At the end of August, the first X-ray photons were detected with a prototype of the focal plane of the MXT camera (the engineering model). This is an important step in validating the design of the detection chain.
The design of the MXT camera began in 2014. It contains a pixelated silicon detector of pnCCD type with 256×256 pixels, similar to the one integrated in the eRosita instrument of the Russian Spectrum-Röntgen Gamma (SRG) satellite to be launched next year.
The X-ray detector and its front-end electronics are mounted on a multilayer ceramic circuit to ensure good thermal dissipation. This set, called the focal plane, was placed in a cryostat to allow laboratory tests to validate the operation of the detection chain at the target operating temperature of -65°C.
The integration of the focal plane into the clean room cryostat involves several complex steps, which required great rigour and several repetitions, as shown in the video below.
Once the cryostat was closed, the detector was then cooled to -60°C and illuminated with a Cobalt-57 radioactive source. Tests carried out at the end of August showed that it correctly detects X-ray photons, thus validating all the work carried out by the various teams over the past 4 years.
First detected X-ray photons. Cobalt-57 source, -60°C, actual acquisition rate.
Each event (point on the image) represents the detection of a photon from the source by the focal plane. By extracting the energy deposited in the detector for each impact, a spectrum can be constructed. The following figure represents the first spectrum obtained on a Cobalt-57 source by the prototype of the MXT focal plane. The Cobalt-57 low energy lines at 6.4 keV, 7 keV and 14 keV are recognized in this spectrum.
Next Steps
The next step, in early 2019, will be to irradiate this prototype of the focal plane with protons to simulate space conditions.
In one year, the flight model of the focal plane will be integrated into the flight model of the camera.
The CEA Irfu is in charge of the design and construction of the X-ray camera of the MXT telescope.
