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Health Physics > Interactions of Radiation with Matter > Flashcards

Interactions of Radiation with Matter Flashcards

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1
Q

What is the energy of a thermal neutron?

A

0.025 eV

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2
Q

What is the energy range of a slow neutron?

A

1 - 10 eV

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3
Q

What is the energy range of a fast neutron?

A

1 - 20 MeV

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4
Q

What are three slow neutron interactions?

A
  1. Radiative Capture
  2. Charged Particle Emission
  3. Fission
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5
Q

State the general equation for radiative capture (slow neutron interaction) and give an example (H-1).

A
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6
Q

State the general equation for charged particle emission (slow neutron interaction) and give an example (B-10).

A
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7
Q

State the general equation for fission (slow neutron interaction) and give an example (U-235).

A
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8
Q

____________ is the mechanism responsible for about 80% of fast neutron dose to human tissue.

A

Elastic scattering is the mechanism responsible for about 80% of fast neutron dose to human tissue.

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9
Q

What are the two fast neutron interactions?

A
  1. Elastic scattering
  2. Inelastic scattering
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10
Q

What is mean free path?

A
  • The average (mean) distance of travel in the medium between interactions.
  • Over the distance of one mean free path, each photon in the beam has only a 50% chance of interacting.
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11
Q

Demonstrate elastic and inelastic scattering of fast neutrons.

A
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12
Q

What is inelastic scattering?

A
  • A fast neutron collides with a target nucleus, and transfer some kinetic energy to the nucleus, raising it to a higher energy level.
  • When the nucleus de-excites, it will emit a gamma-ray.
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13
Q

What are three photon interactions?

A
  1. Photoelectric Effect
  2. Compton Scattering
  3. Pair Production
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14
Q

Describe photoelectric effect.

A
  • The incoming photon transfers its full energy to an inner shell orbital electron, almost always one in the K-shell.
  • The energized electron uses the excess energy to escape from the nucleus (i.e., gives up the binding energy).
  • The electron escapes the atom with the remaining energy given to it by the photon (photon energy less electron binding energy).
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15
Q

What is the relative probability of photoelectric effect interaction per gram of absorber?

A

Z3/E3

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16
Q

Describe Compton Scattering.

A
  • The incoming photon transfers a portion of its energy to an orbital electron.
  • A lower energy photon (a Compton photon) then leaves, in a different direction, with the remaining energy.
  • The Compton electron is ejected from the atom with the energy it receives minus the electron binding energy.
  • Any orbital electron which has a binding energy less than about 10% of the photon energy is available to interact by a Compton process.
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17
Q

What is the relative probability of a Compton scattering interaction?

A

The interaction is independent of atomic number, and decreases with E.

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18
Q

Describe Pair Production.

A
  • In the vicinity of the nucleus of an absorber atom, an incoming photon suddenly disappears, and in its place, appears an electron/positron pair.
  • The rest mass for both the electron and positron is 0.511 MeV; therefore, a minimum of 1.022 MeV is required to undergo pair production.
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19
Q

What is the relative probability of Pair Production interaction?

A

The probability of pair production occurring, per gram of absorber, is directly proportional to the atomic number, Z, and increases with E.

20
Q

What happens to the positron created by Pair Production?

A
  • The positron eventually collides with an electron due to attraction of opposite charges.
  • The two particles annihilate each other and create two photons each carrying 0.511 MeV of energy traveling in exactly opposite directions.
21
Q

What are the linear and mass energy absorption coefficients?

A
  • The linear energy absorption coefficient (μen) represents the fraction of energy actually removed from photons in the beam per unit distance.
  • The mass energy absorption coefficient (μen/ρ) gives the fraction of energy removed per unit density of absorber.
22
Q

What interaction cofficient is used in shielding calculations? Why?

A
  • Absorption Coefficient
  • Because dose is energy deposited per unit mass, calculations using absorption coefficients instead of attenuation cefficients more closely estimate the reduction in dose rate as a result of adding shielding around a photon radiation source.
23
Q

What are three charged particle energy loss mechanisms?

A
  1. Ioniziation
  2. Excitation
  3. Bremsstrahlung
24
Q

What is ionization?

A
  • The complete removal of orbital atomic electrons as a result of the Coulomb force between the charged particle and the orbiting electrons.
  • This process removes charge from a neutral atom and so it becomes an ion.
25
Q

What is an ion pair?

A

The combination of an electron removed through ionization and the residual positive ion.

26
Q

In most materials, the amount of energy needed to produce an ion pair (W) is about ___ to ___ eV.

A

In most materials, the amount of energy needed to produce an ion pair (W) is about 30 to 40 eV.

27
Q

What is the W-value for air?

A

33.9 eV

28
Q

What is excitation?

A
  • In contrast to ionization, insufficient energy is transferred to the orbiting electron to break the electrical binding force, so the electron merely jumps up to a higher atomic energy level rather than leaving the atom entirely.
  • In this case, the electron is still bound to the atom so that the electrical neutrality of the atom is not disturbed.
29
Q

What is bremsstrahlung?

A
  • When a charged particle travels past a nucleus, Coulombic forces cause a deflection in the path of the charged particle.
  • The directional change and the speed of the particle is reduced due to the energy loss.
  • The energy lost by the particle is emitted as electromagnetic energy in the X-ray region of the electomagnetic spectrum.
30
Q

What is stopping power (S)?

A
  • Units: MeV cm-1
  • The average energy lost by a charged particle per unit distance of travel.
  • Analogous to the concept of the attenuation coefficient for gamma rays, and describes how effective the absorber is in removing energy from a beam of charged particles.
31
Q

What is mass stopping power (S/ρ)?

A

The stopping power per unit density.

32
Q

What is specific ionization?

A
  • The average number of ion pairs produced per unit distance of travel of a charged particle.
  • Specific ionization = S/W (ion pairs cm-1)
33
Q

Rule of Thumb: Alpha particles and the skin

A

Alpha particles up to 7.5 MeV are stopped in the dead layer of normal human skin.

34
Q

Rule of Thumb: Beta particles in air

A

Beta particles will penetrate about 4 meters in air per MeV of energy.

35
Q

Rule of Thumb: Beta particles in soft tissue

A

Beta particles will penetrate about 0.5 cm in soft tissue per MeV of energy.

36
Q

Rule of Thumb: Beta particles and the human skin

A

Beta particles up to 70 keV are stopped in the dead layer of normal human skin.

37
Q

What is linear energy transfer (LET)?

A
  • Units: MeV cm-1
  • The average energy locally deposited in an absorber per unit distance of travel of a charged particle.
  • Analogous to the energy absorption coefficient for gamma rays.
38
Q

What is the chief difference between linear energy transfer (LET) and stopping power (S)?

A

LET is concerned primarily with the energy left behind in the absorber while S focuses on the energy retained by a charged particle.

39
Q

Describe

Coherent (Rayleigh) scattering

A
  • All energy of the photon is maintained.
  • Photon interacts coherently (uniformly) with orbital electrons, without a change in wavelength.
  • Complex models of photon interactions must take into account scattering angles from coherent scattering.
40
Q

Graph

Probabilities of photon interaction as a graph of Absorber Z vs. Energy of Photon (MeV)

A
41
Q

Graph

Photon interactions as a function of attenuation coefficient (cm2 g-1) vs. Energy (MeV)

A
42
Q

What condition must exist for absorbed dose to be approximately equal to KERMA?

A
  • Radiative energy losses by liberated charged particles must be negligible ⇒ Eab = Etr
  • Eab ⇒ Average energy absorbed per interaction.
  • Fluence of indirectly ionizing radiation must be uniform.
  • Photon energy cannot be so large that liberated electrons only travel in the forward direction.
  • Photon energy must be large enough so their mean free path exceeds the range of liberated electrons.
43
Q

What is the purpose of a build-up cap on an ionization chamber?

A
  • Assures equilibrium distribution of electrons is present in the wall (at wall/gas interface).
  • Ensures dose is produced in the gas.
44
Q

At what point (depth) in a material is the largest absorbed dose?

A

The peak would be expected where

  • the increase in charged particle population AND
  • the increase in absorbed dose

is balanced by

  • the decrease in dose associated with the attenuation of the photon beam
45
Q

In estimating the shielding for scattered X-ray radiation, the reflection coefficient α (albedo factor) can be used. List four parameters that may affect the reflection coefficient.

A
  • Energy of incident photons
  • Angle of incidence of photons on the scatterer surface
  • Angle of reflection from the scatterer surface
  • Material of construction of the scatterer

Health Physics (17 decks)

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