Types of Radiation

There are various ways in which nuclei can decay through emitting radiation -

Alpha Decay

Alpha decay only occurs in the heaviest nuclei with Z > 83 or A ≥ 200.

As an alpha particle is a helium nucleus 4He, the mass number (A) of the daughter nucleus is reduced by 4 and the atomic number (Z) is reduced by 2 thus

(A, Z) (A - 4, Z - 2) + α

Examples of alpha decay are

238U 234Th + α t1/2 = 4 x 109 years

216Ra 212Po + α t1/2 = 45 μs

Alpha particles are stopped by a few cms of air, a sheet of paper or human skin.

Beta Decay

Beta decay occurs for all values of mass number. It is the emission of an electron (known as β- decay) or a positron (known as β+ decay) with a resulting change in atomic number, Z, but not mass number, A. Neutrons or protons in the nucleus decay by emitting electrons via the weak interaction. This decay is always accompanied by emission of a neutrino or anti-neutrino.

β- Decay

(A, Z) (A, Z + 1) + e-

n p + β- + anti-neutrino

27Mg 27Al* + β- + anti-neutrino

β+ Decay

(A, Z) (A, Z-1) + e+

27Si 27Al* + β+ + neutrino

Beta radiation is stopped by a thin piece of wood or plastic.

Gamma Decay

Gamma radiation is high energy electromagnetic particles/waves. They are emitted by nuclei in an excited state attempting to return to their ground state. There is no accompanying change in their mass number, A, or the atomic number, Z, with a half llfe usually less than 1µs.

Gamma radiation can only be stopped by a substantial thickness of heavy material such as lead.

Other types of nuclear activity include

Neutron release

This is mostly due to nuclear fission but can also be caused by cosmic radiation. Fast neutrons are very penetrating but can be moderated/thermalised/slowed down by graphite or water.

Spontaneous fission

This can occur for any nucleus with A ≥ 230. The parent nucleus splits into two almost equal fragments. The resultant nuclei are left in an excited state which then emits γ rays to decay to a non-exited state.