Electron screening: can metals simulate plasmas

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Electron screening can metals simulate plasmas?
Marialuisa Aliotta School of Physics -
University of Edinburgh

• electron screening
• d(d,p)t reaction in deuterated materials
• experimental results and interpretation
• testing the model
• overview of final results

International Workshop XXXIV on Gross Properties
of Nuclei and Nuclear Excitations Hirschegg,
Kleinwalsertal, Austria, January 15 - 21, 2006
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Electron screening
assumption
bare nuclei
2?? Z1Z2(m/E)½
Ec
bare
Coulomb potential
E Ue
screened
E
Ratomic
0
Rn
Rt
in the lab and in stellar plasmas interaction is
affected by presence of electrons
Energy gain SCREENING POTENTIAL Ue typically
tiny amount ( 10-100 eV) ? corrections
typically negligible ? except for ultra-low
energies
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Screening potential experimental approach
typically, experimental investigations
Ue in excess of theoretical limit !
ideally one would use a plasma to investigate
screening effects in plasmas can we use metals
instead?
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100 kV accelerator Ruhr-Universität Bochum
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Experimental procedure
target preparation

• Kr sputtering at E 35 keV (remove 200
  mono-layers)
• D implantation (at Ed 5-30 keV until
  saturation)
• stoichiometry MxD attained

experimental run and data analysis
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Anomalous behaviour of Ue in deuterated metals
compared to D2 gas target (Ue ? 30 eV) anomalous
enhancements observed for some materials but not
for others WHY?
F. Raiola et al. Phys. Lett. B547 (2002) 193 F.
Raiola et al. Eur. Phys. J A19 (2004) 283
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Results overview
55 samples in total
F. Raiola et al. Eur. Phys. J A19 (2004) 283

• FEATURES
• elements in same group show similar Ue values
• exceptions group 13 (B insulator) and group
  14 (C, Si, Ge semiconductors)
• large effect 300 eV ? metals with low H
  solubility (1/x)
• metallic character retained
  during implantation with D
• small effect 30 eV ? metals with large H
  solubility
• metallic character lost during implantation
  with D

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Temperature dependence of H solubility
at room temperature metals of group 3 and 4 and
lanthanides all have HIGH hydrogen solubility
y1/x
solubility decreases with temperature ? repeat
measurements at T 200 oC
group 4
Ti group 4
solubility y drops to a few percent increase in
screening potential Ue
similarly for all elements of groups 3 4 and
Lanthanides
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overview of final results
enhancement clearly linked to properties of the
metallic environment
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A possible classical explanation?
A SIMPLE MODEL
following Debyes plasma theory free
electrons in metals cluster around deuterons in
lattice at radius
neff number of quasi-free electrons/atom
(typically 1) ?a atomic density (typically
6x1028 m-3)
for T 300 K ? RD 1/10 Ra
Ue,D ? Z1Z2e2/RD ? Ue,D 300 eV
CRITICAL TESTS
TEMPERATURE DEPENDENCE CHARGE DEPENDENCE
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Temperature dependence of Ue
need elements with almost constant solubility at
all T examples Pt and Co range of T 20
200 oC
group 10
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Target-charge dependence of Ue
Debye radius scales inversely with nuclear charge
Zt of target atoms
expect increased effect in screening potential
with Zt
example
7Li(p,a)4He
in H2 gas target ? UA 300160 eV
in Li metal neff(Li) 0.80.2 ? UD 820100 eV
expect Ue UA UD 1120260 eV
in PdLix alloy neff(Pd) 6.31.2 ? UD
2800280 eV (for x lt few percent)
expect Ue UA UD 3100440 eV
in insulators neff 0 ? UD 0
expect Ue UA UD 300160 eV
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Results
J. Cruz et al. Phys Lett B 624 (2005) 181
similar results observed for 6Li(p,a) (Zt 3)
J. Cruz et al. Phys Lett B 624 (2005) 181
D. Zahnow et al. Z. Phys. A359 (1997)211
9Be(p,a)6Li and 9Be(p,d)8Be (Zt 4)
C. Rolfs, (2005) private communication
50V(p,n)50Cr (Zt 23) 176Lu(p,n)176Hf (Zt71)
C. Rolfs, (2005) private communication
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Summary

• enhanced electron screening in metals explained
  using Debye model
• temperature dependence of Ue verified
• target-charge dependence of Ue verified
• need for improved theory

another crucial prediction of Debye model a
metallic environment should alter the half-lives
of radioactive decay? measurements currently in
progress at Bochum
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the collaboration
F.Raiola1, J.Cruz2, G.Gyürky3, Z.Fülöp3, S.Zeng4,
M.Aliotta5, H.W.Becker1, B.Burchard1,
C.Broggini6, A.Di Leva1, A.DOnofrio7,
M.Fonseca2, L.Gang4, L.Gialanella8, G.Imbriani8,
A.P.Jesus2, M.Junker9, K.U.Kettner10, B.Limata8,
H.Luis2, J.P. Ribeiro2, V.Roca8, C. Rolfs1,
M.Romano8, D. Schürmann1, E.Somorijai3,
F.Strieder1, F. Terrasi7
1 Institut für Physik mit Ionenstrhalen,
Ruhr-Universität Bochum, Germany 2 Centro de
Fisica Nuclear, Universidade de Lisboa,
Portugal 3 Atomki, Debrecen, Hungary 4 China
Institute of Atomic Energy, Beijing, P.R.China 5
School of Physics, University of Edinburgh, UK 6
INFN, Sezione di Padova, Padova, Italy 7
Dipartimento di Scienze Ambientali, Seconda
Università di Napoli, Caserta, Italy 8
Dipartimento di Scienze Fisiche, Università
Federico II and INFN, Napoli, Italy 9 Laboratori
Nazionali del Gran Sasso dellINFN, Assergi,
Italy 10 Fachhochschule Bielefeld, Germany
My special thanks to Francesco Raiola and João
Cruz for much of the material presented
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investigate 6,7Li(p,a) reactions in different
materials to test Zt dependence of UD
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Temperature dependence of H solubility
at room temperature metals of group 3 and 4 and
lanthanides all have HIGH hydrogen solubility
y1/x
in general, hydrogen solubility decreases with
temperature
modified setup to investigate effects of
temperature dependence
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Additional remarks
previous studies of 9Be(p,a)6Li and 9Be(p,d)8Be
reactions D. Zahnow et al. Z. Phys. A359
(1997)211 on metallic Be targets led to Ue
90050 eV, not understood at that time
with neff(Be) 0.210.04 T 293 K Zt 4
UD 87080 eV
UA 240 eV
Ue UA UD 111080 eV
consistent with observation and further
supporting Zt scaling of Debye model
Zt scaling recently verified also for Zt23
50V(p,n)50Cr and Zt71 176Lu(p,n)176Hf
(Rolfs, private communication)