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The European Photon Imaging Camera (EPIC) onboard XMM-Newton
The XMM-Newton spacecraft is carrying a set of three X-ray CCD cameras, comprising the European Photon Imaging Camera (EPIC). Two of the cameras are MOS (Metal Oxide Semi-conductor) CCD arrays (referred to as the MOS cameras). They are installed behind the X-ray telescopes that are equipped with the gratings of the Reflection Grating Spectrometers (RGS). The gratings divert about half of the telescope incident flux towards the RGS detectors such that (taking structural obscuration into account) about 44% of the original incoming flux reaches the MOS cameras. The third X-ray telescope has an unobstructed beam; the EPIC instrument at the focus of this telescope uses pn CCDs and is referred to as the pn camera.
The 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/Delta E ~ 20-50) and angular resolution (PSF, 6 arcsec FWHM).
All EPIC CCDs operate in photon counting mode with a fixed, mode dependent frame read-out frequency, producing event lists, i.e. 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 energies. The two types of EPIC, however, differ in some major aspects. This does not only hold for the geometry of the CCD arrays and the instrument design but also for other properties, like e.g., their readout times.
Another experiment on board of 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 MOS EEV CCD22 is a three-phase frame transfer device on high resistivity epitaxial silicon with an open-electrode structure; it has a useful quantum efficiency in the energy range 0.2 to 10 keV. The low energy response of the conventional front illuminated CCD is poor below ~700 eV because of absorption in the electrode structure. For EPIC MOS, one of the three electrodes has been enlarged to occupy a greater fraction of each pixel, and holes have been etched through this enlarged electrode to the gate oxide. This gives an "open" fraction of the total pixel area of 40%; this region has a high transmission for very soft X-rays that would have otherwise be absorbed in the electrodes. In the etched areas, the surface potential is pinned to the substrate potential by means of "pinning implant". High energy efficiency is defined by the resistivity of the epitaxial silicon (around 400 Ohm-cm). The epitaxial layer is 80 microns thick (p-type). The actual mean depletion of the flight CCDs is between 35 to 40 microns: the open phase region is not fully depleted.
The schematic view looking into the pn-CCD introduces intuitively the advantages of the concept: X-rays hit the detector from the rear side. In the event of an X-ray interaction with the silicon atoms, electrons and holes are generated in numbers proportional to the energy of the incident photon. The average energy required to form an electron-hole pair is 3.7 eV at -90° C. The strong electric fields in the pn-CCD detector separate the electrons and holes before they recombine. Signal charges (in our case electrons), are drifted to the potential minimum and stored under the transfer registers. The positively charged holes move to the negatively biased back side, where they are 'absorbed'. The electrons, captured in the potential wells 10 microns below the surface can be transferred towards the readout nodes upon command, conserving the local charge distribution patterns from the ionization process. Each CCD line is terminated by a readout amplifier.
The EPIC cameras allow several modes of data acquisition. Note that in the case of MOS the outer ring of 6 CCDs remain in standard full-frame imaging mode while the central MOS CCD can be operated separately. The pn camera CCDs can be operated in common modes in all quadrants for full frame, extended full frame and large window mode, or just with one single CCD (CCD0 in quadrant 1 = CCD4 according to SAS numbering conventions) for small window, timing and burst mode.
Further details about the EPIC science modes are given in the XMM-Newton Users' Handbook.
One of the factors to be taken into account when determining the effective area of the EPIC cameras is their quantum efficiency.
The quantum efficiency of the MOS CCDs varies a little from CCD to CCD at very low energies. It is a smooth function except near the edges of silicon and oxygen: the carbon and aluminum edges are apparent in the thin and medium filter responses, tin appears as well in the thick filter (a description of the filters is given below, the gold edges of the mirror are apparent in the overall quantum efficiency). The response near these edges was measured using different beams at the Orsay synchrotron. The measurements have been linked together using celestial sources.
The fully depleted 280 µm of silicon determines the pn detector efficiency on the high energy end, while the quality of the radiation entrance window is responsible for the low energy response. The absolute quantum efficiency calibration was performed at PTB (BESSY synchrotron in Berlin) and the Orsay synchrotron. The drop of quantum efficiency at the lowest energies is caused by the properties of the silicon L-edge. The drop of about 5% of quantum efficiency at 528 eV is due to the additional absorption in the SiO2 passivation on the detector surface. The other prominent feature is the typical X-ray absorption fine structure (XAFS) behavior around the silicon K-edge at 1.838 keV, enlarged in the inset. At higher energies the solid line nicely fits the photon absorption data for 300 µm of silicon. The solid line is a fit to the measured data with a depletion thickness of 298 µm. The quantum efficiency is not expected to change during the XMM-Newton lifetime under nominal conditions.
The EPIC background can be divided into two parts: a cosmic X-ray background (CXB), and an instrumental background. The latter component may be further divided into a detector noise component, which becomes important at low energies (below 200 eV) and a second component which is due to the interaction of particles with the structure surrounding the detectors and the detectors themselves. This component is characterized by a flat spectrum and is particularly important at high energies (above a few keV). The particle induced background can be divided into two components: an external 'flaring' component, characterized by strong and rapid variability, which is often totally absent and a second more stable internal component. The flaring component is currently attributed to soft protons (with energies smaller than a few 100 keV), which are funneled towards the detectors by the X-ray mirrors. The stable component is due to the interaction of high energy particles (with energies larger than some 100 MeV) with the structure surrounding the detectors and possibly the detectors themselves.
Further details about the origin of the EPIC background are given in the document XMM-SOC-CAL-TN-0016. The many diverse aspects of the XMM-Newton radiation environment were discussed extensively at a Workshop held at the XMM-Newton SOC in December 2000. More details on the instrumental background are given in the XMM-Newton Users' Handbook.
As the EPIC detectors are not only sensitive to X-ray photons but also to IR, visible and UV light, the cameras include aluminised optical blocking filters to reduce the contamination of the X-ray signal by those photons.
There are four filters in each EPIC camera. Two are thin filters made of 1600 Å of poly-imide film with 400 Å of aluminium evaporated on to one side; one is the medium filter made of the same material but with 800 Å of aluminium deposited on it; and one is the thick filter. This is made of 3300 Å thick Polypropylene with 1100 Å of aluminium and 450 Å of tin evaporated on the film. The filters are self-supporting and 76 mm in diameter. The remaining two positions on the filter wheel are occupied by the closed (1.05 mm of aluminium) and open positions, respectively. The former is used to protect the CCDs from soft protons in orbit, while the open position could in principle be used for observations where the light flux is very low, and no filter is needed.
The EPIC MOS effective area for each of the optical blocking filters and without a filter
The EPIC pn effective area for each of the optical blocking filters and without a filter
Combined effective area of all telescopes assuming that the EPIC cameras operate with the same filters, either thin, medium or thick
The EPIC Radiation Monitor (ERM) experiment is a part of EPIC. The ERM has a function of detection of radiation belts and solar flares in order to supply particle environment information for the correct operation of the EPIC camera. Moreover, the ERM is used for the detail monitoring of the space radiative environment, constituting a reference for the development of detectors to be used in future missions. A technological experiment is also included in the ERM: Evalcomp. The purpose of this experiment is to evaluate the susceptibility to single event upsets (SEU) of various type of memories.
Three parts constitute the ERM experiment:
7.1 ERM detectors
The ERM contains two types of silicon diode detectors, one for low energy and one for high energy, fully redundant, and assembled on a single mechanical structure as the electronic box:
a) Low Energy detector characteristics:
A Silicon detector, 500 mm of thickness, 0.85 cm2 of surface with one programmable low threshold from 0 to 256 keV.
b) High Energy detector characteristics:
Two silicon detectors, 500 mm of thickness, 0,85 cm2 of surface with one programmable low threshold from 0 to 512 keV.
The block diagram given here after shows the ERMD functions:
7.2 The ERM signal "Warning Flag"
This signal is an alert signal for EPIC and will be delivered to the telemetry every 4 second, by the ERM. This signal is the results of computation done on the detector counters and is used as a trigger to close the EPIC cameras protecting them during high radiation intervals, e.g. in the Earth's radiation belts or during solar flares.
The computing method is the following: each counting rate is compared to a 16 bit reference word, set by telecommand. Each 4 sec, if at least one of the detector rates is greater than its reference word, then a counter will be incremented by one unit and, if not, the counter will be reset. When the content of this counter is equal to a commendable value N, i.e. when the condition "at least one counting rate is higher than its reference for 4 sec" is met N successive times, then the signal "Warning Flag" will be generated, until a command, sent from the ground, reset the "Warning Flag", or when complementary conditions occurs. This flag is set if N successive times one of the detector counting is found greater than his associated threshold. The flag is cleared if N successive times the detector counting are less than their associated threshold or if the Warning Flag reset telecommand is received.