Kazuhiko Kawai

Improved Fabrication Method for Nb/Al/AlOx/Al/Nb Superconducting Tunnel Junctions as X-Ray Detectors

Niobium-based superconducting tunnel junctions (STJs) with aluminum trapping layers as X-ray detectors were fabricated. We applied a fabrication technique for reducing the leakage current and improving the energy resolution for X-rays at around 6 keV. The edges of two Al layers sandwiching a tunneling barrier were oxidized by plasma discharge. The performance of these new STJs was compared with other types of STJs to which thermal oxidization in an O2 atmosphere had been applied. The oxidization process in O2 plasma significantly improved the energy resolution for X-rays, which reached values (FWHM) of 41 eV, 58 eV, 65 eV, and 129 eV for STJs with the size of 20×20 µm2, 100×100 µm2, 200×200 µm2, and 500×500 µm2, respectively.

Development of superconducting tunnel junctions with an aluminum-oxide insulation layer for X-ray detection

Superconducting tunnel junctions (STJ), with a buffer layer between the silicon substrate and junction, are being developed for use as high-resolution X-ray detectors. Aluminum-oxide (Al/sub 2/O/sub 3/) is employed as the buffer layer in order to suppress the phonon mediated background from the Si substrate. The extent of phonon insulation was studied by measuring the X-ray spectra of STJs as a function of buffer layer thickness. The phonon insulation ability of Al/sub 2/O/sub 3/ was compared to that of magnesium-oxide. The Al/sub 2/O/sub 3/ layer should be a good phonon insulator, with the ability to suppress phonons with a single buffer layer.

Properties of Substrate Phonon Events in Superconducting Tunnel Junctions Induced by X-Ray Absorption.

The spectra of substrate phonon events induced by illuminating with X-rays obtained with Nb-based superconducting tunnel junctions (STJs) fabricated on a sapphire substrate were examined. We constructed a geometrical model in which the collection efficiency of phonons is proportional to the solid angle subtended to the STJ from the position of the absorption. By using the Monte-Carlo simulation, we succeeded in fairly reproducing the features of the spectra obtained by our measurements. A comparison of the model spectra with the measured ones revealed that a mismatch of the acoustic impedance at the junction-substrate interface and the spatial spread of a phonon cloud play an important role in the determination of the spectra. By including these effects, we show that the measured spectra can be well reproduced by adjusting only one free parameter, the radius of the phonon cloud. We estimated this radius in a sapphire substrate to be ~20 µm under 5.9 keV X-ray illumination.

A microstrip-coil integration on superconducting tunnel junctions for X-ray detection

We report the first demonstration of X-ray detection by using a Superconducting Tunnel Junction (STJ) with a microstrip-coil. The STJ was fabricated based on Nb/Al/AlO/sub x//Nb integration process technology using a 2 /spl mu/m design rule. Magnetic field was applied into the STJ by the microstrip-coil to suppress the dc Josephson current instead of the conventional electromagnet. The output waveform was successfully observed by irradiation of 5.9 keV X-ray under a condition of microstrip-coil current of 20 mA at 0.4 K.

Development of superconducting tunnel junctions for ultra soft X-ray detectors

Nb-based superconducting tunnel junctions (STJs) with Al trapping layers have been developed for X-ray detectors. One applications for STJs is a low energy radiation detector having energy resolution and a high photon counting rate. STJs have good potential in the band of energies from 40 eV up to 1 keV, because STJs materials have good absorbing properties in this region. The energy resolution was measured by using the Photon Factory beamline BL-12A of High Energy Accelerator Research Organization (KEK) in Japan, and for 200 eV photon, 11 eV Full Width Half at Half Maximum (FWHM) with a 20/spl times/20 /spl mu/m/sup 2/ junction were achieved. In this article, the fabrication technique and preliminary results of measuring the response for this band of energies are reported.

On-chip coil integrated superconducting tunnel junction for a high performance X-ray detector

Abstract On-chip coil integrated superconducting tunnel junction (OC 2 -STJ) has been demonstrated for high performance X-ray detector. The STJ was fabricated by an improved fabrication technique based on a high quality Nb/Al/AlO x /Nb junction technology. The STJ shows high quality current–voltage characteristics with an ultra-low leakage current of a few nA at 0.4 K for 100 μm ×100 μm junction. Typical critical current of the coil was achieved more than 50 mA for 2 μm line width, which can generate a magnetic field of about 10 mT into the STJ. It was found that the OC 2 -STJ successfully responded to the irradiated 5.9 keV X-ray.

Development of superconducting tunnel junctions as an energy-dispersive detector for UV and visible photons

Abstract We are developing an energy-dispersive detector for UV and visible photons that uses superconducting tunnel junctions (STJs) with Al trapping layers. As a first step to achieve this, we evaluated the performance of the detector for extreme ultraviolet (EUV) photons by using a synchrotron radiation facility. We achieved an energy resolution of FWHM=17.8±0.6 eV (including electrical noise of 17.6±0.2 eV) for 55 eV EUV photons with a 100×100 μm2 STJ. The electrical noise was the main component of the energy resolution. We achieved the electrical noise of FWHM=2.1±0.1 eV for a 20×20 μm2 STJ by replacing the cables connecting the STJ to a preamplifier operated at room temperature. A stronger signal resulting from a thinner tunnel barrier will lead to better energy resolution for detecting UV and visible photons.

Development of superconducting tunnel junctions as energy-dispersive detectors for EUV radiation

An energy-dispersive detector of extreme ultraviolet (EUV) photons that uses superconducting tunnel junctions (STJs) is being developed. The performance of the STJs for detecting EUV photons was evaluated at a synchrotron radiation facility. An energy resolution of 17.8±0.6 eV in FWHM (including external noise of 17.6±0.2 eV) for 55 eV EUV photons was obtained with a 100×100 μm2 STJ.

A high-resolution X-ray detection system using STJ and SQUID amplifier

We succeeded in separating the STJ upper electrode events from other electrode events. This study is for X-ray spectroscopy of slow ions. With a SQUID amplifier, it is clearly seen that the upper electrode signal has a sharp peak with a narrow base, while the other has a relatively gentle peak with a wide base. The difference in signal width was used to separate the upper electrode events from other electrode events. Then the X-ray spectroscopy of slow highly charged ions was carried out at RIKEN. The results are shown here.

Superconducting Tunnel Junctions as General Purpose Detectors

Superconducting tunnel junctions exhibit excellent performance in energy resolution together with the high rate counting capabilities which cannot be realized with conventional radiation detectors. We discuss possible improvements in astronomy and other research fields by employing superconducting tunnel junctions as detectors of X-rays and visible photons.