Dose Course 2018_Flipping book

Monitor back scatter

0 b M M M A M b A          = = + + 0 0 0 0 1 ( ) 1 ( )

• 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.

monitor

2 mon 2 coll

z

( ) b A k F = back

z



z z coll −

3

mon

cos



F

A

=

d



2

(

)

z

z



−

Irradiated collimator area

coll

mon

Irradiated collimator surface

42

PMB 44 p R99-R155