  The back-streaming white neutron source (Back-n) is a comprehensive neutron
experimental platform for nuclear data measurement, nuclear astrophysics,
neutron irradiation, detector calibration, etc. In order to meet a variety of
experimental requirements, an attempt of various combinations of collimators
has been made for the current collimation system. The basic parameters such as
beam flux and beam spot characteristics under different beam operation modes
have been studied. According to the change of the CSNS proton beam operation
mode, the influence of different proton beam spots on the Back-n beam is also
studied. The study finds that some beam modes with new collimator combinations
can meet the experimental requirements and greatly shorten the experimental
time. In addition, we have found a parameterized formula for the uniform beam
spot of the high-power proton beam based on the beam monitor data, which
provides a more accurate proton-beam source term for future white neutron beam
physics and application researches.
