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