  A white neutron beam line using the back-streaming neutrons from the
spallation target at China Spallation Neutron Source (CSNS) is under
construction. Different spectrometers mainly for nuclear data measurements to
be installed in the so-called Back-n beam line are also under development in
phases. The physics design of the beam line includes the overview of the
characteristics of the neutron beam including energy spectrum, flux and time
structure, and the optimizations of neutron beam spots and in-hall background
with the help of a complicated collimation system and a sophisticated neutron
dump. The wide neutron energy range of 1 eV~100 MeV is very good to support
different applications, especially for nuclear data measurements. At
Endstation#2 where is about 80 m from the target, the main properties include
neutron flux of 106 n/cm2/s, time resolution of a few per mille over almost the
whole energy range, in-hall background about 0.01 /cm2/s for both neutron and
gamma. With its completion in late 2017, Back-n will be not only the first
high-performance white neutron source in China, but also among the best white
neutron sources in the world.
