Technical Description of the
OnGuard Continuous Radon Monitor (CRM)

The Radon Monitor is a patented, state-of-the-art continuous radon monitor designed to provide high accuracy, reliability and simplicity of use. The Radon Monitor is fully electronic and operates on non-rechargeable, non-user replaceable alkaline and lithium batteries. The Radon Monitor is always ready to conduct a radon test because no AC power is ever needed to operate the Radon Monitor or to recharge its batteries. The batteries are sized to continuously power the Radon Monitor between annual calibration intervals. Figure 1 illustrates the Radon Monitor’s mechanical layout. Its sensor is a current-type ion chamber. Radon enters a decay volume within the Radon Monitor by the passive diffusion of air through filtered, hidden, recessed openings in the Radon Monitor’s sides. The filters allow only radon gas to enter the decay volume.

The alpha and beta particles from the radioactive decay of the radon and its progeny penetrate a mylar window on the upper surface of the ion chamber and ionize the air inside the ion chamber. The charges generated by that ionization and by the ionization from background gamma ray and cosmic radiation are collected and measured by a charge amplifier circuit that is one-thousand times more sensitive than any available amplifier. The output is sampled ten times per second and is analyzed in real-time. Signal processing electronics separate the output signal into alpha events and the level of radiation from the beta particles, gamma rays and cosmic radiation. Individual alpha events are validated, counted and quantified by their magnitude and collected in a histogram of the number of alpha decay events by their magnitude. Figure 2 is an electronic block diagram of the radon monitoring system.

The Radon Monitor can be likened to an electret that never has to be recharged and whose output voltage is constantly measured in real-time and analyzed in exquisitely fine detail. When the voltage on the Radon Monitor’s collecting electrode (its “electret”) reaches a predetermined level, the Radon Monitor reverses an applied electric field bias voltage, e.g., from positive to negative. The collecting electrode’s output voltage begins to change in the direction opposite to what it was before the bias reversal because it is now collecting charges of the opposite sign. When the collecting electrode’s voltage reaches a second predetermined level, the Radon Monitor again reverses the applied electric field bias voltage and the output voltage again changes in the opposite direction. In this way, the Radon Monitor operates continuously without ever having to recharge its “electret”.

The alpha histogram, background radiation level, ambient temperature measurements, and other Radon Monitor performance data, including a noise signature, are accumulated in hourly records, beginning from the time the Radon Monitor is commanded by the user software to start collecting data for a radon test. Note, this detailed information is used by the user software and is not available to the end user. Figure 3 shows a typical alpha event histogram. This is from a 71 hour test with an average radon level of around 58 picocuries per liter per hour. The alpha event histogram is generated and recorded for each of the ion chamber electric field bias directions. The hourly records are stored in non-volatile memory with capacity for 240 hours of this and other data. The Radon Monitor stops recording hourly data when commanded to do so by the user software or when 240 hours of data have been collected. This ensures that no data is ever overwritten. Once the test has been stopped, the recorded information in the Radon Monitor is downloaded to either version of the user software. The primary tester software or the secondary tester server sums the alpha histogram and calculates the hourly and average radon levels by applying Radon Monitor-specific calibration coefficients that are downloaded from the Radon Monitor with the test data. The data download speed is about 0.3 seconds per hourly record, so that a typical 48-hour radon test is downloaded in about 15 seconds. The download process is fail safe. If for some reason, a data download was unsuccessful, it can be repeated as many times as necessary.

The Radon Monitor operates continuously, even when it is not formally gathering data for a radon test. This means that self-checks of the ion chamber and its electronics can be constantly performed, providing high assurance that accurate test results can be obtained. For example, the Radon Monitor operates reliably and accurately as long as the ion chamber’s output operates between the two set points, and the Radon Monitor’s circuitry constantly monitors the operating range. Other self-checks ensure that every necessary communication within the Radon Monitor is taking place as expected and that sufficient battery power exists to complete a standard test. If any of these checks fail, the Radon Monitor cannot be used to start a new radon test or complete an ongoing radon test.. As additional operating experience with the Radon Monitors is gained, the alpha histogram, background radiation and noise signature data will be used for even more sophisticated operability self-checks.

The Radon Monitor displays radon levels from 0 pCi/l to 99.9 pCi/l and temperatures from 0°F. The Radon Monitor is capable of measuring higher radon levels, but because of inaccuracies introduced by the multiple alpha decay events that occur simultaneously at higher levels, the higher levels of radon are not reported. The Radon Monitor’s sensitivity is 0.2 alpha events per minute per picocuries of radon per liter of air. The Radon Monitor is 7 1/4 inches high, including its handle, and is 4 3/8 inches wide across the flats. It weighs less than 1.5 pounds. Its housing is made from injection molded cycoloy, a blend of ABS and polystyrene. The handle and shock absorbing bumpers are strong santoprene rubber.

Figure 1
OnGuard Systems Professional Radon Monitoring System
Radon Monitor Mechanical Drawing

Figure 2
OnGuard Systems Professional Radon Monitoring System

Figure 3
OnGuard Radon Monitoring System

Alpha Event Histogram

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