Betavoltaic devices, also known as betavoltaic cells, are generators of electric current, in effect a form of battery, which use energy from a radioactive source emitting beta particles (electrons). A common source used is the hydrogen isotope, tritium. Unlike most nuclear power sources, which use nuclear radiation to generate heat, which then is used to generate electricity (thermoelectric and thermionic sources), betavoltaics use a non-thermal conversion process; converting the electron-hole pairs produced by the ionization trail of beta particles traversing a semiconductor.
Contents
Betavoltaic power sources (and the related technology of alphavoltaic power sources) are particularly well-suited to low-power electrical applications where long life of the energy source is needed, such as implantable medical devices or military and space applications.
History
Betavoltaics were invented over 60 years ago. Early pacemakers used betavoltaics based on promethium, but were phased out as cheaper lithium batteries were developed.
Early semiconducting materials weren't efficient at converting electrons from beta decay into usable current, so higher energy, more expensive—and potentially hazardous—isotopes were used. The more efficient semiconducting materials used today can be paired with relatively benign isotopes such as tritium, which produce less radiation.
The Betacel was considered the first successfully commercialized betavoltaic battery. The use of diamond-encapsulated carbon-14 to be extracted from nuclear waste has recently been proposed as a very long lived betavoltaic source.
Applications
The primary use for betavoltaics is for remote and long-term use, such as spacecraft requiring electrical power for a decade or two. Recent progress has prompted some to suggest using betavoltaics to trickle-charge conventional batteries in consumer devices, such as cell phones and laptop computers. As early as 1973, betavoltaics were suggested for use in long-term medical devices such as pacemakers.
Although betavoltaics use a radioactive material as a power source, the beta particles used are low energy and easily stopped by shielding, as compared to the gamma rays generated by more dangerous radioactive materials. With proper device construction (that is, proper containment), a betavoltaic device would not emit dangerous radiation. Leakage of the enclosed material would engender health risks, just as leakage of the materials in other types of batteries leads to significant health and environmental concerns.
Drawbacks
As radioactive material emits, it slowly decreases in activity (refer to half-life). Thus, over time a betavoltaic device will provide less power. For practical devices, this decrease occurs over a period of many years. For tritium devices, the half-life is 12.32 years. In device design, one must account for what battery characteristics are required at end-of-life, and ensure that the beginning-of-life properties take into account the desired usable lifetime.
Liability connected with environmental laws and human exposure to tritium and its beta decay must also be taken into consideration during risk assessment and product development. Naturally, this increases both time-to-market and the already high cost associated with tritium. A 2007 report by the UK government's Health Protection Agency Advisory Group on Ionizing Radiation declared the health risks of tritium exposure to be double those previously set by the International Commission on Radiological Protection located in Sweden.