The atomic number and atomic mass
- the atomic number Z is the atomic number of protons in the nucleus
- every element has its own Z number
- A is the total number of protons and neutrons within the nucleus of the atom
charges for protons, electrons and neutrons
Protons have a positive charge
Electrons have a negative charge
Neutrons have no charge
Isotape
protons and neutrons in the nucleus are not of equal charge
Radioisotapes
isotapes with unstable nuclei which undergoes radioactive disintegration (radioactive)
Electrons
Have a negative charge
they orbit the nucleus in shells, each shell has a different energy level
K - 2
L - 8
M - 18
N - 32
O - 58
The K shell electron has the least energy and is nearest to the nucleus
Electrons can jumps from one shell to another but cannot exsist between shells (forbidden zone)
Electrons are bound to the nucleus by electromagnetic force - binding energy
The atomic structure
- atoms are neutral in the ground state and are the smallest component
- if an electron is removed the atom becomes positively charged and becomes a POSITIVE ION
- the unit of energy in the atomic system is called the electron volt (eV)
Ionistation
The process of removing the electron from the shell
excitation
if an electron is moved from an inner shell to an outer shell the atom remains neutral
Heat producing collisions
- the incoming electron is deflected by the cloud of outer shell tungsten electrons with a small loss of energy or the incoming electron
or - the incoming electorn collides with an outer shell tungsten electron displacing it to a peripheral shell (excitation) or displaces it completely from the atom (ionistaion)
- each electon can undergo many collisions
- the heat producing interations are most common, 99% of collisions are heat producing
X-ray producing collisions
the incoming electron penetrate the outer electron shell and passes close to the nucleus of the tungsten atom. the incoming electron is dramatically slowed down and deflected by the nucleus with a large loss of energy which is emitted in the form of x-rays
or
the incoming electron collides with an inner shell tungsten electron displacing it to an outer shell (excitation) or displaces it completely from the atom (ionisation) with a large loss of energy and emmision of xrays
each electron can undergo many collisions
ONLY 1% OF COLLISIONS PRODUCE XRAYS
Xray spectra
the two diffrent types of xray collisions result in different xray spectra:
CHARACTERISTIC SPECTRUM
CONTINUOUS / BREMSSTRAHLUNG SPECTRUM
Characteristic spectrum
- if the electron has enough energy it will knock an orbital electron out of the inner electron shell of the atom leaving a vacancy in the electron shell
- the outer shell electron of the atom wil move in to fill the lower shell with the emission of one or more characteristic xrays
- intensity peaks appear in the spectrum
- an electron interacts with the nucleus of a target atom
- it completely avoids the orbiting electrons and may come close enough to the nucleus to come under the influence of its electric field
- the electron is slowed down and its course is changed, it leaves with reduced kinetic enery in a different direction
- this loss of kinetic energy reappears as an xray
- the maximum photon energy possible is Emax which is related to the size of the kV (potential difference) across the xray tube
Combined spectra
is the total spectrum of useful xray beam by combining the continuous and characteristic spectra using equipment operating at 69.5kV or over
Kilovoltage (Kv)
voltage across the tube determines the quality of the beam
increase the Kv the contrast is decreased = longer grey scale and the dose is reduced
Milliamperage (mA)
electrical current flowing through the tube - determines the quantity of the beam
increase the mA the image will be darker and the higher the dose
Time (s)
the duration of the exposure time
increase the time the image will be darker and the dose will be higher
Xray interactions with matter - the four outcomes
- Scattered - the electrons are deflected from their original path and carry on travelling in a different direction with no loss of energy
- Absorbed - the electrons are absorbed with a total loss of energy
- Attentuated - a combination of absorption and scattering which will reduce the intensity of the beam
- The electrons do not interact and so are transmitted unchanged.
these interactions depend upon the amount of energy the photons possess.
Interactions of xrays at the atomic level
- Absorption - pure absorption - the ‘photoelectric effect’
- attentuation - scatter and absorption - the ‘compton effect’
There is also pure scatter known as unmodified or rayleigh scattering and pair production from pure absorption
Pure absorption - low energy photons
The incoming photon and inner shell electron collide - the energy of the incoming xray photon needs to be equal to or greater than that of the binding energy for ejection to take place 69.5Kv
the inner shell electron is ejected with considerable energy into the tissues - this ejected electron is now called a photoelectrons (further interactions will take place with the photoelectrons)
Xray photons has deposited all its energy - pure absorption - xray photon no longer exists
vacancy within the electron shell has to be filled - outer shell electrons fall in to replace ejected photoelectron
The electrons moving from one shell to another and release very low energy radiation in the form of light which is quickly absorbed
the atom has to return to neutral state - it captures a free electron to achieve stability
the ejected photo electons have higher energy, they will behave in same manner as the original xray photon and will interact with other electrons which cause ionisation within the tissue and damage
Contrast in tissues
As the density of an object increases (the atomic number Z) so does the number of bound inner shell electrons
soft tissue has an atomic number of 7
whereas bone has an atomic number of 12
ALUMINIUM Z is 13
COPPER Z is 29
lead is used in radiation protection as Z is 82
compton effect is absorption and scattering
interactions from high energy photons
- collision between incoming photon and outer shell electron
- electron is ejected (compton recoil electron) and the incoming photon loses some energy - there is some absorption
- the remaining incoming electron is deflected (scattered) from its original path and becomes a scattered photon
the scattered photon can go on to
- make further compton interactions
- produce photoelectric interactions
- escape - scatter radiation (dangerous in clinical environment)
- achieve atomic stability by capturing another free electron
