Keisuke Maehata

Microcalorimeter-type energy dispersive X-ray spectrometer for a transmission electron microscope

A new energy dispersive X-ray spectrometer (EDS) with a microcalorimeter detector equipped with a transmission electron microscope (TEM) has been developed for high- accuracy compositional analysis in the nanoscale. A superconducting transition-edge-sensor-type microcalorimeter is applied as the detector. A cryogen-free cooling system, which consists of a mechanical and a dilution refrigerator, is selected to achieve long-term temperature stability. In order to mount these detector and refrigerators on a TEM, the cooling system is specially designed such that these two refrigerators are separated. Also, the detector position and arrangement are carefully designed to avoid adverse affects between the superconductor detector and the TEM lens system. Using the developed EDS system, at present, an energy resolution of 21.92 eV full-width-at-half maximum has been achieved at the Cr K alpha line. This value is about seven times better than that of the current typical commercial Si(Li) detector, which is usually around 140 eV. The developed microcalorimeter EDS system can measure a wide energy range, 1-20 keV, at one time with this high energy resolution that can resolve peaks from most of the elements. Although several further developments will be needed to enable practical use, highly accurate compositional analysis with high energy resolution will be realized by this microcalorimeter EDS system.

Design chart of gas-cooled current leads made of copper of different RRR values

Abstract The generalized pressure drop of the cooling gas of current leads is obtained using the effective friction factor λ p = 0.0715 , which has been obtained in experiments on spiral-fin current leads. The design chart of current leads made of RRR = 92.5 copper is improved with respect to the generalized pressure drop of the cooling gas. For various copper materials with different values of RRR approximate expressions are considered to evaluate the electrical resistivity and the thermal conductivity as a function of temperature. The generalized characteristics of current leads made of copper of several RRR values are summarized in the design chart.

Test operations of the VENUS superconducting magnet at KEK

Abstract The superconducting magnet of the VENUS detector was successfully operated with a central field of 0.75 T. A cryogenic system kept the coil temperature to below 4.5 K. When a coil quench was induced by built-in heaters, the stored energy of 11.7 MJ was safely extracted from the magnet to the outside dump resistor. The iron structure of the magnet yoke supported the magnetic force of about 230 t with a maximum elastic deformation of 0.4 mm. The maximum leakage field at the location of the barrel electromagnetic calorimeter was 33 G. The magnetic field was mapped in the solenoid bore by an NMR probe and by three-dimensional Hall probes with an accuracy of order 10−4. The field was confirmed to be uniform within 0.3% deviation in the spatial region of a central drift chamber.

Development of high temperature superconducting current feeders for a large-scale superconducting experimental fusion system

The National Institute for Fusion Science (NIFS), in collaboration with universities and laboratories in Japan, the Forschungszentrum Karlsruhe (FZK) and the Max-Planck Institut fur Plasma Physik (IPP) in Germany, is planning to develop high temperature superconducting (HTS) current feeders for large-scale superconducting coils. Two programs are being progressed: one is a current feedthrough for superfluid helium (He II) cooled superconducting coils; the other is current leads for experimental fusion system. The paper describes the present status of the two programs, including joint experiments of a 20 kA Bi-2223 current lead developed by FZK and development of a prototype YBCO bulk current feedthrough for He II cooled superconducting coils. In addition, the test results of Bi-2212 tubes fabricated by diffusion process, applicable to HTS current leads, are described Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/.

Effect of the Critical Current Density and the Junction Size on the Leakage Current of Nb/Al–AlOx/Nb Superconducting Tunnel Junctions for Radiation Detection

Nb/Al–AlOx /Nb superconducting tunnel junctions (STJs) designed for X-ray detection have been fabricated. The behavior of the low-temperature subgap leakage current, which severely limits the energy resolution obtained in such devices, is investigated. From trends in the dependence of the leakage currents on the critical current density and the size of the STJ, as well as from the low-temperature current-voltage characteristics, and an analysis of the base electrode surface morphology, it is concluded that physical defects in the barrier region are the most probable cause of the leakage currents. Suggestions are given for optimization of the device processing.

Development of GM cryocooler separate type liquid-helium-free 3He-4He dilution refrigerator system

We developed the new liquid-helium-free dilution refrigerator system, in which the Gifford-McMahon (GM) cycle cryocooler and dilution refrigerator (DR) unit are separated. We obtained the base temperature below 50 mK in this DR system. In usual liquid-helium-free DR systems, the DR unit directly couples with GM-cryocooler in the same vacuum chamber. Therefore the mechanical vibration of GM-cryocooler is hardly removed from DR unit. In order to eliminate the vibration problem, the separated vacuum chamber contacting the GM-cryocooler is connected with the DR unit chamber by the flexible hose with length of about 1 meter. Thin flexible tubes used for circulation of the refrigerant gas and radiation shield are installed in the connection hose. The 4He gas, cooled in the GM-cryocooler unit, transfers to the DR unit throw the thin flexible tubes. After cooling the DR unit, the gas returns to GM-cryocooler unit with cooling of the radiation shield. We expect that our separate-type dilution refrigerator becomes a useful piece of apparatus for the low temperature experiments.

High quality Nb/Al–AlOx/Nb superconducting tunnel junctions for radiation detection

Nb/Al–AlOx/Nb tunnel junctions designed for x‐ray detection have been manufactured and characterized at temperatures down to 0.35 K. At a bias voltage of 0.5 mV, a leakage current of the order of 0.1 pA/μm2, and a dynamic resistance exceeding 106 times the normal state resistance, were achieved in junctions with dimensions up to 200×200 μm2 and critical current densities of order 200 A/cm2. The combination of high critical current density, low leakage current, and large area represents an important step towards the development of superconducting tunnel junction detectors with improved spectroscopic performance.

Development of Hard X-Ray and Gamma-Ray Spectrometer Using Superconducting Transition Edge Sensor

Superconducting transition edge sensors (TES) are used for high-resolution X-ray spectroscopy. In our group, we have developed Ir/Au-TESs. We have recently started to develop a high-resolution X-ray and gamma-ray spectrometer using our Ir/Au-TES. In this study, we plan to accelerate the response time of the detector system, thus we have designed a new Sn-absorber coupled TES detector system by improving the thermal contact between the TES and the absorber. A superconducting tin absorber is coupled to an Ir/Au superconducting film with a gold (Au) bump post, instead of an epoxy post, which is used conventionally. The pulse height signal, which we obtained from the Au bump post TES, is more than twice as large as that from the epoxy post system, and the slow component of the decay time constant improved to 8.0 ms from 22.0 ms. The energy resolution of the 59.5 keV photoelectron peak is 280 eV FWHM, which is currently dominated by the mechanical vibration of our pulse-tube refrigerator.

Large-Area Tunnel Junction Exhibiting Two Operating Modes for X-ray Detection

High quality Nb-based tunnel junctions with surface areas of 100×100 µm2 and 200×200 µm2 were tested by irradiating x-rays. The 200×200 µm2 junction exhibited better x-ray response than that of the 100×100 µm2 one, due to better collection of quasiparticles. The junction detected 5.9-keV x-rays with an energy resolution (FWHM) of 93 eV and the energy required for single electron collection was 6.5 meV. In the operation of the 200×200 µm2 junction, two kinds of operation modes with different pulse heights were found by applying a magnetic field with different ramping speeds. The individual operation modes gave different current-voltage characteristics. The Sine-Gordon equation was employed to calculate the current-volt-age characteristics for the large junction. From calculated results, it was confirmed that the higher pulse height operation mode was related to a large reduction of dc current through the junction.

Operational performance of spiral-fin current leads

Abstract The profile of the pressure drop of cooling gas was measured in operational tests of 7 and 10 kA current leads. The current leads have spiral-shaped fins which permit both high heat exchange efficiency and reliability against thermal runaway. A general design chart was employed to determine the geometrical dimensions and to estimate the operational performance of the leads. The spiral-fin current leads exhibited good performances in the operational tests. The heat flow from the cold end of the 7 and 10 kA leads was 7.47 and 10.4 W, respectively, in the self-sustained condition. A constant friction factor of 0.0715 was found to reproduce the experimental pressure drop profile along the cooling path. This friction factor can be employed to estimate the pressure drop of the cooling gas of current leads in general designs.