X-Ray Tube (OCR A Level Physics)

Structure of an X-ray Tube

• An X-ray tube is a device that converts an electrical input into X-rays

• It is composed of four main components:

  • A heated cathode

  • An anode

  • A metal target

  • A high voltage power supply

• The production of X-rays has many practical uses, such as in:

  • Medical imaging (radiography)

  • Security

  • Industrial imaging

The main components of an X-ray Tube are the heated cathode, anode, metal target and a high voltage supply

The Role of the Components

Heated Cathode

• At one end of the tube is the cathode (negative terminal) which is heated by an electric current

  • The heat causes electrons to be liberated from the cathode, gathering in a cloud near its surface

  • This process of thermionic emission is the source of the electrons 

Anode

• At the other end of the tube, an anode (positive terminal) is connected to the high voltage supply

• This allows the electrons to be accelerated up to a voltage of 200 kV 

  • When the electron arrives at the anode, its kinetic energy is 200 keV (by the definition of an electronvolt)

• Only about 1% of the kinetic energy is converted to X-rays

  • The rest is converted to heat energy

  • Therefore, to avoid overheating, the anode is spun at 3000 rpm and sometimes water-cooled

Metal Target

• When the electrons hit the target at high speed, they lose some of their kinetic energy

  • This is emitted as X-ray photons

• A heat-resistant block of metal, usually Tungsten, is embedded at the end of the anode, facing the cathode

  • This is the material that the electrons collide with and X-rays are generated in

High Voltage Power Supply

• The high voltage supply creates a large potential difference (> 50 kV) between the cathode and the target

  • This causes electrons in the cloud around the cathode to be accelerated to a high velocity towards the target, which they strike, creating X-rays

Other Components

• X-rays are produced in all directions, so the tube is surrounded by lead shielding

  • This is to ensure the safety of the operators and recipients of the X-rays

  • An adjustable window allows a concentrated beam of X-rays to escape and be controlled safely

• The anode and cathode are housed inside a vacuum chamber

  • This is to ensure that the electrons do not collide with any particles on their way to the metal target

Production of X-ray Photons

• When the fast-moving electrons collide with the target, X-rays are produced by one of two methods

  • Method 1: Bremsstrahlung

  • Method 2: Characteristic Radiation

Method 1: Bremsstrahlung

• When the high-speed electrons collide with the metal target, they undergo a steep deceleration

  • When a charged particle decelerates quickly, some of the energy released is converted into a photon

• A small amount of the kinetic energy (~ 1%) from the incoming electrons is converted into X-rays as the electrons decelerate in the tungsten, due to conservation of energy 

  • The rest of the energy heats up the anode, which usually requires some form of cooling

• The energy of the X-ray photon can be of any value, up to the original kinetic energy of the electron, giving a spread of possible X-ray energies

  • These X-rays cause the continuous or ‘smooth hump shaped’ line on an intensity wavelength graph

• When an electron is accelerated, it gains energy equal to the electronvolt, this energy can be calculated using:

E_max = eV

• This is the maximum energy that an X-ray photon can have

• The smallest possible wavelength is equivalent to the highest possible frequency and therefore, the highest possible energy

  • This is assuming all of the electron’s kinetic energy has turned into electromagnetic energy

• Therefore, the maximum X-ray frequency f_max, or the minimum wavelength λ_min, that can be produced is calculated using the equation:

• The maximum X-ray frequency, f_max, is therefore equal to:

• The minimum X-ray wavelength, λ_min, is therefore equal to:

• Where:

  • e = elementary charge (C)

  • V = potential difference between the anode and cathode (V)

  • h = Planck's constant (J s)

  • c = the speed of light (m s⁻¹)

Method 2: Characteristic Radiation

• Some of the incoming fast electrons cause inner shell electrons of the tungsten to be ‘knocked out’ of the atom, leaving a vacancy

  • This vacancy is filled by an outer electron moving down and releasing an X-ray photon as it does (equal in energy to the difference between the two energy levels)

  • Because these X-rays are caused by energy level transitions, they have only specific discrete energies

  • They cause sharp spikes on an intensity wavelength graph

  • The number of spikes depends on the element used for the target - there are two sets of spikes for a tungsten target, representing two sets of possible energy transitions