When several nuclides are to be simultaneously determined (e.g. mixtures of ^99mTc, ¹²³I, ¹²⁵I, ¹³¹I, ⁸⁹Sr, ¹⁸⁶Re, ⁵⁷Co, ⁶⁷Ga, ¹¹¹In, ²⁰¹Tl, ⁴⁰K, ²²⁶Ra, ....) or when the activity of the desired nuclide is very low, the measurements should be made with a high-resolution semiconductor detector (High-Purity Germanium, HPGe). Because of its superior resolution in energy and the high efficiency of modern systems, the detection limits of a HPGe system are typically 15 times lower than that of low-resolution NaI(Tl) systems.

 

Detector:
The detector typically has an efficiency of 25% relative to a standard 3"*3" NaI(Tl) detector and a resolution (FWHM for the 661.6 keV line of ¹³⁷Cs) of better than 0.22%. During the measurement the detector must be cooled down to very low temperature with liquid nitrogen (LN₂). The cooling process consumes LN₂ at a rate of less than 25 liters LN₂ per week. When the detector is not used for an extended period of time, say for more than 4 weeks, one can terminate the LN₂ supply and let the system warm up. The cooling-down time of the detector before the next measurement is typically 8 hours (overnight).

Multichannel analyser:
Recent developments in MCA technology actually come back to NIM-based analog electronics because it has been found that these systems yield better resolution than fully digital systems. Thus, in a modern stationary system one will have NIM units for the high-voltage supply, the spectroscopy amplifier and the ADC, whereas the MCA can be another NIM unit or it is a small attachment to the ADC bus connector.
Most modern ADCs are connected to the controlling PC via USB or RJ-45 (network) cables. All hardware settings are defined by buttons and switch on the NIM level whereas all spectrometric functions are software controlled and they can be automatically controlled via batch files. Some NIM high-voltage units allow software controlled settings and ramping.

Shielding:
The lead castle usually has a shielding thickness of 10 cm very low activity lead and its weight is about 1300 kg. The shielding is sufficiently thick to reduce 1 MeV gamma-rays from the external background by a factor of about 1000. The inside of the lead castle has an extra lining made of copper and acrylic sheets for the effective suppression of x-ray fluorescence radiation from the lead shielding material. The top cover of the lead castle is made of sliding lead doors which sit on ball bearings for very easy handling. (see next page)

Analysis software:
Gamma-ray spectra measured with a HPGe detector are analysed with Gamma-W or Gamma-W for Windows. GAMMA-W is ideal because of its high-precision analysis of low-level spectra and reproducible resolution of complex multiplets.

The GAMMA-W code was rated among the top spectrometry software packages on the world market in IAEA intercomparisons studies
 "The 1995 IAEA intercomparison of gamma-ray spectrum analysis software", M. Blaauw et al., Nucl. Instr. Meth, A387 (1997) 416)
The program contains all options needed for the quantitative analysis of HPGe gamma-ray spectra and all algorithms which are required by german or international law (detection limit, radioactive decay control, uncertainties, documentation, ...etc.)
The program comes with a set of nuclide libraries designed for specific purposes (e.g. SIPPING analysis in NPPs, nuclear medicine, neutron activation, natural radioactivity); user specific libraries are supplied on request at no extra cost.

 HPGe spectrometer systems [2]

 Special Systems

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