Nuclear Decay

There are four main types of nuclear decay

1. Alpha Decay
2. Beta- Decay
3. Beta+ Decay
4. Electron Capture

Alpha decay is prominent in heavy atoms as emission of an alpha particle (a He-4 nucleus) reduces the size of the nucleus by two neutrons and two protons. Beta- decay occurs with nuclei that are "neutron rich". The net result of beta- decay is the conversion of a neutron to a proton. Beta+ decay and electron capture both result in the opposite: a proton converting to a neutron. In all four cases, the decay is generally accompanied by the emission of high energy gamma radiation.

Alpha Decay

Alpha decay is a process in which a nucleus decays via the emission of an alpha particle. An alpha particle is a helium-4 nucleus (2 protons and 2 neutrons bound in a nucleus). Alpha decay is typical of very heavy atoms (the radioactive isotopes found at the bottom of the periodic table). It results in the change of 4 in the mass number and 2 in the charge number.

Below is an example of the alpha decay of Po-210 to Pb-206

\[\rm{^{210}_{84}Po \; \rightarrow \;^{206}_{82}Pb \;+\; ^4_2He}\]

Beta Decay (-/+)

In beta decay, the nuclear decay is associated with the release of a beta particle.  This could be a negative beta particle (or electron) or a positive beta particle (a positron).

This results in no change in the mass number, and a change in the charge number of    +/- 1.

Below is an example of beta - decay

\[\rm{^{14}_6C\; \rightarrow\; ^{14}_7N \;+ \;^0_{-1}\beta}\]

Isotopes that are "above" the band of stability have a neutron to proton ratio that is too large (too many neutrons).  Therefore these isotopes are candidates for beta - decay as this converts a neutron to a proton.

Below is an example of beta + decay which is almost always called "positron emission"

\[\rm{^{11}_6C\; \rightarrow\; ^{11}_5B \;+ \;^0_{1}\beta}\]

Isotopes that are "below" the band of stability have a neutron to proton ratio that is too small (too many protons).  Therefore these isotopes are candidates for emission of a positron (beta +) as this converts a proton to a neutron. The particle that is emitted is sometimes denoted by the a lower case beta, β, and sometimes by a lower case e.

Electron Capture

Electron capture is a form of radioactive decay that is very similar to positron emission.  It occurs for isotopes that have a neutron to proton ratio that is "below" the band of stability (that is "too many protons").  Electron capture happens for isotopes for which positron emission is energetically not possible.  In this form of decay, a core electron from the atom is captured by the nucleus and a proton is subsequently converted into a neutron.

Below is an example of electron capture

\[\rm{^{57}_{27}Co\; + \;^0_{-1}e\; \rightarrow \;^{57}_{26}Fe}\]

Other

Other more unusual decay processes are the direct emission of a neutron or proton from the nucleus. Alternatively some isotopes (particularly heavy ones) undergo spontaneous fission.