Resident Physics LecturesResident Physics Lectures

• Christensen, Chapter 2C

Production of X-Rays

George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia

The Atomic NucleusThe Atomic Nucleus

• Protons+ Charges

# protons = atomic # (Z)

• Neutrons No charge Mass about the same as proton

Atomic Weight(mass)= # protons + # neutrons

+ +

+

~ ~

~

kVp = kilovolts peakkVp = kilovolts peak

• peak kilovoltage applied across x-ray tube

• voltage applied across x-ray tube pulses and varies single phase three phase

*

Single Phase

Three Phase

kVp

kVp

keV = kilo-electron voltkeV = kilo-electron volt

• energy of an electron

• Kinetic energy

• Higher energy electron moves faster

• Electrons can be manipulated by electric fields Accelerated Steered

+

Orbital ElectronsOrbital Electrons• Electrons

- chargesvery small mass compared with protons /

neutrons

• Electrons reside only at certain energy levels or ShellsShells

Designations start at K shellK shell closest to nucleusL shell next closestShells proceed up from K, L, M, N, etc.Except for K shell, all shells contain sub-shells

-

-

-

++

~~

+~

K

L

Binding EnergyBinding Energy

• energy required to remove orbital electron from atom

• Negative electrons attracted to positive nucleus

• more binding energy for shells closer to nucleus

K shell has highest binding force

• higher atomic # materials (higher Z) result in more binding energy

more positive charge in nucleus

-

-

-

++

~~

+~

K

L

Electron Shells (cont.)Electron Shells (cont.)

• Electrons can only reside in a shell electron has exactly the energy associated with

its shell electrons attempt to reside in lowest available

energy shell

-

-

-

++

~~

+~

K

L

-

The Shell GameThe Shell Game• Electrons can move from shell to

shell

• to move to higher energy shell requires energy input equal to difference between shells

-

-

-

++

~~

+~

K

L

-

Requiresenergyinput!

*

The Shell Game (cont.)The Shell Game (cont.)• to move to a lower energy shell

requires the release of energy equal to the difference between shells characteristic x-rays

---

-

++

~~

+~

K

L

Energyreleased

X-Ray Production(cont.)X-Ray Production(cont.)

• X-Rays are produced in the x-ray tube by two distinct processes Characteristic Characteristic

radiationradiation BremsstrahlungBremsstrahlung

Characteristic RadiationCharacteristic Radiation

• Occurs whenever electrons drop into lower shell

• Inner shell has lower energy state

• Energy difference between shells emitted as characteristic x-ray 0-28% of total x-ray beam energy

--

-

++

~~

+~

K

L

-

Characteristic RadiationCharacteristic Radiation• High speed electron

from cathode slams into target knocking out inner shell orbital electron

• orbital electron removed from atom

• electrons from higher energy shells cascade down to fill vacancies

• Characteristic x-rays emitted.

--

-

++

~~

+~

K

L

-

+

Characteristic RadiationCharacteristic Radiation• Consists only of discrete x-ray

energies corresponding to energy difference between electron shells of target

• Specific energies are characteristic of target material

• for tungsten 59 keV corresponds to the difference in energy between K and L shells

-

-

-

++

~~

+~

K

L

-

Characteristic Radiation (cont.)Characteristic Radiation (cont.)

• threshold energy required for incident electron (from cathode) to eject orbital electron = electron’s binding energy

++

~~

+~

K

L

-

-

BremsstrahlungBremsstrahlung• interaction of moving electron with

nucleus of target atoms• Positive nucleus causes moving

electron to change speed / direction• Kinetic energy lost• Emitted in form of Bremsstrahlung x-ray

-

-

-

++

~~

+~

K

L

-

Bremsstrahlung (cont.)Bremsstrahlung (cont.)• Bremsstrahlung means braking braking

radiationradiation

• Moving electrons have many Bremsstrahlung reactions

» small amount of energy lost with each

-

-

-

++

~~

+~

K

L

-

Bremsstrahlung (cont.)Bremsstrahlung (cont.)• Energy lost by moving electron is

random & depends on distance from nucleus charge (Z) of nucleus

• Bremsstrahlung Energy Spectrum0 - peak kilovoltage (kVp) applied to x-ray tube most x-ray photons low energy lowest energy photons don’t escape tube

» easily filtered by tube enclosures or added filtration

Beam IntensityBeam Intensity

• Product of # photons in beam energy per photon

• Units Roentgens (R) per unit time Measure of ionization rate of air

• Depends on kVp mA target material filtration waveform

Intensity & Target MaterialIntensity & Target Material

• higher target atomic # results in greater x-ray production efficiency

higher positive charge of nucleus causes more Bremsstrahlung

• discrete energies of characteristic radiation determined by anode material

Energy differences between shells molybdenum used in mammo

» characteristic radiation of 17 & 19 keV

Intensity & TechniqueIntensity & Technique

• beam intensity proportional to mA

• beam Intensity ~ proportional to kVp2

+ filamentvoltagesource

highvoltagesource