Promethium is a radioactive rare earth element used primarily in nuclear batteries, thickness-measuring instruments, and luminous paints. It is the only lanthanide element with no stable isotopes, meaning every form of it is radioactive and must be produced artificially. The most useful form, promethium-147, emits low-energy beta radiation with a half-life of about 2.6 years, making it practical for applications that need a steady, gentle source of radiation over several years.
Nuclear Batteries
The most notable use of promethium-147 is in betavoltaic batteries, sometimes called atomic batteries. Small samples of promethium are embedded in a semiconductor material, and the beta particles (electrons) released during radioactive decay knock electrons loose in the semiconductor, generating a small but steady electric current. These batteries deliver low power compared to lithium-ion cells or thermoelectric nuclear batteries, but they can run continuously for years without recharging or replacement.
Promethium microbatteries produce an average power density of about 5 milliwatts per cubic centimeter and can last up to five years. That makes them candidates for devices like implantable pacemakers, where reliability matters far more than raw power. Current betavoltaic batteries generally max out at about ten years of service, though newer designs aim to extend that to fifteen years. The tradeoff is simple: you get far less energy than a conventional battery, but you get it for a very long time in a very small package.
Industrial Thickness Gauges
Promethium-147’s low-energy beta radiation is well suited for measuring the thickness of thin materials. In a typical setup, a promethium source directs beta particles at a sheet of plastic, metal foil, or coating. A detector on the other side (or the same side, in backscatter designs) measures how much radiation passes through or bounces back. Thicker material absorbs more radiation, so the reading translates directly into a precise thickness measurement.
One specific industrial application involves measuring the graphite coating thickness on the inner surface of zircaloy cladding tubes used in nuclear fuel assemblies. Oak Ridge National Laboratory, currently the only domestic producer of promethium-147, reports that one of its three current buyers uses the isotope specifically for measuring the thickness of thin plastic films. Because promethium’s beta particles carry only about 224 keV of energy, they are easily absorbed by even modest material thicknesses, which makes them ideal for gauging very thin layers that higher-energy sources would pass straight through.
Luminous Paints and Dials
Promethium-147 has been used in self-luminous paint for watch dials, clocks, and safety devices. The beta radiation from the promethium excites a phosphor coating, causing it to glow without any external power source. Compared to tritium, another common choice for luminous paint, promethium produces a brighter glow. Its 2.6-year half-life is long enough for items like watches to maintain useful brightness for several years before fading.
Regulatory history around this use is complicated. In the 1960s, the U.S. Atomic Energy Commission considered exempting promethium-activated timepieces from licensing requirements, as it had done for tritium watches. The commission estimated that someone wearing a promethium watch continuously might receive up to 800 millirem of radiation per year to a small area of the wrist, though the dose to the rest of the body would be far lower (around 2 millirem per year to reproductive organs). Ultimately, regulations allowed up to 100 microcuries of promethium-147 per watch and 200 microcuries per other timepieces, with strict limits on external radiation levels. Aircraft safety devices were permitted to contain up to 100 millicuries, reflecting their critical role in emergencies and the limited time people spend near them.
X-Ray Sources and Measurement Instruments
Promethium can serve as a portable source of X-rays and general radioactivity for measurement instruments. When its beta particles strike a metal target, they produce low-energy X-rays, a process that could power compact, portable X-ray devices. This application has not been widely developed commercially, but the principle is straightforward and the low energy of promethium’s emissions makes shielding relatively simple compared to stronger radioactive sources.
How Promethium Is Produced
Promethium does not exist naturally in any useful quantity. It is harvested as a byproduct of nuclear reactions. At Oak Ridge National Laboratory, promethium-147 is extracted from the waste stream generated when plutonium-238 (the fuel used to power deep space probes) is separated from irradiated neptunium-237 targets. The waste contains promethium mixed with other rare earth elements and radioactive contaminants.
Purifying it is a multi-step process. The initial separation takes place in heavily shielded hot cells because the liquid waste is highly radioactive. A key step involves removing curium, another radioactive element, from the mix. ORNL recently switched from a 1960s-era extraction method called TALSPEAK to a newer approach using a chemical compound called camphor-BTP. The older method pulled promethium out while leaving curium behind. The newer method does the opposite, removing curium from the mixture first, which allows a more efficient series of follow-up purifications. The final steps take place in a glove box, where promethium-147 reaches the purity levels needed for research and commercial use.
ORNL is currently the only U.S. producer of promethium-147, and supply is limited. As of recent reporting, only three buyers purchase the isotope: two for research purposes and one for industrial thickness measurement.
A 2024 Breakthrough in Understanding Promethium
Despite decades of use, promethium’s basic chemical properties remained poorly understood because it is so difficult to produce in pure form. In 2024, an ORNL-led team published a landmark study in Nature after successfully creating a promethium chemical complex and characterizing it in solution for the first time. They bound promethium atoms with specially designed organic molecules and used X-ray spectroscopy to measure the length of promethium’s chemical bond with neighboring atoms, a value that had never been directly determined.
The findings filled in the last missing piece of a well-known trend called lanthanide contraction, in which atoms across the rare earth series gradually shrink. The ORNL team showed that the contraction of the chemical bond accelerates through the first part of the lanthanide series but slows considerably after promethium. This shift changes how chemists understand bonding behavior across the entire group of 15 lanthanide elements and could influence how rare earths are separated and used in technology, from magnets to catalysts.