  The transport of neutrons in long beamlines at spallation neutron sources
presents a unique challenge for Monte-Carlo transport calculations. This is due
to the need to accurately model the deep-penetration of high-energy neutrons
through meters of thick dense shields close to the source and at the same time
to model the transport of low-energy neutrons across distances up to around 150
m in length. Typically, such types of calculations may be carried out with
MCNP-based codes or alternatively PHITS. However, in recent years there has
been an increased interest in the suitability of Geant4 for such types of
calculations. Therefore, we have implemented supermirror physics, a neutron
chopper module and the duct-source variance reduction technique for low-energy
neutron transport from the PHITS Monte-Carlo code into Geant4. In the current
work, we present a series of benchmarks of these extensions with the PHITS
software, which demonstrates the suitability of Geant4 for simulating long
neutron beamlines at a spallation neutron source, such as the European
Spallation Source, currently under construction in Lund, Sweden.
