Betavoltaics

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Betavoltaics are generators of electrical 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 generates electricity (thermoelectric and thermionic sources), betavoltaics use a non-thermal conversion process.

The functioning of a betavoltaic device is somewhat similar to a solar cell, which converts photons (light) into electric current. In a betavoltaic, when an electron strikes a particular interface between two layers of material (a p-n junction), a current is generated.

Betavoltaics were invented over 50 years ago. In 2005 a new betavoltaic device using porous silicon diodes was proposed to increase their efficiency. This increase in efficiency is largely due to the larger surface area of the capture material. The porous silicon allows the tritium gas to penetrate into many pits and pores, greatly increasing the effective surface area of the device.

The primary use for betavoltaics is for remote and long-term use, such as spacecraft requiring electrical power for a decade or two. The recent progress in technology 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, it is important to note that beta particles are low energy and easily stopped by shielding, as compared to the gamma rays generated by more dangerous radioactive materials. With proper device construction (i.e.: shielding), a betavoltaic device would not emit any dangerous radiation. Leakage of the enclosed material would of course engender health risks, just as leakage of the materials in other types of batteries lead to significant health and environmental concerns.

Betavoltaic devices suffer internal damage to their components as a result of the energetic electrons. Additionally, as the radioactive material emits, it slowly decreases in activity (refer to half-life). Thus, over time a betavoltaic device will output less and less power. 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 insure that the beginning-of-life properties take into account the desired useable lifetime.