UNIVERSITY OF IOANNINA
To perform radon measurements, we use both passive monitoring techniques (SSNTDs and E-PERM detectors) and continuous monitoring techniques (Solid State Silicon and Germanium Detectors).
Solid State Nuclear tracks Detectors (SSNTDs) are plastic materials, widely used
for integrated measurements of radon isotopes. Alpha particles generated by the
decay of radon and its progeny, irradiate the plastics and produce damage
tracks, which can be enlarged to visible size by chemical etching.
We use the most popular of the SSNTDs, CR-39 films (Pershore
Mouldings Ltd).
CR-39 films
The CR-39 homopolymer is formed by polymerization and cross-linking of the oxydi-2,1-ethanediyl di-2-propenyl ester of carbonic acid. The resulting product is a thermoset plastic, which is characterized by being hard, totally amorphous, resistant to all solvents and very sensitive to heavy-ion damage.
Following exposure to radiation, the CR-39 films are etched in a 5 M NaOH solution, at 80ºC for 8 hours. They are then rinsed under tap water, dried and analyzed by an automated optical scanning system (video image analyzer), using the computer code TRACKA ã .
The detectors are being used in various setups, which have been calibrated in the NRPB radon chamber. Independent calibrations are also performed in a homemade radon chamber, using a 226Ra liquid source.
TRACKA: Track Analysis software
The computer code TRACKA is used to automatically count the number of tracks per optical field, by calculating the picture elements (pixels) with previously specified gray-scale values. For the purpose of alpha-tracks measurements, a 640x480 resolution is chosen and the dynamic range of the image is converted to an 8-bit (256 shades) gray-scale.
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Before enumeration of the tracks, a calibration procedure is carried out in order to correlate the average number of pixels of a track image with the gray-scale values. In this procedure the gray-scale values of individual pixels of a specified number of tracks are summed and an average track gray-scale sum (AT) value is computed to represent the track. The corresponding gray-scale sum value (GSV) is then calculated for all the tracks in the optical field. The number of tracks is estimated by dividing this GSV value by the AT value. For the computation of GSV and AT sums, limitations on the pixel gray-scale can be imposed to define sums representative of tracks. |
Alpha tracks on CR-39 |
Electret ion chambers are commonly used for passive radon measurements. We use two types of E-PERM devices (the S- and H-chambers) purchased from RadElec Inc www.radelec.com).
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The "S" and "H" E-PERM chambers |
A schematic diagram of the "H"- chamber |
The E-PERM detector consists of an electrically charged dielectric material that produces a strong electrostatic field inside a small chamber. Radon gas is allowed to diffuse inside the chamber and the ions produced upon its decay are attracted to the electret’s surface, thus causing a decrease in its charge. After proper calibration and by measuring the surface potential both before and after the exposure, the radon flux can be calculated.
An ALGADE Barasol unit ( www.algade.com) is operating for continuous soil radon monitoring in an ongoing seismic study.
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The measuring unit consists of a cylindrical chamber, 57 cm long and 60 mm in diameter, equipped with
an implanted silicon junction for the detection of radon alpha particle
emissions. A micro-processor is used to integrate and store the data, acquired
during user-preset measuring intervals between 15 and 240 min. |
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CAM-PIPS Detector
A Continuous Air Monitoring (CAM) Passivated Implanted Planar Silicon (PIPS) Detector from Canberra Industries (www.canberra.com) is used for radon studies in the Laboratory. The CAM-PIPS technology offers the advantage of light tight, moisture resistant detectors that can be operated under environmental conditions. The detector has an active surface area of 300 mm2, is equipped with an aluminum window and operates under a 25 V voltage. The signal is processed through a Canberra amplifier and is accumulated to a PC-based multi-channel analyzer.
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Radon concentrations may be deduced by measuring the gamma-rays emitted from the radon progeny. A High-Purity Germanium (HPGe) detector (Princeton Gamma-Tech) with a 1.9 keV resolution (for the 661.65 keV line of Cs-137) and an 18% intrinsic efficiency, is used for this purpose. The detector is located inside a 5-cm lead well to shield against background radiation. Standard electronics are used for processing the signal and spectra are stored in an IBM compatible PC. |
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