Cooling System for a Frame-Store PN-CCD Detector for Low Background Application

The astroparticle physics experiment CERN Axion Solar Telescope (CAST) aims to detect hypothetical axions or axion-like particles produced in the Sun by the Primakoff process. A Large Hadron Collider (LHC) prototype superconducting dipole magnet provides a 9 T transverse magnetic field for the conversion of axions into detectable X-ray photons. These photons are detected with an X-ray telescope and a novel type of frame-store CCD detector built from radio-pure materials, installed in the optics focal plane. A novel type of cooling system has been designed and built based on krypton-filled cryogenic heat pipes, made out of oxygen-free radiopure copper, and a Stirling cryocooler as cold source. The heat pipes provide an efficient thermal coupling between the cryocooler and the CCD which is kept at stable temperatures between 150 and 230 K within an accuracy of 0.1 K. A graded-Z radiation shield, also serving as a gas cold-trap operated at 120 K, is implemented to reduce the surface contamination of the CCD window and suppress background radiation.