leftover question-- Do photons have mass? Depends on what you mean by mass.

1. 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- no Remember simulation- light is oscillation in the electric force field that is spreading out from oscillating electron. Only exists as entity moving at speed of light. Photon is just little chunk of light, so can never exist as stationary object. Cannot be weighed. speed c 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”) “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.

2. Phys 1020 – April 8, 13 Physics of Medical Imaging I: X-rays (today) II. Ultrasound III. MRI magnetic resonance imaging book: 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 not • have time to cover, but simple old stuff, learn from notes)

3. 2000 V 0 V 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. e e 2. an x-ray machine a. 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 of atoms in the body? a. hydrogen, b. carbon, c. calcium, d. oxygen. answers. 1-a, 2-d, 3- a

4. How many have had an x-ray? Did you look at it? x-ray source Photographic film 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-rays going from source to film. Why show up bones well and flesh poorly? a. bones harder, b. bones are stiffer, c. bones are made out of different kinds of atoms than flesh, d. bones are denser.

5. 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). More electrons, more electrons in deep energy levels. highest energy level with el. stuck to nucleus. Atoms with more protons and electrons. More electrons with energy level spacings in range to absorb EM radiation of x-ray energy. H lot more deep energy levels with electrons in them, x-ray energy ends up just a little bit extra energy, easy to happen. If lots of extra energy, usually will not go. Why?? Ca

6. 1) this is what experiments show. 2) cew best attempt at explanation… quantum wave function of free electron with lots of energy. Big change in shape. Less probable. quantum wave function of free electron with a little energy. Not too big change in shape. Easier, more probable. free quantum wave function of electron in atom

7. 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.

8. 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

9. If used x-ray with 10 times higher energy it would a. 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.

10. If used x-ray with 10 times higher energy it would a. 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. very high energy e in both cases. 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 weakly absorbed by all. Would not give good x-ray picture. Poor absorption and poor contrast. H

11. How to make x-rays. (Rontgen, 1895, playing around putting big voltages on electrons) Discovery… anode 87,000V+ Glowing screen!! Stick hand in front of it?? Shadow of hand on screen!! V = 0 e Cardboard box Unknown radiation could penetrate paper!! And lots of other things too. What was it?? Called it x-radiation, or x-rays.

12. How to make x-rays. 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 treatment 1,000,000 volts. anode 87,000V+ electron beam V = 0 e lead box

13. Like all other light, by accelerating electrons How are x-rays formed? nucleus X-ray 1) accel incoming electrons: -- “bremsstrahlung” (“braking radiation”) 2) accel. electrons in atom. =jump between energy levels “X-ray fluorescence” • incoming elec. knocks electron out of its level • higher-lying electron falls into available spot. • Photon emitted • BIG energy difference high-energy photon = x-ray. incoming e- nucleus X-ray incoming e-

14. 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, give electrons 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 EM photons/radiation. 2) Bremsstrahlung process much weaker at lower energies, and if hit lighter atoms.

15. Bremsstrahlung: • 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!. x ray out. Can have range of energies depending on how close comes to nucleus. Highest is incoming electron energy. e

16. 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 electron smashing in with enough energy.

17. X-ray machine

18. 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 = 0 V+ = 87,000 V e- beam Anode X-rays Energy 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

19. How x-rays detected. With film, just like light. Breaks apart molecules. Chemically react. Then add chemicals. red-green molecules react to make black stuff. Intact red-green-blue molecules react to make white stuff. photographic film- paper covered with special molecules

20. 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. 1–D image finding swallowed piece of metal. X-rays Spine

21. Why do they put heavy apron over patient getting x-rayed? a. Doesn’t actually do anything, just makes patients feel better. b. holds them down to keep them from moving c. 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 film from being overexposed. e. keeps them warmer and hence happier ans. c. Would tin work as well?

22. Questions on x-ray imaging? How x-rays can cure cancer? How x-rays can cause cancer? physics exactly like sunburn- UV kills cells. UV damages DNA molecules.

23. X-ray therapy- use x-rays to kill cells. (hopefully mostly cancer tumor) • Use more energy. Absorption less likely, damage greater. • x-ray million eV, much more energy than electron levels. So much • energy just gives kick to electron in passing and knocks it out of atom • with lots of energy. rampaging energetic electron, off to smash up other atoms and molecules. Killing cells. e

24. after c c c c c c c c c c c c H H H H H H H H H H H H H H H H H H H H H H H H e e typical biomolecule in cell before electron or x-ray hits it e broken molecule, dead cell. Usually extra electron also kicked out “ionizing radiation

25. Delivering the x-rays to the tumor: X-rays Intensity highest at skin Tumor • Bad idea. Spine Damage occurs to skin more than tumor. Better idea: Shine x-rays on tumor from all sides. Hopefully kill mostly tumor.

26. after c c c c c c c c c c c c H H H H H H H H H H H H H H H H H H H H H H H H e How x-rays cause cancer. Exactly like UV light with skin cancer, except x-rays go deeper into body. e typical DNA in cell before electron or x-ray hits it e broken DNA molecule, can very occasionally become cancerous.

27. 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.