The thorium series is one of three classical radioactive series beginning with naturally occurring thorium-232. This radioactive decay chain consists of unstable heavy atomic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved. In the case of the thorium series, the stable nucleus is lead-208.
Since alpha decay represents the disintegration of a parent nucleus to a daughter through the emission of the nucleus of a helium atom (which contains four nucleons), there are only four decay series. Therefore, the mass number of the members within each series may be expressed as four times an appropriate integer (n) plus the constant for that series. As a result, the thorium series is known as the 4n series.
The activity of Natural Samples – Thorium Series
Thorium cascade significantly influences radioactivity (disintegrations per second) of natural samples and natural materials. All the descendants are present, at least transiently, in any natural thorium-containing sample, whether metal, compound or mineral. For example, pure thorium-232 is weakly radioactive (proportional to its long half-life). Still, a thorium ore is about 10 times more radioactive than the pure thorium-232 metal because of its daughter isotopes (e.g., radon, radium, etc.) it contains. Not only are unstable radium isotopes significant radioactivity emitters, but as the next stage in the decay chain, they also generate radon, a heavy, inert, naturally occurring radioactive gas. Moreover, the decay heat of thorium and its decay products (e.g., radon, radium, etc.) contributes to heating the Earth’s core. Together with uranium and potassium-40 in the Earth’s mantle, these elements are the main source of heat that keeps the Earth’s core liquid.
Types of Decay in Thorium Series
Within each radioactive series, there are two main modes of radioactive decay:
- Alpha decay. Alpha decay represents the disintegration of a parent nucleus to a daughter through the emission of the nucleus of a helium atom. Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it heavy ionizes material and has a very short range.
- Beta-decay. Beta-decay or β decay represents the disintegration of a parent nucleus to a daughter through the emission of the beta particle. Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles have a greater range of penetration than alpha particles but still much less than gamma rays. The beta particles emitted are a form of ionizing radiation, also known as beta rays. The production of beta particles is termed beta decay.