III. Description of the new data logger
From Intamap
III. Description of the new data logger
A new type of data logger for gamma dose rate measurements has been developed in the last 3 years to replace the old systems from the mid 80's. Within a few years about 1400 of these systems will be installed the dose rate network.
Hardware
The system consists of a LINUX based data logger, an intelligent main power supply unit buffered with rechargeable battery for un-interruptible power supply and fixed wired or mobile modem. In the standard configuration one dose rate tube that consists of two Geiger-Müller tubes (GS05 by TechniData) equipped with a new developed microcontroller based counting device (QIS) that registers the count rates from high and low dose GM tube and sends them over a serial line to the linux data logger. Additionally to the gamma rate counts the QIS provides the temperature in the tube, echo counts (two counts within a defined period of time) and coincidence counts for quality assurance and error detection. Using a RS485 interface for data exchange and power supply of the tube, the distance between both can be more then 1000 meter.
The embedded LINUX data logger offers the possibility to use standard protocols for the hardware interfaces (2x RS232, 1x RS485) and the data transfer. The board is equipped the 64 MB RAM and 8 MB flash memory, the CPU works with a frequency of 100 MHz. In standard case about 70% of the RAM is in use. The mean load of the system tends to zero, thus the system offers many possibilities for further applications.
For the data transfer a wide range of possibilities are implemented dependent on the environment at the measurement site:
- Mobil modems (SIEMENS MC35i) for GPRS and GSM modem - modem connections as backup solution and direct access.
- Fixed telephone network for internet by call and modem - modem connections as backup solution and direct access.
- Direct internet connection (provider DSL, German research network, German weather forecast network) with or without direct access to the system from BfS systems.
Additionally to the standard set-up with one dose rate tube, up to two other measurement devices are supported. Productively a system with two parallel tubes and a system with the combination of one tube and a spectroscopic system is use. Other applications like ground humidity measurements and dose rate measurement using a NaI-detector are in preparation.
Software
As well as the hardware of the data logger, the software on the system is also developed as in-house work from the BfS. The access to the program source code ensures fast and cheap bug fixing and addition of new features. Most of the programs are written in C, some tools are written as bash shell scripts.
The main data collection software consists of 3 threads, for the interaction with the tubes, for hardware interaction (status LED, spontaneous message tip switch) and for data validation, accumulation, storage, system check and transfer initiation. It is highly configurable with a CGI-Webinterface as well as remotely by sending request files to the system. The Webinterface is mainly used for on-site configuration, interaction with the system and control of system. Moreover it provides X-Y-plots of the sampled data, a live monitor and background information about the BfS measurement network.
In contrast to the old system, the gamma dose rate is not calculated on the data logger. Only measured counts and temperatures are stored on the system as raw data. Using a new quality management, each tube has it's specific calibration coefficients in the central database, that is used to calculate the gamma dose rate on the database server after the data transfer.
The data transfer can be initiated from the data logger system automatically in defined intervals, a data query from the database server is also supported. In case of exceeding occurrences the system automatically contacts the database server to report the status immediately. Possible spontaneous reasons are:
- No power supply for hours
- Battery defect
- Self switch off due to energy problems
- No dose rate counts (hardware defect or broken cable)
- Dose rate counts over threshold value
- Manual initiation of a data transfer
A hand shake mechanism has been implemented to ensure a correct data transfer. The system sends the data files as long as it gets a command to remove them. The remove command is provided in a request file, that is copied to the system and is interpreted before the data from the system is copied to the database server.
Due to the fact that some systems work behind a provider firewall, that do not allow direct interaction or updates, an update daemon has been implemented to keep the software up to date. Using the request mechanism of the system it is also possible to send LINUX shell scripts to the data logger. After interpretation of the request file, the shell script is provided to the update daemon, who execute it. This option, using standard shell scripts offers the possibility to update or reconfigure the complete system, if desired for the whole network over night.
The autarcic probe
In 2005 the definitions of a new so called autarcic probe were specified and since end of 2006 30 probes of this type are operational. The probes are manufactured by Genitron and called XL2. With respect to the physical properties these probes are identical to the standard probes since the same type of GM tubes are used. Also from the technical point of view these probes integrate well in our network design, since the datagram of the linux data logger and the XL2 probe are identical. The advantage of the XL2 probe is, that it operates without connection to any external infrastructure for more than 3 years. Data is transferred using GPRS and GSM modem connections and a GPS receiver is installed primarily to synchronize the probes in time, but with the advantage to have to coordinates available of the actual positions, too. The probe typically is in sleep mode and wakes up only for data transmission. This is done every 4 hours under routine conditions. When the count rate exceeds a predefined threshold, the transmission intervals are reduced to 10 minutes. Since the probes can be installed without any further restrictions, the installations costs are minimised and the probes can easily be move to any other location.
The detector
A dose rate measuring probe used in the German GDR network consists of a probes with two Geiger-Mueller counters, the so-called low-dose and the high-dose counters. The entire measuring range of the probe extends from some 10 nSv/h up to some Sv/h, whereby the high-dose counter measures reliably above a dose rate of about 100 µSv/h at a measuring interval of 1 minute.
The natural environmental radiation essentially consists of two components: terrestrial radiation, owing to the radioactive isotopes in the soil (e.g. 40K), and secondary cosmic radiation, which develops as a consequence of the penetration of the primary cosmic radiation into the earth's atmosphere.
The two components differ substantially in their composition. Whereas terrestrial radiation exclusively consists of gamma radiation with energies up to 2.6 MeV, the ambient dose equivalent owing to secondary cosmic radiation at sea level is created to approximately 78% by muons and electrons, to approximately 20% by neutrons and to about 2% by gamma radiation (this is the reason why NaI detectors only see 10 % of the cosmic contribution). The energy range of the particles extends over several orders of magnitude, from some keV up to some hundred GeV, even exceeding the TeV range.
Response to Secondary Cosmic Radiation
The response to secondary cosmic radiation is an important characteristic for dose meters used to measure environmental radiation. In the GDR network the measurement sites are installed at different altitudes in the range up to 1500 m above sea level and only one site at the Zugspitze in the Alps has a much higher altitude of approx. 2900 m. Hence it is not possible to treat the response of the detectors to the secondary cosmic radiation as a constant. With increasing altitude, the air pressure decreases and the local dose rate owing to the cosmic contribution increases. To determine the response to secondary cosmic radiation has either to at a lake platform, with sufficient depth of water to shield the terrestrial component (for example we examined the effect on the Stechlinsee and on the Schluchsee). Or the measurements have to be done on board of an airplane, assuming the terrestrial radiation is sufficiently shielded by the air (we performed measurments at altitudes up to 3000 m on board of sport airplane).
From these investigations we obtaind the dependency of the detector response to the secondary cosmic radiation and we confirmed that the GS05 probes overestimate the cosmic radiation by about 20 %, which at see level in Germany and an air pressure of 1000 hPa takes 45 nSv/h.
It has to be pointed out, that the response of detectors to the secondary cosmic radiation observed strongly depends on the type of detector. Szintiallation detectors like NaI, CsI or LaBr3 for example underestimate the effect and only report 10 percent of the secondary cosmic radiation.
Self-effect of dose rate detectors
The self-effect of a probe can for example be determined at the low dose laboratory UDO in the Asse salt mine (490m-level) in North Germany, where the flow density of myons is reduced by 4 orders of magnitude, and, since almost no terrestrial and cosmic contributions are to be detected, the ambient dose rate is reduced to about 2.5 nSv/h. In such an environment the self-effect of a probe can easily be determined, simply measuring the dose rate for a period long enough, so that the statistical error becomes sufficiently low. For the GS05 probe the self-effect is rather high and takes 38 nSv/h with a probe dependend variation of a about 12 percent. This behaviour is well known, since we observed frequently a step in the mean value of the measured dose rate after the replacement of a GS05 probe with an other one. The step can be as large as about 10 nSv/h.
Again it has to be pointed out, that the self effect varies strongly from detector type to another one. For example the porportional counters of the BITT company have a much lower self-effect of only 5 nSv/h. The intercomparison of dose rate data from different networks on the european scale has to deal with these effects, since self-effect and response to the secondary cosmic radiation must be known. Forthermore it must be known if the different effects are compensated in the final results provided from different countries. As has been shown in the AIRDOS project, without this knowledge it becomes very difficult to compare the ground level data of different european countries.
Back to WP5 discussion.
