POLDI: Pulse Overlap time-of-flight Diffractometer

 

 

 

 

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The POLDI instrument - performance


Figure 1. FWHM as function of the scattering vector Q for several settings of the chopper speed and the width of the diaphragm D1.

POLDI shows the best performance
The advantages of POLDI cannot be fully used

Resolution

The neutron intensity after the chopper depends only on the ratio between duty slits and "black" rest of the chopper ring in beam position and not on the chopper speed. On the other hand the resolution of POLDI depends on the geometrical resolution and on the time resolution. The time resolution depends linearly on the chopper speed. As higher the chopper speed as smaller the instrumental line broadening related to the time. The geometric resolution of POLDI depends on the horizontal divergence of the primary beam. With the diaphragm D1 placed 3m in front of the sample (see also instrument description) the beam divergence can be influenced. This is demonstrated in figure 1.

Example

Figure 2 displays the diffraction spectrum of Al2O3, measured with a chopper speed of 15000 rpm and 6 mm width of diaphragm D1. The total investigated sample volume was about 100 mm3. The complete spectrum contains 26000 data points. Click on the diffraction pattern to get an enlarged view. Notice that the double peak at Q= 5.08 1/ (d=1.23) can be very well resolved.

Figure 2. Diffraction spectrum of Al2O3.