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Measurement techniques for radon progenies
About 10 % of lung cancer diseases result from inhalation of the radioactive nobel gas Rn-222 and its decay products.
The most dangerous nuclides are the alpha-emitters Po-218 and Po-214 because of the high energy transfer to lung tissue.
In the sense of ray protection is the measurement of radon as useful as the measurement of its decay products.
So this decision will depend on the properties of the available measuring methods. The following chapter gives an overview of existing common measuring methods.
1. Passive measuring methods
These methods work without power supply during sampling. Great number of pieces can be used at low costs.
On the other hand may exist great laboratory expense to get measuring results. All common devices measure radon gas:
- Adsorption method with charcoal for radon gas
After a sampling period of hours to about three days the charcoal and adsorpted radon as well as the decay products are measured with liquid scintillation or gamma-spectroscopy.
Various influences from atmospheric pressure, humidity, the type of charcoal... must be taken into account.
- Method using nucleus traces for radon gas
A measuring chamber with inlet filter is equipped with a synthetic detector foil. Nuclei from decays inside the chamber damage the foil.
By corroding procedures these damages result in visible tiny holes where its number is proportional to the radon concentration.
- Lucas-method for radon gas
Radon decays cause scintillation effects in a chamber with activated (ZnS) walls. These scintillations are counted by use of a photo-multiplier.
2. Active measuring methods
In general these methods use pumps and electronic detectors for automatic operation. The existing systems for radon gas and decay products measurements are mentioned below.
For radon gas the double-filter method, diffusion-chamber or puls-ionisation chamber are preferred methods. For decay products the filter method in various applications is well known.
- Double-filter method for radon gas
Air is pumped through a measuring chamber with a volume of some liters passing an inlet filter.
New decay products generated in the chamber are sampled and measured on an outlet filter.
- Diffusion-chamber method for radon gas
This method corresponds to the nucleus-trace method. Instead of a foil electronic detectors and additional concentration systems are used.
Decay products deposit on the walls and the detector. Often an electrical field between detector and chamber is used for higher deposition and resp. measurement efficiency.
- Ionisation-chamber method for radon gas
Charge pulses from decays in a ray sensible volume are counted. At high radon levels a direct current can be measured instead of single pulses.
- Filter method for radon decay products
Air is sucked through a filter while decay products deposit on. A detector measures radioactivity on the filter.
Various methods exist with different design, flowrate, detectors, sampling and measuring intervals and last but not least calculation algorithms.
The use of filter ribbons and automatic transport systems will allow long-term operation.
Continuously working devices on the basis of the filter method can supply good time-resolution of the Potential Alpha Energy Concentration (PAEC).
This method also supplies single nuclide activity concentrations of the decay products. Calculation errors may occur at fast progenies concentration changes.
Fast changes can be observed at low measuring intervals p.e. 15 min and attaching of new filters. For that case automatic step filter systems are available.