Beta Attenuation Technology for Particulate Matter Measurement
When beta rays strike a material, they can be absorbed, reflected or pass directly through. The attenuation of intensity in beta rays is proportional to the amount of material present. The attenuation through most materials is relatively consistent and is based on the electron density of the material (calculated by dividing the atomic number by the atomic mass). The attenuation for most materials is about 0.5, except for hydrogen and heavy metals.
The principle behind beta attenuation particulate sampling instruments (beta gauge) is that energy is absorbed from beta particles as they pass through particulate matter (PM) collected on a filter media. Beta gauge instruments have been designed to take advantage of this scientific principle to monitor/measure PM concentrations. The attenuation due to only the PM is measurable if a baseline beta count through just the filter can be established prior to sampling. The difference between the baseline beta count and the beta count after sampling is directly proportional to the mass of PM in the sample.
The two main components of a beta attenuation measuring system are the beta source and the detector. The beta source must be selected so that: it has an energy level high enough for the beta particles to pass through the air volume, collection media (i.e., the filter tape) and the particulate; it has enough source material so that a high count rate is present; it is stable over long periods of time; and it does not present a danger to the health of personnel that come into contact with the instrument. The source of choice has been Carbon-14 because: it has a safe yet high enough energy level; it has a half-life of 5,568 years; and it is relatively abundant. Many different types of detectors can quantify beta particle counts, but the ones most widely used are the Geiger Mueller counter or a photodiode detector.
The Thermo Scientific Model 5028i Continuous Particulate Monitor uses the radiometric principle of beta attenuation through a known area on a fibrous filter tape to continuously detect the mass of deposited ambient particles.
The beta gauge works by measuring beta counts before and after collecting PM on a filter media. The instrument will measure a clean area of the filter media for a fixed period to determine the baseline (e.g., 2 minutes), then it will advance that area of the filter to a sampling apparatus for another set period of time (e.g., 8 to 9 minutes), and finally steps back to the detector for a final reading. The difference in the beta count can be directly correlated to particulate mass through calibration of the instrument using a filter media containing a known mass of a particulate-like material. This step wise measurement must occur every 15 minutes to meet the data reporting requirements.
The beta gauge instrument is designed to provide a mass concentration. The instrument measures the volume of gas extracted from the stack/duct for each sample interval and calculates mass concentration in the specified units (e.g., mg/dscm).
From the above description of beta attenuation technology, the primary drawback is that it is a non-continuous monitoring technology. Typically only 4 PM reading per hour can be collected. The technology is also fairly expensive limiting its market acceptance. Its radioactive source also limits acceptance.