High-Z electron screening: the cases 50V(p,n)50Cr and 176Lu(p,n)176Hf

The electron screening in 50 V(p,n) 50 Cr has been studied at E p = 0.75 to 1.55 MeV for different environments: VO 2 insulator, V metal and PdV 10% alloy. Relative to the insulator we find for the metal and alloy a large screening energy U D = 27 ± 9 and 34 ± 11 keV, respectively. We also studied 176 Lu(p,n) 176 Hf at similar proton energies for a Lu 2 O 3 insulator, a Lu metal and a PdLu 10% alloy; here we observed a narrow resonance at E pr = 0.81 MeV exhibiting a prominent Lewis peak and a shift in proton resonance energy of U D = 32 ± 2 and 33 ± 2 keV for the metal and alloy, respectively, relative to the insulator. The data demonstrate that the electron screening occurs across the periodic table and is not restricted to reactions among light nuclides studied so far. The two reactions with neutrons in the exit channel demonstrate furthermore that the electron screening is an effect in the entrance channel of the reaction and is not influenced by the ejectiles of the exit channel, i.e. by the charged particles of the exit channel studied so far. The U D values can be explained by the plasma model of Debye applied to the quasi-free electrons in the metallic samples. The data together with previous studies of the fusion reactions d+d, Li+p and Be+p in metals verify the Debye model prediction, i.e. a scaling of U D with the nuclear charge of the target atoms. Finally, we discuss the possibility for an enhanced α-decay rate of transuranic waste in metals.