Chapter 3: Fundamentals of Dosimetry Flashcards
What is Dosimetry?
Dosimetry is the measurement and calculation of the amount of energy imparted to matter by radiation.
What kind of changes may occur in matter following irradiation?
Changes in matter caused by IONIZING radiation can be broadly classified as:
(1) Physical (< 10^-15 s) - Ionization, pair production, photo-nuclear disintegration, rise in temperature .
(2) Chemical (< 10^-5 s) - Ion radicals, Free radical formation, damage to DNA molecule (critical target), breaking of bonds.
(3) Biological (hours, days, months, years, generations) - Damage to cells, organs and systems, carcinogenesis, hereditable effects
Historically, what dosimetric quantity was mainly used to quantify the passage of radiation through matter? What is its replacement?
Exposure has been historically used to quantify the passage of ionizing photon radiation ( X and gamma rays) in air by measuring the amount of ionization. “It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air.”
X= dQ /dm
Its SI unit is coulomb per kilogram (C/kg). A previously used unit was the roentgen (R).
Conversion:
1 R = 0.000258 C/kg
1 C/kg= 3876 R
Recently, exposure has been replaced by kerma (kinetic energy released per unit mass).
What is the dosimetric quantity that best describes the EFFECTS of radiation on matter and human beings?
Absorbed Dose best describes the effects of radiation. Its SI unit is J/kg or the Gray (Gy). Another popular unit is the rad. Conversion: 1 rad= 0.01 Gy 1 Gy= 100 rads
What dosimetric quantities are used to quantify the radiation field?
Fluence and Energy fluence.
” A radiation field at a point P can be quantified by the physical non-stochastic
quantity fluence, which is usually expressed in units of m–2 or cm–2, and is given
by the relation:
Φ = dN /da
where dN is the differential of the expectation value of the number of particles
(photons or massive particles) striking an infinitesimal sphere with a great circle
area, da, surrounding point P.”
“The concept of energy fluence follows easily, by summing the radiant
energy of each particle that strikes the infinitesimal sphere:
Ψ = dR /da
dR is the differential of the radiant energy R — kinetic energy
of massive particles, energy of photons — that impinges on the infinitesimal
sphere. The SI unit of energy fluence is joules per square metre (J/m^2).
If the radiation field is composed of particles each with the same energy E, the energy fluence is related to the fluence (or particle fluence) through the
simple expression: Ψ = Φ E “
Define the dosimetric quantity of fluence. What is it used to quantify?
Fluence is used to quantify the radiation field at a point P. Fluence is the differential of the number of photons or particles striking the greater area of the sphere, da, surrounding point P.
Φ = dN /da
Its unit is m^-2 or cm^-2.
Define the dosimetric quantitiy of energy fluence. What is it used to quantify?
Energy fluence, like fluence, is used to quantify the radiation field at a point P. Energy fluence is “ the differential of the radiant energy R — kinetic energy
of massive particles, energy of photons — that impinges on the infinitesimal sphere.
Ψ = dR /da
The SI unit of energy fluence is joules per square metre (J/m^2).
If the radiation field is composed of particles each with the same energy E, the energy fluence is related to the fluence (or particle fluence) through the
simple expression: Ψ = Φ E .”
Define the dosimetric quantities:
(1) Energy transferred
(2) Net energy transferred
“In a volume, V, of material, the energy transferred (εtr) is given by the sum of all the initial kinetic energies of charged ionizing particles liberated by the uncharged particles in the volume V.”
” When the energies of the bremsstrahlung photons (hν_brem) and the
annihilation photons (T_ann) are subtracted from etr, another quantity is defined —
the net energy transferred — εtr net:
ε_tr net = ε_tr − ∑ hv_brem − ∑ T_ ann
.”
Explain how the formula for the dosimetric quantity - net energy transferred- is altered for photons in the diagnostic energy range incident on low Z materials.
Pair production is threshold limited to > 1.02 MeV. Therefore is does not occur in the diagnostic energy range.
Also in low Z materials, Bremsstrahlung radiation is very unlikely.
We have then that the net energy transferred is equal to energy transferred.
Define the dosimetric quantity: Energy imparted. Give its formula.
“This quantity is defined for any
radiation (charged or uncharged) and is related to the part of the radiant energy
that can produce effects within an irradiated volume. If V is the irradiated volume, R_in is the radiant energy that enters the volume and R_out is the energy that leaves the volume, the energy imparted is defined as:
ε = R_in − R_out + E_m→ R − E_ R →m
The quantities E_m→R and E_R→m are the changes in energy when
the rest mass of a particle is converted to radiant energy (m→R) or the energy of a
photon is converted to the mass of particles (R→m) inside the volume V. “
Explain how the formula for the dosimetric quantity - energy imparted- is altered for photons in the radiation in the diagnostic energy range.
ε =R_in − R_out
The quantities E_m→R and E_R→m becomes negligible in the diagnostic energy range.
Define kerma.
Kerma stands for the kinetic energy released per unit mass.
“ The physical, non-stochastic quantity kerma (K) is related to the energy
transferred from uncharged particles to matter. It is defined as:
K= dε_tr /dm
where the quantity dε_tr is the expectation value of the energy transferred from
indirectly ionizing radiation to charged particles in the elemental volume dV of
mass dm. The SI unit of kerma is joules per kilogram (j/kg), which is given the
special name gray (gy).”
True or False: The dosimetric quantities Kerma and Absorbed dose have the same SI units.
True.
The SI unit for Kerma and Absorbed dose is J/kg or Gy.
True or False: Kerma is defined for ionizing and indirectly ionizing radiation.
False.
Kerma is defined for indirectly ionizing radiation like photons and neutrons.
What are the two components of Kerma? What is the relationship between these components?
(1) Collision Kerma - the energy spent by secondary charged particles in collisions and ionizations. It is the differential of the NET energy transferred per unit mass.
K_col= dε_tr_net /dm
(2) Radiative Kerma - kinetic energy of secondary charged particles that is converted back to photon energy. Eg. through bremsstrahlung production and electron-positron annihilation.
K_rad= K - K_col
K= K_col + K_rad
