X-ray Microcalorimeter Spectrometer (XMS)
For the XMS we need to develop detectors with enough sensitivity, dynamic
range, speed, angular resolution and uniform performance to meet Gen-X
calorimeter array requirements. The strategy for achieving these goals is
described in the
Microcalorimeter Development and Roadmap document.
The operational principle of a microcalorimeter is depicted in the figure
to the right. The general principle is to measure the temperature rise of the
calorimeter when an x-ray photon is absorbed. When an x-ray is absorbed, the
temperature in the absorber increases by an amount E/C, where E is the x-ray
energy, and then recovers back to the steady state temperature. The thermal
conductance of the weak link is engineered such that enough time is allowed for
proper thermalization of the photon energy in the device, but fast enough such
that the device recovers back to its base temperature in time to absorb the
A thermometer is integrated into the device to measure the temperature rise. To
ensure high spectral resolution, the thermometer must be very well coupled to
the absorber, and should be extremely sensitive to temperature changes. For
this Gen-X, there are currently two candidate technologies for the thermometer:
transition-edge sensors (TESs) and magnetic microcalorimeters (MMCs). In a TES
the heat deposited by an individual X-ray photon changes the impedance of a
superconducting bilayer held just below its transition temperature. In an MMC
the heat from an X-ray photon changes the magnetization of a paramagnetic
film. Both of these devices are read out using SQUID amplifiers. Currently TES
microcalorimeters have provided the best spectral resolution that has been
achieved (1.8 eV FWHM at 6 keV; Bandler et al., 2007), in arrays of pixels such
as the one shown in the photograph to the right.
Microcalorimeters are usually designed to thermalize as quickly as possible to
avoid degradation in energy resolution from position dependence to the pulse
shapes. Each pixel consists of an absorber and thermometer, both decoupled from
the cold bath through a weak thermal link. Each pixel requires a separate SQUID
readout channel. For Gen-X, where we require millions of resolution elements,
having an individual SQUID readout channel for each pixel becomes
difficult. Furthermore, at a focal length of 50 m and an impressive 0.1"
point-spread function, the Gen-X mirror requires small pixels at the focal
plane. A Gen-X focal plane with 0.1" pixels would have pixels that are 30 um to
a side. For comparison, the IXO X-ray Microcalorimeter Spectrometer uses 300 um
pixels. Thus the area of Gen-X pixels is 100 times smaller than those for
IXO. Fabricating microcalorimeter pixels that are two orders of magnitude
smaller than the current state of the art is a technological challenge.
a) Schematic diagram of the 4-abosorber sensor design. b) Average pulse shapes
for 5.89 keV photons absorbed in each of the four absorbers.
Both of these challenges (number of SQUID readout channels and pixel size), can
in principle be reduced by an order of magnitude using position-sensitive
microcalorimeters (Smith et al, 2008). These devices typically use one
thermometer coupled to a number of different absorbers through thermal
conductances that are engineered to give position dependence in the pulse
shape. In the figure above, a single TES sensor is connected to four different
absorbers. When x-rays are absorbed into different absorbers, the TES produces
different pulse shapes depending on the absorption location. Thus each TES
read-out element can be subdivided into several resolution elements. For Gen-X,
we may have sensors with over ten absorbers attached to each sensor.
The most significant technical challenge in the development of
microcalorimeters for Gen-X is to raise the pixel count from the current state
of the art (a few dozen pixels), beyond the number needed for the International
X-ray Observatory (a few thousand pixels) to a number in the range of a few
million pixels. Very low-noise multiplexing is required if a reasonable number
of signal paths are to carry the signals from every pixel in the cryogenic
focal plane to the processing electronics. Currently the use of time-division
multiplexing is a promising approach for arrays of medium size, such as IXO.
The ability to scale this approach to much larger arrays, such as is needed for
Gen-X, is limited by both power dissipation and the bandwidth of SQUID read-out
approach being used. However, a read-out consisting of a microwave multiplexer
offers a path to meeting the Gen-X requirements. Several different versions of
this type of multiplexer exist, depending upon whether it is adapted for the
read-out of TESs, MMCs or KIDs. For TESs and MMCs, unshunted, non-hysteretic rf
SQUIDs are incorporated into the read-out which have negligible power
dissipation even for extremely large arrays (Mates et al., 2008). Furthermore,
rf SQUIDs can be coupled to high-Q microwave resonant circuits fabricated from
superconducting coplanar waveguides with resonant frequencies of several
gigahertz. In this approach, the bandwidth of each microcalorimeter is limited
by the resonant circuit after amplification by the rf SQUID. A single high
electron mobility transistor (HEMT) amplifier has the bandwidth and dynamic
range to read out many hundreds of rf SQUIDs operated in superconducting
microresonators tuned to different frequencies, all coupled to the same
coplanar-waveguide feedline. This essential technology that must be developed
in order to make this microcalorimeter feasible is microwave multiplexing.
The table below illustrates a plausible arrangement for the cryogenic
microcalorimeter within the 50 m focal length telescope configuration.
|Design Parameter || Design Options
| Field of View || 3x3 arcmin (54 x 54 mm) |
| Pixel Size
||Single sensor position-sensitive microcalorimeter:
- 324,000 sensors
- 0.1" x 1" (/10 via subpixel event location)
- 30 x 300 um
| Number of Resolution Elements|| 3.24 x 106|
| Multiplexing Scheme
- 3.24 x 105 sensors
- 3000 RF SQUIDs multiplexed on each HEMT amplifier
- 108 HEMT amplifiers to obtain 3.24 x 105 readout channels
"Performance of TES X-ray Microcalorimeters with a Novel Absorber"
S.R. Bandler, R.P. Brekosky, A.-D. Brown, J. Chervenak, E. Figueroa-Feliciano,
F. Finkbeiner, N. Iyomoto, R.L. Kelley, C.A. Kilbourne, F.S. Porter,
J. Sadleir, S.J. Smith J. of Low Temp. Physics 151 (1/2), 400-405 (2008).
"Development of arrays of position-sensitive microcalorimeters for Constellation-X"
S.J. Smith, S.R. Bandler, R.P. Brekosky, A.-D. Brown, J.A. Chervenak, M.E. Eckart, E. Figueroa-Feliciano, F.M. Finkbeiner, R.L. Kelley, C.A. Kilbourne, F.S. Porter, J. Sadlier
Proc. SPIE 7011, 701126-1 - 701126-8, (2008).
"Demonstration of a Multiplexer of Dissipationless SQUIDs"
J.A.B. Mates, G.C. Hilton, K.D. Irwin, L.R. Vale, K.W. Lehnert
Appl. Phys. Lett. 92, 023514, (2008).