Tasuku HAYASHI

We herein report a concept study of a transition edge sensor (TES) X-ray microcalorimeter array with two different thickness absorbers. We developed an energy-dispersive X-ray spectroscope (EDS) with a 64-pixel TES array and installed it on a scanning transmission electron microscope (STEM) for material analysis. One of the key applications of the proposed system is the microanalysis of astromaterials, for which the relative abundance of light elements such as boron, carbon, and oxygen against silicon are crucial. However, the line sensitivity below similar to 500 eV for the our STEM TES EDS system was not enough to detect the X-ray from light elements because of the relatively high continuum emission and low detection efficiency, which occurs due to the X-ray window and the optical blocking filters. A simple solution to increase line sensitivity at low energy is the adoption of thin X-ray absorbers that leads to an improvement in the energy resolution. However, doing so causes the sensitivity to decrease for high energy lines. Utilizing the spot-size dependence of the polycapillary X-ray optics on energy, which are used in the STEM TES EDS system, we studied a design in which thin absorbers are distributed on the outer area of detector. We optimized the design using the raytracing analysis of optics. A thin (300 nm) absorber is placed on the 52 outer pixels, while a thick (3.5 mu absorber is placed on the central 12 pixels. The thin pixels detect approximately 50-60% of the total counts in 0.1-2 keV, while the central thick pixels detect approximately 50-80% of the total counts in 2-10 keV. We also demonstrated the fabrication process of two-thickness absorber arrays.

A quantitative microanalysis of astromaterials (e.g., meteorite, returned samples from asteroids) is a key technology to understand the history of our solar system formation. To fulfill this, we developed an energy-dispersive X-ray spectroscopy (EDS) using a transition-edge sensor (TES) microcalorimeterarray on a scanning transmission electron microscope (STEM) for material analysis. To reduce the systematic errors of a spectral analysis, we investigated and constructed the response function of the STEM-EDS system, which consists of detection efficiency and a two-dimensional response matrix. The latter represents the pulse-height redistribution functions of the incident photons of different energies. Using the constructed response function, we demonstrated the quantitative determination of SiO2 film and confirmed that the number-density ratio of oxygen to silicon (=2.29(-0.29)(+0.32)) is consistent with the expected value of 2 within the statistical errors. We further study the systematic errors of the concentration determination with simulations. We analyze the simulated spectra of TES-EDS and SDD (silicon drift detector)-EDS without a priori knowledge about the continuum spectra and find that the systematic deviations of parameters from the model values are smaller than 1% for TES-EDS and larger than 10% for SDD-EDS.

A detector head for an energy-dispersive X-ray spectroscopy (EDS) for a scanning transmission electron microscope (STEM) was designed, fabricated, and tested. A 64-pixel TES X-ray microcalorimeter and 64 SQUID array amplifiers (SAAs) are mounted on a detector head which is cooled to about 100 mK. The body of the detector head is a copper rod of about 1 cm(2) cross section and 10 cm length with 3 cm cubic structure at the bottom. The TES microcalorimeter is mounted at the top of the rod while the SAAs are mounted on the four side surfaces of the cubic structure. In order to reduce the number of wire bondings, we adopted a flip-chip bonding for the SAAs. In order to reduce the stress imposed on the flip-chip bondings due to the difference in the linear thermal expansion of the SAA chip and the mounting surfaces, we mounted the SAAs and connectors to the room-temperature electronics on sapphire circuit board and mounted the SAAs and connectors using a superconducting flip-chip bonding technology. Then, both the TES and the sapphire circuit board were mounted on the rod and are connected to the print circuit like superconducting wires, which are created on the multiple surfaces of the rod, with A1 wire bondings. We reduced the number of wire bondings from 768 to 256. The yield of the flip-chip bonding was not perfect but relatively high. We installed the detector head in the STEM EDS system, confirmed that the energy resolution and counting requirements, Delta E < 10 eV with 5 kcps were fulfilled.