Ionization Chamber
Pocket dosimeter
1
Ionization Chamber
Pocket dosimeter
2
Radiation Quantities and Units
Radiation measurements require
specification of the radiation field at
various points



At the source – Activity, mA, kVp
In flight – Exposure, fluence (dN/dA),
energy fluence (dE/dA)
At the first interaction point – kerma
 Kinetic Energy Released in Matter

In matter – Absorbed dose, equivalent
dose, effective dose
 Radiation dosimetry is concerned with a
quantitative determination of the energy
deposited a medium by ionizing radiation
3
Radiation Quantities and Units
Pictorially
Energy
Deposition
Source
Transport
First
Interaction
4
Radiation Units
Activity

1 Bq (bequerel) == 1 disintegration / s
 A common unit is MBq = 106 Bq

1 Ci (curie) == 3.7x1010 disintegrations /s
 An earlier unit of activity and used in EPP
 A typical HDR brachytherapy source is 10-20 Ci
 A typical radioactive source is the lab is ~
10μCi
 40K in your body is 0.1 μCi = 3700 Bq
5
Radiation Units
 Exposure



Defined for x-ray and gamma rays < 3 MeV
Measures the amount of ionization (charge Q) in a
volume of air at STP with mass m
X == Q/m
 Assumes that the small test volume is embedded in a
sufficiently large volume of irradiation that the number of
secondary electrons entering the volume equals the
number that leave (CPE)

Units are C/kg or R (roentgen)
 1 R (roentgen) == 2.58 x 10-4 C/kg
 Somewhat historical unit (R) now but sometimes still
found on radiation monitoring instruments
 X-ray machine might be given as 5mR/mAs at 70 kVp at
100 cm
6
Radiation Units
 Absorbed dose




Energy deposited by ionizing radiation in a volume
element of material divided by the mass of the
volume
D=E/m
Related to biological effects in matter
Units are grays (Gy) or rads (R)
 1 Gy = 1 J / kg = 6.24 x 1012 MeV/kg
 1 Gy = 100 rad

1 Gy is a relatively large dose
 Radiotherapy doses ~ 50 Gy
 Diagnostic radiology doses 1-30 mGy
 Typical background radiation ~ 6 mGy
7
Radiation Units
Equivalent dose



Not all types of radiation cause the same
biological damage per unit dose
Dense ionization (high LET) along a track
causes more biological damage than less
dense (low LET)
HT=D x wR
8
Radiation Units
 Effective dose

Not all tissues are equally sensitive to ionizing
radiation
E   HT  wT
T

Used to compare the stochastic risk from an
exposure to a specific organ(s) in terms of the
equivalent risk from an exposure of the whole body
 The stochastic risks are carcinogenesis and hereditary
effects
 Not intended for acute effects
 In practice, most exposures are whole body
9
Radiation Units
 Tissue weighting factors

Sums to 1
Tissue or Organ
Gonads
Bone marrow – red
Colon
Lung
Stomach
Bladder
Breast
Liver
Oesophagus
Thyroid
Skin
Bone surface
Remainder
Tissue weighing factor - w T
0.20
0.12
0.12
0.12
0.12
0.05
0.05
0.05
0.05
0.05
0.01
0.01
0.05
10
Radiation Units
Units of equivalent dose and effective
dose are sieverts (Sv)

1 Sv = 100 rem (roentgen equivalent in
man)
 3.6 (6.2) mSv / year = typical equivalent dose
in 1980’s (2006)
 15 mSv/ year = Fermilab maximum allowed
dose
 20 mSv/year = CERN maximum allowed dose
 50 mSv/year = US limit
 3-4 Sv whole body = 50% chance of death (LD
50/30)
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Background Radiation
Average equivalent dose (1980’s)
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Background Radiation
Average equivalent dose (2006)
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Background Radiation
1980’s versus 2006
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Radiation in Japan
20 mSv / yr
= 2.3 mSv/hr
3/28
update

Reactor 2
@ 1 Sv /
hr !!!
15
Fission Yield
Some of the
more harmful
fission products
are 90Sr (29y),
106Ru (1y), 131I
(8d), 132Te (3d),
133Xe (5d), and
137Cs (30y)
16
Natural Radioactivity
17
Natural Radioactivity
 Terrestrial



Present during the formation of the solar system
Uranium, actinium, thorium, neptunium series
40K
 Cosmogenic

Radionuclides produced in collisions between
energetic cosmic rays and stable particles in the
atmosphere (14C, 3H, 7Be)
 Human produced

Nuclear medicine, fission reactors, nuclear testing
 Cosmic rays

~270 μSv / year (a bit more in Tucson)
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Natural Radioactivity
 Radon
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Radon
 222Rn (radon) is produced in the 238U decay series
 222
Rn →
218Po
+ α (t1/2=3.8 days)
 218
Po →
214Pb
+ α (t1/2=3.1 minutes)
 Radon is a gas that can easily travel from the soil
to indoors
 Air pressure differences
 Cracks/openings in a building
 218Po can be absorbed into the lungs (via dust,
etc.)
 The decay alpha particles are heavily ionizing
 The ionization in bronchial epithelial cells is
believed to initiate carcinogenesis
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Radiation Units
Kerma





Kinetic energy released per unit mass
Defined for indirectly ionizing energy
(photons and neutrons)
Mean energy transferred to ionizing
particles in the medium without concern as
to what happens after the transfer
K=Etr/m
Units are grays (Gy)
 1 Gy = 1 J / kg
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Radiation Units
The energy transferred to electrons by
photons (kerma) can be expended in
two ways



Ionization losses
Radiation losses (bremsstrahlung and
electron-positron annihilation)
Thus we can write
K  K col  K rad
K col  K 1  g 
g is thefractionof energy transferredto
electronsthatis lost through radiativeprocesses
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Photon Attenuation
Coefficients Review
I  I 0e
 mx
m is thelinear attenuation coefficient
m
m m  is themass attenuation coefficient

men is theenergyabsorptioncoefficient
mtr is theenergy transfer coefficient
men  mtr 1  g  where g is thefractionof energy
thatis lost in radiativeprocesses
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Compton Scattering
C   
tr
C
sc
C
T
hv  hv
  C
 C
h
h
hv 
sc
C  C
h
similarlyfor t hemass energy t ransfer
at t enuat ion coefficient
tr
C
m
T mC
T N Av C


 h  h A
tr
C
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Kcol and D as a function of depth
25
Relations
Kerma and energy fluence

For a monoenergetic photon beam of
energy E
 mtr 
K  Y 
  E

The energy fluence Y units are J/m2
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Relations
 Exposure and kerma
 e
X  K col air  
 Wair



Wair 33.97eV
1.602 1019 J / eV


e
ion pair 1.602 1019 C / ionpair
 33.97J / C


Wair includes the electron’s binding energy, average
kinetic energy of ejected electrons, energy lost in
excitation of atoms, …
On average, 2.2 atoms are excited for each atom
ionized
27
Relations
 Absorbed dose and kerma
D  K col  K 1  g 
g is theradiativefraction
g dependson theelectronkineticenergyas well as
thematerialunder consideration
T heaboverelationassumes CPE
 In theory, one can thus use exposure X to
determine the absorbed dose


Assumes CPE
Limited to photon energies below 3 MeV
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Kcol and D as a function of depth
b=D/Kcol
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Kcol and D as a function of depth
In the TCPE region, b = D/Kcol > 1


Photon beam is being attenuated
Electrons are produced (generally) in the
forward direction
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Bragg-Gray Cavity Theory
The main question is, how does one
determine or measure the absorbed
dose delivered to the patient (to within
a few percent)


The answer is to use ionization in an air ion
chamber placed in a medium
The ionization can then be related to
energy absorbed in the surrounding
medium
31
Bragg-Gray Cavity Theory
 Assumes


Cavity is small (< Relectrons) so that the fluence of
charged particles is not perturbed (CPE)
Absorbed dose in the cavity comes solely by
charged particles crossing it (i.e. no electrons are
produced in the cavity or stop in the cavity)
Dmed
S 
 Dcav  
   med
S 
/  
   cav
S is theaverageunrestricted mass collisionstoppingpower
Dcav
Q W  IP  eV   eV 
eV
    
for air
;
  33.97
m e  kg  IP   IP 
IP
32
Bragg-Gray Cavity Theory
 Spencer-Attix modification


Accounts for delta rays that may escape the cavity
volume
In this case, one uses the restricted stopping power
(energy loss)
L
L
Dmed  Dcav   /  
   med    cav
L is theaveragerest rictedmass collisionstoppingpower
33
Calibration of MV Beams
 Protocols exist to calibrate the absorbed dose
from high energy photon and electron beams


End result is a measurement of dose to water per
MU (monitor unit = 0.01 Gy)
For a reference depth, field size, and source to
surface distance (SSD)
 TG-21


Outdated but conceptually nice
Based on cavity-gas calibration factor Ngas
 TG-51


New standard
Based on absorbed dose to water calibration
factor ND,w for 60Co
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Ionization Chamber
 Ionization chambers are a fundamental type
of dosimeter in radiation physics
 Measurement of the current or charge or
reduction in charge gives the exposure or
absorbed dose




Free-air ionization chamber
Thimble chamber
Plane parallel chamber
Pocket dosimeter
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Ionization Chamber
Current mode

Current gives average rate of ion formation
of many particles
Pulse mode

Voltage gives measure of individual
charged particle ion formation
36
Ionization Chamber
Free-air chamber
37
Ionization Chamber
 Used as a primary standard in standards
laboratories
 Used to measure X
Q
 mx
X R  
e
4
AP L  2.5810
 Guard wires and guard electrodes produce
uniform electric field
 E ~ 100-200V/cm between plates
 Assumes CPE
 Limited to E<3 MeV (if pressurized) because of
electron range
38
Ionization Chamber
Free-air chambers are not so practical
however

Instead one uses an ion chamber with a
solid, air equivalent wall
39
Ion Chambers
EXRADIN A12 Farmer
EXRADIN A17 Farmer
EXRADIN A12 thimble
EXRADIN A3 Spherical Chamber
EXRADIN 11 Parallel Plate Chamber
EXRADIN mini thimble
40
Ionization Chamber
Capintec Inc.
Vendors
Nuclear Associates
VICTOREEN INC
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Ionization Chamber
0.6 cm3 Farmer chamber
42
Ionization Chamber
Cavity
Electrode
Sleeve
43
Ionization Chambers
 Materials used
Central Electrode
Aluminum
Graphite



Wall
A150
C552
PMMA
Graphite
Sleeve
PMMA
A150 = Tissue equivalent plastic
C552 = Air equivlaent plastic
PMMA = Polymethyl-methacrylate (lucite)
44
Ionization Chamber
Farmer chamber




Farmer type has a graphite wall and
aluminum electrode
For CPE , amount of carbon coating and
size of aluminum electrode is adjusted so
that the energy response of the chamber is
nearly that of photons in free air over a
wide range of energies
Since an exact air equivalent chamber and
knowledge of V is difficult, in practice they
must be calibrated against free air
chambers for low energy x-rays
Nominal energy range is 60 keV – 50 MeV
45
Ionization Chamber
Correction factors





Saturation
Recombination
Stem effects
Polarity effects
Environmental conditions
46
Ionization Chamber
Need to ensure chamber is used in the
saturation region
47
Ionization Chamber
 Stem irradiation can cause ionization
measured by the chamber so a correction
factor will be needed

Found by irradiating the chamber with different
stem lengths in the radiation field
48
Ionization Chamber
The collection efficiency can be
measured by making measurements at
two different voltages (one low and one
nominal)
Polarity effects can be measured by
making measurements at both polarities
and taking the average
Environmental conditions are corrected
to STP by
49
Beam Calibration with Water Phantom
50
Electrometer
This device displays the measured values of dose and
dose rate in Gy, Sv, R, Gy/min, Sv/h, R/min.
51
Ion Chamber and Electrometer Setup
PTW Ion Chamber
Electrometer
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Ion Chamber and Electrometer Setup
53
Calibration Summary
54
Verification of the
dose
for treatment plan
55
Calibration of Novalis System
56
Novalis System
at
Department of
Radiation
Oncology, UA
57
Calibration of Novalis System
58
Ionization Chamber
Plane parallel chamber
59
Ionization Chamber
 Roos or advanced Markus type


Used for precise dose measurements of electron
beams
 Nominal useful electron energy from 2 to 45
MeV
For surface dose from gammas, current arises
from backwards Compton scattering
60
Ionization Chamber
Smoke detector
61
Ionization Chamber
As with the proportional chamber,
charge is induced by the drifting charge
carriers

Can be both ions and electrons or only
electrons
Reasoning goes as follows


If response time > collection time, energy
is conserved
Energy to move the charges comes from
the stored energy in the capacitor
62
Ionization Chamber
 Consider
63
Ionization Chamber
1
1
CV02  n0eEv t  n0eEvt  CVch2
2
2
Following Knoll,VR  V0  Vch is given by
no e 

v  v  t
dC
As we saw with theproportion
al tube, the
VR 
motionof thecharges generatesa thesignal
by inducing a charge on theelectrodes
no e 

After t heelectronsare collectedVR 
v t  x
dC
no e
d  x  x 
After t heionsare collectedVR 
dC
ne
So Vmax  o
C
64
Ionization Chamber
 In order to minimize the deadtime, we usually
don’t wait for the ions to drift to the electrodes

Then
no ex
Vmax 
Cd
 But in this case, the amplitude depends on the
position of interaction
65
Ionization Chamber
 The solution to this feature is the Frisch grid


The motion of the ions to the cathode and of the
electrons to the grid is ignored because of the
location of the load resistor
Once the electrons pass the grid, using arguments
as before
n0e 
n0e
VR 
dC
v t and Vmax 
C
66
Radiation Units
Particle fluence and flux




Fluence F = N/A
Flux (fluence rate) f = N/At
Usually used to describe photon beams but
may also be used in describing charged
particle beams
One can think of the particles being
incident on a sphere of cross-sectional area
A
 Hence fluence is independent of incident angle

Units are m-2 (fluence) and m-2s-1 (flux)
67
Radiation Units
Energy fluence and flux



Energy fluence Y = E/A
Energy flux y = E/At
Units are J/m2 (energy fluence) and W/m2
(energy flux)
Although photon and energy fluence
and flux are used in calculations, they
are not easily measured
68
Radiation Units
 Most realistic beams are polyenergetic and a
spectrum must be used for fluence and energy
fluence
dF
E 
F E  
dE
dY
dF
E   E E
Y E  
dE
dE
69