditions enables GRR to serve as remote and environmentally stable sensors of RPR and SPR. The time-dependence of GRR is consistent with that expected from variations in the presence of radon and radon progeny in the atmosphere. Change in the amplitude of the diurnal cycle may serve as a monitor of radon concentration. A γ-ray detector integrates over a large volume with good time resolution and thus may be responsive to real-time changes in radon concentrations due to increased exhalation from tectonic activity.

Predicting terrestrial <sup>222</sup>Rn flux using gamma dose rate as a proxy

“…It suddenly increased after the first rain event from 85 nSv/h to 110 nSv/h, an increase of 29%. This effect is caused by outwash of particles from the lower atmosphere, carrying previously absorbed 222 Rn progeny back to the soil surface (Greenfield, 2002(Greenfield, , 2003. The cummulative half-life of the short-lived 222 Rn progeny is about 50 min.…”

“…At the same time, low soil moisture leads to reduced shielding of gamma-rays and a larger proportion of them can be detected in the atmosphere above the ground. Diurnal changes in the amplitude of GDR during periods without precipitation are supposed to be influenced by changes in Rn and Rn-progeny concentrations in the near surface air, where they accumulate during atmospherically stable conditions at night (Greenfield, 2002(Greenfield, , 2003.…”

Determination of rain age via γ rays from accreted radon progeny

“…A diurnal cycle of exhaled radon and its progeny gives rise to a diurnal cycle of atmospheric GRR, which at ground level has measured amplitude on the same order of magnitude as the background from all other sources combined. 13 This activity peaks just before sunrise and is then largely carried to higher elevations via vertical convective and turbulent transport of the vapor and/or particulate matter to which it is attached. This cycle of exhaled radon progeny is the source of the GRR that we use as a natural tracer of precipitation, yet atmospheric GRR from this cycle can mostly be eliminated as a source of background when measurements are made above about 15 m rather than at ground level.…”

“…Measurements of atmospheric GRR at International Christian University 13 ͑ICU͒ began shortly after a nuclear accident in Tokaimura, Japan in 1999. A diurnal cycle of exhaled radon and its progeny gives rise to a diurnal cycle of atmospheric GRR, which at ground level has measured amplitude on the same order of magnitude as the background from all other sources combined.…”

“…13 The age of the rain drop is determined from its accreted activity, which is thus proportional to the integral surface area of the rain drops. From the drop size distribution, see Fig.…”