Our Gamma-Ray Spectrometers are state of the art software-controlled PC-based units.
There are two basic spectrometer designs to cover all quantitative gamma-ray measurements:
- for the quantitative measurement of simple spectra from samples containing only a few nuclides one may use scintillator spectrometers with NaI(Tl) or CeBr3 detectors. One should note that many applications where HPGe spectrometry was once mandatory can nowadays be solved with room-temperature (no LN2) NaI(Tl) or CeBr3 detectors.
- for measuring sources at low activity levels (such as mBq/kg) or when very complex spectra from mixtures of many nuclides are to be expected one must use a high-resolution HPGe detector. Detection limits of HPGe systems for identical sample sizes and counting times are typically a factor of 15 lower than those of NaI(Tl) systems.
- Both systems include modern detectors, low background lead shielding, and the most modern PC analysers. High-precision analysis software packages containing all algorithms needed for the quantitative analysis of nuclides from singlet or complex multiplet peaks are the key to the success of the systems.
These spectrometry systems are mostly used for:
- High-precision measurements in research and development
- (Neutron) Activation Products measurement
- Release measurements in waste disposal systems in nuclear medicine departments
and radiological clinics
- Survey of low- and high-level radioactive waste
- Quantification of wipe-tests
- Environmental surveys and pollution measurements
- Measurements of geological samples
- Certification measurements for QA and export documentation
- Control measurements of radioactive substances and devices
- Education and training
For systems that supervise radioactive waste from diagnostic and therapeutical applications in nuclear medicine departments which is collected and treated until short-lived activities have decayed to acceptable levels below release activity limit see paper on Online Monitoring Systems
When several nuclides are to be simultaneously determined (e.g. mixtures of Tc-99m, I-123, I-125, I-131, Sr-89, Re-186, Co-57, Ga-67, In-111, Tl-201, K-40, Ra-226, ….) 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.
The detector typically has an efficiency of 25% to 45% relative to a standard 3″*3″ NaI(Tl) detector and a resolution (FWHM for the 1332.5 keV line of Co-60) of better than 0.2%. There are detectors available that have significantly higher relative efficiency, however, for almost all applications there is no real advantage of size. The higher the efficiency, i.e. the bigger the detector crystal, the more pronounced are disturbances to the measured peakshape and resolution. Some disturbances such as ballistic deficit can be partly corrected by digital signal processors but in general one finds poorer resolution in very large detectors. The key advantage of HPGe detectors is their very good resolution, therefore it is not recommended to use very efficient units whose resolution is not so good. It seems that the best compromise between efficiency, resolution and price lies somewhere between 40% and 50% relative efficiency.
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 around 25 liters per week.
Recent developments in MCA technology have actually returned 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 either another NIM unit or 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.
A completely new development is the PX5-HPGe stand alone multichannel analyser from AMPTEK which contains all digitally controlled components (High Voltage, Preamplifier, Amplifier, Digital ADC, Multichannel analyser, PC interface) in one small low-power box for easy high-resolution spectrometry.
For quantitative and low-level applications the HPGe detector must be shielded against external radiation from terrestrial and cosmic sources. An effective shield is provided by a 10 cm thick layer of special very low-activity lead (Pb) encircling the detector head. The shielding is sufficiently thick to reduce 1 MeV gamma-rays from the external background by a factor of about 1000. The very heavy (ca. 1300 kg) shield sits on a solid steel table and the top rolling doors are easily moved for safe and easy access. Special “lead castles” with 15 cm thick walls (or more) made of extremely low activity materials are also available. The inside of the lead castle can have an extra lining made of copper and acrylic sheets for the effective suppression of x-ray fluorescence radiation from the lead shielding material (not really recommended). The top cover of the lead castle is made of sliding lead doors which sit on ball bearings for very easy handling.
Gamma-ray spectra measured with a HPGe detector are analysed with the Gamma-W for Windows software. GAMMA-W is ideal because of its high-precision analysis of low-level spectra and reproducible resolution of complex multiplets.