It is impossible for radiation detection systems to “see” 100% of all the disintegrations (whether counting alphas, betas or gammas) produced by a given radioactive sample. Different instruments offer varying degrees of counting efficiency. Counting efficiency is defined as the fraction of the total radioactive decay events detected and recorded by a radiation detection instrument. It is determined by comparing the number of counts per minute (cpm) a detector is capable of seeing to how many dpm (disintegrations per minute) are actually being emitted from a known activity. For radon/RDP measurements, efficiency may be expressed in cpm per pCi or cpm per dpm.
Once the efficiency is calculated, a calibration factor (CF) is derived. The CF is then applied to the gross counting rate for converting raw counts per unit time to pCi or WL. Depending on the method used, the CF (Calibration factor) may vary with the time that has elapsed since the sample was taken.
Radon/RDP measurement must account for sample decay rate. Once a sample is collected, or in the process of being collected, it will begin to decay at a rate relative to its radioactive half-life. Based on that half-life and elapsed time before analysis, a decay factor must be applied to the measurement calculation. If the measurement technician improperly records the start/stop date and time of the sampling period, the calculated decay factor will be incorrect and the measurement rendered invalid.
In addition to determining the counting efficiency, and decay factor, the measurement process almost always includes a correction for background radiation. Background activity can be caused by low-level ambient radiation, a residue from a previous sample or even electronic noise within the counter. Background counts must be distinguished from those that are generated only by the sample. To do this, a blank or empty sample is counted and the background cpm are subtracted from the gross cpm of the sampling being analyzed, yielding the net cpm attributed to the sample activity.
Thoron, radon-220, is an isotope of radon that may interfere with some types of radon-222 measurements. The parent isotope for thoron is thorium-232, a primordial element that is widely spread in soils and rocks. Thorium-232 has a half-life of 1.41 x 1010 years (4.1 billion years). See Figure 4-3.for the decay chain of thorium.
Measurement devices vary greatly and can affect the accuracy of the radon measurement. EPA and ANSI/AARST require all radon detection devices to have an accuracy factor of +/- 25% of the radon concentration in known environment such as a radon chamber.
The precision of the measurement device directly correlates with the accuracy of final result. Precision measurements are conducted in the field as part of a quality control program and are also called duplicate measurements. Precision measurement are conducted with 2 devices located side-by-side, approximately 4 inches apart.
The ability to track hourly readings on a graph allows the professional and the consumer visual evidence of radon spikes or ventilation effects that contribute to false-high or false-low results.
Approximately 5% to 10% of radon in the environment will be the isotope radon -220 or thoron. This means there is the potential of overestimating the amount of radon-222 in a measurement by inadvertently including the decays from radon-220 (and its decay products) in a radon-222 calculation. Fortunately, the relatively short half-life of radon-220 (55.6 seconds) and the long half-life of lead-212 (10.6 hours) means that radon-220 and its decay products will not interfere in most measurements which sample for radon gas, i.e. grab samples, continuous radon monitors, and passive integrating devices.
|Approximately 5% to 10% of radon in the environment will be the isotope radon -220 or thoron.|
When measuring decay products, however, the long-lived lead-212 in the air being sampled will be collected onto the filter and will subsequently decay into polonioum-212 an alpha emitter. Some continuous working level monitors have a built-in correction for subtracting the polonium-216 and polonioum-212 contribution, but some do not. For continuous working level devices that do not self-correct and for grab working level devices a correction can be made by waiting several hours and recounting the filter. All the new counts are assumed to be attributed to polonium-212 and these counts are used to calculate the radon-220 that is then subtracted from the radon-222 calculated previously.
Interference-Resistant Testing Devices – Optional/Best Practice
A particularly effective interference prevention and detection is the use of interference-resistant testing devices. It is particularly important to use due diligence in preventing interference with the test environment to ensure the most accurate radon readings are disseminated. Many factors, intentional or unintentional, can interfere with this process such as weather, homeowners, mechanical equipment, building contractors, or measurement operator error.
Detecting False-High or False-Low Test Results
According to EPA, using equipment with tamper-detection features provides a greater chance of detecting interference and can also limit liability, provide credibility in test results, and give the measurement provider peace of mind during the test. The following are factors that help prevent or detect false test results:
Ability to Measure Environmental Factors
The ability to track temperature, barometric pressure, and/or humidity in the test environment provides the measurement expert an opportunity to detect many variables that can cause radon spikes or drops. Some scenarios might include:
Ability to Integrate Radon Measurements Over Short Intervals Of Time
In order to meet the single test device option for measurements conducted in the context of a real estate transaction the testing device must be able to meet the following two requirements:
Ability to Graph and Print Consecutive Hourly Readings
Testing devices that provide software or laboratory reports itemizing and graphing hourly readings can assist even the most experienced radon professional in detecting hidden spikes or excessive ventilation of radon. Most important, these graphs can substantiate interference and might ensure payment for retesting services. It can be difficult to advocate for the test environment and ultimately the client when there’s no physical proof of interference.
Tilt indicators and power on/off indicators also present evidence of physical tampering. Although interference may appear intentional, the circumstances may be entirely accidental. The Pre-test Consultation and the signed Non-Interference Agreement are critical in these situations. The signed Non-Interference Agreement can also be a useful and necessary tool for the radon professional for receiving payment for retesting services.