phys586-lec19-ion2
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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
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Radiation Quantities and Units
Pictorially
Energy
Deposition
Source
Transport
First
Interaction
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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
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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
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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
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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
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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
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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
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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 !!!
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Fission Yield
Some of the
more harmful
fission products
are 90Sr (29y),
106Ru (1y), 131I
(8d), 132Te (3d),
133Xe (5d), and
137Cs (30y)
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Natural Radioactivity
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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
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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 1019 J / eV
e
ion pair 1.602 1019 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
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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
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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
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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
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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
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Ionization Chamber
Free-air chamber
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Ionization Chamber
Used as a primary standard in standards
laboratories
Used to measure X
Q
mx
X R
e
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AP L 2.5810
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
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Ionization Chamber
Free-air chambers are not so practical
however
Instead one uses an ion chamber with a
solid, air equivalent wall
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Ion Chambers
EXRADIN A12 Farmer
EXRADIN A17 Farmer
EXRADIN A12 thimble
EXRADIN A3 Spherical Chamber
EXRADIN 11 Parallel Plate Chamber
EXRADIN mini thimble
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Ionization Chamber
Capintec Inc.
Vendors
Nuclear Associates
VICTOREEN INC
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Ionization Chamber
0.6 cm3 Farmer chamber
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Ionization Chamber
Cavity
Electrode
Sleeve
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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)
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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
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Ionization Chamber
Correction factors
Saturation
Recombination
Stem effects
Polarity effects
Environmental conditions
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Ionization Chamber
Need to ensure chamber is used in the
saturation region
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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
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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
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Beam Calibration with Water Phantom
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Electrometer
This device displays the measured values of dose and
dose rate in Gy, Sv, R, Gy/min, Sv/h, R/min.
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Ion Chamber and Electrometer Setup
PTW Ion Chamber
Electrometer
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Ion Chamber and Electrometer Setup
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Calibration Summary
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Verification of the
dose
for treatment plan
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Calibration of Novalis System
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Novalis System
at
Department of
Radiation
Oncology, UA
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Calibration of Novalis System
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Ionization Chamber
Plane parallel chamber
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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
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Ionization Chamber
Smoke detector
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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
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Ionization Chamber
Consider
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Ionization Chamber
1
1
CV02 n0eEv t n0eEvt 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
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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
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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
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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)
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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
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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
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