POLDI: Pulse Overlap time-of-flight Diffractometer





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Instrument description - collimators

Figure 1. Scheme of the scattering geometry with the gauge volume.

The collimator together with diaphragm 2 defines the sampled gauge volume. This is demonstrated in figure 1. Diaphragm 2 determines the gauge volume in two directions. It is mounted at an optical bench. Due to the horizontal and vertical beam divergence (1) it is recommended to minimize the distance between slits and sample. If the diaphragm 2 cannot be fixed close to the sample, the vertical divergence can be reduced by mounting of an additional slit or small collimator between neutron guide and monitor, or an additional slit after the diaphragm.

The third dimension of the gauge volume is determined by the radial collimator. An overview of the available collimators at POLDI is given in the table below. The collimators with an acceptance of 1.5 mm and 3.8 mm are permanently installed at the collimator holder (see figure 2), whereas an 0.6 mm collimator can be installed at the front side of the collimator holder (see figure 3). All collimators have an angular range of 30. The transmission functions of these three collimators are close to a triangle shape, as demonstrated in figure 4. The collimators have a quite large distance to the gauge volume. This allows, in combination with the well collimated primary beam to investigate also large samples and keeping still a good spatial resolution.

Table 1. Properties of the POLDI radial collimators.
Collimator FWHM (mm) Distance to specimen (mm) Transmission (%)
1 3.8 410 78
2 1.5 230 74
3 0.6 70 72

Figure 2. Collimators with 1.5mm (top) and 3.8mm (bottom) spatial resolution.

Figure 3. Collimators with 0.6mm spatial resolution.

Figure 4. Transmission functions for the three radial collimators.