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Physics > Interactions with matter > Flashcards

Interactions with matter Flashcards

1
Q

Name the 3 Key Interactions that occur in matter

A

Transmitted
Absorbed
Scattered

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2
Q

What 4 things does attenuation depend on?

A

The beam energy; the energy of the X-ray photons from the tube
The density of the tissue
The atomic number
The thickness of the tissue

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3
Q 
Place these is order of energy - lowest first:
A: Megavoltage
B: Mammography
C: Kilovoltage radiotherapy
D: Diagnostic x-rays
A

Mammography - lowest energy
Diagnostic x-rays
Kilovoltage radiotherapy
Megavoltage

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4
Q

List the 4 types of interactions which occur in matter?

A

Elastic Scattering
Photoelectric Absorption
Comptom Effect
Pair Production

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5
Q

Describe Elastic scattering

A

Photon energy is significantly less than electron binding energy i.e. it occurs with low energy radiation

Photon is deflected as it passes close to electron

There is no loss of energy

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6
Q

Describe Photoelectric Absorption

A

Photon collides inelastically with an electron in an inner shell

If the photon has an energy greater than the binding energy of the electron it will escape

Electron is ejected from the atom as a photoelectron

Vacancy is filled by electron cascade releasing characteristic radiation as they move down the fill the vacancy.

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7
Q

How is an Auger electron produced?

A

In materials with a higher atomic number it is possible an additional interaction may occur.

Incoming X-ray photon ionises a K-shell electron

The K-shell vacancy is filled by an electron from a higher shell

The energy emitted in this transition impacts on another electron, if the energy exceeds the binding energy it too is ejected from the atom

This second ejected electron is called the Auger electron

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8
Q

What energy level does the Photoelectric absorption occur between?

A

Occurs between 40 – 80 kVp

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9
Q

What relationship does Photoelectric absorption have to the Beam energy?

A

Photoelectric absorption is inversely proportional to energy cubed

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10
Q

What relationship does Photoelectric absorption have to the Atomic number?

A

Photoelectric absorption is proportional to Atomic number cubed

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11
Q

Describe Compton Scattering

A

Photon energy is significantly higher than electron binding energy

Photon gives up proportion of its energy to electron and electron is ejected

Photon continues in different direction

Recoil electron ejected at between 1 - 90° relative to the incident photon

Scattered photon can be scattered in any direction

Higher photon energies result in lower scattering angles

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12
Q

What relationship does Compton Scattering have to the Beam energy?

A

Compton scattering is inversely proptional to Beam energy

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13
Q

What relationship does Compton Scattering have to the Atomic number?

A

Compton scattering is independant of Atomic number

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14
Q

What is Compton scattering proptional to?

A

Compton scattering is proptional to the electron density i.e. density of the attenuator

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15
Q

Describe Pair Production

A

Occurs when photon energy is > 1.02MeV

Photon interacts with electrical field of nucleus

Photon is turned into Positron and Negatron

Negatron is slowed down and Positron will eventually
collide with an electron

This results in annihilation radiation i.e. Mass is converted back to energy

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16
Q

What relationship does Pair Production have to the Beam energy?

A

Pair Production is proptional to the Beam energy

17
Q

What relationship does Pair Production have to the Atomic number?

A

Pair Production is proptional to the Atomic number

18
Q

Which interaction is the most prevalent at the following energies?
• Kilovoltage Diagnostic x-rays
• Kilovoltage Radiotherapy treatment
• Megavoltage Radiotherapy treatment

A

Kilovoltage Diagnostic x-rays:
- Low energy: i.e. mammography: 75% interactions in soft tissue will be that of photoelectric with Compton scatter playing a relatively minor role
- Beam energy 30-40keV: Photoelectric=Compton scatter in soft tissues
- High energy: i.e. radionuclide imaging: 10-15% interactions in soft tissue will be that of
photoelectric with Compton scatter playing a dominant role

Kilovoltage Radiotherapy treatment:
- High energy kilovoltage: Compton scatter playing a dominant role

Megavoltage Radiotherapy treatment:
- Compton effect and pair production are the dominant attenuation processes.

19
Q

True Or False:

Elastic scattering causes no change to the energy of the incoming photon

A

True

20
Q

What conditions are required for the exponential relationship of an electromagnetic beam to attenuator thickness?

A

Homogenous beam
Homogenous attenuator material
Radiation beam must be parallel

21
Q

What Greek letter represents total linear attenuation coeffiecient?

A

µ

22
Q

What is Half-value thickness/layer (HVL/HVT)?

A

The half-value thickness/ layer is that thickness of a substance which will transmit one half of the intensity of radiation incident upon it

23
Q

What are Homogenous and Monochromatic beams?

A

An electromagnetic beam with the photons having the exact same energy

24
Q

What is a Polychromatic beam?

A

An electromagnetic beam with a spectrum of photon energies

25
Q

What 2 things increase Beam quality?

A

A larger HVL/HVT

A larger filter thickness

26
Q

Why is half value thickness not a constant value in a polychromatic beam?

A

Due to beam hardened, causing the quality of the
beam to improve, therefore the beam becomes more
penetrating, therefore requiring greater thickness of
HVL

27
Q

Suggest an appropriate metal that can be utilised as an attenuator for DR & kV Superficial?

A

Aluminium

28
Q

Suggest an appropriate metal that can be utilised as an attenuator for kV Orthovoltage?

A

Copper

29
Q

What is the Law of Radioactive Decay?

A

The rate of decay of a nuclide is proportional to the

number of nuclei left in the sample

30
Q

What is Half life?

A

The time required for the activity of the radioactive sample to decay to one half of it’s original value

31
Q

What 2 things can you work out from Half life?

A

What the activity was at a previous time

What the activity will be in the future

32
Q

What is the mass and charge of an Alpha particle?

A

6.645 x 10^-27kg

+2 charge

33
Q

What is the penetration in air of an Alpha particle?

A

0.5cm

34
Q

What is the mass and charge of an Beta minus and plus particle?

A
  1. 11 x 10-31 kg

- 1 or +1 charge

35
Q

What is the penetration in air of an Beta particle?

A

3.5m

36
Q

What is the mass and charge of an Gamma particle?

A

No Mass

Neutral

37
Q

What is the penetration in air of an Gamma particle?

A

8,200m *

Physics (10 decks)

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