AstroPhysics Notes Nuclear Physics Dr. Bill Pezzaglia Nuclear Physics A. Nuclear Structure B. Nuclear Decay C. Nuclear Reactions Updated: 0Feb07 Rough draft A. Nuclear Structure. Parts of Atom. Parts of the Atom. Isotopes. Nuclide Table Electrons (negative charge) orbital diameter approximately 0-0 m Nucleus size 0-5 m Nucleus made of Protons ( charge) Neutrons (neutral). Isotopes 5 b. Nomenclature 6 H H H Isotopes have same atomic number (number of protons) Z: Atomic Number Number of Protons Tells what is chemical X N: Neutron Number Number of Neutrons A: Mass Number Number of Nucleons A=ZN Don t really need Z : You know Carbon has 6 protons, because its carbon. A X Z C
c. Atomic Mass 7. Nuclide Table G. Seaborg 90 8 AMU: Atomic Mass Unit Carbon is exactly amu Atomic number is on vertical axis, Neutron number on the horizontal Or mole of C is grams [ mole=6.0 0 atoms] Isotopes: same Z C, C Naturally occurring carbon 98.9% C (.00000 amu).% C (.005 amu) Average:. 0(.005).989() =.0amu Isotones: same N C, N 5, O 6 Isobars: same A C, N, O Nuclide Table (Small Z) 9 Nuclide Table (BIG Z) 0 97 96 95 9 9 9 9 B. Nuclear Decay. Radioactivity. Activity. Decay Law. Modes (Alpha, Beta, Gamma). Dosage (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) Antoine Henri Becquerel (85-908), 90 Nobel Prize for discovery of radioactivity (c) Decay Constant Activity is proportional to number of nuclei present N Activity = λn Decay Constant λ is probability of decay per second.
. Decay Law. Decay Modes 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 λt This emination law showed radioactive decay was not deterministic, but statistical (indeterminant) in nature. 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.. Decay Modes 5 a. Alpha Decay Alpha particle is a Helium Nucleus α = He 6 Example: Pu U 8 9 9 α Most alpha emitters are heavy nuclei b. Beta Decay 7 b. More Beta Decay 8 Beta particle is actually an electron, identified in 897 by Thomson. Beta decay involves a neutrino (described by Enrico Fermi in 90s) 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 Example: Neutron decays to proton (plus beta and neutrino) with min halflife n p β ν C 6 N 7 β ν
b. Inverse Beta Decay 9 c. Gamma Decay 0 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). Hydrogen fusion in sun changes protons into neutrons to make helium: ( H ) He β ν p n β ν 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 β β γ C. Nuclear Reactions. Stability. Fission & Fusion. Efficiency. Nuclear Stability (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. Mass Defect: m=(zm p Nm n -m atom ) Binding Energy: BE= m (9.9 MeV/u) b. Binding energy per nucleon 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 c. Nuclear Force 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
b Fusion 5 c Fission 6 Combine two (or more) small nuclei to make a bigger, more stable, nuclei Large (bigger than iron), unstable nucleus is split into two (or more) smaller, more stable nuclei Fusion of Hydrogen to Helium is how sun produces energy Fission can be induced by tossing a slow neutron at a nucleus. Fusion of Helium to Carbon is how red giants create energy During fission, often or more neutrons are released, which can create more fissions (chain reaction) All elements up to iron in the universe were made this way inside of stars ( nucleosynthesis ). Nuclear reactors generate power from fission of U 5.. Efficiency The reaction that the sun uses to generate energy is to fuse (four) hydrogen into helium. 7 References/Notes Physics Today, Feb (996) -6, The Discovery of Radioactivity 8 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. 5