Neutron Scattering

whereby E and v,, denote energy and speed ofthe backscattered neutrons and vD die velocity
of the Doppler drive . Moving a crystal - set to neutrons with ~ z 6.3- 10-1° m by using the
Si(111) reflection - with a velocity amplitude of 2 .5 m/s results in an energy range of
f 15 p eV .
Supermirror
Guide
Deflector
Monitor
Chopper
Sample
Detectors
Doppler
Monochromator Drive
Neutron Guide
Fig.
17) layout of the backscattering spectrometer in Jülich
Fig. 17) now displays the realisation ofthe above considerations by means of an experimental
facility . Not unexpectedly, this set up is quite different from that of triple axis spectrometer .
The backscattered and Doppler shifted neutrons have to be deflectod by a second crystal off
from the neutron guide towards the Sample position . This so called deflector is positioned next
to the neutron guide which entails a slight deviation from perfect backscattering and thus the
optimum energy resolution . Exact backscattering has been attempted in the first experimental
set up by placing a deflector covering 1/10 11' of the beam size inside the neutron guide . Despite
the somewhat better resolution this option results in too a low flux for most applications . After
deflection, the neutrons travel through a conical shaped, supermhror coated neutron guide
which focuses the beam onto the Sample. A chopper wich about 50% dead time interrupts the
continuous beam and triggers the gate of the counters . Thereby one avoids counting of those
neutrons being scattered directly from the Sample into the nearby counters, whereas neutrons
having passed the analyzers are detected according to the Doppler velocity . The analyzers
(elastically curved Si single crystals) are arranged at a fixed radius around the Sample and focus
the backscattered neutrons on the associated detectors . Note, that the energy resolution of
the backscattering instrument also depends on the flight path. It increases wich increasing
flight path since the detectors have a finite volume which means slightly different detection
times. The accuracy of the counting electronic may thereby be considered as perfect .
9- 15