Axion helioscopes like CAST or the future IAXO need to detect x-rays with very low background level. A very promising technological choice for x-ray detection in helioscopes are TPCs like the ones developed within T-REX.
Solar axion-induced x-rays can be detected by small TPCs in which the cathode is made of a x-ray-transparent foil (x-ray window), and the drift distance is sufficiently thick to stop 1-10 keV photons. The current design choice in CAST is 3 cms of drift at 1.3 bar pressure. The foil must withstand the pressure difference to the vacuum of the magnet bore, so it is held on a metallic strongback. The TPC readout is a highly pixelised microbulk Micromegas. A design of a recent version of this type of detectors is shown below.
Background levels below 10-6 ckeV-1cm-2s-1 have been achieved in the last runs at CAST, and close to 10-7 ckeV-1cm-2s-1 in special shielding environment underground at the Laboratorio Subterráneo de Canfranc. These record values are achieved by a combination of strategies:
- Detector shielding from external radiation
- Use of offline discrimination using detailed topological information provided by the pixelised readout
- Use of radiopure materials for the detector components
Picture of the detectors installed at the CAST “sunset” side and their (partially built) shielding
You can find technical and scientific results about our detectors for axion research in several T-REX publications. And also in the
- CAST UNIZAR web,
- CAST CERN web,
- and the forthcoming IAXO web at CERN.