Neutron Scattering

if an increase in resolution may be achieved by stricter wavevector 1 selection (velocity,
direction) and by reduction of the chopper pulse length, it is offen not advisable to
enhance all elements of the resolution up to the technological limits because this goes along
with a drastic loss of intensity (detector count rate) . Design of (neutron)-spectremet ers
means the search for the Best compromise between resolution and intensity .
The optimum
depends on the nature of the problem, i .e . the features and structures expected in S(Q, 4 .
TOF spectrometers as described in this chapter are preferentially used to investigate
isotropie to weakly anisotropie samples with only weak structures in S(Q) . This enables
the utilization of a large solid angle for detectinn which compensates for the loses caused
by energy analysis . In the schematics of the spectrometers this is already indicated by
the large number of detectors . Modern TOF instruments contain more than 1000 single
counting tubes covering a detecting area of 30 x lcm 2 each . The total area coverd by
1000 detectors is about 3m2 , for a flight path of 3 m this corresponds to a solid angle
of 0.333 or 1000 degrees squared . Thereby an intesity gain of a factor 500 is obtained
compared to a triple axis spectrometer with a detecting area of 2 degrees squared, the
loss caused by the fact that the chopper opens only for about 1% of the time is more
than compensated . In addition the TOF instrument has a multiple advantage : all energy
transfers are detected simultaneously and not sequentially as in the case of a triple-axis
spectrometer . However it is seldom useful to surr the data of ALL detectors into one
spectrum but different scattering angle regions have to be evaluated separately. Still
they are measured all at the saine time! Generally the TOF instrument is more efficient
than a triple-axis spectrometer for isotropie samples . As soon as single crystals or very
anisotropie samples with a strong dependence of the spectra on Q are to be investigated
the conventional triple-axis spectrometers are better suited .