Q:

What properties of Germanium make it suitable for Dark Matter detectors?

What properties of Germanium make it suitable for Dark Matter detectors?
I tried googling but there was too many results describing the use of Germanium Chrystals at low tempretures for Dark Matter detaction but not the why of it.
Or alternatively what are the properties that make a good dark matter detector? in this case Germanium just gets to fit the requirement and have a look at other possible candidates.
Assuming Germanium is rare and hard to get I prompted this question, on the other hand if it was copper that was being used for such a task, it would have gone completely unnoticed.

A:

WIMP's are hypothesized to interact only by the weak force, thus will interact only with the nucleus. The WIMP will hit the nucleus, slow down and change direction. The nucleus will recoil, and introduce vibrations (phonons) into the lattice. Those vibrations create heat which is measured. The crystal must me kept at 10 mK in order to reduce thermal noise. Additionally, the recoil of the nucleus lifts some electrons into the conduction band where they too can be measured.
Interactions by alpha, beta and gamma rays generate more ionization than temperature rise. WIMP interactions will generate a large nuclear recoil and many phonons but will not interact directly with the shell electrons thus very few electrons will enter the conduction band. By looking for a large phonon signal and a relatively small ionization signal, WIMPS can be sorted out.
But why Germanium?  Because Ge has the smallest band gap of the semiconductors. An incident particle will lift more electrons out of the ground state and put them into the conduction state. The signal reaching the preamp is larger with Ge than any other semiconductor. Germanium is the gold standard for the detection of gamma rays. Silicon has a larger band gap thus is not as sensitive. 

