  Back-streaming neutrons from the spallation target of the China Spallation
Neutron Source (CSNS) that emit through the incoming proton channel were
exploited to build a white neutron beam facility (the so-called Back-n white
neutron source), which was completed in March 2018. The Back-n neutron beam is
very intense, at approximately 2*10^7 n/cm^2/s at 55 m from the target, and has
a nominal proton beam with a power of 100 kW in the CSNS-I phase and a kinetic
energy of 1.6 GeV and a thick tungsten target in multiple slices with modest
moderation from the cooling water through the slices. In addition, the
excellent energy spectrum spanning from 0.5 eV to 200 MeV, and a good time
resolution related to the time-of-flight measurements make it a typical white
neutron source for nuclear data measurements; its overall performance is among
that of the best white neutron sources in the world. Equipped with advanced
spectrometers, detectors, and application utilities, the Back-n facility can
serve wide applications, with a focus on neutron-induced cross-section
measurements. This article presents an overview of the neutron beam
characteristics, the experimental setups, and the ongoing applications at
Back-n.
