32 IONIZING RADIATION, NUCLEAR ENERGY, AND ELEMENTARY PARTICLES

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1 32 IONIZING RADIATION, NUCLEAR ENERGY, AND ELEMENTARY PARTICLES 32.1 Biological Effects of Ionizing Radiation γ-rays (high-energy photons) can penetrate almost anything, but do comparatively little damage. β-particles (electrons) are more easily stopped, but do more damage if they get into the tissue. α-particles (He nuclei) are very easily stopped, but do a lot of damage if they get into the tissue. neutrons (and protons) are difficult to stop and can be very damaging. neutrinos are almost impossible to stop but they do no damage. Approximately 100,000,000,000,000 neutrinos pass through your body every second! (10 14 )

2 32.2 Induced Nuclear Reactions When two particles collide at high enough energy a nuclear reaction can take place. Examples include: Nuclear reactors Nuclear bombs (fission and fusion) The interior of stars Cosmic rays in the upper atmosphere Nuclear reactions are characterized by creation of new elements and release of energy.

3 Induced nuclear reaction (example)

4 32.3 Nuclear Fission

5 Critical mass

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7 32.4 Nuclear Reactors

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9 32.5 Nuclear Fusion

10 EXAMPLE: 2 1 H H 4 2 He n + ΔE ISOTOPE MASS (u) MASS (KG) Deuterium Tritium Helium Neutron x atomic mass unit x mass defect = u = x Kg ΔE = x J = 2.82 pj

11 OTHER EXAMPLES: Two protons collide to form deuterium: H + 1H 1H + 1e + +υ A proton & a deuterium nucleus collide to form tritium: H + 1H 2He + γ ' s Two of these tritium nuclei collide to form helium: He + 2He 2He+ 1H + 1H + γ ' s FROM HERE ON TAKE VERY GOOD NOTES. MUCH OF WHAT WE ARE GOING TO COVER IS NOT IN THE TEXT!!!

12 32.6 Elementary Particles

13 ELEMENTARY PARTICLES STANDARD MODEL Three Groups of Elementary Particles: Leptons Quarks Gauge Bosons LEPTONS 6 Leptons (+6 Antileptons) 3 Generations (flavors) 2 Leptons (+ 2 Antileptons) in each generation QUARKS 6 Quarks (+6 Antiquarks) 3 Generations 2 Quarks (+2 Antiquarks) in each generation

14 GAUGE BOSONS The Gauge Bosons are associated with the four fundamental forces 6 Gauge Bosons (perhaps) Gluon associated with the Strong Force W ±, Z 0 associated with the Weak Force Photon associated with the electromagnetic force Graviton (?) associated with the gravitational force (postulated; but so far never observed). Photon and Graviton are massless. Others are quite heavy.

15 LEPTONS Flavor Particle Sym. Q Mass * c 2 L e L μ L τ 1 electron e MeV 1 electron neutrino ν 0 <0.6 ev muon μ MeV muon neutrino ν μ 0 <0.6 ev tauon τ MeV 3 tauon neutrino ν τ 0 <0.6 ev Q = charge (in units of 1 electron charge), L= Lepton number Since the neutrinos can transform into each other (neutrino oscillation) it is inferred that they have some mass.

16 QUARKS Q = charge B = Baryon Number S = strangeness c = charm t = topness b = bottomness Gen Name Q c S t b Mass MeV 1 Up u +⅔ Down d -⅓ Charm c +⅔ Strange s -⅓ Top t +⅔ Bottom b -⅓ All Quarks have B = ⅓, All Leptons have B, c, S, t and b = 0

17 SOME MORE PARTICLE NAMES and FACTS Baryons include nucleons and other heavy particles. They are composed of 3 quarks. Mesons are particles composed of a quark-antiquark pair. Hadrons include baryons and mesons collectively but not leptons. Leptons and baryons are called Fermions and have ½-integer spin. Mesons and particles comprising even numbers of baryons are called Bosons. They have integer spin. Fermions obey the Pauli Exclusion Principle; bosons do not

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21 CONSERVATION LAWS Energy Momentum Angular momentum Charge Baryon number Lepton Number (L e, L μ, L τ individually in strong interactions) Strangeness Charm Topness Bottomness Spin Parity (in strong interactions)

22 32.7 Cosmology STELLAR EVOLUTION: BIRTH AND DEATH OF STARS Star Formation: Stars are formed when large amounts of gaseous matter (mostly hydrogen) collect under the influence of their mutual gravity. As the atoms fall together they generate heat. The hot atoms loose their electrons and become protons. What keeps a star burning? Principal reaction in stars: Two protons collide to form deuterium: 1 1 H H 2 1 H + e + +υ A proton & a deuterium nucleus collide to form 3 He H + H He + γ Two of these 3 He nuclei collide to form 4 He He+ 2He 2 He+ 1H + We started with 6 protons and ended up with only 2. What happens when we run out of protons? H

23 Death of a Star: In a star there is a delicate balance between gravity and thermal energy. As the fuel runs out the star collapses under its own gravity. Possible fates include: Supernova Neutron star Black hole Brown dwarf White dwarf What about our sun? Red Giant, then White dwarf

24 GENERAL RELATIVITY AND THE CURVATURE OF SPACE The Postulate of General Relativity: An observer in an enclosed room cannot distinguish whether he is in a gravitational field or whether he is experiencing acceleration. This leads to viewing gravity as a warping of Space- Time.

25 EXPANDING UNIVERSE: THE RED SHIFT The vast majority of stars and galaxies are traveling away from us. (Only a very few stars are traveling toward us and they are nearby.) The farther away from us the stars are, the faster they are receding. The velocity is directly proportional to distance. (Hubbell s Law) v = H r where H 63 (Km/s)/Mpc 1 Mpc = 3.804x10 19 Km = x 10 6 ly

26 STEADY-STATE vs. BIG BANG Evidence for the Big Bang model: Uniform expansion of the universe (red shift + Hubble s law) Age of oldest stars as calculated from Hubble s Constant agrees with age as calculated from stellar properties. Helium-Hydrogen ratio Cosmic background radiation Clustering of galaxies.

27 REVIEW OF BLACKBODY RADIATION P = σεat 4 8π h f u( f, T ) 3 h f / c e = kt 3 1

28 BIG BANG AND COSMIC BACKGROUND RADIATION Discovery of the background radiation 1965 Radiation (almost)uniformly distributed in all directions Essentially perfect fit to blackbody radiation curve for T = K

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30 Experimental error in the measured values is smaller than the size of the dots in the figure!

31 MORE RECENT DATA EXTENDS THE CURVE TO EVEN LOWER FREQUENCIES

32 LOOKING BACK IN TIME Uniformity due to Inflation? (Unexplained rapid expansion by a factor of between and s after the Big Bang

33 DARK MATTER Most of the matter in the universe is of such a nature than we cannot detect it directly. Dark Matter was first postulated to explain the observed rotation of galaxies. What constitutes dark matter? We don t know. Possibly neutrinos account for part of it. Possibly Black Holes account for part of it. Possibly WIMPS (Weakly Interacting Massive Particles) but as yet they are undetected. DARK ENERGY Recent experiments indicate that the expansion of the universe is accelerating. Current theory cannot explain this. An otherwise unknown energy source (called dark energy) is postulated to explain these results What constitutes Dark Energy? -- We don t know.

34 COMPOSITION OF THE UNIVERSE We presently believe that the universe is composed of: 73% Dark Energy 23% Dark Matter 4% Baryonic Matter i.e., matter as we know it! Only 10% of the Baryonic Matter is shining so that we can see it. Therefore: WHEN WE EXAMINE THE SKY WITH OUR VERY BEST INSTRUMENTS WE ACTUALLY DETECT ONLY ABOUT 0.4% OF WHAT IS OUT THERE!!!