RAD 354 Chapt. 9Interaction with Matter

• Five Interactions to know– Coherent (classical, unmodified, Thompson)– Compton– Photo Electric– Pair Production– Photodisintigration

Coherent

• Incoming photon interacts with the nuclear force field of the atom and changes direction, BUT has NO CHANGE in energy– Just change in direction – thus the name

“UNMODIFIED”

Compton

• Incoming photon ejects an orbital electron (orbital electron loses some of it’s “on-board” energy to overcome the binding energy of the electron it ejects), and continues on in a deviated direction MINUS the energy required to overcome the binding energy of the ejected electron

Thoughts on Compton

• Compton REDUCES radiographic contrast• The GREATER the angle of the ejected

“Compton electron,” the MORE energy transferred from the “incident photon”

Photo Electric

• Incident photon MUST be at or just slightly higher energy of the orbital electron binding energy it ejects. THE INCIDENT PHOTON CEASES TO EXIST!

• Any and ALL of it’s energy is used to overcome the binding energy of the electron it ejects

• ANY SURPLUS energy (over the binding energy) the incident photon had is held by the ejected electron in the form of POTENTIAL energy

P.E.

PE Tidbits

• As kVp INCREASES, the likelihood of P.E. DECRREASES

• P.E. can occure at ANY orbital level, BUT the MAXIMUM energy exchange is ALWAYS at the K-shell (as k-shell binding energy is the highest binding energy of any atom’s shell)

Useful for Diagnostic X-ray

• ONLY P.E. and COMPTON are useful in the diagnostic energy range (Coherent is too low of energy to be os use and Pair Production and Photodisintegration are TOO HIGH of energy to be in the diagnostic energy range)

Pair Production

• Incident photon (with min. of 1.02 MeV) interacts with the FORCE FIELD of the nucleus

• The photon disappears and TWO electrons (one negative and one positive “POSITRON”) appear – EACH with HALF of the original photon energy (i.e. = .51 MeV each)

Pair Production

The POSITRON produces “ANNIHILATION RADIATION”

• The positron will interact with an electron, converting the mass of BOTH the electron and positron to a photon with an energy of the SUM of the two particle’s energy

• Useful in PET scanning energy range