X-ray production process (week 9)

Incident/projectile electrons:Electrons traveling from cathode to anode that are apart of the x-ray tube current 

• What percentage of interaction goes towards heat? Photons? 99% → heat, 1% to photons

• How is anode heat created? i.e., how is IR radiation generated?

Kinetic energy → heat; 

Projectile e- interact with the outer shell e- of the tungsten atoms but do not transfer enough energy to these outer shell e- to ionize them, so they cause atomic excitation. The e- drops back to outer shell and releases Infrared radiation → heat

• Bremsstrahlung radiation vs characteristic radiation

  • Bremsstrahlung interactions are more common than characteristic x-ray interactions, and most of the x-ray beam is made up of bremsstrahlung x-rays. 

  • Chance, moving at great velocities, e- direction and attraction to nucleus, influence of surrounding e- determine which x-ray production takes place

• How are photons generated in each scenario?

Brem’s: The closer a projectile or incident electron gets to the nucleus, the more energy it loses and therefore the higher the energy of the resultant x-ray photon; the e- can convert all, some, or very little amount of its kinetic energy into x-ray photon energy

Characteristic: Incident or projectile electron interacts and knocks out an (usually k level) inner-shell e- rather than an outer shell e-, kinetic energy must be above 69.5 to be considered as characteristic. It leaves a hole in the inner shell, a lower shell will fill the void and release kinetic energy, releasing a characteristic photon

• Are the photon energies unpredictable/varied or specific?

Unpredictable for brem’s, specific for characteristic

• Relationship between kVp, KE and Eb

Characteristic: 

  • kVp: needs to be 70+ or KE will not be high enough to produce characteristic xrays

  • KE: needs a high enough KE that the e-  will kick out the k-shell e- out to produce characteristic radiation

  • Binding energy: enough between shells to have a strong enough photon released to be qualified as characteristic

Brem’s

  • kVp: depends on initial KE of projectile e- and energy the e- loses - how close to the nucleus

  • KE: needs enough energy it does not only produce heat  - needs to be closer to nucleus (increase electrostatic attraction = more KE lost)

    • Projectile e- may come and produce an xray from 0-70kev (closer to the nucleus = increased energy lost and increase energy of photon)

  • Binding energy: not dependent on it due to brem’s not being associated with a shell

• Calculating photon energy in characteristic radiation

The photon is equal to the difference in binding energy between the higher electron orbit and the new 

• Which type of interaction is most common? Brem’s

• Characteristic cascade

Stabalizes the atom as much as possible → when e- attracts a “free” (ejected or projectile) e- to fill the void in the outermost shell or can be to fill the innershell; its random

  • Easy to attract free e- because the atom is ionized and every atom in nature does not want a charge

• What happens to the projectile electrons and the ejected electrons

Projectile electrons: continues in altered path, may interact with other atoms (causing other ejections or atomic excitations) and slows down until comes to rest; conducted to anode circuitry or with another atom missing an e-

Ejected electrons: carries away and leftover KE (anything that wasn't lost to the x-ray or projectile e-) and interacts with different atoms until it loses all or most of the KE

• What does the word Bremsstrahlung mean? Breaking

• How is the energy of a Brems photon determined?

By how close it gets to the nucleus; the closer to the nucleus, the more kinetic energy lost, the stronger the photo released; from initial KE of the projectile e- and the energy it loses

  • Can predict energies from x-ray emission spectrum