The baseline technology for the BabyIAXO and IAXO detectors are Micromegas detectors. These detectors have been developed over the years in the context of low-background experiments, like CAST itself. These development have been the main focus of the detector research at the CAST/IAXO group at the Universidad de Zaragoza.
These detectors are small gaseous time projection chambers, read by Micromegas readout planes. The cathode of the detector is made of a very thin window so that it is transparent to the incoming signal x-rays, while at the same time supports the overpressure of the gas inside the chamber.
The x-rays ionize the gas, creating some primary charge that drifts towards the anode of the chamber where the readout plane is. The charge is then amplified inside the Micromegas plane. Micromegas are a particular class of micropattern gas detectors (MPGD) that consist of a very thin micromesh suspended over a pixelated anode by only 50-100 microns tall insulator pillars. The mesh is put at a voltage difference with the anode, creating the neccesary electron-amplifying field in the gap.
The readout is divided in an array of 500 microns wide pixels, that are interconnected in X-Y columns. Therefore the detector provides a X and a Y projection of the event electron cloud in the gas. Time of arrival are every strip gives relative Z position. The detector thus provides a 3D image of the primary ionization cloud in the gas.
The detector parts are built with selected materials with ultra-low levels of radioactivity, like pure copper, teflon or kapton. Moreover, it is surrounded by a shielding made of lead an copper bricks, plus muon detectors (muon vetos) that tags these penetrating particles present in the cosmic rays. These techniques, together with the capability to record event topology in the detector gas, have allowed these detectors to enjoy the lowest detector levels for x-ray detection.
This is a picture of the IAXO-D0 detectors at the U. of Zaragoza. The latest state-of-the-art prototype of Micromegas detectors for IAXO (muon vetoes not shown)