  The European Spallation Source is being constructed in Lund, Sweden and is
planned to be the world's brightest pulsed spallation neutron source for cold
and thermal neutron beams ($\le$ 1 eV). The facility uses a 2 GeV proton beam
to produce neutrons from a tungsten target. The neutrons are then moderated in
a moderator assembly consisting of both liquid hydrogen and water compartments.
Surrounding the moderator are 22 beamports, which view the moderator's outside
surfaces. The beamports are connected to long neutron guides that transport the
moderated neutrons to the sample position via reflections. As well as the
desired moderated neutrons, fast neutrons coming directly from the target can
find their way down the beamlines. These can create unwanted sources of
background for the instruments. To mitigate such a kind of background, several
instruments will use curved guides to lose direct line-of-sight (LoS) to the
moderator and the target. In addition instruments can also use shielding
collimators to reduce the amount of fast neutrons further traveling down the
guide due to albedo reflections or streaming. Several different materials have
been proposed for this purpose. We present the results of a study of different
options for collimators and identify the optimal choices that balance cost,
background and activation levels.
