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Betaelectric Generators

February 14, 2022

A heat engine generates work from a temperature difference, and discovery of this essential fact is what drove the development of the science of thermodynamics. This principle applies not just to steam engines and automobile engines, but to thermoelectric devices. Thermoelectrics have powered our planetary probes in the form of radioisotope thermoelectric generators in which the heat generated by the decay of the radionuclide, plutonium-238 (238Pu), works against the ultra cold of outer space.

NASA has developed a number of radioisotope thermoelectric generators, such as the General Purpose Heat Source Radioisotope Thermoelectric Generator used on the Cassini-Huygens spacecraft to Saturn. This generator contained 32.7 kg of plutonium-238 as the heat source and produced 300 watts peak electrical power. Since the efficiency of thermoelectric conversion is low, 4,400 watts of thermal energy were required to produce these 300 electrical watts. Current NASA missions, beginning with the Mars Curiosity Rover and then the Perseverance Rover, use the multi-mission radioisotope thermoelectric generator (see figure).

NASA's Multi-Mission Radioisotope Thermoelectric Generator

Simplified schematic diagram of NASA's multi-mission radioisotope thermoelectric generator.

The heat source, appropriately rendered in a reddish-orange color, is constructed from eight general-purpose heat source (GPHS) modules. The fins provide a thermal sink to the environment.

The GPHS modules are made from Pu-238 dioxide, but it appears that NASA would rather say GPHS, and not plutonium. However, they can't be faulted for using plutonium, since this is the only feasible energy source for planetary missions.

These eight GPHS modules generate about 2 kilowatts of thermal power initially, to produce 125 watts of electrical power at the start of a mission, falling to about 100 W after 14 years.

Spot Quiz - Based on these numbers, what's the half-life of 238Pu?

Wikimedia Commons image (simplified) by Ryan Bechtel of the United States Department of Energy.

The cost of a kilogram of plutonium, once you have established a production capability, is about ten million dollars. The Cassini-Huygens Radioisotope Thermoelectric Generator used 32.7 kg of plutonium, and the smaller Multi-Mission Radioisotope Thermoelectric Generator contains 4.8 kg of plutonium dioxide. However, if your objective is a low power source for simple sensors in inaccessible places, there's a less expensive and less dangerous radioisotope alternative; namely, a betavoltaic device. The betavoltaic concept was invented more than a half century ago, but its low efficiency and small power output relegated the concept to footnotes until the advent of low power electronics and the desire to power sensors in inaccessible places.

Unlike the alpha decay of plutonium-238, in which an alpha particle (a bare 4He nucleus) is emitted to form 234U, a beta emitter has an electron (noted as a β- particle) as a decay product. For example,
8Li -> β- + 9Be

In this case, a neutron is decomposed into a beta particle and a proton, so the atomic number of the nucleus is increased by one, from lithium to beryllium. Beta decay is interesting, since it's a natural generator of electrons; and, since betavoltaic devices function by absorbing the beta radiation, they can be designed to be intrinsically shielding.

One way to generate electricity from beta radiation is to generate electron-hole pairs at a semiconductor junction, similar to the way that a photovoltaic cells operates, but with beta electrons acting as photons would.[1] Tritium, which has a half-life of 12.32 years, is a convenient beta source for such devices, and device architectures such as photovoltaic junctions fabricated as high aspect-ratio pillars allow the tritium gas to circulate between the pillars, thereby increasing device efficiency.[1-2]

Another way to harvest energy from beta decay is the radioisotope piezoelectric generator, as shown in the figure.[3-4] Since beta particles are electrons, these electrons can be used to induce a charge in a cantilever beam. Since charge is conserved, the beta radiator develops an equal and opposite charge, so the cantilever beam is attracted to it. When the beam touches the radiation source, the charges are neutralized, and the cantilever beam springs back to its original state, ready for another cycle. The energy derived from the rapid release from bending is converted to a voltage by a piezoelectric material. Such devices have been produced with 7% efficiency with a cycle frequency of a few hertz. 63Ni can be used as the beta source to give a half-life of more than 100 years.

Cantilever beta energy harvester

A cantilever beam beta decay energy harvester. Beta particles are easier to shield than other forms of radiation, and their energy is not large enough to penetrate skin.[3-4]

The prototype device, as described in a recently expired patent, has a copper cantilever, 1 millimeter wide, 2 centimeters long and 60 micrometers thick positioned above a thin film of radioactive nickel-63.[4]

(Created using Inkscape.)

Scientists at the US Army Research Laboratory (Adelphi, Maryland), Oak Ridge Associated Universities (Oak Ridge, Tennessee), and Oak Ridge National Laboratory (Oak Ridge, Tennessee) have been developing a novel method of harvesting beta decay energy.[5-6] Nickel chloride, created using the 63Ni isotope, was combined with the common green phosphor, ZnS doped with copper and aluminum, and the optical energy produced by the beta excitation of the phosphor was converted to electricity by an indium gallium phosphide (InGaP) photovoltaic cell.[5] The photovoltaic cell was optimized for the spectrum of the phosphor and the small radioluminescence obtained.[5]

Beta photovoltaic cell

Extracting beta decay energy by exciting a phosphor, then using a photovoltaic cell to convert the radioluminescence to a voltage. (Created using Inkscape from schematic diagram in ref, 6.[6])

Such betavoltaic devices can store a thousand times the energy of a chemical battery, so they can power small sensors for many years.[6-7] The cells used 15 mCi of 63Ni and achieved an efficiency of up to 0.289%.[5] A similar device using 147Pm instead of 63Ni produced a betavoltaic device that could provide milliwatts of power continuously for at least 3 years in a 1000 cm3 total volume.[7]


  1. Mark Paulson, "Nano-Nuclear Batteries," Student Report from the University of Wisconsin-Madison, 2005.
  2. M.V.S. Chandrashekhar, Christopher Ian Thomas and Michael G. Spencer, "Betavoltaic cell." US Patent no. 7,939,986, May 10, 2011.
  3. Bill Steele, "Tiny atomic battery developed at Cornell could run for decades unattended, powering sensors or machines," Cornell University Press Release, October 16, 2002.
  4. Amit Lal, Hui Li, James P. Blanchard, and Douglass L. Henderson, "Direct charge radioisotope activation and power generation," US Patent no. 6.479.920, November 12, 2002.
  5. Johnny Russo, Marc Litz, William Ray II, Brenda Smith, and Richard Moyers, "A radioluminescent nuclear battery using volumetric configuration: 63Ni solution/ZnS:Cu,Al/InGaP," Applied Radiation and Isotopes, vol. 130 (December, 2017, pp. 66-74, https://doi.org/10.1016/j.apradiso.2017.09.018.
  6. Neil Taylor and Marc Litz, "New Approach to Radioisotope Power Sources for Improved Efficiency and Long Life," Isotope R&D and Production, December 13, 2021.
  7. M. Litz, J. Russo, and B. Smith, "Demonstration of a Radioisotope Power Source Using Promethium-147 Chloride and 4H-SiC Betavoltaic Cell, ARL-TR-9197," Technical paper presented at the IDETC/CIE 2021 Virtual Conference (2021).

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