IB Physics Atomic Nuclear and Particle Physics Nucleus
IB Physics: Atomic, Nuclear, and Particle Physics • Nucleus – Nucleons (A) = Protons (Z) + Neutrons (N) – Mass and Atomic Numbers – Number of protons & neutrons in nucleus is limited. • Radioactivity is the decay of nuclei to more stable element via emission of “radiation” (α or β particles, rays, etc. ). • Half-Life (2 n exponential decay) • Isotopes - 3000 known nuclei, but only 266 stable ones! • Radioactive Processes – α, β, and γ-rays – Natural radioactivity > At. No. 83 – Fusion (Joining) v. Fission (Splitting) of Atoms – both release energy. Page 1
Nucleus: Particle Composition • Z protons + N neutrons = A nucleons (1 – 10 fm 10 -15 diam. ). • 1920: Ernest Rutherford – Bombarded Au foil with Alpha particles – Most of atom is empty space with massive + charged nucleus. • 1932: James Chadwick discovered neutron (bombarded Be with α). • Isotope: same Z (# protons), different N (# neutrons). – 15 O and 16 O … or … 12 C and 14 C … or … 238 U and 235 U … Page 2
The Atom: Particle Properties Particle Charge Proton +1. 6 x 10 -19 1. 007276 Neutron Electron 0 amu 1. 008665 – 1. 6 x 10 -19 5. 4858× 10 -4 kg 1. 67 x 10 -27 ~1. 67 x 10 -27 9. 11 x 10 -31 Recall the 4 Models: • Single Indivisible Particle • Plum-Pudding • Planetary-Quantum (Bohr Model) Page 3
Nucleus: Binding Energy • Nuclear mass is slightly less than mass of constituent protons and neutrons due to nuclear binding energy. • Binding energy per nucleon peaks at A = 56 (~8 Me. V/nucleon) and slowly decreases. • Energy is released when a heavy nucleus (A~200) fissions into lighter nuclei near A~60. Binding Energy / Nucleon ( Me. V) • Mass is converted to energy when a nucleus is formed, E = mc 2. Peaks at Fe (A = 56) Fission (A ~ 200) Nucleon Number A Page 4
Radioactivity: Historical Overview • 1896: Becquerel accidentally discovered that a mysterious rock emitted invisible radiation onto a photographic plate. • 1898: Marie and Pierre Curie discovered polonium (Z=84) and radium (Z = 88), two new radioactive elements. • 1903: Becquerel and the Curie’s received the Nobel prize in physics for radioactive studies. • 1911: Marie Curie received a 2 nd Nobel prize (in chemistry) for discovery of polonium and radium. • 1938: Hahn (1944 Nobel prize) and Strassmann discovered nuclear fission - Lisa Meitner played a key role! • 1938: Enrico Fermi received the Nobel prize in physics for producing new radioactive elements via neutron irradiation, and work with nuclear reactions. Page 5
Contributors to the Study of Nuclear Physics The Nobel Prize in Physics 1903 "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel" Pierre Curie France Marie Curie France 1/4 of the prize "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity" Antoine Henri Becquerel 1/2 of the prize (France) Page 6
Recall: Nuclear Physics. Page 7
Radioactive Decay Examples = + Page 8
Types of Nuclear Radiation ØALPHA (a) PARTICLE is identical to helium nucleus. It has 2 protons and 2 neutrons, mass number of 4 and atomic number of 2. ØBETA (b) PARTICLE is a high-energy Electron. It has a negative charge and mass number of 0. ØGAMMA ( ) RAYS are high-energy radiation, like X-rays. They contain no mass or charge, only energy. λ = 10 -10 to 10 -15 m Page 9
Alpha Emitters Note: An Alpha particle has the same structure as a Helium nucleus. Page 10
Alpha Emitters: Ø 226 Ra 88 ? + 42 He ØFirst, let’s figure out the identity of the new nucleus. How? Determine the atomic number: 88 – 2 = 86. The new element is Rn. ØNext, figure out the mass number of the new nucleus: 226 – 4 = 222. Ø 226 Ra 88 222 Rn 86 + 42 He Page 11
Beta Emitters Note: During Beta Decay, a Neutron spontaneously changes to a. Page Proton. 12
Beta Emitters • 14 C 6 X + 0 -1 e • To find the new mass # we take 14 -0 = 14 • To find the new atomic # we take 6+1 = 7 • The element with atomic number 7 is Nitrogen • 14 C 6 147 N + 0 -1 e Page 13
Producing Radioactive Isotopes: ØTRANSMUTATION is the process of changing one element into another (can be via bombardment OR emission of radiation). ØThis can be natural or artificial. ØA stable atom can be bombarded with fast-moving a particles, protons, or neutrons. ØA radioactive isotope is called a RADIOISOTOPE. Page 14
A stable atom can be bombarded with fast-moving a particles. Page 15
Radiation Exposure: ØBACKGROUND RADIATION is the radiation that is in the environment. ØBackground radiation can come from food, building materials, cosmic rays, etc. ØThe air molecules in the atmosphere block out some cosmic rays. Page 16
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Radiation Exposure: ØRadiation sickness from too much radiation. Nausea, vertigo, and fatigue (side effects of chemotherapy). More exposure can lead to death. ØExposure is measured by LD 50 or lethal dose that is expected to cause death in 50% of the people receiving that dose. ØTo minimize problems, workers often wear badges to monitor the maximum permissible dose. Page 18
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a basements of buildings Note: Chart displays average values of doses for common isotopes. Page 20
Radioactivity End of Part 1. Page 21
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