A new method to quantify multiple elements by pulsed epithermal neutron transmission spectroscopy

To perform quantitative analysis of constituent elements, including light atoms, by time-of-flight (TOF) type neutron imaging, a new analysis method using epithermal neutrons excluding resonance dips has been proposed. The method is based on fitting the theoretical neutron transmission spectrum calculated from evaluated nuclear data to a measured neutron transmission spectrum. Compared to conventional imaging methods such as those using neutron radiography or neutron resonance analysis, it has the advantage of being able to evaluate multiple/light elements. We applied this technique to a pulsed neutron transmission experiment at the Hokkaido University Neutron Source. A stack of iron and carbon plates was measured as a model sample consisting of heavy and light elements. The evaluated elemental densities show reasonable agreements within an error of 10%

Neutron spectrum change with thermal moderator temperature in a compact electron accelerator-driven neutron source and its effects on spectroscopic neutron transmission imaging

Recently, Compact Accelerator-driven Neutron Sources (CANSs) are attracting attention. In CANSs, a simple thermal neutron moderator such as polyethylene is often used from the viewpoints of cost, simplicity and maintainability. In most cases, the temperature of such a moderator has not been controlled although it is natural that the moderator temperature and the neutron spectrum will change with accelerator-operation. Thus, we simultaneously measured neutron spectra and the temperature of a polyethylene moderator at the Hokkaido University Neutron Source (HUNS) driven by a compact electron accelerator to observe the effect of any temperature change on the reliability of spectroscopic transmission measurement. The ratio of the neutron effective temperature and the moderator temperature was constant in HUNS case, although both increased by 4-5 K within one hour after the start of accelerator-operation. This indicated that the neutron effective temperature was well estimated by the moderator temperature. The effect of the temperature change can be easily avoided by excluding data collection before the moderator warms up. These results suggested that the monitoring of moderator temperature is recommended in compact electron accelerator-driven neutron sources with a thermal neutron moderator to guarantee reliability of spectroscopic transmission measurement without sacrifices of cost, simplicity and maintainability

High wavelength-resolution Bragg-edge/dip transmission imaging instrument with a supermirror guide-tube coupled to a decoupled thermal-neutron moderator at Hokkaido University Neutron Source

Bragg-edge neutron transmission imaging is one of several useful material characterization tools available at a compact-acceralator driven pulsed-neutron source (a pulsed CANS). Quantitative imaging experiments for crystalline phase, crystallographic texture, and crystallite size have been successfully performed at a pulsed CANS using a coupled (high intensity type) cold-neutron moderator. However, imaging experiments for strain and grain orientation have not been achieved due to the low wavelength-resolution of the coupled moderator. In this study, we demonstrated that both strain imaging using the Bragg-edge transmission method and grain-orientation imaging using the Bragg-dip transmission method are feasible at a pulsed CANS; both types of imaging are made possible with an efficient neutron beam transport system using a supermirror guide-tube combined with a decoupled thermal-neutron moderator (300 K polyethylene), which can supply short neutron pulse. Using this system, we achieved high wavelength-resolution (about 0.5%) Bragg-edge/dip neutron transmission imaging experiments, which correctly visualized the strain values and grain orientations in several polycrystalline materials. On the other hand, it was also found that the neutron flux and the neutron beam angular divergence (L/D) were insufficient with this approach. For this reason, we performed Monte-Carlo simulation studies to investigate a new geometry of moderator system which achieves not only high wavelength-resolution (short pulse width) but also high neutron brightness which is necessary for a high L/D experiment. The simulation results suggest that the most promising candidates use a thin and low-height cold-neutron moderator (20 K methane) with decoupled pre-moderators or poisoned pre-moderators with large solid-angle coverage for fast neutrons emitted from a neutron production target. This system offers higher peak intensity than a coupled moderator for cold neutrons emitted from the highest brightness region on the moderator surface, while achieving narrow pulse widths and decay times as fast as those of decoupled/poisoned moderators

Bragg-edge neutron transmission spectrum analysis using a high-speed-camera-type time-of-flight neutron imaging detector

Thus far, quantitative imaging of crystallographic information using a time-of-flight (TOF) neutron Bragg-edge transmission method has been performed using counting-type neutron TOF-imaging detectors. However, at intense pulsed neutron beam facilities, the limit of the maximum counting rate of the detectors restricts acceptable neutron intensity. A camera-type neutron imaging detector can accept considerably higher neutron intensity than counting-type detectors. For this reason, a camera-type detector applicable for the TOF measurement has been developed. However, the camera-type detector has not been applied to quantitative analysis of crystallographic information thus far. As the neutron spectrum data obtained by a camera-type detector may have different characteristics compared with those obtained by a counting-type detector, it is important to experimentally demonstrate the applicability of the camera-type detector for quantitative crystallographic analysis. Thus, in this study, we performed a demonstration experiment using a steel knife specimen at a beam-line connected to a coupled-type neutron moderator of the Hokkaido University Neutron Source (HUNS). We applied the Rietveld-type data analysis method to measured Bragg-edge neutron transmission spectra in order to obtain quantitative crystallographic information, and we then conducted the crystalline phase imaging. Finally, using the new detector system along with the spectral analysis, the results were obtained non-destructively; the results showed that the crystalline phase distribution of the steel knife composed of two phases was changed gradually, and the crystallographic texture and crystallite size distributions were almost uniform. In addition, the new detector system provided the best spatial resolution of 520 mu m at a field-of-view of 13 cm x 13 cm