Not All Radioisotopes Are Equal

Nuclear microbatteries contain only small amounts of radioactive material, but safety is nonetheless a crucial issue. It is important first to note that not all radioisotopes are alike. The level of radioactivity depends on the type and amount of the radioisotope.

Images: SRB Technologies Inc.; Hemeva Technologies/Alamy

Radioisotopes are unstable atoms that spontaneously emit high-energy particles as they decay to a more stable state. Most emit gamma rays, which are essentially high-energy X-rays that can penetrate most materials, including human flesh. But other radioisotopes emit alpha particles (an aggregate of two protons and two neutrons) and beta particles (high-energy electrons) that can't penetrate as deeply and therefore pose less risk.

The nuclear microbatteries we are developing contain 1 to 10 millicuries of nickel-63 or tritium, whose beta particles have relatively low energy and can be blocked by a layer of 25 to 100 micrometers of plastic, metal, or semiconductor; they are also blocked by the thin dead-skin layer covering our bodies.

Other than shielding considerations, safety concerns also involve the possibility of a release of the radioisotope into the environment and subsequent inhalation or ingestion. Again, by limiting the amount of radioisotope and by using the proper packaging, it is possible to ensure that such nuclear microbatteries offer minimal risk to the public.

In fact, radioisotopes have been used for decades in commercial applications. Many smoke detectors contain 1 to 5 microcuries of americium-241, used to ionize the air between a pair of parallel plates. (The detector measures the degree of ionization between the plates; when smoke enters the gap, it changes the ionization, which activates the alarm.) And some emergency exit signs in public buildings, schools, and auditoriums that have to remain visible during power outages contain 8 to 10 curies of tritium, whose emitted electrons excite phosphor atoms, illuminating the sign.

The amount of radioactive material in the nuclear batteries we are developing falls between those in a smoke detector and in an exit sign. And for whatever amount, any commercial application of such nuclear batteries would have to take into account all required safety measures, including designing safe packaging and following regulations about handling and disposing of the device and its components.

—A.L. and J.B.