## 1 Introduction  
The Micro-Computed Tomography (MCT) Beamline is a 1.3 T bending-magnet-based beamline at the third generation 3 GeV (200 mA) Australian Synchrotron that is operated by the Australian Nuclear Science and Technology Organisation (ANSTO). It is the first of the eight new beamlines being constructed as part of the BRIGHT project and can produce nearly parallel polychromatic and monochromatic X-ray beams with energies ranging from 8 to 40 keV through the use of a double multilayer monochromator (DMM). The MCT user experimental program will encompass widespread applications from material to life sciences. In most cases, the experiments are performed under ambient conditions. However, for some material-science applications in the near future, the beamline will incorporate specialized sample stages for heating, cooling, or performing in-liquid experiments. In situ loading during measurements can also be subjected to the sample, including tensile stress, compression, etc. Several imaging modalities are offered at MCT for both 2D and/or 3D imaging, including propagation-based phase-contrast techniques [1; 2; 3; 4] at the current stage of development and, in the future, grating-based techniques [5; 6], speckle-based techniques [7; 8; 9], dark field imaging [10], and nano-CT imaging [11]. The choice of technique depends mainly on the specific application and science drivers. Dynamic or high-speed imaging (as a function of time) [12; 13; 14] will also be available due to the comparatively high X-ray flux at MCT relative to laboratory-based sources. There are three hutches at the MCT beamline. Hutch A is the beam-conditioning hutch and houses the photon delivery system (PDS), including key X-ray optical elements. The experiment hutches are B and C, in order of increasing distance from the source, with a maximum distance of over 34 m from the source at the end of hutch C. There is a removable beam stop at the end of hutch B. This allows access to hutch C while an experiment is being conducted inside hutch B. The beam size in the sample plane inside Hutch B (22 m from the source) is 44 mm (horizontally) \(\times\) 6.6 mm (vertically), as defined by the mask in the PDS. In hutch C (32 m from the source), the beam size is 64 mm (horizontally) \(\times\) 9.6 mm (vertically). The PDS in hutch A will deliver nearly parallel X-rays. Four different beam modes will be available, i.e., white beam, pink beam, and monochromatic beam with and without harmonic suppression. A schematic spectrum of the four-beam modes is shown in Figure 1. The white beam (as shown in Figure 1a) is a polychromatic unfiltered beam directly from the first bending magnet source in sector 9 of the Australian Synchrotron storage ring. The pink beam (as shown in Figure 1b) is achieved by cutting off the high energy end of the white beam using a Vertical Bounce Mirror (VBM). A double-multilayer monochromator (DMM) is used to produce a monochromatic beam with the energy bandpass of 0.5% or 3% depending on the DMM stripe used. A monochromatic beam is schematically shown in Figure 1c with the presence of a small contribution from the third harmonic. The small, undesired third harmonic component from the monochromatic beam can be removed by using the VBM if necessary, as shown in Figure 1d. The beamline layout for MCT is shown in Figure 2. There are four vacuum sections in PDS (in Hutch A). PDS vacuum section #1 consists of mask, white beam slit A, and filters with five paddles. These filters are useful to harden the beam or remove the low energy parts. The DMM is in PDS vacuum section #2. The substrate material of the DMM is single-crystal silicon. There are three stripes in the DMM with the coating material of Ru/C (for 8-23 keV, 3% bandpass), V/B\({}_{4}\)C (for 8-25 keV, 0.5% bandpass), and W/B\({}_{4}\)C (21-40 keV, 3% bandpass). The first multilayer of the DMM is at a fixed location of 13 m from the source while the second multilayer can be moved up-stream/down-stream during the operation.