POLDI: Pulse Overlap time-of-flight Diffractometer

 

 

 

 

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Instrument description - chopper


Figure 1. Chopper disc with 32 slits

The chopper is an essential part for the determination of the neutron energy (see also the section about the basic principle of POLDI). It is essentially a disc rotated at high speeds with some apertures through which neutrons may only pass during certain periods of the discs rotation. This produces a pulsed beam where the 'start time' for each neutron pulse is known.


Two disc choppers with a diameter of 700 mm are available at the POLDI facility. One chopper is used for standard applications. It has 32 slits resulting a 5.8% duty cycle. Eight pseudo-randomly distributed distances between the slits are repeated each quarter round (see figure 1). Additionally a one-slit chopper is available for applications where pulse overlapping has to be minimized or excluded. The entrance slit of the chopper and the slits in the chopper itself have a width of 4 mm and a height of 35 mm. The chopper speed can be set until 15000 rpm. Because of resonances some speed ranges are not valid.