POLDI: Pulse Overlap time-of-flight Diffractometer

 

 

 

 

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Instrument description - optics


Figure 1. Neutron mirror at POLDI.

The small chopper slit is imaged onto the sample position by an elliptical neutron mirror in order to minimize the loss of neutrons (see figure 1). The distance between chopper and sample is 11.8 m and the length of the neutron mirror is 6 m. The neutron mirror forms one side of the neutron guide. The mirror itself and the top and bottom plates have a super mirror coating with a maximum reflecting angle of 3 times that of natural Ni. The glass plates on the opposite site of the mirror are non-reflecting.


Figure 2 displays a 2D image of the intensity profile of the beam at the sample position. The beam profile is asymmetrical due to the cut off at the inner side of the neutron mirror.


The neutron wavelength distribution at the sample position is shown in figure 3. Note the sharp cut-off of the intensity at about 1.1 which is a property of the neutron mirror. The beam collimation can be adjusted with a diaphragm at the end of the mirror 2.9 m in front of the sample, the beam size and position can be selected with a second diaphragm close to the sample (see also instrument description).


Figure 2. The intensity profile of the beam at the sample position.

Figure 3. Neutron wavelength distribution at sample position.