Nuclear Physics Since Rutherford

Transcription

1 Nuclear Physics Since Rutherford and his legacy to science Jim Al-Khalili University of Surrey, UK Nuclear and Particle Physics Divisional Conference 4-7 April 2011, University of Glasgow Nuclear and Particle Physics Divisional Conference, 4-7 April 2011, University of Glasgow

2 "The phenomena exhibited by the Frederick Soddy ( ) radioactive bodies are English Chemist worked with extremely complicated Rutherford at McGill and some form of theory is essential to connect in Glasgow ( ) an intelligible manner the mass of experimental Then Aberdeen and Oxford facts that have now been accumulated". Nobel Prize for Chemistry in 1921 Coined term isotopes From Radio-activity, Rutherford s 1904 textbook. 1908: Nobel Prize for chemistry (for transmutation ) 2

3 Two proton decay in 54 Fe 3 "Two-proton radioactivity - caught in the act", P.M. Walker and R.C. Johnson, Nature Physics 3 (2007) April 5, 2011 Nuclear and Particle Physics Divisional Conference, 4-7 April 2011, University of Glasgow

4 The nucleus is the playground of quantum physics. No single theoretical model can describe all nuclei. Each has its appropriate domain of applicability. Single protons and neutrons are described with a quark model using Quantum Electrodynamics (QCD). Light nuclei described using microscopic models starting from the basic interaction between nucleons. medium mass nuclei described using the nuclear shell model Heavy nuclei described using mean field models that describe the behaviour of the nucleus as a whole. 4

5 Age of the Earth Kelvin had measured age of earth to be million years old Also, by measuring amount of helium trapped in rocks, he could work out age of those rocks based on estimate of half life of radioactive materials. Rutherford showed that taking into heat generated by radioactive decay would increase this to 100s of millions of years. "Kelvin had limited age of earth provided no new sources of heat found. That prophetic utterance referred to what we are now considering!" 5

6 Rutherford returned from Canada to England and University of Manchester in Rutheford (right) with Hans Geiger in Manchester. 6

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8 It took Rutherford 18 months to figure out Geiger and Marsden s results. Then. December 1910: He figures out notion of single scattering and borrows from Newton s Principia idea of hyperbolic trajectory for alpha particle March 1911: He delivers talk to Manchester Literary and Philosophical Society April 1911: He submits paper to Philosophical Magazine May 1911: Paper published - Philosophical Magazine, volume 21 (1911), pp Charles Galton Darwin 8

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12 Deriving Rutherford scattering quantum mechanically: k ( projectile target In Born Approximation, Scattering amplitude is k' θ V << E, E = h2 k 2 2µ f ( θ) = µ 2πh 2 q e iq r q = k k q = 2k sin 1 2θ V( r)dr ( µ - reduced mass) For simple Coulomb potential with shielding, V( r) = Z Z 1 2 e2 can derive analytic expression for scattering amplitude r e r / a, 12

13 ( ) = 2µZ 1 Z 2 e2 f θ q 2 + ( h a) 2 and can drop 2 nd term in denominator. The Rutherford differential cross section is therefore dσ dω = ( ) 2 = 4µ 2 Z 1 Z 2 e 2 f θ ( ) 2 ( ) 2 = µ 2 Z 1 Z 2 e 2 q 4 4k 4 sin 4 1 2θ Compare with formula (from classical theory of electrostatic deflection of charge) appearing in Rutherford s 1911 paper for number of alpha particles scattered to angle Φ : 13

14 The first artificial nuclear reaction 1914 Ernest Marsden performs experiment scattering alpha particles from light atoms sees many more hydrogen atoms than expected suspects contamination of source Rutherford works out what is going on: 14 N(α,p) 17 O E. Rutherford, Philos. Mag. Ser. 6, v. 37, Collision of a Particle with Light Atoms. IV. An Annomalous Effect in Nitrogen. By Professor Sir E. Rutherford, F.R.S. University of Cambridge. (Received 1919) 14

15 The Neutron In his Bakerian lecture at the Royal Society in 1920 Rutherford writes of...the possible the existence of an atom of mass nearly 2, carrying one charge, which is to be regarded as an isotope of hydrogen He then deduces from this... the possible existence of an atom of mass 1, which has zero charge He even coined the term neutron for this particle. James Chadwick confirms existence of neutron in

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17 The same physics that led Rutherford to conclusion from Geiger-Marsden exp that atoms had internal structure (concentration of mass in nucleus) was used again: In late 1960s, James Bjorken suggested could probe internal structure of protons from deep inelastic scattering of electrons SLAC => quarks 17

18 The legacy of Rutherford scattering today: NIM, A 637 (2011) e+a scattering at FAIR 18

19 Facility for Antiproton and Ion Research (FAIR) Charge density distributions of exotic nuclei from elastic scattering New collective modes of nuclear excitation via inelastic scattering Internal nucleon-nucleon correlations via quasi-free scattering such as nucleon knockout (e,e N) or cluster knockout (e,e α) Electron-Ion Collider New Experimental Storage Ring (NESR) Electron-Antiproton Ring (EAR) 19

20 Moonshine Early 1930s Rutherford publically dismissive of the energy locked up within nucleus The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine. In 1932, privately, to Maurice Hankey, Britain s man of secrets, (Secretary of the Committee of Imperial Defence in World War I). Hankey: He told me the experiments on nuclear transformation which he was supervising in Cambridge might one day turn out to be of great importance to the defence of the country. He did not quite know in what way this would be so.but warned me that someone should keep an eye on the matter. 20

21 Moonshine Further evidence: Great interest in Fermi s experiments in Rome with slow neutrons. In the last year of his life, Bohr and Wheeler wrote their seminal paper on the liquid drop model of the nucleus. After a lecture on this topic by Bohr, Rutherford told him: If in a nuclear reaction mass disappears, energy will appear and, ultimately, whatever its initial form, be degraded to heat. It might be used. 21

22 Science funding Main influence on science funding and policy came through the DSIR (Department of Scientific and Industrial Research), founded in Today, the DSIR has been replaced by the Research Councils. Rutherford began tapping the DSIR for funds around 1924: o Secured the salary of Chadwick at the Cavendish for the post of deputy director o Applied for and received 8000 worth of funding for equipment. This marked the launch of his research in 'Big Science In 1937, tried to create a research council for physical sciences, but it would take several further decades before something like what we have today appeared. 22

23 Finally, a quick plug: The deadline for abstract submission has been extended to 22 April. 23

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