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2
mon
back
2
coll
z
b A k F
z
z z
coll
mon
• 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
3
2
Irradiated
coll
mon
collimator
area
cos
d
F
A
z
z
Irradiated collimator surface
monitor
PMB 44 p R99-R155
Monitor back scatter