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Experimental Signatures of a New Channel of the Deuteron-Deuteron Reaction at Very Low Energy

R. Dubey1,*, K. Czerski1, Gokul Das H.1, A. Kowalska2, N. Targosz-Sleczka1, M. Kaczmarski1, and M. Valat1

  • *Contact author: rakesh.dubey@usz.edu.pl
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Phys. Rev. X 15, 041004 – Published 7 October, 2025

DOI: https://doi.org/10.1103/chlp-b215

Abstract

The discovery of a new, e+e− reaction channel in deuteron-deuteron (DD) fusion at very low energies might have major implications for understanding primordial and stellar nucleosynthesis, where electron-positron reaction channels are typically not considered. It could also enable research on metal hydride fusion, potentially paving the way for the design and construction of next-generation fusion energy sources. Following the first experimental indications of electron emission, we present here an extensive experimental study confirming emission of high-energy electrons from DD reaction at very low energy. A simultaneous use of Si charged particle detectors of different thicknesses and large-volume NaI(Tl) and HPGe detectors has allowed the determination of the branching ratios between emitted protons, neutrons, and e+e− pairs for deuteron energies down to 5 keV. The high-energy positrons could be unambiguously detected by their bremsstrahlung spectra and annihilation radiation, supported by the geant4 Monte Carlo simulations. The theoretical calculations, based on a destructive interference between the threshold resonance and the known broad resonance in He4, agree very well with experimentally observed increase of branching ratios for lowering projectile energies. The partial width of the threshold resonance for the internal e+e− pair creation should be at least 10 times larger than that of the proton channel.

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Popular Summary

Deuteron-deuteron fusion reactions are essential for understanding the formation of the first light elements in the early Universe and for explaining how stars generate energy. However, the recently suggested threshold resonance in helium-4 can strongly increase their reaction rates at very low energies. In our study, we present extensive experimental evidence confirming a previously unobserved channel for this reaction: At deuteron energies below 5 keV, deuteron-deuteron fusion is dominated by the formation of this resonance, which then decays by emitting an electron-positron pair.

We use a low-energy accelerator to bombard a deuterium-implanted zirconium target with a deuteron beam, employing multiple detector types to simultaneously observe all reaction products. Thick silicon detectors enable us to distinguish the partial energy signature of the high-energy electron-positron pairs from the protons produced in conventional deuteron-deuteron fusion. For the first time, we also use large volume NaI(Tl) and HPGe detectors to observe the γ rays produced. These detectors provide unambiguous evidence of positron emission by measuring an excess of 511 keV photons from positron annihilation, as well as the bremsstrahlung radiation from the high-energy electron-positron pairs.

Theoretical calculations show excellent agreement with our data, indicating that the probability of this electron-positron decay is at least 10 times that of the standard proton channel. Taken together, our findings suggest that internal pair creation should be incorporated into future models of stellar nucleosynthesis. This work could also pave the way for new, clean fusion energy sources by using materials that enhance these low-energy reactions.

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Phys. Rev. X 15, 041004– Published 7 October, 2025
  • Received 15 December 2024
  • Accepted 25 August 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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