Germanium Array Detectors
Introduction
The broad-band x-ray flux from synchrotron radiation sources has revitalized the relatively old experimental technique known as x-ray absorption spectrometry. X-ray absorption spectroscopy measures the attenuation of an x-ray beam passing through a sample, just as do the more familiar infrared or UV-visible techniques. Typical x-ray energies are on the order of 300 eV to 30 keV, compared to visible light of 2-3 eV and infrared energies of about 0.05-0.5 eV. High energy x-ray absorption transitions involve core electrons which are only slightly perturbed by chemical changes in the valence electrons, hence each element has characteristic absorption edges at which the x-ray energy is just sufficient to liberate a particular type of core electron. Since edges are generally well separated in energy, X-ray absorption is a technique which can uniquely probe the environment of any element from carbon through the transuranics. A generalized x-ray absorption spectrum is illustrated below.
Schematic representation of a typical X-ray absorption
spectrum. The edge, or XANES, region extends over 25 to 50 eV and the EXAFS is typically
observable over several hundred to 1000 eV.
CANBERRA has been the leader in the development and production of Germanium Array Detectors for this application. Herein you will find a brief summary of our capabilities and products.
The 13-Element Detector
CANBERRA developed the first Ge Array Detector for EXAFS in 1987. This 13-element detector was an instant success. It was developed and produced in record time, it worked immediately after it was installed at the Brookhaven National Synchrotron Light Source, and it turned out data of unprecedented quantity and quality.
In succeeding years CANBERRA has produced many 13-element detectors in essentially the original design. These detectors have been the real work horses in EXAFS, with extremely high up-time and utilization for such sophisticated, high-performance systems.
13 ElementEXAFS Array Detector
The WRULEAD
In 1988 CANBERRA introduced the Ultra-LEGe detector which was the first commercial Ge detector to demonstrate good resolution, peak shape, and efficiency for soft x rays. The Ultra-LEGe proved that germanium can compete favorably with silicon at low energies and, as expected, far exceed the performance of silicon at high energies. The Ultra-LEGe extended the energy range of Ge detectors from a low of about 3 keV to less than 300 eV.
Almost immediately, interest in the Ultra-LEGe surfaced among our EXAFS customer base. We quickly developed a 7-element low-energy array detector called the WRULEAD (Windowless, Retractable, Ultra Low Energy Array Detector) which, despite the generally delicate nature of windowless detectors, has been deployed successfully for several years. Versions of the WRULEAD ranging from three to thirteen elements have been built since.
7 Element WRULEAD
Other Array Detectors
In addition to the 13-element hard x-ray detectors and the 7- to 13-element WRULEAD, CANBERRA has built many single- and multi-element detectors with special geometries for specialized applications. This includes linear arrays with rectangular elements, circular arrays, and arrays with element sizes ranging up to 500 mm2.
Array Patterns
And Now - Advanced Array Detectors
The CANBERRA LEGe and Ultra-LEGe detectors are notable for high rate performance compared to that of Si(Li) and Planar Ge detectors. This is due in large part to their low capacitance and to the absence of "dead" material in the elements.
The Advanced Array Detector takes these advantages to the extreme and, in addition, employs a high-rate preamplifier. In combination, these improvements lead to as much as a 4:1 improvement in throughput rate compared to our previous array detectors. Advanced Array Detectors give excellent resolution with amplifier time constants of 1/8 microsecond -comparable to Si(Li) and Planar Ge detectors operating with 1 microsecond shaping time constants (for an 8:1 increase in throughput rate).
Advanced Array Performance
Advanced Array Detectors are available in 7, 13, 19 and even in 30 element versions. Shown below is a compact 13 element unit and a 30 element WRULEAD.
13 Element Compact
30 Element WRULEAD
Complementary electronics including a fast amplifier - three channel analyzer (Model 2016) are available to ensure that the full capability of the Advanced Array Detector is realized in practice.
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