Characteristic or Compton: Which One is it? Upon completion of this presentation, participants will be able to Identify the specific location of characteristic and bremsstrahlung radiation production Identify the general location of the photoelectric effect and the Compton effect Recognize characteristic and bremsstrahlung radiation production from a diagram Recognize the photoelectric effect and the Compton effect from a diagram Contrast the process of radiation production with the given x-ray interactions with matter Upon completion of this presentation, participants will be able to
Compare and contrast the photoelectric effect and the Compton effect Compare and contrast characteristic and bremsstrahlung radiation production Thermionic Emission
High speed electrons are boiled off of the cathode filament (-) and projected across the tube to the target anode (+) Projectile electrons bombard the target anode, lose kinetic energy, and may ionize the atoms of the target Produces Anode heat AND Bremsstrahlung radiation OR Characteristic radiation X-Ray Production
X-ray production occurs in the x-ray tube
Denise Moore, Sinclair Community College X-Ray Production
Denise Moore, Sinclair Community College Tungsten Target Atom Z = 74
K-shell: 69.5 keV M-shell: 3 keV
L-shell: 12 keV N-shell: 1 keV
O-shell: 0.1 keV
Denise Moore, Sinclair Community College Bremsstrahlung Radiation Production The projectile electron interacts with the nuclear force field of the target tungsten atom The electron (-) is attracted to the nucleus (+) The electron DOES NOT interact with the orbital shell electrons of the atom Always produced = 100% of time
http://www.internaldosimetry.com/courses/i ntrodosimetry/images/ParticlesBrem.JPG Brems Radiation Production As the electron gets close to the nucleus, it slows down (brems = braking) and changes direction
The loss of kinetic energy (from slowing down) appears in the form of an x-ray
The closer the electron gets to the nucleus the more it slows down, changes direction, and the greater the energy of the resultant x-ray
The energy of the x-ray can be anywhere from almost 0 (zero) to the level of the kVp Brems Radiation Production
Denise Moore, Sinclair Community College Results of Brems Radiation Production
1. Electron scattered in different direction with less energy
2. X-ray Energy dependent upon location of electron to nucleus and degree of deceleration Characteristic Radiation Production
The projectile electron interacts with and ejects an inner shell electron of the target atom The inner shell vacancy is filled by an outer shell electron in an effort to stabilize the atom This electron transmission from outer to inner shell gives rise to an x-ray The energy of this x-ray is difference in binding energies between the two electrons involved Only k-characteristic x-rays of tungsten are useful in diagnostic imaging Characteristic Radiation Production
Denise Moore, Sinclair Community College Results of Characteristic X-Ray Production
1. Ion – atom is now ionized because an electron was ejected 2. X-ray Energy dependent upon target atom and electron binding energies of that atom (hence, the term characteristic) Tungsten Target Atom Z = 74
K-shell: 69.5 keV M-shell: 3 keV
L-shell: 12 keV N-shell: 1 keV
O-shell: 0.1 keV
Denise Moore, Sinclair Community College Diagnostic X-Ray Interactions w/ Matter
Occur in biologic tissue (patient, radiographer) Photoelectric Effect
Incoming photon is completely absorbed by an inner shell electron This excess energy in the inner shell electron causes it to be ejected from the atom The ejected electron (photoelectron) leaves with energy equal to the incident x-ray energy minus the electron binding energy There is now an inner shell vacancy… Results of Photoelectric Effect Atom is now an ion because electron was ejected Free electron = photoelectron Secondary (characteristic) x-ray produced Patient dose due to incoming x-ray absorption Radiographic contrast due to absorption of incoming x-ray (white areas on image) Dowd, S.B. Practical Radiation Protection and Applied Radiobiology Compton Effect Incoming photon interacts with, and transfers some of its energy to, an outer shell electron (low binding E) This excess energy in the outer shell electron causes it to be ejected from the atom (recoil, Compton, or secondary electron) The ejected electron leaves with energy equal to the energy of the incoming x-ray minus the electron binding energy (low E) The x-ray now has decreased energy so it changes direction (scatter radiation) and exits the atom – usually retains most of the E Compton Effect Results
Atom = ion because electron was ejected Free e- = recoil Scattered x-ray Decreased contrast Operator dose
Source: Dowd, S.B. Things to Remember About Diagnostic Radiation Production Diagnostic radiation production happens in the x-ray tube Two kinds of diagnostic radiation production: characteristic and bremsstrahlung Incoming electron Interacts with: inner shell electron = characteristic radiation nuclear force field = bremsstrahlung radiation Things to Remember About Diagnostic Radiation Production
Results in: ion and x-ray w/specific (discrete) energy = Characteristic scattered e- and x-ray w/varying (continuous) energy = Bremsstrahlung
Things to Remember About X-ray Interactions with Matter
Interactions with matter occur in biologic tissue Two interactions with matter important in general radiography: photoelectric & Compton Incoming photon or x-ray Interacts with: inner shell electron = photoelectric effect outer shell electron = Compton effect Things to Remember About X-ray Interactions with Matter Results in: Increased patient dose = photoelectric effect Increased operator dose = Compton effect Increased image contrast = photoelectric effect Decreased image contrast = Compton effect Characteristic x-ray = photoelectric effect Scattered x-ray = Compton effect Photoelectron = photoelectric effect Recoil/secondary/Compton electron = Compton effect 1. What is This? 2. What is This? 3. What is This? 4. What is This? 5. What is This? 6. What is This? 7. Where Does This Occur? 8. Where Does This Occur? 9. Where Does This Occur? 10. Where Does This Occur? 1. What is This?
Characteristic Radiation Production 2. What is This?
Photoelectric Effect 3. What is This?
Bremsstrahlung Radiation 4. What is This?
Characteristic Radiation 5. What is This?
Bremsstrahlung Radiation 6. What is This?
Compton Effect 7. Where Does This Occur?
In Biologic Tissue – Photoelectric Effect 8. Where Does This Occur?
In X-Ray Tube - Bremsstrahlung 9. Where Does This Occur?
In Biologic Tissue – Compton Effect 10. Where Does This Occur?
In X-Ray Tube - Characteristic Can You Keep Them Straight?
Do You Get It? Questions?
Julie Gill, PhD, RT(R)(QM) University of Cincinnati Blue Ash College [email protected]