Junichi H. Kaneko

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.

Fusion Neutronic Source deuterium–tritium neutron spectrum measurements using natural diamond detectors

Two natural diamond detectors (NDDs) operating at room temperature were used for Fusion Neutronics Source (FNS) deuterium–tritium (DT) neutron spectra measurements at different points around the tritium target and for different deuteron beam energies. Energy resolution of both NDDs were measured, with values 1.95% and 2.8%. Due to the higher energy resolution of one of the two NDDs studied it was possible to measure the shape of the DT neutron energy distribution and its broadening due to deuteron scattering inside the target. The influence of pulse pileup on the energy resolution of the combined system (NDD+electronics) at count rates up to 3.8×105 counts/s was investigated. A 3.58% energy resolution for the spectrometric system based on NDD and a 0.25 μs shaping time amplifier has been measured at a count rate of 5.7×105 counts/s. It is shown that special development of a fast pulse signal processor is necessary for NDD based spectrometry at count rates of approximately 106 counts/s.

Diamond radiation detector using a synthetic IIa type mono-crystal

A diamond radiation detector using a synthetic IIa type mono-crystal was developed. An energy resolution of 0.81%, 45 keV, was achieved for 5.486 MeV α particles in a vacuum environment. The ionization energy, ∈Diamond, was estimated as 13.07 eV by calibration with a silicon surface barrier detector. On a preamplifier output signal, a trapping and detrapping effect on holes was observed.

Response function measurement of layered type CVD single crystal diamond radiation detectors for 14 MeV neutrons

Response function measurement of layered-type chemical vapor deposition single crystal diamond radiation detectors for 14 MeV neutrons was carried out. The detector had a layered structure that was composed of a boron-doped diamond layer of 0.5 μm in thickness and a nondoped diamond layer of 20 μm on an inexpensive high pressure and high temperature-type Ib diamond substrate. The detector had energy resolution of 2.6% for 5.5 MeV α particles. This experiment was mainly carried out in order to understand the present status of the detector as a 14 MeV neutron spectrometer and an extent of charge trapping. As result, a peak caused by the 12C(n,α0)9Be reactions was clearly observed; the best energy resolution of 6% as for a synthetic diamond radiation detector was achieved. Detection efficiency was 3.2×10−7 counts/unit neutron fluence. However, taking the energy resolution for α particles, etc., into account, the energy resolution for 14 MeV neutrons was not so high. Further improvement based on better crysta...

Radiation distribution sensing with normal optical fiber

The purpose of this study is to develop a radiation distribution monitor using a normal plastic optical fiber. The monitor has a long operating length and can obtain continuous radiation distributions. A principle of;the position sensing is based on a time-of-flight technique. The monitor is sensitive to beta rays or charged particles, gamma rays, and fast neutrons. The spatial resolutions for beta rays (/sup 90/Sr-/sup 90/Y), gamma rays (/sup 137/Cs) and D-T neutrons are 30 cm, 37 cm and 13 cm, respectively. The detection efficiencies for the beta rays, the gamma rays and D-T neutrons are 0.11%, 1.6/spl times/10/sup -5/% and 1.2/spl times/10/sup -4/%, respectively. The effective attenuation length of the detection efficiency is 18 m. In this paper, we describe the basic characteristics of this monitor.

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}

The transmissivity of KU-1 quartz which is a candidate of the window material for UV and visible spectroscopy in ITER was measured under the irradiation of 14 MeV neutrons up to the fluence of 7.4×1019 n/m2 in the UV range (200–400 nm). Significant transmission loss was observed in the wavelength range of 200–300 nm. Two absorption peaks were identified: an E′-center at 215 nm and Si(III) defects at 245 nm. The E′-center absorption is much larger than the Si(III) defect absorption. The transmission loss increased with the neutron fluence, decreased during the irradiation breaks and returned almost to the same level as before when the irradiation resumed. This phenomenon indicates that an absorption induced by the neutron irradiation is caused by stable scattering and short lifetime centers.

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

The response function of a diamond radiation detector using a synthetic type IIa single diamond crystal for 14 MeV neutrons was measured. There was severe trapping of electrons in the synthetic diamond crystals used in this study. Accordingly, a prototype diamond radiation detector using the synthetic type IIa single diamond whose thickness was 0.1 mm was used with high bias voltage up to 1000 V, i.e., electric field of 100 kV/cm. The measurement resulted in the first observation of a 14 MeV neutron peak corresponding to 12C(n,α)9Be reactions by a synthetic diamond radiation detector. Energy resolution of the prototype detector for 14 MeV neutrons was 9%. Experimental detection efficiency of the detector was consistent with calculated detection efficiency. Synthetic diamond crystals which do not have trapping of charge carriers are indispensable in order to improve energy resolution for 14 MeV neutrons.

In situ transmissivity measurements of KU1 quartz in the UV range under 14 MeV neutron irradiation

The performance of a diamond detector made of single-crystal diamond grown by chemical vapour deposition was studied for heavy ions, having energy of 3 MeV. Energy peaks of these low-energy ions were clearly observed. However, the pulse height for individual incident ion decreases with increasing atomic number of the ions. For understanding this pulse height reduction effect, we calculated the amount of ionizing and non-ionizing energy loss of incident ions in the diamond detector. The results of our calculation suggest the contribution of charge loss mechanisms other than the recombination effect of electron-hole pairs produced along the ionized track. We also mentioned the incomplete charge collection near the boundary region between the metal electrode and the diamond surface.

Response function measurement of a synthetic diamond radiation detector for 14 MeV neutrons

Abstract The D–T neutron skyshine experiments have been carried out at the Fusion Neutronics Source (FNS) of JAERI with the neutron yield of ∼1.7×1011 n/s. The concrete thickness of the roof and the wall of a FNS target room are 1.15 and 2 m, respectively. The FNS skyshine port with a size of 0.9×0.9 m2 was open during the experimental period. The radiation dose rate outside the target room was measured a maximum distance of 550 m from the D–T target point with a spherical rem-counter. Secondary gamma-rays were measured with high purity Ge detectors and NaI scintillation counters. The highest neutron dose was about 9×10−22 Sv/(source neutron) at a distance of 30 m from the D–T target point and the dose rate was attenuated to 4×10−24 Sv/(source neutron) at a distance of 550 m. The measured neutron dose distribution was analyzed with Monte Carlo code MCNP-4B and a simple line source model. The MCNP calculation overestimates the neutron dose in the distance range larger than 230 m. The line source model agrees well with the experimental results within the distance of 350 m.