When radioactivity levels in several inaccessible locations are to be measured, or when the extraction of radioactive samples for measurement is prohibited or undesired, the in-situ installation of a multiple-detector-system is an effective solution. In these systems there is one detector mounted into each measuring site (for example in the tubing inside a sewage tank) and wired to the PC with special HV and signal cables. The MCA can be operated in a selecting, a sequential, or in a parallel mode, depending on the desired operation. In addition one can use the system for on-line controls with reaction times of much less than 1 second.

The selecting mode:

The PC contains one TISA MCA card together with a special signal switcher which selects the desired detector by user or programme definition. The connected detector is then used to measure a spectrum which is analysed and nuclide activities are quantified with SODIGAM. This system is the most price effective setup to measure spectra from several locations but using only one single MCA. The system runs under complete user control where one location is selected at a time for measurement. SODIGAM can carry out automatic batch-mode analyses thus running continuous measurements over any desired time interval.

The sequential mode:

The PC contains one TISA card together with a special signal switcher. A special option in the SODIGAM software alternatingly routes at given time intervals one detector after the other to the TISA MCA card and measurements are made for a pre-determined time. Each spectrum is immediately analysed and the results are provided to the user on the monitor and/or in the printout. By use of extra 20 mA interfaces the actual activity levels can be transferred to a monitoring site (e.g. to the control room). By user intervention the sequential mode can be interrupted for measurements in the selecting or other mode. On return to the sequential mode, the system will restore the results of the previous sequential measurements; thus there is no confusion or interruption of the monitor information.
The sequential mode is often applied in radioactive sewage decay storages of nuclear and radiological clinics. Several tanks are equipped with one detector each and the actual inventory of e.g. ¹³¹I in each tank is constantly monitored and indicated. When the activity in one tank is low enough for release the system is used in the selecting mode and a longer, very precise measurement of the releasable activity is made in order to confirm the limit value of e.g. 5 Bq/l for ¹³¹I. During the pumping out of the low activity sewage the outlet is monitored in the fast on-line mode (see below) and the effluent is stopped if the wrong tank is erroneously pumped out. After the release the system is set back to the sequential mode for continuous monitoring.

The parallel mode:

The PC contains one separate TISA card for each detector. Spectra from all detectors are measured simultaneously and in parallel with separate timing control in a synchronized mode or without any synchronization. Each spectrum is analysed immediately after the measurement and the results are provided to the user. By use of extra 20 mA interfaces the actual activity levels can be transferred to a monitoring site (e.g. control room).
By use of special software options several individually measured spectra can be manipulated (normalization of the energy scale, adding, subtraction, ....) and these manipulated spectra are analysed. The latter setup is particularly useful for measurements in which several locations must be measured simultaneously or where several detectors are used to assay one object. Typical applications are in multi-detector whole-body counters or in systems for the nuclide detection and quantification in waste barrels where spectra from several detectors are simultaneously processed for improved statistics.

Fast on-line control:

The fast on-line control measurements can be made by software selection with any of the above measuring systems. The SODIGAM software will control the MCA and read and analyse a continuously measured spectrum every ca. 30 ms. The resulting current activity value for one nuclide is displayed on the screen and a logical (+5V) output signal is activated for external control purposes when a user-defined limit value is exceeded. The measured activity profile is internally stored on a file in 1-second increments for later documentation. The fast control system is often used for the on-line control of sewage during the release, in order to make sure that the correct tank is pumped out. Erroneous and illegal release can thus be avoided. The system is also used to monitor processes with fast varying activity gradients.

 NaI spectrometers [1]

  Special Systems