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Physics for FRCR CO1 > 4.1 Physics relevant to radiotherapy > Flashcards

4.1 Physics relevant to radiotherapy Flashcards

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1
Q

What is an atom?

A

Smallest particle of an element that can exist

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2
Q

What is a Molecule?

A

2 or more atoms bonded together

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3
Q

What is the top right number of an element notation?

A

A = Mass number

Remember: A = Atomic mass

The number of protons and neutrons

Can vary for the same element - the same elements with different mass numbers are isotopes

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4
Q

What is the bottom left number in element notation?

A

Z = atomic number

Number of protons
This is unique to and defines each element

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5
Q

What is a Nuclide?

A

Atoms of the same element with differing numbers of neutrons. Relative number of protons to neutrons determines how stable the nucleus is.

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6
Q

What are Energy shells?

A

Electrons are arrnaged in shells
Each shell can have a maximum number of electrons

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7
Q

How many electrons does each shell hold?

A

K - 2
L - 8
M -18
N - Q = 32

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8
Q

What is the Binding Energy of an electron?

A

The energy required to overcome the attraction of an electron to the nucelus and pull it away

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9
Q

Which elements have high binding energy?

A
  • Smaller elements - positive charge is not as strong
  • Less reactive elements (right side of table)
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10
Q

What is Ionisation?

A

An outer electron gains enough enegery to overcome the bidning eneregy and escape the atom

Leaves the atom with a positive charge

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11
Q

What is an alpha particle?

A

Made up of 2 protons and 2 neutrons
Positive charge

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12
Q

What is a Beta particle?

A

A high-speed
high-energy
electron
Negatively charged

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13
Q

Where do alpha and beta particles come from?

A

Emitted from large nuceli during radioactive decay

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14
Q

What causes an electromagnetic wave?

A

The transfer of enegery by oscilating electrical or magnetic fields

A moving electric field generates a varying magnetic field and vice versa

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15
Q

What are the types of EM radiation? (In order of wavelength)

A
  • Radiowave
  • Microwave
  • Infrared
  • Visible light
  • UV
  • X-Ray
  • Gamma
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16
Q

What is the Wavelength?

A

The distance peak-peak or trough-trough

17
Q

What is the Frequency of a wave?

A

The number of waves passing a point per unit of time

18
Q

How are Wavelength and Frequency related?

A

Speed = Wavelength x Frequency
c = λ x v

Speed is constant (speed of light) so wavelength and frequency have to be inversely proportional
As wavelength increases frequency has to decrease

19
Q

What is wave-particle duality?

A

Electromagnetic radiation behaves as both a wave and a particle

20
Q

What is a Photon?

A

Discrete package of energy (particle/quanta) that behaves like a wave

21
Q

How does energy relate to frequency?

A

Energy is directly proportional to frequency

E = h (plan’s constant) x v

22
Q

Which parts of the atom are x-rays and gamma rays released from?

A

X-rays from electrons
Gamma rays from nucleus

23
Q

Where does the energy for x-rays come from

A

As electrons drop down an energy level the extra energy is released as x-rays

24
Q

How is a radiation beam produced?

A

A negatively charged cathode is heated causing it to release electrons
The electrons are attracted to a positively charged tungsten anode and so accelerate towards it
The electrons then interact with the atoms in the anode and produce radiation

25
Q

What happens if the electrons interect with orbiting electrons of atoms?

A
  1. The incoming electron interacts with an inner shell electron of an atom
  2. The incoming electron gives he inner shell electron enough energy to escape
  3. On outer shell electron drops down into the inner shell to fill the space
  4. The energy released as the outer electron drops down is released as an x-ray photon
  5. The energy released is equal to the difference between the energy levels of the two shells
  6. Energy levels of atoms are fixed so photons have corresponding fixed energies = characteristic x-rays
26
Q

What are charcateristic x-rays?

A

Atoms have fixed energy levels so when electrons in an atom drop down levels they produce specific amounts of energy
This produces characteristic x-rays

27
Q

What happens if the electrons interact with the nucleus?
What name is given to the radiation produced?

A
  1. The incoming electron is attracted to the nucelus
  2. As it nears the nucelus it slows down and changes direction
  3. The energy lost in the slowing and direction change is released as an x-ray photon

Known as Bremstrahlung or ‘breaking’ radiation

Photons are produced at a large range of energies which produces a continuous spectrum of energy

28
Q

What is Beam Energy?

A

maximum possible energy of a photon in the beam

29
Q

What is Beam Quality?

A

the penetrating power of the beam
directly related to the range of energy of the beam

(measured by physicists for quality assurance of the machine)

30
Q

What is Beam Fluence?

A

the number of photons passing through a unit areas

φ = dN/dA
(number of photons/area)

31
Q

What is field size of the beam?

A

The beam diverges as it travels
The field size of the beam is determined at a set point (100cm from the source)

32
Q

What is the Inverse Square Law?

A

intensity drops off according to inverse square law

2 x distance = 1/4 intensity

33
Q

What are the 5 ways that radiation beams can interact with matter?

A
  1. Pass through with nothing happening
  2. Scatter with no energy change
  3. Scatter with energy change
  4. Absorption - photoelectric effect
  5. Absorption - pair production

A combination of these interactions results in beam Attenuation

34
Q

What does final beam intensity (after passing through a material) depend on?

A

Attentuator thickness (x) and what material it is

35
Q

What is attenuation?

A

Reduction in the amplitude/intensity of a beam

36
Q

How does attenuation change with material thickness?

A

Attenuation increases as material thickness increases

I = Io x exp (- μX)

37
Q

What is Linear Attenuation Coefficient (μ)?

A

How much attenuation the material causes to the beam
Reduction in intensity of the beam per unit length of material

38
Q

What is Mass Attenuation Coefficient?

A

μ/p

Reduction in intensity of beam per unit mass of material

39
Q

What is the Half Value Layer?

A

Value that describes how attenuating a mateial is to a particular beam of radiation

Thickness of material needed to half the beam intensity - can draw this off a graph where Y is intensity and X is thickness

HVL for a paritcular known material can be used to describe beam quality

Physics for FRCR CO1 (8 decks)

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