Ultracold Neutron Production in a Pulsed Neutron Beam Line

We present the results of an Ultracold neutron (UCN) production experiment in a pulsed neutron beam line at the Los Alamos Neutron Scattering Center. The experimental apparatus allows for a comprehensive set of measurements of UCN production as a function of target temperature, incident neutron energy, target volume, and applied magnetic field. However, the low counting statistics of the UCN signal expected can be overwhelmed by the large background associated with the scattering of the primary cold neutron flux that is required for UCN production. We have developed a background subtraction technique that takes advantage of the very different time-of-flight profiles between the UCN and the cold neutrons, in the pulsed beam. Using the unique timing structure, we can reliably extract the UCN signal. Solid ortho-D$_2$ is used to calibrate UCN transmission through the apparatus, which is designed primarily for studies of UCN production in solid O$_2$. In addition to setting the overall detection efficiency in the apparatus, UCN production data using solid D$_2$ suggest that the UCN upscattering cross-section is smaller than previous estimates, indicating the deficiency of the incoherent approximation widely used to estimate inelastic cross-sections in the thermal and cold regimes

Crystalline phases involved in the hydration of calcium silicate-based cements: Semi-quantitative Rietveld X-ray diffraction analysis

Chemical comparisons of powder and hydrated forms of calcium silicate cements (CSCs) and calculation of alterations in tricalcium silicate (Ca3SiO5) calcium hydroxide (Ca(OH)2) are essential for understanding their hydration processes. This study aimed to evaluate and compare these changes in ProRoot MTA, Biodentine and CEM cement. Powder and hydrated forms of tooth coloured ProRoot MTA, Biodentine and CEM cement were subjected to X-ray diffraction (XRD) analysis with Rietveld refinement to semi-quantitatively identify and quantify the main phases involved in their hydration process. Data were reported descriptively. Reduction in Ca3SiO5 and formation of Ca(OH)2 were seen after the hydration of ProRoot MTA and Biodentine; however, in the case of CEM cement, no reduction of Ca3SiO5 and no formation of Ca(OH)2 were detected. The highest percentages of amorphous phases were seen in Biodentine samples. Ettringite was detected in the hydrated forms of ProRoot MTA and CEM cement but not in Biodentine

Fast Neutron Irradiated Magnetite and Haematite Investigated by Neutron Diffraction

In this paper, we present new experimental results about crystallographic and magnetic properties obtained on magnetite Fe3O4 and haematite α-Fe2O3 samples irradiated by fast neutrons. Polycrystalline rods were irradiated at 350K by fast neutrons inside the core of Melusine nuclear reactor (CENG) in Grenoble at various fluences (4 1019, 9.2 1019 and 20 1019 neutrons/cm2).The irradiated samples have been studied by the neutron diffraction technique. Contrary to the case of gamets ferrites, where amorphization is observed at such fluences, no similar effect is observed in magnetite : one just observes a very weak swelling. Results about haematite are also discussed

Order and disorder in solid oxygen: Neutron scattering investigations

Solid oxygen exists at zero pressure in three crystallographic phases, denoted α, β, γ. Various results have been obtained by neutron scattering in the two disordered β and у phases. Plastic γ-O₂ has a A15 structure with 8 molecules per unit cell. The molecules, located at 2α and 6c sites, exhibit two different types of disorder of molecular axes: a spherical-like disorder for the 6c molecules, and a disc-like disorder for the 6c molecules. A precise diffraction analysis on large single crystals shows that the disorder of molecular axis of the molecules can be interpreted in terms of localized forbidden orientations, or orientational holes, with a wide disorder among the other orientations. An important contribution of the translation-rotation coupling must be taken into account for the 6c molecules. Analysis of diffuse scattering by both an approximate analytical model and molecular dynamics simulation shows an important contribution to the orientational correlations between 6c-6c and 2a-6c molecules. Magnetic disorder can be investigated by neutron polarization analysis of the paramagnetic scattering. The 3D long range antiferromagnetic order of the α phase becomes a 2D short range helicoidal order in the β phase, with a very short 0.5 nm correlation range. In the γ phase, one observes, as in the liquid, a strong antiferromagnetic correlation, with a suggestion of a ID order along the linear chains of 6c molecules. In all the measured phases, the variation of the scattering intensity with scattering vector q cannot be reproduced at high q by the known magnetic form factor. This suggests the need for a better description of the л orbital and the theoretical magnetic form factor of the molecule

SANS study of structural changes in irradiated FeCu dilute alloys

Copper-rich precipitates formed in iron based dilute alloys by irradiation or thermal ageing, have been investigated using small angle neutron scattering (SANS). In particular, FeCu1.5% has been studied after either 2.5 MeV electron irradiations at 300°C or thermal ageing at 500°C. Size distribution, composition and number density of the clusters were deduced from SANS data. In addition, microhardness measurements allowed to correlate precipitates configuration with alloys hardening. The precipitation mechanism looks the same for both kinds of ageing, but irradiation induces an additional contribution to hardening. A linear correlation between duration of thermal ageing and irradiation fluence can be established

Early-age volume changes of extrudable reactive powder concrete

This article presents a study on the early-age autogenous deformations of Extrudable Reactive Powder Concretes (ERPCs), especially designed for the making of concrete pipes by extrusion. Different ERPC mixtures, with variable amounts of polycarboxylate superplasticizer (SP), have been investigated. Results on 28-day mechanical properties, early-age hydration rate, autogenous shrinkage and premature cracking risk are analyzed and discussed in relation with the ERPC mix parameters

Magnetic dynamics in condensed oxygen: recent experimental results

The spin dynamics in the condensed phases of ambient-pressure molecular oxygen (liquid, plastic-crystalline (γ-), magnetically disordered (β-) and fully ordered α-O2) iS investigated by means of the concurrent use of neutron scattering and muon-spin relaxation. Above the α → β transition the magnetic dynamics is governed by fast paramagnetic fluctuations whose spectrum is determined from the S(Q, ω) dynamic structure factors accessible from inelastic neutron scattering. Such information is shown to provide a key to understand the non-trivial temperature dependence of longitudinal relaxation rates found in muon spin relaxation measurements. Recent neutron scattering measurements performed within the magnetically ordered α-phase under high-resolution conditions reveal the presence of a low-energy excitation of magnetic origin, unnoticed in a previous polarized-neutron experiment, which corresponds to the continuation to longer wavevectors of the spin-wave mode detected in antiferromagnetic resonance (AFMR) experiments carried out by optical means.This research is supported by DGICYT (Spain) grant No. PB95-0075- C03-01. Work performed at ANL was supported by the U.S. Department of Energy, BES - Materials Sciences, under Contract No. W-31-109-ENG- 38.Peer Reviewe

Radioactive waste conditioning: the choice of the cement matrix versus irradiation

International audienceThe aim of this study is to compare the behavior of Portland and Ciment Fondu® cement under irradiation taking into account both real cement pastes and synthetic hydrates. First, γ-irradiations were performed using a 60 Co source (dose rate: 0.17-0.25 Gy.s-1, dose: up to 500×10 3 Gy). Thermogravimetric analysis measurements were performed in order to determine the amount and the type of water involved. H2 gas production was measured by gas chromatography. Regardless of the water to cement ratio (W/C) chosen (0.2, 0.4 and 0.6), it is shown that Ciment Fondu® pastes produce less H2 under irradiation. Moreover, the H2 production of portlandite and gibbsite, constituent hydrates of Portland and Ciment Fondu® cements respectively, shows that the amount of gas produced depends on the hydrate nature. Secondly, portlandite and gibbsite were electron-irradiated up to 300 MGy and 3 GGy using the LSI SIRIUS accelerator platform. X-Ray Diffraction analyses were performed before and after irradiation in order to investigate structural damage. Only limited structural disorder was observed, which confirms the good stability of both hydrates under present irradiation conditions