Neutron Scattering

From the thermal spectrum of the neutron beam entering from the left a monochromator
(of any type) filters a limited wavelength band .\+0A .
By doing this typically a bandwidth
of AA/A ~ 10-2 is achieved .
The thus monochromatized beam enters a so-called chopper
which opens the beam path periodically for a short moment .
Typical frequencies are
between 20 and 200 Hz, the ratio of time-open:time-closed is around 1 :100 .
The resulting
pulse widths are of the order of several (tens) of microseconds . After an as short as
possible flight path the neutron bunches hit the sample and are scattered according to
the double differential cross section of the Sample material .
In this process some neutrons
exchange kinetic energy with excitations in the Sample, i .e . change their velocities . After
scattering into different directions the neutrons transverse the flight space between sample
and the detectors .
The path length between sample and detector is usually kept the saute
for all detectors placed at the periphery of the flight space. The detectors most often
consists of 'He (-10 bar) filled counting tubes of 30 . . . 40 cm length . Up to 1000 (and
more) tubes are used in some installations to cover as much solid angle as possible .
The elastically scattered neutrons (like those from the direct beam) reach the detectors
after the time tcp + t o = dchopper-Sample /vo + L/vo , those scattered inelastically arrive
earlier (energy gain of the neutron) or later (energy lors of the neutron) . Each pulse
from a counting tube causes via the associated electronics an increment of one Gell in
the histogrammic memory. The address of this cell is derived from the time difference
between chopper opening and arrival time of the neutron (pulse), i .e . TOF, and the
detector number (----> scattering angle) . Thus the distribution of flight times evolves as a
histogram of 512 . . . 2048 channels with a width of around 10ps each . For each detector
(resp . group of detectors) such an histogramm vs . time is obtained . A monitor (Mo) in
the direct beam serves to normalize the histograms to the incoming neutron flux . 2
2A "monitor" is a detection device (counting "tube") that covers the beam cross section and has a
high transmission for neutrons (> 90%) and low detection probability (10-3 . . . 10 -7) . In the figure only
one monitor behind the sample is shown for clarity . In general another monitor (more important) is
located between chopper and sample, it measures the incoming flux without the influence of the sample
transmission .
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