  A compact photoneutron source (PNS), based on an electron linac was designed
and constructed to provide required nuclear data for the design of Thorium
Molten Salt Reactor (TMSR). Many local shielding are built to reduce the
background of neutron and {\gamma} rays, making the location of the time of
flight (TOF) detector be fixed at 6.2 m place. Under the existing layout, some
physical parameters are very difficult to get by the experiments, while can be
obtained by the Monte Carlo simulation method. However, for the deep
penetration problem of the neutron and {\gamma} rays transport in the channel
of PNS with complex geometry, the normal Monte Carlo method is inefficient
since electron transport calculation need a large amount of computing time and
neutrons have little contribution to the detector in far-source region. In this
work, the subsection method is applied in the simulation for PNS, which divide
the simulation process in two steps, recording the neutron and {\gamma} rays
information passing through the source window in the first step and adopting
the covariance reduction techniques in the second step. The simulated neutron
flux and energy spectrum at the TOF detector place with the relative error 1.6%
are well agreement with the experimental results, achieving an efficiency 23
times better than the normal method. This method is fast and efficient in
predicting the physical parameters, providing a required verification and
initiating the foreseen physics experiment.
