Physics and Chemistry of Radiation Absorption

Question 1

Directly ionizing radiation

Answer 1

causes ionization immediately and continuously as it passes through material (charged particles: e.g. electrons, alpha particles, protons

Question 2

Indirectly ionizing radiation

Answer 2

only causes ionization after some type of interaction with the material (no charge: photons)

Question 3

Particulate radiation

Answer 3

(atomic or subatomic particles e.g. electrons, protons)

Question 4

Electromagnetic radiation

Answer 4

Photons

Question 5

X-rays

Answer 5

photons emitted by electrons transitioning between atomic energy levels (characteristic) or slowing down in a coulomb force field (bremsstrahlung)

Question 6

Photoelectric interactions

Answer 6

The likelihood of a photoelectric interaction is approximately proportional to 1/E3, and to Z3
Dominant photon interaction in tissue below 30 keV

Question 7

Compton interactions

Answer 7

Independent of Z
Depend on electron density
100 keV - 10 MeV

Question 8

For every 1 Gy of radiation, there are more than _____ ionizations within a 10um diameter cell

Answer 8

10^5

Question 9

LET

Answer 9

the average energy transferred per unit track length of the radiation (keV/μm)
Low LET radiation is sparsely ionizing (photons),
high LET radiation is densely ionizing (heavy charged particles, neutrons)
– 250-kV x-rays = 2.0 keV/μm
– Cobalt-60 gamma rays (average energy 1.25 MeV) = 0.2 keV/μm
– 14 MeV neutrons = 100 keV/μm (energy average)
– 2.5 MeV alpha particles = 166 keV/μm

Question 10

Spur

Answer 10

60-100 eV, 3 ion pairs, 4nm

95% of energy deposition by photons is spurs

Question 11

“Local multiply damaged site or “clustered lesion”

Answer 11

any of various complex lesions including base damage as well as DSBs, produced by spurs and blobs from a high LET track, less frequently by low LET photons
Clustered lesions are unique to ionizing radiation and are difficult for the cell to repair
As electrons slow down and near the end of their range, their LET increases
The resulting increased density of ionizations leads to clusters of damage, spread out over up to 20 base pairs
Clusters are a small % of damage caused by photons (low LET)

Question 12

Hydroxyl radical

Answer 12

·OH most effective at DNA damage, removes an H from DNA, producing a DNA radical R·
·OH lifetime of 10-9 secs, can diffuse to DNA from within a cylinder with a diameter of 4nm
Approximately 2/3 of the DNA damage to mammalian cells from low LET radiation caused by ·OH (indirect action)
Hydroxyl radicals have a lifetime of 10-9 seconds