HPGe spectrometry

Spectrometric measurement of gamma-rays with high-resolution detectors is a standardized method since over 50 years in many areas of nuclear research and applications. The first high-resolution germanium detectors being presented in 1961 were lithium-drifted germanium crystals which required continuous cooling with liquid nitrogen.

Gamma-Ray Spectrometer
A modern high-resolution gamma-ray spectrometer consists of a cooled hyperpure germanium crystal (HPGe) in vacuum which is connected to a suitable preamplifier (PA), a high voltage power supply (HV), a spectroscopy amplifier (LA), an analog-to-digital converter (ADC), a multichannel analyzer (MCA) as well as emulation software for on-line display of the measured spectrum, control of live measuring time and storage of the data. For reduction of external background radiation the detector is normally enclosed in a lead shield made of low-activity lead in suitable thickness.

During measurement the detector crystal must be cooled down to a temperature below -160 °C, either by liquid nitrogen (LN2) contained in an isolating DEWAR container or by electric cooling. LN2-cooling guarantees a more stable and lower crystal temperature than electric cooling. There are closed hybrid systems available where a reservoir of LN2 is maintained by electric cooling. Part of the PA electronics also must be cooled, this is one of the reasons why the PA is normally mounted very close to the crystal.

The high-purity germanium crystal (crystal defects and impurities <10-10) can be produced in different variants (p-type and n-type) as well as in different geometries and structures. This leads to many types of HPGe detectors which are suitable for different applications or special research options.

The “standard” coaxial p-type detector serves for measurement of gamma-radiation having energies between approx. 40 keV and 3 MeV; big detector crystals can measure up to 10 MeV with good full-energy peak yield.

The “standard” coaxial n-type detector has an endcap made of beryllium or plastic (Carbon) which are very transparent for low-energy gamma-rays. It serves for the measurement of gamma-rays and X-rays in the energy range from approx. 3 keV to 3 MeV
There is a large selection of crystal profiles and geometries yielding detectors for special applications. We will be glad to consult you when it comes to finding the best suitable detector for your specific task.

All components coming after the PA can be either built from „Nuclear-Instrumentation-Modules” (NIM) or they are “stand-alone” units or they are built as a mixture of NIM and external units.Operation of HPGe detectors with PC plug-in cards which contain all electronic components is more or less not available any more.

HPGe spectra are normally measured in 4096 (4k) channels, some applications, however, may require 8k spectrum length. Only for very special applications such as measurement of prompt gammas one makes measurement of 16k spectra. Basically one should measure HPGe spectra on as few channels as possible. When measuring the same energy range on double spectrum length then one will distribute the same number of counts on twice as many channels – this reduces the statistical significance of each channel contents by a factor of 1.4. A longer spectrum only leads to more numbers having less significance. As long as a peak is defined over 5 channels or more it can be reliably fitted with any current algorithm.

Environmental and radiation protection measurements
For measurements in radiation protection and environmental supervision the energy range of the spectrometer is normally set so that the 46.5 keV peak from 210Pb is visible around the lower energy end and the 2614.6 keV line of 232Th progeny (the line comes from decay of 208Tl) is well measured at the high-energy end. In a 4096-channel spectrum this yields a calibration of 0.63 keV per channel which means that the FWHM of the 46.5 keV line is around 2 channels and the base width of the peak is around 5 channels. This is well sufficient for a good fit.

Measurements in research, industry and medicine
For dedicated measurements in research applications, or when certain nuclides like U or Th are to be surveilled, or in industrial and medical applications a selected energy range is normally chosen for measurement which is optimized for the nuclides of interest. Some multichannel analyzers provide an offset function for this purpose on the ADC or in the MCA hardware, or one can make use the lower and upper level discriminator functions.

In most applications it is essential to provide additional shielding for the detector in order to reduce the influence of external background radiation to the measured spectrum. The economically most effective shieding material is lead, the typical lead shield is also denoted a “lead castle”. There is a variety of lead castles for many different detector types and geometries. For HPGe detectors a lead thickness of 10 cm low-activity lead is recommended; the shielding should cover 4-π, even the feedthrough of the dipstick from DEWAR vessel to the detector head should be well shielded. There are lead shields with graded absorber materials such as tin, copper and plastic lining the inner lead walls. The lining is meant to shield against induced X-rays and to “clean” the spectrum from the Pb-X-ray peaks. This “cleaning” works well, however, at the expense of an elevated continuous background around lower energies which makes quantification of small peaks below approx. 70 keV such as from 210Pb and others difficult.
Proper selection of a lead shield is a small scientific endeavor – we will consult you well.