M. Ohkubo

Thicker, more efficient superconducting strip-line detectors for high throughput macromolecules analysis

Fast detectors with large area are required in time-of-flight mass spectrometers for high throughput analysis of biological molecules. We fabricated and characterized subnanosecond 1×1u2002mm2 NbN superconducting strip-line detectors. The influence of the strip-line thickness on the temporal characteristics and efficiency of the detector for the impacts of keV accelerated molecules is investigated. We find that the increase of thickness improves both efficiency and response time. In the thicker sample we achieved a rise time of 380 ps, a fall time of 1.38 ns, and a higher count rate. The physics involved in this behavior is investigated.

Hot-Spot Detection Model in Superconducting Nano-Stripline Detector for keV Ions

Superconducting nano-stripline detectors (SSLDs) are promising for realizing ideal ion detection in mass spectrometry. We have investigated the ion detection efficiency of a niobium nitride (NbN) SSLD, measuring the bias current dependence of the detection efficiency for Ar+, Ar2+, and Ar3+ ions accelerated by static voltages between 0.5 and 3 kV. The bias current dependence exhibited a distinct plateau in a high bias region and an abrupt reduction at a certain bias current (threshold bias current) that decreased with increasing kinetic energy. It was found that the kinetic energy dependence of the threshold bias current is consistent with a hot-spot model.

Superconducting Tunnel Junction Detectors for Analytical Sciences

Superconducting tunnel junction (STJ) detectors exhibit superior detection performance for photons and particles at a high spectroscopic resolution of ~ 10 eV, a short dead-time (decay time) of ~ μs, a high quantum efficiency of ~ 100%, and a low detection threshold energy of less than 1 eV, which cannot be achieved by conventional detectors. The outstanding detection performance originates from a small superconducting energy gap of ~ meV, which is three orders of magnitude smaller than ~ eV in semiconductors. This paper reports our recent progress in two applications of STJ detectors to fluorescence-yield X-ray absorption fine structure (XAFS) spectrometry for trace light elements in matrices and mass spectrometry (MS) for ions with the same mass/charge-number ratio (m/z) but different charge states and neutral fragments.

Ion-induced dynamical change of supercurrent flow in superconducting strip ion detectors with parallel configuration

Superconducting strip ion detectors are promising for realizing ideal ion detection in time-of-flight mass spectrometry. To realize large sensitive area for practical use, parallel configurations of superconducting strips are mandatory. In a previous parallel configuration design, however, we have found that a non-negligible number of ion impact events were lost because a large number of output current pulses for single ions were fatally small. An alternative parallel configuration design has solved this critical problem. It has been revealed that ion impact events induce dynamical change of bias current flow among parallel superconducting strips. Furthermore, output current distributions of larger bias current have shown another dynamical phenomenon: multi-strip switching triggered by single ion impact.

Design and high-speed tests of a single-flux-quantum time-to-digital converter for time-of-flight mass spectrometry

We are developing a single-flux-quantum (SFQ) time-to-digital converter (TDC) for time-of-flight mass spectrometry (TOF MS) system. In this study, we designed and tested a 24-bit SFQ TDC with a 3 × 24-bit FIFO buffer using the AIST Nb standard process (STP2), whose time resolution and dynamic range are 100 ps and 1.6 ms, respectively. The TDC design was improved to reduce the total junction numbers and the bias current in order to install it in a 4.2 K cryo-cooler. We confirmed the operation of the TDC and evaluated the jitter by measuring a histogram of TDC read out.

Superconducting tunnel junction (STJ) detectors for ion-charge-number sensitive mass spectrometry

Mass spectrometry (MS) is one of the indispensable methods in analytical chemistry. Conventional ion detectors installed in MS instruments solely count individual keV ions or measure ion flux, which leads to no information on the charge states of the individual ions. Superconducting tunnel junction (STJ) detectors, on the other hand, have sensitivity to the ion-charge-number through kinetic energy measurement of keV ions. The STJ detectors overcome such limits in conventional MS instruments as mass/charge-number (m/z) overlap that occurs between, for example, 14N+ and 14N22+ (m/z 14), and neutral loss that means that neutral fragments produced from a precursor ion cannot be analyzed directly in tandem mass spectrometry.