PHY122_L25 - PHY122 Physics for the Life Sciences II...

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PHY122 – Physics for the Life Sciences II Lecture 25 Nuclear Physics and Radioactivity Note: Clicker Channel 21 12/06/2011 Lecture 25 2
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The Atomic Nucleus Rutherford’s 1911 experiment: – scattering of a beam of “ α ” particles (He nuclei) on a thin gold foil findings: – Almost all α ’s scatter with angles 1 ; –Bu t ~1 10 4 scatter at large angle, up to 180 ! –th is can be understood only if the nucleus is tiny, massive, & has all the positive charge ! – Excellent agreement between measurements and expectation assuming point-like nucleus of charge Ze and point-like α of charge 2 e ! Atom: mostly empty space! “A fly in the Cathedral.” 12/06/2011 Lecture 25 3 1898, J.J.Thomson: “Plum Pudding model” of the atom: electrons as negative “plums” in the positive “pudding” of the atom 1911, E.Rutherford: “Solar model” of the atom: electrons “circle” a small but massive and positive nucleus According to Rutherford: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge.” +
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Content Nuclear components: the proton (mass m p ) and the neutron (mass m n ) – masses, spin, neutron decay, … – the strong nuclear force – the nuclear shell model – binding energy m (He) – 2 m H –2 m n < 0 (The bound whole has less mass than the sum of its parts! This is true for any bound system.) – binding energy/nucleon – fission and fusion Radioactivity – Isotopes and stability – from instability towards stability – types of radiation – decay activity, half-life and decay constant Applications and Effects of Radiation – radiation effects and radiation sources in nature – nuclear medicine – nuclear energy 12/06/2011 Lecture 25 4
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Protons and Neutrons 1917: Rutherford identified the proton (i.e. the H -nucleus) in “nuclear reaction” α + 14 N p + 17 O . – Initially it was assumed that the extra atomic weight A – Z was from p-e pairs inside the nucleus, but QM made that idea untenable In 1932 Chadwick in a series of experiments proved that the neutral penetrating radiation produced when α ’s bombarded certain targets ( B , …) consisted of “ neutrons – mass slightly larger than m p • FREE neutrons will decay! n p + e + ν e (~900 s) – protons and neutrons see the NUCLEAR force: SHORT range ( ~1 fm ) VERY strong attractive up to ~0.5 fm same for p & n Nuclear masses: – note: 1 MeV/c 2 = 5.34×10 –22 J·s 2 /m 2 = 5.34×10 –22 kg !
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PHY122_L25 - PHY122 Physics for the Life Sciences II...

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