Cryogenic Imaging Spectrometer

Future X-ray astronomy missions, like NASA’s Constellation–X and ESA’s XEUS (X-ray Evolving Universe Spectroscopy Mission) require detectors with very challenging specifications. The high energy (0.2 to 10 keV) spectrometer for XEUS aims at: high energy resolution, DEFWHM< 2 eV at 1 keV and DEFWHM< 5 eV at 7 keV, high absorption efficiency, > 90% up to 7 keV, a counting rate > 4 kHz, and an imaging capability of typically 32x32 pixels. The type of detector considered most promising is an array of voltage biased superconducting transition edge microcalorimeters, operated at sub-Kelvin temperatures. The array will be read out using SQUID amplifiers. Based on a successful development of single pixel microcalorimeters, SRON recently demonstrated an energy resolution DEFWHM< 4.5 eV at 5.9 keV combined with an effective time constant of 150 µs.

The aim of the current project is to demonstrate this performance in a small array of 5x5 pixels (and later, 32x32 pixels). At MESA+, careful options are considered for micromachining the imaging array. Since the array will operate at sub-Kelvin regime, the design must take into account the thermal constraints at such low temperature. These physical considerations have already been taken (theoretically) including the thermal properties of silicon and silicon nitride at low temperatures. The design of such an array of pixel is however bound by a number of additional constraints:

1.	Each pixel must have the same thermal link to the outside world, independent of the number of pixels in the array.

2.	Given the tight efficiency requirements, the pixels should be closely packed together, so that the space available for electrical wiring and thermal connections to the heat bath is limited.

3.	The electrical and thermal crosstalk between the pixels must be small enough not to degrade the energy resolution.

Fabrication feasibility and ruggedness against thermal cycling play a larger role than for single pixels.

Fabricated prototype of a 5x5 pixel array.

The project is a joint work between MESA+¹ for the micromachining part and SRON² for the cryogenic and sensing part.

¹MESA⁺, University of Twente, Enschede, The Netherlands
²Space Research Organization Netherlands, Utrecht, The Netherlands

Part of the project is supported by a research grant from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) and by a contract of the ESA technological research programme for the development of an imaging microcalorimeter array for XEUS.

Contacts

▪ John van Baar

▪ Remco Wiegerink

Publications▪ 
M.P.Bruijn, et al., "Micro-machining of a cryogenic imaging array of transition edge X-ray microcalorimeters," Proc. of the Sensor Technology Conference 2001, Enschede, The Netherlands,  May 14-15, 2001, pp. 179-184.
▪ 
Z.Moktadir, M.P.Bruijn, R.J.Wiegerink, M.Elwenspoek, M.Ridder, W.A.Mels, "Limitations of heat conductivity in cryogenic sensors due to surface roughness," Proc. IEEE Sensors 2002, paper 37.3, Orlando, June 11-14, 2002.
▪ 
M. P. Bruijn, et al., "Development of Arrays of Transition Edge Sensors for Application in X-ray Astronomy," Proceedings of SeSens 2002, Veldhoven, The Netherlands, November 29, 2002, pp. 598-602.
▪ 
M.L. Ridder, et al., "Modelling and thermal characterization of an array of microcalorimeters at sub-Kelvin temperatures," IEEE Sensors Conference, Toronto, October 22-24, 2003.
▪ 
P.A.J. de Korte, et al., "A cryogenic imaging X-ray spectrometer for XEUS read out by frequency-division SQUID multiplexers," SPIE 2004, Glasgow, Scotland, United Kingdom, June 21-25, 2004.
▪ 
R.J. Wiegerink, J.J. van Baar, J.H. de Boer, M.L. Ridder, M.P. Bruijn, A. Germeau, H.F.C. Hoevers, "Cryogenic imaging X-ray spectrometer," MEMS05, Miami, Florida, Jan. 30 - Feb. 3, 2005.

<b></b>