Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

The Complex, Unique, and Powerful Imaging Instrument for Dynamics (CUPI2D) at the Spallation Neutron Source (invited)

The Oak Ridge National Laboratory is planning to build the Second Target Station (STS) at the Spallation Neutron Source (SNS). STS will host a suite of novel instruments that complement the First Target Station's beamline capabilities by offering an increased flux for cold neutrons and a broader wavelength bandwidth. A novel neutron imaging beamline, named the Complex, Unique, and Powerful Imaging Instrument for Dynamics (CUPI2D), is among the first eight instruments that will be commissioned at STS as part of the construction project. CUPI2D is designed for a broad range of neutron imaging scientific applications, such as energy storage and conversion (batteries and fuel cells), materials science and engineering (additive manufacturing, superalloys, and archaeometry), nuclear materials (novel cladding materials, nuclear fuel, and moderators), cementitious materials, biology/medical/dental applications (regenerative medicine and cancer), and life sciences (plant-soil interactions and nutrient dynamics). The innovation of this instrument lies in the utilization of a high flux of wavelength-separated cold neutrons to perform real time in situ neutron grating interferometry and Bragg edge imaging-with a wavelength resolution of δλ/λ ≈ 0.3%-simultaneously when required, across a broad range of length and time scales. This manuscript briefly describes the science enabled at CUPI2D based on its unique capabilities. The preliminary beamline performance, a design concept, and future development requirements are also presented.

Short-Pulse Laser-Driven Moderated Neutron Source

Neutron production with laser-driven neutron sources was demonstrated. We outline the basics of laser-driven neutron sources, highlight some fundamental advantages, and quantitatively compare the neutron production at the TRIDENT laser sources with the well-established LANSCE pulsed neutron spallation source. Ongoing efforts by our team to continue development of these sources, in particular the LANSCE-ina-box instrument, are described. The promise of ultra-intense lasers as drivers for brilliant, compact, and highly efficient particle accelerators portends driving next-generation neutron sources, potentially replacing in some cases much larger conventional accelerators.

Investigation of dissimilar metal welds by energy-resolved neutron imaging

A nondestructive study of the internal structure and compositional gradient of dissimilar metal-alloy welds through energy-resolved neutron imaging is described in this paper. The ability of neutrons to penetrate thick metal objects (up to several cm) provides a unique possibility to examine samples which are opaque to other conventional techniques. The presence of Bragg edges in the measured neutron transmission spectra can be used to characterize the internal residual strain within the samples and some microstructural features, e.g. texture within the grains, while neutron resonance absorption provides the possibility to map the degree of uniformity in mixing of the participating alloys and intermetallic formation within the welds. In addition, voids and other defects can be revealed by the variation of neutron attenuation across the samples. This paper demonstrates the potential of neutron energy-resolved imaging to measure all these characteristics simultaneously in a single experiment with sub-mm spatial resolution. Two dissimilar alloy welds are used in this study: Al autogenously laser welded to steel, and Ti gas metal arc welded (GMAW) to stainless steel using Cu as a filler alloy. The cold metal transfer variant of the GMAW process was used in joining the Ti to the stainless steel in order to minimize the heat input. The distributions of the lattice parameter and texture variation in these welds as well as the presence of voids and defects in the melt region are mapped across the welds. The depth of the thermal front in the Al-steel weld is clearly resolved and could be used to optimize the welding process. A highly textured structure is revealed in the Ti to stainless steel joint where copper was used as a filler wire. The limited diffusion of Ti into the weld region is also verified by the resonance absorption.

Flexible sample environment for high resolution neutron imaging at high temperatures in controlled atmosphere

High material penetration by neutrons allows for experiments using sophisticated sample environments providing complex conditions. Thus, neutron imaging holds potential for performing in situ nondestructive measurements on large samples or even full technological systems, which are not possible with any other technique. This paper presents a new sample environment for in situ high resolution neutron imaging experiments at temperatures from room temperature up to 1100 °C and/or using controllable flow of reactive atmospheres. The design also offers the possibility to directly combine imaging with diffraction measurements. Design, special features, and specification of the furnace are described. In addition, examples of experiments successfully performed at various neutron facilities with the furnace, as well as examples of possible applications are presented. This covers a broad field of research from fundamental to technological investigations of various types of materials and components.

High-Resolution Strain Mapping Through Time-of-Flight Neutron Transmission Diffraction

The spatial resolution of time of flight neutron transmission diffraction was recently improved by the extension of photon/electron counting technology to imaging of thermal and cold neutrons. The development of novel neutron sensitive microchannel plates enables neutron counting with spatial resolution of ~55 um and time-of-flight accuracy of ~1 us, with efficiency as high as 70% for cold and ~40% for thermal neutrons. The combination of such a high resolution detector with a pulsed collimated neuron beam provides the opportunity to obtain a 2-dimensional map of neutron transmission spectra in one measurement. The results of our neuron transmission measurements demonstrate that maps of strains integrated along the beam propagation direction can be obtained with ~100 microstrain accuracy and spatial resolution of ~100 um providing there are sufficient neutron events collected. In this paper we describe the capabilities of the MCP neutron counting detectors and present the experimental results of 2-dimensional strain maps within austenitic steel compact tension (CT) crack samples measured at the ENGIN-X beamline of the ISIS pulsed neutron source.