Radiometric surveying on a rainy day...
Rain is not the best weather condition for performing a survey, especially when you are driving a quad. You can use good clothing to prevent te surveyor from being wet, but how does it affect the measurements?
The effect of soil moisture on the concentrations
In other words: what is the effect of soil moisture content on radionuclide concentrations measured in the field? This question has been studied in some detail for instance by Alma de Groot in her thesis research performed at the University of Groningen. In one of her papers, she describes how soil moisture does affect the signal intensity. An increase of 10% in soil moisture results in a 10% decrease in intensity of the signal, but this has no effect on the spectral shape (an ratio of the concentrations). In general soil moisture differences are small and not expected to be more than 10%.
Radon in the air
Radon (222Rn) originates in the decay of radium (226Ra, half live 1600 y) that is present in all uranium bearing soils. The 226Ra is either trapped inside soil and rock particles or dissolved in the soil water phase after leaching from the soil particles. After decay of 226Ra the radon atom obtains some recoil energy that may allow the atom to escape (emanate) from the soil particle. Only radon atoms that result from decaying radium atoms close to the surface of the soil particle may emanate to the soil pore space. Typically, the fraction of radon atoms that reaches the soil pores is between (0.01 and 0.4). The radon emanation factor for uranium ore samples is reported as 0.3.
In the soil pores radon partitions between the water and the air phase, also a fraction will be adsorbed on the surface of the soil particles. The ratio of radon concentrations in the air and water phase is described by a Henry’s type of law. The ratio is slightly dependent on temperatureiii.
Migration of radon through soils is a multiphase (water, air and possibly adsorbed to soil surfaces) phenomenoniv driven by pressure (advection) and concentration (diffusion) differences and/or buoyancy forces leading to geogas (N2 CO, CO2, Rn) bubbles that ascend through to water phase.
The half live of radon is 3.8 days. This limits the time to transport radon between places. For diffusive transport in dry soil, transport lengths are typically 1-2 m. For moist soils these lengths are in the order of 10’s of cm.
When radon is released from the soil, it will accumulate in the air at higher elevations. When it starts raining, this radon will be rained down onto the surface of the soil and onto the detector! Especially after a period of drought, the amount of radon transported by the rain can be high. However, when it has been raining for some time, the transport of radon by rain will be small.