H.F.C. Hoevers

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.

Radiative ballistic phonon transport in silicon-nitride membranes at low temperatures

We studied the phonon transport in free-standing 1 µm thick silicon-nitride membranes at temperatures around 100 mK. By varying the geometry of the membranes and the dimensions of the heater element, we are able to distinguish between radiative and diffuse phonon transport. The data indicate that the transport is radiative ballistic with a lower limit to a phonon mean-free path of about 1 mm and that the probability for specular reflection from the surface is at least 0.99. The tested silicon-nitride membranes were grown on Si(100), Si(110), and polycrystalline-Si and the transport properties show no dependency on the substrate

Niobium and Tantalum High Q Resonators for Photon Detectors

We have measured the quality factors and phase noise of niobium and tantalum coplanar waveguide microwave resonators on silicon. The results of both materials are similar. We reach quality factors up to 105. At low temperatures the quality factors show an anomalous increase, while the resonance frequency remains constant for increasing power levels. The resonance frequency starts to decrease at temperatures around a tenth of the critical temperature. The phase noise exhibits a 1/f like slope. We attribute this behavior to the silicon dielectric.

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.

Frequency Domain Multiplexed Readout of TES Detector Arrays With Baseband Feedback

SRON is developing an electronic read-out system for an array of Transition-Edge Sensors (TES) which combines the techniques of Frequency Domain Multiplexing (FDM) with Base-Band FeedBack (BBFB). Its potential astronomical applications are in the read-out of soft X-ray microcalorimeters of the XMS instrument on the International X-ray Observatory (IXO) and the far-IR bolometers of the SAFARI instrument on the Japanese-European mission SPICA. In this paper we focus on the experimental verification of the system, demonstrating for 16 pixels that simultaneous read-out does not degrade the noise performance, and that crosstalk between pixels is close to the requirements for IXO. A detailed analysis of the BBFB system is presented, identifying its limitations and routes for improvement to the levels required for implementation on both missions.

Performance of an x-ray microcalorimeter under ac biasing

Frequency domain multiplexing (FDM) is an attractive option for the readout of imaging arrays of microcalorimeters. Implementation of FDM requires ac biasing of the individual microcalorimeters. In this letter we present a small signal model for the behavior of a microcalorimeter under ac bias. Moreover, we have measured the behavior of the same microcalorimeter under ac (at 46 kHz) and dc bias. These experiments show that the performance of the device is very similar in terms of energy resolution, pulse shapes, and current–voltage characteristics. The measured energy resolution at 5.89 keV is 6.9 eV for ac bias and 5.5 eV for dc bias. The effective time constant in both cases is 100 μs.

Study of the Dependency on Magnetic Field and Bias Voltage of an AC-Biased TES Microcalorimeter

At SRON we are studying the performance of a Goddard Space Flight Center single pixel TES microcalorimeter operated in an AC bias configuration. For x-ray photons at 6xa0keV the pixel shows an x-ray energy resolution ΔEFWHM=3.7xa0eV, which is about a factorxa02 worse than the energy resolution observed in an identical DC-biased pixel. In order to better understand the reasons for this discrepancy we characterised the detector as a function of temperature, bias working point and applied perpendicular magnetic field. A strong periodic dependency of the detector noise on the TES AC bias voltage is measured. We discuss the results in the framework of the recently observed weak-link behaviour of a TES microcalorimeter.

Fluctuation-limited noise in a superconducting transition-edge sensor.

In order to investigate the origin of the until now unaccounted excess noise and to minimize the uncontrollable phenomena at the transition in X-ray mi-crocalorimeters we have developed superconducting transition-edge sensors into an edgeless geometry, the so-called Corbino disk (CorTES), with super-conducting contacts in the centre and at the outer perimeter. The measured rms current noise and its spectral density can be modeled as resistance noise resulting from fluctuations near the equilibrium superconductor-normal metal boundary.

Performance of a microcalorimeter with a superconducting transition edge thermometer

Abstract The performance of a single-pixel microcalorimeter is described and analyzed. The microcalorimeter has a Cu absorber which is isolated from the heat bath by means of a Si x N y membrane and is connected to a voltage-biased Ti/Au proximity bilayer thermometer with a transition temperature of about 150xa0mK. The measured FWHM energy resolution for 5.9xa0keV X-ray photons is 12.4xa0eV for this sensor while 2.36( k B T 2 C ) 1/2 equals 9.1xa0eV. The detector time constant is about 1xa0ms. The measured current–voltage characteristic and current noise spectra are analyzed in detail in the context of the energy resolution. High frequency excess noise is measured in the superconducting transition and is briefly discussed.

Weak-Link Phenomena in AC-Biased Transition Edge Sensors

It has been recently demonstrated that superconducting transition edge-sensors behave as weak-links due to longitudinally induced superconductivity from the leads with higher