H. Pressler

Experimental imaging diagnosis of superconducting tunnel junction x-ray detectors by low-temperature scanning synchrotron microscope

Imaging diagnosis of superconducting tunnel junction x-ray detectors has been performed by an apparatus called the low-temperature scanning synchrotron microscope (LTSSM) using an x-ray microbeam with a diameter of 5–10 μm originated from synchrotron radiation. Quasiparallel intense synchrotron radiation enables one to obtain the full two-dimensional images of junctions with dimensions of 200×200 μm2 in about 1 h. The LTSSM results indicate that the standard quasiparticle diffusion and edge loss model for the spatial distribution of the junction response to x rays is evidently inadequate for intermediate or large junctions (with respect to a Josephson penetration depth). On this basis, it is argued that the models proposed for the signal creation and loss mechanism should be reconsidered.

Development of an X-ray imaging microcalorimeter with a pixel-type Ir transition edge sensor

Abstract We are developing an X-ray microcalorimeter with high energy resolution and position sensitivity by using an Iridium transition edge sensor. The Ir-TES with a new pixel-type geometry has ten small pixel TES array. Each pixel size is 80 μm ×200 μm . This device has shown a good energy resolution of 26 eV (FWHM) at 5.9 keV . Furthermore, the Ir-TES film was scanned along each pixel by a collimated X-ray microbeam and signal pulses were measured at each incident position. The pulse height and rise time map has shown ten different regions where pulses are concentrated which correspond to each pixel. This might be useful for an imaging TES array.

Spatially resolved study of superconducting tunnel junctions X-ray detectors by low temperature scanning synchrotron microscopy

A low temperature scanning synchrotron microscope (LTSSM) has been developed for spatial analysis of superconducting tunnel junction X-ray detectors. One and two-dimensional images of the detector response to X-rays were measured by scanning the junctions kept at a working temperature of about 0.4 K with a highly collimated synchrotron radiation beam. The collimation was performed by inserting a pinhole mounted on a scanning unit into the synchrotron radiation in the range of 3-6 keV. The spatial resolution of the LTSSM is between 5 and 10 micrometer. The present results indicate a large discrepancy between the experimental spatial response and a quasiparticle diffusion and edge-loss model. The LTSSM plays an important role in the development of cryogenic X-ray detectors.

TES x-ray microcalorimeters with single element superconductors

To develop high energy resolution x-ray microcalorimeters, we have fabricated single element superconductor TES detectors with W and Ir. In the W-TES, the detector is operated in the saturation mode because of high sensitivity of the W film. Energy resolution of 34.2eV (FWHM) at 5.9 keV is obtained by integrating the reduced current signals. In the Ir-TES detector, a position dependent response of pulse height distribution was observed, however, 15.4 eV (FWHM) energy resolution and 60μsec response time are obtained for collimated 3 keV x-rays.

Intermediate states of superconducting thermometers for x-ray microcalorimeters

Intermediate states near the normal–superconducting transition at ∼100 mK, important for a sensitive thermometer in x-ray calorimetry, have been studied by an imaging technique based on low temperature scanning synchrotron microscopy (LTSSM). In conjunction with iridium superconducting transition-edge sensors (TES) operating in electrothermal feedback mode, the LTSSM visualizations reveal that a normal–superconducting phase separation actually takes place in the intermediate states.

Imaging analysis of superconducting transition edge sensors for calorimeters

The superconducting-transition-edge temperature sensors (TES) for calorimeters operate in an intermediate state within normal-superconducting transition. The spatial profiles of a response to an x-ray microbeam exhibit that normal-superconducting phase separation occurs in an iridium TES with electrothermal feedback, as has been observed in many current-carrying conductors with self-heating. The resistive domain (normal region) in the 500 /spl mu/m-square TES grows as applied bias voltage increases. This observation agrees reasonably well with a one-dimensional model of the self-heating. On the other hand, two-dimensional features are explained by nonuniform supercurrent distribution in the superconducting domain. These findings are important for developing detectors with a large detection area for energy dispersive x-ray spectroscopy.

Characteristics of Ti films for transition-edge sensor microcalorimeters

Abstract We are developing X-ray microcalorimeters using superconducting transition-edge sensors (TESs), which can be operated at relatively high base temperatures of a 3 He cryostat. For this purpose, we have selected Ti films to be used as TESs. The Ti films were deposited on different substrates by RF-sputtering. It was found that the superconducting properties of the Ti films depended on Ar pressure, film thickness, and substrate surface roughness.

High count rate x-ray detector using a superconducting tunnel junction with current readout method

We developed a high count rate x-ray detector using a superconducting tunnel junction (STJ) having a structure of Nb/Al/AlOx/Al/Nb with a fast current readout system. The STJ has a current rise time of 100 ns, a current decay time of 160 ns and resistance of 20 Ω at 0.4 K. High count rate capability of the x-ray detector has been measured by synchrotron radiation. The energy resolution is ∼230 eV at 100 cps and ∼265 eV at 100 k cps for 4 keV x-rays.

Fabrication of bridge-type microcalorimeter arrays with Ti-Au transition-edge-sensors

The present stage of cryogenic detector development is focused on fabrication of arrays. Arrays of superconducting-transition-edge-sensor (TES) microcalorimeters have generally been fabricated on SiNX membranes. In conventional membrane structures fabricated by backside etching, it is difficult to realize arrays having both mechanical toughness and a large filling factor. It is possible to overcome these difficulties by using a bridge-type membrane structure, in which SiNX membranes are raised above the Si substrate with a gap of 10–50 μm. In order to carry out etching and other processes, we have fabricated a small scale array with a bridge-type membrane structure. A preliminary version of a TES microcalorimeter array with a 5×5 pixel structure had dimensions of 6×10 mm2. Each pixel consisted of a Ti/Au bilayer with a size of 0.5×0.7 mm2 on a 1 μm-thick SiNX membrane of 0.7×2 mm2. Although the current array exhibited a small filling factor of about 0.1, it is possible to obtain a high filling factor by f...

Factors of inhomogeneous spatial response of superconducting tunnel junction detectors

Spatial uniformity in the response of superconducting tunnel junction detectors (STJ) to photons is a main concern in the development of these devices. Low Temperature Scanning Synchrotron Microscopy (LTSSM) has been used to directly image spatial profiles of the response of Nb-based STJ to X-ray photons. Scanning an X-ray microbeam with a diameter of 5–10 μm enables visualization of the spectroscopic properties of STJ X-ray detectors at an actual working temperature of about 0.4 K. We have found that the inhomogeneity of the junction response strongly depends on the junction size, the bias points, and the strength of applied magnetic field parallel to the insulation barrier. It is proposed that some resonance phenomena in the junction may influence signal creation processes.