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• Usually a very small effect
• Dominated by low energy charged particles, can be stopped by a protection sheet.
• Can be modelled by a monitor’s-eye-view of the collimators:
- Distance source to monitor and collimator.
- Shape of visible collimator surface.
- Empirical constant, 0.3 to 0.4.
0
0
0
0
0
1
( )
1 ( )
b
M M M A M b A
Ψ Ψ Ψ
=
=
+
+
Monitor back scatter
42
( )
2
mon
back
2
coll
z
b A k F
z
=
θ
z
z
coll
mon
−
(
)
3
2
Irradiated
coll
mon
collimator
area
cos
d
F
A
z z
θ
π
=
−
∫∫
Irradiated collimator surface
monitor
PMB 44 p R99-R155