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Canberra Unattended Spectrometric Aerosol Monitor (CUSAM)
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Cusam is an on-line spectroscopy system to measure gamma, alpha and beta activity in the environment.
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Features
- Continuos Air Monitoring
- Alpha, Beta and Gamma measurement unit
- Different alarm functions
- Canberra Genie software
- Quality Assurance features
- Transfer of data to a remote Communication Control Center via network or modem
- Synchronised measurements with variable measurement time.
- M.D.A.: typical 0.005 Bq/m³ (60Co) for a complete day. 0.03 Bq/m³ for a 4 hour measurement. The detection limits for the alpha and beta activities are respectively 0.01 and 0.03 Bq/m³
Introduction
The past decades the world has become very concerned with respect to environmental pollution. One of the main domains on this subject is the danger of radioactivity. Due to the Chernobyl accident several countries started programs to monitor the radioactivity in the air on their territory. Canberra Eurisys Benelux has developed an alpha/beta/gamma air monitor spectrometer system. Several countries have integrated such a system in their national environmental monitoring network.
The spectrometric monitor continuously measures air, sampled on a filter, and deduces nuclide activities, based on gamma radiation, and total alpha and beta activities. The results are communicated on-line or off-line to a remote computer system of the national monitor network.
Description
The drawing shows the main components of the CUSAM. The cabinet contains the air sampling system, a CAM PIPS detector, possible NIM electronics and cabinet control electronics. The cabinet also holds the gamma measurement unit: Ge-detector with cooling unit and possible ICB NIM electronics or a DSA-2000 unit. A PC with screen, mouse, keyboard and printer is placed near the cabinet. Via an Ethernet network or modem it is possible to obtain on-line or off-line analysis results and measurement conditions from the system, for example for further processing in a spreadsheet application.
The main part of the sampling system is the filter unit, which accumulates the particles, present in the air. The filter band shifts each time a new measurement is started. A pump inside the cabinet circulates the air from the small chimney at the top, through the filter paper band, back to the top of the cabinet, where the air leaves the cabinet via a small hole.
The principle part of the alpha/beta measurement unit in the cabinet is the CAM PIPS detector, which allows a reasonable discrimination between beta and alpha activity. The energy proportional detector signals, induced by the impinging alpha/beta particles, originating from radio-active air particles, are processed by (ICB) NIM electronics or a MCA board (inside the PC) and finally the energy dependent events are stored in the MCA board or AIM NIM module, a networked blind MCA. Special algorithms deduce the natural and artificial alpha activity and the (artificial) beta activity from the registered alpha/beta spectrum (counts in specific energy windows).
The main component of the gamma measurement unit in the cabinet is the Ge-detector, which is cooled by the liquid nitrogen, contained in the dewar. As an alternative a cryolectric cooled detector type can be used for unattended systems. The energy proportional detector signals, induced by the impinging gamma rays, originating from radioactive particles, are processed by the NIM module electronics and finally the energy dependent events are stored in an AIM module (MCA). A DSA-2000 unit can replace the networked NIM and MCA. Special algorithms, based on the Genie software, deduce the activity of the identified nuclides from the resulting MCA spectrum.
The computer drives the complete system by means of the customised Canberra Genie software. The standard features of the Genie software allow to calibrate the PIPS and Ge-detection line, to collect spectra, to visualise spectra and to derive activities from these spectra. The Genie algorithms are fine-tuned to the special environmental conditions. The menu’s of the special Cusam program, built with the Genie Visual Basic Components, allow the user to interact with the system in order to display MCA and file spectra, start and stop the monitoring, to perform calibrations, to perform QA checks, to adjust system parameters,...
Through digital and analogue interaction with the cabinet, the PC watches and controls several measurement conditions: air flow, air and cabinet temperature, belt movement, air pressure, detector leakage current, ... An alarm is generated by the computer in case of abnormal conditions.
Important information, such as alarm conditions, normal and daily mean reports, ... can be dumped on-line on the connected printer. Off-line analysis reports, calibration reports, QA reports, QA graphs, results listings, data graphs, ... can be generated on the printer by means of the special Cusam application in order to check the performance of the alpha/beta and gamma measurement unit.
During monitoring the system continuously measures the material, deposited on the filter. In order to avoid a saturation of the filter, the filter band is advanced about every 4 hours, and a new measurement is started on the clean filter spot. The alpha/beta and gamma INPUT are started/stopped simultaneously at the start/end of each measurement. For a proper integration of the monitor data into the national monitor network, the start of each measurement is synchronised to the hour.
At the end of each measurement the alpha/beta/gamma analysis results and the measurement conditions are displayed on the main screen of the Cusam program. The main screen also shows the current status and alarm conditions of the system, which makes it very easy to check the functioning of the system.
The same data can also be sent out via a COM port of the PC. By connecting a modem to this RS-232 output, the remote communication control center (CCC) can receive the actual activities and the current system status at the end of each (4 hour) measurement.
The analysis results of all normal and daily mean spectra are maintained on the PC. Via the Cusam program it is possible to review, check and list the stored data off-line. The program also has an option to generate graphs, showing the trending of activities, measurement conditions and ratio’s in function of time.
The proper functioning of the system is continuously checked. Hardware and software problems, abnormal measurement conditions, alarm flags, ... are dumped on-line on the PC screen, on the printer and in a logbook. This logbook can be consulted off-line to verify the system’s functioning over a longer period.
A weak K-40 gamma source is mounted close to the detector. The resulting peak at 1461 keV is used to software stabilise the gamma detection line and to check the efficiency. Whenever the energy shifts exceeds a certain limit, the gain of the amplifier is adjusted. This feature allows the monitor to run continuously without any energy recalibration at all.
At the end of each measurement the K-40 peak information (peak position, width and count rate) is transferred to a QA file. Using the Canberra Genie QA package, graphs and reports in function of time on these K-40 peak parameters can be generated off-line. Hence the proper functioning of the gamma detection line in all his aspects can be verified quite easily.
The accumulated alpha/beta and gamma spectra are maintained on the hard disk of the computer in a cyclic buffer, which can typically hold the data for about a year (depending on the size of the hard disk). The measurement results (activities) and the complete system status are added to these spectra. Hence complete analysis reports can always be generated off-line at a later moment in order to investigate a given period in more detail.
By means of the PCAnyWhere host software, which supports modem and network communication, it is possible to interact with the system remotely. As a results all local options of the Cusam program are also available remotely, allowing to check the system from any remote PC, having PCAnyWhere Client loaded. The PCAnyWhere file transfer option allows to retrieve Excel spreadsheets with analysis results, created by means of the Cusam program.
Sensitivity
At the end of each day a daily mean report is generated. All the gamma spectra of a given day are summed. The measurement conditions are totalled and averaged. The final gamma spectrum is analysed. By summing the spectra, better detection limits can be achieved. Using a 25 % Ge-detector this gives a typical Co-60 detection limit of 0.005 Bq/M3 for a complete day. For a 4-hour measurement this is about 0.03 Bq/M3. The detection limits for the alpha and beta activities are respectively 0.01 and 0.03 Bq/M3.
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