The question of which element holds the title of “most radioactive” is complex, depending entirely on the specific definition used to measure this property. The most powerful and readily discussed substance is defined by the sheer intensity of its decay, causing a massive output of energy from a minuscule amount of mass. The answer lies in an isotope that is both naturally occurring and can be produced synthetically in measurable quantities, making its intense radiation a tangible force for study.
Understanding Radioactivity Metrics
Radioactivity describes the process where an unstable atomic nucleus releases energy and matter, known as radiation, to achieve a more stable state. To accurately compare the intensity of different radioactive materials, scientists rely on two primary metrics: half-life and specific activity.
The half-life of a radioactive isotope is the time required for half of the atoms in any given sample to undergo radioactive decay. Half-life is a measure of stability; a shorter half-life indicates a less stable, more rapidly decaying isotope.
Specific activity defines the intensity of radiation, measuring the amount of decay occurring per unit mass of the substance. It is commonly expressed in units like Curies (Ci) or Becquerels (Bq) per gram. A fundamental relationship exists between these two concepts: a shorter half-life means a larger proportion of atoms decay every second, which translates directly into a higher specific activity.
Defining the Most Radioactive Element
When “most radioactive” is interpreted as having the highest specific activity among isotopes that can be produced in measurable amounts, the answer is Polonium-210 (\(\text{Po}\)-210). This isotope is a potent alpha-emitter, ejecting highly energetic particles during its decay. Its half-life of approximately 138 days is short enough to generate tremendous energy output from a small mass.
Polonium-210 exhibits an incredibly high specific activity, roughly 166 terabecquerels per gram (\(\text{TBq/g}\)). This intensity means that a single milligram of \(\text{Po}\)-210 emits as many alpha particles per second as five grams of Radium-226 (\(\text{Ra}\)-226). Other contenders, such as artificially created super-heavy elements, may have shorter half-lives, but they decay so instantaneously that they cannot be collected or measured in bulk, disqualifying them from practical consideration. Francium-223, a naturally occurring element, has a half-life of only 22 minutes.
The Unique Characteristics and Source of Polonium-210
Polonium is element number 84 on the periodic table. All of its isotopes are radioactive, and \(\text{Po}\)-210 is the most abundant naturally occurring one, with a half-life of 138.376 days. It is a product of the natural uranium-238 (\(\text{U}\)-238) decay chain, appearing just before stable lead-206 (\(\text{Pb}\)-206).
While it occurs naturally in trace amounts in uranium ore, isolation is extremely difficult as natural sources contain less than 0.1 milligrams per ton. Today, \(\text{Po}\)-210 is predominantly produced synthetically by bombarding the stable isotope Bismuth-209 (\(\text{Bi}\)-209) with neutrons in a nuclear reactor. This process creates Bismuth-210, which then rapidly decays into \(\text{Po}\)-210. The intense decay rate generates approximately 140 watts of thermal power per gram, a property exploited for its use as a lightweight heat source in radioisotope thermoelectric generators for space applications.
Health Risks of Internal Alpha Emitters
The primary danger of Polonium-210 stems from its near-pure emission of alpha particles. An alpha particle is essentially the nucleus of a helium atom. These particles are highly energetic but have a very short range, meaning they can be stopped easily by a sheet of paper or the outer layer of human skin.
This low penetrating power means that \(\text{Po}\)-210 poses virtually no external radiation hazard. The danger becomes catastrophic only if the element is internalized, such as through ingestion, inhalation, or an open wound.
Once inside the body, the alpha particles release all their energy within a very small localized area, causing extreme cellular damage, fragmenting DNA, and inducing cell death. Because \(\text{Po}\)-210 is soluble, it can spread through the bloodstream and concentrate in soft tissues like the spleen, liver, and kidneys, delivering a massive, localized radiation dose.