In order to see where the background comes from, the beam line from the exit of
main linear accelerator(linac) to the interaction point(IP) is shown in
Fig.2 for JLC-I(
500GeV). [2]
A bending magnet of +20mrad, which is the so-called big bend, is needed because the
background from the upstream linac can not directly hit a detector. The second bending
magnet of -16mrad makes a horizontal crossing of 
mrad between two beams at IP.
There are two major sections of 1200m collimation and a 600m final focus
system in the beam line, where (1)muons and (2)synchrotron radiation are produced,
respectively. At IP, (3)
pairs and (4)minijets are created in beam-beam
interactions. In the subsequent sections these four kinds of background are
briefly discussed together with the possible designs of an interaction region against
them.
Figure 2:
Top view of the beam line from the exit of the main linac to the
interaction point (IP) of JLC-I(500GeV).
pairs from Beam-beam Interaction and Masking