Beamline& Endstation
Beamline
XTM beamline located at BL1.2W between the other two beamlines (BL1.1W: MXT and BL1.3W: SAXS), takes the center of the fan beam radiation generated from the 2.2-Tesla multipole wiggler for the high photon intensity to benefit x-ray imaging. The XTM beamline is composed of 2 key optical elements: 1) Focusing mirror (FM); and 2) Double crystal monochromator (DCM). The x-ray beam is focused with a toroidal mirror located 8900 mm from the radiation source. Then, the x-ray beam is transferred to the DCM with a pair of Ge 111 crystals located at 33000 mm downstream. Between the focusing mirror and the DCM, there is the x-ray attenuator that opts a set of filters to attenuate low energy x-ray photon from the spectrum.
The XTM beamline functions in two distinct modes: 1) the polychromatic x-ray beam mode, and 2) the monochromatic x-ray beam mode. In polychromatic mode, also referred to as White beam mode, the x-ray beam bypasses the DCM, capturing a broad spectrum of x-ray photons for absorption-contrast imaging. This mode is advantageous for rapid data acquisition but requires consideration of potential heat-induced radiation damage to the sample. To mitigate this, an x-ray attenuator is often employed to eliminate low-energy x-rays that contribute to radiation dose without improving image quality. Conversely, in monochromatic mode, a set of Ge (111) crystals selectively diffracts the x-ray beam, yielding a monochromatic beam between 5 to 18 keV with a bandwidth of 0.1%. This mode is optimal for elemental-specific imaging and quantitative analysis, such as K-edge subtraction, offering significant benefits for examining organic or polymer samples. Efforts to enhance the monochromatic x-ray beam mode at the XTM beamline are in progress.
Endstation
The experimental station is designed using microtomography geometry principles. As the X-ray beam exits the transfer tube, it projects onto the rotating sample, and the X-ray images are captured by the detection system, as illustrated in the image. The sample's movement is controlled by a 6-axis goniometer, which allows rotation and translation along the X, Y, and Z axes. Mounted on the goniometer is a head that holds the sample; this can be moved in the X and Y axes within a ±5 mm range, enabling precise centering of the sample on the rotation axis. Following the sample, the detection system captures the X-ray images, consisting of a YAG-Ce scintillator, a lens-coupled microscope (Optique Peter, France), and a Andor NeO 5.5 scientific CMOS camera (2560 x 2160 pixels chipset). The detector system can be adjusted along the Y-axis to fine-tune the distance between the sample and detector from 0 to 100 cm using a sliding guide. With two observation cameras, the experiment can be monitored externally from the enclosure.
The operation of the XTM beamline and its experimental station adheres to the radioprotection principle of ALARA (As Low As Reasonably Achievable), as regulated by the Safety Section. To prevent radiation exposure to users, the XTM beamline operation is controlled by an interlock safety system.
Optical performance
The optical performance of the XTM beamline primarily depends on the objective lens-coupled microscope and the chipset of the Andor NeO 5.5 camera. The table lists the field of view (FOV) and the expected pixel size provided by the available objective lenses, which include 2X, 5X, 10X, and 20X.
