Production Flashcards
State the five components of an xray tube
Cathode Anode Filament Envelope Vacuum
Explain Thermionic emission?
3
The filament is heated by an electric current which increases the vibration of atoms within the
filament so much that it emits heat and light. These are energy forms converted from the energy
of electrons through the filament. So much heat energy is acquired by the atoms that some of
their electrons (outer, negatively charged particles) can break free and temporarily leave the
filament’s surface.
Explain why there is a space cloud near the cathode filament 1
The filament now becomes positively charged. Loss of electrons is therefore
followed by re-attraction
State the charge for anodes and cathodes 2
anode positive
cathode negative
What kind of energy is given to the electrons to propel them from cathode to anode 1
potential energy which is converted to kinetic energy
Explain how the beam is focused from the cathode
draw diagram
4
Electrons are forced to form a narrow beam, converging on to a specific part of the anode, the
target.
The high negative potential of the focusing cup repels the electrons into a converging
beam directed at the target.
Explain how it may be possible for electrons to change direction
2
tungsten. Its atomic number is 74 and therefore
there is a positive charge of 74 units on the nucleus of every tungsten atom. The target’s nuclei
can exert strong, positive, attractive forces on the electrons passing through on their way from the
filament. These are the forces which cause the sudden change in direction.
explain three ways in which electrons from the cathode may lose its energy 3
Because of interactions between them and outer electrons surrounding target atoms.
This is emitted as infra-red radiation in the form of heat (95-99%)
- Because of interactions between them and the nuclei of target atoms (Bremsstrahlung).
- Because of interactions between them and individual inner electrons of the target atoms
(Characteristic)
(
When they reach the target only a minority of electrons will pass close enough to the nuclei
of target atoms to experience the force of attraction. What percentage is this? 1
(5%)
Explain how a nucleus of tungsten is involved with the change in direction of the oncoming electron
Draw diagram
3
As they are momentarily drawn towards an orbital path around the nucleus,
a sudden change in direction converts some of their kinetic energy into X-ray energy.
Explain the phenomena of Bremsstrahlung? 3
diagram
“braking radiation” or “deceleration radiation”, is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus.
diagram including cathode
anode vacume focusing cup filament
Why is Bremsstrahlung refereed to as braking radiation? 2
These are referred to as ‘braking radiation’ because the loss of kinetic energy causes the electron
to slow down. Influenced by the nuclei of the tungsten atom (74)
State the relationship between intensity, energy and atomic number 1
i proportional ZE2
Explain how characteristic xrays are created 4
If an electron from the
filament has more energy than the orbiting electron with which it collides, the collision removes
the electron from its orbit. This creates a vacancy and is filled with an outer shell electron. This
downward transition of an electron involves the release of energy because the inner shell
represents a lower energy state than the upper shell. The energy is released either by
transferring the energy to another orbital electron which is ejected (then known as Auger electron)
or by emission of a photon of EM radiation
How would the characteristic xray energy be calculated? 2
If an X-ray photon is emitted, its energy is equal to the
difference in electron binding energies between the inner shell in which the vacancy is created
and then filled, and the outer shell from which the electron travels to fill the vacancy
What is the binding energy for a tungsten k shell electron?1
69.5 keV
Explain the nature of characteristic xray photons in relation to elements. 1
Characteristic X-ray photons have higher energies
when the target material has a higher atomic number
Illustrate how different k shell characteristic xrays are labled
K alpha
K beta
K gamma
In radiography which shell based characteristic xrays are of any significance
K L
Rest are too low energy
Draw a line spectrum and label accordingly
y intensity
x energy photon
area under curve - bremsstrahlung
k lines
l lines
low and high cut off
