The Chopper parameters

By "chopper parameters" we mean the wavelength, the master speed, the frame overlap speed ratio denominator, the resolution mode, and the histogram delay time. These parameters determine the speeds, phases and pickup pulse delays for the seven choppers.

We usually talk of selecting or setting the wavelength as a shorthand for the chopper parameters: this usage may be adopted in what follows.

In this section we shall discuss the meaning of the chopper parameters. (For more detailed explanations please talk to your local contact). In later sections we shall offer advice on how to choose the chopper parameters and we'll describe how to implement these choices ( Deciding the choppers parameters .

The wavelength is self-evident. A neutron with wavelength (in ) has an energy E (in meV) given by . Furthermore its speed v (in mm/s) is approximately 4/. Since the DCS sample-detector distance is about 4 m, a neutron with wavelength takes a time T=1000 (s) to travel that distance. Click here for a conversion table.

In normal operation six of the seven choppers run at the so-called master speed _o. In principle _o can take any value from 1200 to 20,000 rpm but in practice we almost always operate with a master speed of 20,000 rpm. The remaining chopper (called the "frame overlap chopper" (chopper 5 - choppers being numbered from North to South, i.e. chopper 1 being closest to the sample) is operated at the same speed _FO = _o or else at a lower speed, either _FO = _o/m or _FO = _o(m-1)/m, where the frame overlap speed ratio denominator m is a small integer greater than 1. The corresponding frequency of pulses at the sample, _s, is _o if _FO = _o, otherwise _o/m if _FO = _o/m or _FO = _o(m-1)/m.
The period of a chopper turning at 20,000 rpm (333.333 Hz) is 3000 s.

Each of the pulsing and monochromating disks has three slots of differing widths. By suitably phasing the choppers we can choose among three distinct intensity/resolution conditions without having to change the wavelength or the speed of the choppers. These three conditions, or modes, are known as "low resolution", "medium resolution" and "high resolution".

The histogram delay time, generally called "tsdmin" (minimum time from sample to detector), is the time of flight from sample to detector corresponding to the start of the first histogram time channel. Its significance should become apparent in the following discussion ( Deciding the chopper parameters ).

Go to Deciding the chopper parameters | Go to Setting the choppers parameters

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Last modified 07-March-2006 by website owner: NCNR (attn: John Copley)