Answer to Question #10548 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

The following question was answered by an expert in the appropriate field:

I have a problem with a radon source, PYLON RN-1025. How can I calculate the radon concentration in "continuous operation" mode? In the manual, there is the term t1, but I don't understand what it is.
I cannot respond precisely to your question because I am not sure about the kind of system into which the radon is being introduced. I will try to provide some comments that may be useful.

The manufacturer should have specific information available as to the expected generation rate of radon by the RN-1025 source. These dry type sources presumably release all of the radon produced by the parent 226Ra decay, so that the production rate and the release rate are well correlated. I assume that you are flowing the radon into a mixing chamber of some sort, and it is the radon concentration in this chamber that is of interest to you. If air is being pumped continuously through the pylon source, the radon input rate into the mixing chamber can be reasonably estimated from the manufacturer’s given information. If the input rate is given by P (e.g., in Bq s-1), the radon gas is being pumped into the chamber continuously, and the chamber is closed except for the radon input line, the radon concentration, C, in the chamber at any time, T, following the start of input may be estimated from

C = (P/Vk)(1 – e-kT),

where V is the combined volume of the chamber plus any void volumes associated with tubing, the source holder, etc., and k is the effective removal rate constant for the radon in the chamber (e.g., s-1). This equation comes from simple considerations regarding assumed first order linear kinetics in which it is assumed that the removal rate of radon is proportional to the quantity of radon present in the system at any time. There is an implied assumption of uniform and instantaneous mixing of radon with all air in the system. In reality this assumption may not apply at all times, depending on the efficiency of mixing in the chamber, and some time may be required before uniform mixing applies.

If there are no removal mechanisms other than radioactive decay, the value of k in the equation is equal to the radioactive decay constant for radon—i.e., ln2/(3.824 day)(24 h day-1)(3600 s h-1) = 2.098 x 10-6 s-1. This same equation would apply if the radon is being pumped one way into the chamber as long as the chamber volume is very large compared to the volume of air that was introduced into the chamber through the source. If this was not the case, the buildup of backpressure in the chamber could affect the input rate of radon from the source. If the chamber had an air outlet that was connected in a closed loop to the air intake of the source holder, this condition (backpressure) would not be a factor of concern.

Concentrations are best confirmed using appropriate measurement techniques. These include the use of such devices as calibrated Lucas-type flow-through scintillation cells (examples can be seen on Pylon’s website, - cells designated as 110A, 300A, 200A). Other instrumental means, including devices such as calibrated ionization chambers and solid state electret devices have also been used for concentration measurements.

I hope this is helpful to you.

George Chabot, PhD
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