Athena Project

The X-ray Integral Field Unit (X-IFU)

Please take a look at the paper below

— The X-ray Integral Field Unit (X-IFU) for Athena+ (Barret, den Herder, Piro et al.)

The X-IFU is a 2-D imaging camera, which allows the identification and characterization of the different ionization stages in hot plasmas thanks to its excellent spectral resolution (2.5 eV at 6 keV). This enables detailed diagnostics of abundances, temperatures, densities and bulk or turbulent velocities of hot plasma’s in extended objects. It is an integral-field spectrometer operating in the soft X-ray band. The technology for this detector has been under development over many years and capitalizes on long term investments at the member states. ESA has been developing the technology for the cooling system.

X-IFU main characteristics

  • Detector

The microcalorimeter consists of a cryogenically cooled detector that works by sensing the heat pulses generated by X-ray photons when they are absorbed and thermalized. The temperature increase is directly proportional to the photon energy and is measured by the change in electrical resistance of the sensor. The resulting signal represents the total energy deposited. For a few eV resolution the sensor has to be cooled to <100 mK and is biased in its transition between super conducting and normal resistance.

Principle of a micro-calorimeter. The absorption of an X-ray photon heats both the absorber and the sensor. The resulting signal represents the total energy deposited. The system goes back slowly to its original state through a weak thermal link with a heat sink.

The detector consists of an array 3840 absorbers which are 250 µm squared composed of 1 µm Au and 4 µm Bi to achieve the correct stopping power at 6 keV and low heat capacitance required for high energy resolution. The energy resolution has been demonstrated using different read-out schemes (Frequency Domain Multiplexing, Time Domain Multiplexing and Code Division Multiplexing). Multiplexing of 40 pixels per read-out channel, the total number channels can be limited to 96 for which cooling can be provided. At higher count rates the resolution will degrade (events are too close in time to determine the integrated flux accurately). For a 10 mCrab point source 80% of the events will have their nominal resolution. For higher count rates either a band pass filter is available or defocussing optics. As the response of each pixel is independent, the 10 mCrab limit applies to each beam. Also some narrow band polarization sensitivity will be provided by using filters made from dichroic materials. Under the sensor, an anti-coincidence detector is placed to reduce the effect of charged particles.

A small subarray (10 x 10 arcsec) could be considered to implement the goal energy resolution of 1.5 eV for point sources. Such array will use smaller pixels (e.g. 50 microns) and will be optimized for a lower maximum energy (2 keV).

Picture of the X-ray Microcalorimeter Spectrometer. The central part of the detector shows an array of absorbers that are read-out by Transition Edge Sensors.

Spectral resolution obtained by FDM read-out. Note that different read-out schemes are available and provide somewhat better resolution.

  • Cooling

The sensor is coupled to a 50 mK bath. The instrument life time will not be limited by consumables as a combination of different, cryogen free, cooling techniques will be used. The cooling chain is split into two subsystems: the prime cooling chain for cooling part of the instrument from room temperature to 4 K and the last stage cooler for 4 K to 50 mK. There are multiple solutions for the cooling system, which are under development at ESA (e.g. sorption/ADR, Joule-Thomson coolers, Stirling coolers, Dilution Refrigerators). The selection of the cooling system will be based on the technological level at the time of the mission proposal, but the required resources have been assessed based on a full design for the coolers on Athena/IXO.

The focal plane assembly (left) including harness, suspension and magnetic shielding and a possible implementation of the cryostat (right) based on the Athena proposal using active cooling.

  • X-IFU consortium

The X-IFU is being developed by a consortium of European research institutions which include IRAP (France), INAF (Italy), SRON (The Netherlands), CSL (Belgium), CEA/Saclay & CEA/Grenoble (France), MSSL & Leicester University (UK), ISDC (Switzerland), CSIC (Spain) and Erlangen University (Germany).


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