Moxon-Rae detectors consist of a converter, typically of carbon or bismuth, to "convert" the capture - rays into electrons by Compton scattering, pair production, or the photo effect. These electrons are either absorbed in the converter or detected in a thin plastic scintillator, which is mounted between the converter and a photomultiplier (see figure). The efficiency of such a detector increases almost linearly with - ray energy. This property results in an efficiency for neutron capture events, which is nearly proportional to the neutron binding energy and independent of the capture - ray cascade, if the probability of detecting more than one - ray out of a cascade is negligible.
Figure: Schematic scetch of the experimental setup.
In measurements with Moxon-Rae detectors, the main systematic uncertainty
is due to deviations from the ideal proportionality between efficiency
and - ray energy. Therefore,
the slope of the efficiency curve has to be measured. With these data,
a reliable efficiency correction can be derived (reference
for more details).
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