The TopHat detectors are monolithic silicon bolometers. These devices measure the intensity of radiation by absorbing it and converting it to a temperature rise in the absorber material.
Bolometers are the most sensitive detectors for broadband low-background radiation.
With these detectors mounted in a 3He cryostat, TopHat will be able to make observations of cosmic microwave background anisotropy of unprecedented sensitivity, in spite of the relatively short integration time available with a balloon-borne telescope. The central disk is coated with a layer of bismuth which absorbs the incident radiation. The disk, which is only 5 microns thick, has a very low heat capacity, so even small amounts of radiation result in a significant temperature increase. This figure s hows an actual TopHat detector. The thin supporting legs are 10 microns wide by 5 microns thick.