308 Physics II - Q1 (x-ray circuit, tube, production, emission spectrum, interactions)

anode

the positively charged electrode in the x-ray tube; houses the target where filament electrons decelerate to produce x-rays

binding energy (electron)

energy that keeps an electron orbiting around a nucleus; higher for inner shells and determines characteristic photon energies

bremsstrahlung (Brems)

‘‘braking’’ radiation produced when a filament electron is deflected by the nuclear force of the target; yields a continuous, polyenergetic spectrum with peak amplitude around one-third kVp

cathode

negatively charged electrode containing the filament and focusing cup; source of the electron cloud for x-ray production

characteristic radiation

discrete-energy photons produced when an inner shell vacancy (commonly k-shell) is filled by an electron from a higher shell; energy equals the difference in binding energies

electromagnetic radiation (EMR)

radiation consisting of photons with no mass and velocity of 3×10⁸ m/s; x-rays are ionizing EMR described by photon energy (eV/kEv)

stator

the electromagnetic ______ is outside the vacuum tube

isotropically

x-rays are produced ______: equal intensity in all directions

1mGy/hr at 1m

housing must limit leakage radiation to what

1. thermionic emission (mA) at filament creates free electrons (concentrated)

2. kVp accelerates electrons to anode

3. sudden deceleration of the electrons (anode)

3 conditions necessary to produce x-rays

mA

controls filament current (high amps needed- step down transformer) to create enough electrons thu thermionic emission

2200°C

how much heat is needed in thermionic emission to create an electron cloud

focusing cup

negatively charged to repel the electron cloud into a tight beam and concentrate it on the anode target

heat

_______ is created when the filament e-’s interact with outer shell e-’s of the tungsten atoms but do not have enough energy to remove them; rather they are just ‘‘excited’’ to a higher energy level

infrared radiation/heat

when the tungsten e-’s drop back into their normal energy level, they emit _______ _______

doubles heat produced

increasing tube current (mA)

more than 70 kVp

when does characteristic interaction occur

1. incident e- knocks out target’s K shell e- out of orbit

2. both e-s leave, creating a ‘‘hole’’ in the K-shell (unstable)

3. another e- ‘‘drops in’’ the hole, emitting excess energy as a characteristic photon

4. the new hole created will be filled by another e- from a different shell (or free e-) producing a characteristic cascade

explain characteristic interactions

69.5

binding energy of tungsten for the K-shell

30%

at > 69.5kVp, ________ of the photons emitted from the tube at diagnostic kVp are characteristic

1. an incident e- passes close enough to the nucleus to be influenced by it & it ‘‘brakes’’ (slows down) due to electrostatic attraction

2. the incident e- changes direction, leaving the e- with less KE than it had before

3. the lost energy is given off as Brems photon with energy equal to the difference in the incident e-’s initial KE and the final KE of the e- as it leaves

explain Bremsstrahlung (brems) interactions

1/3

usually brems photon energy is about _____ of the max energy of the initial electron

1. only characteristic photons w/ enough energy to overcome the k-shell are useful

2. the incident e- is more likely to miss the orbital e-’s of the target and be attracted to the nucleus because e-’s are in constant motion and the shells are mostly empty space

In a tungsten target most of the photons produced are brems for 2 reasons:

1. how fast the filament e- is traveling (determined by kVp)

2. how close the filament e- comes to the nucleus

   • Max energy photon is produced when e- is absorbed into nucleus or comes very close (high energy) and nearly stops & deflects back closer to where it entered the atom

   • low energy photon produced when the e- is far away from & has weak attraction to the nucleus and is only slightly deflected

3. the Z of the target atom

Final KE of the brem’s photon emitted is dependent on: