Radon measurement
The Radon measurement is used to determine the proportion of the radioactive noble gas radon in the air we breathe. There are various measuring devices for the measurement, depending on the application. They range from the smallest passive radon exposimeters to very complex, computer-controlled collection devices.
Radon gas measurement or decay product measurement
From the point of view of radiation protection, measurements of the radon gas and also the radon decay products are useful. The noble gas radon is the cause of lung exposure and the second most common cause of lung cancer after smoking . The decay product measurements record the effect on humans as well as possible.
When deciding between measuring radon or its decay products, it is often the special properties of the available measuring methods that have to be taken into account, rather than the aspects of the radiation effect. Radon measuring devices are usually easier to use and cheaper than devices for decay products. In order to keep the entire effort low with a large number of measurements, one begins with a radon gas measurement. Only if the measurement result is close to a legal or recommended limit value is a more complex and meaningful measurement of the decay products carried out. The radiation protection requirements for mines always provide for decay product measurements.
Passive and active measurement methods
There are a large number of different measuring devices for radon measurements. It is common to classify them according to whether they measure radon gas or decay products and whether they work actively or passively.
Small passive measuring devices are often referred to as radon dosimeters or exposimeters. They work during the measurement without operating energy and can therefore be used inexpensively in large numbers. This is offset by a large amount of laboratory effort to get the measurement results indirectly contained in the devices. Passive devices practically only measure radon. This is one of the reasons why, with a few exceptions, radon gas is measured and assessed in residential buildings. Various measuring laboratories send them by post and evaluate them later in a laboratory.
Active measuring devices mostly work with pumps and electronic radiation detectors and therefore require electrical energy during the measurement. This makes them suitable for automatic operation. An evaluation laboratory or separate evaluation computers are rarely required.
Microelectronics enables electronic devices that can measure with a battery for up to a year. In terms of handling, they correspond to passive devices, but do not require an evaluation laboratory.
Principles of radon gas measurement
overview
There are a large number of different measuring devices for measuring radon gas, but only a few measuring principles. Most of the processes are based on the idea of the double filter process. It is quite difficult to detect individual radon atoms in a volume of air. That is why it is not the radon itself that is measured, but the decay products resulting from it in an apparatus.
Virtually all radon gas detectors need to be calibrated . This happens in radon chambers with a known concentration of radon gas. Special radium preparations are used to generate radon.
Double filter process
The double filter process is available in numerous variants. A pump draws air through a measuring chamber with a volume of a few liters. An inlet filter holds back decay products that are already in the room air. In the chamber, new decay products are formed from the radon gas, which are deposited on an outlet filter (measuring filter). A radiation detector measures the radioactivity of the outlet filter. In short-term operation of up to a few hours of collection time, relatively complex evaluation calculations are required. In continuous long-term operation, the counting rate of the radiation detector is proportional to the radon gas concentration.
Diffusion chamber process
The diffusion chamber method is most widely used for radon gas measurements . The idea corresponds to the double filter process, but the natural gas diffusion replaces the pump. The measuring chamber is much smaller than with the double filter method. The measurement sensitivity is usually low, which is why this method is only suitable for long-term measurements from around one month.
Radon gas diffuses through an inlet filter into a measuring chamber with a volume of approximately 10 cm 3 . Decomposition products that form in the chamber are deposited on the chamber walls and on a built-in radiation detector. The radon gas in the chamber also makes a contribution to the measurement signal. Depending on the radiation detector, there are active and passive versions.
The active version mostly uses a semiconductor detector made of the semiconductor material silicon for the alpha radiation generated during decay . The measuring chamber can be enlarged to increase the sensitivity. In addition, a high voltage field between the chamber walls and the detector increases the counting yield. Radon decay product atoms freshly formed in the chamber are electrically charged and can therefore be directed specifically to the detector surface.
In the passive version has the nuclear track method or Ätzspurverfahren proven. The detector consists of a radiation-sensitive polycarbonate film . Alpha radiation that hits it destroys some of the chemical bonds. The plastic gradually dissolves in a caustic bath made from sodium or potassium hydroxide . This happens particularly quickly at the point where the alpha particles hit. This is why there are etching traces that can be optically recognized and counted with a microscope. This passive procedure is standardized in DIN 25706-1.
Adsorption process
The adsorption process or activated carbon process for radon gas is standardized in DIN 25706-2. Activated carbon deposits radon gas from the ambient air on its surface. The amount of radioactivity collected by the activated carbon is a measure of the radon concentration in the ambient air.
DIN calls these devices radon collectors (also called activated carbon dosimeters , activated carbon exposimeters ). They are tin cans with approx. 100 g activated carbon or plastic tubes with approx. 5 g content. Tin cans can be evaluated with a gamma spectrometer . In this case, the spectrometer measures the gamma radiation of the decay products created from the radon gas inside the can. To evaluate the tubes, their carbon content is mixed with a liquid scintillator that reacts to alpha and beta radiation.
Ionization chamber process
The clever arrangement of electrodes with a relatively low operating voltage creates a radiation-sensitive volume of one liter and more in an ionization chamber . Every nuclear disintegration within this volume delivers a very weak charge pulse to the electrodes, which is amplified, processed and counted in a complex process (impulsion chamber). In the case of very high activity concentrations, a direct current can be measured instead of the pulses.
Principles of Decay Product Measurement
The acquisition of the necessary raw data is easier with measurements of the radon decay products than with radon gas. That is why there are not so many measurement methods. Practically all measuring devices use the filter method . A pump draws ambient air through a measuring filter. The decay products contained in the air are deposited on the filter surface. A radiation detector measures the radioactivity on the filter. There are numerous variants of the filter process depending on the arrangement, pump output, detector type, operational sequence and evaluation method. The most common are short-term systems that initially vacuum the measuring filter for a few minutes. The radioactivity measurement of the filter is then carried out. Continuous processes measure the filter activity during the collection process.
Depending on the complexity of the radiation measurement and evaluation calculation, a certain arrangement can only provide a measure (mostly the potential alpha energy concentration) for the decay product mixture or the exact composition of the decay products. Devices with automatically running filter belts enable maintenance-free operation over many months and good temporal resolution.
The calibration of a measuring device for radon decay products can in principle be carried out purely by calculation. The first steps are to determine the air throughput through the measuring filter and the response probability of the radiation detector. The much more complex second step is the calculation of the activity concentrations in the air from the filter activities. For this, the solution of the differential equations is necessary, which describes the conversion of the different decay products on the filter according to the decay law , as well as the collection process.
Several authors provide evaluation formulas for special, fixed time sequences from sampling and radioactivity measurement . More modern devices have their own microcomputers which record the necessary data during the collection process and solve the associated system of equations in real time. They show the calculated measurement result already during the measurement.
There are few large radon test chambers for testing and comparing measuring devices with one another . The mixing ratio of the decay products in it can be influenced but not individually adjusted. A filter measurement is always necessary in order to know the exact contents of the chamber. The procedure for this is geared towards minimal measurement errors and not, like most commercial devices, easy handling or automatic operation.
Measurements for radon and its decay products
As with most radioactive materials, the usual amount of radon gas is the activity in Becquerel (Bq). The unit of measurement for the radon concentration in air is accordingly Bq / m 3 . The mean radon concentration outdoors is around 10 Bq / m 3 , in living rooms it is around 50 Bq / m 3 and in the air pores in the ground it is around 20,000 Bq / m 3 .
The time integral of the radon concentration is called radon exposure with the unit Bq h / m 3 . It is practically the accumulated radon concentration to which a person or a measuring device is exposed during a certain period of time.
A meaningful quantitative statement of the radon decay products is much more complicated , because it is a mixture of several different emitters. Professional radiation protection in uranium ore mines has produced a whole series of unusual measurands for this, the most common of which is the potential alpha energy concentration in J / m 3 or MeV / l. The equilibrium-equivalent radon concentration also describes the concentration of the radon decay products and corresponds to the potential alpha energy concentration. It has the unit Bq / m 3 and must not be confused with the radon gas concentration.
Radon measurements in buildings
In order to stay safely below a radon concentration above which damage to health can be proven, the responsible authorities are oriented towards setting the value of 100 Bq / m³ as the target value for the maximum permissible radon concentration in common rooms. The experience of international measurement comparisons over the last 20 years has shown that the deviation of measured values from different measuring devices can be more than 100%. In this context, it is not only useful, but also necessary to use measurement methods that are also suitable for the respective measurement purpose. In addition, special emphasis must be placed on the measurement accuracy, i. H. a valid calibration of the measuring devices used by an accredited calibration body.
In the following, it will be discussed whether overview measurements are sufficient for an initial assessment or whether more precise measurements, for example assessment measurements, are necessary for a decision on remedial measures.
Overview measurements
An overview measurement (screening measurements) is used to decide whether evaluation measurements must follow. An overview measurement of the radon concentration should be carried out in one of the common rooms in the basement of a house. If possible, measurement times of more than a week, preferably one to two months, are chosen. For measurement times of up to three days, windows and doors should remain closed as early as possible one day before the start of the measurement and also during the measurement in order to be able to evaluate the measurement result in a conservative way from the point of view of radiation protection. Short-term measurements are not allowed on days with strong wind or extreme heat.
For decision-making, for example in connection with the sale of a house / apartment, overview measurements with measurement times of a few days are not suitable. However, if the radon concentration measurement result from an overview measurement is less than a quarter of the decision threshold, it can be assumed with a high degree of probability that the decision threshold will not be exceeded.
Evaluation measurements
Evaluation measurements provide data for the decision on a possible renovation. In order to evaluate the radon exposure of the residents of a house or property, the long-term mean value of the radon concentration must be determined. For this purpose, measurements over twelve consecutive months are required. Alternatively, measurements can be made over a period of three months, preferably during the transition periods of spring and autumn. The measurements themselves should be carried out in three separate rooms such as living room, bedroom and children's room.
Continuous measurements
Continuous measurements record the course of the radon concentration over time. Most of the electronic devices for radon gas or radon decay products are continuously measuring. The time resolution of the individual device types is different and usually varies between a few minutes and a day. The saved measurement data can be read out after the measurement is finished.
Continuous measurements over a few months can provide information as to whether renovation can be dispensed with. If all peak values are below the target value, it is very likely that the annual mean will not exceed it either.
Evaluation of measurement methods
The following overview shows the most common methods of radon measurements with information on their suitability for various measurement tasks.
| Measurement method | Overview measurement | Evaluation measurement |
|---|---|---|
| Activated charcoal tubes | suitable | not suitable |
| Activated charcoal cans | suitable | not suitable |
| Nuclear track method | suitable | suitable |
| Electret process | conditionally suitable | suitable |
| Active radon gas measuring devices | suitable | suitable but laborious |
| Radon Decay Product Meters | suitable | suitable but laborious |
When selecting the measuring points in buildings, a few basic aspects must be observed:
- Selection of two common rooms on the ground floor and, if available, one room in the basement
- Installation if possible in the middle of the room at a height of approx. 1 to 2 m above the floor;
- Minimum distance of 30 cm from walls, ceiling and floor (disturbing influence of the thoron);
- Measuring devices must not be placed in closed cabinets during the measuring period.
Measurement practice
The measuring devices most used worldwide are passive radon exposimeters according to DIN 25706-1, which are radon diffusion chambers with nuclear track detectors. These measuring devices are suitable for both overview and evaluation measurements with typical measuring times of one month to one year.
The continuously measuring devices of electronic design (active devices) can record the course of the radon concentration time-resolved. However, due to the currently (2006) still comparatively high costs compared to the radon diffusion chambers, these devices are only conditionally suitable for measurements in houses.
If an increased radon concentration is determined with a long-term integrating radon diffusion chamber, the use of continuously measuring devices with time-resolved display of the course of the radon concentration is unavoidable in order to find the entry path of radon into the building concerned.
No house is like the other. From the measured value of the radon concentration in one house, no conclusions can be drawn about the expected radon measured values in the neighboring houses.
Quality assurance and measurement certainty
The statistical character of the radioactive decay manifests itself in repeated measurements in fluctuations in the measured values. The numerical values of the activity concentration measured variable fluctuate from one measurement to the next. The smaller the measured variable, the greater the fluctuations in the measured values. For this reason, radon diffusion chambers based on nuclear track detectors should not be used to carry out measurements with measurement times of less than one month if it is expected that the radon concentration in the building will be below 100 Bq / m³.
The same applies to the continuously measuring active devices: Here, measurements should be made over a period of at least twelve hours at comparatively low radon concentrations. With these devices it should also be noted that after starting a measurement, i.e. after switching on these devices, the first two or even three displayed measured values are not always representative.
A very important aspect when evaluating measurement results is quality assurance for all radon measuring devices that are used. For the determination of the radiation exposure during work according to § 95 Annex XI Part A of the amended Radiation Protection Ordinance, d. H. For employees in systems for water extraction, treatment and distribution, radon therapeutic baths and mines, the “Guideline for Monitoring Radiation Exposure” has been in effect since December 15, 2003. Accordingly, continuously measuring devices with direct display are suitable for the measurement purpose if they have been calibrated by an accredited calibration center (Physikalisch Technische Bundesanstalt PTB Braunschweig or Federal Office for Radiation Protection BfS Berlin) and this is not older than two years.
Radon diffusion chambers are suitable for the measuring purpose if the measuring point with the measuring device types issued by it participates in a review by the BfS in accordance with ISO / IEC 17025, which for test laboratories goes beyond the requirements of ISO 9001, and the suitability is determined by the BfS becomes. Since the beginning of 2006 there has even been a requirement that the relevant measuring points have to submit an accreditation from an evaluated accreditation body or a recognition as a measuring point from the BfS control center to monitor the environment for natural radioactive substances.
The remediation of buildings with radon is usually associated with a comparatively high level of structural engineering effort and consequently high costs. In this context it is advisable to also use the above mentioned for measurements outside the scope of § 95 StrlSchV. Set standards in order to exclude possible legal consequences from the outset as a result of improperly calibrated measuring devices.
Summary
The criteria for the selection of a suitable measuring system for determining the radon concentration include not only metrological requirements for the individual processes, but also economic aspects. In this context, the measurement accuracy plays a decisive role. The duration of a measurement, i.e. H. The decision-making whether short-term measurement or long-term measurement is an essential criterion for the determination of representative results of the radon concentration. In addition, radon measurements should only be carried out with measuring devices that have a valid calibration by one of the above. evaluated positions.
Individual evidence
- ↑ a b Publications of the Radiation Protection Commission; Volume 47: Guide to the measurement of radon, thoron and their decay products . Urban and Fischer, Munich; 2002, ISBN 3-437-21478-0
- ↑ Radiation Protection Commission: Radiation protection principles to limit radiation exposure to radon and its decay products in buildings . ( Memento of the original from September 27, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Publications of the Commission on Radiological Protection, Volume 36, 1994.
- ↑ Guideline for the monitoring of radiation exposure during work according to Part 3 Chapter 2 Radiation Protection Ordinance; Internal report BMU (?) RS II 3 - 15506/9 (2003)
See also
literature
- Publications of the radiation protection commission; Volume 47: Guide to the measurement of radon, thoron and their decay products . Urban and Fischer, Munich; 2002, ISBN 3-437-21478-0
- GSF Research Center for Environment and Health: Radiation in Everyday Life . Munich 1991, ISSN 0175-4521
Web links
- Federal Office of Public Health FOPH: Measure radon concentration
- Federal Office for Radiation Protection: What is radon?