X-ray Microcalorimeter Spectrometer (XMS)
The purpose of the XMS is to provide spectra with a resolving power E/δE of several hundred in the energy range 0.2 – 6 keV, simultaneously with images of a modest (2 arcmin) field of view. This is achieved by time resolved photon counting.
The XMS is based on an array of micro-calorimeters (see Figure below) which will be configured as an inner array (40x40 pixels on a pitch of 300 μm) providing high energy resolution (2 eV FWHM) and an outer array (of larger pixels on a 600 μm pitch) with lower energy resolution (<10 eV FWHM).
Each pixel in the array consists of a Bi/Cu X-ray absorber of typically 300 × 300 μm² size and 7 μm thickness of which the temperature is read-out by a normal-to-superconducting phase transition thermometer with a critical temperature Tc ≈ 100 mK, usually referred to as a Transition-Edge-Sensor (TES). The absorber-thermometer combination is weakly coupled to the 50 mK base temperature of the cryostat by means of a Si3N4 membrane. Each pixel in the array is voltage-biased thereby raising the device temperature into the transition at about 100 mK. Subsequently the current signals are amplified by cryogenic SQUID-amplifiers (50 mK – 2.5 K), which are linearized by room temperature feedback electronics.
The detector operates at 50 mK and will be cooled to this temperature by a multistage adiabatic demagnetization refrigerator (ADR) operating in series with a mechanical cryo-cooler. No expendable cryogens are foreseen for the XMS.
The XMS is based on TES for which there has been extensive development work both in Europe and the US. Institutes on both sides of the Atlantic have demonstrated significant progress in developing sensors that can achieve the energy resolution required for IXO. While the basic principles have been clearly demonstrated the next step is to show that large arrays can be fabricated which maintain the performance requirements. Another key technology which needs further development is the multiplexing of the sensor’s signals to minimize the heat load caused by thermal conduction through the harness to the cold finger.
Additional development remains to qualify a cryo-chain to achieve the detector operating temperature of 50 mK. All of the individual elements that are needed to develop the optimum cryo-chain for the IXO payload configuration already exist.
(Editor's Note: During an early study phase of IXO the XMS was known as the CIS (Cryogenic Imaging Spectrograph). )