European Photon Imaging Camera (EPIC)
The main focal plane instrument on XMM-Newton, providing imaging and spectroscopy, is the European Photon Imaging Camera (EPIC). The three EPIC cameras offer the possibility to perform extremely sensitive imaging observations over the telescope's field of view (FOV) of 30 arcmin and in the energy range from 0.15 to 15 keV with moderate spectral (E/∆E ~ 20-50) and angular resolution (PSF, about 6 arcsec FWHM).
All EPIC CCDs operate in photon counting mode with a fixed, mode dependent frame read-out frequency, producing event lists; that is tables with one entry line per received event, listing (among others) attributes of the events such as the position at which they were registered, their arrival time and their energy.
Another associated experiment on board XMM-Newton is the EPIC Radiation Monitor (ERM). The main function of the ERM is the detection of the radiative belts and solar flares in order to supply particle environment information for the correct operation of the EPIC camera. In addition, the ERM provides detailed monitoring of the space radiative environment constituting a reference for the development of detectors to be used in futures missions.
The EPIC consortium is made up of ten institutes in four nations: the United Kingdom, Italy, France and Germany.
Charge-coupled devices (CCD)
EPIC consists of three cameras, at whose heart are charge-coupled devices (CCD) which register and record the energy of incoming X-ray photons. Two of the cameras employ metal oxide semi-conductor (MOS) CCDs, developed jointly by Leicester University and English Electric Valve (EEV, Chelmsford, UK) while the third uses a new type of CCD (pn) and was built by the Max Planck Institute of Extraterrestrial Physics in Garching and the Astronomical Institute in Tübingen, both in Germany.
The MOS detectors register photons in the 'soft' portion of the X-ray spectrum with good energy resolution. With only a 40-micron sensitive depth of silicon, the detectors are less responsive to high energy or 'hard' X-rays. This upper part of the spectrum is covered better by the EPIC 'pn' CCD, which has a 300-micron thickness. The spatial resolution of the cameras at 1.5 keV is 6.0 and 4.5 arcsec FWHM for the two EPIC MOS detectors, and 6.6 arcsec FWHM for EPIC pn.
The only preceding mission using X-ray CCDs (the ASCA satellite) proved its efficiency to detect hard X-rays, but also highlighted its vulnerability to radiation damage. In order to protect all the detectors, the three cameras are well shielded with a 3 cm-thick piece of aluminium. One position on each camera filter wheel can also occult the sensitive detector. All the cameras have a large radiator that cools down the CCDs to their nominal operating temperature of -100 °C. In November 2002, the temperature for the MOS detectors was lowered to -120 °C to move important detector characteristics back to the values which they had earlier in the mission, improving the performance.
Each EPIC-MOS CCD consists of seven silicon chips, which are made up of a matrix of 600 × 600 pixels. Each CCD reads out in a couple of seconds and the image data is processed and compressed electronically in the instrument electronic boxes, so as to be compatible with the spacecraft's telemetry, sending the data back to the Earth ground stations.
New technology 'pn' detector
The development of the 'pn' detector was a seven-year effort by the semi-conductor laboratory of the Max Planck Institute for Extraterrestrial Physics, followed by over two years of integration work and testing on the camera itself. The result is an EPIC pn detector with a single chip, whose design represents a radical departure from the integrated circuit conception of current MOS CCDs.
The silicon wafer has been manufactured 'in-house' with extreme precision to maintain the very high resistivity of the 300 micron-thick double-sided processed silicon detectors. This fact is responsible for the efficient detection of the high energy X-rays.
As the pn-CCD is illuminated from the rear side, which does not have insensitive layers or coatings, the X-ray detection efficiency is extremely high and homogeneous from the very low to the highest XMM-Newton energies (over 90 per cent from 0.5 to 10 keV). The parallel readout of 768 independent channels enables the camera to be operated quickly: less than 80 ms are needed to acquire one picture or frame. Special readout modes allow the observation of transient objects with a time resolution of only 40 ms.
In contrast to the MOS camera, the pn detector has a 400 × 384 pixel matrix of 6 cm × 6 cm, monolithically fabricated on a ten centimetre high purity silicon wafer. The 36 cm² sensitive area made it the largest X-ray CCD detector built for space at the time, and it remained so for many years.