Tatsuya Nakamura

High-speed readout method using wavelength-shifting fibers for an imaging plate

A novel readout method using wavelength-shifting fibers aimed at real-time imaging for X-ray and neutron was developed. It is confirmed that the PSLs can be detected by wavelength-shifting fibers with sufficient sensitivity and a radiation image can be measured with good position resolution by the readout method.

X-ray Detection Characteristics of Nb-Based Superconducting Tunnel Junctions

Detailed X-ray detection characteristics for Nb-based superconducting tunnel junctions (STJs) were measured with cooled 4-JFETs low noise charge sensitive amplifier that we developed. We studied the detection characteristics for a wide energy range of X-rays and signal production processes for quasiparticles in Nb-based SIS junction and substrate phonons, and phonon detection.

Development of flexible and wide-range polymer-based temperature sensor for human bodies

In this paper, we propose a thin, flexible, polymer PTC sensor with a wide temperature measurement range. A polymer PTC sensor consists of several polymers and conductive filler and its resistivity changes with an increase in temperature. Conventionally, it has been proposed as a flexible temperature sensor. However, no research has yet achieved enough flexibility and a wide temperature measurement range that is sufficient for measuring the human body temperature; furthermore, polymer PTC sensors have not yet been put to practical use. In this research, by using two specific monomers as the raw materials of the polymers and by using several of such polymers for fabricating one sensor, we succeeded in making a new polymer PTC sensor which has not only a wide temperature measurement range (25°C to 45°C) but also enough flexibility to function with a very small thickness (33 μm). Resistivity of the proposed polymer PTC changes by 4 orders in accord with an increase in the temperature from 25°C to 45°C. These characteristics are more suitable for the wearable temperature sensors and medical devices than those of conventional polymer PTC. Wide temperature measurement range helps us to measure temperature at several body parts in different environments, and flexibility increases the adhesiveness of the sensor to the human body. Moreover, high sensitivity makes readout circuits simple and it is easy to calculate temperature from the resistivity of the sensor.

An MRI-readable wireless flexible pressure sensor

We developed a magnetic resonance imaging (MRI) -detectable wireless flexible pressure sensor with pressure-sensitive LC resonator fabricated on a flexible film substrate. Measuring pressures in the body such as blood vessels and cardiac ventricle are very important in making diagnoses and in postoperative observation. However, conventional wired pressure sensors have difficulty in maintaining their connections to external readout equipment, and they also increase the risk of infection during and after implantation. In this study, to read the pressure wirelessly using an MRI system, an LC resonator consisting of a spiral coil and a pressure-sensitive capacitor was designed resonate at 300 MHz which corresponds to the Larmor frequency in an external magnetic field of 7-T. In order to validate the operating principle of the fabricated device, the frequency-impedance characteristics were measured by changing the pressure. The resonance frequencies of complemented LC circuits were lower by approximately 10% than those of nonpressured conditions. After implanting these devices in a 1% agarose gel, MR images were acquired by inducing pressures close to blood pressures of 20 kPa. As a result, contrast changes in the MR images were observed around the integrated spiral coils. This result indicated that the developed flexible pressure sensor has sufficient sensitivity to measure physiological pressure such as blood pressure of 20 kPa wirelessly in the body. In the future, quantitative pressure sensing will be evaluated by comparing it to the contrast changes in MR images with flip angle mapping.

Basic characteristics of implantable flexible pressure sensor for wireless readout using MRI

Measuring the local pressure in blood vessels is valuable in the postoperative monitoring of aneurysms. However, implanting a conventional pressure sensor equipped with power and signal cables causes difficulties during the operative procedure and carries a risk of infection after the implantation. In this study, we developed a wireless, implantable, and flexible pressure sensor. A magnetic resonance imaging (MRI) system reads out the sensor output. The proposed wireless sensor is based on an LC resonant circuit with a spiral coil and a pressure-sensitive capacitor. The pressure-dependence of the capacitance affects the magnetic field produced by the spiral coil, changing the magnetization of the nearby sample that can be observed as a signal variation by MRI. We fabricated a prototype sensor using a capacitor with a silicone elastomer as the dielectric and a spiral coil made of gold. The maximum change in the capacitance was 8% under an external pressure of 20 kPa. A change in the thickness of the dielectric elastomer caused the capacitance to change, resulting in a signal variation detectable by MRI.

Theoretical Analysis on Photostimulated Luminescence Phenomenon of Imaging Plate.

The photostimulated luminescence (PSL) characteristics of an imaging plate (IP) irradiated with α-ray has been experimentally investigated in order to construct a theoretical model on PSL phenomenon. In this way the nonlinearity of PSL intensity to α-ray dose, the dependence of PSL intensity decay on PSL excitation light power and the saturation of PSL intensity with increasing PSL excitation light power have been found.We propose a rate equation that describes the time evolution of the number of holes and F centers in the phosphor with α-ray dose and show that the nonlinearity of the PSL intensity to α-ray dose is caused by hole and F center recombination losses.Next we construct a theoretical model for the interaction of the PSL excitation light photon with the F center and the recombination of the conduction band electron with the hole. This shows that the constructed PSL model reproduces the decay of PSL intensity and the relationship between α-ray dose, PSL excitation light power and PSL intensity.Finally, fading experiments of IP have been performed. With elapsed time there was a rapid decrease of PSL intensity with a corresponding increase in its fluctuation. We propose a theoretical model for the fading of the IP and thereby show that the fading phenomenon is caused by the production of a large and fluctuating number of unstable F (Br-) centers.

Quantitative Evaluation of the Quantum Efficiency of a Superconducting Tunnel Junction with the Multiple Scattering Process Model of Primary Photoelectrons

The electron multiple scattering process of primary photoelectrons generated absorbed X-rays was studied so as to developed a clear model of a signal producing process of a STJ, and the intrinsic peak efficiency P1 of the STJ was calculated as a function of X-ray energy. X-rays of 5 to 18 keV were measured by a 178 µm×178 µm Nb/Al-AlO2/Al/Nb superconducting tunnel junction with a Nb-electrode, 200nm thickness and the P1 agreed well with the measured one in X-ray energy range of 5 to 12 keV.