Historical research guide · 0727 · 2020
Screening energy for low energy nuclear reactions in condensed matter
J. Kasagi · Y. Honda · K. Fang
In one minute
Jirohta Kasagi has spent two decades on one measurement: fire a very slow deuteron beam at a metal and count the fusions. In free space, two deuterons at a few thousand electronvolts almost never reach each other — their positive charges hold them apart. Inside a metal, they fuse far more often than that. The electrons and ions of the host crowd around each deuteron and cancel part of its charge, and the size of that cancellation is a single number, the screening energy. Kasagi and colleagues at Tohoku University measure it. In titanium it is about 27 electronvolts, roughly the value for deuterium gas; in palladium about 310, in palladium oxide about 600 — and at a bombarding energy of 2.5 kiloelectronvolts, palladium oxide fuses about a hundred times more often than the bare-nucleus rate. His verdict is the important part: one of the two miracles of cold fusion is, experimentally, no longer a miracle.
Connection
Chapter 12 rests on whether the effective barrier between two nuclei can be changed by their environment, and this is the accelerator physics that says it plainly: the same reaction, the same energy, a hundredfold difference in rate depending only on what solid the deuterons are sitting in.
From the source
The enhancement is real and has been seen by independent groups. In Kasagi’s summary of the field, the strong enhancement of the reaction rate for deuteron-deuteron reactions in metals has been independently observed in low-energy beam experiments by three groups; the values obtained for the various host metals do not completely agree between data sets, but the conclusion that a large screening mechanism exists for the reaction in metals stands on all of them.Kasagi, ICCF-14 review, Summary, opening paragraph
The screening energy differs enormously from host to host, and the numbers are large. Yuki and colleagues first deduced it for the deuteron-deuteron reaction in metal: 27 electronvolts in titanium, close to the 25 electronvolts of a deuterium gas target, but 80 electronvolts in ytterbium, which is a reaction rate about 1.5 times higher at 3 kiloelectronvolts. Subsequent experiments found anomalously large values — 600 electronvolts for palladium oxide, 310 for palladium, 200 for iron — and for palladium oxide at a bombarding energy of 2.5 kiloelectronvolts the reaction rate became about a hundred times the normal rate. Czerski and colleagues reported about 300 electronvolts in tantalum, confirmed by Raiola and colleagues.Kasagi, ICCF-14 review, section 2, Overview of DD reactions in metal
Existing theory accounts for only part of it. Ichimaru and colleagues calculated the effective static potential for hydrogen in titanium and palladium and obtained 51 and 75 electronvolts; Czerski and colleagues, treating electron screening as a static polarisation of the metallic medium induced by the deuteron and including cohesion effects, obtained about 130 electronvolts for palladium — less than half the observed value. Kasagi states the position exactly: the mechanism of the enhancement is not fully understood, especially where the screening energy exceeds 200 electronvolts.Kasagi, ICCF-14 review, section 2, closing paragraphs
Melting the target raises the screening sharply, which points at the missing mechanism. Kasagi’s group measured alpha yields from lithium-6 plus deuteron and lithium-7 plus proton reactions on the same lithium target in its solid phase near 330 kelvin and its liquid phase near 520 kelvin, at bombarding energies from 22.5 to 70 kiloelectronvolts. Fitting both data sets gives a screening potential of 400 plus or minus 50 electronvolts for the solid and 700 plus or minus 60 for the liquid, a difference of 300 plus or minus 30 electronvolts — about 1.75 times the solid value.Kasagi, ICCF-14 review, section 3, Liquid Li plus p,d reactions, and Figures 1 and 2
Mobile positive ions screen more strongly than electrons do. Liquid lithium can be treated as a low-temperature dense plasma of classical lithium ions and quantum electrons at about five times ten to the twenty-second per cubic centimetre. The Debye screening length for the mobile ions is about 6.7 picometres at 520 kelvin, far shorter than anything the degenerate electrons provide, giving a predicted ionic contribution of 645 electronvolts against a measured 580 — while the electronic contribution alone predicts 194 electronvolts for the solid. Kasagi’s conclusion is that ionic screening is much stronger than electronic screening in low-temperature dense plasmas.Kasagi, ICCF-14 review, section 3, screening-length estimates, and Summary, third paragraph
What to watch: whether deuterons inside palladium behave like that liquid. Kasagi puts the test himself — if deuterons in palladium metal behave like a liquid, then the screening effect of the deuteron ions must be included and the screening potential would be much larger than the electronic screening alone. He also notes what the whole line of work is aiming at: a simple extrapolation of the screening potential down to the thermal energy region, from the kiloelectronvolt beam experiments, predicts the occurrence of cold-fusion deuteron-deuteron reactions. The measurement that would settle it is a direct determination of the screening potential for deuterons in a highly loaded palladium lattice at thermal energies.Kasagi, ICCF-14 review, Abstract, sentence 2, and Summary, final paragraph
The way in
https://doi.org/10.1016/b978-0-12-815944-6.00010-5How to cite it
J. Kasagi, Y. Honda, K. Fang (2020) Screening energy for low energy nuclear reactions in condensed matter doi:10.1016/b978-0-12-815944-6.00010-5