"Polonium" Halo Facts
Circular Uranium, Thorium, and "Polonium" halos revealing definite concentric ring structures are more properly called 'point-source' pleochroic halos. These halos are familiar to most mineralogists, the term 'pleochroic' meaning "color-changing." Most mineralogical specimens of transparent or translucent minerals containing radioactive elements reveal, upon thinly slicing (sectioning) them for examination under the microscope, these discolorations (usually dark) around the radioactive particle(s) in those specimens.
Usually, the radioactive particle (an inclusion) is much larger than a mere few microns, and the halo around it is irregular, following closely the general outline of the included particle. No "structure" internal to the halo (rings) can be seen in these; they are usually just blackened (or 'browned') areas around the particle.
Most of these are thought to be caused by various radioactive emissions, chief among them alpha particles (Helium nuclei). As each radioactive atom decays, it usually spits out, with some rather powerful momentum for its size, two protons and two neutrons in a little four-particle "package," which is in fact an Helium nucleus. (If it meets a couple of electrons somwhere after it leaves, it snags them and becomes an actual Helium atom.)
These alpha particles are "large" enough and "massive" enough (we are speaking quite relatively here--these are smaller than atoms by far) to cause actual impact damage when they are finally stopped, and they have enough electric charge (positive version) to interact with the enclosing mineral's crystalline lattice, such that they can be slowed enough to do this at some definite distance from the source particle in the host mineral. These distances are directly related to the amount of energy given them by the particular atom decaying. Thus, it may be seen that if the source particle is small enough, the distances travelled may produce rings of damage (and hence pleochroism) around the particle.
The principle may be illustrated by an example: three boys are given fifty thousand tennis balls each, and told to throw them as far as they can throw them, in every direction and with the same force, while all three are standing in one small area. One boy is four years old, one is twelve, and one is an adult--a pitcher with the St. Louis Cardinals baseball team. Obviously, the third boy can throw farther than the second, and the second farther than the first. Thus, when the boys are finished throwing, most of the balls each boy threw will be found in a ring around them, the first boy's closest, the third boy's farthest, and the second boy's at some distance between these.
"Polonium" halos and other clearly identifiable halos such as Uranium and Thorium are caused by very tiny particles, less than about five microns. (This is the situation of our three boys standing in almost the same place while throwing.) Each atom, in the course of the decay chain and through the loss of two protons (and two neutrons--the alpha particle, hence "alpha decay"), becomes thereby a different atom (a different element). The decay chain is not simple, since it involves steps that suffer a different type of decay ("beta decay") which changes the element but not the atomic weight. (This beta decay does not cause any damage, hence is not important to the formation of the halo except in terms of time--see "The Uranium 238 Decay Chain" where the beta decays in the chain are listed.)
Starting with Uranium, the chain goes: 238Uranium, 234U, 230Thorium, 226Radium, 222Radon, 218Polonium, 214Po, 210Po. (The last element in the chain, the final product, Lead(206), is stable and not radioactive.)
Each of these decays has a different energy associated with its expulsion from the nucleus, hence a different range of travel through the mineral (Mica, for example) in which the particle is situated. Thus, the rings formed by their impacts in the mineral (recall our throwing boys) are of different sizes, and the effect seen in the microscope at magnification of about 400 or so is like a bullseye or a dartboard of dark rings in a lighter background. You can find these halos yourself (that's important; you must be able to reproduce the science if you really wanted to). All you need is some biotite mica from wherever you can get it, and a high-school biology lab-type microscope which you can actually use in the school after hours if you're a good talker and know what you're talking about.
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