From the definition of the measurement parameter PAEC (Potenial Alpha Energy Concentration) the simple measurement principle becomes clear::
- Draw air through a measurement filter that collects the decay products as completely as possible;
- Record and count the alpha decays;
- Apply an algorithm to calculate the PAEC or EEC from the counting results.
According to the definition, PAEC could thus be measured by drawing a (measured) volume of air through a filter within a specified time interval and allowing the collected decay products’ activity to completely decay. This occurs after approximately 3 hours (see half-lives). Alpha decays must be recorded from the start of pumping until complete decay is achieved. For precision, the two nuclides Po-218 and Po-214 would be registered separately, multiplied by their respective energies, and divided by the air volume. However, I will not discuss this discontinuous method here.
Even if the filter exposure occurs continuously and the alpha decays are counted continuously without separating the nuclides, the measurement parameter PAEC (or EEC) can still be determined with sufficient accuracy. This is achieved by converting the primary measurement result—the number of alpha decays per unit of time—into PAEC/EEC. The practical measurement method I am proposing is based on continuous measurement.
Note:
The measurement of radon decay products has a long tradition in radiation protection in mining. In the past, numerous short-term measurement methods were developed for this purpose. However, in recent years, these measurements for practical radiation protection have largely been replaced, at least in Germany, by the measurement of radon gas itself.
Even though the measurement principle is very simple, a few critical points must be considered:
- A high-efficiency filter is required,
- The airflow rate must be reliably determined, and
- The detection efficiency of the alpha radiation must be known.
I hope this does not discourage anyone who has already been considering building a measurement apparatus. There are achievable solutions for all of these requirements.
Here is the similarly simple equation for calculating the measurement parameter EEC.
Equation (1):
with:
- a: constant that depends on the units used,
- N: number of counted pulses,
- η: detection efficiency of the alpha particles (ratio of generated alpha particles to those detected by the detector; determination is described in the practical section),
- Q: airflow rate (determination is described in the practical section),
- T: measurement time or counting interval.
If the units
- T in seconds (s) and
- Q in liters per minute (l/min)
- EEC in Bq/m3
are used, the constant is a=13.2 To calculate the result as PAEC in the unit nJ/m3, simply use the constant a=13.2×5.56=73.39.
After the start of pumping, the collection of decay products on the filter begins, meaning that the accumulated activity on the filter increases. At the same time, however, the decay of the collected activity also begins. This results in an exponential increase until a state of equilibrium is reached, provided the measurement parameter remains constant during the observed period. For radon decay products, this equilibrium state is reached after approximately 3 hours.
Strictly speaking, Equation (1) is only valid after equilibrium is established. This should be taken into account when interpreting measurement results. Now the question arises: what if the concentration of radon decay products changes during the measurement? Due to the time-dependent behavior, changing concentrations are effectively “smoothed out” over the measurement period. For averaging over a longer period (e.g., 12- or 24-hour averages), this poses no problem; only the first 3 hours of measurement might be disregarded. A time correction for the first 3 hours is also possible but is not necessary for the simple DIY approach intended here.
That’s enough theory for now. I will address the measurement of thoron decay products when describing the analysis process.
Measurement technology
The measurement is conducted in principle in the same way as is common in nuclear radiation measurements. Here, I would like to focus only on the most important points for the project.
The first stage of detecting radioactive radiation involves a detector that produces a signal, which can be electronically processed, in response to the radiation. Typically, these signals are so small that they cannot be directly processed by subsequent electronics. Therefore, the detector signal is routed through a so-called preamplifier for further signal processing.
The decay products deposited on filter emit both alpha radiation as well as beta and gamma radiation. For measuring radon and thoron decay products, detecting alpha radiation is the most practical and commonly used method due to its advantages, both in terms of defining the measurement quantity PAEC and for technical benefits (such as minimal or negligible background noise and a higher signal amplitude compared to beta and gamma radiation). Typically, professional setups use so-called semiconductor detectors, which are essentially large-area silicon diodes designed so that alpha radiation can reach the depletion zone of the diode. When an alpha particle strikes the sensitive layer, it generates charge carriers in the depletion zone of the diode, which are collected due to the applied voltage, creating a current pulse. The time integral of this pulse is proportional to the amount of generated charge and, thus, to the energy of the alpha particle. However, this pulse must then be converted into a usable voltage pulse. This is achieved by a so-called preamplifier. In the preamplifier, the charge-dependent signal is converted into a voltage pulse via the input capacitance and an amplification stage, with the pulse amplitude being proportional to the generated charge.
In our DIY version, pulse height analysis is not included. Instead, all alpha particles that reach the detector, regardless of their energy, will be registered—a method also known as “gross alpha counting”.
In brief:
- To measure radon decay products, air is continuously drawn through a measurement filter.
- A detector positioned opposite the filter registers a known and constant fraction of the alpha radiation emitted by the activity collected on the filter as counts.
- Subsequent evaluation electronics store the count results N for the counting period T.
- The result for the measurement quantity EEC (PAEC) is then calculated using Equation (1).
It is possible that I didn’t explain the basics of the measurement in enough detail in the introduction to fully convey the measurement principle. I apologize for this. I would be grateful for any suggestions to improve clarity.