Radioactivity describes the spontaneous emission of particles or energy from unstable atomic nuclei. Understanding what makes certain substances intensely radioactive involves delving into their atomic structure and decay mechanisms.
Identifying the Substance
Polonium-210 (Po-210) is among the most intensely radioactive substances known. Marie and Pierre Curie first identified it in 1898. This element stood out due to its exceptionally high specific activity, a measure of radioactivity per unit mass.
Polonium-210 has a specific activity of approximately 166 terabecquerels per gram (TBq/g). To put this into perspective, a milligram of Po-210 emits as many alpha particles as 5 grams of radium-226, making it thousands of times more radioactive by mass.
The Science of Extreme Radioactivity
Extreme radioactivity is fundamentally linked to a substance’s nuclear properties. A primary factor is a short half-life, which is the time it takes for half of the radioactive atoms in a sample to decay. Substances with short half-lives undergo rapid disintegration, releasing a large amount of radiation over a brief period.
Polonium-210 exhibits a relatively short half-life of about 138 days. The type of radiation emitted also plays a role in perceived radioactivity; Po-210 primarily undergoes alpha decay. Alpha decay involves the emission of an alpha particle, which consists of two protons and two neutrons, effectively a helium nucleus.
Alpha particles, despite their low penetrating power, are highly energetic and can cause significant damage if the substance is internalized. The energy released by its decay is substantial; one gram of Po-210 generates approximately 140 watts of heat.
Unique Characteristics and Decay
Polonium-210 is a low-melting, silvery metal that is quite volatile. Its intense radioactivity causes self-heating, with a half-gram sample capable of reaching over 500°C. The primary decay process for Po-210 is alpha decay, where it transforms into a stable isotope of lead, specifically lead-206 (Pb-206).
This decay involves the emission of an alpha particle, which is essentially a helium-4 nucleus. While alpha decay is the predominant mode, about one in 100,000 decays also results in the emission of a low-energy gamma ray. Polonium-210 occurs naturally in minute quantities as part of the uranium-238 decay chain, forming from the beta decay of bismuth-210.
The trace amounts found in nature, such as in uranium ores, are not sufficient for practical applications. Most Po-210 used today is produced artificially by bombarding bismuth-209 with neutrons in a nuclear reactor. This process converts bismuth-209 into bismuth-210, which then undergoes beta decay to form polonium-210.
Real-World Presence and Safety
Due to its extreme radioactivity, polonium-210 has very limited real-world presence and applications. Annually, only about 100 grams are produced worldwide for various purposes. Its high alpha particle emission and heat generation make it useful as a lightweight heat source for thermoelectric cells in space applications, such as powering components in the Lunokhod rovers.
Polonium-210 is also utilized in antistatic devices, where the alpha particles ionize the surrounding air to neutralize static electricity. These devices are found in industrial settings, like paper and plastic manufacturing, and in brushes for cleaning photographic films. Handling Po-210 requires stringent precautions, even for minute quantities.
External exposure to Po-210 is generally not a concern because alpha particles have low penetrating power and can be stopped by a few centimeters of air or the outer layer of skin. However, it poses a significant health hazard if ingested, inhaled, or introduced through an open wound. Once inside the body, the alpha particles can directly damage tissues and DNA.