Sample quantity and multiple scattering

Inelastic neutron scattering is a rare process in part because neutron sources are so weak. Thus one might be inclined to load as much sample as possible into the beam. Within the limits set by multiple scattering, this is an excellent idea. Multiple scattering arises when a neutron is scattered twice (or more) from separate atoms within the body of the sample (compare 2.5.1.3, Table 2.1). The most likely process is two elastic scattering events but elastic scattering is of no consequence for the energy transferred in an INS experiment. 

This simple analysis suggests that the double inelastic event is detrimental to the spectra collected on all instruments, whereas the (elastic + inelastic) case is detrimental only for direct geometry instruments. This is true even for powder samples, since the magnitude of the momentum transfer is also lost. This contamination is most problematic for data obtained from the low scattering angle detectors. The data in these detectors are nominally obtained at low Q but multiple scattering injects high Q information into their data. 

This is a much less serious problem for indirect geometry instruments with low final energies. These spectrometers work close to the maximum 

Instrument Location Type Energy range/ cm Resolution/(%AE/ r) Status Ref. 

Filter Detector BARC (India) Be filter 80 - 2000 20 Operational [32] 

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