Interactions (week 7-8)

Types of interactions with matter-

• Tranmission

• Photoelectric absorption

• Compton (scatter)

Radioopique

PE (whites and brights)

Radio- Lucent

Transmission

Other names for Coherent

• Classical

• Thompson

• General

Whats coherent interactions do

• Energy in = energy out

Direction or photon gets changed at same energy

• Cant minimize

At what energy do coherent interactions happen

Low energy (cant knock out electron)

What color does coherent interactions appearlike on image

Grey (10% noise)

What happens in a PE interaction

• Photon hits electron k-shell

• Photons completely absorbed

• Electron gets kicked out with energy

• Electrons move to fill missing electron space

• Characterstic xrays form (low energy and rarely hit image: dose for patient)

K-shell vacancy forms?

• Radiative transition

• Non radiative transition (Augar)

Radiative transition

Higher shell electron drops down to k-shell.

• Forms characterstic x-ray

Non-radiative transition

• Auger (rare)

Dosent form a x-ray, instead ejects another electron

K-Edge

Spike in PE due to reaching k-shell binding energy

Emission spectrum

Amount + energy of photons being produced

How to produce best image

• Match emisssion spectrum to k-edge

Because thats where most PE occurs and xrays that form contribute to image contrast/whites

What affects absorobtion

• Mass density

• Atomic number

Density Affect

High density Absorbs more due

• Low density just passes through (less interactions)

Atomic number affect

High atomic number = high amount of protons = more absorbtion

Who gets dose from Compton

Us, its out occupational dose (scatter)

Hows Compton scatter occur

• Photon hits outer shell electron

• Electrons leaves

• Forms scattered xray (lower energy then OG)

Probability of interactions at high energy

High= compton

• Do note that its just because PE dramatically decreases

Low= PE

Compton is independent of?

• Energy ( decreases due to tranmission/less chance of it happening)

• Atomic number

Scatter probability higher at?

The front

Probability of scatter lowest at?

90 degress

How can energy of scatter be deflected

Increase angle

• 180* = least intense

(But 90* has least chance of it occuring)

What happens to ejected electrons

Causes more interactions (less energy each time)

Scatter direction (high energy)

Straight

Scatter direction (low energy)

Backwards

Pair production happens how?

• High energy photon passes through nucleus

• Energy creates two particles (+/-) that split into different directions

How much energy is needed for Pair Production

1.02 MeV

Photodisintagration

• High energy beam hits nucleus

• Nucleus absorbs energy

• Knocks out nuclear fragment