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The Nucleus, almost the final frontier IIIC Specialist Lecture 2 28 September 2011 Prof Carter School of Physics, Wits Geiger, Marsden, Rutherford expt. – (Geiger, Marsden, 1906 - 1911) (interpreted by Rutherford, 1911) – get α particles from radioactive source – make “beam” of particles using “collimators” (lead plates with holes in them, holes aligned in straight line) – bombard foils of gold, silver, copper with beam – measure scattering angles of particles with scintillating screen (ZnS) Geiger Marsden experiment: result – most particles only slightly deflected (i.e. by small angles), but some by large angles - even backward – measured angular distribution of scattered particles did not agree with expectations from Thomson model (only small angles expected) – but did agree with that expected from scattering on small, dense positively charged nucleus with diameter < 10-14 m, surrounded by electrons at ≈10-10 m Ernest Rutherford 1871-1937 Proton • “Canal rays” – 1898: Wilhelm Wien: opposite of “cathode rays” • Positive charge in nucleus (1900 – 1920) – Atoms are neutral • positive charge needed to cancel electron’s negative charge • Rutherford atom: positive charge in nucleus – periodic table ⇒ realized that the positive charge of any nucleus could be accounted for by an integer number of hydrogen nuclei -- protons Neutron • Walther Bothe 1930 – bombard light elements (e.g. neutral radiation emitted 9 4 Be) with alpha particles ⇒ • Irène and Frederic Joliot-Curie (1931) – pass radiation released from Be target through paraffin wax ⇒ protons with energies up to 5.7 MeV released – if neutral radiation = photons, their energy would have to be 50 MeV -- puzzle • puzzle solved by James Chadwick (1932) – “assume that radiation is not quantum radiation, but a neutral particle with mass approximately equal to that of the proton” – identified with the “neutron” suggested by Rutherford in 1920 – observed reaction was 4He + 9Be → 13C* → 12C + n 4 6 6 Structure of nucleus • size (Rutherford 1910, Hofstadter 1950s) R = r0 A1/3, r0 = 1.2 x 10-15 m = 1.2 fm; i.e. ≈ 0.15 nucleons / fm3 • generally spherical shape, almost uniform density • made up of protons and neutrons -- “nucleons” fermions (spin ½), have magnetic moment • nucleons confined to small region (“potential well”) occupy discrete energy levels two distinct sets of energy levels, one for protons, one for neutrons proton energy levels slightly higher than those of neutrons – due to electrostatic repulsion spin ½ ⇒ Pauli principle Nuclear Sizes - examples 1 3 r = ro (A ) ro = 1.2 x 10-15 m Find the ratio of the radii for the following nuclei: 1H, 12C, 56Fe, 208Pb, 238U 1 3 1 3 1 3 1 3 1 : 12 : 56 : 208 : 238 1 3 1 : 2.89 : 3.83 : 5.92 : 6.20 A, N, Z • for natural nuclei – Z range 1 (hydrogen) to 92 (Uranium) – A range from 1 (hydrogen) to 238 (Uranium) • N = neutron number = A-Z • N – Z = “neutron excess” increases with Z • nomenclature EB/A vs A “Fireflies or great balls fire in the sky: stars and stardust” Bioluminescence to attract mates or prey, with a lighting efficiency of up to 96%. “Fireflies or great balls fire in the sky: stars and stardust” Star gazing from Lion King. Pumbaa: Timon? Timon: Yeah? Pumbaa: Ever wonder what those sparkly dots are up there? Timon: Pumbaa. I don't wonder; I know. Pumbaa: Oh. What are they? Timon: They're fireflies. Fireflies that uh... got stuck up on that big... bluish-black... thing. Pumbaa: Oh. Gee. I always thought that they were great balls of gas burning billions of miles away. Timon: Pumbaa, wit' you, everything's gas. Amenhotep IV reigned 1379 to 1362 BC. Suppressed ancient religion of Egypt and instituted a monotheistic worship of the Sun God. Succeeded by his queen, Nefertiti. The old gods were reinstated. Nefertiti Akhenaton, Nefertiti and Sun God The Sun is a normal G2 star, one of more than 100 billion stars in our galaxy. Diameter: 1,390,000 km Solar mass: 1.989x1030 kg Temperature: 5,800 K (surface) 15,600,000 K (core) Electromagnetic Spectrum Soft X-Ray Telescope Extreme UV Imaging Telescope Fe IX. X 17.1 nm Extreme UV Imaging telescope Fe XII 19.5 nm Extreme UV Imaging Telescope Fe XV 28.4 nm Extreme UV Imaging Telescope He II 30.4 nm Hydrogen alpha 656.3 nm Spectroheliogram He I 1083 nm Infant stars in the Milky Way Sun Facts Solar age = 4.57x109 y Solar radius = 695,990 km Surface density = 2.07x10-7 g/cm3 Surface temperature = 5770 K Surface composition = 70% H, 28% He, 2% (C, N, O) by mass Solar mass = 1.989x1030 kg Central density = 150 g/cm3 Central temperature = 15,600,000 K Central composition = 35% H, 63% He, 2% (C, N, O) by mass The Proton-Proton Cycle + 1H → 2H + e+ + ν e+ + e- → γ + γ 2H + 1H → 3He + γ 1H 1 pp collision in 1022 → fusion! 3He + 3He → 4He + 1H + 1H 4H → 4He Deuterium creation 3He creation 4He creation Stardust 7.65 MeV above 12C ground state Sir Fred Hoyle 1915-2001 Stardust – II 7.12 MeV 7.19 MeV History of Sunspots 1613 – Galileo discovers sunspots 1859 – Heinrich Schwabe announces discovery of the sunspot cycle 1859 – Richard Carrington discovers solar Flares Galileo Galilei (1564– 1642) Galileo claimed to have been the first to discover sunspots, although he did not immediately recognise their significance. Sunspot Observations by Galileo The division of sunspots into umbral and penumbral regions is clearly apparent in this drawing from Galileo’s Istoia (1613) Christopher Scheiner (1575 – 1650) The frontispiece of Fr. Scheiner’s Rosa Ursina sive Sol (1630) shows Scheiner making observations with the help of an assistant. An early example of a photograph of the Sun taken by L.M. Rutherford on 21 September 1870. There are numerous clearly defined sunspot groups. THEORY OF SUNSPOT FORMATION Early in the 19th century Sir William Herschel, discoverer of Uranus, then a leading observer postulated that sunspots were holes in the fiery luminous outer layers of the Sun. Through these holes it was possible to see towards the solid surface on which, he believed, living creatures almost certainly existed. THEORY OF SUNSPOT FORMATION Leighton-Babcock Model (1960’s) Weak magnetic fields, 100 G, generated in Convective Zone about 200,000 km below Photosphere by circulating currents. Differential rotation of the Sun, 25 day period at equator and 28 days towards the poles, winds up the initially weak fields. • Flux tubes are formed 100’s km diameter. • Bundles of flux tangle together producing fields 2000 – 4000 G. • Magnetic pressure makes flux bundles buoyant, so float to surface. • Flux bundles pierce the Photosphere, field spreads out to form sunspot. • Winding up starts spot formation at about 40 deg. N and S. Following spot F opposite polarity to pole moves towards pole to neutralise field resulting in “unwinding” of previously wound up lines to produce straight field lines again. Sunspot maximum every 11 y observed. Photospheric Magnetogram Magnetogram Ca II 8542 A Rudolf Wolf devised the basic formula for calculating sunspots in 1848: R = k (10g +s), where R is the sunspot number, g is number of sunspot groups, s is number individual spots all groups and k is scaling factor (usually <1). Wavelet Analysis Wavelet Analysis Wavelet Analysis Wavelet Analysis WAVELET ANALYSIS 11 year Sunspot Cycle Scale years Date years WHY ARE SUNSPOTS DARK? Debate has been going on for centuries! • The intense localised magnetic field in the umbra reduces the flow of hot material to that region of the Photosphere. • Alternatively, strong magnetic fields enhance flow of heat but converts about 80% into “hydromagnetic waves” which propagate through the Photosphere without dissipating significant amounts of energy. Instead, contributes to heating up the solar atmosphere higher up above the sunspot group. Extended, complex group of sunspots Nuclear Giant Resonances simple collective excitations Isoscalar Isovector ∆T = 0 ∆S = 0 ∆T = 1 ∆S = 0 Monopole ∆L = 0 Dipole ∆L = 1 Quadrupole ∆L = 2 Courtesy of P. Adrich 1p-1h excitations in 208Pb Z=82 N=126 GQR Systematics Bertrand et al., NP A354(1981)129c Fine Structure of Giant Resonances High energy resolution is crucial Possible probes: electron and hadron scattering Fine Structure of Giant Resonances High energy resolution is crucial Possible probes: electron and hadron scattering Darmstadt 1980’s ∆E = 50 keV IUCF K600 1991 ∆E = 50 keV iTL K600 2001 ∆E < 40 keV Nucleus as a container that rotates slowly containing nucleons travelling at one third speed of light. Like a bee hive containing fast moving bees. No long range order so moment of inertia between that of a solid and that of shell containing fluid. Surface Waves on the surface of the Sun Bumps on the surface are due to sound waves generated and trapped inside the Sun. What does an ENGINEER do? ENGINEERING What does a BOTANIST do? BOTANY What does a CHEMIST do? CHEMISTRY What does a PHYSICIST do? EVERYTHING Snake of Sizes PHYSICISTS do everything under the Sun Supernova Cycle Main sequence Stars Physicists can do everything, even ride a bicycle!