The technological challenges for detection

All space observatories like ISO, Spitzer, or Planck are based on cryogenic instruments with cooled sensors, sometimes down to a fraction of a degree above absolute zero. The goal is to improve sensitivity by using sensors cooled to the lowest possible temperatures in order to measure an infinitely small variation in signal. One challenge for future space missions is to replace cryogenic liquid tanks by mechanical cryo-coolers. This disruptive technology means space and weight savings and, above all, it increases the lifetime of the missions considerably. New cryogenic sensors sensitive to polarised light and operating at 50 mK to reach a very high sensitivity have been developed and are now produced directly on top of a cryogenic read-out integrated circuit. At the other end of the electromagnetic spectrum, CEA is developing hard X-ray sensors (from keV to MeV) based on uncooled CdTe (cadmium telluride) semiconductors, associated with application specific integrated circuits (ASIC). These small heavy and dense crystals allow to stop each X-ray photon and to measure their energy individually. The photon fluxes from very high-energy gamma sources (energies larger than 50 GeV) are very small, less than one particle per km² per second. They interact in the atmosphere and create relativistic particle showers emitting Cherenkov radiations. Focal planes of Cherenkov telescopes are now equipped with cameras with several thousand photomultipliers, able to detect the photons individually. CEA is involved in designing, assembling and testing these NectarCAM cameras. They are equipped with 1800 photomultipliers (pixels) giving a field of view of 8 degrees for mapping extended sources such as supernova remnants. The NectarCAM can read 10.000 image sequences per second, with each sequence lasting 60 ns. (A.C.)