This invention relates in general to atmospheric physics and chemistry studies, especially those studies relating to the transport of continental gases and particulates. In particular, this invention relates to apparatus for monitoring the concentration of radon gas in airborne particulate matter.
Atmospheric investigations at sea have shown that the composition of the near-surface air changes rapidly and that radon (.sup.222 Rn) is a good indicator of the recent continental history of oceanic air masses. Radon gas concentrations can be used to indicate short-term variations in atmospheric composition due to the origin and nature of the air and weather phenomena. They may also be used to study the continental contribution to the atmospheric characteristics in coastal or oceanic areas and to determine air-mass transit times, and have been suggested as being useful in earthquake prediction.
Although radon exists as a gas at ordinary temperatures, airborne radon decays to produce solid radioactive decay products accompanied by corresponding emission of alpha, beta, and gamma radiation, the decay products of radon being generally referred to as "radon daughter products". Since radon and its daughter products are generally assumed to be in equilibrium in the air and the daughter products generally attach themselves to particulate matter, the daughter products may be collected by filtering the particulate matter from the air. Assuming that radon and its daughter products are in equilibrium, the radon content of the air may then be determined by monitoring the emissions produced by the decay of the daughter products. In particular, the beta emissions from radium B (.sup.214 Pb) and radium C (.sup.214 Bi) may be utilized to determine atmospheric radon concentration.
In general, prior art methods of monitoring the radon concentration are of two types. In the first type individual samples of the aerosol particules are collected on a section of filter paper and the single section is manually transferred to a detection device to determine the radioactive content. In the second type, the continual build up and decay of radioactivity on a single section of filter paper is monitored. The former method provides time resolution as good as desired for concentration resolution on the order of a few picocuries per cubic meter (pCm.sup.-3), but requires extensive personnel, particularly for round-the-clock operation. The latter method enables unattended monitoring for a day or more depending on the load build-up on the filter, but exhibits poor sensitivity for time or concentration resolution since rapid decreases in concentrations are masked by the effective half-life (approximately one hour) of the radon daughter products. Other prior art methods, such as double filtering, or collecting radon on activated charcoal with subsequent direct measurement, provide good sensitivity but require extensive personnel time and the data are not available until some time after collection.
It is desirable to be able to monitor radon concentrations on the order of a few picocuries per cubic meter for trace gas measurements and with time scales on the order of a few minutes to detect the arrival and measure the extent of different air masses or frontal systems over oceanic areas. The frequent measurements are necessary to help understand atmosperic phenomena such as the scale of turbulent eddies, aerosol distributions and the onset of fog. Short-term, real-time radon concentration data can also be used to optimize other experiments, such as finding the best start and stop times for longer-term experiments or sample collections. For example, this may be done to restrict these samples to one air mass or to a single frontal passage.