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Abstract: Superconducting detectors are ideal for detecting low-energy particles from atoms to proteins of which kinetic energies range from less than 1keV to ∼30keV compatible with analytical instruments for ordinary laboratory use. The superconducting detectors have the advantages of detecting low energy quanta, because a threshold equivalent to the gap parameter (Δ) is extremely small: ∼meV that is one-thousandth of the band gap of semiconductor detectors. Any low-energy quanta including phonons created by individual particle hit-events on the detector surface can be detected through Cooper-pair breaking. Such detectors as superconducting tunnel junctions (STJs) and superconducting stripline detectors (SSLDs) are promising for developing advanced analytical instruments for nuclear physics, basic chemistry, and biology. Especially, mass spectrometry (MS) equipped with the superconducting detectors has been producing the remarkable results, which have never been obtained by conventional MS instruments. [Copyright &y& Elsevier]

Abstract: Fragmentation of precursor ions of peptides or proteins is very promising for sequencing with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (TOF-MS). Neutral fragments as well as ions produced by post-source decay (PSD) were measured by a superconducting tunnel junction detector in a linear TOF-MS. It has been demonstrated that the PSD fragments are observable as event bands with different kinetic energies at the same TOF as that of the precursor ions of polystyrene with a nominal mass of 200kDa and bovine serum albumin of 66.43kDa. [Copyright &y& Elsevier]

Abstract: Fluorescent-yield X-ray absorption spectroscopy (XAS) taken with a superconducting tunnel junction (STJ) detector has been employed to study the oxygen K-edge structure of Hf–Al oxide gate insulators for next-generation metal-oxide-semiconductor (MOS) devices. Thermal stability is a critical issue in the MOS fabrication processes. We show that O K-edge XAS indicates that no change in the electronic band structure occurs due to thermal annealing. [Copyright &y& Elsevier]

It has been demonstrated by Low-Temperature Scanning Synchrotron Microscope that the spatial profiles of Nb/Al/AlOx/Al/Nb detectors are abnormally large in an energy range between 2 and 10 keV, and thus the detectors produce no clear total absorption peaks. Below 2 keV, on the other hand, full illumination experiments have afforded the evidence that the spatial uniformity is significantly improved, for example, an 100 μm square junction has an intrinsic energy resolution of 5.4 eV at 200 eV. These results reconfirm that carefully designed junction detectors with a reasonable size are ideal for energy-dispersive spectroscopy in a soft X-ray range. [Copyright &y& Elsevier]

Abstract: Energy sensitive particle detectors for low kinetic energy of <10keV are required to realize high throughput ion scattering spectrometry, charge discriminated mass spectrometry, neutral mass spectrometry without re-ionization, and plasma particle spectroscopy. Superconducting tunnel junction (STJ) detectors are promising for these applications, because STJ detectors can measure kinetic energy less than 10eV in principle. The performance of an 200μm-square STJ detector having a layer structure of Nb/Al/AlO x /Al/Nb set at 0.4K was evaluated by using protons with a kinetic energy range of 1–3keV. The sensitivity was approximately 2×102 electrons/eV. The kinetic energy resolution for the 3keV protons was 132eV in full width at half maximum with a readout noise of 64eV, which is far superior compared with conventional particle detectors such as semiconductor detectors and proportional counters that cannot measure kinetic energies of individual ions in this energy range. [Copyright &y& Elsevier]

Abstract: Fluorescent yield X-ray absorption fine structure (XAFS) spectroscopy is widely used for measuring chemical or physical states of particular elements in materials. A superconducting tunnel junction (STJ) detector is promising for X-ray absorption spectroscopy, especially in a soft X-ray region below 1keV, because of an excellent energy resolution of 10–20eV, which is crucial for resolving the characteristic X-ray lines from dilute light elements in materials, and a high photon-counting-rate. We constructed an X-ray absorption spectrometer using a 100-pixel STJ array detector mounted on a cryogen-free 3He cryostat and a parallel data acquisition system. It has been confirmed that the STJ detector produces a single peak for the O-Kα line (525eV) with energy resolution of 22eV in full width at half maximum. An XAFS spectrum only for the oxygen atoms from a SiN/SiO2 thin film sample was successfully obtained by using the clear separation between the O-Kα line and the N-Kα line (392eV), eliminating the effect of the N-Kα background. [Copyright &y& Elsevier]

Abstract: Superconducting tunnel junction (STJ) detectors are promising for energy-dispersive X-ray spectroscopy. We are developing array detectors with a sensitive area of 4mm2, which consist of 100 of Nb/Al/AlO x /Al/Nb STJs having a size of 200×200μm2 in order to obtain a good statistic for a short measurement time while keeping an excellent energy resolution for element selection in X-ray material analysis. In the course of this development, we analyzed the microstructures of the layers of Nb, Nb/Al, Nb/Al/AlO x /Al and Nb/Al/AlO x /Al/Nb deposited on Si substrates by using AFM (Atomic Force Microscope) and cross-sectional TEM (Transmission Electron Microscope). We found that the grain sizes of the polycrystalline films and the interface boundaries in the top electrodes are quite different from those in the bottom electrodes in a nanometer scale. It is expected that these different microstructures cause different heights of the pulse signals for the X-ray photon absorption events in the top and bottom electrodes. [Copyright &y& Elsevier]

Abstract: Time-of-flight mass spectrometry (TOF-MS) with matrix assisted laser desorption/ionization (MALDI) has become an essential spectrometry for a research field of bioscience. However, the mass limit of the conventional TOF-MS spectrometer frequently prevent us from measuring the mass values for large biomolecules without chemical or enzymatic digestion. Overcoming the circumstance, a TOF-MS instrument with superconducting tunnel junction (STJ) detectors (Super TOF-MS) were developed. The direct observation of the kinetic energy for each ion by Super TOF-MS leads to the two considerable capabilities: the observation of large proteins up to 1MDa and the ionic charge-state discrimination of them, which are impossible with conventional ion detectors. In this paper, it is demonstrated that the these advantages of Super TOF-MS is a powerful tool at performing a fragment analysis of, for example, immunoglobulin G (IgG) which is a large protein playing a key role in an immune system. [Copyright &y& Elsevier]

Abstract: In order to develop fabrication processes for superconducting tunnel junction (STJ) array detectors for time-of-flight mass spectroscopy, we have focused on a roughness of film surface and a residual stress of each deposited film in a layer structure of Nb/Al/Al2O3/Al/Nb. The roughness of each film depending on pre-treatments of silicon substrates and deposition conditions of Al and Nb films was studied by using an atomic force microscope (AFM). The dependence of the residual stress of the Al and Nb films on Ar gas pressure was evaluated by using an interference microscope. It was found that the Al thin film has a very smooth surface with a roughness of 0.79nm in case of deposition on silicon substrates cleaned by a wet and dry process and in contrast, the surface roughness of the Nb and Nb/Al film are the smallest of 0.4nm and 1.0nm, respectively, on the Si substrate cleaned by only a dry process. We found appropriate sputtering conditions at which Al and Nb films have the stress less than 10MPa, respectively. Those low stress conditions have to be realized in order to achieve a high reproducibility of STJ characteristics at each process run. The optimum deposition conditions for Nb and Al films were obtained for the STJ fabrication. [Copyright &y& Elsevier]

Abstract: The spatial resolution of a cryogenic neutron detector comprising two superconducting tunnel junctions (STJs) on a single crystal of Li2B4O7 was measured using a collimated neutron beam. Neutrons are captured in the crystal by the nuclear reactions with 6Li or 10B atoms, and excited non-thermal phonons are propagated in the crystal to the STJs. The incident position of the neutron can be evaluated from the pulse–height correlation between the two STJs. A collimated cold-neutron beam of size 0.4×1.1mm2 was incident equidistant from two STJs separated by 1.3mm. The measured counts distribution in the pulse–height correlation plot between the two STJs was consistent with the results calculated using a solid-angle model, confirming the position sensitivity of the developed detector. Moreover, the events caused by the 6Li(n,α)t reaction showed signal heights that were 1.8 times larger than those caused by the 10B(n,α)Li reaction, which indicated that the signal-to-noise ratio of the detector is higher for a 6Li-enriched crystal than for a 10B-enriched crystal. [Copyright &y& Elsevier]

Abstract: Superconducting tunnel junction (STJ) detectors have a fast time response and a low threshold to detect such quanta as phonons. These characteristics are advantageous to their applications in mass spectrometry for large molecules. However, one drawback is small active area, which cancels out the high quantum efficiency for large molecules. In this study, therefore, arrays of 100 niobium-based proximized STJ elements with a size of 200×200μm2 have been fabricated to improve effective detection area to achieve a practical short measurement time. [Copyright &y& Elsevier]

We demonstrate that neutrons and gamma rays can be discriminated by a neutron detector comprising a superconducting tunnel junction on a single crystal of Li2B4O7. The neutron detector was irradiated with X-rays, gamma rays and neutrons, and the signal pulses were analyzed in terms of pulse heights and rise times. It was found that most of the signal pulses for neutron events exhibited slower rise times than those for X-rays and gamma rays, indicating that neutrons and gamma rays could be discriminated by the detector on the basis of the rise times of the signal pulses. [Copyright &y& Elsevier]

We succeeded in detecting neutrons using superconducting tunnel junctions (STJs) fabricated on a single crystal of Li2B4O7. Neutrons are captured in the crystal by the nuclear reactions 6Li+n→T+alpha+4.78 MeV and 10B+n→7Li+alpha+2.3 MeV, which excite a large number of phonons in the substrate. The phonons propagate in the absorber and are measured by the STJs. We selected a single crystal of Li2B4O7 as the absorber material because of properties such as the large neutron cross-section of 6Li and 10B, low γ-ray sensitivity, short particle range in the substrate, and fast phonon velocity. Series-connected or multiple STJs on the crystal would enable two-dimensional neutron imaging with high detection efficiency, low γ-ray background, and a high spatial resolution of a few microns. In this paper, we demonstrate neutron detection by STJs and report the basic characteristics of the fabricated STJs, including their X-ray response and neutron detection. The correlation in pulse heights between two junctions located ∼1.3 mm apart clearly indicates the possibility of neutron imaging. [Copyright &y& Elsevier]