Radiation Physics

Question 1

What is radiation?

Answer 1

• Radiation is the transmission of energy through space and matter.
• Occurs as particulate and electromagnetic radiation

Question 2

What are the two types of radiation?

Answer 2

1. Particulate Radiation

• Alpha particles
• Beta particles

2. Electromagnetic Radiation

• Radio waves
• Microwaves
• Infrared waves
• UV light
• gamma radiation
• x-radiation

Question 3

What are the two theories of Electromagnetic Radiation?

Answer 3

1. Quantum Theory:

Considers electromagnetic radiation as small discrete particles of energy called photons.

Electron volt (eV) is the unit of energy = the amount of energy acquired by one electron accelerating through a potential difference of one volt.

2. Wave Theory:

Movement of photon energy through space as a combination of electric and magnetic fields.

Fields oriented in planes at right angles to one another that oscillate perpendicular to the direction of motion.

Question 4

What are the properties of x-rays?

Answer 4

x-ray photons have high energy and short wavelengths

• Composed of photons (a quanta of energy) • Invisible
• High energy (short wavelengths & high frequencies)
• No mass or charge
• Travel at the speed of light • Each x-ray photon travels in a straight line

• Cannot be focused from their point of origin; they diverge

• X-ray beam can be deflected and scattered
• Can penetrate or may be absorbed by matter (other factors also involved) • Cause some substances to fluoresce

• They are ionising radiation & can cause biological damage • Can affect photographic film

Question 5

What is the atomic number of an atom?

Answer 5

Number of protons

Question 6

What is ionisation?

Answer 6

a process when an atom loses an electron and becomes a positive ion

To ionise an atom, sufficient energy is required to overcome the electrostatic force (binding energy of the electrons to the nucleus)

Question 7

How is the binding energy of an electron is related to the atomic number of the atom and the orbital type.

Answer 7

Large atomic number = large no. of protons in the nucleus

• High binding energy of electrons
• High energy is required to IONIZE atoms

Question 8

The production of x-ray photons requires 3 important processes:

Answer 8

1. Production of electrons
2. Acceleration of these electrons such that they have high kinetic energy
3. Impact of these electrons into other atoms

Question 9

What are the primary components of a tube head:

Answer 9

X-ray tube and power supply

Question 10

What are the other components of the tube head?

Answer 10

• An electrical insulating material, usually oil, surrounds the x-ray tube (conducts heat away from the x-ray tube).
• Lead Casing/Glass: Removes scattered x-rays.
• Aluminium Filter: removes soft (low energy) x-rays from the primary beam.
• PID (position indicating device; 200mm in length) - provides optimum distance (least divergence & unclearness)
• Collimator (opening) à restricts beam to usable size
• Entire tube head is supported by an arm that is mounted on a wall.
• Control panel available for adjusting: duration of exposure, and energy and exposure rate of the x-ray beam

Question 11

What is the x-ray tube composed of?

Answer 11

A cathode (-ve) and an anode (+ve) within an evacuated glass/tube (vacuum chamber)

Question 12

What is the cathode?

Answer 12

The cathode (-ve) consists of a tungsten filament and a molybdenum focusing cup. The filament is the source of electrons within the x-ray tube.

• The filament is heated by the flow of current from the low-voltage source (~ 10 volts) and emits electrons at a rate proportional to the temperature of the filament.
• Focusing cup is also negatively charged.
• Concave shape and charge serve to focus the electrons into a beam that is aimed at the focal spot on the anode. (electron clouds)
• Electrons are then accelerated (gaining kinetic energy) by the second high-voltage source (60-100 kilovolts) and move / are attracted to the positively charged anode (focal spot).

Question 13

What is the anode?

Answer 13

Consists of a tungsten target in a copper stem.

• Typically, the target is placed at an angle (inclined about 20 degrees) to the electron beam
• to generate a much smaller (outgoing) beamàthe more ‘focused’ the beam, the sharper the radiographic image at the expense of generating more heat
• Target converts the kinetic energy of the colliding electrons into x-ray photons.
• Conversion of the kinetic energy of the electrons into x- ray photons is an inefficient process as 99% of the electron kinetic energy converted to heat.
• A target made of a high atomic number material is most efficient in producing x rays.
• Copper stem removes heat from the tungsten, reducing the risk of the target melting.
• Additionally, the insulating oil carries heat away from the copper stem.

Question 14

How is heat produced in the tungsten target (anode)?

Answer 14

Most high-speed electrons interacting with the tungsten target release their energy as heat (99%)

Heat is produced the following ways:

1. The incoming electron is deflected by the cloud of outer-shell target electrons with a small loss of energy in the form of heat
2. The incoming electron collides with an outer shell target electron displacing the target electron to a more peripheral shell (excitation) or displacing the target electron from the atom (ionisation) with a small loss of energy in the form of heat

The remaining 1% of the high-energy electrons convert their kinetic energy into 2 types of x-ray photons: Bremsstrahlung radiation and characteristic radiation.

Question 15

what is Bremsstrahlung Radiation?

Answer 15

• More than 70% of dental x-rays
• Electrons’ sudden stopping or decrease in speed by tungsten nuclei causes the electrons to lose kinetic energy - producing bremsstrahlung photons
• The closer the electron approaches the nuclei, the greater the braking effect, and the greater the energy of the resulting bremsstrahlung photons.
  
  • High Z (atomic number) metals (more protons) are more effective in deflecting the path of the incident electrons.
  • Large deflections produce high-energy photons
  • Small deflections produce low-energy photons

Question 16

What is characteristic radiation?

Answer 16

• Only a small fraction of dental x-rays
• Occurs when an incident electron (from the incoming beam) ejects an inner orbit electron from the tungsten target.
• The electron vacancy is filled by an electron from an outer orbit.
• A photon is emitted with energy equal to the difference in energy between the two orbits.
• The energies of characteristic photons are discrete because they represent the difference of the energy levels of electron orbital levels and hence are characteristic of the target atom.

Question 17

How can an x-ray beam be modified?

Answer 17

altering the:

• beam exposure duration (timer)
• exposure rate (mA)
• energy (kVp and filtration)
• shape (collimation)intensity (target-patient distance)

Question 18

Which 6 factors control the x-ray beam?

Answer 18

1. Tube Voltage (kVp)
2. Filtration
3. Tube Current (mA)
4. Duration (time in seconds)
5. Collimation
6. Target-Patient Distance - Inverse-square law

Question 19

What happens if tube voltage (kVP) increases?

Answer 19

There is a corresponding increase in:

1. the mean energy of the beam,
2. the total number of photons emitted, and
3. the maximum energy of the photons.

Question 20

What happens if you filtrate an x-ray beam with aluminium?

Answer 20

Results in the preferential removal of low- energy photons, reducing the intensity of the beam but increasing its mean energy. (also decrease in no. of photons)

Question 21

What happens if the exposure time increases?

Answer 21

As the exposure time increases the total number of photons increases, but the mean energy and maximum energy of the beams are unchanged.

Question 22

What is a collimator?

Answer 22

A collimator is a metallic barrier with an aperture in the middle used to reduce the size of the x-ray beam

Question 23

What do collimators do?

Answer 23

Affect shape of the beam

• Dental x-ray beams are usually collimated to a circle (7 cm in diameter) - Less frequently, there are rectangular collimators
• Typically, round collimators are built into open-ended aiming cylinders.
• Rectangular collimators further limit the size of the beam to just larger than the x-ray film (intraoral x-ray films are rectangular), thereby further reducing patient exposure.

Question 24

What is the inverse square law?

Answer 24

Inverse square law = for a given beam, the intensity is inversely proportional to the square of the distance from the source

Question 25

How does the distance affect the intensity of an xray?

Answer 25

the intensity of an xray beam (the number of photons per cross sectional area per unit of exposure time) depends on the distance of the measuring device from the focal spot 2 x distance = ¼ x radiation --\> for diagnostic purposes you want the radiation beam to be as close as possible to the area of interest