NE Introduction to Nuclear Science Spring 2012 Classroom Session 2: Natural Radioactivity Chart of the Nuclides Nuclear Stability (Binding Energy, BE) Radioactive Decay Types Radioactive Decay and Growth Isotopes and Decay Diagrams Nuclear Reactions Energy of nuclear reactions Neutron Cross Sections Activation Calculations

Prelude… Please bring clickers to class. Please bring calculators. Get in pairs/groups, feel free to talk across tables. Time on task please. I want everybody working to get answers. 2

Radioactivity in Nature Discovered in 1896 by Henri Becquerel Using photographic plates Notice: no natural sensors for radiation Marie Curie: differences between radioactivity of Uranium and Uranium minerals Discovered Polonium then Radium (both 1898) Main radioactive minerals are: 3

Main Radioactive Minerals 4

Main Naturally Occurring Isotopes: Trivia: 14 C production: ~22,000 at/s/m 2 of earth surface 3 H production: ~2,500 at/s/m 2 Global Planetary Equilibrium Inventory of: 63 tons of 14 C 3.5 kg of 3 H 5

A Bit of History… Filling the Periodic Table The Periodic Table of the Elements 1869 by Dmitri Mendeleev a) Elements Known: 1 – H to 83 – Bi. Plus U and Th. Some stable elements later (Hf -1922, Re- 1925) b) Natural Unstable Elements: Po, Rn, Fr, Ra, Ac, and Pa (mainly by Curies) c) Artificial Elements 43-Tc, 61-Pm, 85-At, and Transuranics (Np, Pu, Am, etc) 6

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Isotopes… 40 kinds of atoms with different half- lives were found Problem: Only 13 spots available in the Periodic Table Frederick Soddy in 1913: Isotopes = “In the Same Place” Nobel Prize 1921 It was later discovered that stable atoms also can have isotopes (e.g. Sn has 10 stable and 18 unstable ones). 8

9 Atomic and Nuclear Nomenclature X element symbol (H, He, Na, Xe, U, etc.) represents the number of charges in the nucleus (thus electrons, thus chemical behavior). A = Mass number = protons + neutrons = Nucleons. Z = atomic number = protons. X and Z are redundant and normally we only write A X. e.g. 235 U, 16 O, 3 H, 14 C

Nuclides Atoms differing by atomic number of mass number Radionuclide: Unstable nuclide Table of nuclides Provide accurate mass of each nuclide 10

Chart of the Nuclides 11

12 Chart of the Nuclides Z N Isobars Isotopes Isotones

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Trivia: Nuclides known: Found in Nature: Main Nat. Radioactive: Produced by cosmic rays: 14 ~ Stable 25 with Z<80 reported radioactive (7 not confirmed) 235 U, 238 U, 232 Th and decay families. 14 C, 10 Be, 7 Be, 3 H

Nuclear Isomers Certain Nuclides with different physical properties Half-lives Modes of decay Metastable states of the same nuclide Either decay by  -emission Transmute to other nuclides ( ,  decay, etc) Suffix “m” next to A. (i.e. 60m Co, 99m Tc) 15

Download RadDecay, by Charles Hacker Play with it: 1. How to find nuclides 2. Half-life 3. Understand Decay products a)  ’s b)  ’s c)  ’s d) X-Rays e) electrons 16

Nuclear Stability 17

18 Stability of Nuclei – General Observations In general, nuclei with an even number of protons or neutrons are more stable As the protons increase electromagnetic forces increase and more neutrons are needed to provide attractive nuclear force to keep the nucleus together Certain Z or N numbers are particular stable (Higher BE) 2, 8, 20, 28, 50, 82, 126 Magic Numbers

19 Stability with Even or Odd Z

20 Stability with Even or Odd N Even Z, Even N = 159 Even Z, Odd N = 53 Odd Z, Even N = 50 Odd Z, Odd N = 4

Let’s do some accounting… For example: Mass of Oxygen Atom: Mp= amu Mn= amu Me=5.48e-4 amu 21 Mass Defect = Binding Energy (BE) Energy (BE) 1 amu = MeV

22 Nuclear Stability and Binding Energy (BE) Higher BE = Higher Stability Compare BE of 12 C and 14 C: To understand stability and radioactive decay, please calculate BE for the following ISOBARS (A=73): 73 Zn, 73 Ga, 73 Ge, 73 As, 73 Se, 73 Br, 73 Kr Everybody!

Isobars = A 23

Notice radioactive decay stabilizes atoms: Question: Do fission products normally have  - or  + decay? 24

25 Chart of the Nuclides Z N Z=N i.e. Fission products are neutron rich

Another Example: Notice several stable possibilities 26

If we graph naturally occurring atoms… Fission Fusion

Reaction Energetics Reaction reactants and products If  E is positive: reaction exothermic releases energy If  E is negative, reaction endothermic requires energy Endoergic and exoergic is sometimes used A + B  C + D +  E

Please remember… BALANCE! Before starting to work

Balancing Reactions nucleons  = 16+1 Charges (+)  = (-)  =  e - missing 0 1 So in reality the reaction is: Calculating Q…

Q-value for the reaction is: Using atomic mass tables: Endothermic reaction. Only a few fission neutrons can do it