Chemistry 130 Nuclear Chemistry Dr. John F. C. Turner 409 Buehler Hall

Chemistry 130 Radioactivity The majority of the chemical elements are stable in that their nuclei do not show any form of decay. In general, from hydrogen (Z = 1) to bismuth (Z = 83) all elements have at least one stable nucleus with the exceptions of technetium (Z = 43) and promethium (Z = 61). After Bi, all the elements do not have a stable nucleus. Even for the elements that have at least one stable isotope, many radioactive isotopes are also known and are produced artificially. Examples include

Chemistry 130 The nucleus The nucleus is a very complicated object. Part of the difficulty of describing the nucleus is the strength of the forces involved and the level at which they couple to each other. The two forces in the nucleus that are important in terms of stability are the strong nuclear force and the electromagnetic force. Electrons are bound in the atom by the electromagnetic force and the equilibrium size of the atom represents the magnitude of the force involved – atoms have a radius of ~ 1-2 Å or ~ m. The nuclear radius is ~ 1-2 fm or ~ m or 5 orders of magnitude smaller than the atom.

Chemistry 130 The nucleus The constituents of the nucleus are the neutron and the proton. The neutron has no electric charge, a spin of ½ and a mass very similar to that of the proton. The proton has a single positive charge and a spin of ½ as well. Both the neutron and the proton are therefore fermions and obey the Pauli principle in a manner analogous to electrons in the atom: The wavefunction of a fermion must change sign on interchange of particles or No more than two fermions can occupy the same state

Chemistry 130 The nucleus Though a description of the nucleus is extremely hard, several features are immediately obvious. There is a force that is attractive and extremely powerful – the strong nuclear force. The strong force is powerful enough to overcome the electrostatic repulsion of the protons in the nucleus and so the nucleons in the nucleus are bound – elements after hydrogen are usually stable up to Z = 83. The size of the nucleus is dictated by the balance between these two forces. The definition of the nuclear surface and the nuclear radius and size is ambiguous and depends on the particle.

Chemistry 130 The nucleus For protons, there is a large Coulombic barrier or repulsion outside the nuclear surface that will repel a positive charge. For neutrons, no such barrier exists. However, there is a spin-dependency based on the Pauli principle which complicates this picture for both neutrons and protons.

Chemistry 130 The nucleus The size of the nucleus is determined by the balance between the strong force between nucleons and the Coulombic repulsion between protons. Spin and the Pauli principle also provide rules that govern the spatial segregation of the nucleons, which will also affect the Coulombic repulsion. Spin is not the only source of angular momentum in the nucleus – there is also orbital angular momentum, though the quantum numbers that label the orbitals have different relationships than those that apply to the electrons in the atom.

Chemistry 130 The nucleus The angular momentum rules provide a rationale for the observation of highly stable nuclei – the so-called 'magic' or 'doubly magic' nuclei Using the notation where M is the mass number, Z the atomic number and X the chemical symbol, special stability is seen when the number of neutrons or the number of protons are 2, 8, 20, 28, 50, 82 or 126 – the so- called 'magic numbers. An isotope that has a magic number for both the neutron number and the proton number is termed 'doubly magic'. is doubly magic, whereas Sn (Z = 50) is singly magic and has 10 stable isotopes.

Chemistry 130 The nucleus The stability of the nucleus is determined by summing the rest mass of the individual nucleons and subtracting the mass of the nucleus. The difference is the binding energy of the nucleus, calculated via E = mc 2. The maximum of the curve occurs at Fe and maxima are seen at the magic numbers such as He and O. Elements heavier than iron will release energy when they undergo fission; elements lighter than iron with release energy when they fuse.

Chemistry 130 The nucleus Not all nuclei are stable. There is a band of nuclear stability. Above and below the stable nuclei lie proton rich nuclei; below lie neutron rich nuclei. The 'drip lines' mark the limits of stability before emission of a proton or a neutron spontaneously. Alternatively, neutrons and protons can no longer be bound into the nucleus for a given Z or neutron number.

Chemistry 130 The nucleus Unstable nuclei emit three primary types of radiation: These are the common types of radiation, though there are many others. Alpha particles are usually emitted by elements at the high mass end of the stability diagram – typically the transuranics and unstable elements heavier than iron. The alpha is a helium nucleus and is emitted probablistically by tunneling through the nuclear potential. Alpha decay takes place through the strong nuclear force.

Chemistry 130 The nucleus Unstable nuclei emit three primary types of radiation: Beta particles are electrons emitted when a neutron converts into a proton inside the nucleus: Beta decay takes place through the weak nuclear force and changes the atomic number by 1. An antineutrino is emitted at the same time, which ensures that angular momentum is conserved.

Chemistry 130 The nucleus Unstable nuclei emit three primary types of radiation: Gamma radiation occurs when a very high energy photon is emitted from the nucleus. This corresponds to a change in the energetic state of a nucleus, usually from an excited state, analogous to emission of a photon from an excited atom.

Chemistry 130 Radiation and matter There are many natural sources of radiation: 'Primordial sources' include 235 U7.04 x 10 8 yr 238 U 4.47 x 10 9 yr 232 Th1.41 x yr 226 Ra1.60 x 10 3 yr 222 Rn3.82 days 40 K1.28 x 10 9 yr and the associated daughter nuclides. These are isotopes that have half- lives on the same order of magnitude as the age of the earth. Cosmic rays also generate radioactivity – primarily 14 C and 3 H Man-made sources include 3 H, 131 I, 129 I, 137 Cs, 90 Sr and isotopes of Pu. These are fission fragments from bomb tests or from processing weapons grade materials and fission reactors

Chemistry 130 Radiation and matter Atmospheric testing of nuclear weapons has added to the fallout inventory; two tests (Ivy Mike (US) and Tsar Bomba (USSR)) in particular were very large – 10.4 Mtons and Mtons and made significant contributions (at full yield, the Tsar Bomba (100 Mtons) would have increased all man-made emissions by 25%) Of particular interest are the nature of the fission fragments. 131 I and 90 Sr are either used in the body (I) or mimic Ca (Sr) and so are readily incorporated into tissue, causing radiation damage to the surrounding tissue. Radiation from these isotopes is intense – half-lives of days – 10's of years – which increases the harm that can potentially be done.

Chemistry 130 Radiation and matter Penetration depths of the principle types of radiation vary: Alphaouter layer of skin2-3 sheets of paper Betainner layers of skin2-3 cm of paper 5 mm Al 1 mm Pb Gamma 2 m concrete10 cm Pb These are estimates and will depend on the energy of the radiation and therefore the source

Chemistry 130 Radiation and matter The destructive nature of radiation with respect to tissue can be harnessed for benign purposes. Focused or collimated beams of X-rays or gamma's are used in cancer therapy; implants of radioisotopes into tumours are effective in certain cases – e.g. in prostate cancer. Radiolabels, often 99 Tc, are used to image specific tissue types.