Photon/Optical Detectors
HPGE
Germanium is a lot like silicon: It's a semiconductor from which you can build complex electronic circuits, and in bulk form it's a dark, shiny crystalline yet sort of metallic solid. It's quite expensive, much more expensive than silicon, and not nearly as commonly used. Germanium can also be used as an x-ray lens, because even though it's opaque to visible light, it transmits and can focus x-rays. Silicon and Germanium are available in higher purity grades than virtually any other elements. Silicon in particular can be had in large quantities at reasonable prices, at a purity that exceeds anything else achieved by the hand of man. Germanium is similar, just more expensive. The reason for this is not that other elements are necessarily more difficult to purify (some are, some aren't), it's that there is a huge market demand for hyperpure silicon and to a lesser extent germanium for the semiconductor industry. Additional information available at "http://www.theodoregray.com/PeriodicTable/Elements/032/"