2. The
ion-‐chamber
should
be
tested
along
with
the
electrometer
and
the
cable
it
is
going
to
get
used
with
For
reproducibility
of
results
the
ion-‐chamber
should
also
be
tested
against
different
electrometers
and
the
results
should
have
minimum
variability
It
is
important
to
be
aware
of
various
sources
of
noise
in
the
system
that
contribute
to
charge-‐
collection
at
the
ion-‐chamber
electrodes
in
addition
to
charge
collected
due
to
ionizations
by
radiation
3. Acceptance
testing
is
recommended
when
the
chamber
is
back
from
ADCL
before
using
it
for
reference
or
absolute
dosimetry.
4.
PTW
0.6cc
SN1315
farmer
type
ion-‐chamber
by
PTW
FREIBERG.
Model
#
TN
30013-‐1315
5.
6. IC
set-‐up:
100
cm
SSD;
ion-‐chamber
active
volume
at
the
center
of
a
20cmx20cm
field;
solid-‐water
phantom
with
1.5
cm
build-‐up
and
5
cm
back
scatter
Electrometer:
-‐300
V
(-‐100%)
bias
Energy:
6
MV
photon
beam
Procedure:
Record
charge
collection
measurements
for
MU
delivery
ranging
between
2
MU
to
200
MU
7.
8. The
uncertainty
in
the
charge
collection
due
to
stem
effect
should
be
less
than
0.5%
This
can
be
checked
by
taking
exposures
using
a
field
size
that
irradiates
just
the
thimble
and
comparing
it
with
charge
collection
reading
taken
when
the
whole
stem
is
in
the
field.
Set
up:
100
cm
SSD;
FS
5cmx30cm;
tape
the
IC
in
two
orientations
–
IC
parallel
to
the
30
cm
dim
of
field,
IC
perpendicular
to
it
with
only
thimble
inside
direct
radiation
beam
9.
stem
effect
slightly
greater
than
0.5%
but
5cmx30cm
has
greater
penumbra
uncertainties
repeat
test
using
7cmx30cm
or
10cmx30cm
FS
stem
effect
is
0.2%,
within
manufacture’s
specifications
10. Collect
charge
readings
using
both
bias
polarities
with
all
other
set-‐up
parameters
constant
11. Which
means
check
the
cylindrical
symmetry
of
the
ion
chamber’s
active
volume’s
construction
Ion
chamber
suspended
in-‐air
with
the
ion
chamber’s
build-‐up
cap
on
This
provides
same
build-‐up
from
all
different
directions
of
irradiation
12. (left)
Ion
chamber
major
axis
perpendicular
to
the
CAX.
(right)
Ion
chamber
major
axis
parallel
to
the
CAX.
14. Ion chamber response for an orientation parallel to the central beam axis
(Set up: 10 x 10 field, 100 SAD, 06 MV X-rays, 100 MU, CAX parallel)
15. Measurements
involving
ionizing
current
at
2
different
voltages
are
used
to
assess
the
collection
efficiency
of
an
ion
chamber
[Boag
2-‐volatge
technique]
This
is
‘Pion’
or
recombination
correction
factor
[TG
51]
Use
electrometer
100
%
and
50
%
voltage
settings
using
negative
and
positive
bias
(Vh
and
Vl
settings)
17.
A
feel
of
your
ion
chamber
Physicist’s
extended
hand
and
mind
in
clinical
reference
dosimetry
JEB’s
way
of
keeping
things
in
head
for
an
easy
reference
*Average
nC/cc
MU
for
a
0.6
cc
farmers
chamber
=
0.2092
nC/cc/
MU
*Average
nC/cc
MU
for
a
0.125
cc
farmers
chamber
=
0.1833
nC/cc/
MU
*Average
nC/cc
MU
for
a
0.015
cc
farmers
chamber
=
0.1714
nC/cc
/MU
OR,
simply
put…
….the
average
charge
collected
per
cubic
centimeter
of
the
chamber’s
active
volume
for
a
monitor
unit
of
radiation
is
roughly
~0.2
nC
So,
expected
charge
collection
for
100
MU
~
20
nC…..right,
Happy
Physicist!
*
Averages
derived
from
detailed
linearity
measurements
using
3
chamber
types
18. The
SNR
is
derived
from
the
same
irradiation
data
set
over
the
three
ion-‐chambers
SNR
=
mean
signal
(your
data)/
Std
Deviation
SNR
test
Results
observations:
SNR
generally
increases
with
the
increase
in
Signal
Normalized
SNR
values
show
that
the
SNR
decreases
with
the
decrease
in
ion
chamber
collection
volume
(reason
I
wouldn’t
try
to
use
a
0.015
cc
ion-‐chamber
to
assess
doses
at
very
low
signals)
At
10
MU
using
0.015
pin-‐point
-‐
noise
and
the
detected
signal
are
almost
equal
19.
1.
2.
3.
To
quantify
the
leakage
from
various
components
the
following
were
quantified:
Ionizations
in
the
ion-‐chamber
cable
in
the
field
Ionizations
in
the
triax
cable
in
the
field
Leakage
when
no
irradiation
but
power
supply
is
on.
This
was
quantified
with
and
without
the
ion-‐
chamber
being
connected
to
the
electrometer.
Leakage
only
due
to
electrometer
electronics
can
be
characterized
by
letting
the
electrometers
run
on
battery
for
a
given
duration
of
time
without
any
connecting
wires
or
ion-‐chamber.
