Introduction
In recent years, large-area immission monitoring of the radioactivity has gained a high priority. This is true not only for Germany, but also for some European countries, such as the Netherlands, Switzerland, France, Spain and Italy. In addition to low-priced measurements of the environmental dose rate, which are carried out using several measuring stations to cover large areas, nuclide-specific measurements have gained in importance. The purpose of the measurement is to monitor nuclear facilities, both local and foreign, and to provide automatic, rapid and reliable detection of airborne contamination. Moreover it has been designed to be integrated into immission monitoring networks, which require data conversion and storage as well as telecommunication interfaces. BERTHOLD has designed, tested and employed measuring systems for this application.

Sampling of Particulates
There are basically two different methods to sample airborne particultes on a filter. The first is with moving filter the second with fixed filter A sample of air is drawn via a sampler head through a glass fiber filter tape, which retains particulates. Downstream of the filter a flow measurement system is located and a lateral blower pump. The filter tape is advanced in the same manner as the tape on a tape recorder. The difference between the tape recorder and the particulate monitor lies in the position of the detector head and the dual function of the capstan. The capstan of the moving filter monitor is provided with slits, which allow the sample air to pass through them at the collection area. At the same time it ensures a continuous motion of the filter tape and thus a homogeneous deposition of particles, both radioactive and non-radioactive. The advantage of this technique lies in the fact that, with proper choice of filter speed and flow rate, clogging problems no longer occur. The disadvantage of fixed filters which require frequent changing ; hence mechanisms to overcome the radon in growth to equilibrium problem (false alarms ! ), is eliminated with the use of a moving filter tape. Another feature of interest for stand-alone systems is the autonomy of more than 200 days obtained by the use of a 50-m filter tape.

Alpha-Beta Activity Concentrations
The direct alpha / beta detector is installed in the sample head above the dust collection area at close distance (typically 4 - 7 mm). The detector consists of a ZnS / plastic sandwich scintillator coupled to a 2” photomultiplier tube or of a proportional counter tube. The separation between alpha and beta disintegrations is done by means of energy discriminators. The signals derived from the detector are then further treated to strip the unwanted Radon influence by means of beta to alpha ratio or pseudocoincidence (electronic gating). The obtained net count rates are due to artificial radioactivity with a certain statistical confidence level determined by the applied measuring times. Further treatment in the microprocessor electronics results in gross beta and alpha concentrations (Bq/m³) and derived units s.a. release rate (Bq/h) and total activity (Bq)

Compensation of Natural Background
Artificial airborne radioactivity can easily be masked by natural activity concentrations of radon, thoron and their progeny. There is a strong demand for low or even extremely low detection limits. This requires efficient methods to compensate for the natural component. Most efficient is the pseudocoincidence methode. This technique utilizes the fact, that in the radon and in the thoron decay chain there are daughter nuclides with extremely short halflifes. Consecuting alpha and beta decays appear to happen simultaneously. This is a characteristic feature of these decay chains and can be used to identify and measure radon or thoron levels. The rate of pseudocoincident alpha-beta events is proportional to the natural contribution and can be used to subtract it.

Nuclide Specific Air Monitoring
A reliable estimate of the dose and the decay product dose is possible only if the nuclide composition is known. Knowing the nuclide composition, one can make inferences about the type of accident and its development, moreover, for a reliable estimate of the dose it is essential to know the nuclide composition, since different nuclides yield different doses. The BAI 9100DG is one of the recent developments in the field of automatic monitors for airborne radioactive particulates. It features both alpha / beta and gamma spectroscopy detectors, a high sample air throughput and a fully automated evaluation electronics. The essential principle of measurement is to accumulate air-borne radioactive aerosols on moving filters and to measure them using highest grade Germanium detectors.

The detector geometry is such that it allows the user to perform direct measurement of the concentration of particulates. The alpha / beta system permits the assessment of alpha and beta emitters, whereas the disturbing influence of Radon descendants is overcome with either beta to alpha ratio or pseudocoincidence compensation techniques. The on-line gamma spectroscopy system permits identification and quantification of any gamma emitter present in the air sample. A flow measurement system combined with a high flow pump enables the system software to express the measurement result in volumetric concentrations (Bq/m³) and to ensure optimal sensitivity and detection limits. The basis of the nuclide-specific measuring system is the Alpha / Beta Particulate Monitor BAI 9100D, of which more than 200 units were sold worldwide. By integrating the Germanium detector into the dust collection unit one gets a completely automatic monitoring system which is also capable of performing the very sensitive measurement of the artificial beta activity concentration according to the alpha-beta ratio method. This combination of alpha, beta and gamma measurement permits the detection of the relevant nuclides or nuclide groups with very high sensitivity.

Data Acquisition
An important feature of this new generation of nuclide-specific measuring systems is the use of standard personal computers, with co-processor and RAM, for the analysis and processing of spectra rather than using special computers operating with more of less obscure programming languages. The GammaVision system is used for the hardware analysis of the spectrum. In combination with the GIMRAD system one can now build up a monitoring system suitable for operation in a measuring network which even permits the remote transfer of the spectrum. A spectroscopy system on PC basis utilising the reliable and improved software GammaVision provides the user with a monitoring system, which is very simple and safe to operate.