Historical research guide · 0599 · 2007
Measurement of the enhanced screening effect of the d + d reactions in metals
A. Huke · K. Czerski · P. Heide
In one minute
Inside a star, free electrons crowd around nuclei and soften their mutual repulsion, so fusion runs far faster than the bare Coulomb barrier would allow. Armin Huke, Konrad Czerski and Peter Heide asked whether the electron gas inside a metal does the same job for deuterium, treating the metal as a cold, dense, strongly coupled stand-in for stellar plasma. They fired deuterons from an electrostatic accelerator into metal targets that the beam itself had loaded with deuterium, and measured the fusion yield down to a few thousand electron volts. The yield is higher than the same reaction measured in deuterium gas, and the screening energy that describes the enhancement comes out around ten times the gas value — hundreds of electron volts against twenty-five — and well above what theory predicts. Much of the paper is about what nearly ruins such a measurement: deuterium is extremely mobile in metals, and carbon and oxide layers grow on the target under the beam.
Connection
Chapter 12 turns on a published, checkable fact — that a metal lattice changes the effective barrier two deuterons face — and this is one of the measurements that fact rests on. It belongs to chapter 1 too, because the authors show that the standard way of analysing this experiment gives wrong screening energies, and replace it with a method that does not depend on stopping powers or on knowing the absolute deuteron density.
From the source
Deuteron fusion measured in self-implanted metallic targets shows an enhancement of the reaction cross-section compared with gas target experiments, and the electron screening energies extracted from it are about one order of magnitude larger than the gas target values.Abstract; Section 1
The measured screening energies from the first campaign are 322 plus or minus 15 electron volts for tantalum, 297 plus or minus 8 for zirconium and 190 plus or minus 15 for aluminium, against 25 plus or minus 5 electron volts for a gaseous deuterium target.Table 1; Section 1, comparison with the gas target value
The measured values exceed the theoretical predictions significantly: the calculations available to the authors give screening energies smaller by a factor of two than the experimental ones, so the size of the effect in metals is not yet accounted for by theory.Section 1; Abstract, closing sentences
The authors replace the standard total-yield analysis with a differential data acquisition and analysis method that reads the screening effect only from the discontinuities in reduced reaction yield at each change of beam energy. It allows on-line monitoring of the deuteron density, is independent of the imprecise stopping power coefficients and of the absolute deuteron number density, and lets measurements with unwanted shifts in the density depth profile be recognised and rejected — whereas the standard procedure, they state, produces fatal errors in the extracted screening energies.Section 7, Conclusion; Abstract
Surface layers grown under the beam from the residual gas dominate the systematic error: dedicated experiments on tantalum with deliberately varied surface composition returned screening energies from 210 to 460 electron volts, and a few tens of atomic layers of carbon or metal oxide are enough to obliterate the screening effect entirely — so the published values are lower limits.Section 7, Conclusion; Figure 11, tantalum surface compositions A to E
The measurement that would settle the number is named: a precise determination of the screening energies requires ultra-high vacuum systems held well below ten to the minus tenth hectopascal, where only hydrogen and noble gases remain in the residual gas, equipped with in-situ target diagnosis. Until then, the authors say, the great screening enhancement is sustained as a fact while no further assertion about its cause can be made from other material properties.Section 7, Conclusion, final sentences
The way in
https://doi.org/10.1016/j.nimb.2007.01.082How to cite it
A. Huke, K. Czerski, P. Heide (2007) Measurement of the enhanced screening effect of the d + d reactions in metals doi:10.1016/j.nimb.2007.01.082