Abstract
Abstract
Objectives. To derive a collection efficiency formula,
f
Gauss
, for cylindrical ionization chambers in pulsed radiation beams from a volume recombination model of Boag et al (1996 Phys. Med. Biol.
41 885–97) including free electrons. To validate
f
Gauss
and a parallel plate chamber formula
f
exp
using an ion transport code and calculate changes in collection efficiencies caused by electric field charge screening at 0.1–100 mGy doses-per-pulse. And to determine collection efficiencies
C
E
∞
predicted at infinite voltage in the absence of avalanche effects by fitting scaled formulae to efficiencies computed for 100–400 V chamber voltages and 10 and 100 mGy doses-per-pulse. Approach. Calculations were performed for an idealized parallel plate chamber with 2 mm electrode separation
d
, and for an idealized cylindrical chamber with 0.5 and 2.333 mm inner and electrode radii
r
in
and
r
out
. Main results.
f
Gauss
and
f
exp
predict the same collection efficiencies for cylindrical and parallel plate chambers satisfying
d
2
=
(
r
out
2
−
r
in
2
)
ln
(
r
out
/
r
in
)
/
2
, an equivalence condition met by the chambers studied. Without charge screening, efficiencies computed using the code equalled
f
Gauss
and
f
exp
. With screening, efficiencies changed by ⩽0.03%, ⩽1.1% and ⩽21.3% at 1, 10 and 100 mGy doses-per-pulse, and differed between the chambers by ⩽0.9% and ⩽19.6% at ⩽10 and 100 mGy dose-per-pulse. For fits of
f
exp
and
f
Gauss
,
C
E
∞
values were ⩽1.2% and ⩽17.6% from unity at 10 and 100 mGy per pulse respectively, closer than for other formulae tested. Significance. Allowing for screening,
f
Gauss
and
f
exp
described computed collection efficiencies to within 0.03%, 1.1% and 21.3% at doses-per-pulse ⩽1, 10 and 100 mGy. Equivalence of the two chambers broke down at 100 mGy per pulse. Departures of
C
E
∞
values from unity suggest that collection efficiencies determined experimentally by fitting
f
Gauss
or
f
exp
to readings made at multiple voltages will be accurate to within 1.2% and 17.6% at 10 and 100 mGy per pulse respectively.