Dr. Bill Pezzaglia Nuclear Physics Updated: 2010May17 Modern Physics Series 1 INCOMPLETE 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 Electron orbit diameter approximately m Nucleus size m Nucleus made of  Protons  Neutrons 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 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) 10

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

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) DISCUSSED IN CLASS 16

3c. Gamma Decay “Gamma Rays” discovered 1900 by Villard (later identified as high energy photons, which were what Becquerel originally saw) For example: A beta+ (positron) which annihilates with beta- (electron) will create 2 gamma rays 17

4. Dosage Radiation does damage to tissue: Dose: energy absorbed per mass New SI unit: Gray: 1 Gy = 1 Joule/ke Old Unit: rad: 1 rad = 0.01 Gy RBE: Relative Biological Effectiveness Gamma 1, Beta 1-2, Alpha Dose Equivalent: Dose equiv = Dose x RBE SI unit: sievert: 1 Sv = Gy x RBE Old unit: rem: 1 rem = 0.01 Sv= rad x RBE 18

C. Nuclear Reactions 1.Stability 2.Fission 3.Fusion 19

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) 20

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 21

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 22

2. Nuclear Reactions (a) Terms Q=(mass reactants-mass products)c 2 Efficiency: Q/(mass reactants)c 2 23

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”). 24

2c Fission Large, 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. Shell Model of Nucleus 26

3c. Shell Model Note prediction of stable nuclei for Z=126 (not discovered yet!) [highest element so far is what?] Nuclei with “magic” numbers of neutrons and protons are very stable: 2

Notes 28 Knight: does not cover “Q” is chap 30, although it appears in several problems. Certainly does not do the kinematics correctly to get the KE

References/Notes 29 Physics Today, Feb (1996) 21-26, “The Discovery of Radioactivity”