Uranium 238 (atomic number 92) consists of 92 protons and 146 neutrons, giving a total number of 238 nucleons forming the nucleus of the atom. This number is known as its atomic mass. Around the nucleus circles a cloud of electrons equal in number to the quantity of protons present. It is the number of protons in the atom which determines what element it is. Whilst the number of protons and electrons remains constant for a particular element, the number of uncharged neutrons can vary, giving rise to varying mass numbers for different forms of the same element. These are referred to as isotopes. Thus U 234 is another isotope of uranium, containing 142 neutrons, 92 protons and 92 electrons.

Natural radioactivity is the process in which unstable nuclei attempt to evolve into a more stable atom by the ejection of mass, energy or both at a fixed rate for the particular isotope formed. Each time the atom loses protons, as in the release of an alpha particle, it becomes a different element. Uranium 238 will eventually become Lead 206, a stable isotope of lead. This is known as the Uranium 238 decay series.

Radioisotopes are atoms with unstable nuclei which undergo nuclear changes involving the spontaneous ejection of fast moving particles (particulate radiation), high-energy electromagnetic radiation or both. This form of radiation is called ionising radiation because it has enough energy to strip electrons off atoms it encounters to form positively charged ions. This can result in the formation of free radicals which cause minute chemical changes in living cells and can thus damage them.

The main types of radiation emitted are:

Alpha particles consisting of two protons and two neutrons. These have a net double positive charge, there being no electrons to offset the positively charged protons.

Beta particles which are negatively charged electrons.

Gamma radiation which is a high energy electromagnetic radiation.

The Half Life of a radioisotope is the time it will take for a sample of that isotope to lose half its radioactivity, or in the case of decay by alpha emission, for half the sample to change into a different element. It requires some ten half lives for the radioactivity to approach zero for a particular isotope sample.

Half lives for elements in the U238 decay series:

U238......4,510,000,000yrs .....(Uranium).... ag

Th234....24days ......................(Thorium)...bg

Pa234.....1min...................(Protactinium)....bg

U234......252,000yrs......................................ag

Th230.....8,000yrs.........................................ag

Ra226.....1,600yrs.....................(Radium)....ag

Rn222.....3.8days.........................(Radon)...ag

Po218.....3.1mins....................(Polonium)...a / b

Pb214.....27mins.............................(Lead)....bg

At218.....2secs.........................(Actinium)...a

Bi214.....20mins.......................(Bismuth)..bg / a

Po214.....0.000164secs..................................ag

Ti210......1.3mins.....................(Titanium)....bg

Pb210.....22yrs...........................................bg / a

Bi210.....5days.............................................b / a

Hg206....8mins.........................(Mercury)....bg

Po210.....138days..........................................ag

Ti206.....4mins.................................................b

Pb206.....Stable