  The spatial and energy distribution of spallation neutrons have an effect on
the performance of Accelerator Driven Subcritical systems. In this work, the
spatial, energy distribution of spallation neutrons and the effect of these
factors on proton efficiency was studied. When the radius of spallation region
increases, backward neutrons were found to have a rather big ratio and have a
positive effect on proton efficiency. By making better use of these neutrons,
we may increase the radius of target to satisfy some other requirements in the
design of subcritical core.
