The European Photon Imaging Camera (EPIC) onboard XMM-Newton
2. Chip Geometry
2.1. MOS CCDs
2.2. pn CCDs
3. The Camera Concept
3.1. The basic principle of the MOS CCDs
3.2. The basic principle of the pn-CCDs
4. Operating Modes
5. Instrument Characteristics
5.1. Quantum Efficiency
5.1.1. EPIC MOS
5.1.2. EPIC pn
6. Filters and Effective Area
7. EPIC Radiation Monitor
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
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.
In this mode, all pixels of all CCDs are read out and thus the full FoV is covered
In a partial window mode the central CCD of both MOS cameras can be operated in a different mode of science data acquisition, reading out only part of the CCD chip: in small window mode an area of 100 x 100 pixels is read out, whereas in large window mode an area of 300 x 300 pixels is active.
In large window mode only half the area of all 12 CCDs is read out, whereas in small window mode only the part of CCD0 in quadrant 1 at the focal point is used to collect data
a) MOS + pn
In timing mode, imaging is made only in one dimension, along the column axis. Along the row direction, data from a predefined area on one CCD chip are collapsed into a one-dimensional row to be read out at high speed
b) pn only
A special flavour of the timing mode of the EPIC pn camera is the burst mode, which offers very high time resolution, but has a low duty cycle of 3%
It is the quantum efficiency of the EPIC-MOS chips that limits the energy passband at its high energy end, while the pn camera can detect photons with high efficiency up to 15 keV.
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.
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.
If such photons are registered by the EPIC detectors, the data analysis would be impeded in three ways:
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
Three parts constitute the ERM experiment:
7.1 ERM detectorsThe 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.
It registers events in the energy range from 50 keV to 5 MeV. Dynamic range of the preamplifier from 40 keV to 5 MeV with a conversion factor of 4 MeV/3 V. The range is digitised in the range from 0 to 3 V full scale with a resolution of 256 channels.
The maximum counting rate for electrons is 106 e-/s and for the protons 2 x 104 p+/s.
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.
It registers events in the energy range from 500 keV to 12 MeV. Dynamic range of the preamplifier from 130 keV to 12 MeV with a conversion factor of 8 MeV/3 V. The range is digitised from 0 to 4.4 V full scale with resolution of 256 channels.
The maximum counting rate is less or equal 5 x 105 count per second. After sum of the pulses, the resulting count rate is 106 count per second, 10% of which are in coincidence.
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.
- The MOS camera system was produced by a consortium which includes Leicester University (CCDs and camera head), the University of Birmingham (thermal control system) and CEA Service d'Astrophysique Saclay (control and event recognition electronics).
- The pn CCD array was fabricated in the MPI-semiconductor laboratory, the pn camera was built by MPI für extraterrestrische Physik and Astronomisches Institut Tübingen.
- The EPIC Radiation Monitor was developed by CESR Toulouse.
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