Schematic overview of the ACAR spectrometer. (a) Total view of the spectrometer, including detectors. The baseline of the Anger cameras is 16.5 m in order to achieve the desired angular resolution. (b) Cut-view of the central assembly. The cold finger of the cryostat is fed into the sample chamber from below. (c) Detailed cut-view of the source-sample chamber with the custom-built pole piece assembly. Sample and source are positioned symmetrical with respect to the pole pieces.
Magnetic field configuration. FEM simulation of the magnetic flux density inside the sample chamber. Between the two pole pieces a region with a homogeneous field is created. Sample and source are placed inside this region for optimal positron transport.
Magnetic field configuration. Measured magnetic field in sample plane. The sample position is located on the plateau in the center. The flux density decreases rapidly outside the pole piece configuration.
Dependence of the magnetic flux density at the sample position on the electric current through the coils. At a current of I coil ≈ 45 A the soft iron is beginning to saturate.
Calculated positron momentum distribution on the sample, obtained by a Monte-Carlo simulation. For high flux densities almost the full spectrum is mapped onto the sample.
Measured beta efficiency in comparison with the expectation derived from the simulation. The measured values exceed the expectation indicating that positrons which are emitted into the upper half-space are effectively reflected downwards onto the sample by the Ta reflector.
Dependence of the spot diameter on the magnetic flux density at the sample position for two arbitrary orthogonal axis.
Contour plot of the ACAR spectrum of single crystalline α-quartz.
Horizontal cut through the ACAR spectrum along p y = 0.
Vertical cut through the ACAR spectrum along p x = 0.
Article metrics loading...
Full text loading...