Gamma flux is monitored continuously on the St Paul Campus Climatological Observatory using sodium iodide scintillation counters mounted 2 meters above the surface. Total counts are recorded every 15 minutes. Two counters are employed so as to confirm observations of large short period fluctuations and to provide some backup for cases of instrument failure. The purpose of this study is to obtain a quantitative description of the gamma background and its temporal variation and to gain an understanding of the physical processes producing this structure.
Since the termination of nuclear bomb testing most of the gamma flux at the earth's surface originates in the decay of naturally occurring radioactive elements in the uranium and thorium decay series as well as radioactive potassium, K40. The gamma flux background in the absence of precipitation arises primarily from decays in the upper 30 cm of the earth surface. Radon (Rn222 a radium daughter in the uranium series) is a gas with a half-life of 3.82 days. The decay of radium atoms near the surface of soil grains releases radon which may escape to the atmosphere; the flux rate over land at midlatitude is estimated to be of the order of 1 atom/cm2 sec. It is the decay of the second and third generation progeny of Rn222 originally termed radium B and C (RaB and RaC) that is the principal source of gamma flux from the atmosphere.
If periods of precipitation are excluded the contribution to the gamma flux at the earth's surface by decays in the atmosphere may be no greater than 10%. However, since the source of radon and moisture in the atmosphere is at the earth's surface, rising air currents which produce the cooling necessary for condensation and precipitation thus have relatively high concentrations of radon as well as water vapor. Precipitation particles formed in this environment therefore contain a relatively large concentration of radon progeny which decay after reaching the surface. The resulting gamma flux at the surface during a summer rain may reach a level twice that of the normal background. An example of high gamma activity during a rain is illustrated in a time series histogram of fifteen-minute observations of gamma counts and precipitation for the 24 hours starting at 12:00 CST, 21 August 1999 (Figure 1).
Once precipitation ends the effective decay time of the excess gamma flux usually falls in the range of from 50 to 40 minutes which corresponds to that for a mixture of RaB and RaC with an initial concentration of RaB greater than 50% and the return to gamma background level is essentially complete within two hours. If there has been very heavy precipitation or if the precipitation is snow there may be a layer of H2O on the surface which absorbs gammas from the soil and the background gamma flux decreases. An example of the time series in which gamma counts decreased by 20% after the snowstorm 8-9 March 1999 is shown in Figure 2. Gamma counts are recorded every 15 minutes. Hourly precipitation values are estimates derived from the Local Climatological Data for Minneapolis-St Paul which recorded a water equivalent precipitation of 1.20 inches for these two days and a snow depth of 13 inches at 06:00 on the 9th.
A time series of gamma counts for the period 16-17 August
2000 illustrates the decrease in background after a moderate rain which
followed a dry spell (Figure 3). The decrease
in background is 6% in this case and the return to the higher background
occurs within 24 hours.
A further example of meteorological factors controlling the gamma flux is that of diurnal variation. The diurnal variation of heating and cooling at the earth surface results in a diurnal cycle of convection and mixing in the atmospheric boundary layer. Radon escaping from the soil accumulates in the lower boundary layer during the night when convective mixing is reduced thus increasing the gamma flux at the surface. The time series of gamma counts for both instruments for the three days, 10-12 July 1999 illustrates the magnitude of this effect (Figure 4: note the change in scale). The trough to peak amplitude from afternoon of the 11th to 06:00 of the 12th represents an increase in the gamma background of 9%. The linear best-fit line illustrates the overall increase in background of 7% over the three-day period. This longer period increase is believed to be due to drying of the upper layer of the soil after the shower on the afternoon of 8 July.