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In order to increase the count rate capability of a neutron detection system as a whole, we propose a multi-stage neutron detection system. Experiments to test the effectiveness of this concept were carried out on Fusion Neutronics Source. Comparing four configurations of alignment, it was found that the influence of an anterior stage on a posterior stage was negligible for the pulse height distribution. The two-stage system using 25 mm thickness scintillator was about 1.65 times the count rate capability of a single detector system for d-D neutrons and was about 1.8 times the count rate capability for d-T neutrons. The results suggested that the concept of a multi-stage detection system will work in practice. [ABSTRACT FROM AUTHOR]

Abstract: Under the ITER/ITA task, we have conducted the neutron streaming experiment simulating narrow and deep gaps at boundaries between ITER vacuum vessel and equatorial port plugs. Micro-fission chambers and some activation foils were used to measure fission rates and reaction rates to evaluate the relative fast and slow neutron fluences along the gap in the experimental assembly. The MCNP4C, TORT and Attila codes were used for the experimental analysis. From comparing our measurements and calculations, the following facts were found: (1) in case of a such narrow and deep gap structure, the calculation with MCNP, TORT and Attila codes and FENDL-2.1 is sufficient to predict fast neutron field inside the gap; (2) by scattering neutrons in the experimental room, experimental error considerably increased at the deeper region than 100cm; (3) angular quadrature set of upward biased U315 and last collided source calculation on TORT and Attila were very important technique for accurate estimation of neutron transport. [Copyright &y& Elsevier]

There are several vacant channels for diagnosis, RF heating and so on through the shielding structure in fusion reactors. Some of them consist of dogleg ducts, through which neutrons stream in a complex manner. An experiment was conducted with the Fusion Neutronics Source (FNS) facility at JAERI to study the behaviour of neutrons in the duct and assess the reliability of calculation methods for the design of fusion reactors such as ITER. The assembly was an iron slab 180 cm in thickness with a doubly bent duct 30 × 30 cm² in cross section. The experiment was analysed using a simple design code for radiation streaming, DUCT-III, and the Monte Carlo code MCNP. The results indicate that the simple design code is reliable enough to be used for shielding design analyses as well as the Monte Carlo method, which showed excellent agreement between calculated and measured values. [ABSTRACT FROM AUTHOR]

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 crystal growth is indispensable. [ABSTRACT FROM AUTHOR]

A neutron monitor using micro-fission chambers to be installed inside the vacuum vessel has been designed for the International Thermonuclear Experimental Reactor (ITER). The monitoring system needs to be insensitive to the changes of the plasma position and the profile, and the locations behind upper and lower outboard blankets were selected as appropriate based on the neutron transport calculations with the Monte Carlo code for neutron and photon transport (MCNP). Employing both pulse counting and Campbelling modes in the electronics, the ITER requirement of 10[sup 7] dynamic range with 1 ms temporal resolution will be accomplished. The system meets the 10% accuracy required for the fusion power monitor. A set of a [sup 235]U micro-fission chamber with 12 mg UO[sub 2] and a fissile-material-free "blank" detector to eliminate noise issues arising from γ rays, etc. were fabricated based on the design. The vacuum leak rate of the chamber with the mineral insulated (MI) cable, resistances between the central conductor and outer sheath, and mechanical strength up to 50 G acceleration were tested to meet the design criteria. The output signals for y rays were measured with the [sup 60]Co γ-ray irradiation facility at Japan Atomic Energy Research Institute (JAERI)-Takasaki and the influence was estimated to be less than 0.1% of the signals for neutrons. Excellent linearities between count rates, square of Campbelling voltage, and neutron fluxes were confirmed in the temperature range from 20 °C (room) to 250 °C with the Fusion Neutronics Source (FNS) facility of JAERI. The influence of the surrounding material was studied with the shielding blanket mock-up, and it was verified that the chamber provides an effective response although the sensitivity was enhanced by slow-downed neutrons. As a result, it was concluded that the present micro-fission chamber is applicable to ITER power monitoring. [ABSTRACT FROM AUTHOR]

99Mo was produced by the 100Mo(n, 2n) 99Mo reaction using 14-MeV neutrons from the D(³H, n)4 He reaction at Fusion Neutronics Source Facility (FNS) at JAEA. A target of titanium oxide gel with molybdenum oxide was irradiated and used as the 99mTc/99Mo generator. The growth curve of 99mTc in the titanium gel target and the elution curve of 99mTc from the 99Mo/99mTc generator were obtained. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

Abstract: Thin Gap Chambers (TGCs) will be used as a muon trigger device in the end-cap regions of the ATLAS detector. The detector will be placed in an experimental cavern approximately 100m below the ground surface. A large amount of neutron background is expected in the cavern. Recoiled nuclei, mostly protons, emerging from the neutron interaction provide a much larger energy deposit in the gas volume than that of the minimum ionizing particle (MIP). This might cause aging and improper operation of the TGC. In order to investigate such influences, we performed a test on the operation stability of a real-use TGC under a much higher neutron flux environment than that expected in the ATLAS cavern. This paper describes the experimental results of the irradiation test using 14-MeV neutrons. The maximum flux was , which was effectively approximately nine times higher than that at the worst TGC location. It was proved that the TGC was immune to such a flux and instantaneous large signals originating from the neutrons. The mean output charge from the recoil proton at the nominal operation voltage was found to be approximately 30 times larger than that of MIP. The total fluence was , up to which the TGC operated stably and no detector deterioration was observed. This corresponds to approximately nine months of ATLAS operation at the nominal luminosity of . [Copyright &y& Elsevier]

Abstract: Double-differential charged-particle emission cross section (DDXc) is a fundamental value to estimate nuclear heating and material damages in fusion reactor development. In the present study, measurements of DDXc for several light elements have been carried out with a newly developed spectrometer. In the spectrometer, a good signal-to-noise ratio and very low minimum measurable energy were realized by using a pencil-beam DT neutron source of FNS in JAERI and a counter-telescope system with very thin silicon surface barrier detectors of appropriate thicknesses. The detailed DDXc data in a wide emission energy range were obtained for 27Al, 9Be, natC and 19F. For 27Al(n,xα) reactions, the measured total cross section agreed well with the evaluated data. For 9Be(n,xα) reactions, there were differences in the α-particle emission DDX between measurement and the evaluated data in the detail of energy structure. For natC(n,xα) reaction, a large discrepancy between the present result and the previous experimental data appeared. For 19F(n, charged-particle) reactions, some peaks corresponding to excited states of the residual nuclei were observed in the energy spectrum of emitted proton, deuteron, and triton particles. [Copyright &y& Elsevier]

Abstract: V–4Cr–4Ti, F82H, Ni and Cu were irradiated with fission and fusion neutrons at room temperature and 473K. Defect structures were analyzed and compared using positron annihilation lifetime measurement, and microstructural evolution was discussed. The mean lifetime of positrons (the total amount of residual defects) increased with the irradiation dose. The effect of cascade impact was detected in Ni at room temperature. The size and the number of vacancy clusters were not affected by the displacement rate in the fission neutron irradiation at 473K for the metals studied. The vacancy clusters were not formed in V–4Cr–4Ti irradiated at 473K in the range of 10−6−10−3 dpa. In F82H irradiated at 473K, the defect evolution was prevented by pre-existing defects. The mean lifetime of positrons in fission neutron irradiation was longer than that in fusion neutron irradiation in V–4Cr–4Ti at 473K. It was interpreted that more closely situated subcascades were formed in the fusion neutron irradiation and subcascades interacted with each other, and consequently the vacancy clusters did not grow larger. [Copyright &y& Elsevier]