An alpha particle is simply the nucleus of a helium atom emitted in a nuclear decay by a heavier radioactive nucleus. This type of decay by heavy elements naturally occurring deep underground is the source of all helium available on Earth, which becomes trapped in natural gas deposits. Large amounts of helium exist in the universe, it is the second most abundant element after hydrogen, but the Earth's gravity is insufficient to hold large amounts of it in the atmosphere. Coincidentally, the primary supply of helium on Earth is Amarillo, Texas (not far north of the HT3R project), where the concentrations of helium in the natural gas deposits are high enough to make it economical to extract from the natural gas itself.
As seen in this image, alpha decay is the process by which a heavy, unstable nucleus shifts to a lower-energy state by emitting an alpha particle. Shown below is a representation of the man-made super heavy element Seaborgium decaying via emission of an alpha particle into Rutherfordium. Shown in red and green are the number of protons and neutrons present in the nuclei, respectively. The white number represents the total.
Alpha particles typically have high energies, around 5 million electron-Volts! In contrast, a typical chemical reaction releases only a few electron-Volts of energy. Yet these high-energy particles are easily stopped in materials as thin as human skin or a sheet of paper. The particles lose energy and slow down by colliding with electrons in the atoms of the materials they pass through, and the enormous number of collisions allows them to deposit all their energy in a very short distance. Alpha decay, combined with beta decay, allow unstable heavy elements (like uranium) to eventually decay into stable elements such as lead or thallium, as shown here:
At higher energies the particles appear to "shrink" due to quantum effects, lowering the amount of energy they release and increasing the amount of material they can pass through and cause little ionization. As they do slowly lose energy, however, the ionization rate of an alpha particle dramatically increases, depositing most of its energy at the very end of its passage through a material, as if it were crashing to a halt. These energies are not available in nuclear decay, however, and require special particle accelerators. Capitalizing on their stopping characteristics, scientists have devised means of treating cancer tumors by targeting and destroying the tumors with high-energy beams of alpha particles while causing little radiation damage to the tissues they pass through. Elements which undergo alpha decay are also used to reduce static charge, increase the efficiency of lightning rods, and power (through the heat they generate as they release energy) deep space probes.