L 4: Interaction of Ionizing radiation

Question 1

Photodisintegration

Answer 1

• It is Interaction of energetic photon with the nucleus results in emission of atomic neutron.
• It does not contribute considerably to attenuation in a patient.
• It becomes more efficient in high Z materials of the treatment head.
• It can cause single event pacemaker dysfunction.
• Hence >10MV beams are not used to treat patients with a pacemaker
• Neutrona are produced in high energy x-ray beams by this process as the x-ray beams hit the target, FF, collimators = Neutron contamination

Question 2

Gamma Decay

Answer 2

A and Z stays the same
Occurs in pico seconds

Question 3

Naturally occuring Isotopes

Answer 3

C-14
U-288
K-40
Th-232
Ra-226

Question 4

Alpha Decay

Answer 4

• Changes both atomic mass and atomic number of the molecule, both A & Z

Question 5

Beta + Decay

Answer 5

• Electron capture
• Atomic number decreases by 1

Question 6

Beta -ve Decay

Answer 6

• Increases Atomic number by 1
• Aneutron decays into proton, electron and anti neutrino.
• So daughter nuclei will have Z+1

Question 7

Compton effect

Answer 7

incoming photon
Low energy - scatter photon is isotrophic
High energy - Scatter photon is forward peaked

Question 8

QA for linacs major categories

Answer 8

• Mechanical
• Dosimetric
• Safety
• Imaging

Question 9

Flattening Filter

Answer 9

• Flattens the beam at 10cm depth
• Can create horns at the periphery at shallow depth.
• So Periphery may have more internsity than the central axis

Question 10

FFF

Answer 10

Less attenuation = higher dose rate

Question 11

Cherenkov radiation

Answer 11

Blue flashes seen by the patient when eyes closed.

Question 12

Problem

Answer 12

Question 13

Z dependent

Answer 13

Photoelectric effect
Pair production

Question 14

Gold fiducials

Answer 14

are visible on CT scan due to Photoelectric effect
Gold has high atomic number and high density

Question 15

Pair production

Answer 15

• Interacts with the nucles to form a pair of electron-positron
• Needs 1.022 MeV energy
• Interacts with the nuclear coulomb forces/field

Question 16

Electron energy at different angles

Answer 16

Question 17

Photo Electric effect for diagnostic x-rays

Answer 17

Question 18

Internal Photoelectric effect

Answer 18

A characteristic x-ray produced by the atom is absorbed by an outer shell elctron, ejecting the electron.

Question 19

Delta rays

Answer 19

An electron ejected through electron collision, forming its own ionizaition track

Question 20

Collision loss

Answer 20

Question 21

Radiative loss

Answer 21

Question 22

LET

Answer 22

• Directly related to charge
• Inversely related to velocity
• eg: Electron LET is higher at lower energy

Question 23

Secular equilibrium graph

Answer 23