Neutron Scattering

times is performed at the detector distance dcp + L .
A possible resulting "spectrum" is
indicated on top of the detector distance line.
The extrapolation of the dashed line to the
detector position indicates the location of the "elastic line", i.e .
the time channel where
neutrons without energy transfer arrive 4 .
The left vertical edge of the triangle defines
the "beginning" of the time-of-flight spectrum, i .e .
the earliest possible time of arrival
occurring only for nearly infinite energy gain of the neutron .
In reality there are no infinite
large energy gains and the proper spectrum Starts somewhat later .
The right edge of the
"velocity fan" symbolized by the triangle is less well defined, in principle the neutron may
transfer only a part of its energy but also virtually all of its energy to the Sample, thus
virtually hortizontal path-time curves may result . Le . strictly speaking the spectrum does
not end at some maximum channel number . However it is immediately recognizable that
fortunately this effect goes along with a corresponding "dilution"
of the intensity which
becomes virtually structureless on the scale of the histogram channel Windows ("frames")
causing a constant background in all channels of a "frame" that may be subtracted during
data treatment . Thereby it becomes possible to repeat the uptake of a "frame" with a
frequency S2 and to accumulate the spectra into the histograms mentioned above . Each
chopper pulse resets the clock to time zero and the channels of the histogram cover the
time interval T in terms of bins of width O7K ~ TIN .
If due to resolution requirements
5 or due to other technical demands 9 lias to be chosen such that the above discussed
"frame"-overlap effect still distorts the spectra, it is possible, as indicated in fig . 10 .4, to
use an additional coarse chopper to transmit only every 2nd or n-th pulse .
By doing this
sufficient spacing between "frames" may be gained to collect
an undistorted spectrum,
however with an n-fold reduction of effective data collection rate .
10 .2 .2 Time-of-flight spectra
In fig. 10 .5 several spectra as they are accumulated in the histogrammic memory are
displayed .
The time-of-flight scale of the horizontal axis refers to the distance L between
Sample and detectors . This time-of-flight is directly proportional to the wavelength A'
of the scattered neutrons as is indicated by the diagonal representing this linear relation
4 Generally the scattering without (sizeable) energy transfer is the most probable process .
5The pulse width (opening time) of a chopper depends -for a given design- on its rotation frequency.
10-6