lecture 9 Flashcards
E/M radiation for photon E
> 100 ev starts in UV
What is the absorbed dose
the measure of the amount of energy from ionizing radiation deposited in a mass of some material (delta E/delta M)
Does the biological effect depend only on dose
No, it is dependent on the nature of radiation
Old unit of Absorbed dose
RAD (radiation absorbed dose) = 100 ergs/g
1 erg=10^-7 J
SI unit for absorbed dose
Gy (Gray)=1J/kg=10^7 erg/1000g=10000erg/g=100rad
For what type of radiation Gy is used
all types of radiation and does not describe the biological effects (health effects depend on how radiation interacts with material on micro-level)
Absorbed dose: is it a macroscopic concept
yes, it is not related to microdosimetry
Is energy delivered by radiation is continuous and uniform?
No, it is delivered not uniformly through the entire mass
Why do we care about secondary electrons?
They produce additional ionization and excitation until the energies fall below the threshold for further interaction
What paradox illustrated irradiation of viruses
both sensitive and highly resistant, some of them were killed at low dose. On average they tolerated very large doses: actually receiving a dose is not equal to the dose delivered, bioeffects can be very different at the same dose. Random statistical hit.
How many J in ev
1.6*10^(-19)
Absorbed energy depends on
number of particles traversing the mass and energy that they deposit in tissue
Main problems with the absorbed dose concept
neglects the density of energy absorbed per unit mass in a point in the medium,
on a small scale (micrometer) the energy distribution is non-uniform,
energy is localized near tracks,
E transfer takes place in discrete amounts,
statistical quantity gives a global value (large enough that statistical fluctuations are not significant)
The biological effect is related to the dose
deposition at micro-level (cell, DNA), depends on nature nad E of particles
Can we use exposure for neutrons?
No, only for gamma and x-rays.
What affects the absorbed dose?
type and E of Radiation, depth and elementary composition of the material.
How do we characterize dose from X-ray fields?
exposure
What is exposure/ what unit is used?
amount of E transferred from the X-ray field to a unit mass of AIR. 1 [X] unit = 1C/kg air = 34 Gy/air (production of single ion pair = 34 ev)
Charge of single ion?
1.6*10^(-19) C
What unit is used for exposure (non SI)
R = quantity of gamma radiation that produces ions carrying one SC (3*10^9 C) per cm^3 at T=0C, and P = 760mm. [R] = 1 [sc/cm^3]
convert 1 X unit to R
1 X = 3881 R
1R = 2.58*10^-4 X
Dose to air from 1R
1R= 0.877 rad = 87.7 erg/g air = 8.7 *10^-4 Gy
Formula and units for exposure rate
dX/dt = Fluence rate ([photones/(cm^28s) * E (Mev) 1.610^(-13) J/Mev* mu (air/cm)]/density of air34) [C/kgs]
Formula and units for dose rate
dD/dt = Fluence rate ([photones/(cm^28s) * E (Mev) 1.610^(-13) J/Mev* mu (/cm)]/density of medium*1) [Gy/s]
The formula for a point source of gamma rays Dose rate
dD/dt = E fluence rate* mass E abs coefficient (cm^2/g)= CE(mu.en/rho)/4Pir^2), where C is activity
Dose rate from exposure rate can be found as
34Xrate mu/density if medium divided on mu/density of air
Dose for a beam of photons
D = mu.en/rho * photon fluence (/cm^2) * E
Two reactions of tissue with neutrons
capture (thermal) and scattering (higher E)
Dose from thermal neutrons formula
D=fluence (/cm^2)*atom density (/cm^3) reaction cross sectionE /tissue density
name major neutron capture reactions is tissue
14N(n, p)14c, 1H(n, gama)2H
name 4 principal tissue components
H, O, C N
Average E lost by fast neutron
4MmE/2(m+M)^2
first collision dose
represent the abs dose when mean free path»_space; target size;
D delivered by individual neutron;
formula and units for first collision dose
D=atom densityscattering cross sectionAverage E lost by fast neutron/density;
[Gy*cm^2]
What is KERMA
(for indirectly ionizing radiation - no charge) the measure of all the E transferred from the uncharged particle (gamma, neutron to primary ionizing particle (e, photo e, Compton e, scattered nuclei and etc) per unit mass
Provide examples of radionuclides with specific metabolic pathways?
I- thyroid
Ra, Sr -bone,
3H - water in body,
Cs- throughout the whole body
Range of alpha particle
short, E deposited locally
Rate of E absorption per gram tissue for alpha particle
dD/dx = AE [Mev/g*s]
Dose formula for alpha particle
D= fluence * mu.rho *E absorbed
Formula for KERMA
D= fluence * mu.rho *E transferred
10 MEV photon penetration in 100 g tissue
single interaction produces beta - and beta + with 4.5 MEV each, that dissipate all E through Brems giving us Kerma os 2*4.5Mev / 0.1kg
