7. The Radioactive Series

A radioactive atom is unstable and will eventually eject a particle and/or a photon to attain a more stable state. Certain atoms are still unstable even if radiation has been emitted. Uranium is a typical example shown in Figure 3.2.

Type of radiation	Isotope	Half-life
α	○ Uranium-238	4.47	billion years
β	○ Thorium-234	24.1	days
β	○Protactinium-234	1.17	minutes
α	○Uranium-234	245,000	years
α	○Thorium-230	77,000	years
α	○Radium-226	1600	years
α	● Radon-222	3.82	days
α	● Polonium-218	3.05	minutes
β	● Lead-214	26.8	minutes
β	● Bismuth-214	19.8	minutes
α	○ Poionium-214	0.164	milliseconds
β	○ Lead-210	22.3		years
β	○ Bismuth-210	5.01		days
α	○Polonium-210	138.4		days
	○ Lead-206	Stable

FIG 3-2.

Uranium-radium-series. The start of the series is U-238 and the end point is Pb-206. The first isotope has the longest half-life, 4.47 billion years. Radon and the radon decay are encircled (notice the short half-lives).

The chemical symbol for uranium is U. The start of this decay series is the isotope U-238 or ²³⁸U. When this isotope emits α-particle, it is changed into thorium-234.

Th-234 is also unstable and it emits β-particle, forming a new decay product, protactinium-234. The new product is still not stable and the decay processes continue step by step until Pb-206 is reached. Altogether, 14 disintegrations take place before U-238 ends up as a stable lead isotope (the whole series is shown in Figure 3.2). A series of unstable atoms where one atom changes into another is called a radioactive family or simply a radioactive series. Altogether, there are 4 naturally occurring radioactive families on Earth. Two of these have almost disappeared and only the uranium-radium series and thorium series are still active.

A radioactive source consists of a large number of unstable atoms. For example, one gram of the iodine isotope I-131 consists of 4.6 • 10²¹ atoms. All these atoms will sooner or later emit radiation, but these emissions do not take place simultaneously. It is a statistical process, with one atom decaying every now and then. When one half of the atoms have decayed the source has gone through what is called one "half-life". Not all atoms have decayed after two half-lives, ¼ of the unstable atoms remain.

This uranium-radium series has been present from the beginning of the Earth. The first step in this series has a very long half-life of almost 5 billion years, the present age of the Earth. Thus, we are only now into the second half-life of the uranium-radium series. The two radioactive series that have almost disappeared have done so because the half-lives are much shorter.