Akifumi Koike

Electron Optical Properties of Microcolumn with Field Emitter

We present the electron optical properties of a newly designed microcolumn. The microcolumn consists of an objective lens and an electron gun, which is composed of a microscale field emitter and a condenser lens. An acceleration lens was used as the objective lens. Each component was optimized to maximize its own function while minimizing its impact on the functions of the other components. The current–voltage characteristics of each electrode were evaluated. The current variation characteristics of each electrode indicated that each optimized structure could be used to control the electron beam. The objective lens could also focus the electron beam to a diameter of approximately 40 µm at a working distance of 2 mm and 400× magnification when no acceleration electric field was applied between the microcolumn and the anode.

Atomic Number and Electron Density Measurement Using a Conventional X-ray Tube and a CdTe Detector

In order to apply the dual-energy technique to material identification, a new computed tomography scanning system was proposed using a conventional X-ray tube and a CdTe detector. This system can provide information of projection data at two distinct energy bands for scanned materials. After introducing an approximation, the measured projection data were reconstructed to obtain the distributions of the X-ray linear attenuation coefficients of the materials at two different energies. Then, the corresponding atomic number and electron density can be derived with the dual-energy X-ray computed tomography (DXCT) method adopted. By comparing the obtained results with theoretical ones, the feasibility of using this system for identifying low-Z materials was demonstrated in this study.

Emission and Focusing Characteristics of a Quintuple-Gated Field Emitter Array

A field emitter array (FEA) with a five-stacked gate electrode, that is, a quintuple-gated FEA with a four-stacked electrode lens, was fabricated. The four-stacked electrode forms an einzel lens, where the second and third electrodes are connected through a contact hole. Because the electron velocity of the einzel lens in a quintuple-gated FEA is smaller than for the quadruple-gated FEA, the quintuple-gated FEA has a stronger electron convergence. The slit sweeping method confirmed that a beam crossover is formed for the quintuple-gated FEA.

Material discriminated x-ray CT by using conventional microfocus x-ray tube and cdte imager

Material discriminated X-ray CT system has been constructed by using conventional micro-focused X-ray tube (small white X-ray source) and photon counting type CdTe X-ray imager with energy-band discriminate function. We have already reported material identify X-ray CT using K-shell edge data. In this report the principle of material discrimination was adapted the separation of electron-density and atomic number from attenuation coefficient mapping in X-ray CT reconstructed image in dual energy X-ray CT method. The dual energy X-ray method was obtained good electron-density mappings by using two monochrome X-ray source, however, it is difficult for very large-scale equipment such as an accelerator to be necessary and to apply this system to homeland security field at the airport and so on. The measurement sample was prepared as three kinds material rods (aluminum(Al), titanium(Ti) ,iron(Fe) of 6mm-diameter). We could observed material discriminated X-ray CT reconstructed image, however, the discrimination properties were not good than two monochrome X-ray CT method. This results was could be explained because X-ray scattering, beam-hardening and so on based on white X-ray source, which could not observe in two monochrome X-ray CT method. However, since our developed CdTe imager can be detect five energy-bands at the same time, we can use multi-band analysis to decrease the least square error margin. We will be able to obtain more high separation in atomic number mapping in X-ray CT reconstructed image by using this system.

Carrier transportation and polarization properties in CdTe diode detectors

The carrier transportation properties of a P-intrinsic-N (PiN) type CdTe diode detector for radiation spectrography was evaluated under the non-polarized and polarized condition with high speed signal processing method for a practical photon-counting system. The rise-up time distribution in each energy region was obtained by our system in each operation condition. These results suggest that the polarization affects transportation speed of holes slower than electrons, because the rise-up time distribution in same detection efficiency of the polarization condition (passage of time 90 min at bias voltage 200 V), and the non-polarization condition with lower bias voltage (passage of time 1 min at bias voltage 50 V) shows obviously different tendency by energy region. We believe our new approach could give key information for analyzing the polarization phenomenon in CdTe diode detectors.

Field Emitter Equipped With a Suppressor to Control Emission Angle

We introduce a suppressor electrode into a micro-sized field emitter to control the emission angle, as an alternative to using an aperture. The suppressor electrode is fabricated between the emission tip and the extractor by using an etch-back technique. The suppressor height is determined to be 100 nm below the tip. This structure is expected to control the emission angle to be narrower when applying a low voltage at the suppressor electrode. The emission characteristics of the fabricated device closely follow the behavior of the simulated results. It is shown that the emission angle is narrowed by applying a negative voltage at the suppressor electrode.

Characteristics and stability of diode type CdTe-based x-ray and gamma-ray detectors

The electrical and spectral properties of the M-p-n structured In/CdTe/Au detectors have been investigated in comparison with the characteristics of In/CdTe/Pt Schottky diode detectors. The high-resistivity (111) oriented Cl-compensated p-like CdTe single crystals were used in both the cases. The CdTe diode structures with a p-n junction were fabricated by the laser-induced doping procedure. The M-p-n and Schottky diodes were examined by measurements of I-V and C-V characteristics and spectra excited by a 57Co radioisotope at room temperature. From the obtained spectrum measurement data, FWHM, shift and peak-to-valley ratios concerning the 122 keV and 14.4 keV peaks were calculated and the time dependences of these parameters were analyzed. Both types of CdTe-based diode detectors showed high rectification properties that allowed to apply higher bias voltage to reach full charge carrier collection thanks to extension of the depleted layer and hence to obtain high energy resolution.

Dynamical properties of electron and hole generated in CdTe radiation detector probed by optical pulses

Characteristics of CdTe radiation detectors are studied using laser pulses. To improve the spatial and time resolution of carrier generated position and time, optical laser pulses were used for carrier generation and the electrical response from the CdTe radiation detectors were detected as a function of irradiated position and time. The detector measured in this study was Schottky contact CdTe detectors with a thickness of 0.5 mm and a size of 2 × 1 mm2. Laser light from laser diode with a wavelength of 850 nm were modulated to pulses by using pulse generator and were irradiated to the side surface of the CdTe detectors. The pulse width and the period were 2 ns and 2 ms, respectively. The incident light power was 30 nW, which correspond to one photon energy of γ or X lay with a energy of around 70 keV. Time traces of output voltage amplified by pre-amplifier were measured by oscilloscope. Pulse height and rise-up time of the output voltage depended on the distance from the anode plane of the detector. These dependences were changed by changing applied bias voltage. It is concerned that the depletion layer are increased with increasing the bias voltage and the distribution of the output voltage value in the detector are changed. This probing technique should be suitable for measuring the carrier distribution and transportations in the CdTe radiation detectors.

Photon counting x-ray CT with 3D holograms by CdTe line sensor

The novel 3-D display system is required in the medical treatment field and non-destructive testing field. In these field, the X-ray CT system is used for obtaining 3-D information. However, there are no meaningful 3-D information in X-ray CT data, and there are also no practical 3-D display system. Therefore, in this paper, we propose an X-ray 3-D CT display system by combining a photon-counting X-ray CT system and a holographic image display system. The advantage of this system was demonstrated by comparing the holographic calculation time and recognizability of a reconstructed image.

10.1: New functioned field emission array to controllable initial emission angle by introducing a suppressor gate

We have proposed the new micro column electron beam microscope fabricated with multi-gated field emitter array based on the first result that a crossover successfully observed with quintuple-gated FEA. In order to realize the fine focused electron beam less than several ten nm in diameter, the initial emission angle after emitting from tip surface has to be controlled narrowly. In this report, it was described that we have introduced a suppressor gate which was located below tip and fabricated double-gated field emitter array with extractor. It was demonstrated that the suppressor gate had an emission angle narrowed successfully.