Samenvatting

SRON is the Dutch national expertise institute for scientific space research and is part of NWO. It plays an important role in delivering contributions to the national and international space-research communities. Currently one of SRON’s involvements is in the X-IFU instrument for the Athena space X-ray observatory.

The X-IFU instrument on Athena consists of a molybdenum-gold Transition Edge Sensor (TES) array at its heart. These TES-detectors make use of the sharp superconducting transition and have a high sensitivity to small temperature changes. Each of the TES microcalorimeters in the array is referred to as a pixel. Each of
the pixels is cooled to sub kelvin levels and readout under Frequency Domain Multiplexing (FDM).

The pixels have to deliver a high spectral energy resolution of 2.5 eV up to energies of 7.0 keV. To do this, the pixels are biased at specific frequencies and voltages to work under FDM and to select a point in their super to normal transition.

To verify the X-ray energy resolution of a TES microcalorimeter, a lengthy measurement has to be taken which can take over an hour for each time a parameter value is changed. Finding the optimum parameter values for one pixel can take up to a full day. For this reason, methods in quickly identifying the best values for the parameters without the need of taking long X-ray resolution measurements are searched for.

One method tested in this thesis is to look at the characteristic IV-curve of the TES-sensor, together with a phase curve that shows oscillations present in the uperconducting transition. A circuit scan can also be made which shows important properties of the components connected to the TES, such as the LC-resonator resonance frequency. Statistics regarding the X-ray energy resolution dependency on different values of the tuning parameters were searched for in this thesis. Measurements of the X-ray energy resolutions dependency on the AC-bias frequency, voltage and slope in the superconducting transition were performed.

The correlation between the X-ray energy resolution and the AC-bias frequency showed that the best X-ray energy resolutions are achieved right at or close to the LC-resonator resonance frequency. The correlation was not able to be described by a fit since the scatter in the obtained measurements is too large. If this scatter is caused by a systematic error then it could be removed by repeating the measurements several times and averaging the results.

When the bias voltage across the TES is changed, the X-ray energy resolution of the TES becomes more sensitive to the AC-bias frequency. This is because a lower voltage brings the TES at a lower point in its transition and gives it a lower resistance. This causes the Q-factor of the circuit to increase and sharpens the peak at the resonance frequency of the LC-resonator.

The slope in the phase curve of the TES seems to be unimportant in determining the X-ray energy resolution. It was tested for a TES which works at a relatively low AC-bias frequency. For pixels working at higher frequencies, the effect could become of bigger importance as the oscillation caused by the weak link effect becomes larger. Keywords: TES, microcalorimeter, X-ray, energy, resolution, X-IFU, Athena, characteristics, dependency, correlation.