ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2023 20 correspond to better time resolution at the expense of neutron intensity. The cave flight tubes are filled with helium (He) to minimize neutron scattering in air and decrease the aluminum window thickness (another source of background due to gammas being generated by neutrons at the window) that is closest to the sample area. The VENUS cave is accessible during operating hours once the beamline shutter is in the closed option (i.e., no radiation going into the cave) and it is safe to enter the area when checking with a handheld radiation monitor. Due to the beam height, a 3 ft (~91.4 cm) high platform is being built around the sample table/detector area, as illustrated in Fig. 2. The platform is accessible from both sides to give maximum flexibility to setup sample environment and large size samples. The cave is equipped with a 2-ton crane used to lift heavy equipment to the sample position. The detector suite is also visible. Detectors are mounted on a rail system such that changing detector configuration is swift, thus greatly reducing time reconfiguring the instrument. Neutrons that have not interacted with the sample are attenuated by a beam stop placed downstream of the cave. Table 1 summarizes VENUS’ main capabilities, including spatial and wavelength resolutions, and the suite of detectors that can be divided in two categories: TOF (hyperspectral or wavelength-resolving) detectors (microchannel Timepix)[17], and the high spatial resolution detector with no TOF capabilities (ANDOR iKon-XL 230 and ZWO ASI6200MM). Moving forward, this article will focus on the TOF detectors and capabilities. Neutrons of different wavelengths/ energies can be measured using the TOF technique. TOF neutron imaging obeys the Lambert-Beer law, which states that for a homogeneous sample, the wavelength-dependent measured intensity, I, can be given by where x is the sample thickness (in cm), I0 is the incident intensity, and µ is the linear attenuation coefficient (in cm-1) given by where σT is the total (absorption and scattering) neutron cross-section, NA is Avogadro’s number, and ⍴ and M are the sample density and molar mass, respectively. A transmission pattern, I(λ) or I(E), can be measured on each detector pixel. The observed pattern depends on the neutron incident energy. The measured intensity in the radio- graphy is sensitive to the sample’s total cross section, which includes diffraction contrast (Bragg edges) and nuclear absorption (resonances), as illustrated in Fig. 3. These two techniques and their contribution to the field of materials science and engineering are detailed in the section titled “Examples of Nondestructive Measurements and Interpretation of Results.” ACCESSING THE VENUS FACILITY VENUS is installed at the SNS, one of the U.S. Department of Energy (DOE) scientific user facilities available to researchers and engineers from academia, industry, and national laboratories. Access to a user facility is commonly done through the general user program, an external peer-reviewed proposal system, which ranks research projects based on their scientific merit. Access is free of charge with the condition that the research performed at the facility is published, and thus available to the scientific and engineering community. Proposed work that is not publishable and/or requires rapid access can be submitted at any time. These proposals are reviewed internally, and some fees apply. For more information about the different modes of access, please visit neutrons.ornl.gov. EXAMPLES OF NONDESTRUCTIVE MEASUREMENTS AND INTERPRETATON OF RESULTS The past two decades have seen advances in pulsed neutron source fluences[18,19] and imaging detector technology, namely the microchannel plate (MCP) Timepix (TPX) detectors[17,20-22]. These efforts have led to the development of TOF imaging beamlines at major large-scale facilities across the world[23-29]. VENUS is optimized for two TOF/hyperspectral capabilities: Bragg edge and resonance imaging. Bragg’s law dictates that the neutron wavelength, λ, is given by where dhkl is the lattice spacing, <hkl> are the Miller indices of a family of lattice planes, and θ is the Bragg angle between the beam and the lattice planes. TABLE 1 — VENUS KEY CAPABILITIES AND EQUIPMENT Parameter Bell furnaces Source power and repetition rate 2 MW and 60 Hz Neutron wavelength Epithermal, thermal, and cold Source-to-detector distance 25 m Wavelength resolution ~0.15% Spatial resolution 50-100 µm Maximum single shot radiograph field-of-view 20 x 20 cm2 for thermal and cold neutrons 4 x 4 cm2 for epithermal neutrons Detectors ANDOR iKon-XL 230, ZWO ASI6200MM, Microchannel plate Timepix detector Sample stage 1 m3 open area 500 kg maximum weight capacity
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