Space Technology Special Report - Version B. July 2023 - 28

TECH BRIEFS
I
nnovators at the NASA Langley
Research Center have developed
the Multi-Layer Nuclear Thermionic
Avalanche Cell (NTAC), a novel
electrical generator, which transforms
nuclear gamma-ray photon energy
directly to electric power by liberating
intra-band atomic inner shell electrons.
The invention consists of several
NTAC layers arranged in a radially
concentric series separated by a
vacuum gap space. It is comprised of a
gamma-ray source and various layers
of emitters, collectors, and insulators.
Ideal emitter materials include elements
with high atomic numbers, while ideal
collector and insulator materials include
Multi-Layer Nuclear Thermionic Avalanche Cell
The device provides a compact, reliable, and continuous electrical source with high power
density capable of long-life operation without refueling.
Langley Research Center, Hampton, VA
elements with low atomic numbers.
A high-energy gamma-ray (tens
of keV to MeV) is used to liberate a
large number of intra-band, innershell
electrons from atoms within the
emitter material for power generation
through the primary interactions of
photoelectric, Compton scattering,
photonuclear, and electron/positron
pair production processes. Secondary
and tertiary electrons are liberated
in the avalanche process as well.
If a power conversion process
effectively utilizes all liberated electrons
in an avalanche mode through a power
conversion circuit, the power output is
drastically increased. Because power
Gamma-Ray Liberated
Electron Populated
(Photoexcited Energetic Electrons)
Thermalized Electrons
Thermoelectric
Generator
TE
Power
Avalanche Cell
EF
Radioactive Element
EF,n= EF + ∑ EI +κ Tc∑ In(nI/neq)
Where EF + A = EF + χ
I
EI +Φ = EF + χ
EM+ M = EF + χ
Φ
Φ
I
I
El
(Intra-band)
(Intra-band)
Thermal Emission
Gamma Ray Emission
EM
α- Particle Emission
β- Particle Emission
Energy diagram of photoexcitation and thermalization processes. Photoexcitation and thermalization processes initiated by gamma-ray and beta
particles from radioactive materials increase the conduction band population, creating a large thermionic current. The thermal energy generated by
radioactive coupling and decaying processes is converted by the TE device. (Image: NASA)
28 JULY 2023
SPACE TECHNOLOGY SPECIAL REPORT
EgM
Intra-band
Egl
χ
ΦA
ΦI
ΦM
Avalanche
Cell Power
Vout
Collector
Φa
conversion is determined by the
absorption rate of high energy photons,
increasing power output requires either
thicker collector material or a sufficient
number of layer structures to capture
the high energy photons, leaving
no liberated electrons escaping (i.e.,
minimizing the leak of radioactive rays).
The selection of materials, the
thicknesses of the emitter, collector,
and insulator, as well as the number of
NTAC layers required are all determined
by the energy of photon source. The
thermal energy from radioactive decay
can also be converted to electricity
using a thermoelectric device to
further increase power output.
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