AstroPhysics Notes Tom Lehrer: Elements Dr. Bill Pezzaglia Nuclear Physics Updated: 0Feb Rough draft Nuclear Physics A. Nuclear Structure A. Nuclear Structure B. Nuclear Decay C. Nuclear Reactions. Parts of the Atom. Isotopes. Nuclide Table. Parts of Atom 5. Isotopes 6 Electrons (negative charge) orbital diameter approximately 0-0 m Nucleus size 0-5 m Nucleus made of Protons (+ charge) Neutrons (neutral) H H H Isotopes have same atomic number (number of protons)
b. Nomenclature 7 c. Atomic Mass 8 Z: Atomic Number Number of Protons Tells what is chemical X N: Neutron Number Number of Neutrons A: Mass Number Number of Nucleons A=Z+N Don t really need Z : You know Carbon has 6 protons, because its carbon. A X Z C AMU: Atomic Mass Unit Carbon is exactly amu Or mole of C is grams [ mole=6.0 0 atoms] Naturally occurring carbon 98.9% C (.00000 amu).% C (.005 amu) Average:. 0(.005) +.989() =.0amu. Nuclide Table G. Seaborg 90 Atomic number is on vertical axis, Neutron number on the horizontal 9 Nuclide Table (Small Z) 0 Isotopes: same Z C, C Isotones: same N C, N 5, O 6 Isobars: same A C, N, O Nuclide Table (BIG Z) B. Nuclear Decay 97 96 95 9 9 9 9. Activity. Decay Law. Modes (Alpha, Beta, Gamma). Dosage
. Radioactivity (a) Phenomena 898 Term coined by Pierre & Marie Curie (radiation-active) 896 Becquerel discovers radioactive emissions ( Becquerel Rays ) of uranium salts (using photographic plates) (b) Units Activity: decays per second (emissions per second) new SI unit Bq=becquerels= decays per second Old Unit: Curie: Ci =.7 0 0 Bq (activity of gram of radium 6) (c) Decay Constant Activity is proportional to number of nuclei present N Activity = λn Decay Constant λ is probability of decay per second. Antoine Henri Becquerel (85-908), 90 Nobel Prize for discovery of radioactivity 90 Rutherford & Soddy realized that all radioactive decays obeyed the same exponential decay law N( t) = N 0 e Half Life: time for half of sample to decay. It is related to decay constant λ: Ln() t / = λ This emination law showed radioactive decay was not deterministic, but statistical (indeterminant) in nature.. Decay Law λt. Decay Modes 5. Decay Modes 6 Rutherford (897) clarifies that there are two types of Becquerel Rays, alpha (which he identifies as a Helium nucleus), and beta which is 00x more penetrating. By emitting any of these, the element undergoes transmutation into another element. a. Alpha Decay Alpha particle is a Helium Nucleus Example: Pu 8 9 U9 + α α = He Most alpha emitters are heavy nuclei 7 b. Beta Decay Beta particle is actually an electron, identified in 897 by Thomson. Beta decay involves a neutrino (described by Enrico Fermi in 90s) Example: Neutron decays to proton (plus beta and neutrino) with min halflife n p + β +ν 8
b. More Beta Decay 9 b. Inverse Beta Decay 0 Nuclei with neutron excess will change a neutron into a proton by beta decay, emitting an anti-neutrino and beta minus (aka an electron ) Example: Carbon decay C 6 N 7 + β +ν Nuclei with proton excess will change a proton into a neutron by inverse beta decay emitting a neutrino and beta plus (aka positron or anti-electron). + p n + β +ν Hydrogen fusion in sun changes protons into neutrons to make helium: + ( H ) He + β + ν c. Gamma Decay C. Nuclear Reactions Gamma Rays discovered 900 by Villard (later identified as high energy photons, which were what Becquerel originally saw) For example: If a β - (electron) combines with its antimatter particle, the β + (positron), they will annihilate, creating two gamma rays. Stability. Fission & Fusion. Efficiency β + β + γ. Nuclear Stability b. Binding energy per nucleon (a) Binding Energy: the energy required to remove one nucleon from the nucleus The mass of an atom is LESS than the sum of its parts due to negative potential energy of nuclear force. Low Z: more nucleons means more nuclear force, hence more stable High Z: nuclear force is short range, big nuclei unstable Iron is most stable nuclei Mass Defect: m=(zm p +Nm n -m atom ) Binding Energy: BE= m (9.9 MeV/u)
c. Nuclear Force 5 b Fusion 6 Aka strong force. This is what holds the protons together in a nucleus Nucleons attract each other Force is short range, hence big nuclei are unstable Combine two (or more) small nuclei to make a bigger, more stable, nuclei Fusion of Hydrogen to Helium is how sun produces energy Fusion of Helium to Carbon is how red giants create energy All elements up to iron in the universe were made this way inside of stars ( nucleosynthesis ). c Fission 7. Efficiency 8 Large (bigger than iron), unstable nucleus is split into two (or more) smaller, more stable nuclei The reaction that the sun uses to generate energy is to fuse (four) hydrogen into helium. Fission can be induced by tossing a slow neutron at a nucleus. During fission, often or more neutrons are released, which can create more fissions (chain reaction) Nuclear reactors generate power from fission of U 5. The mass of Hydrogen s (protons) is: (.00785) =.000 amu This is more than the Helium (.0060), so there was a small amount of mass converted to energy m=(.000 -.0060)=0.087 amu Converted to a percentage: 0.087/.000 = 0.007 or 0.7% of the mass was converted to energy. This is called the efficiency. References/Notes 9 Physics Today, Feb (996) -6, The Discovery of Radioactivity 5