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Nuclear Spectroscopy: From Natural Radioactivity to Studies of Exotic Isotopes. Prof. Paddy Regan Chair of Radionuclide Metrology, Department of Physics University of Surrey, Guildford, & Radioactivity Group, National Physical Laboratory, Teddington [email protected] Outline of talk • • • • • • Elements, Isotopes and Isotones Alpha, beta and gamma decay Primordial radionuclides…..why so long ? Internal structures, gamma rays and shells. How big is the nuclear chart ? What could this tell us about nucleosynthesis? Darmstadtium Roentgenium Copernicium The Microscopic World… •ATOMS ~ 10-10 m •NUCLEI ~ 10-14 m •NUCLEONS-10-15 m •QUARKS ~? Nuclear Isotopes Not all atoms of the same chemical element have the same mass (A) Frederick Soddy (1911) gave the name isotopes. (iso = same ; topos = place). Results for natural terrestrial krypton Krypton, Z=36 Mass Spectrograph (Francis Aston 1919) Atoms of a given element are ionized. The charged ions go into a velocity selector which has orthogonal electric (E) and magnetic fields (B) set to exert equal and opposite forces on ions of a particular velocity → (v/B) = cont. N = 42 0.4% 44 2.3 The magnet then separates the ions according to mass since the bending radius is r = (A/Q) x (v/B) Q = charge of ion &7 46 47 48 50 A is the mass of the isotope 11.6 11.5 57.0 17.3 Nuclear chart Atomic Masses and Nuclear Binding Energies M(Z,A) = mass of neutral atom of element Z and isotope A energy M(Z,A) m ( 11H ) + Nmn - The binding energy is the energy needed to take a nucleus Bnuclear of Z protons and N neutrons apart into A separate nucleons Mass of Z protons + Z electrons + N neutrons (N=A-Z) = binding energy (nuclear + atomic) Mass of neutral atom MeV eV 9 Radioactivity….. The science of decay… increasing binding energy = smaller mass increasing Z → increasing Z → A=125, odd-A even-Z, odd-N or odd-Z, even N 125Sn, Z=50, N=75 A=128, even-A even-Z, even-N or odd-Z, odd- N 125Xe, Z=54, N=71 ISOBARS have different combinations of protons (Z) and neutrons (N) but same total nucleon number, A → A = N + Z. (Beta) decays occur along ISOBARIC CHAINS to reach the most energetically favoured Z,N combination. This is the ‘stable’ isobar. This (usually) gives the stable element for this isobaric chain. 11 125 A=125, stable isobar is Te (Z=52, N=73); Even-A usually have 2 long-lived. Mass (atomic mass units) A=137 Mass Parabola 137Xe 83 137Ba 81 137Cs 82 b- decay: 2 types: 1) Neutron-rich nuclei (fission frags) n → p + b- + n 2) Neutron-deficient nuclei (18F PET) p → n + b+ + n Nucleus can be left in an excited configuration. Excess energy released by Gamma-ray emission. ‘signature’ 1461 keV 1461 Note, the number of 40K decays would then be equal to the number of 1461 keV gamma rays emitted, divided by the ‘branching ratio’ which is 0.1067 in this case. gamma Some (odd-odd) nuclei can decay by competing types of beta decay (a) p → n + b- + n ; (b) n → p + b+ + n ; (c) p + e- → n+ v ). Decay rate depends on energy released (Qb value) and CONSERVATION OF ANGULAR MOMENTUM. Big change in angular momentum and small Qb →long half-life. Nuclei can also decay by a emission.. ejection of a 4He nucleus…. Depends (again) on binding energies & masses Before… After… a 232Th, Z = 90 N = 142 228Ra, Z = 88 N =140 4He, Z=2 N=2 Radioactive decays occur as a result of conservation of mass/energy E=Dmc2 M(232Th) = 232.038055 u = mass / energy before alpha decay. M(4He) = 4.002603 u + M(228Ra) = 228.031070 u = mass after. 1 u = 1 atomic mass unit = 931.5 MeV/c2 Dmc2 = M(232Th) – [ M(228Ra) + M(4He)])c2 Dmc2=0.004382 uc2 = 4.08 MeV 4.08 MeV of ‘binding energy’ from 232Th is released in its decay to 228Ra by the emission of a 4He nucleus (a particle). Due to conservation of linear momentum, this energy is split between the energy of the emitted alpha particle (4.01 MeV) and the recoil energy of the residual 228Ra nucleus (0.07 MeV). Geiger-Nuttall rule links Qa values to explain long lifetimes of 232Th, 238U compared to other ‘heavy’ nuclei. ‘Classic’ evidence for quantum mechanical ‘tunnelling’ effect through a barrier. Alpha decay can also leave daughter in excited states which can then decay by (characteristic) gamma emission. a •Radiation occurs in nature…the earth is ‘bathed’ in radiation from a variety of sources. •Humans have evolved with these levels of radiation in the environment. Naturally Occurring Radioactive Materials These include Uranium-238, which has radioactive half-life of 4.47 billion years. 238U decays via a series of alpha and beta decays (some of which also emit gamma rays). These create radionuclides including: • Radium-226 • Radon-222 • Polonium-210 •Radiation occurs in nature…the earth is ‘bathed’ in radiation from a variety of sources. •Humans have evolved with these levels of radiation in the environment. Naturally Occurring Radioactive Materials These include Uranium-238, which has radioactive half-life of 4.47 billion years. 238U decays via a series of alpha and beta decays (some of which also emit gamma rays). These create radionuclides including: • Radium-226 • Radon-222 • Polonium-210 (all of which are a emitters). Other NORM includes 40K (in bones!) Bateman equations, for ‘secular equilibrium’, The activity (decays per second) of cascade nuclide equals the activity of the ‘parent’. How do you measure the gammas? i.e., How do you see inside the nucleus? Little ones…single hyper-pure germanium detector, CNRP labs, U. of Surrey Bigger ones…the RISING array at GSI-Darmstadt, Germany, 105 Germanium detectors (see later)… How do you know how much radioactive material is present? Activity (A) = number of decays per second The activity (A) is also equal to the number of (radioactive) nuclei present (N), multiplied by the characteristic decay probability per second for that particular nuclear species (l). A =lN l is related to the half-life of the radioactive species by l = 0.693 / T1/2 One signature that a radioactive decay has taken place is the emission of gamma rays from excited states in the daughter nuclei. If we can measure these, we can obtain an accurate measure of the activities of the different radionuclides present in a sample. Not all the gamma rays observed have to originate from the same radionuclide. 226Ra Different radionuclides are identified by their characteristic gamma-ray energies. 228Ac 40K Making a Radiological Map of Qatar • Arabic Gulf state, • Oil Rich (oil industry all around) • To host World Cup (2022) 662 keV Characteristic gamma signatures can be used to measure emissions of radionuclides from ‘man-made sources’ such as Fukushima, Chernobyl, nuclear weapons tests…etc. – Nuclear Fission fragments: • • 137Cs (T1/2 = 30 years) 131I (T 1/2 = 8 days) – Neutron-capture on fission products in reactors • 134Cs (T1/2 = 2 years) Summary • Radionuclides (e.g. 235U, 238U, 232Th, 40K) are everywhere. • Radioactive decays arise from energy conservation and other (quantum) conservation laws. • Characteristic gamma ray energies tell us structural info.