X-ray Microcalorimeter System Technology
The X-ray Microcalorimeter Spectrometer (XMS) on Constellation-X will provide imaging and high-resolution spectroscopy in the 0.6-10 keV band. The XMS uses an X-ray microcalorimeter to sense individual X-ray photons as heat, and determine their energy with high precision. Thermodynamic limits determine the spectral resolution and drive the need for operation at a temperature below ~0.1 K. Although extraordinarily cold, such temperatures can be readily achieved and maintained using flight-proven techniques.
Microcalorimeter
The Suzaku/XRS microcalorimeter array. The pixels are each 625 micrometers an based on ion-implanted Si with HgTe absorbers.
The Constellation-X XMS reference design consists of superconducting transition-dege sensor (TES) microcalorimeters. TES microcalorimeters are operated in the narrow temperature range between the onset of non-zero resistance and the fully normal state. The change in resistance is measured by monitoring the current through a voltage-biased TES using a superconducting quantum interference device (SQUID).
The reference design consists of 64 × 64 pixels, consisting of a high-performance core array, surrounded by a field-of-view extension. The core array consists of a 32 × 32 array of 250 × 250 micron pixels (5 arcsec × 5 arcsec). Each pixel consists of a TES (which acts as the calorimeter thermometer), an X-ray absorber, and a membrane thermal link to the 50 mK heat sink. The basic design for the extension utilizes imaging TES detectors that will have at least eight imaging elements per pair of TES. Position information is obtained by comparing the relative signals on the two TESs while energy is inferred by summing the signals. Both parts of the focal plane will be read using multiplexed SQUID amplifiers. In total there will be ~1800 TES microcalorimeters that will be read out using a SQUID multiplexer.
Cooling System
Photo of technology demonstration Continuous Adiabatic Demagnetization Refrigerator
The XMS cooling system, consisting of a Continuous Adiabatic Demagnetization Refrigerator (CADR) and cryocooler, has no stored cryogens, thus maximizing the lifetime/mass ratio for the instrument.
Cooling of the detector stage will be achieved using a multistage CADR, which provides the necessary cooling power down to 50 mK. The warmer stages of the CADR are sequentially linked through heat switches and then cycled to transfer heat to the relatively warm cryocooler interface. A mechanical cryocooler will provide the <5 K heat sink for the CADR and will actively cool several thermal shields within the cryostat.
