1. 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 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

2. 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 ( ) (in progress)
screening in gas
screening in metals
>

3. 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!!

4. 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

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

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

7. Summary
Controllare gli errori della tabella

8. 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!