From CEvNS to Energy Conversion: A Physical Continuity

In August 2017, the COHERENT collaboration at Oak Ridge National Laboratory announced the first direct observation of coherent elastic neutrino-nucleus scattering — CEvNS. The process had been predicted by Daniel Freedman in 1974, and its detection closed a 43-year gap between theory and experiment. The measurement matched Standard Model expectations within experimental uncertainties and has since been extended to multiple target nuclei, confirming the predicted $N^2$ scaling with neutron number.

For most of the physics community, CEvNS is primarily a new platform for precision Standard Model tests and for constraining non-standard neutrino interactions. But CEvNS has a second significance that is less often discussed: it is the physical process on which applied research into energy conversion from the invisible radiation spectrum fundamentally rests.

This article traces the continuity. It begins with the measured physics — what CEvNS is, what COHERENT showed, what JUNO’s 2025 data now constrains. It ends with the engineering framework, the Master Equation, that packages these processes into a form suitable for device-design analysis.

The physics: what CEvNS does

When a neutrino of energy $E_{\nu }$ scatters coherently off an atomic nucleus of mass $M$ through $Z^0$-exchange, the momentum transferred to the nucleus produces a recoil of kinetic energy up to $T_{\max }=\frac{2E_{\nu }^2}{M+2E_{\nu }}$ For a 30 MeV neutrino on a CsI nucleus ($M\approx 130$ u), this is about 14 keV. At reactor energies of a few MeV on germanium, the maximum recoil is of order 100 eV. The cross-section at low momentum transfer takes the coherent form $\sigma \propto G_F^2{Q}_W^2{E}_{\nu }^2$ where $Q_W\approx -N$ for heavy nuclei. The $N^2$ enhancement is what makes CEvNS the largest neutrino cross-section at sub-50-MeV energies.

The experimental confirmation

COHERENT’s 2017 paper reported a 6.7σ observation using a 14.6 kg CsI[Na] scintillator crystal placed in “Neutrino Alley” at the Spallation Neutron Source. The measured cross-section matched Standard Model expectations at the 10% level, with the residual uncertainty dominated by the nuclear quenching factor (the recoil-to-scintillation conversion).

Follow-up measurements with liquid argon (N=22) and germanium (N=42) targets confirmed the $N^2$ scaling across the periodic table. Reactor-based CEvNS experiments — CONUS in Germany, CONNIE in Brazil, Dresden-II in the US, RED-100 in Russia — are pursuing CEvNS at lower energies where the cleaner antineutrino spectrum enables precision Standard Model tests and beyond-SM searches.

The physical process is no longer in question. Its characterisation is now an active frontier, but its existence is solid experimental fact.

Flux: what the universe delivers

The other ingredient in any energy-conversion calculation is the incoming flux. For neutrinos, multiple contributions overlap at any terrestrial location:

Solar neutrinos — the dominant terrestrial flux, $\sim 6.5\times 10^{10}$ cm⁻² s⁻¹, peaked at sub-MeV energies from the pp fusion chain. Measured and characterised by Homestake, SAGE/GALLEX, Super-Kamiokande, SNO, and Borexino. The 2020 Borexino detection of CNO-cycle neutrinos closed the last unobserved solar branch.

Atmospheric neutrinos — cosmic-ray-induced, spanning from sub-GeV to TeV, $\sim 1$ cm⁻² s⁻¹ in the GeV range. Measured comprehensively by Super-K and IceCube.

Reactor antineutrinos — very large fluxes near commercial reactors, falling as $1/r^2$. Characterised by Daya Bay, KamLAND, and now JUNO’s 2025 first data.

Geoneutrinos — from ${}^{238}$U and ${}^{232}$Th decay chains in Earth’s crust and mantle, $\sim 6\times 10^6$ cm⁻² s⁻¹ above the IBD threshold. Characterised by KamLAND and Borexino.

Supernova neutrinos — extremely intense but transient; a galactic core collapse would deliver $\sim 10^{11}$ cm⁻² s⁻¹ for a few seconds.

The cosmic neutrino background — the relic flux from the Big Bang, $\sim 336$ cm⁻³ with thermal velocities much below detection threshold for any conventional process.

For any given location and time, the effective flux ${\Phi }_{\text{eff}}$ at energies relevant to CEvNS is a weighted sum over these contributions. The dominant terrestrial component is solar.

The engineering question

Take these two established quantities — the CEvNS cross-section ${\sigma }_{\text{CEvNS}}(E)$ and the effective flux ${\Phi }_{\text{eff}}(E)$ — and ask: how much power could a practical device extract from the combination?

The rate of nuclear-recoil events per unit target mass is $R=\int {\Phi }_{\text{eff}}(E)\cdot {\sigma }_{\text{CEvNS}}(E)\cdot n_{T}dE$ where $n_T$ is the number density of target atoms. For solar neutrinos on, say, a few cubic metres of heavy-nucleus-loaded target, this gives of order $10^{-3}$ to $10^{-2}$ events per second — far too small to be useful by itself.

The applied-research question is whether the effective rate can be enhanced, and whether other components of the invisible radiation spectrum — cosmic-ray muons, ambient electromagnetic fields, thermal gradients — can contribute alongside the neutrino term in an integrated device. Each of these contributions is a well-characterised physical process in its own right; the engineering question is whether they can be harvested together in a single conversion system.

The Master Equation

The formalism developed by Holger Thorsten Schubart in collaboration with the Neutrino Energy Group packages this integrated approach. The expression reads $P(t)=\eta \cdot {\int }_V{\Phi }_{\text{eff}}(r,t)\cdot {\sigma }_{\text{eff}}(E)dV$ where $P(t)$ is the device output power; $\eta$ is the conversion efficiency of the material system; ${\Phi }_{\text{eff}}(r,t)$ is the effective flux at position $r$ and time $t$ — a weighted sum of neutrino, muon, electromagnetic, and thermal contributions; ${\sigma }_{\text{eff}}(E)$ is an energy-dependent effective cross-section; and $V$ is the active material volume.

The equation is not a new physical proposition. It is an engineering packaging of processes each separately established in the peer-reviewed literature. For the neutrino channel, ${\sigma }_{\text{eff}}$ is fundamentally the CEvNS cross-section observed at COHERENT and being refined by the reactor-CEvNS programme. For the muon channel, it reduces to the ionisation energy loss $dE/dx$ that has been measured to percent precision for decades. For the electromagnetic and thermal channels, it captures well-understood solid-state couplings.

What the Master Equation provides is a common notation in which all of these processes can be added, optimised against, and compared. It is an engineering tool, not a physics claim.

Where the applied research stands

Candidate materials for CEvNS-era applied research include multilayer graphene-silicon nanostructures — the same family of materials being developed for direct dark-matter detection and for cryogenic bolometry. The requirements overlap substantially: both need low-threshold charge or phonon readout of small-momentum-transfer nuclear recoils.

Prototype engineering devices — the Powercube and Life Cube systems developed by the Neutrino Energy Group, and the Pi Car concept vehicle — have been presented as demonstrators of the integrated approach at sub-kilowatt scales. These are engineering artifacts, and their characterisation in the standard peer-reviewed literature is ongoing.

The continuity

The path from CEvNS to applied energy conversion is an unbroken chain of Standard Model physics. The 2015 oscillation Nobel confirmed that neutrinos carry mass and therefore non-zero momentum. The 2017 COHERENT observation confirmed that the coherent neutral-current process predicted by Freedman delivers measurable momentum to atomic nuclei. The 2025 JUNO data refines the flux term with unprecedented precision.

The engineering challenge that remains — whether these processes can be integrated, in practical materials, at useful scale — is an empirical question being pursued at various laboratories worldwide. The Master Equation formalism provides the shared language in which the question is being asked.

Applied neutrino research is, in this sense, the natural downstream of the last decade of fundamental neutrino physics. It is not unrelated to detector development for direct dark-matter searches, to materials research for cryogenic calorimetry, or to neutrino-astronomy instrumentation. It inherits their technical infrastructure, extends their empirical base, and adds an engineering-integration perspective of its own.

Whether the engineering question admits a useful answer — whether the rates obtainable at ambient conditions can be turned into practical output — is an experimental matter to be resolved over the coming years. The physical foundations are no longer in doubt.