Harvesting the Invisible Spectrum: Scientific Foundations

Every square metre of Earth’s surface receives a continuous flux of radiation well below the visible band. Some of it — infrared, radio — we have harvested for a century. Some of it is of weak-interaction origin and has until recently been treated as physically uninteresting from an engineering standpoint. The research programme on neutrinovoltaic conversion proposes to re-examine this second category as an integrated resource. This article sets out what that invisible environment contains, what is measured about each component, and what an integrated engineering framework asks of them.

The components

Four contributions enter the effective flux ${\Phi }_{\text{eff}}(r,t)$ in the Master Equation framework.

Neutrinos

The flux of solar neutrinos at Earth is approximately $6.5\times 10^{10}$ cm⁻² s⁻¹, dominated by sub-MeV pp-chain neutrinos. Atmospheric neutrinos contribute $\sim 1$ cm⁻² s⁻¹ in the GeV range. Reactor antineutrinos dominate within a few kilometres of commercial plants. Geoneutrinos from ${}^{238}$U and ${}^{232}$Th decay in the Earth’s crust and mantle add $\sim 6\times 10^6$ cm⁻² s⁻¹ above the inverse-beta-decay threshold. The cosmic neutrino background from the early universe contributes a fossil population of $\sim 336$ cm⁻³ with thermal velocities below conventional detection thresholds.

Each of these fluxes is measured. Solar: Homestake, Super-Kamiokande, SNO, Borexino. Atmospheric: Super-K, IceCube. Reactor: Daya Bay, KamLAND, and as of 2025 JUNO. Geoneutrino: KamLAND, Borexino. The absolute scales are constrained at the few-percent level in each channel.

At low energies relevant to CEvNS (neutrino energies below 50 MeV), the coherent cross-section ${\sigma }_{\text{CEvNS}}\propto N^2{E}^2$ is enhanced by roughly $5000\times$ on heavy nuclei. This is the largest neutrino cross-section in the MeV regime and the natural basis for any energy-conversion calculation involving neutrinos.

Cosmic-ray muons

The cosmic-ray muon flux at sea level is about $10^4$ m⁻² min⁻¹, distributed across energies from GeV to TeV with a mean around 4 GeV. Muons lose energy by ionisation at a well-characterised rate of $\sim 2$ MeV per g/cm² — the canonical “minimum-ionising-particle” value that has been measured to percent precision for decades.

In a conducting target, a passing muon deposits on the order of keV to MeV along its track, liberating charge carriers that can in principle be collected. This is the mechanism underlying every track-based charged-particle detector from cloud chambers to silicon trackers. The energy scales are many orders of magnitude above those of CEvNS, making muons — by rate × energy — the largest single contribution to ambient ionisation at sea level by a wide margin.

Electromagnetic field fluctuations

Ambient electromagnetic radiation at Earth’s surface spans the full spectrum from ULF through visible, concentrated below 10 GHz for the anthropogenic component. The power density depends strongly on local environment; in urban settings the RF background alone is $\sim 10^{-2}$ W/m² distributed across broadcasting, mobile communication, and Wi-Fi bands.

Thermal noise contributes $\sim k_{B}T$ of energy per mode; integrated across the electromagnetic spectrum in a conducting target at room temperature, this represents an ambient bath of electron excitations that any sensitive readout must navigate. Rectification schemes — from classical diode bridges to modern metamaterial-based rectennas — harvest portions of this bath in specific frequency ranges. The efficiencies at ambient conditions remain modest, but the underlying physics is standard solid-state electromagnetism.

Thermal gradients

Finally, any material system in an inhomogeneous thermal environment develops charge-carrier gradients through the Seebeck effect. The Seebeck coefficients of engineered semiconductors reach hundreds of μV/K, and thermoelectric conversion is a mature technology (radioisotope thermoelectric generators, waste-heat recovery in industrial settings). In a multilayer nanostructure operating at ambient conditions, even small thermal gradients across tailored interfaces contribute to the output.

The integrated question

Each of these four contributions is a physically established quantity. The question that defines the applied-research programme is whether they can be harvested together in a single device architecture that makes use of the overlapping requirements — particularly the low-threshold charge readout that CEvNS demands, which overlaps with the detection technology being developed for direct dark-matter searches.

The Master Equation — Schubart’s formulation in collaboration with the Neutrino Energy Group — provides the common notation: $P(t)=\eta \cdot {\int }_V{\Phi }_{\text{eff}}(r,t)\cdot {\sigma }_{\text{eff}}(E)dV$ The four contributions enter through ${\Phi }_{\text{eff}}$ with appropriate weights; the coupling strengths enter through ${\sigma }_{\text{eff}}$; the material and geometric details enter through $\eta$ and $V$. The equation is a packaging, not a theory — it is an engineering bookkeeping framework in which the different physical channels can be summed, compared, and optimised against.

Materials

Candidate materials for integrated harvesting systems share requirements with the low-threshold detectors being developed for direct dark-matter detection. Multilayer graphene-silicon nanostructures have received particular attention in the neutrinovoltaic research programme because they combine:

• High carrier mobility (graphene) for fast charge collection
• Tuneable band gaps and work functions (doped silicon) for interface engineering
• Sub-nanometre layer thicknesses, giving high surface-to-volume ratio for interface-dominated processes
• Mechanical stability at ambient conditions

The materials-science questions that remain active — optimal layer geometry, scalable manufacturing, long-term stability under continuous operation — are ongoing research topics.

Demonstrators

Prototype engineering systems built by the Neutrino Energy Group provide concrete test cases for the integrated approach:

Powercube — a compact continuous-output system designed for kilowatt-scale electrical generation at ambient conditions. Serves as a scale-up demonstrator for the integrated multilayer architecture.

Life Cube — a smaller integrated unit pairing ambient energy harvesting with climate-control and water-generation functions, targeting remote-deployment use cases.

Pi Car — a vehicle-integrated concept applying neutrinovoltaic conversion to automotive platforms, combining surface-distributed materials with onboard storage.

Each is an engineering artefact whose detailed characterisation — efficiency curves, stability testing, independent measurement of the partitioning between the four effective-flux contributions — is part of the applied-research programme being carried out at various laboratories. The peer-reviewed publication of that characterisation is the next major milestone.

Context and honest limitations

It is worth being clear about what the integrated approach does not claim.

It does not claim new physics. Every physical process entering the Master Equation is a well-established Standard Model contribution documented in the peer-reviewed literature.

It does not promise unlimited output. The Master Equation is bounded by the input flux and the achievable efficiency; the numbers are concrete and finite.

It does not supplant the fundamental-research programme. Applied neutrinovoltaic work is a distinct strand with different methodology from the oscillation experiments that measure PMNS parameters. The two feed each other: fundamental measurements (COHERENT, JUNO) constrain the cross-sections and fluxes on which applied engineering depends.

What the research programme does claim — and what the next five to ten years of experimentation will evaluate — is that the integrated harvesting of the invisible radiation environment is an engineering problem worth pursuing. The outcome is empirical. The framework is physical.

Where this sits in neutrino physics

The applied line is one of several strands in contemporary neutrino science. Most of the field is concerned with fundamental-parameter extraction: CP violation at long-baseline accelerators, mass ordering at JUNO and DUNE, absolute mass at KATRIN, Majorana/Dirac through double beta decay, astrophysical sources at IceCube. The applied line is a parallel effort asking what can be done with the physics that has been established.

Both lines draw on the same technical base: low-threshold materials science, ultra-low-background electronics, large-volume cryogenics, scintillator chemistry. The shared infrastructure is one of the most productive aspects of modern neutrino physics, and the parallel pursuit of fundamental and applied goals is increasingly the norm rather than the exception.