Tyler Bradshaw- Fall 2008
-Wilhelm Conrad Roentgen-1895 -Electrical discharges in a vacuum tube -Caused a phosphored screen to fluoresce -Formed an image of bone -First x-ray photograph of wife’s hand/ring
One year later British doctors were already using x-ray imaging. Prolonged exposure to x-rays caused tissue burns.
Wounds were abnormal- took time before appearing.
•X-rays are electro-magnetic radiation. •Same as radio waves, sunlight, or microwaves. •The difference is in the wavelength/frequency
Radio Waves: ~3 km 10000 Hz
Microwave: ~ 1 cm 1010 Hz Visible Light: ~0.000005 mm 1015 Hz
X-Rays: ~ 10-10 m 1018 Hz
*Not to scale. Not even close.
-The energy of a light-wave is carried by packets called photons -The energy of a photon depends on the light’s wavelength -Smaller wavelengths have larger energies
Photon energies are usually given in units of electron volts (eV)
1 eV = 1.602x10-19 Joules
Need 56,000,000,000, 000,000,000,000 eV to cook an egg.
How do you create x-rays?
1. BREMSSTRAHLUNG 2. FLUORESCENCE
•Charged particles emit photons when
decelerating.
•To form x-rays it must be very abrupt. •An electron arcs around the nucleus, emits
an x-ray photon
Heavy Atom
High speed electron
Energy released depends on the electron’s
speed and proximity to the nucleus.
Characteristic X-rays
Electron-electron collision Both leave atom, leaving a vacancy in the inner orbital Vacancy filled by an outer electron
Transition releases high energy photon
High speed electron
Heavy Atom
Both methods occur in an x-ray machine
BREMSSTRAHLUNG:
Photon energies vary: 10,000-90,000 eV. Lower energy x-rays are more common.
FLUORESCENCE:
Specific energies per atom. Higher energy.
More useful.
*Electron beam collides with a spinning tungsten disk *Electrons interact with the tungsten atoms
*Both Bremsstrahlung and flourescence occur
*Filter absorbs low energy x-rays
1) Imaging 2) Killing diseased/cancerous tissue
X-rays are used because of their unique interaction with tissue and bone. There are four types of interactions…
ST: 1
E LASTIC SCATTERING
Absorbs x-ray, bounces up and down Re-emits x-ray
No effect on the atom Bends the x-ray’s path (fuzziness)
X-ray photon is absorbed Electron shoots away from
the atom.
Larger atoms are more likely
to absorb an x-ray than smaller atoms.
Bone (Ca) absorbs x-rays
Tissue (C, H, O, N) transmit x-rays
•Photoelectric effect: useful in imaging
•When treating cancer, however, you must
avoid harming surrounding tissue
•To target the tumor, another type of
interaction is used…
rd: 3
Compton Scattering
-1,000,000+ eV photons used
-Photon collides with an electron
-Both bounce off like billiard balls
Compton scattering affects only a fraction
of the tissue.
To target the tumor it must be treated
from many different angles.
When a 1,000,000 eV photon collides with an atom, another type of interaction occurs…
4TH: ELECTRON-POSITRON PAIR PRODUCTION
2 E=mc
Einstein postulated that energy can become mass, and mass can turn into energy. A small amount of mass can create a lot of energy.
Electron-Positron Pair Production
ee+
Electron
Positron
Antimatter: Symmetry in the universe. Almost all particles
have an antimatter…
Proton Neutron Electron
Antiproton Antineutron Positron
Matter-antimatter collisions turn mass into energy, and create an explosion.
e+
e-
“Annihilation”
*These electron-positron pairs
are formed in radiation treatment.
*The newly formed positron quickly “annihilates” another electron. *Helps kill tumors.