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The ANTARES facility for neutron imaging
Author: Burkhard Schillinger
The ANTARES facility is situated at beam line SR4b, facing the cold source. It is equipped for various forms of neutron and X-ray radiography and computed tomography. Ii consists of a collimator, selector wheel, filter wheel, evacuated flight tube, beam size limiter, block house, sample manipulator and various detection systems. The ANTARES facility has two selectable collimators with collimation ratios L/D=400 and L/D=800, with neutron fluxes of 1*108 n/cm2s and 2.6 *107 n/cm2s. The maximum beam cross section at the sample position is 40 cm x 40 cm. It can be reduced to the actual sample size by variable beam size limiters.
The sample manipulator is designed to carry samples up to 500 kg of weight and 1m of size, fitted to carry a car engine. Samples can be rotated, translated and lifted. Smaller samples are put onto a smaller rotation table close to the detector.
The standard detection system is a 2048 x 2048 pixel cooled CCD camera. It is used in combination with a mirror, a neutron or a X-ray scintillation screen, the sample manipulator and turntable to perform neutron radiography or neutron computed tomography.
Typical exposure times are in the range of 1-20 seconds, depending on the adjusted projection ratio and scintillator size. The second detection system is a intensified gate-able 1024 x 1024 pixel cooled CCD camera, which can be used for low-level or stroboscopic imaging on repetitive motions. Gating times can be in the microsecond range, on-chip accumulation is possible.
A combination of a standard video camera and night vision scope allows for the examination of slow motions at video rate (25 fps.) A Fuji BAS 5000 Image Plate reader with imagig plates allows for high-resolution neutron radiography with 25 micron pixel size on an area of 25 cm x 20 cm.
A 300 kV X-ray tube can be shifted into the beam path when the neutron shutter is closed. The tube is mounted at the beginning of the flight tube and produces exactly the same beam geometry as the neutron collimator. Images recorded with the X-ray tube and scintillator can be immediately overlaid to neutron images without further registration and adjustment.
At the beginning of the flight tube, a selector wheel allows for insertion of several smaller diaphagms to increase the collimation and lateral coherence of the beam. With the smallest, 2 mm opening, phase contrast and diffraction enhanced imaging can be performed for edge enhancement of foams or detection of pure phase-shifting objects.
The selector wheel contains a position with a Cadmium sheet of 2mm thickness. With this filter in the beam, all thermal and cold neutrons with energies below 0.4 eV are cut off, and radiography can be performed with the remaining epithermal part of the spectrum, giving better penetration for large samples.
Resolution in neutron imaging is limited by the scintillation screen, as the reaction products of the detection reaction have a certain range. Typical resolution is in the order of 100-200 microns. Neutron imaging can penetrate most metals while being sensitive to even small amounts of hydrogen. Typical maximum penetration lengths are: Aluminium 20 cm, steel 3-5 cm, water 0.5-1 cm, plastics 0.5-2 cm