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Thick target yield of the ²H(d, p)3H reaction has been measured at very low energies under ultra-high vacuum conditions using deuteron-implanted Zr targets. Increase of enhancement factors observed for lowering deuteron energies could not be explained only by the electron screening effect. Assuming an additional contribution resulting from a single-particle threshold resonance, we are able to describe the energy dependence of the experimental reaction yield correctly. The theoretical calculations performed within the T-matrix approach allows also to study interference effects between different reaction amplitudes and predict a saturation of the threshold resonance strength at deuteron energies below 5 keV. [ABSTRACT FROM AUTHOR]

Nuclear reactions at very low energies can be strongly enhanced due to screening of the Coulomb barrier by surrounding electrons. In the past decade, this effect was intensively studied for the d + d reactions taking place in metallic environments as a model for dense astrophysical plasmas, where the reaction rates can be increased even by many orders of magnitude. The experimentally determined screening energies corresponding to the reduction of the Coulomb barrier height are, however, much larger than the theoretical predictions. New experimental data obtained under ultra high vacuum conditions additionally increases this discrepancy, the origin of which remains still unknown. One of a possible explanation is the excitation of a hypothetical threshold resonance in the 4He nucleus. As the energy dependence of the resonant reaction cross section differs to that of the electron screening effect, one can distinguish between both processes expanding measurements down to the deuteron energies of 1 keV. Because of very high enhancement factors (of the order of 106) a new high current accelerator facility, being now under construction at the University of Szczecin, will make it possible to measure for the first time the reaction cross sections at so low energies. [ABSTRACT FROM AUTHOR]