Fumiyuki Fujita

Floating zone growth and scintillation characteristics of cerium-doped gadolinium pyrosilicate single crystals

Growth of cerium-doped gadolinium pyrosilicate (Gd₂Si₂O₇:Ce) single crystals, which show 2.5 times greater light output for gamma-rays and five times greater light output for alpha particles than GSO single crystals, is accomplished using floating zone growth method (FZ method). Although growth of Gd₂Si₂O₇ (GPS) single crystal is considered to be difficult because it melts incongruently according to the phase diagram in a Gd₂O₃-SiO₂ system, we attempted crystal growth of Ce:GPS because the possibility exists that heavy Ce doping changes the phase diagram. Transparent single crystals were obtained, but cracks were observed in the crystals. The crystal structure was triclinic with space group P_/1 and density of 5.5 g/cm³. Two peaks were observed by photoluminescence spectrum measurement at 374 nm and 394 nm caused by 5d-4f transition in Ce³⁺ ion. Decay times of Ce:GPS were 46 ns for gamma-ray and 39 ns for alpha particles; its density was 5.5 g/cm³. We consider that the energy resolution of 23% will be improved by fabrication of large crystals and improvement of crystal perfectability.

Determination of Electron Energy Distribution Function of Plasmas by Digital Processing from Langmuir Probe Characteristic

Electron energy distribution functions of plasmas were calculated by digital processing from Langmuir probe characteristics. The method used is based on the least squares priciple for fitting the experimental data to a polynomial curve, and performs the differential operation by a convolution of data with weighting functions. The computer program developed in accordance with this method was applied to the probe characteristics of positive column plasmas. The results were compared with the Maxwellian and Druyvesteyn distribution functions.

Scintillation Characteristics of

Cerium-doped gadolinium pyrosilicate single crystals Ce:Gd2Si2O7 (Ce:GPS) with various Ce concentrations of 2.5-30 mol% with respect to the total rare-earth sites were prepared by the floating zone method. Their scintillation performances were investigated under irradiation of gamma rays of 137Cs. The Ce concentration dependence of scintillation characteristics and high-performance scintillation characteristics were obtained: 3-6 times greater light output than that of BGO single crystals, rapid decay, and good energy resolutions of 5.1-8.4%. Furthermore, X-ray diffraction patterns of Ce:GPS revealed Ce concentration dependence of the crystal structure of Ce:GPS.

{\rm Ce}:{\rm Gd}_{2}{\rm Si}_{2}{\rm O}_{7}

Dependence of crystal structure and luminescence property of Gd2Si2O7(Ce:GPS) was investigated using poly-crystalline samples synthesized by a solid state reaction method. Three types of crystal structures and different luminescence spectra were observed according to concentration of cerium. The largest luminescence intensity was observed for orthorhombic structure whose cerium concentration was between 0.75 and 2.5 mol%.

(Ce 2.5–30 mol%) Single Crystals Prepared by the Floating Zone Method

When electrons at relativistic velocities pass through a crystal plate, such as silicon, photons are emitted around the Bragg angle for X-ray diffraction. This phenomenon is called parametric X-ray radiation (PXR). The monochromaticity and directivity of PXR are adequate and the energy can be changed continuously by rotating the crystal. This study measured the mass attenuation coefficient around the K-shell absorption edge of Nb, Zr and Mo as a PXR application of monochromatic hard X-ray radiation sources.

Dependence of crystal structure and luminescence property of Gd2Si2O7 on cerium concentration

Response function measurement of Gd2Si2O7 (GPS) scintillator for thermal neutrons was carried out. GPS single crystals exhibits light output twice higher than GSO single crystals for thermal neutrons. In addition, similar experiments with GPS scintillators of 50 mum in thickness were conducted to improve signal-to-noise ratio between neutrons and gamma-ray. GPS powder samples, size of each particle was 50 mum, showed lower light output than that of GPS single crystals, because absorption and scattering of scintillation light at the sample surface reduced the correction efficiency of scintillation light. Optimization of the particle size, reflectors and techniques to fix the GPS sample will improve the powder density and detection efficiency because powder samples were directly coated on a glass plate in this experiment.

Measurement of mass attenuation coefficients around the K absorption edge by parametric X-rays

A quick measurement and data processing system for Langmuir probes was developed based on a digital technique. This system enabled us to avoid the effect of probe contamination and plasma-state variations on probe characteristics. For noise reduction, the system had an averaging function by digital accumulation of data from multiple scanning. A set of personal computer programs which automatically determine the electron temperature and density from probe-characteristic data was also developed. The system was applied to a negative glow plasma, and the plasma parameters were successfully determined.

Response function measurement of Gd2Si2O7 scintillator for neutrons

Single crystal Gd<inf>2</inf>Si<inf>2</inf>O<inf>7</inf>:Ce (GPS:Ce) scintillators were grown by a top seeded solution growth (TSSG) method and their crystal structures and scintillation properties were measured. Single crystal GPS with several cerium concentration; (Gd<inf>1‒x</inf>Ce<inf>x</inf>)<inf>2</inf>Si<inf>2</inf>O<inf>7</inf> (x=0.025, 0.1, 0.15), were synthesized with a RF-Czochralski furnace. For GPS:Ce 2.5 at.%, a crystal structure was orthorhombic. For GPS:Ce 10 and 15 at.%, these were triclinic. In PL measurements, broad emissions around 350 to 450 nm correspond to 5d–4f transitions of Ce³⁺ were observed. In gamma-ray measurements, each scintillator shows light output larger than that of BGO. High energy resolution of around 5% was obtained for GPS:Ce 2.5 and 10 at.%. GPS:Ce 2.5 at.% shows ca. 4.4 times light output larger than that of BGO. Decay times of triclinic GPS were faster than that of orthorhombic GPS.

Quick measurement and digital data handling for Langmuir probes

A cellulose nitrate track detector, LR-115 (Type II), was studied experimentally to observe its neutron energy response when a radiator, such as fissionable material, is absent. The detector was exposed to monochromatic neutrons from the D (d, n) He reaction produced by a Van de Graaf accelerator. Through-etched and almost through-etched tracks in the detector were counted by two different methods: 1) optical microscope counting with an automatic image analyzing system and 2) spark counting. The detection sensitivity was obtained in the neutron energy region lower than 5 MeV and it was found that only α-particles from (n, α) reactions could be detected in both counting methods. We derived the counting probability of α-tracks induced in the detector and the threshold energies for neutron detection.

Dependence of scintillation properties on cerium concentration for GPS single crystal scintillators grown by a TSSG method

Radiation detectors were fabricated with layered structure diamonds comprising several micrometers of a boron-doped single crystal diamond and 20 μm of insulating single crystal diamond layers grown on a HP/HT-type Ib diamond single crystal substrate. To improve the yield rate of CVD diamond radiation detectors this approach was adopted. The detectors had rectification characteristics because of their boron-doped diamond contact and aluminum Schottky contact. The leakage current of one detector was less than 0.2 pA at reversed bias voltage of +50 V. Energy resolutions of 2.6% and 2.8% for alpha particles from 241 Am were achieved using these detectors. The rise time of a leading edge of an output signal from one detector was less than 20 ns, which was the fastest rise time of the measurement system; drift velocities of charge carriers were estimated to be faster than 1 × 105 cm/s. In addition, cathode luminescence spectra of insulating diamond layers were measured; peaks caused by free exciton recombination and ‘band A’ were observed. Improved crystallinity of the boron-doped diamond layer helps realization of a layered-type diamond radiation detector with higher energy resolution.