Neutron Scattering

constant offset to the TOF . To be able to correct the TOF-histograms with respect to
the spectral distribution of the incoming pulse, it is important to detected the incoming
neutron flux by a monitor (Mo) located close to the sample position accumulated in a
separate TOF-histogram . This type of TOF-spectrometer is preferentially used at pulsed
sources, ideally it may be possible to omit the first chopper by taking the pulse as generated
by the pulsed source (spallation target) . e All neutrons from the pulse (flying
into the right direction) are utilized . This method has a few specific advantages and
disadvantages, advantages are :
1 . The pulse contains neutrons with high incident energy that may perform energy
loss scattering, the resolution at high energy loss is relatively good, especially if the
pulsed source supplies very short pulses ai high neutron energies .
2 . If the analyzers are used in near backscattering configuration high resolutions (comparable
to those on truc backscattering spectrometers, See chapter 9) can be attained
.
The utilization of the full spectrum has however also disadvantages :
1 . The full "white" pulse enters the shielded sample detector spacc . Any parasitic
scattering and any imperfection of the analyzing filters or failurc to absorb neutrons
of "unused" final wavelength or direction leads to ilicreased background . In addition
the background depends on the sample which makes correcting subtractions difficult .
2 . Samples are hit by a higher integral flux and therfore become more radioactive .
In general also "normal" TOF instruments perform better on a pulsed source in comparison
with a reactor (if the average fluxes are equal) silice by synchronizing source and
chopper only a small part of the generated neutrons (with the desired wavelength) are net
used . The distance source-chopper may serve at the saine time as TOF-monochromator .
In total the efficiency is comparable with the inverted type because in both cases on
one ride of the sample the spectrum is restricted by filters/monochromators and on the
other side the full spectrum is utilized and sorted according to the TOF . Energy transfer
analysis without filter on any ride is impossible .
'Te suppress background or to prevent "frame overlap" it may be nevertheless advisable to use an
additional chopper .
10- 19