2.3 GEM Choppers
GEM has two disc choppers and one nimonic chopper, located at the following distances from the moderator:
| Chopper: | disc | nimonic | disc |
| Distance(m): | 6.5 | 9.45 | 9.75 |
| Standard phase setting (microseconds): | 19302 | 19393 | |
| Standard angle setting (degrees): | 112 |
The nimonic chopper is used to stop the prompt pulse of fast neutrons which is produced shortly after the proton pulse hits the ISIS target. These neutrons can moderate within the sample environment equipment and the beamline shielding etcetera, giving rise to a high background at later times.
The disc choppers are used to prevent frame-overlap problems by limiting the wavelength range of the neutrons which arrive at the sample.
With the standard setting of the disc choppers (i.e. entirely within the first frame) and the nimonic chopper turned off, the wavelength range of GEM is 0.1 - 3.5 Å.
The nimonic chopper is usually turned off for experiments on disordered materials which need to access high Q-values.
Click here for a schematic of the GEM beamline.
Last updated on 25 Feb 2003 by Alex Hannon (a.c.hannon@rl.ac.uk)
