A Study of the Transition-Edge Sensor Based X-ray Microcalorimeter (PhD Defense)

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 Time:  Tuesday, May 28, 2019, 04:15pm
 Title:  A Study of the Transition-Edge Sensor Based X-ray Microcalorimeter (PhD Defense)
 Speaker:  MS. Yu Zhou (DoA student)



Transition-edge sensor(TES) based X-ray microcalorimeter is the most advanced X-ray cryogenic detector in terms of the potential unprecedented energy resolution. With its unique capability of applying high energy resolution spectroscopy and imaging to the extended soft X-ray emitting intergalactic medium in space, the transition-edge sensor based X-ray microcalorimeter will be able to address the astrophysics problems that are of key importance, such as ’the missing baryon problem’.

To improve the energy resolution of the transition-edge sensor with conditionally large detector area has became the main challenge of the field. There are five main factors that determine the energy resolution of the TES microcalorimeter : the operational temperature of the detector, the heat capacity of the absorber, the temperature sensitivity(α) and current sensitivity(β) of the TES and the noise equivalent power of the circuit. The operational temperature of the detector is designed by the cooling technique in space and the heat capacity of the absorber should meet the requirement of the detection efficiency of the telescope. Therefore, the most effective way of enhancing the energy resolution of the detector should be achieved by optimizing the α and β of the TES in the superconducting transition. The energy resolution of the TES detector is correlated with the figure of merit \sqrt(1+2β)/α. However, due to the complication of the TES resistance in the transition as a α function of the temperature, current and magnetic field, it is unclear how to minimize the figure of merit \sqrt(1+2β)/α to optimize the energy resolution. I thus conducted the experiment α to map the α and β through the whole TES transition and studied their relation with TES resistance and bias power to find the optimized operational point. The results has suggested the current sensitivity only depends on the TES resistance, which is consistent with the prediction of the two fluid model. The two fluid model has succeeded in explaining the overall correlation between α and β, but can not explain how α and β oscillate with the magnetic field, which is due to the Josephson effect of the TES. With an alternative B-field applied perpendicularly onto the TES, I was able to observe the ’Shapiro step’ in the I−V curve due to the inverse AC Josephson effect. By taking the advantage of the presence of the ’Shapiro step’as the most precise voltage standard in modern physics, I invented a new method to calibrate the shunt resistance(R_{shunt}) with better precision of less than 1 μΩ, which is about at least one order of magnitude higher than the traditional measurement with parametric amplifiers (about 20 μΩ precision). The important TES resistance related value, such as α and β, is also determined with better accuracy with more precise measurement of R_{shunt} . Fitting the complex admittance of the TES provides an alternative way to measure α and β. The fitting results are in consistency with the measurements of α and β calculated from the derivative of the I−V curves within 40% systematic error. During the fitting procedure I found the two thermal block model is needed to fit the complex admittance and studied the fitted thermal conductances and heat capacities as they evolve during the transition. The geometry design of the TESs can tune the detector behavior from multiple aspects. By comparing the TES performance with different geometry design, I found the TESs with pure bilayer design has better energy resolution than the standard ones, which has updated our stereotype of the TESs and will help simplify the fabrication procedure of the TESs.