Radiation Physics

Question 1

Where is the mass of an atom concentrated?

Answer 1

Central nucleus (protons + neutrons)

Question 2

What is A?

Answer 2

Mass number of nucleons (protons + neutrons)

Question 3

What is Z?

Answer 3

Atomic number of the element (number of protons)

Question 4

What is binding energy?

Answer 4

Energy (eV) expended in removing an electron from the atom against the attractive force of the positive nucleus.

Question 5

What does binding energy depend on?

Answer 5

• The shell (Ek>El>Em)
• Element (increases as atomic number increases)

Question 6

When is an atom excited?

Answer 6

When an electron is raised from a shell to one further out. In this state the whole atom has more energy than normal.

Question 7

What is electromagnetic radiation?

Answer 7

Term given to energy travelling across empty space. All forms travel with same velocity (c) as light when in vacuo, 3x10^8 m/s

Question 8

Where do Xrays and Gamma rays come from/ how do they differ?

Answer 8

• Xray tubes
• Radioactive nuclei

Same properties, differ only in their origin.

Question 9

What are the two different aspects of EM radiation?

Answer 9

• Quantum aspects - particle like properties, stream of photons that travel in straight lines
• Wave aspects - can also be regarded as having sinusoidally varying electric and magnetic fields, which travel with velocity (c) in vacuo. The vectors point at right angles to each other, and to the direction of wave travel

Question 10

What is amplitude?

Answer 10

Peak field strength

Question 11

What is wavelength?

Answer 11

Distance between successive crests of the wave

Question 12

What is frequency?

Answer 12

The number of crests passing a point in a second

Question 13

What is the equation linking wavelength and frequency?

Answer 13

wavelength x f = velocity (c)

Question 14

What are the equations involving wavelength/frequency that link quantum/classical theory

Answer 14

E=hf or E = h/wavelength
Photon energy is proportional to the frequency, and therefore also to wavelength
E (KeV) = 1.24/ wavelength (nm)

Question 15

What is a beam?

Answer 15

A collimated set of radiation rays. A beam may contain photons of different energies

Question 16

What is photon and energy fluence?

Answer 16

• Photon fluence is the number of photons passing through the cross section at a time
• Energy fluence is the sum of photon energy through the cross section per unit area for the given time.

Question 17

What is beam intensity?

Answer 17

It is the energy fluence rate at the point, i.e. the sum photon energy, per unit area passing through the cross section per unit time.

Question 18

In wave theory, what is beam intensity proportional to?

Answer 18

The square of the amplitude, measured from the peak of the wave to the axis.

Question 19

What is an easier, indirect measurement of energy fluence/ beam intensity?

Answer 19

Air kerma
Air kerma rate

Question 20

What is meant by the inverse square law?

Answer 20

• Because radiation travels in straight lines, the dimensions of a beam are proportional to the distance from the point source. As a result, the square of a distance from the point source is proportional to the area of the beam. A∝ d^2
• Therefore the air kerma/ beam intensity is inversely proportional to the square of the distance from the source.
  intensity ∝ 1/A, or intensity ∝1/d^2
  air kerma at B/ air kerma at A= (distance to A)^2/ (distance to B)^2

Question 21

In basic terms, how are xrays produced?

Answer 21

When fast moving electrons are stopped by impact on a metal (tungsten) target, the KE of electrons is converted into Xrays (~1%) and heat (~99%).

Question 22

Describe/draw an Xray tube

Answer 22

Two electrodes are sealed into a glass envelope vacuum:
- Cathode (negative electrode) - incorporates a fine tungsten coil or filament, a focusing cup surrounds the filament
- Anode (positive electrode) - incorporates a smooth flat metal (tungsten) target

There is a window in the glass envelope which the XR beams pass out of.
There is a lead casing which surround the glass envelope

Question 23

What are the two sources of electrical energy in XR and what are they derived from?

Answer 23

They are derived from the alternating current (AC) mains by means of transformers.
1. Filament heatwave voltage (10V), and current ~10A
2. Tube potential,kilovoltage or kV; which is the accelerating voltage between the anode and cathode (normally 30-150 kV). It drives the current of electrons flowing between the anode or cathode (which is typically 0.5 - 1000 mA), it is also known as the tube current, milleramperage or mA.

Question 24

How is the tube current/mA controlled?

Answer 24

By adjusting the filament voltage and current, and therefore temperature. A small increased in temperature produces a large increase in tube current.

Question 25

If you alter the tube voltage, does it affect the tube current?

Answer 25

No, which is unlike most electrical components.
It is also designed so that the kV is unaffected by changes in mA.
The two factors can therefore be varied independently.