Electron Screening in Metals Ruhr-Universität Bochum Lehrstuhl für Physik mit Ionenstrahlen (EP III) Raiola Francesco, Fakultät für Physik und Astronomie Ruhr-Universität Bochum, D-44780 Bochum, Germany Electron Screening in Metals Today, I will talk about electron screening effect in metallic environments. I will show you several examples where this effect is visible. At the end I will discuss about some possible ways to understand this problem. Well, let’s follow me. ©Francesco Raiola 12.Marz.2001 Münster

D(d,p)t reaction > Bochum (1995) offset Russian group (1998) in gas target Bochum (1995) Thermonuclear reactions play a key role in understanding energy production and nucleosynthesis of the elements in stars. After the Burbidge, Burbidge, Fowler, and Hoyle (so-called B²FH postulate) work which they postulate a series of more or less discrete nucleosynthetic process might have taken place, subsequent experimental and teoretical analysis were did in these years. In particular a part of astrophysics researchers had studied the electron screening effect. Nuclear reactions between charged particles in astrophysical environment proceed by tunnel effect because of the average kinetic energy of interacting particles (Ecin =kT, e.g. p+p at EC = 6,4 T9) is always lower (by some order of magnitude) than their Coulomb barrier. Then Gamow and independently Condon showed that the nuclear process proceed by so-called tunnel effect which penetration probability is ------- The quantity eta is called the Sommerfeld parameter. We must notice that the exp factor is assumed that coulomb potential between the interacting nuclei that deriving from nuclei bare, that is they are completely devoid of their orbital electrons. Primarily because of the exponential behavior of the probability for tunneling , the cross section for charged-particle-induced nuclear reaction drops rapidly for energies below the Coulomb barrier. Then for no-resonant reactions the cross section factor could be write as: Sigma(E)=1/E exp(- 2pi eta)S(E), where the function S(E) defined by this equation, contains all the strictly nuclear effects and for no-resonant reactions it a smoothly varying function of energy. In this formula it was assumed that the Coulomb potential of the target nucleus as seen by the projectile is that resulting from a bare nucleus and thus would extend to infinity. For nuclear reactions induced in the laboratory, the target nuclei are in the form of atoms. The atomic electron cloud surrounding the nucleus acts as a screening potential. As a result, the total potential goes to zero outside the atomic radius The penetration through a shielded Coulomb potential at projectile energy Es is equivalent to that of bare nuclei at energy E=Es+Ue. below the Coulomb barrier discrepancies between experiment and theory ( factor 2) are not yet understood offset Ue = 25 ±5 [eV] (from U. Greife et al. (1995) work) Uad = 14 [ev] ( factor 2 is not yet understood!!) in deuterated-metal target Russian group (1998) Berlin group (2001) Bochum group (2001-2002) (in progress) screening in gas screening in metals >

1 order magnitude larger than in gas target status quo Screening effect in deuterated metals 1 order magnitude larger than in gas target an environment effect no systematics!!

set-up at Bochum sample spectrum at Ed=48 keV d(d,p)t t Ni foil liquid-nitrogen-cooled Si Cu pipe -200 V Ta target deuteron beam P= 2x10-8mbar q = 130° Si Ni turbo pump 450 l/s aperture 8 mm F x/y – scanners methanol cooling -10°C p d(d,p)t t sample spectrum at Ed=48 keV

production of deuterated Ta Ta foil bombarded with 10 keV deuterons saturation ERDA range (Ed=100 keV)

differentation method of Y(E0) differentiation thin-target data Ue= 309±12 [eV] bare E [keV] result for Ta (Berlin) confirmed

Summary Controllare gli errori della tabella

confirmed Ti, Au and Pd of Russian group, Screening effect in deuterated metals - russian confirmed Ti e Au - Berlin confirm for Ta and not for Al and Zr - open quest: stopping power below the Bragg peak - similar Ue of the same grou of periodic table - Fermi shuttle mechanism - question markers -periodic table in una slide considerations & open questions confirmed Ti, Au and Pd of Russian group, Berlin confirmed for Ta and not for Al and Zr. open quest: stopping power below the Bragg peak, similar Ue of the same group of periodic table!