Kahn Outline Chapter 5: Interactions Of Ionizing Radiation

Front 1

What is ionizing radiation?

Back 1

Ionization is the process by which a neutral atom acquires a positive or a negative charge.

Stripping electron from an atom gives rise to an ion pair.

The stripped electron quickly attachesitself to a neutral atom to create a negative ion and the atom from which the electron has beenstripped becomes the positive ion.

Front 2

What is the linear attenuation coefficient and units? State what it is dependent on.

Back 2

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It is dependent on the material composition (density and atomic number) of the absorber or medium.

Front 3

List the photon beam attenuation formula.

Back 3

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Front 4

How does the first HVL compare to the second in a poly energetic beam? Mono energetic beam?

Back 4

For a heterogeneous beam, the first HVL is less than the subsequent HVLs. As the filter thickness increases, the average energy of the transmitted beam increases or the beam becomes increasingly harder.

If I(x) is plotted as a function of x for a narrow monoenergetic beam, a straight line will be obtained on semilogarithmic paper, showing that the attenuation of a monoenergetic beam is described by an exponential function.

Front 5

Define an HVL and calc it for a linear attenuation coefficient of 0.021 per cm.

Back 5

Half-value layer (HVL)is the thickness of an absorber required to attenuate the primary beamintensity to half its original value.
HVL = 0.693 / 0.021
HVL = 33 cm

Front 6

What are the energy transfer and energy absorption coefficients?

Back 6

The energy transfer coefficient describes the fraction of photon energy transferred into kinetic energy of charged particles per unit thickness of absorber.

The energy absorption coefficient is defined as the product of energy transfer coefficient (1 - g) where g is the fraction of the energy of secondary charged particles that is lost to bremsstrahlung in the material.

Front 7

What is the mass attenuation coefficient and what is it composed of?

Back 7

(u /r)

Since the attenuation produced by a thickness x depends on the number of electron present in that thickness, u depends on the density of the material. Thus, by dividing the u by density r, the resulting coefficient (u/r) will be independent of density.

Front 8

Describe Coherent scattering and state its respective energy ranges. Also list what it is dependent on.

Back 8

-Coherent scattering interaction (also known as clas-sicalscattering or Rayleigh scattering) consists of aphoton setting an orbital electron into oscillation.The oscillating electron reradiates energy at the samefrequency as the incident photon

-No energy is absorbed in the medium from coherentscattering. The only effect is the scattering of photonat small angles.

-Coherent scattering is probable in high atomic materials and with photons of low energy.

Front 9

Describe Photoelectric effect and state its respective energy ranges. Also list what it is dependent on.

Back 9

-In the photoelectric process, the entire energy hν of the photon is first absorbed by the atomand then transferred to an orbital electron. The kinetic energy of the ejected electron (called thephotoelectron) is equal to hν−EB, where EB is the binding energy of the electron. Interactions of this type can take place with electrons in the K, L, M, or N shells.

-Ejection of a photoelectron may cause the emission of characteristic x-rays and Auger electron.

-Photoelectric is predominant for beams of < 75 kV.

Front 10

Describe Compton effect and state its respective energy ranges. Also list what it is dependent on.

Back 10

-The Compton interaction involves a collision between the photon and a “free” electron. The term
free here means the binding energy of the electron is much smaller than the energy of the bombarding photon.

-In the Compton interaction, the electron receives some energy fromthe photon and is emittedat an angle θ. The photon, with reduced energy, is scattered at an angle φ

-Compton interaction probability in water increases with photon energy from 10 to 150 keV. It then decreases with further increase in energy. However, it is the predominant mode of inter-action in water for 30 keV to 24 MeV. That includes all photon beams used in radiation therapy.

-Compton is predominant for beams of ~75 kV to 70 MV

Front 11

Describe Pair Production and state its respective energy ranges. Also list what it is dependent on.

Back 11

-Pair production involves a high-energy photon interaction with the electromagnetic field of anucleus. The entire energy of the photon is converted in creating a pair of electron (e−) andpositron (e+) and providing them with kinetic energy (0.51 MeV each)

-Pair production is predominant for beams of >70 MV.

-Pair production threshold is at 1.02 MeV but its contribution is relatively small in clinical beams(~5% at 10 MV to ~20% at 25 MV).

Front 12

Describe Photo-disintegration and state its respective energy ranges. Also list what it is dependent on.

Back 12

Photodisintegration involves a photon creating a nuclear reaction. In most cases it results in the emission of a neutron. The process is importantonly at high photon energies and is responsible for neutron contamination of therapy beams ofenergy greater than 10 MV.

Front 13

Describe electron attenuation

Back 13

In water or soft tissue, electrons, like other charged particles, lose energy predominantly by ionization and excitation. This results in deposition of energy or absorbed dose in the medium.

If the energy transferred to the orbital electron is not sufficient to overcome the binding energy, it is displaced from its stable position and then returns to it; the effect is call excitation.

An electron may interact with the electromagnetic field of a nucleus and be decelerated so rapidly that a part of its energy is lost as bremsstrahlung.

The rate of energy loss as a result of bremsstrahlung increases with the increase in the energy of the electron and the atomic number of the medium.

Front 14

Describe neutron interactions

Back 14

Neutrons are indirectly ionizing.

Neutrons interact by two processes: (a) recoiling protons from hydrogen and recoiling heaving nuclei from other elements, and (b) nuclear disintegrations. Energy transfer is very efficient if the colliding particles have the same mass. However, the neutron loses very little energy when colliding with a heavier nucleus. The, the most efficient absorbers of a neutron beam are the hydrogenous materials such as paraffin wax or polyethylene.

Dose deposited in tissue from a high-energy neutron beams is predominantly contributed by recoil protons.

Front 15

Interpolate the following PDD for a 11.2 cm2 fs and 8.4 cm depth.
Field size= 8 cm depth 10 cm depth
11 cm2 77.1 70.5
12 cm2 78.2 71.3

Back 15

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Front 16

How many HVLs are there when 45% of the beam is transmitted through a material?

Back 16

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Front 17

What is the μ when you have an HVL of 2.5 mm?

Back 17

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Front 18

What is the wavelength and frequency of a 125 Kev photon beam?

Back 18

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