Alpha Decay
Protons carrying the same positive charge inside the nucleus of an atom tend to repel each
other. This repulsive force between protons is usually overcome by the Strong Force, which
keeps them bound together. However, when there are a large number of protons, the
electrostatic force between the protons will become so large that the Strong Force may not
hold the protons together, resulting in an alpha decay. An alpha particle, which is a helium
nucleus with 2 protons and 2 neutrons, will be emitted during the process. The radioactive
isotope will change into another element. This could be shown by the following example:
Note that the daughter nucleus has an [atomic number] 2 less than its [parent nucleus] and a
[mass number] 4 less. It is found that the total masses of the daughter nuclei and the helium
nucleus is slightly less than the mass of the parent nucleus. By Einstein's famous equation
E=mc2, the loss in mass is converted into kinetic energies carried away by the decay
products. The energy released during this process is fixed, depending on the type of
radioisotope involved.
Beta Decay
When an unstable nucleus decays by beta emission, a fast moving [electron](beta particle) is
emitted from the nucleus. But where does this electron come from? The nucleus consists
only of protons and neutrons, so it seems impossible to have an electron coming from the
nucleus. In fact, the electron comes from an unstable neutron, which transforms into a proton
and an electron. Due to its small mass, the electron is emitted as a beta particle while the
heavy proton remains in the nucleus. The following shows the process of beta decay:
As the daughter nucleus has one more proton, its atomic number increases by one. However,
the mass number of the daughter nucleus remain changed because the mass of the lost
neutron and that of the new proton is more or less the same, while the emitted beta particle
has negligible mass. When conservation of mass is considered, there is always a loss of
energy. The missing energy is due to the emission of an elementary particle called neutrino.
A neutrino has no charge and zero rest mass and travels at the speed of light.
Gamma Emission
Gamma rays are a type of [electromagnetic wave]. For complex nuclei there are many different
possible ways in which the neutrons and protons can be arranged within the nucleus. Gamma
rays can be emitted when a nucleus undergoes a transition from one such configuration to
another. For example, this can occur when the shape of the nucleus undergoes a change,
implying that gamma rays can also be emitted during an alpha or beta decay. Since gamma
rays carry no mass or charge, they do not change the atomic number or the mass number of
the daughter nucleus. There is no equation which can represent the gamma emission alone,
but an asterisk (*) written on the top right corner of the parent nucleus indicates the emission
of gamma ray.
Similar to alpha decay, the amount of energy released in gamma emission from a particular
radioisotope is specific.
*back* (C) 1999 ThinkQuest Team 27954
Click here for an animated explanation.