Nuclear spectrometry using room-temperature detectors has the big advantage that no extra cooling of the detector is required and thus both the cost of the detector and the running expenses are low. Formerly, NaI(Tl) or CsI(Na,Tl) scintillation detectors were only used for quantitative assay of sources containing one or at most two radionuclides. This restriction was overcome in the early `80s when we succeeded in developing the physically correct peakshape description of peaks from scintillation detectors and proportional counters. Using this correct peakshape one can unfold spectra from very complex mixtures of nuclides, even those taken with low-resolution NaI(Tl) detectors ( SODIGAM).

Dr. Westmeier GmbH provides a line of products based on low-resolution NaI(Tl) scintillation detectors and designed specifically for your applications. The system components typically include:

Detector:

The low-resolution system is based on a well shielded, large (3"*3") SCIONIX NaI(Tl) detector with <7.5% resolution (FWHM for the 661.6 keV line of ¹³⁷Cs). The shield is a lead castle made from very low activity material and specifically designed for NaI detectors. The absolute efficiency of the detector, as defined for a point source in 25 cm distance and for the 661.6 keV line of ¹³⁷Cs, is 0.23%. The detector is hermetically sealed together with the photomultiplier and it is operated by an integrated voltage divider with cable driver. A 2048-channel MCA connected to the system. When the distance between the detector position and the MCA exceeds ca. 8 m a special cable driver is used which can drive over 200 m of signal cable. The cable driver units are a special development built for operation with an extra external +12V power supply for the preamplifier.

Multi-Channel-Analyser:

The multi-channel-analyser (MCA) is a model TARGET TISA-2048 plug-in card for any standard PC with a 16 bit ISA slot and colour graphics. The TISA card is a 2/3-length card containing on board the complete electronics, such as HV, preamplifier, linear amplifier, Analog-to-Digital Converter (ADC) and the multichannel-analyser with spectrum memory and real/live timing control. The HV bias supply is a software-controlled unit delivering up to 1250 Volts positive bias voltage with a current sufficient to drive PM-tubes up to 5". The preamplifier is a fixed x3 amplifier (software controlled) with potentiometer-controlled LLD and ULD. The linear amplifier is software-controlled in 16 coarse and 1000 fine steps. The ADC is a 112 MHz Wilkinson ADC with a fixed conversion of 2048 channels. All settings and reference values are stored in a setup batch file and automatically loaded when the MCA is started. Thus all measuring conditions are exactly reproduced for each measurement by default.

The TISA card is equipped with a spectrum stabilizer which is suitable to stabilize the position of a reference peak in the spectrum to within fractions of 1 channel.

The minimum PC hardware for the TISA MCA system generally consists of :
>200 MHz Pentium II PC
>64 MB RAM, 20 GB HDD, 1.44 MB FDD, CD-ROM
SVGA graphics card, >4 MB, 17" SVGA Monitor
1 parallel and a USB port, mouse, keyboard, inkjet printer
WINDOWS 98, Utilities
 

Lead shield:

The detector is contained in a special lead castle for NaI(Tl) detectors made of very low activity lead of <25 Bq/kg. The 5 cm thick castle will reduce the intensity of 1 MeV gamma radiation by more than a factor of 32 and it completely absorbs photon energies below ca. 250 keV. 

NaI spectrometer systems [2]

 Special Systems

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