leftover question-- Do photons have mass?Depends on what you mean by mass.1. Official physics definition:mass is a property you can weigh if you stop something- noRemember simulation- light is oscillation in the electric forcefield that is spreading out from oscillating electron. Onlyexists as entity moving at speed of light. Photon is just little chunkof light, so can never exist as stationary object. Cannot be weighed.

2. mass is measure of how big a thump you (or electron) get when something hits you - yes (technically this is not “mass”, but “momentum”)

speed c

“momentum” for rock and most other things is = mass x velocity, for photon it is different, is Energy/c = h/wavelength. Short wavelength (gamma ray)= high energy = big kick.Long wavelength (radio wave) = low energy = small kick.

Phys 1020 – April 8, 13 Physics of Medical Imaging I: X-rays (today) II. UltrasoundIII. MRI magnetic resonance imagingbook: way too many topics, with too much jargon and too shallow. Only expected to know material covered in class. (few demos -- fun, but complicated and dangerous)

today X-rays--All your questions about them and•What are they?•Why can they be used to see bones and guns?•How are they made and detected?•How they can cause and treat cancer. (maybe nothave time to cover, but simple old stuff, learn from notes)

reading quiz.

1. two kiloelectron volts of energy is the energy: a. one electron picks up in going between two places that have a potential energy difference between them of 2000 volts.b. one coulomb of electrons pick up in going between two places with a potential energy difference between them of 2000 volts.c. the energy an electron has sitting on the terminal of a 2000 V battery.

2. an x-ray machinea. slams high energy photons into material to produce x rays.b. gives off energetic electrons (called “x rays”).c. slams many low energy electrons into atoms to produce x rays.d. slams high energy electrons into atoms to produce energetic photons.

3. An MRI machine primarily detects the locations of what kind ofatoms in the body?a. hydrogen, b. carbon, c. calcium, d. oxygen.

answers. 1-a, 2-d, 3- a

0 V 2000 V

e e

Photographic film

x-ray source

Why show up bones well and flesh poorly?a. bones harder, b. bones are stiffer, c. bones are madeout of different kinds of atoms than flesh, d. bones are denser.

film gives negative of x-ray exposure.Black- lots of x-rays hit, grey- some hit, white- few hit film. Its all about probabilities for blocking/absorbing x-raysgoing from source to film.

How many have had an x-ray? Did you look at it?

Why show up bones and not flesh? c. bones are made out of different kinds of atoms. Calcium (20 P, 20 el)and phosphorus (15 P, 15 el). Tissue carbon (6 el), H (1el), 0 (8 el). Moreelectrons, more electrons in deep energy levels.

Atoms with more protons and electrons.More electrons with energy level spacingsin range to absorb EM radiation of x-ray energy.H

Ca

lot more deep energy levels with electrons inthem, x-ray energy ends up just a little bitextra energy, easy to happen. If lots ofextra energy, usually will not go. Why??

highest energy level with el. stuck to nucleus.

quantum wave function of electron in atom

quantum wave function of free electron with a little energy. Not too big change in shape.Easier, more probable.

quantum wave function of free electron with lots of energy. Big change in shape. Less probable.

1) this is what experiments show.2) cew best attempt at explanation…

free

x-ray imaging is like finding spoon in dark jello.Shoot high power rifle bullets into it?Shoot slow b.b.s into it? What would work best?

darkens film where hit.

x-rays: basically wimpy, but not too wimpy bullets. powerful enough to get through the light atoms, but not the heavier atoms in bone. Energetic (but not too energetic) electromagnetic radiation.

X-rays are part of the electromagnetic spectrum• Just like light, and radio waves, and microwaves...

About 100 times the energy/photon of visible light.that means wavelength is shorter

If used x-ray with 10 times higher energy it woulda. be highly absorbed by both H, C, and Ca.b. be highly absorbed by Ca, but weakly by H and C.c. be weakly absorbed by H, C, and Ca.d. be weakly absorbed by Ca, but highly absorbed by H and C.e. not be absorbed at all by Ca, C, or H.

If used x-ray with 10 times higher energy it woulda. be highly absorbed by both H, C, and Ca.b. be highly absorbed by Ca, but weakly by H and C.c. be weakly absorbed by H, C, and Ca.d. be weakly absorbed by Ca, but highly absorbed by H and C.e. not be absorbed at all by Ca, C, or H.

c. Now would have to give the electron of Ca lots of energy, just like lower x-ray did for H and C electrons. So would be weaklyabsorbed by all. Would not give good x-ray picture. Poor absorption and poor contrast.

H

very high energye in both cases.

Discovery…87,000V+

anode

V = 0 Cardboard box

Glowing screen!!

Unknown radiation could penetrate paper!! And lots of other things too.

What was it?? Called it x-radiation, or x-rays.

How to make x-rays.

Stick hand in front of it??Shadow of hand on screen!!

e

(Rontgen, 1895, playing around puttingbig voltages on electrons)

87,000V+anode

V = 0

How to make x-rays.

e

Current x-ray sources basically the same.Give electrons lots of energy, smash them into metal plate. x-ray machine for imaging 87,000 V.x-ray machine for cancer treatment1,000,000 volts.

lead box

electronbeam

How are x-rays formed?

1) accel incoming electrons: -- “bremsstrahlung”(“braking radiation”)

2) accel. electrons in atom.=jump between energy levels“X-ray fluorescence”1) incoming elec. knocks

electron out of its level2) higher-lying electron falls

into available spot. 3) Photon emitted4) BIG energy difference

high-energy photon = x-ray.

incoming e-

nucleus

X-raynucleus

X-ray

incoming e-

Like all other light, by accelerating electrons

x-ray source has a lot of the same physics as a. TV, b. neon sign, c. florescent light, d. transformers, e. a,b,c

answer e. like TV puts on big voltage to give beam of electrons lots of energy. 15,000-25,000 TV, 87,000 and up X-ray. Like neon sign, flor. light, and TV, giveelectrons bunch of energy, smash into atoms and convert elec.kinetic energy into electromagnetic radiation.

Differences: 1) x-rays more energy in electrons and higher energy EMphotons/radiation. 2) Bremsstrahlung process much weakerat lower energies, and if hit lighter atoms.

X-ray generated by electron that sling-shots around nucleus. Atom itself does not do anything. Kind of like generating radio waves by shaking electrons in antenna. Just shake REALLY HARD AND FAST!.

Bremsstrahlung:

e x ray out. Can have rangeof energies depending on howclose comes to nucleus.Highest is incoming electron energy.

x ray florescence.Just like for atom discharge lamp. Smash electrons into atom, kick electron to higher level, jumps back down, gives off electromagnetic wave- photon. Big atoms like tungsten good because have lots of very deep electrons, can hop up to pretty high level and still be stuck to nucleus.

Energy comes from incoming electronsmashing in with enough energy.

X-ray machine

X-ray Units of energy: eV

• Electrostatic potential Energy:

• Energy = charge * voltage = q * V• Charge on electron = 1.6 *10-19 C = e • Voltage is accelerating electron.

V+ = 87,000 V

Anode

V = 0

e- beam

X-raysEnergy of e- in beam = q * V = 1 electron * 87,000 V = 87,000 eV = maximum energy of X-ray

Compare to red photon… 1.6 eV, or blue with 3.0 eV

How x-rays detected. With film, just like light.Breaks apart molecules. Chemically react.

Then add chemicals. red-green molecules react to makeblack stuff. Intact red-green-blue moleculesreact to make white stuff.

photographic film-paper covered with specialmolecules

X-rays

Spine

1–D imagefinding swallowed piece of metal.

CAT scans- “Computer aided tomography”. Whole bunch of shadow pictures from different directions. Use computer to figure out 3 dimensional shape.Very simple idea, very complicated computer calculations.

Why do they put heavy apron over patient gettingx-rayed? a. Doesn’t actually do anything, just makes patients feel better.b. holds them down to keep them from movingc. Filled with lead is which is heavy atom and so absorbs x-rays well so do not hit parts of body where they are not needed.d. Lead is heavy atom so absorbs x-rays and keeps filmfrom being overexposed.e. keeps them warmer and hence happier

Would tin work as well?

ans. c.

How x-rays can cure cancer?How x-rays can cause cancer?

Questions on x-ray imaging?

physics exactly like sunburn- UV kills cells.UV damages DNA molecules.

X-ray therapy- use x-rays to kill cells. (hopefully mostly cancer tumor)

e

Use more energy. Absorption less likely, damage greater.x-ray million eV, much more energy than electron levels. So muchenergy just gives kick to electron in passing and knocks it out of atomwith lots of energy.

rampaging energetic electron, off to smashup other atoms and molecules.Killing cells.

cH

H cH

H

cH HcH H

cH HcH H

typical biomolecule in cellbefore electron or x-ray hits it

e

cH

H

cH

H

cH H

cH H

cH

HcH

He

after

e

broken molecule, deadcell. Usually extra electronalso kicked out “ionizing radiation

Delivering the x-rays to the tumor:• Bad idea. X-rays

Tumor

Intensity highest at skin

Spine

Damage occurs to skin more than tumor.

Better idea: Shine x-rays on tumor from all sides.Hopefully kill mostly tumor.

cH

H cH

H

cH HcH H

cH HcH H

typical DNA in cellbefore electron or x-ray hits it

e

cH

H

cH

H

cH H

cH H

cH

HcH

He

after

e

broken DNA molecule, can very occasionally become cancerous.

How x-rays cause cancer. Exactly like UV light with skin cancer,except x-rays go deeper into body.

Are x-rays bad for you?• Damage:

– X-rays can cause mutations of genetic code, and therefore cell death.– With right mutation, cell reproduces uncontrollably -> cancer.

• Facts:– X-rays much lower dose than they used to be:– Pb aprons protect unexposed body parts, but don’t protect the part being

imaged. • Exposure units:

– Radiation Absorbed Dose, or RAD.– Unborn children who receive 2-6 rads have increased risk for developing

cancer.– For adults, “exposure of 5 rad” not harmful.– Natural Exposure: typically 360 mrad / yr

• 70 mrad/yr in Florida• 235 mrad / yr in Colorado

– Medical x-ray:• Lumbar spine x-ray: 280 mrad to skin… 10 mrad to bone marrow

– International flight 3 mrad/flight Facts collected from web to give general idea… Don’t take too seriously.