A new generation of detectors for X-ray imaging and spectroscopy applications
Members of the Astrophysics Division of the Space Science Department at ESTEC Noordwijk have developed a new detector system for X-ray imaging and spectroscopy applications.
Such detectors, based on extremely high-quality Gallium Arsenide (GaAs), a semiconductor material, are under study for future generations of astrophysics missions in space that will follow-on from ESA’s currently operating XMM-Newton Observatory. Prototype devices have already demonstrated the potential to replace the current generation of silicon, gas-filled or crystal scintillation detectors with ones that have 10 times the efficiency for measuring high energy X-rays (5-100 keV).
Small 5×5 pixel imaging arrays (c.f. Figure 1) have been produced to demonstrate potential imaging and spectroscopic capabilities. An energy resolution (i.e. the measure of a detectors ability to determine or discriminate the ‘colour’ of the X-ray) of 1% at 60 keV (c.f. Figure 2) has been achieved with a device operating at room temperature. (For comparison, the human eye has an energy resolution of ~ 5% at optical wavelengths.) By cooling the detector to a modest -30 C, a 70%
improvement in this energy resolution can be achieved. This means these detectors can be operated without the need for expensive and cumbersome cryogenic cooling systems. The next step will be the development of large-format imaging systems of about a million pixels. Because GaAs has the additional advantage of being some ten times faster in response to an incomming X-ray than silicon, the development of a real time X-ray Camcorder can be envisaged.
However such detectors will find much wider applications in the medical field including radiography, in vivo x-ray fluorescence, radioactive tracers and in therapeutic/diagnostic applications of radioactive isotopes. The latter can for example be used in the diagnosis of osteoporosis (a bone disease in which bones become thinner and more porous), through the measurement of the bone mineral content and the determination of the effectiveness of various proposed therapies.
These new compact detector arrays will therefore offer considerable advantages over those currently in use by the medical field including high efficiency, high image contrast through good X-ray ‘colour’ discrimination, portability and operating close to body temperature.
Thus, from future technologies developed originally for space, some more ‘down-to-earth’ practical human benefits could be derived.
* Astrophysics Division website
* The EPIC instrument
[Fig. 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&ooid=24400] A microscope image of a 5×5 pixel array of GaAs detectors. Each detector or pixel is 0.2 mm square and 0.04 mm thick. Devices as thick as 0.4 mm have now been fabricated.
[Fig. 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=1&cid=1&ooid=24395] Recorded spectra of x-ray lines from a single pixel of the array.