Nuclear Physics
  • Home
  • Structure of the Atom
    • Where are the electrons?
    • Structure of the Nucleus>
      • Isotopes
  • Radioactivity
    • Alpha Decay>
      • Smoke Detectors
    • Beta Decay>
      • Electron Capture
    • Gamma Decay
    • Detection of Radioactivity
  • Radioactive Decay
    • Half Life
    • Decay Series
    • Radioactive Dating
  • Effect of Radiation on Humans
    • Measuring Radiation Dose
    • Radiation Damage
    • Radiation Therapy
    • Tracers and Imaging
  • Elementary Particles
  • Contact
  • Useful Information

Decay Series

When a radioactive isotope decays, the daughter nucleus may also be radioactive and decay, and so on. The successive decays are called a Decay Series. Because of decay series like the one shown, some radioactive elements exist in nature that might otherwise not exist. It is believed that when the solar system acquired its present form, about 5 x 10^9 years ago, nearly all nuclides were formed. Those with short half lives decayed quickly and no longer exist, but those with longer half lives still exist and as a result some with shorter half lives are able to be created during their decay. In this decay series, Uranium-238 has a half life of 4.5 x 10^9 years and we would expect about half of the original U-238 to remain. Radium -226 has a half life of 1600 years, so the original Ra-226 would have all disappeared, were it not for the fact that it is constantly being replenished by the decay of U-238.

There are four naturally occuring decay series on earth (there were more but the initial isotopes have all been used up as their half lives were shorter than the age of the earth). They are Uranium-238 to Lead-206,
 Uranium-235 to Lead-207, Thorium-232 to Lead-208 and Neptunium-237 to Bismuth-209. The short half life of Neptunium-237 (2.14 Million years) means it no longer exists naturally, but most of the rest of the decay series continues today. (this decay series is known because Neptunium-237 is created in some atomic reactors, and by decay of Uranium)

Binding Energy and Nuclear Forces

The total mass of a stable nucleus is always less than the sum of the masses of the protons and neutrons in the nucleus. This is because the lost mass has been converted to energy (Einstein suggested that mass might be considered to be a form of energy, convertible to other forms of energy such as Kinetic, Electrical etc). This energy is referred to as the Total Binding Energy and is equal to the energy required to break the nucleons apart.
There is more than one force acting in the nucleus, and the reason the nucleons stay together is due to a force that is very strongly attractive at very small distances, called the Strong Nuclear Force. It is very strong between nucleons if they are less than about 10^-15 metres apart, beyond this it is essentially zero.
This force has some strange quirks, and is not fully understood. There is also a Weak Nuclear Force, and these two, along with the Electromagnetic Force and Gravitational force make up the four known types of force in nature.
If the nuclide has too many or too few neutrons relative to the number of protons, the binding of the nucleons is reduced and the nucleus is unstable, meaning it comes apart, resulting in radioactive decay. Up to an atomic number of about 30-40, the stable nuclei tend to have close to the same number of protons and neutrons. Above this, stable nuclei contain more neutrons than protons. This is becasue the electrical repulsion between the protons increases with more protons, so more neutrons are needed to exert the attractive nuclear force. A point is reached where no number of neutrons can overcome the increasing electrical repulsion, and all nuclides with atomic number greater than Z=82(Lead) are unstable.
Picture
http://ec.europa.eu/research/energy/print.cfm?file=/comm/research/energy/fi/fi_bs/article_1172_en.htm

Powered by Create your own unique website with customizable templates.