New data processing method for nuclear material measurement using pulsed neutrons.

Active neutron methods using pulsed neutrons are used to detect the presence of nuclear materials. In this study, we investigated a new data processing method and compared its performance to a conventional method. It is shown that the new method offers a significant reduction in measurement time and an increased sensitivity.

Effect of sample density in prompt γ-ray analysis.

A high-accuracy analytical method is broadly required to obtain reliable research results. Thus, prompt γ-ray analysis (PGA), one of the most accurate non-destructive analytical methods, has been employed in various fields. However, the measurement accuracy of PGA is also known to degrade in hydrogenous samples. The degradation is caused by variation in the measurement sensitivity (counts per milligram) following the change in neutron energy due to scattering with hydrogen nucleus. Number of scatterings is well known to depend on the hydrogen content in a sample. However, considering multiple scatterings, hydrogen density, which has not been taken into account as yet, may also lead to the accuracy degradation. Here, we show the effect of the hydrogen density in PGA by evaluating the measurement sensitivity of samples with the same hydrogen content and different densities. We find that the measurement sensitivity varies by more than 30% depending on the hydrogen density even at the same hydrogen content. The variation is a particularly serious problem for PGA requiring a few percent accuracy in most cases. Additionally, although the variation is apparently observed in hydrogenous samples, the similar phenomenon can occur in other nuclides with a large scattering cross section; it may affect nuclear cross-section measurements using neutrons in such fields as astrophysics and nuclear energy.

Nondestructive Quantitative Analysis of Difficult-to-Measure Radionuclides 107Pd and 99Tc.

Considering the expanding demand for nuclear waste management of the spent nuclear fuel materials in near future, a nondestructive analytical scheme applicable to one of the most difficult-to-measure nuclides 107Pd, which emits no decay γ-rays and whose half-life is too long to be decayed out during a human lifetime, was designed. The scheme consists of a sophisticated instrument capable of the detection of γ-rays by Ge detectors coupled with time-of-flight measurement of neutrons and a high-intensity pulsed neutron beam and can simultaneously perform time-of-flight-coupled prompt γ-ray analysis (TOF-PGA) as well as PGA and neutron resonance capture analysis (NRCA). The analytical capability for simulated samples of the Tc-platinum group metals (Tc-PGMs) obtained by the group-partitioning process of spent nuclear fuels, which contain not only 107Pd but also 99Tc and other difficult-to-measure fission products, was evaluated. It was confirmed that although PGA and NRCA can accurately analyze both nuclides in individual, single substances, only TOF-PGA can analyze 107Pd as well as 99Tc in the Tc-PGM-simulated sample. The TOF-PGA measurement technique can be widely used for the nondestructive analysis of 107Pd and 99Tc in nuclear wastes.

Synergistic effect of combining two nondestructive analytical methods for multielemental analysis.

We developed a new analytical technique that combines prompt gamma-ray analysis (PGA) and time-of-flight elemental analysis (TOF) by using an intense pulsed neutron beam at the Japan Proton Accelerator Research Complex. It allows us to obtain the results from both methods at the same time. Moreover, it can be used to quantify elemental concentrations in the sample, to which neither of these methods can be applied independently, if a new analytical spectrum (TOF-PGA) is used. To assess the effectiveness of the developed method, a mixed sample of Ag, Au, Cd, Co, and Ta, and the Gibeon meteorite were analyzed. The analytical capabilities were compared based on the gamma-ray peak selectivity and signal-to-noise ratios. TOF-PGA method showed high merits, although the capability may differ based on the target and coexisting elements.

Deep-sea record of impact apparently unrelated to mass extinction in the Late Triassic.

The 34-million-year (My) interval of the Late Triassic is marked by the formation of several large impact structures on Earth. Late Triassic impact events have been considered a factor in biotic extinction events in the Late Triassic (e.g., end-Triassic extinction event), but this scenario remains controversial because of a lack of stratigraphic records of ejecta deposits. Here, we report evidence for an impact event (platinum group elements anomaly with nickel-rich magnetite and microspherules) from the middle Norian (Upper Triassic) deep-sea sediment in Japan. This includes anomalously high abundances of iridium, up to 41.5 parts per billion (ppb), in the ejecta deposit, which suggests that the iridium-enriched ejecta layers of the Late Triassic may be found on a global scale. The ejecta deposit is constrained by microfossils that suggest correlation with the 215.5-Mya, 100-km-wide Manicouagan impact crater in Canada. Our analysis of radiolarians shows no evidence of a mass extinction event across the impact event horizon, and no contemporaneous faunal turnover is seen in other marine planktons. However, such an event has been reported among marine faunas and terrestrial tetrapods and floras in North America. We, therefore, suggest that the Manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not within the pelagic marine realm.

Comparison of multiple prompt γ-ray analysis and prompt γ-ray analysis for the elemental analysis of geological and cosmochemical samples.

Multiple prompt γ-ray analysis (MPGA) and conventional neutron-induced prompt γ-ray analysis (PGA) are nondestructive analytical methods for bulk chemical compositions, and their analytical capabilities were compared for elemental analyses of geological and cosmochemical samples. Detection sensitivities of PGA are often restricted by poor signal-to-noise ratios and interferences from different origins. MPGA can substantially reduce the background level, especially for hydrogenous samples, relative to PGA, which opens up a possibility to use lower energy prompt γ-rays of some trace elements. Although it is one of the major constituent elements of rock samples, Mg is hard to be determined by PGA. With MPGA, Mg contents could be determined with reasonable consistency with their corresponding recommended values in geological and cosmochemical samples by carefully selecting suitable coincident prompt γ-ray energy pairs without interference correction. MPGA was applied to a hydrogenous meteorite, Ivuna, which contains H at 2% mass level. MPGA detection limits for most of the elements studied can be reduced up to 1 order of magnitude when compared with PGA detection limits under the present experimental conditions.

An automatic liquid-nitrogen filling system for multiple Ge detectors.

In order to reduce the time and effort of the daily task of refilling Dewar vessels of Ge detectors with liquid nitrogen (LN(2)), we have developed an automatic LN(2) filling system equipped with a LN(2) plant. With this system, we were freed from the work of LN(2) filling. Such an LN(2) filling system is useful for moderate-scale Ge detector arrays consisting of fewer than 20 Ge detectors.