What Is Polonium?

Polonium is a radioactive chemical element with atomic number 84 and symbol Po, classified among the chalcogens in group 16 of the periodic table. It was discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie while analyzing the radioactive ore pitchblende, and was named in honor of Marie Curie's homeland of Poland. Polonium has no stable isotopes; all 42 known isotopes are radioactive, spanning mass numbers from 186 to 227. Its chemistry and nuclear properties place it at the intersection of radiochemistry, nuclear physics, and radiation safety engineering.

The element behaves chemically as a metalloid and is present in the environment at trace levels as a decay product in the natural uranium and thorium decay chains. Polonium-210 (Po-210), the most studied isotope, occurs in tobacco leaves through uptake from phosphate fertilizers and contributes measurably to the radiation dose received by smokers.

Radioactive Properties and Isotopes

Polonium-210 is an alpha emitter with a half-life of 138.4 days, decaying directly to stable lead-206. It releases alpha particles with an energy of approximately 5.3 MeV and generates heat at a rate of 140 watts per gram, a combination that makes it a compact thermal source. The IAEA technical documentation on polonium-210 characterizes one milligram of Po-210 as emitting alpha particles at a rate comparable to five grams of radium-226, reflecting its specific activity of approximately 166 terabecquerels per gram. Alpha radiation is stopped by a sheet of paper or a few centimeters of air, so Po-210 poses no external radiation hazard through intact skin, but becomes extremely dangerous if inhaled, ingested, or introduced into the bloodstream. Polonium-209, with a half-life of 102 years, is also an alpha emitter but decays partly via electron capture; its longer half-life makes it useful as a reference material in calibration.

Production and Technical Applications

Natural polonium concentrations in the earth's crust are on the order of one part per ten trillion by mass, making extraction from ore impractical. Commercial production relies on neutron irradiation of bismuth-209 targets in nuclear reactors; the reaction produces bismuth-210, which beta-decays with a half-life of 5.01 days to yield polonium-210. The product is dissolved and purified by chemical separation, typically yielding milligram quantities per production cycle. Applications documented in nuclear engineering include its historical use as an initiator in implosion-type nuclear weapons, where beryllium targets bombarded by Po-210 alpha particles generated burst neutron pulses to trigger fission at precisely the right moment. In civilian uses, Po-210 appears in antistatic elimination devices, where its ionizing radiation neutralizes electrostatic charges on paper, photographic film, and plastics during manufacturing. It has also served as a heat source in thermoelectric generators for early space probes, though it has largely been supplanted for this role by plutonium-238 in modern radioisotope thermoelectric generators.

Radiation Hazards and Safety

Polonium-210 is among the most radiotoxic substances known. The IAEA estimates the committed effective dose per unit intake at approximately 2.4 microsieverts per becquerel for ingestion, placing the lethal oral dose for an adult at roughly one microgram. This extreme toxicity reflects the high linear energy transfer of alpha particles within tissue and the long biological half-life of polonium in the body, estimated at 50 days. Regulatory control under radiation protection frameworks classifies polonium as a controlled substance requiring licensed handling.

Applications

Polonium has applications in a range of fields, including:

• Antistatic devices in printing, film processing, and electronics manufacturing
• Neutron source fabrication for nuclear instrumentation
• Radiochemical research as an alpha particle reference source
• Historical nuclear weapons initiator design
• Space probe thermal power research