sulting in the final, stable element lead-206. Where the arrows point downward, those changes result from alpha decay. The arrows pointing upward indicate beta decay. (Alpha decay involves the loss of two protons and two neutrons; beta decay involves splitting of a neutron, the loss of an electron, and the gain of a proton.)
The original (or parent) uranium nucleus contains 92 protons. That’s why uranium is element number 92 on the periodic chart. However, after two protons are ejected in an alpha particle, the nucleus then has only 90 protons, so the uranium has changed into element number 90, which is thorium.
The nucleus of thorium is still unstable, so it decays radioactively. However, it decays by successively splitting and ejecting two electrons, known as beta particles, while gaining two extra protons, giving it a total of 92 again. So the thorium has changed back into the element number 92, uranium.
But this uranium is now four neutrons lighter. Whereas the original parent uranium atom had 146 neutrons and 92 protons in its nucleus (called uranium-238), this uranium atom has only 142 neutrons and 92 protons (called uranium-234).
The nucleus of this uranium-234 atom is still unstable, so it decays. The process continues through what is known as the uranium decay chain, until the stable lead-206 atom is reached, the final end product.
The picture is a little more complicated at the bottom end of this decay chain. At polonium-218 there is “branching,” but most polonium-218 atoms decay to lead-214. Then they quickly decay to bismuth-214, then polonium-214. The main decay path is depicted with solid red arrows. And the main eight alpha-decaying atoms are shown in red.
Figure 1 shows a sample radiohalo that resulted from this process. Notice the eight dark rings. Why eight and not just one? As each of the alphaparticles is “fired” from the nucleus, it has a different energy. Consequently each bullet travels a different distance into the surrounding material before it stops to leave a black mark.
The origin of eight-ring radiohalos is not a mystery for geologists. They must have formed from uranium-238 decay. The mystery is the source of the one-ring, two-ring, and three-ring radiohalos that are found in the same rock specimens (Figures 2–4).
Geologists find four types of radiohalos in granite. One was caused by the decay of uranium, and the others came from the decay of another radioactive element, polonium.
The source of eight-ring radiohalos is still there, but nothing is usually visible at the center of other types of radiohalos.
We can see the source of the eight-ring radiohalos because it is still there—a zircon crystal that still hosts uranium. But nothing is usually visible at the center of the other types of radiohalos. The source has vanished!
Fortunately, it is still possible to determine the source of the rings. By carefully measuring the distance from the center of the radiohalo to each ring, we can identify which type of nucleus formed each ring (Figure 5).1 In each case, the “smoking gun” was a variation of the radioactive element polonium.2 Polonium-218, polonium-214, and polonium-210 generate exactly the right amount of energy to produce the three-ring, two-ring, and one-ring radiohalos.
The problem is that polonium is never found alone in rocks. It is a rare, unstable element that appears quickly during the decay of uranium and then decays into stable elements, such as lead. The only possible source of the polonium was the decay of uranium. But we do not find a uranium source at the center of the one-, two-, and threering radiohalos! The crucial clue is the appearance of polonium during the uranium-238 decay chain. As the uranium atom’s nucleus becomes progressively smaller, three variations of polonium appear briefly in sequence at the end of the chain.
FIGURE 5—Composite schematic drawing of the four types of radiohalos: (a) a uranium-238 radiohalo, (b) a polonium-218 radiohalo, (c) a polonium-214 radiohalo, and (d) a polonium-210 radiohalo. Different types of atoms (nuclides) cause each ring in the 