Topic 4 - Sources of Radation

Radioactivity & Natural Radiation Sources

Radioactivity

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

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 t_½ = 8.5 minutes
  • ⁶⁰Fet_½ = 300,000 years

Nuclear Stability Curve

• Rule of thumb for predicting stability
  • Plot all known isotopes (t_½ > 10^-8 s) as n^o vs p⁺
  • two lines can be drawn to enclose all of the stable nuclei
  • Between these 2 lines lies the Band of Stability
  
• n^o/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 ß ^-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 n’s and p’s 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 n^o 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 n^o 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 n^o 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 n^o
• 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

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 . It’s from neutron capture during a supernova

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 earth’s atmosphere

Cosmic rays

• Altitude dependent
• Latitude dependent

Solar Radiation

• High energy protons and electrons streaming from solar flares hit the earth’s 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

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

Thorium Series (4n)

Neptunium Series (4n+1)

Uranium Series (4n+2)

Actinium Series ( 4n+3)