Radiation Interactions

Question 1

What is the energy of a scattered photon after undergoing a Compton Scatter event?

Answer 1

E’ = E0/(1+E0/0.511*(1-cos(theta)))

Question 2

What is the maximum allowable exposure for an unbadged worker?

Answer 2

Same as the public, TEDE = 1 mSv/year

Question 3

What is the energy of a photon given its wavelength?

Answer 3

E= hc/lambda

Question 4

What is Planck’s constant?

Answer 4

4.1357 eV/s

Question 5

How many Bq per Ci?

Answer 5

1 Ci = 3.7E10 Bq

Question 6

How densely ionizing are alpha particles? (i.e., how many ion pairs per mm?)

Answer 6

5000 ion pairs/mm

Question 7

What is an air kerma strength U?

Answer 7

1 micro-Gy*m^2/h

Question 8

How does the probability of a photoelectric interaction relate to Z of the absorber and E of the photon?

Answer 8

Z^3/E^3

Question 9

What is the conversion between J and eV?

Answer 9

1.6E-19 J/eV

Question 10

How do you calculate the effective Z of a mixture?

Answer 10

x = 2.94, Zeff = rootx( f1Z1^x + f2Z2^x + fn*Zn^x) where f1, f2, and fn are the fraction of protons contributed to the mixture by each component.

Question 11

How does the probability of pair production vary with Z of the absorber?

Answer 11

Z*ln(E) with a minimum E threshold of 1.022 MeV

Question 12

How does the rate of energy loss to bremsstrahlung interactions vary with E and Z?

Answer 12

E*Z^2

Question 13

How does the rate of electron energy loss to collisional interactions vary with E and Z?

Answer 13

E/Z

Question 14

How does the relative probability of radiation losses compared to collisional losses vary with E and Z? (efficiency of the target)

Answer 14

Srad/Scol ~ EZ9E-10

Question 15

How do you convert 1 R to 1 C/kg?

Answer 15

1R = 2.58E-4 C/kg

Question 16

What is the W of air?

Answer 16

34 eV/ion pair

Question 17

What is the rest mass energy of a proton?

Answer 17

938 MeV

Question 18

What is the rest mass energy of an electron?

Answer 18

0.511 MeV

Question 19

How do you calculate electron density?

Answer 19

Na*Z/A

Question 20

What is Na?

Answer 20

6.022E23 atoms/mole

Question 21

How much does a 6 MV beam get attenuated per cm of water?

Answer 21

3%/cm (note this is attenuation only, need to add inverse square on top of this.

Question 22

How are TAR and TMR related?

Answer 22

TMR = TAR/BSF

Question 23

How much more sensitive is a diode than an ion chamber?

Answer 23

20,000 more times

Question 24

How do you calculate optical density for film?

Answer 24

OD = log(I0/I)

Question 25

What is required for transient equilibrium?

Answer 25

T1/2 of the daughter must be shorter than T1/2 of the parent, but not insanely so

Question 26

What is required for secular equilibrium?

Answer 26

T1/2 of the daughter must be much, much shorter than T1/2 of the parent

Question 27

Draw the gas amplification curve

Question 28

What is exposure?

Answer 28

measure if ionization by photons in air - charge produced in air when all electrons liberated by photons in mass dm of air are completely stopped in air.

Question 29

How would you convert dose to air to dose to water?

Answer 29

Multiply by the ratio of mass attenuation coefficients

Question 30

Why is electronic equilibrium (CPE) important?

Answer 30

Because it is a requirement of cavity theory - allows us to measure dose in cavities that aren’t large enough for all ions to be completely stopped and collected in the cavity.

Question 31

What are some common types of tough water?

Answer 31

solid water, plastic water, slab phantom, virtual water

Question 32

How would you get a correction factor for solid water?

Answer 32

Take a measurement under the same conditions, and same depth, with the same chamber in water and in the phantom and your correction factor is the reading in water/reading in phantom

Question 33

How equivalent are solid water phantoms to water?

Answer 33

around +/- 1% different
Readings in phantom are hot for photons (phantom > water) and cold for electrons (phantom < water)

Question 34

What would be an ideal phantom?

Answer 34

One that gives you the same readings as water for all:
-modalities
-energies
-depths

Question 35

What checks would I do for a new slab phantom?

Answer 35

• physical dimensions
• physical density
• CT it to make sure it’s homogeneous
• radiological equivalence with water (either vs water or another slab phantom that has been previously validated)
• measure and save conversion factors - may vary with modality and energy

Question 36

How do you validate the response of your detector/electrometer combinations?

Answer 36

strontium well - beta emitter with a half life of like 30 years and you can stick your chamber in there and expect your response by just accounting for the decay

Question 37

What is the accuracy of TLD?

Answer 37

~2%

Question 38

What is the accuracy of OSLD?

Answer 38

~3%

Question 39

What is a pro of OSLD?

Answer 39

it’s non-destructive readout so can be read out multiple times

Question 40

What is a big con of OSLD?

Answer 40

stronger energy dependence compared to TLD

Question 41

Draw a photomultiplier tube

Question 42

How accurate is TLD/OSLD?

Answer 42

~3%

Question 43

What device do we use for daily proton and photon QA?

Answer 43

daily QA3 - 5 ion chambers for flatness/symmetry/energy, 12 diodes for light/rad field coincidence

Question 44

What is our TrueBeam EPID resolution?

Answer 44

~0.5 mm