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Topic 4 - Sources of Radation

Radioactivity & Natural Radiation Sources

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Radioactivity

  • Spontaneous decay (disintegration) of the NUCLEUS of an atom
  • Involves energy changes in the NUCLEUS
  • Results in:
    • particle + KE
      energy
    Diagram dissecting the atom

Band of Stability

  • The Band of Stability
    There is no known formula to predict if an isotope will be stable.
    • Example 56Fe has highest binding energy per nucleon
    • 53Fehas a half-life of tĹ = 8.5 minutes
    • 60FetĹ = 300,000 years

Nuclear Stability Curve

  • Rule of thumb for predicting stability
    • Plot all known isotopes (tĹ > 10-8 s) as no vs p+
    • two lines can be drawn to enclose all of the stable nuclei
    • Between these 2 lines lies the Band of Stability
    graph: Nuclear Stability Curve
  • no/p+ ratio increases as atomic number increases.
  • As more protons packed into nucleus, larger numbers of neutrons required to compensate strong force - to "dilute" the proton-proton electrostatic repulsions.
  • Isotopes above and to the left tend to Ŗ -emitters.
  • Isotopes below and to the right tend to be positron emitters.
  • Isotopes above atomic #83 tend to be a emitters.

Stability Quirks

Odd Even Rule

  • If nís and pís are both even, isotope is likely stable†
  • Of 264 known stable isotopes
    • only 5 have both odd numbers
    • 157 have both even numbers
    • 102 have both an odd and an even number.
  • The odd-even rule is related to the spins on the nucleons:
    • Both p+ and no have spins. When two like particles have paired spins, combined energy is less than when their spins are not paired.
    • When even numbers of p+ and no all the spins can be paired and the system has less energy (i.e. more stable) then when an odd proton or odd neutron is present.

Magic Numbers

  • Isotopes with specific numbers of protons and neutrons are more stable then the rest.
    • Numbers are 2, 8, 20, 28 , 50, 82, and 126.
    • When both the numbers of the p+ and the number of no are the same magic number the isotope can be quite stable.
    • eg. 42He, 168O, 4020Ca
    • All these isotopes are extremely stable.
    • †20882Pb has 82 p+ and 126 no
  • Existence of magic numbers supports the hypothesis that there are nuclear energy levels similar to electron energy levels.

The facts of life

  • Where do radioactive (and other) elements come from?
    • Universe is primarily hydrogen and helium
    • Big Bang produced mainly H and He, with trace amounts of some lighter elements
    • No appreciable production of C, N, O, Fe, Mg, Si, ... , and other elements heavier than iron.
    • Where did these elements come from?
    • Most of the elements lighter than iron and nickel can be built up from successive rounds of thermonuclear fusion burning in the cores of stars.

Cosmic Abundance

graph: Cosmic Abundance

Elemental Distribution

  • Extreme abundance of hydrogen
  • General decrease with Z
  • Low abundance of some elements (Li, Be, B, Sc)
  • High abundance of Fe, Ni, Pb
  • Greater abundance of even Z vs odd Z
  • The process of creating the heavier elements is called nucleosynthesis . Itís from neutron capture during a supernova

Crustal Abundance of Elements

graph: Crustal Abundance of Elements

Radiations/Radionuclides

  • Solar/Cosmic Radiations
  • Terrestrial radionuclides
    • Primordial
    • Chain decay
    • Other

Natural Sources of Ionizing Radiation

  • Cosmogenic radionuclides, 3h, 14C, 7be,
  • Cosmic radiation, P+, e-, …
  • Terrestrial radiation, 232Th, 238U, 226Ra, 40K, 87Rb

Cosmic Radiation

  • High energy particles
    • 87 % protons
    • 1% Electrons
    • 11% alpha
    • Muons
    • 1% Heavy particles (4 < Z < 26)
  • Interact with earthís atmosphere

Cosmic rays

  • Altitude dependent
  • Latitude dependent
Figure 6-10 The geomagnetically trapped corpueculer radiations.

graph: Radiation Dose

Solar Radiation

  • High energy protons and electrons streaming from solar flares hit the earthís outer atmosphere

Cosmogenic Nuclides

  • Induced Radionuclides
    • Produced by cosmic ray interactions with atmospheric nuclei
    • Includes 3H, 14C, 7Be.
    • Also† 10Be, 22 Na, 32P, 35S, 39Cl
Nuclide Half-Life Source
14C 5730y 14N(n,p)14C
3H 12.3y Cosmic ray interactions with N and O spallation from cosmic-rays, 6Li(n,alpha)3H
7Be 53.28 d Cosmic ray interactions with H and O

Primordial Radionuclides

  • Primordial radionuclides are left over from when the world and the universe was created.
  • They are typically long lived, with half-lives often on the order of hundreds of millions of years.

Radiation From the Earth

  • Naturally occurring radionuclides present in rocks, soils, plants, water, air, and building material
  • Major nuclides include
    • Uranium (U)
    • Thorium (Th)
    • Radium (Ra)
    • Radon

Primordial nuclides - examples

Nuclide Half-Life Typical Activity
235U 1.04 * 108 yr 0.72% of all natural uranium
238U 4.47 * 109 yr 99.2745% of all natural uranium; 0.5 to 4.7 ppm total uranium in the common rock types
232Th 1.41 * 1010 yr 1.6 to 20 ppm in the common rock types with a crustal average of 10.7 ppm
40K 1.28 * 109 yr soil - 1-30 pCi/g (0.037-1.1 Bq/g)

Thorium Series (4n)

graph: Thorium Series

Neptunium Series (4n+1)

graph: Neptunium Series

Uranium Series (4n+2)

graph: Uranium Series

Actinium Series ( 4n+3)

graph: Actinium Series

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