  In this paper, we study a monitoring method for neutron flux for the
spallation target used in an accelerator driven sub-critical (ADS) system,
where a spallation target located vertically at the centre of a sub-critical
core is bombarded vertically by high-energy protons from an accelerator. First,
by considering the characteristics in the spatial variation of neutron flux
from the spallation target, we propose a multi-point measurement technique,
i.e. the spallation neutron flux should be measured at multiple vertical
locations. To explain why the flux should be measured at multiple locations, we
have studied neutron production from a tungsten target bombarded by a 250
MeV-proton beam with Geant4-based Monte Carlo simulations. The simulation
results indicate that the neutron flux at the central location is up to three
orders of magnitude higher than the flux at lower locations. Secondly, we have
developed an effective technique in order to measure the spallation neutron
flux with a fission chamber (FC), by establishing the relation between the
fission rate measured by FC and the spallation neutron flux. Since this
relation is linear for a FC, a constant calibration factor is used to derive
the neutron flux from the measured fission rate. This calibration factor can be
extracted from the energy spectra of spallation neutrons. Finally, we have
evaluated the proposed calibration method for a FC in the environment of an ADS
system. The results indicate that the proposed method functions very well.
