  Current interferometric gravitational wave detectors (IGWDs) are operated at
room temperature with test masses made from fused silica. Fused silica shows
very low absorption at the laser wavelength of 1064 nm. It is also well suited
to realize low thermal noise floors in the detector signal band since it offers
low mechanical loss, i. e. high quality factors (Q factors) at room
temperature. However, for a further reduction of thermal noise, cooling the
test masses to cryogenic temperatures may prove an interesting technique. Here
we compare the results of Q factor measurements at cryogenic temperatures of
acoustic eigenmodes of test masses from fused silica and its crystalline
counterpart. Our results show that the mechanical loss of fused silica
increases with lower temperature and reaches a maximum at 30 K for frequencies
of slightly above 10 kHz. The losses of crystalline quartz generally show lower
values and even fall below the room temperature values of fused silica below 10
K. Our results show that in comparison to fused silica, crystalline quartz has
a considerably narrower and lower dissipation peak on cooling and thus has more
promise as a test mass material for IGDWs operated at cryogenic temperatures.
The origin of the different Q factor versus temperature behavior of the two
materials is discussed.
