Screening energy for low energy nuclear reactions in condensed matter

Coulomb screening in nuclear reactions is considered to play an important role in cold fusion, since the screening effect causes considerable enhancement of reaction rates in the ultra-low energy region. This chapter is an overview of results obtained so far in low energy beam experiments of the d+d and Li+d reactions in metal environments. In various metals, extraordinary large screening effects were found in the d+d reaction. The observed enhancement in the low energy region was interpreted as electronic screening, mainly due to conduction electrons in metal. In the Li+d reaction, ionic screening was found in addition to the electronic one, by using liquid Li target. Comparing the obtained screening energy with the expected one from simple models, we found the following. The screening energy due to conduction electrons is several times larger than the calculation by the Thomas-Fermi model. On the other hand, the screening energy due to Li⁺ ions is about 60% of the prediction by the Debye model, which explains observed temperature dependence of the screening energy. Irradiation of liquid metals with a deuteron molecular beam has led to the discovery of a new process, a cooperative collision mechanism (CCM) of the d+d reaction, by which the screening energy was determined for several metals. Since the CCM is effective in accurately estimating the number of deuterons involved in the d+d reaction, it is expected to determine the screening energy more reliable.