  The back-streaming neutrons from the spallation target at CSNS are very
intense, and can pose serious damage problems for the devices in the
accelerator-target interface region. To tackle the problems, a possible scheme
for this region was studied, namely a specially designed optics for the proton
beam line produces two beam waists, and two collimators are placed at the two
waist positions to maximize the collimation effect of the back-streaming
neutrons. Detailed Monte Carlo simulations with the beams in the two different
CSNS phases show the effectiveness of the collimation system, and the radiation
dose rate decreases largely in the interface section. This can ensure the use
of epoxy coils for the last magnets and other devices in the beam transport
line with reasonable lifetimes, e.g. thirty years. The design philosophy for
such an accelerator-target interface region can also be applicable to other
high-power proton beam applications.
