Monday, February 4, 2008

Kinetics of Radioactive Decay

Kinetics of Radioactive Decay

Half-Life Concept

Radioactive decay proceeds according to a principal called the half-life.

The half-life (T½) is the amount of time necessary for one-half of the radioactive material to decay.

For example, the radioactive element bismuth (210Bi) can undergo alpha decay to form the element thallium (206Tl) with a reaction half-life equal to five days.

If we begin an experiment starting with 100 g of bismuth in a sealed lead container, after five days we will have 50 g of bismuth and 50 g of thallium in the jar. After another five days (ten from the starting point), one-half of the remaining bismuth will decay and we will be left with 25 g of bismuth and 75 g of thallium in the jar. As illustrated, the reaction proceeds in halfs, with half of whatever is left of the radioactive element decaying every half-life period.

Stability of nuclei with respect to proton-neutron ratio

The stability of nucleus depends upon the neutron to proton ratio.

When the number of neutrons (n) are plotted against number of protons (p) for nuclei of various items, it has been observed that most of the stable (non-radio active) nuclei fall in a zone or belt. This zone is called stability zone or belt.

a. Fornuclei having atomic number up to 20 the stability belt(n/p ratio) is very close to one.
b. For nuclei having atomim number more than 20, the ratio increases up to 1.5 for heavier nuclei.
c. When the ratio is different from the stability ratio, the nuclei emit alpha or beta particles to move into stability zone.

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