Nucleus & Nuclear Radiation AGEN-689 Advances in Food Engineering Nuclear Structure The nucleus of an atom of atomic number Z and mass number A consist of Z protons and N = A-Z neutrons A gives the total # of nucleons (protons and neutrons) Nuclide: a species of atom characterized by its nuclear constitution (its value of Z and A (or N)) •Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in Isotopes the form of particles or electromagnetic waves. Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. Hydrogen For example, the most (stable) common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and Deuterium another, tritium, with two (stable) neutrons Tritium 1 2 H1, deuterium is H1, 3 (radioactive) and tritium is H1 Isotones Nuclides having the same number of neutrons 206 204 Pb82 and Hg80 • Lead and mercury are isotones with N = 124 Isobars Nuclides that have the same A’s but different Z’s: 64 64 Ni28 and Zn30 • Nickel and zinc are isobars with A = 64 and but different Z’s Nuclear Mass & Biding Energy Nuclear reactions can be either exothermic (releasing energy) or endothermic (requiring energy to take place) Energies associated with nuclear changes are usually in the MeV range – 106 times greater than energies associated with the valence electrons involved in chemical reactions Nuclear Mass & Biding Energy Energies from exothermic nuclear reactions comes from mass conversion to energy Mass loss = ∆m then energy released, E: E = (∆m)c2 Atomic mass units (amu) & Energy (MeV) By definition: 12 C atom has a mass of exactly 12 amu Since its gram atomic weight is 12 g: 1 1 amu = =1.66×10−24 g 6.02×1023 using Einstein relation with c = 3×1010 cm / s 1 amu = (1.66×10−24 )(3×1010 )2 =1.49×10−3 erg 1.49×10−3 erg(MeV) 1 amu = = 931.48MeV 1.6×10−6 erg Mass Defect Is the difference between the atomic mass (measured mass) sometimes called isotopic mass, M, and the mass number, A: ∆ = M − A Mass Defect Actually, the mass of a proton is 1.00728 amu; a neutron is 1.00866 amu; a electron is 0.0005485 amu. The standard is that one atom of carbon 12, the isotope of carbon with 6 protons, 6 neutrons, and 6 electrons, has a mass of exactly 12 amu. If you add up 6 protons, 6 neutrons and 6 electrons, you get more than 12 amu: 6(1.00728) + 6(1.00866) =12.0956 The mass of 6 protons, 6 electrons, and 6 neutrons is 12.0956 amu, to be precise--but the mass of a carbon nucleus is less than the sum of its parts. Binding energy The "binding energy" of a particular isotope is the amount of energy released at its creation; You can calculate it by finding the amount of mass that "disappears" and using Einstein's equation. The binding energy is also the amount of energy you'd need to add to a nucleus to break it up into protons and neutrons again; the larger the binding energy, the more difficult that would be. Binding Energy 24 For the Nuclide of 11Na the total mass in AMU is: 11(1.00728) +13(1.00866) +11(0.00055) = 24.199 From appendix D, ∆ = -8.418 MeV = 0.0090371 AMU = M-A So, M=24-0.0090371=23.991AMU BE = 24.199-23.991 = 0.208AMU = 194 MeV This is the total BE of the atom – nucleons + electrons BE/nucleon = 194/24 = 8.08 MeV Radioactivity The property that some atomic species, called radionuclides, have of undergoing spontaneous nuclear disintegration All of the heaviest elements are radioactive 209 Bi83 is the only stable nuclide with Z > 82 Radionuclide May emit alpha or beta particles when the ratio of neutrons or protons is unfavorable for the state of stability If after the emission of the particle, the nuclide is still in an energetically unstable state, it may emit gamma ray Nuclear emissions have high kinetic energy (MeV) Unit of Radioactivity Becquerel = 1 disintegration per second It measures only the rate of nuclear transformation It does not deal with kinetic energy released in the process Nucleus Decay In nuclear decay, an atomic nucleus can split into smaller nuclei. A bunch of protons and neutrons divide into smaller bunches of protons and neutrons Particle Decay It refers to the transformation of a fundamental particle into other fundamental particles. The end products are not pieces of the starting particle, but totally new particles. Types of radiation Alpha particles are helium nuclei (2 p, 2 n): p n n p Beta particles are speedy electrons: Gamma radiation is a high-energy photon: Differences among Radiation Can be distinguished by a magnetic field The positively-charged alpha particles curve in one direction, The negatively-charged beta particles curve in the opposite direction, The electrically-neutral gamma radiation doesn't curve at all. Alpha Decay Some heavy isotopes decay by spitting out alpha particles. These are actually helium 4 nuclei--clumps of two neutrons and two protons each. A typical alpha decay looks like this: 238 234 4 U92 => Th90 + He2 Alpha Decay Heavy elements with Z>83 226 222 4 88 Ra→ 86 Rn+2 He Q = M Ra,N − M Rn,N − M He,N Qα = ∆ p − ∆ D − ∆α parent daughter Use Appendix D to find these values Energy Q It is shared by the alpha particle and the recoil (daughter) nucleus Parent nucleus was at rest, so the momenta of the 2 decay products must be equal and opposite: mv = MV Recoil nucleus Alpha particle 1 1 mv2 + MV 2 = Q 2 2 Eα and EN 2MQ v2 = m(m + M ) 1 MQ E = mv2 = α 2 m + M 1 mQ E = MV 2 = N 2 m + M Eα + EN = Q Beta Decay-electron Suppose an atom That's the case with tritium, 3H . has too many 1 3 H metamorphoses into helium neutrons to be 3, it also gives off an electron-- stable. which has hardly any mass, and is endowed with a In beta decay, a negative charge that exactly nucleus cancels one proton. 3 3 0 H => He + e simultaneously emits 1 2 -1 The nuclear reaction involved an electron, or in the beta decay of tritium by negative beta giving the electron a "mass •Note that the mass & chargesnumber" are concervedof 0 and an "atomic number" of -1 particle •It must be true in any nuclear reaction!! Beta Decay A nucleus simultaneously emits an electron, or negative beta particle and an antineutrino Beta decay Beta decay can be seen as the decay of one of the neutrons to a proton via the weak interaction Weak interaction diagram Beta Decay A nucleus simultaneously emits an electron, or negative beta particle and an antineutrino 60 60 0 0 27 Co→28 Ni+−1β +0 v Q = M Co,N − (M Ni,N + m) Q = M Co,N + 27mCo − (M Ni,N + 28mNi ) − m Q − β = ∆ p − ∆ D Note that here we are neglecting the differences in atomic-electron BEs Energy Q It is shared by the beta particle, antineutrino and the recoil (daughter) nucleus The nucleus, because of its large mass, receives negligible energy E − + E = Q β v These are initial kinetic energies of the electron and antineutrino Beta Decay-positron Suppose an atom That's the case with beryllium 7, 7Be - It decays to lithium has not enough 4 7--so a proton turns into a neutrons to be neutron stable. So a positron is emitted--a particle that's just like an In beta decay, a electron except that it has nucleus opposite electric charge. In simultaneously emits nuclear reactions, positrons are written this way: 0e an positron, or 1 7 7 0 Be => Li + e positive•Note beta that the particle mass & charges are concerved4 3 1 •It must be true in any nuclear reaction!! Positron Decay A nucleus simultaneously emits a positron, or positive beta particle and a neutrino 22 22 0 0 11 Na→10 Ne+1β +0 v Q = M Na,N − M Ne,N − m Q = M Na,N +11mNa − (M Ne,N +10mNe ) − 2m 2 Qβ + = ∆ p − ∆ D + 2mc Positron Decay For positron emission to be possible: The mass of the parent atom must be greater than that of the daughter by at least 2 ∆ p > ∆ D + 2mc 2mc2 =1.022MeV Gamma-ray After alpha or beta decay, a nucleus is often left in an excited state--that is, with some extra energy. It then "calms down" by releasing this energy in the form of a very high- frequency photon, or electromagnetic wave, known as a gamma ray. Gamma Ray One or more gamma rays can be emitted from the excited states of a daughter nuclei following radiation decay Transition that results in gamma emission leave Z and A unchanged and are called isomeric Nuclides (initial and final states) are called isomers Gamma Rays 137 137 0 0 Cs137 55 Cs→ 56 Ba+−1β +0 v 55 1.174 Q = ∆ p − ∆ D 0.512 1.174 β− 95% Q = −86.9 + 88.0 =1.1MeV 0.662 5% From appendix: Decay by this mode take places 5% of time – releasing 1.174 MeV β− γ 95% of the cases leaves the daughter 85% nuclei in an excited state with energy 1.174-0.512=0.662 MeV A photon with this energy is shown with 85% frequency 137 56Ba 0 Internal conversion occurs in 95- 85=10% of the disintegration Ba X-rays are emitted following the inner-shell vacancies created in the atom by internal conversion Internal Conversion Is the process in which the energy of an excited nuclear state is transferred to an atomic electron, a K or L shell electron, ejecting it from the atom This process is not the same as emitting a gamma ray which knocks an electron out of the atom It is also not the same as beta decay, since the emitted electron was previously one of the orbital electrons, whereas the electron in beta decay is produced by the decay of a neutron.