Germanium Ge Detectors

Germanium detectors are fabricated in many different geometries, thus offering devices that can be tailored to the specific needs of the measurement. Two examples, the coaxial and the well-type detector, are shown in Fig. 7.24. 

More details about these detectors are presented in Chap. 12 in connection with y-ray spectroscopy. 

The major disadvantage of lithium-drifted detectors is the requirement for continuous cooling. In the case of Ge detectors, the requirement for cooling 

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Alpha-particle detector Beta-particle detector Gamma-ray detector proportional counters silicon (Si) diode with spectrometer proportional counters Geiger-Muller counters liquid scintillation (LS) counters thallium-activated sodium iodide (Nal(Tl) detector with spectrometer germanium (Ge) detector with spectrometer... 

The gas-filled recoil separator RITU (Recoil Ion Transport Unit) and the recoil mass separator FMA (fragment mass analyzer) in conjunction with germanium (Ge) detector arrays... 

Germanium metal is also used in specially prepared Ge single crystals for y-ray detectors (54). Both the older hthium-drifted detectors and the newer intrinsic detectors, which do not have to be stored in hquid nitrogen, do an exceUent job of spectral analysis of y-radiation and are important analytical tools. Even more sensitive Ge detectors have been made using isotopicahy enriched Ge crystals. Most of these have been made from enriched Ge and have been used in neutrino studies (55—57). 

The classical coaxial Ge detector is made of p-type germanium, and is used for spectroscopy of gamma rays. It covers the energy range from 100 keV to several MeV. On the low energy side, its efficiency is limited by the fact that low energy gamma rays... 

Instead of a cylindrically shaped detector, one can take only a slice of the monocrystal. This will make a planar Ge detector. With a beryllium entrance window, it can be used for X-rays. The last addition to the world of Ge detectors is the Low Energy Germanium detector. If made by Canberra, it will be called LEGE. It is excellent for low energy gamma-rays its energy range extends from 10 to 300 keV. This is indicated in Fig. 5.32. 

The availability of high flux thermal neutron irradiation facilities and high resolution intrinsic Ge and lithium drifted germanium (Ge(Li)) or silicon (Si(Li)) detectors has made neutron activation a very attractive tool for determining trace elemental composition of petroleum and petroleum products. This analytical technique is generally referred to as instrumental neutron activation analysis (INAA) to distinguish it from neutron activation followed by radiochemical separations. INAA can be used as a multi-elemental method with high sensitivity for many trace elements (Table 3.IV), and it has been applied to various petroleum materials in recent years (45-55). In some instances as many as 30 trace elements have been identified and measured in crude oils by this technique (56, 57). 

As mentioned in Sec. 7.5.5, the Ge(Li) detectors have been replaced by Ge detectors, which are devices that use hyper pure germanium (impurity concentration 10 atoms/m or less). The main advantage of Ge over Ge(Li) detectors is that the former should be kept at low temperatures only when in use the latter must be kept cool at all times. 

For truly multielement determinations, increased selectivity is required. It is offered by semiconductor detectors, e.g., by lithium-drifted germanium [Ge(Li)] or intrinsic germanium (high-purity) [HP-Ge] types with a resolution - full width at half maximum or FWHM... 

Accurate measurements of the concentrations of trace elements on atmospheric particulates are difiicult enough to make in polluted urban atmospheres but even more so in clean marine or polar atmospheres because of the minute quantities of material that can be collected in a reasonable time. For these measurements, one needs a sensitive analytical technique that is free from interference by other elements present. Recently, the use of lithium-drifted germanium [Ge(Li)] y-ray detectors in neutron activation analysis has greatly improved analytical sensitivities and accuracies for such studies (J). 

Lithium-drifted detectors are made either from silicon (Si(Li) detectors) or from germanium (Ge(Li) detectors). The latter has a higher atomic number and density than silicon and is therefore preferable for 7-spectrometry. For 60 keV X-rays, the efficiency of a Si(Li) detector may be 5%, while for a comparable Ge(Li) detector it may be 100%. At lower energy the Si(Li) detector is preferable, especially if the measurements are carried out in a high 7-background. Si(Li) detectors are of particular importance in X-ray fluorescence analysis (cf. 6.8.4). 

FIG. 8.14. Cut-away view of a Ge-detector showing Dewar, cold-finger, preamplifier and germanium crystal. (Acc. to ORTEC.)... 

The excellent resolution provided by the more recently developed lithium-drifted silicon [Si(Li)] and germanium [Ge(Li)] semiconductor detectors (see Chap. 

The ground-based prototype consists of a crystal lens holding small cubes of diffracting germanium crystals and a 3x3 germanium array that detects the concentrated beam in the focal plane. Measured performances of the instrument at different line energies (511 keV and 662 keV) are presented and compared with Monte-Carlo simulations. The advantages of a 3x3 Ge-detector array with respect to a standard-monoblock detector have been confirmed. 

The largest applications for semiconductors use extrinsic material. The entire electronic materials industry is built around doped silicon. However, there are applications that require intrinsic semiconductors. One such application is X-ray detectors used on transmission electron microscopes (TEMs) and scanning electron microscopes (SEMs) for chemical analysis. Unfortunately it is essentially impossible to produce pure silicon. Even electronic grade silicon contains small amounts of boron (a p-type dopant). To create intrinsic material a dopant is added that produces an excess of electrons that combine with the holes formed by the residual boron. The process involves diffusing lithium atoms into the semiconductor. Ionization of the lithium produces electrons that recombine with the holes. It is possible to produce germanium crystals with much higher purity, and intrinsic Ge detectors are used on some TEMs. 

HP-Ge detectors have the advantage that they can be stored at room temperature and are cooled only during operation to reduce tbe problem of thermal excitation of electrons. Coaxial p-type detectors and n-type detectors are available. In the n-type detector the inner contact is made by diffused lithium, while the outer contact is achieved by ion implantation, which ensures a very thin entrance window (0.3 pm). Thin planar germanium detectors (5-20 mm) can also be used... 

Germanium detectors are characterized by three parameters resolution, peak-to-Compton ratio, and efficiency. The resolution is typically given for the 1332-keV Co line and varies from 1.8 keV for the very best to 2.3 keV for the very large detectors. The peak-to-Compton ratio is measured as the ratio of the number of counts in the 1332-keV peak to the number of counts in a region of the Compton continuum. Values vary from 30 to 90 for the most expensive model. The efficiency is expressed as a relative efficiency compared with the 7.5x7.5-cm Nal(Tl) scintillation detector. Relative efficiencies of HP-Ge detectors vary from 10% up to 150%. The dead time of semiconductor detectors is low, so the count rate is limited largely by the electronic circuit. 

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