W.A. Mels

Thermal fluctuation noise in a voltage biased superconducting transition edge thermometer

The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addition to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermometer is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise. A simple model provides quantitative agreement between the observed and calculated noise spectra for all bias points in the superconducting transition.

Performance of X-ray microcalorimeters with an energy resolution below 4.5 eV and 100 μs response time

The performance of X-ray microcalorimeters with a Ti/Au Transition Edge Sensor (TES) and a Cu absorber has been investigated. With these microcalorimeters we achieve energy resolutions below 4.5 eV FWHM for 5.89 keV X-rays in combination with a response time of about 100 μs. The measured energy resolution is analyzed and discussed based on spectral and RMS noise measurements. Excess noise was present which, in one case, could be fitted by noise proportional to 1α, suggesting the presence of thermal fluctuations in the TES. Variation in geometry and illumination was examined but did not influence the energy resolution.

Fabrication and characterization of infrared and sub-mm spiderweb bolometers with low-T/sub c/ superconducting transition edge thermometers

Spiderweb bolometers with a superconducting transition edge thermometer combine a high sensitivity and absorption efficiency for far infrared and sub-mm radiation with a low susceptibility to high energetic particles. They are among the most promising detectors today for future astronomy applications. In this paper we present our fabrication process for a fully integrated bolometer consisting of absorber, thermometer and suspension structure, based on micromachining of a silicon nitride membrane and various thin film deposition and etching steps. The optimization of Ti/Au transition edge thermometers is described, as well as electrical measurements on a fully integrated bolometer, indicating an electrical noise equivalent power of 2/spl times/10/sup -17/ W//spl radic/Hz at an operating temperature of 270 mK.

Noise and energy resolution of X-ray microcalorimeters

Two type of sensor geometries of voltage-biased X-ray microcalorimeters with a phase-transition thermometer have been built and tested. Both devices show, in addition to the well-known noise sources of thermal fluctuation noise or phonon noise from the heatlink to the bath and Johnson noise from the thermometer resistance, also thermal fluctuation noise from the thermometer itself. In both cases however the measured energy resolution is limited by other sources. The energy resolution of the asymmetric lateral type of sensor, 12 to 15 eV @ 5.9 keV, is limited by a position dependent heatleak of the absorber to the bath. The energy resolution of the symmetric lateral sensor, 6.8+/-0.3 eV FWHM @ 5.9 keV, is limited by excess noise at frequencies below 1000 Hz. The origin of this noise component is unknown so far.

Limitations of heat conductivity in cryogenic sensors due to surface roughness [X-ray detection]

The limitation of heat conductivity in cryogenic sensors due to surface roughness was discussed. It was found that at macroscopic scale and high temperatures, the transport coefficients were characteristic properties of the material and were independent of the shape and size of specimen. An experiment was demonstrated to indicate the size dependent effects. The estimation of the performance of cryogenic sensor by solving the non-linear heat equation was also elaborated.

PERFORMANCE OF SUPERCONDUCTING TRANSITION EDGE BOLOMETERS FOR INFRARED PHOTOMETRY AND X-RAY MICRO-CALORIMETRY

The performance of voltage biased superconducting transition edge bolometers for infrared photometry and X-ray micro-calorimetry is described. Experimental results for the resistance–temperature curve of the superconducting phase thermometer, the current–voltage characteristic of the bolometer, and its current noise are presented and evaluated. Excess current noise turns out to be present when the bolometers are biased in the superconducting transition. The possible origin of this excess noise is shortly discussed. For the infrared bolometer an NEP of 2 at 270 mK is obtained. The X-ray micro-calorimeter resolution is 60 eVFWHM at 155 mK for 5.9 keV X-rays, a factor 2.6 above the thermodynamical limit.

A cryogenic imaging x-ray spectrometer for XEUS readout by frequency-division SQUID multiplexers

A micro-calorimeter array with superconducting transition-edge sensors read out by a SQUID-based frequency-domain multiplexer is under development for the X-ray imaging spectrometer on board ESA’s X-ray Evolving Universe Spectroscopy (XEUS) mission. The XEUS requirements are 2 and 5 eV FWHM energy resolution for 2 and 7 keV X-rays, respectively. An array of 32 x 32 pixels with 250 micron square pixels is envisaged. SRON and MESA+ have developed 5 x 5 imaging micro-calorimeter prototype arrays along a bulk micromachining and a surface micromachining route. The present state of array design and development with emphasis on pixel-to-pixel performance measurements of thermal and I-V characteristics, sensor noise and energy resolution are presented. SRON and VTT are developing frequency-domain multiplexing with SQUID current amplifiers to read out the 32 x 32 array. The concept for the frequency division multiplexing read-out will be presented and its performance characteristics discussed. Recent results of sensor operation under AC-bias (500 kHz) are presented.