Promethium

Promethium (symbol Pm, atomic number 61) is a rare, radioactive metal belonging to the lanthanide series of the periodic table. It is the only lanthanide that is entirely radioactive, with no stable isotopes. Promethium is silvery in appearance, similar to other rare earth elements, and emits beta radiation as it decays. Because of its scarcity, radioactivity, and short half-life, it has very limited practical uses and is primarily employed in scientific research and a few niche industrial applications. Despite its obscurity, promethium represents an important link in understanding nuclear stability, energy generation, and materials science.

Discovery and Occurrence

Promethium was first conclusively identified in 1945 by scientists Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell at the Oak Ridge National Laboratory in the United States. It was discovered as a fission product of uranium fuel from a nuclear reactor.
Natural promethium is exceedingly rare; trace amounts exist in uranium ores as a result of spontaneous fission, but the total natural abundance in the Earth’s crust is estimated to be less than a gram. Therefore, all usable promethium is synthetically produced by neutron irradiation of neodymium or uranium in nuclear reactors.
The most stable isotope, Promethium-145, has a half-life of about 17.7 years, while Promethium-147 (half-life 2.6 years) is the most commonly used isotope for practical applications.

Physical and Chemical Characteristics

Promethium is a soft, metallic, and silvery element that glows faintly in the dark due to beta radiation exciting nearby air molecules. It behaves chemically like its neighbouring lanthanides—neodymium and samarium—forming trivalent ions (Pm³⁺) in compounds.
Its melting point is about 1042 °C, and its estimated boiling point is around 3000 °C. As a radioactive element, promethium must be handled with extreme care to avoid radiation exposure and contamination.

Everyday Applications

Promethium has almost no direct everyday use, owing to its radioactivity and scarcity. However, it appears indirectly in technologies and devices that people use or encounter through its specialised applications:

• Luminous paint and dials: Promethium-147 has been used in self-luminous paint for watch hands, aircraft instruments, and control panels. Unlike older radium-based paints, promethium emits only beta particles, which are less penetrating and therefore safer when properly contained.
• Battery components: In small quantities, promethium can be used in nuclear batteries (betavoltaic cells) to generate electricity for devices where long-term, maintenance-free power is needed.
• Portable lighting: Early experimental devices explored using promethium for portable, long-lived light sources, although safer isotopes and LEDs have since replaced such applications.

Although largely historical, these uses illustrate promethium’s once-promising role in radiation-based illumination and compact energy systems.

Industrial Applications

The limited industrial use of promethium stems from the controlled availability of its isotopes and the challenges of handling radioactive materials. The most notable applications include:

1. Betavoltaic Power Sources
   • Promethium-147 serves as a beta emitter in betavoltaic (nuclear) batteries.
   • These batteries convert the kinetic energy of beta particles into electricity using semiconductor materials.
   • Such systems are useful in remote sensors, space equipment, and military technology, where regular maintenance or battery replacement is impractical.
   • For example, early spacecraft experiments considered promethium batteries as lightweight, stable power sources for instruments.
2. Thickness and Material Gauges
   • Promethium-147 is used in beta-ray thickness gauges, which measure the thickness or density of materials such as metal foils, paper, plastics, and fabrics in industrial production lines.
   • The isotope’s moderate radiation intensity makes it suitable for portable or precision measurement systems.
3. Calibration of Instruments
   • Promethium’s well-defined beta emission energy allows it to be used as a calibration source for detectors and radiation-measuring instruments.
4. Scientific Research
   • In nuclear and chemical laboratories, promethium is used to study radioactive decay patterns and to refine models of nuclear structure among lanthanide elements.
   • It also assists in investigating beta-voltaic efficiency and energy conversion technologies for advanced power systems.

Economic Importance

Promethium’s economic impact is minimal compared with other rare earth elements, as its applications are restricted to niche scientific and technical uses.

• Production Scale: Global production of promethium is very limited and measured in grams per year, derived primarily from spent nuclear fuel.
• Cost: Due to its rarity and radioactivity, promethium is expensive to isolate and handle safely, with production costs dominated by radiochemical separation and containment procedures.
• Strategic Significance: Despite its small-scale use, promethium is of scientific and strategic interest in nuclear research, particularly in developing small, long-lasting power sources and understanding radioactive isotopes of lanthanides.
• Economic Substitution: In most industrial contexts, promethium has been replaced by safer or more available alternatives such as tritium, strontium-90, or chemical batteries.

Thus, while promethium’s direct economic contribution is negligible, its indirect value lies in research and innovation in nuclear materials and energy technologies.

Environmental and Health Considerations

As a radioactive substance, promethium requires strict handling and disposal measures. Promethium-147, the most used isotope, emits beta radiation that can penetrate skin and cause tissue damage if not properly shielded. However, it poses little external danger when contained in sealed devices.

• Environmental impact: Promethium is not naturally concentrated in the environment, and its controlled production means environmental contamination is minimal.
• Health precautions: Workers handling promethium use protective gear and shielding, while devices containing the element are encased in corrosion-resistant materials to prevent leakage.

Because of these safety requirements, promethium applications are confined to controlled laboratory or industrial settings under regulatory supervision.