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Energy dependence due to changing

Stopping-power ratios

6 MV photon beam

n=narrow beam (

=0.5 cm)

b=broad beam (

=10 cm)

Heydarian

et al

Cylindrical air ion chambers can

be used without correction for

energy variations in high

energy photon beams, incl. Co-

60 (IAEA TRS-398).

The secondary electron energy

spectrum is not changing with

depth.

Ratio narrow/large field depth dependence

Difference <0.5%