Dr. Bill Pezzaglia Nuclear Physics Updated: 2011Feb07 AstroPhysics Notes 1 Rough draft

Nuclear Physics A. Nuclear Structure B. Nuclear Decay C. Nuclear Reactions 2

A. Nuclear Structure 1.Parts of the Atom 2.Isotopes 3.Nuclide Table 3

1. Parts of Atom Electrons (negative charge) orbital diameter approximately m Nucleus size m Nucleus made of  Protons (+ charge)  Neutrons (neutral) 4

2. Isotopes 5 Isotopes have same atomic number (number of protons)

2b. Nomenclature 6 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 14 has 6 protons, because its carbon.

2c. Atomic Mass 7 AMU: Atomic Mass Unit Carbon 12 is exactly 12 amu Or 1 mole of C 12 is 12 grams [1 mole=6.02  atoms] Naturally occurring carbon 98.9% C 12 ( amu) 1.1% C 13 ( amu) Average:

3. Nuclide Table G. Seaborg 1940 Atomic number is on vertical axis, Neutron number on the horizontal Isotopes: same Z C 12, C 13 Isotones: same N C 14, N 15, O 16 Isobars: same A C 14, N 14, O 14 8

Nuclide Table (Small Z) 9

Nuclide Table (BIG Z)

B. Nuclear Decay 1.Activity 2.Decay Law 3.Modes (Alpha, Beta, Gamma) 4.Dosage 11

1. Radioactivity (a) Phenomena 1898 Term coined by Pierre & Marie Curie (radiation-active) 1896 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: 1 Ci = 3.7×10 10 Bq (activity of 1 gram of radium 226) (c) Decay Constant Activity is proportional to number of nuclei present “N” Activity = N Decay Constant “ ” is probability of decay per second. 12 Antoine Henri Becquerel ( ), 1903 Nobel Prize for discovery of radioactivity

2. Decay Law 1902 Rutherford & Soddy realized that all radioactive decays obeyed the same exponential decay law Half Life: time for half of sample to decay. It is related to decay constant : This “emination law” showed radioactive decay was not deterministic, but statistical (indeterminant) in nature. 13

3. Decay Modes Rutherford (1897) clarifies that there are two types of “Becquerel Rays”, alpha (which he identifies as a Helium nucleus), and beta which is 100x more penetrating. By emitting any of these, the element undergoes “transmutation” into another element. 14

3. Decay Modes 15

3a. Alpha Decay Alpha particle is a Helium Nucleus Example: Most alpha emitters are heavy nuclei 16

3b. Beta Decay Beta particle is actually an electron, identified in 1897 by Thomson. Beta decay involves a “neutrino” (described by Enrico Fermi in 1930s) Example: Neutron decays to proton (plus beta and neutrino) with 12 min halflife 17

3b. More Beta Decay 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 14 decay 18

3b. Inverse Beta Decay 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 2 protons into neutrons to make helium: 19

3c. Gamma Decay “Gamma Rays” discovered 1900 by Villard (later identified as high energy photons, which were what Becquerel originally saw) For example: If a  - (electron) combines with its “antimatter” particle, the  + (positron), they will annihilate, creating two gamma rays 20

C. Nuclear Reactions 1.Stability 2.Fission & Fusion 3.Efficiency 21

1. 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  ( MeV/u) 22

1b. 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 23

1c. 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 24

2b Fusion Combine two (or more) small nuclei to make a bigger, more stable, nuclei Fusion of 4 Hydrogen to Helium is how sun produces energy Fusion of 3 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”). 25

2c Fission Large (bigger than iron), unstable nucleus is split into two (or more) smaller, more stable nuclei Fission can be induced by tossing a slow neutron at a nucleus. During fission, often 2 or more neutrons are released, which can create more fissions (chain reaction) Nuclear reactors generate power from fission of U

3. Efficiency The reaction that the sun uses to generate energy is to fuse (four) hydrogen into helium. The mass of 4 Hydrogen’s (protons) is: 4( ) = amu This is more than the Helium ( ), so there was a small amount of mass converted to energy  m=( )= amu Converted to a percentage: / = or 0.7% of the mass was converted to energy. This is called the efficiency. 27

References/Notes 28 Physics Today, Feb (1996) 21-26, “The Discovery of Radioactivity”