Takahiro Yamada

Electron emission from conduction band of diamond with negative electron affinity

Experimental evidence explaining the extremely low-threshold electron emission from diamond reported in 1996 has been obtained [K. Okano et al., Nature (London) 381, 140 (1996)]. Direct observation using combined ultraviolet photoelectron spectroscopy/field-emission spectroscopy proved that the origin of field-induced electron emission from heavily nitrogen (N)-doped chemical-vapor deposited (CVD) diamond was at conduction-band minimum utilizing negative-electron affinity (NEA). The significance of the result is that not only does it prove the utilization of NEA as the dominant factor for the extremely low-threshold electron emission from heavily N-doped CVD diamond but also strongly implies that such low-threshold emission is possible from other types of diamond and even other materials having NEA surface. The low-threshold voltage, along with the stable intensity and remarkably narrow energy width, suggests that this type of electron emission can be applied to develop a next generation vacuum nanoelectronic devices with long lifetime and high-energy resolution.

A 10 V programmable Josephson voltage standard circuit with a maximum output voltage of 20 V

A 10 V programmable Josephson voltage standard (PJVS) circuit with a maximum output voltage of 20 V and a microwave bias of 18.5 GHz was designed and fabricated. Although an attempt was made to improve the fabrication yield for the 10 V PJVS circuit, it was not sufficiently large to reproducibly fabricate a perfect chip without any defects. The redundant arrays were additionally integrated to increase the maximum output voltage, which contributed to an increase in the number of available chips under the limited fabrication yield. Fortunately, most of the defective arrays had a zeroth Shapiro step, while the first step was very small or zero. If the array was not dc biased, it did not generate any voltage, because it worked as a superconductive wiring. In this case, the maximum output voltage became smaller than that of a perfect chip, but it functioned as a 10 V PJVS, due to the back-up arrays.

Absolute measurement of 166mHo radioactivity and development of sealed sources for standardization of γ-ray emitting nuclides

Holmium-166m has a long half life (1200 yr) and emits a large number of gamma-rays between 80 and 1400 keV. These characteristics are very suitable for gamma-ray calibration sources, therefore, the absolute activity of 166mHo was measured and several sealed sources were produced to be used as reference sources for the secondary standardization systems for gamma-ray emitting nuclides. In this project, seven metal sealed sources and ten point sources were produced and several of these sources were transferred to the secondary standard laboratories to complete the traceability scheme.

Comparison of a Multichip 10-V Programmable Josephson Voltage Standard System With a Superconductor–Insulator–Superconductor-Based Conventional System

We developed a 10-V dc programmable Josephson voltage standard (PJVS) using a multichip technique. The PJVS was based on NbN/TiNx/NbN junctions and operated using a 10-K compact cryocooler. We carried out an indirect comparison with a superconductor-insulator-superconductor-based conventional Josephson voltage standard (JVS) by measuring the voltage of a 10-V zener diode reference standard. The combined standard uncertainty of the comparison was u c = 0.03 muV(k = 1), and the relative combined standard uncertainty was 3 times10-9.

Evaluation of low-frequency characteristic of a thermal converter using a programmable josephson voltage standard

A new dual-heater thermal voltage converter (DH- TVC) with improved low-frequency performance has been developed at the AIST. Frequency characteristic of the DH-TVC was evaluated using a dual-output programmable Josephson voltage standard(DO-PJVS), and was confirmed to have low-frequency flatness better than 10-6 down to 1 Hz.

Standardization of 18F using the 4π(β+γ) integral counting technique

Alpha 4pi(beta+gamma) integral counting technique using a 4pibeta-4pigamma detector configuration was adopted for the standardization of (18)F. In this technique, the beta-detector is composed of two thin plastic scintillators sandwiching the source, coupled with a slender photomultiplier tube. The beta-detector part with the source was inserted into a large well-type NaI(Tl) scintillation detector for gamma-ray detection, making a 4pibeta-4pigamma coincidence counting system. In this work, positron particles were detected with high efficiency in the beta-channel and annihilation quanta were also detected with high efficiency in the 4pigamma channel. The very small inefficiency of the 4pi(beta+gamma) integral counter for the beta-plus branch has been confirmed by EGS5 Monte Carlo simulation. The result using this technique agreed within the uncertainties with the result obtained by the conventional 4pibeta-gamma coincidence counting with the efficiency extrapolation technique using the same detector configuration and a conventional 4pibeta-gamma coincidence counter.

Characteristics of Annihilation Photons Emitted From New Types of Point-Like

Purpose: Point-like 22Na radioactive sources have been widely used for checking radiation detectors that measure annihilation photons and for evaluating PET scanners. We developed new types of 22Na point-like radioactive sources that are expected to be more symmetric than conventional source types in the angular distribution of the emitted photons. The purpose of this study is to use Monte Carlo simulation to investigate the basic characteristics of the photons emitted from the new type 22Na point-like radioactive sources. Methods: Two new types of 22Na point-like sources, a cylinder type and a sphere type, were considered in comparison with two conventional source types, a disk type and a cube type. The angular distribution and energy spectra of the emitted 0.511 MeV annihilation photons and 1.275 MeV gamma rays were calculated using a Monte Carlo code based on Geant4. Results: The angular distribution of the emitted photons was more symmetric in the new type point-like sources than in the conventional types. The asymmetry of the angular distribution of the unscattered 0.511 MeV annihilation photon pairs counted in coincidence was 0.1 ±0.1% and 1.6±0.1% in the sphere and cylinder types, compared with 4.1±0.1% and 22.6±0.1% for the cube and disk types. The scatter components around the 0.511 MeV photopeaks in the new source types were typically less than 2/3 and 1/2 of those in the disk and cube types, respectively. Conclusion: The new point-like 22Na sources offer a more symmetric angular distribution of the emitted photons with fewer scatter components. They are useful for applications where the symmetry of the emitted photons is crucial.

^{22}{\rm Na}

We have been developing a new implementation of quantum voltage noise source (QVNS), an integrated QVNS (IQVNS), for Johnson noise thermometry (JNT) in thermal metrology. We designed an IQVNS chip based on rapid single flux quantum (RSFQ) technology. A general formula for the output power spectrum has been derived by using a Fourier analysis method, which shows that the spectrum is calculable with sufficiently small errors for practical JNT measurements.

Radioactive Sources With Symmetric Absorber Designs

We are developing a system for precise ac waveform synthesis in a frequency range below kilo-hertz using a programmable Josephson voltage standard (PJVS). Our PJVS circuit is composed of 65 536 NbN-based over-damped Josephson junctions and operated at a temperature of 9–10 K with a mechanical cooler. We succeeded in generating stepwise-approximated sinusoidal waveforms with peek voltage amplitude of 2 V using a 10-bit PJVS circuit and a bias source with twelve 12-bit digital-to-analog converters. Rough estimations suggest the transient time between each step in the obtained waveforms is less than 5 µs, sufficient for sampling measurements.

Integrated quantum voltage noise source for Johnson noise thermometry

The impact of thin Ga-oxide (GaOx) interlayers on the electrical properties of GaN-based metal-oxide-semiconductor (MOS) devices was systematically investigated. Thin thermal oxides formed at around 900u2009°C were found to be beneficial for improving the electrical properties of insulator/GaN interfaces, despite the fact that thermal oxidation of GaN surfaces at high temperatures proceeds by means of grain growth. Consequently, well-behaved capacitance-voltage characteristics of SiO2/GaOx/n-GaN stacked MOS capacitors with an interface state density (Dit) as low as 1.7u2009×u20091011u2009cm−2u2009eV−1 were demonstrated. Moreover, the Dit value was further reduced for the SiO2/GaOx/GaN capacitor with a 2-nm-thick sputter-deposited GaOx interlayer. These results clearly indicate the intrinsically superior nature of the oxide/GaN interfaces and provide plausible guiding principles for fabricating high-performance GaN-MOS devices with thin GaOx interlayers.