Radiation Biology
Energy Transfer
Particles lose energy in matter.
Energy transferred describes
the kinetic energy gained by
charged particles.
Eventually energy loss is due
to ionization.
Energy imparted is the energy
lost by charged particles.
particle energy
10
MeV
5.4
3.6
energy transferred
9.0
energy imparted
2.8
4.2
2.0
3.3
2.8
1.1
0.1
2.8
1.1
Kerma
Kerma is the energy
transferred per unit mass.
• Kinetic Energy Released per
unit MAss
Radiative kerma is due to
bremmstrahlung and
annihilation.
Collision kerma subtracts the
reradiated photons.
• Net energy transferred per
mass
Etr
K
m
rad
Runc
Kr
m
Etrnet
KC K K r
m
Absorbed Dose
Absorbed dose or dose is
the energy imparted per unit
mass.
E
D
m
Like kerma dose is based on
mean changes in energy.
Two units are used.
• 1 gray (Gy) = 1 J / kg
• 1 rad = 100 erg / g (older)
J 10 7 erg
1 Gy
kg
103 g
erg
10 4
100 rad
g
Lethality
Dose can be compared to
physical effects.
Lethality refers to the likelihood
• Cell death
• Whole body death (see graph
at right)
Lethality %
that a dose will be fatal.
Dose (cGy)
Federation of American Scientists
Exposure
Exposure is defined by the
ionization produced by
photons.
• Gammas and X-rays
• Charge per unit mass in air
The unit of exposure is the
Roentgen (R).
• 1 R = 2.58 x 10-4 C / kg
Useful Conversion
Show that the original
roentgen is equivalent to the
modern one.
Look up constants:
• Density of air at STP is
0.001293 g / cm-3
• 1 esu = 3.34 x 10-10 C
3.34 x 10-10 C / 1.293 x 10-6 kg
= 2.58 x 10-4 C / kg
Radiation Factor
The effect of radiation on
tissue depends on the linear
energy transfer (LET).
• Higher LET is more damaging
Radiation has a weighting
factor based on particle.
• Factor WR or Q
In terms of LET
• LET L (keV / mm in water)
• < 10; WR = 1
• 10 – 100; WR = 0.32L – 2.2
• > 100;
WR 300 / L
In terms of particle
• e, g, m; WR = 1
• n; WR = 5 – 20
• p; WR = 5
• a; WR = 20
Equivalent Dose
The equivalent dose is a
measure that combines the
type of radiation and dose.
HT WR D
Unit is Sievert (Sv)
• 1 Gy equivalent
Older unit is rem
• Roentgen equivalent man
• 1 rad equivalent
• 100 rem = 1 Sv
Natural doses
• Cosmics: 0.3 mSv / yr
• Soil: 0.2 mSv / yr
• Radon: 2 mSv / yr
• Total natural: 3 mSv / yr
Environmental hazards
• Flying at 12 km: 7 mSv / hr
• Chest x-ray: 0.1 mSv
• Mammogram: 1 mSv
• CT scan: 20 mSv
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