Masataka Ohkubo

Time Resolution Improvement of Superconducting NbN Stripline Detectors for Time-of-Flight Mass Spectrometry

We have applied superconducting stripline detectors (SSLDs) for time-of-flight mass spectrometry (TOF-MS) as molecule detectors. Two SSLDs, which consist of 7-nm-thick niobium nitride (NbN) striplines with different linewidths of 200 or 300 nm on a MgO substrate, were fabricated to investigate the effects of kinetic inductance on time resolution. We have observed ultrafast ion detection signals with risetimes of 360–640 ps, and successfully obtained mass spectra for a peptide, Angiotensin I, and a protein, bovine serum albumin (BSA) at an energy of 17.5 keV. It has been confirmed that the response time is governed by the kinetic inductance of the nano-striplines.

1 mm ultrafast superconducting stripline molecule detector

Superconducting stripline detectors (SSLDs) are promising for detecting keV molecules at nanosecond response times and with mass-independent detection efficiency. However, a fast response time is incompatible with practical centimeter detector size. A parallel configuration of striplines provides a means to address this problem. Experimental results and simulation for promisingly large 1-mm-square parallel niobium SSLDs show that nanosecond pulses are produced by superconducting-normal transition within only one of the parallel striplines instead of cascade switching of all the parallel striplines. Successful detection of a series of multimers of immunoglobulin G up to 584 kDa supports the mass-independent efficiency for mass spectrometry.

Subnanosecond time response of large-area superconducting stripline detectors for keV molecular ions

A large-area (200×200 μm2) superconducting stripline detector based on a parallel configuration of superconducting Nb nanowires is presented. We show that the parallel configuration provides a smart way to control the physical nonequilibrium state induced by the molecular impacts, which allows realizing large sensitive area and subnanosecond response at the same time. The experiments were carried out with molecular ions radiation in a keV energy range. The observed rise time was below 400 ps and the relaxation time was 500 ps, the best in this class of superconducting molecular detectors.

Kinetic-energy-sensitive mass spectrometry for separation of different ions with the same m/z value

A double-focusing mass spectrometer (MS) equipped with a superconducting-tunnel-junction (STJ) detector has been applied to measure relative ionization cross-sections for the production of ions that are accompanied by different ion species with the same mass-to-charge (m/z) value. The STJ detector fabricated for this study enables kinetic energy (E) measurement of incoming individual ions at a counting rate of up to approximately 100 k ions/s and an energy resolution (DeltaE/E) of 15%. Both high counting rate and high-energy resolution are necessary to independently determine both m and z and not the m/z value only in ion-counting MS experiments. Ions such as (14)N(2) (2+) and (14)N(+) with the same m/z value can be clearly discriminated using a kinetic-energy-sensitive MS. This fine discrimination capability allows direct determination of relative ionization cross-sections of the homonuclear diatomic ions (14)N(2) (2+)/(14)N(2) (+) and (16)O(2) (2+)/(16)O(2) (+), which are difficult to measure due to the strong interference by the signals of their dissociated atomic ions with noticeably large ionization cross-sections. The new instrument requires no low-abundance heteronuclear diatomic molecules of the forms (14)N(15)N or (16)O(17)O to carry out ionization studies and thus, is expected to be useful in fields such as atmospheric science, interstellar science, or plasma physics.

Superconductivity for Mass Spectroscopy

Time-of-Flight Mass Spectroscopy (TOF-MS) with superconducting detectors has two advantages over MS with conventional ion detectors. First, it is coverage for a very wide range of molecule weight over 1,000,000. Secondly, kinetic energies of accelerated molecules can be measured at impact events one by one. These unique features enable an ultimate detection efficiency of 100% for intact ions and a fragmentation analysis that is critical for top-down proteomics. Superconducting MS is expected to play a role in, for example, the detection of antigen-antibody complexes, which are important for medical diagnosis. In this paper, how superconductivity contributes to MS is described.

Strong, easy-to-manufacture, transition edge x-ray sensor

We developed a membrane structure with a silicon-on-insulator (SOI) wafer by using a micromachining technique to create a transition edge x-ray sensor. In this membrane structure, the part of the SOI layer between the silicon nitride (SiNx) film and the buried oxide layer was etched from the front side to form the SiNx membrane. Advantages of this membrane are that (a) it is stronger than conventional membranes and is therefore suitable for large format arrays, (b) the Si etching time is reduced from 12 h (for conventional etching) to 4 h, and (c) all the fabrication processes are done from the front of the wafer, thus simplifying the manufacturing process.

Titanium Based Transition Edge Microcalorimeters for Optical Photon Measurements

Transition edge sensor microcalorimeters can be used in many optical quantum measurements because of its low dark counts, high quantum efficiency, and high resolving power of a photon number in weak light pulses. In order to increase count rates up to a few MHz, we have developed a titanium transition edge sensor for the optical measurements, and its performances were analysed. Titanium is one of the ideal superconductor because of its higher transition temperature and lower optical reflectance at 1.5 wavelength. Our titanium film was fabricated with electron-beam evaporation, and showed high residual resistance ratio of 3.5. The sharp superconducting transition also was found at 359 mK, which is close to the critical temperature in bulk. The fabricated device showed a fast response to pulsed laser illumination of 1.5 wavelength with the fall time constant of 300 ns. These features are very promising for high-speed single photon detection in many quantum optical measurements.

Charge-state-derivation ion detection using a super-conducting nanostructure device for mass spectrometry

Mass spectrometry (MS) is a method of analyzing ions based on their mass/charge (m/z) ratios. The m/z peak identification requires speculation on the ionic unit-charge states. This problem can be solved by using superconducting junction devices to measure the kinetic energies of single molecules. However, the kinetic energy measurement is followed by the dead time of 1-20 µs, which is fatally slow for modern high-resolution time-of-flight (TOF) analyzers. In this paper, we demonstrate that a superconducting nano-stripline detector (SSLD) composed of a 10-nm-thick and 800-nm-wide NbN strip realizes the charge-state derivation, and furthermore satisfies the ideal MS detector specifications such as a nano-second response, a short recovery time, a wide mass range, and no noise.

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.

Soft X-ray Detection Performance of Superconducting Tunnel Junction Arrays with Asymmetric Tunnel Junction Layer Structure

Superconducting tunnel junction (STJ) array detectors with an asymmetric tunnel junction layer structure have been fabricated for the soft X-ray region between 100 eV and 1 keV. The asymmetric layer design was effective in solving the problem of double peak response to monochromatic X-rays, which is commonly observed in STJ detectors. The sensitive area was patterned by a lift-off technique that ensured no contamination on the top Nb electrode surface. The performance of a 100-pixel STJ array detector was investigated through fluorescent X-ray analysis of oxides and nitrides for the energy region of the K-lines of oxygen, nitrogen, and boron. The STJ array detector exhibited a high energy resolution of <15 eV, which cannot be achieved by semiconductor detectors, and an energy nonlinearity of <1%. It was demonstrated that the performance is suitable for fluorescence-yield X-ray absorption fine structure (XAFS) spectroscopy for light trace elements.