Dosimetry - msf.lu.se · The field of dosimetry • Ionizing radiation may produce biological...
Transcript of Dosimetry - msf.lu.se · The field of dosimetry • Ionizing radiation may produce biological...
Part 1 – Introduction
After this lecture you will be able to:
• Define the scientific field of dosimetry
• Describe the scope of dosimetric quantities
• Outline the content of the following lectures on dosimetry
The field of dosimetry
• Ionizing radiation may produce biological effects in livingmatter
• Dosimetric quantities are introduced to provide a physical measure to correlate with biological effects
• Dosimetry is concerned with the definition, calculation and measurement of dosimetric quantities
The scope of dosimetric quantities
Deterministicbiological effect
Dosimetric quantity
Stochasticbiological effect
Dosimetric quantity
The scope of dosimetric quantities
• Incident ionizing radiation
– Charged particles (e.g. electrons or protons) oruncharged particles (e.g. photons or neutrons)
– Produce ionizations in a medium (directly or indirectly)
– Practical threshold value Eionizing>Ethreshold≈10 eV
Unchargedparticle
Chargedparticle
The scope of dosimetric quantities
• Conversion of energy (blue blobs)
– Energy is transferred to secondary particles
Unchargedparticle
Chargedparticle
The scope of dosimetric quantities
• Conversion of energy (blue blobs)
– Energy is transferred to secondary particles
• Deposition of energy (yellow blobs)
– Not re-emitted by ionizing particles
Unchargedparticle
Chargedparticle
Content of the following lectures
• Definitions
– Dosimetric quantities
– ICRU 85
• Calculations
– The product of radiometric quantities and interaction coefficients
– Radiation equilibrium
• Measurements
– Detectors and cavity theory
– Perturbation factors
Summary
• Dosimetry is concerned with the definition, calculation and
measurement of dosimetric quantities
• Dosimetric quantities describe how the energy of ionizing
radiation is converted to secondary particles and deposited in
matter
• In the following lectures we will define dosimetric quantities
and discuss the fundamentals of radiation equilibrium and
cavity theory.
Part 2 – conversion of energy
After this lecture you will be able to:
• Define the dosimetric quantities given in ICRU 85, especially those relating to the conversion of energy
• Perform simple calculations of conversion quantities
Dosimetric quantities and units
• Conversion of energy (blue blobs)
– Kerma
– Exposition
– Cema
Unchargedparticle
Chargedparticle
Energy transfer
Incomingphoton, ein
Scatteredphoton, e1
Secondaryelectron, e2
Example: Compton scattering
Mean energy transferred: 𝑓𝑖
Energy transfer
Mean energy transferred: 𝑓𝑖
All interaction types: 𝑓 = 𝑖 𝑓𝑖𝜎𝑖 𝑖 𝜎𝑖
𝐸𝑡𝑟 = Φ𝜇𝑉𝜀𝑖𝑛𝑓 = Ψ𝜇
𝜌𝑚𝑓
V
Energy transfer
Mean energy transferred: 𝑓𝑖
All interaction types: 𝑓 = 𝑖 𝑓𝑖𝜎𝑖 𝑖 𝜎𝑖
𝐸𝑡𝑟 = Φ𝜇𝑉𝜀𝑖𝑛𝑓 = Ψ𝜇
𝜌𝑚𝑓
𝐸𝑡𝑟,𝑛𝑒𝑡 = 𝐸𝑡𝑟 1 − 𝑔
V
Kerma
• The kerma, K, for ionizing uncharged particles, is the quotient of dEtr by dm, where dEtr is the mean sum of the initial kinetic energies of all the charged particles liberated in a mass dm of a material by the uncharged particles incident on dm (ICRU 85, 2011)
• Unit: J kg-1
• Special name: Gy (gray)
𝐾 =𝑑𝐸𝑡𝑟𝑑𝑚
Kerma
𝐾 = Ψ𝐸
𝜇𝑡𝑟𝜌𝑑𝐸 𝐾𝑐𝑜𝑙 = Ψ𝐸
𝜇𝑒𝑛𝜌
𝑑𝐸
From : www.nist.govFrom: ESM Ali and DWO Rogers,Phys. Med. Biol. 57 (2012)
Ψ𝐸 for a 10 MV beam𝜇𝑒𝑛𝜌
for water
Kerma rate
• The kerma rate, 𝐾, is the quotient of dK by dt, where dK is the increment of kerma in the time interval dt (ICRU 85, 2011)
• Unit: J kg-1 s-1
• Special name: Gy s-1 (gray per second)
𝐾 =𝑑𝐾
𝑑𝑡
Do you remember?
A water tank is exposed to 1.25 MeV gamma-radiation.At a certain point, the fluence rate is 3.5·109 cm-2s-1.What is the value of Kcol at this point after one hour?
Please write down your answer before moving on to the next page.
Were you right?
A water tank is exposed to 1.25 MeV gamma-radiation.At a certain point, the fluence rate is 3.5·109 cm-2s-1.What is the value of Kcol at this point after one hour?
Solution:Known data:
Ψ = Φ ℎ𝜈 𝑡 = 3.5 ∙ 109𝑐𝑚−2𝑠−1 ∙ 1.25𝑀𝑒𝑉 ∙ 1.6 ∙ 10−13𝐽
𝑀𝑒𝑉∙ 3600𝑠 = 2.520 𝐽𝑐𝑚−2
From www.nist.gov:𝜇𝑒𝑛𝜌
= 2.965 ∙ 10−2𝑐𝑚2𝑔−1 ∙ 1000𝑔
𝑘𝑔= 29.65𝑐𝑚2𝑘𝑔−1
Thus:
𝐾𝑐𝑜𝑙 = Ψ𝜇𝑒𝑛
𝜌= 2.520 𝐽𝑐𝑚−2 ∙ 29.65𝑐𝑚2𝑘𝑔−1 ⇒ 𝐾𝑐𝑜𝑙 = 74.7 𝐺𝑦
Do you remember?
A water tank is exposed to a 14.5 MeV neutron beam.
At a certain point, the kerma is 74.7 Gy.
What is the neutron fluence at this point?
Please write down your answer before moving on to the next page.
Were you right?
A water tank is exposed to a 14.5 MeV neutron beam.
At a certain point, the kerma is 74.7 Gy.
What is the neutron fluence at this point?
Solution:
Known data: 𝐾 = 74.7𝐺𝑦
From Caswell et al., Rad. Res. 83:217, 1980:𝑘Φ = 0.709 ∙ 10−10𝐺𝑦𝑐𝑚2
Thus:
Φ =𝐾
𝑘Φ=
74.7 𝐺𝑦
0.709 ∙ 10−10𝐺𝑦𝑐𝑚2⇒ Φ = 1.05 ∙ 1012𝑐𝑚−2
Exposure
• The exposure, X, is the quotient of dq by dm, where dq is the absolute value of the mean total charge of the ions of one sign produced when all the electrons and positrons liberated or created by photons incident on a mass dm of dry air are completely stopped in dry air (ICRU 85, 2011)
• Unit: C kg-1
𝑋 =𝑑𝑞
𝑑𝑚
Exposure vs. kerma
Mean energy per ion pair: 𝑊
𝑋 =𝑑𝑞
𝑑𝑚=𝑑𝐸𝑡𝑟𝑑𝑚
1 − 𝑔1
𝑊𝑒
= Ψ𝜇𝑡𝑟𝜌
1 − 𝑔1
𝑊𝑒
X = Ψ𝜇𝑒𝑛𝜌
1
𝑊𝑒
= 𝐾𝑎𝑖𝑟,𝑐𝑜𝑙1
𝑊𝑒
𝑊 = 33.97𝑒𝑉
𝑊𝑒 = 33.97 J𝐶−1
Exposure rate
• The exposure rate, 𝑋, is the quotient of dX by dt, where dX is the increment of exposure in the time interval dt (ICRU 85, 2011)
• Unit: C kg-1 s-1
𝑋 =𝑑𝑋
𝑑𝑡
Cema
• The cema, C, for ionizing charged particles, is the quotient of dEel by dm, where dEel is the mean energy lost in electronic interactions in a mass dm of a material by the charged particles, except secondary electrons, incident on dm (ICRU 85, 2011)
• Unit: J kg-1
• Special name: Gy (gray)
𝐶 =𝑑𝐸𝑒𝑙𝑑𝑚
Cema
C=𝑑𝐸𝑒𝑙𝑑𝑚
=Φ𝑆𝑒𝑙𝜌
Example: coulomb interaction
Incomingelectron, ein
Scatteredelectron, e1
Secondaryelectron, e2
Cema
𝐶 = Φ𝐸
𝑆𝑒𝑙𝜌𝑑𝐸 = Φ𝐸
𝐿∞𝜌𝑑𝐸
From : www.nist.govFrom: S Righi et al.,JACMP 14 (2013)
Φ𝐸 for linac electron beams𝑆𝑒𝑙𝜌
for water
Restricted cema
𝐶∆ = Φ′𝐸𝐿∆𝜌𝑑𝐸
e2>D: included in F’E
e2<D: included in LD
Incomingelectron, ein
Scatteredelectron, e1
Secondaryelectron, e2
Example: coulomb interaction
Cema rate
• The cema rate, 𝐶, is the quotient of dC by dt, where dC is the increment of cema in the time interval dt (ICRU 85, 2011)
• Unit: J kg-1 s-1
• Special name: Gy s-1 (gray per second)
𝐶 =𝑑𝐶
𝑑𝑡
Summary
• Conversion of energy takes place when energy of primary
particles is transferred to secondary particles
• Three important dosimetric quantities defined in the ICRU
report 85 for conversion of energy
– Kerma
– Exposure
– Cema
• The restricted cema is applicable when the energy transport
with high-energy delta particles cannot be disregarded
Part 3 – deposition of energy
After this lecture you will be able to:
• Define the dosimetric quantities given in ICRU 85, especially those relating to the deposition of energy
• Perform simple calculations of deposition quantities
Dosimetric quantities and units
• Deposition of energy (yellow blobs)– Energy deposit
– Energy imparted
– Absorbed dose
Unchargedparticle
Chargedparticle
Energy deposit
• The energy deposit, 𝜀𝑖, is the energy deposited in a single interaction, i,
where 𝜀𝑖𝑛 is the energy of the incident ionizing particle (excluding rest energy), 𝜀𝑜𝑢𝑡 is the sum of the energies of all charged and uncharged ionizing particles leaving the interaction (excluding rest energy), and Q is the change in rest energies of the nucleus and of all elementary particles involved in the interaction (ICRU 85, 2011)
• Unit: J
𝜀𝑖 = 𝜀𝑖𝑛 − 𝜀𝑜𝑢𝑡 + 𝑄
Energy deposit
Incomingelectron, ein
Scatteredelectron, e1
Secondaryelectron, e2
Example: coulomb interaction, Q=0
𝜀𝑖 = 𝜀𝑖𝑛 − 𝜀𝑜𝑢𝑡 + 𝑄
Energy deposit
Incomingelectron, ein
Scatteredelectron, e1
Secondaryelectron, e2
hn
EA
𝜀𝑖 = 𝜀𝑖𝑛 − 𝜀𝑜𝑢𝑡 + 𝑄𝜀𝑖 = 𝜀𝑖𝑛 − 𝜀1 + 𝜀2 + ℎ𝜈 + 𝐸𝐴
Example: coulomb interaction, Q=0
Energy deposit
• Energy deposit may appear as– Visible light
– Chemical bindning energy
– Heat
• Stochastic quantity– Subject to random fluctuations
– Associated with a probability distribution
Energy imparted
• The energy imparted, 𝜀, to the matter in a given volume is the sum of all energy deposits in the volume
where the summation is performed over all energy deposits, 𝜀𝑖, in that volume (ICRU 85, 2011)
• Unit: J
𝜀 =
𝑖
𝜀𝑖
Energy imparted
Ψ𝑑 𝐴
𝜀 = 𝑅𝑖𝑛 − 𝑅𝑜𝑢𝑡 + Σ𝑄
𝜀 = − Ψ𝑑 𝐴 + Σ𝑄
𝜀 = − 𝑑𝑖𝑣Ψ𝑑𝑉 + Σ𝑄
𝑑 𝜀
𝑑𝑚= −
1
𝜌𝑑𝑖𝑣Ψ +
1
𝜌
𝑑(Σ𝑄)
𝑑𝑉
Do you remember?
A 6 MeV photon interacts by compton scattering in a given volume. The secondary electron spend half its kinetic energy in electroniccollisions and one fourth in radiation losses. The scattered photoncarries 4 MeV when it escapes the volume. What are the values ofenergy transfer, net energy transfer, and energy imparted?
Please write down your answer before moving on to the next page.
Were you right?
A 6 MeV photon interacts by compton scattering in a given volume. The secondary electron spend half its kinetic energy in electroniccollisions and one fourth in radiation losses. The scattered photoncarries 4 MeV when it escapes the volume. What are the values ofenergy transfer, net energy transfer, and energy imparted?
Solution:
𝜀𝑖𝑛 = 6𝑀𝑒𝑉
𝜀𝑜𝑢𝑡,1 = 4𝑀𝑒𝑉
𝐸𝑡𝑟 = 2𝑀𝑒𝑉
Were you right?
A 6 MeV photon interacts by compton scattering in a given volume. The secondary electron spend half its kinetic energy in electroniccollisions and one fourth in radiation losses. The scattered photoncarries 4 MeV when it escapes the volume. What are the values ofenergy transfer, net energy transfer, and energy imparted?
Solution:
𝜀𝑖𝑛 = 6𝑀𝑒𝑉
𝜀𝑜𝑢𝑡,1 = 4𝑀𝑒𝑉
0.5 𝑀𝑒𝑉
𝐸𝑡𝑟 = 2𝑀𝑒𝑉𝐸𝑡𝑟,𝑛𝑒𝑡 = 1.5 𝑀𝑒𝑉
Were you right?
A 6 MeV photon interacts by compton scattering in a given volume. The secondary electron spend half its kinetic energy in electroniccollisions and one fourth in radiation losses. The scattered photoncarries 4 MeV when it escapes the volume. What are the values ofenergy transfer, net energy transfer, and energy imparted?
Solution:
𝜀𝑖𝑛 = 6𝑀𝑒𝑉
𝜀𝑜𝑢𝑡,1 = 4𝑀𝑒𝑉
1 𝑀𝑒𝑉
0.5 𝑀𝑒𝑉𝜀𝑜𝑢𝑡,2 = 0.5 𝑀𝑒𝑉
𝐸𝑡𝑟 = 2𝑀𝑒𝑉𝐸𝑡𝑟,𝑛𝑒𝑡 = 1.5 𝑀𝑒𝑉
𝜀 = 1.0 𝑀𝑒𝑉
Absorbed dose
• The absorbed dose, D, is the quotient of 𝑑 𝜀 by dm, where 𝑑 𝜀is the mean energy imparted by ionizing radiation to matter of mass dm (ICRU 85, 2011)
• Unit: J kg-1
• Special name: Gy (gray)
𝐷 =𝑑 𝜀
𝑑𝑚
Absorbed dose rate
• The absorbed dose rate, 𝐷, is the quotient of dD by dt, where dD is the increment of absorbed dose in the time interval dt(ICRU 85, 2011)
• Unit: J kg-1 s-1
• Special name: Gy s-1 (gray per second)
𝐷 =𝑑𝐷
𝑑𝑡
Summary
• Deposition of energy takes place when energy is locally
absorbed
• Three important dosimetric quantities defined in the ICRU
report 85 for deposition of energy
– Energy deposit
– Energy imparted
– Absorbed dose
• There is a close relation between radiation transport and
dosimetry
Part 4 – radiation equilibrium
After this lecture you will be able to:
• Define different types of radiation equilibrium and discuss its consequences for absorbed dose calculations
• Perform simple calculations of absorbed dose under different types of radiation equilibrium
Radiation equilibrium
𝑅𝑖𝑛 = 𝑅𝑜𝑢𝑡 (𝑑𝑖𝑣 Ψ = 0)
𝑅𝑖𝑛,𝑖 = 𝑅𝑜𝑢𝑡,𝑖 𝑑𝑖𝑣 Ψ𝑖 = 0
Distance greater thanrange of particle i
Rin Rout
Partial radiation equilibrium
Charged particle equilibrium (CPE): 𝑅𝑖𝑛,𝑐 = 𝑅𝑜𝑢𝑡,𝑐 𝑑𝑖𝑣 Ψ𝑐 = 0
d-particle equilibrium: 𝑅𝑖𝑛,𝛿= 𝑅𝑜𝑢𝑡,𝛿 (𝑑𝑖𝑣 Ψ𝛿 = 0)
Partial d-particle equilibrium: 𝑅𝑖𝑛,𝛿,Δ = 𝑅𝑜𝑢𝑡,𝛿,Δ (𝑑𝑖𝑣 Ψ𝛿,Δ = 0)
Charged particle equilibrium (CPE)
Incoming beam ofuncharged particles
𝑅𝑖𝑛,𝑢
𝑅𝑜𝑢𝑡,𝑢
𝑅𝑖𝑛,𝑐
𝑅𝑜𝑢𝑡,𝑐
𝜀 ≈ 𝐸𝑡𝑟
Distance greater thanrange of charged particles
Charged particle equilibrium (CPE)
Distance greater thanrange of charged particles
Incoming beam ofuncharged particles
𝑅𝑖𝑛,𝑢
𝑅𝑜𝑢𝑡,𝑢,𝑟𝑎𝑑
𝜀 = 𝐸𝑡𝑟 − 𝑅𝑜𝑢𝑡,𝑢,𝑟𝑎𝑑
𝑅𝑜𝑢𝑡,𝑢,𝑛𝑜𝑛𝑟𝑎𝑑
𝑅𝑜𝑢𝑡,𝑢𝑅𝑖𝑛,𝑐
𝑅𝑜𝑢𝑡,𝑐
Charged particle equilibrium (CPE)
𝜀 = 𝑅𝑖𝑛,𝑢 + 𝑅𝑖𝑛,𝑐 − 𝑅𝑜𝑢𝑡,𝑢 − 𝑅𝑜𝑢𝑡,𝑐 + Σ𝑄
𝑅𝑖𝑛,𝑐 = 𝑅𝑜𝑢𝑡,𝑐
𝜀 = 𝑅𝑖𝑛,𝑢 − 𝑅𝑜𝑢𝑡,𝑢 + Σ𝑄
𝜀 = 𝑅𝑖𝑛,𝑢 − 𝑅𝑜𝑢𝑡,𝑢,𝑛𝑜𝑛𝑟𝑎𝑑 − 𝑅𝑜𝑢𝑡,𝑢,𝑟𝑎𝑑 + Σ𝑄
𝜀 = 𝐸𝑡𝑟 − 𝑅𝑜𝑢𝑡,𝑢,𝑟𝑎𝑑
𝜀 = 𝐸𝑡𝑟(1 − 𝑔)
Charged particle equilibrium (CPE)
DepthDepthRange ofcharged particle
Collision kerma, Kcol
Absorbed dose, D
Without attenuation With attenuation
Range ofcharged particle
Charged particle equilibrium (CPE)
Depth
Collision kerma, Kcol
Absorbed dose, D
With attenuation
d
𝐷 𝑑 = 𝐾𝑐𝑜𝑙 𝑑 − 𝑥
𝐾𝑐𝑜𝑙 𝑑 = 𝐾𝑐𝑜𝑙 𝑑 − 𝑥 𝑒−𝜇 𝑥
𝐷 𝑑 = 𝐾𝑐𝑜𝑙 𝑑 𝑒𝜇 𝑥
𝐷 𝑑 = 𝐾𝑐𝑜𝑙 𝑑 (1 + 𝜇 𝑥)
𝑥
Delta-particle equilibrium
Distance greater thanrange of delta particles
𝑅𝑖𝑛,𝑐
𝑅𝑜𝑢𝑡,𝑐
𝑅𝑖𝑛,𝛿
𝑅𝑜𝑢𝑡,𝛿
𝑘 = 1
Charged particlefluence
Partial delta-particle equilibrium
Distance greater thanrange of slow delta particles
Charged particlefluence
𝑅𝑖𝑛,𝑐
𝑅𝑜𝑢𝑡,𝑐
𝑅𝑖𝑛,𝛿,∆
𝑅𝑜𝑢𝑡,𝛿,∆
𝑘 < 1
Do you remember?
A water tank is exposed to 1.25 MeV gamma-radiation.At a certain point, the fluence is 2.1·1011 cm-2.Assuming CPE, what is the absorbed dose at this point?
Please write down your answer before moving on to the next page.
Were you right?
A water tank is exposed to 1.25 MeV gamma-radiation.At a certain point, the fluence is 2.1·1011 cm-2.Assuming CPE, what is the absorbed dose at this point?
Solution:Known data:
Ψ = Φ ℎ𝜈 = 2.1 ∙ 1011𝑐𝑚−2𝑠−1 ∙ 1.25𝑀𝑒𝑉 ∙ 1.6 ∙ 10−13𝐽
𝑀𝑒𝑉= 4.20 ∙ 10−2𝐽𝑐𝑚−2
From www.nist.gov:𝜇𝑒𝑛𝜌
= 2.965 ∙ 10−2𝑐𝑚2𝑔−1 ∙ 1000𝑔
𝑘𝑔= 29.65𝑐𝑚2𝑘𝑔−1
Given that CPE exists, we get:
𝐷 = 𝐾𝑐𝑜𝑙 = Ψ𝜇𝑒𝑛
𝜌= 4.20 ∙ 10−2𝐽𝑐𝑚−2 ∙ 29.65𝑐𝑚2𝑘𝑔−1 ⇒ 𝐷 = 1.25 𝐺𝑦
Do you remember?
A thin disc of lithium fluoride (LiF) is irradiated by an electronfluence of 4.1·109 cm-2 with energy 6 MeV. Assuming delta-particle equilibrium, what is the absorbed dose in the disc?
Please write down your answer before moving on to the next page.
Were you right?
A thin disc of lithium fluoride (LiF) is irradiated by an electronfluence of 4.1·109 cm-2 with energy 6 MeV. Assuming delta-particle equilibrium, what is the absorbed dose in the disc?
Solution:
Known data: Φ = 4.1 ∙ 109𝑐𝑚−2
From www.nist.gov:𝑆𝑒𝑙𝜌
= 1.547 𝑀𝑒𝑉𝑐𝑚2𝑔−1 ∙ 1000𝑔
𝑘𝑔∙ 1.6 ∙ 10−13
𝐽
𝑀𝑒𝑉= 2.48 ∙ 10−10 𝐽𝑐𝑚2 𝑘𝑔−1
Given that delta-particle equilibrium exists, we get:
D = C = Φ𝑆𝑒𝑙
𝜌= 4.1 ∙ 109𝑐𝑚−2 ∙ 2.48 ∙ 10−10 𝐽𝑐𝑚2 𝑘𝑔−1 ⇒ 𝐷 = 1.02 𝐺𝑦
Summary
• Radiation equilibrium can only be approximate in a medium
exposed to external irradiation
• Different types of partial radiation equilibrium have important
consequences for absorbed dose calculations
– Charged particle equilbrium (CPE)
– Delta-particle equilibrium
– Partial delta-particle equilibrium
• Unless CPE exists, absorbed dose calculations need to account
for charged particle interactions
Part 5 – Cavity theory
After this lecture you will be able to:
• Discuss absorbed dose measurements in the context of cavity theory
• Describe ideal cases of large and small cavities, and discuss the case of mid-size cavities in terms of charged particle fluence
• Perform simple calculations based on the Bragg-Gray theory
• Describe the particular case of a gas-filled ion chamber, and explain the concept of perturbation factor
Ideal case – large detector
Charged particle equilibrium (CPE): 𝐷 = 𝐾𝑐 = Ψ𝜇𝑒𝑛
𝜌
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
Ψ𝑚𝑒𝑑𝜇𝑒𝑛𝜌
𝑚𝑒𝑑
Ψ𝑑𝑒𝑡𝜇𝑒𝑛𝜌
𝑑𝑒𝑡
Incoming beam ofionizing particles
Ideal case – large detector
Charged particle equilibrium (CPE): 𝐷 = 𝐾𝑐 = Ψ𝜇𝑒𝑛
𝜌
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
Ψ𝑚𝑒𝑑𝜇𝑒𝑛𝜌
𝑚𝑒𝑑
Ψ𝑑𝑒𝑡𝜇𝑒𝑛𝜌
𝑑𝑒𝑡
Incoming beam ofionizing particles
Ideal case – large detector
Charged particle equilibrium (CPE): 𝐷 = 𝐾𝑐 = Ψ𝜇𝑒𝑛
𝜌
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
Ψ𝑚𝑒𝑑𝜇𝑒𝑛𝜌
𝑚𝑒𝑑
Ψ𝑑𝑒𝑡𝜇𝑒𝑛𝜌
𝑑𝑒𝑡
=
𝜇𝑒𝑛𝜌
𝑚𝑒𝑑
𝜇𝑒𝑛𝜌
𝑑𝑒𝑡
≡𝜇𝑒𝑛𝜌
𝑑𝑒𝑡
𝑚𝑒𝑑
Samefluence
Incoming beam ofionizing particles
Ideal case – small detector
Incoming beam ofionizing particles
Delta-particle equilibrium: 𝐷 = 𝐶 = Φ𝑆𝑒𝑙
𝜌
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
Φ𝑚𝑒𝑑𝑆𝑒𝑙𝜌
𝑚𝑒𝑑
Φ𝑑𝑒𝑡𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
Ideal case – small detector
Incoming beam ofionizing particles
Delta-particle equilibrium: 𝐷 = 𝐶 = Φ𝑆𝑒𝑙
𝜌
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
Φ𝑚𝑒𝑑𝑆𝑒𝑙𝜌
𝑚𝑒𝑑
Φ𝑑𝑒𝑡𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
=
𝑆𝑒𝑙𝜌
𝑚𝑒𝑑
𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
≡𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
𝑚𝑒𝑑
Samefluence
Electron fluence – large detector
Attenuation of primary beam is neglected
DepthFtotal
Fmed
Fdet
Medium Detector Medium
Electron fluence – small detector
Depth
Attenuation of primary beam is neglected
Medium Detector Medium
Ftotal
Fmed
Fdet
Electron fluence – mid-size detector
Depth
Attenuation of primary beam is neglected
Medium Detector Medium
Ftotal
Fmed
Fdet
Do you remember?
An air-filled ion chamber is exposed to 1.25 MeV gamma radiation and the collected charge is 11.55 nC. The gas volume is 0.1 cm3, and the polystyrene wall is thicker than the range of the secondary chargedparticles. Assuming the Bragg-Gray condition is fulfilled, what is the absorbed dose in the adjacent polystyrene wall?
Please write down your answer before moving on to the next page.
Were you right?
An air-filled ion chamber is exposed to 1.25 MeV gamma radiation and the collected charge is 11.55 nC. The gas volume is 0.1 cm3, and the polystyrene wall is thicker than the range of the secondary chargedparticles. Assuming the Bragg-Gray condition is fulfilled, what is the absorbed dose in the adjacent polystyrene wall?
Solution
Known data:
𝑀 = 11.55 ∙ 10−9 𝐶𝑉 = 0.1 𝑐𝑚3
𝜌 = 1.293 ∙ 10−6𝑘𝑔𝑐𝑚3
𝑊𝑒 = 33.97 J𝐶−1
⇒ 𝐷𝑑𝑒𝑡 =𝑀
𝜌𝑉 𝑊𝑒 =3.03 𝐺𝑦
Were you right?
An air-filled ion chamber is exposed to 1.25 MeV gamma radiation and the collected charge is 11.55 nC. The gas volume is 0.1 cm3, and the polystyrene wall is thicker than the range of the secondary chargedparticles. Assuming the Bragg-Gray condition is fulfilled, what is the absorbed dose in the adjacent polystyrene wall?
Solution
From www.nist.gov:𝑓 = 0.467
𝐸𝑒 =1
2ℎ𝜈 𝑓 =
1
21.25 𝑀𝑒𝑉 0.467 = 292 𝑘𝑒𝑉
𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
= 2.11 𝑀𝑒𝑉𝑐𝑚2𝑔−1
𝑆𝑒𝑙𝜌
𝑚𝑒𝑑
= 2.34 𝑀𝑒𝑉𝑐𝑚2𝑔−1
Were you right?
An air-filled ion chamber is exposed to 1.25 MeV gamma radiation and the collected charge is 11.55 nC. The gas volume is 0.1 cm3, and the polystyrene wall is thicker than the range of the secondary chargedparticles. Assuming the Bragg-Gray condition is fulfilled, what is the absorbed dose in the adjacent polystyrene wall?
SolutionGiven that the Bragg−Gray condition is fulfilled:
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
𝑆𝑒𝑙
𝜌 𝑑𝑒𝑡
𝑚𝑒𝑑⇒ 𝐷𝑚𝑒𝑑 = 3.03 𝐺𝑦
2.34
2.11⇒ 𝐷𝑚𝑒𝑑 = 3.36 𝐺𝑦
Ion chamber dosimetry
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑚𝑒𝑑
𝐿Δ𝜌 𝑚𝑒𝑑
𝑑𝐸 +Φ′𝐸,𝑚𝑒𝑑(Δ)Δ
𝑆𝑒𝑙𝜌 𝑚𝑒𝑑
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑑𝑒𝑡
𝐿Δ𝜌
𝑑𝑒𝑡𝑑𝐸 +Φ′
𝐸,𝑑𝑒𝑡(Δ)Δ𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
Ion chamber dosimetry
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡=
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑚𝑒𝑑
𝐿Δ𝜌 𝑚𝑒𝑑
𝑑𝐸 +Φ′𝐸,𝑚𝑒𝑑(Δ)Δ
𝑆𝑒𝑙𝜌 𝑚𝑒𝑑
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑑𝑒𝑡
𝐿Δ𝜌
𝑑𝑒𝑡𝑑𝐸 +Φ′
𝐸,𝑑𝑒𝑡(Δ)Δ𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
𝑆𝑚𝑒𝑑,𝑑𝑒𝑡 =
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑚𝑒𝑑
𝐿Δ𝜌 𝑚𝑒𝑑
𝑑𝐸 + Φ′𝐸,𝑚𝑒𝑑(Δ)Δ
𝑆𝑒𝑙𝜌 𝑚𝑒𝑑
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑚𝑒𝑑
𝐿Δ𝜌
𝑑𝑒𝑡𝑑𝐸 + Φ′
𝐸,𝑚𝑒𝑑(Δ)Δ𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
Ion chamber dosimetry
𝐷𝑚𝑒𝑑
𝐷𝑑𝑒𝑡= 𝑆𝑚𝑒𝑑,𝑑𝑒𝑡 𝑝
𝑆𝑚𝑒𝑑,𝑑𝑒𝑡 =
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑚𝑒𝑑
𝐿Δ𝜌
𝑚𝑒𝑑𝑑𝐸 + Φ′
𝐸,𝑚𝑒𝑑(Δ)Δ𝑆𝑒𝑙𝜌
𝑚𝑒𝑑
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑚𝑒𝑑
𝐿Δ𝜌 𝑑𝑒𝑡
𝑑𝐸 + Φ′𝐸,𝑚𝑒𝑑(Δ)Δ
𝑆𝑒𝑙𝜌 𝑑𝑒𝑡
𝑝 =
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑚𝑒𝑑
𝐿Δ𝜌
𝑑𝑒𝑡𝑑𝐸 + Φ′
𝐸,𝑚𝑒𝑑(Δ)Δ𝑆𝑒𝑙𝜌
𝑑𝑒𝑡
Δ𝐸𝑚𝑎𝑥Φ′𝐸,𝑑𝑒𝑡
𝐿Δ𝜌 𝑑𝑒𝑡
𝑑𝐸 + Φ′𝐸,𝑑𝑒𝑡(Δ)Δ
𝑆𝑒𝑙𝜌 𝑑𝑒𝑡
Summary
• A radiation detector forms a cavity in the medium
• Cavity theory deals with the conversion of absorbed dose in
the detector to absorbed dose in the medium
• While ideal cases are illustrative, the effect on the particle
fluence of a real detector needs to be taken inte account
• This can be accounted for through the concept of
perturbation factors