M. Lindeman

Single Pixel Characterization of X-Ray TES Microcalorimeter Under AC Bias at MHz Frequencies

In this paper, we present the progress made at SRON in the read-out of X-ray Transition Edge Sensor (TES) microcalorimeters under AC bias. The experiments reported so far, whose aim was to demonstrate an energy resolution of 2 eV at 6 keV with a TES acting as a modulator, were carried out at frequencies below 700 kHz using a standard flux locked loop SQUID read-out scheme. The TES read-out suffered from the use of suboptimal circuit components, large parasitic inductances, low quality factor resonators, and poor magnetic field shielding. We have developed a novel experimental set-up that allows us to test several read-out schemes in a single cryogenic run. In this set-up, the TES pixels from a GSFC array are coupled via superconducting transformers to 18 high-Q lithographic LC filters with resonant frequencies ranging between 2 and 5 MHz. The signal is amplified by a two-stage SQUID current sensor and baseband feedback is used to overcome the limited SQUID dynamic range. We measured an X-ray energy resolution of 3.6 eV at 1.4 MHz, which is consistent with the measured integrated Noise Equivalent Power.

Relationships Between Complex Impedance, Thermal Response, and Noise in TES Calorimeters and Bolometers

Complex impedance measurements are widely used to characterize superconducting transition edge sensors (TESs) in bolometers and microcalorimeters. Typically, models are fit to impedance data to find parameters which pertain to the performance of these detectors. After the parameters are determined, the models are then used to compute the response and noise of these devices. In this paper, we present general relationships between the measured impedance, the thermal response to power in the TES, and noise. We describe a method for measuring αI and βI of the superconducting phase transition, which does not require model fitting. We find bolometer response is determined from the impedance provided that the absorber is strongly coupled to the TES electron system. We also demonstrate how to calculate upper and lower limits on the noise directly from the impedance data without modeling. Additionally, the relations can be used to check the validity of the models and to understand what information can and cannot be obtained from measurements of impedance, response, and noise.

A multiplexer for the AC/DC characterization of TES-based bolometers and microcalorimeters

At SRON we are developing the Frequency Domain Multiplexing (FDM) for the read-out of the TES-based detector array for the future infrared and X-ray space mission. We describe the performances of a multiplexer designed to increase the experimental throughput in the characterisation of ultra-low noise equivalent power (NEP) TES bolometers and high energy resolving power X-ray microcalorimeters arrays under ac and dc bias. We discuss the results obtained using the TiAu TES bolometers array fabricated at SRON with measured dark NEP below 5 · 10−19W/ √ Hz and saturation power of several fW.

Effects of Magnetic Fields on Highly Sensitive TiAu TES Bolometers

In the process to develop the transition edge sensor (TES) bolometer technology for the SAFARI FIR Imaging Spectrometer on the SPICA telescope and for future FIR space telescopes, we have recently demonstrated a TiAu TES with a Ta thin film absorber on a thin SiN membrane, which has a dark NEP as low as 4.2 × 10-19 W/√Hz. It is known that superconductors are sensitive to magnetic fields. Therefore, it is important to know how these extremely sensitive TES detectors are affected by external magnetic fields. This is desirable from an application point of view since the design of the SAFARI instrument demands such knowledge and interesting from a fundamental point of view. In this paper, we will describe a detailed experimental study of the effects of magnetic fields, both parallel and perpendicular to the TiAu plane, on the performance of a TES. We investigated the effect on the current, responsivity, speed, and TES parameters of α (dependence of temperature) and β (dependence of current), around the operating points. In order to achieve this highly sensitive measurement, the temperature stability has been improved and a new coil assembly has been designed. The latter allows accurately applying a parallel magnetic field.