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RTM 1688-2
starting/stopping the data acquisition. The data stored within the instrument can be read by
PC even if a measurement is in progress.
The small chamber volume of only 250 ml must be pointed out. The measurement of probes
taken from small and limited volumes is possible due to this property.
The radon/thoron monitor RTM 1688-2 can be powered either by a mains power adapter or
by the internal battery which gives an autonomous operation time up to 14 days.
The instrument is equipped with sensors for temperature, humidity and barometric pressure.
An integrated tilt detector will give a signal if the radon/thoron monitor has been removed
from its original position during the measurement.
The instrument comes with a calibration certificate issued by our DAkkS-accredited Radon
calibration laboratory. The calibration process fulfils the requirements of the DIN ISO EN
17025:2018.
Important Hint
The determination of the activity concentration of Radon is always a radiometric
measurement, meaning a counting experiment. This causes a number of specific
circumstances which have to take in consideration by the one who is carrying out this task.
Only the knowledge of those particularities allows the correct set-up of a test and avoids
misinterpretations of the achieved results.
Please read carefully the next chapters "Theory of Operation" and "Statistical Error" to
become familiar with this kind of radiometric measurements.
Theory of Operation
The Radon (Rn-222) gas concentration will be measured by the short living daughter products,
generated by the Radon decay inside a measurement chamber. Directly after the decay, the
remaining Po-218-nuclei become charged positively for a short period, because some shell
electrons are scattered away by the emitted alpha particle. Those ions are collected by the
electrical field forces on the surface of a semiconductor detector. The number of collected Po-
218 ions is proportional to the Radon gas concentration inside the chamber. Po-218 itself
decays with a half-life time of only 3.05 Minutes and about 50% (particles emitted towards
the detector surface) of all decays will be registered by the detector. The equilibrium between
the Radon decay rate and Po-218 detector activity is given after about 5 half-life times, say 15
Minutes. This time span defines the minimum achievable response time to a Radon
concentration step.
Now, the decay chain is continued by the both beta emitters Pb-214 and Bi-214 followed by
another alpha emitter, the Po-214. That means, each Po-218 decay causes one more
detectable decay by the Po-214 which is delayed about 3 hours because of the superposed
SARAD GmbH 2023
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Manual_RTM1688-2_EN_24-02-2023.docx
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