Sources of Radiation

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Jeffery Underwood
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1 Radioactivity

2 Sources of Radiation

3 Natural Sources Cosmic Radiation The Earth is constantly bombarded by radiation from outside our solar system. interacts in the atmosphere to create secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons.

4 Solar Radiation Includes ionizing and non-ionizing radiation from the sun.

5 Terrestrial Sources Most materials on Earth contain some radioactive atoms in small quantities. Potassium, calcium, carbon isotopes

6 Radon-222 is produced by the decay of radium-226 which is present wherever uranium is found. Since radon is a gas, it seeps out of uranium-containing soils It is often the single largest contributor to an individual's background radiation dose

8 in 1896 Henri Becquerel discovered that uranium and other elements emitted invisible rays that could penetrate solid material Discovery

9 The Becquerel (Bq) is the unit of radioactive decay Equal to 1 disintegration per second

10 Curies Marie and Pierre Curie discovered that the radiation emitted from material depended only on the amount of material, not their chemical state these elements are said to be radioactive

11 nuclei that have too few neutrons in relation to the number of protons are unstable in general, the more protons in a nucleus, the more neutrons that are required to make the nucleus stable (overcome the Coulomb repulsion)

12 Isotope Stability for small stable nuclei, neutrons = protons for large stable nuclei, neutrons > protons for atomic numbers > 83, no stable nuclei (all isotopes are radioactive)

13 Radioactive Decay Rutherford discovered that radioactive thorium emitted a gas that was radioactive An element was changing into another element

14 the changing of one element into another is called transmutation the original nucleus is the parent nuclide and the new nucleus is the daughter nuclide

15 Alpha decay A large nucleus changed into a smaller nucleus and a small positive particle ejected at high speed The particle was called alpha α Later determined to be helium-4 nuclei

16 Example Rn Po He (or could be written as α) 86 2 the sum of the mass numbers on both sides of the arrow must be equal (Conservation of Nucleons) the sum of the atomic numbers on both sides of the arrow must be equal (Conservation of Charge)

17 Example Write the reaction for the alpha decay of Sg Sg He Rf

18 In order for alpha decay to occur, the mass of the parent must be greater than the mass of the daughter and alpha particle combined The excess mass is converted into the kinetic energy of the products

20 Quantum The energy of emitted alpha particles was a mystery to early investigators it seemed that they did not have enough energy to escape the nucleus The nuclear potential energy was several times higher than the observed alpha particle energies.

21 the nuclear potential energy can be represented by a rectangular barrier U(r)

22 according to quantum mechanics, the alpha particle has some probability of tunneling through the potential energy barrier the higher the energy of the alpha particle, the higher the probability of it tunneling through the barrier.

23 Beta negative decay 0 1 β a neutron decays into a proton and an electron which is then emitted from the nucleus

24 Reactions like carbon-14 nitrogen-14 + b- were expected to produce beta particles with identical kinetic energy sometimes a lot of the energy seems to be missing.

25 A New Particle Pauli proposed (1930) that the missing energy and momentum were carried away by a neutral particle with very little mass (neutrino, ν)

26 Beta decay involves the weak nuclear force which acts on electrons and neutrinos Experiments showed that in beta negative decay, antineutrinos are emitted ν

27 Example _ Th 228Pa + 0β + ν

28 Example Write the reaction for the beta-negative decay of cobalt Co 0β Ni 28 _ 60Co 0β + + ν 27-1

29 Beta positive decay 0 +1 β involves emitting a betapositive particle (same mass as an electron, but positive charge) a proton is converted into a neutron

30 A beta-positive particle is the antiparticle of the electron (called a positron) a neutrino is emitted

31 111Sn 111 In β +ν

32 Example Write the reaction for the beta-positive decay of carbon C β C + B 1 β + +ν 5

35 Gamma Decay A nucleus emits gamma radiation when a nucleon drops from an excited energy level in the nucleus to a lower energy level. a gamma ray could be emitted along with an alpha or beta particle

36 Nuclear energy levels are much larger than atomic energy levels

37 Example when a nucleus emits only a gamma ray, the energy of the nucleus is reduced but the mass number and atomic number remain the same 60Co* 60Co + γ Excited nucleus unexcited nucleus

38 Other Decay Modes some radionuclides can transmutate by capturing an electron from the lowest energy level (a proton is converted to a neutron) 41Ca + 0 β 41K + ν

39 Decay Series often a radionuclide will decay into another radionuclide which decays into another radionuclide. 235 U decays by a series of alpha and beta decays to lead-207 (which is stable)

41 Radiation hazards radiation sickness: ionization causes cell damage or death genetic damage: DNA can be damaged by radiation, rapidly dividing DNA has a high probability of being damaged

42 immune system is depressed, so there is extended convalescence and increased risk of infection

43 the charge and energy of radiation determines how ionizing it is alpha particles are very ionizing due to high charge and mass

44 Radiation hazards from sources outside body Radiation Typical Penetration Ionization Hazard Alpha Travels about 5 cm in air, cannot penetrate skin High low Beta Travels about cm in air, little penetration of skin Moderate low Gamma Travels great distances, penetrates entire body low high

45 Penetrating Ability of Radiation

46 α, β, γ particles have energies in MeV, only a few ev are required to ionize molecules Read p

Chapter 30 Nuclear Physics and Radioactivity

Chapter 30 Nuclear Physics and Radioactivity 30.1 Structure and Properties of the Nucleus Nucleus is made of protons and neutrons Proton has positive charge: Neutron is electrically neutral: 30.1 Structure