Radioactivity basics

Nuclear physics is the study of behaviour of the atomic nucleus. It can be subdivided into various categories, such as:

• Radioactivity Basics
• Detectors
• Nuclear Fission
• Nuclear Fusion

Radioactivity Basics

Radioactivity is the tendency of unstable nuclei to emit particles in order to bring it closer to stability. There are 4 main types:

• Alpha radiation - the emission of a $_{2}^{4}He$ nucleus
• Beta Minus (Plus) radiation - the emission of an electron (positron) from the atom
• Gamma radiation - the emission of a photon by the nucleus
• Neutron radiation - the emission of a $_{0}^{1}n$ from the nucleus

These particles are encountered at typically very low level in nature. Moderate to high rates of exposure to these particles can be severely detrimental to organic tissues and life threatening to humans and the rest of the ecosystem.

Sources of Radioactivity

• Minerals containing naturally radioactive elements (potassium, radium, uranium, thorium...)
• Background cosmic rays
• Solar flux
• Nuclear power plants & nuclear fuel cycle plants
• Old watches & clocks (with radioluminescent paints (radium, tritium)
• Nuclear labs
• Radioactive waste
• Nuclear medicine
• Nuclear bomb testing
• Radon gas

. . .

Radioactive Decay

The activity of a radioactive nucleus (the rate of decay with time) can be described by the following equation:

$A={\frac {dN}{dt}}=-\lambda N$

where $\lambda$ is the 'decay constant' of the process in question.

The exponential behaviour of radioactive decay can be derived by separating the variables and integrating:

$\int _{1}^{N}{\frac {dN}{N}}=-\int _{0}^{t}\lambda dt$

$lnN=-\lambda t+C\,$

Setting the initial number of nuclei $N_{0}=e^{C}\,$ results in the final equation:

$N=N_{0}e^{-\lambda t}\,$

Similarly, the decay by two or processes may be described by:

$N=N_{0}e^{-(\lambda _{1}+\lambda _{2}+...)t}\,$

The half-life of a substance is the time that it takes for the number of nuclei to decay to half their initial value. It can be shown that the half life, $t_{(}1/2)$ is given by:

$t_{1/2}={\frac {ln2}{\lambda }}$

Detectors

The detection of radiation is accomplished by several different mechanisms:

• Gas-filled detectors
• Scintillation detectors
• Semiconductor detectors
• Cerenkov detectors

Nuclear Fission

Nuclear fission is the 'splitting' of an atomic nucleus into two or more lighter components. The energy used to run commercial nuclear power plants and to cause the explosion of nuclear weapons is gained from this process. This process takes place with the liberation of enormous amounts of heat.

Nuclear Fusion

Nuclear fusion is the process by which two or more nuclei combine to form one heavier nucleus. If the final product has a mass number lower than Iron (Fe-56), energy is released and can be used to do useful work. It is the aim of several current international projects to refine this process into an economically viable one.

Reference Texts

Knoll, Glenn F. - Radiation Detection and Measurement

Krane, Kenneth S. - Introductory Nuclear Physics