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The Germanium Advantage

Detective Germanium Advantage Image 1ORTEC pioneered the use of HPGe Germanium radiation detectors in homeland security applications in 2004 with the introduction of the Detective series of HPGe Isotope Identifiers. Since that time, many hundreds of these identifiers have been deployed across the world and are in use 24/7 in the war on terror. They are often used as the “final arbiter” in situations where illicit trafficking of radioactive substances is suspected. Germanium has been shown to be far superior to other radiation detector materials in dealing with cases of shielding and of masking.

The first figure shows the comparative energy resolution of HPGe, Sodium Iodide (NaI(Tl)) and PVT, plastic. It is self evident that the HPGe spectrum has considerably more structure than the others, making nuclide identification more reliable. (HPGe provides >40 times better resolution than NaI.)

Detective Germanium Advantage Image 2There are two potential technical drawbacks to the use of HPGe radiation detectors: the first is that they must be cryogenically cooled and the second is that they have relatively low Gross efficiency. The first drawback has been eliminated by ORTEC, through the use of low power Stirling cycle coolers, which have proven extremely reliable in their wide deployment within the ORTEC Detective series.

The second limitation (Gross efficiency) largely rests on a misunderstanding. This misunderstanding is that gross efficiency is an indicator of the ability of a radiation sensor to identify a nuclide. It is, but only in one specific case: namely the case of a single nuclide in a zero-background situation. In real cases, where the nuclide of interest is potentially obscured by other sources of background, the ability to “look for” the tell-tale “spikes” in the HPGe spectrum and ignore the rest of the data hugely benefits the system signal-to-noise ratio as can be seen in the example in the second figure.

15g WGPu with Ba-133

Detective Germanium Advantage Image 3In this simulated masking scenario, the distinction between the weapons grade plutonium target and the Ba-133 mask is clear, and the WGPu may be identified with high confidence; even though in the 5 second acquisition very few counts are detected, they are in a very narrow band of energy, making them, statistically speaking, significantly higher than background. This is the HPGe performance proposition. ORTEC has developed proprietary methods of data analysis, in which data is evaluated continuously, to determine indications of the presence of the target radionuclides. That data set is then passed through a well-tested algorithm developed by Lawrence Livermore National Laboratory and licensed to ORTEC for use in these applications.

The third figure shows a comparison between the 220-280 keV region of a Pu-239 spectrum taken with a NaI and a HPGe radiation detector. The contrast shows clearly the HPGe advantage.