Why do we study heavy elements ? What do we want in Dubna?
Transcript of Why do we study heavy elements ? What do we want in Dubna?
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Chemist’s perspective
Questions:
• Where does the periodic table end ?
• Is the Mendeleev classification still valid ? (relativistic effects)
Language:
• Actinides: Z=89-103
• Transactinides: Z≥104
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Physicist’s perspective
Where does the nuclear chart end ?
Limits of nuclear binding ?
Where are the next magic numbers ?
?
Language: Transfermium Z≥100 (arbitrary)
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Macroscopic view: liquid drop
B(A,Z) = avA volume – attractive nuclear force
-asA2/3 less binding at the surface
-acZ2A-1/3 Coulomb – proton repulsion
-aa(A-2Z)2A-1 asymmetry
+δA-1/3 pairing
Proton separation energy: Sp = B(A,Z) –B (A-1,Z-1)
Neutron separation energy: Sn = B(A,Z) –B (A-1,Z)
If Sp or Sn < 0, we can’t add another proton or neutron, respectively.
The A+1 nucleus will then be unbound.
Question: Who’s the bad guy ?
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Deformed liquid drop: fission
B(A,Z,ε) = avA
-asA2/3 (1 + 2/5 ε2+ …) surface prefers spherical nuclei
-acZ2A-1/3(1-1/5 ε2+…) coulomb favors deformation
-asymmetry –pairing
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2
1a
b−=ε
If B(ε) -B(ε=0)> 0: gain in energy with deformation
→ fission
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Is there hope ?
As long as there are shell effects !
Shell structure depends sensitively on
effective nucleon-nucleon interaction.
Above example: spin-orbit
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Where is the next shell closure ?
• different models and interactions predict
different shell correction energies.
• So which is best?
• Which are the next magic numbers?
• And how much extra stability do they give?
• Could there even be an island of stable
nuclei at the next magic numbers?
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A bit of history
1940 238U92(n,γ) 239U92→239Np93 (Berkeley, neutrons from d+9Be)
1941 238U92(d,2n) 238Np93 →238Pu94 (Berkeley)
1944 239Pu(n,γ) 240Pu(n,γ) 241Pu →241Am95 (reactor neutrons, Argonne)
1944 239Pu(α,n) 242Cm96 (Berkeley)
1949 241Am(α,2n) 243Bk97 (Berkeley)
1950 242Cm(α,n) 245Cf98 (Berkeley)
Significant quantities of these actinides are also bred in reactors
(first chain reaction in Chicago Pile-1 reactor 2.12.1942)
That’s of course where the samples for cyclotron experiments came from.
Questions: How far can we go like this ?
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238U 239U 240U 241U 242U 243U 244U 245U 246U 247U 248U 249U 250U 251U 252U 253U 254U 255U
255Np
255Pu
255Am
255Cm
255Bk
17 (n,γ) reactions
255Cf
255Es
255Fm
8 β- decays
251Cf
α decay
7.1 MeV
T1/2 = 20 h
253Np
253Pu
253Am
253Cm
253Bk
253Cf
253Es
238U 239U 240U 241U 242U 243U 244U 245U 246U 247U 248U 249U 250U 251U 252U 253U
15 (n,γ) reactions
7 β- decays
Very high neutron flux !
Identification via
characteristic α decay
251Bk
α decay6.6 MeV
T1/2 = 20.5 d
samples collected by planes
flying through the mushroom cloud.
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Can even heavier nuclei be made in this way ?
Need even bigger nuclear explosions with higher neutron flux !
are SHE produced
in supernovae ?
can SHE be found in
meteorites ?
no evidence.
Crab nebula
supernovae in A.D. 1054
~ 6500 l.y. distance
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Cold war by hot fusion
253Es(α,n)256Md101 (Berkeley 1955)
last element discovered using chemical separation, 17 atoms
238U(22Ne,6n)254No102 (Dubna 1966)
earlier claims from Sweden and Berkeley ambiguous
252Cf(11B,5n)258Lr103 (Berkeley 1961)
242Pu(22Ne,5n)259Rf104 (Dubna 1966)
249Cf(15N,4n)260Db105 (Dubna 1968)
249Cf(18O,4n)263Sg106 (Berkeley 1974)
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What’s wrong in this picture ?
Glenn T. Seaborg (1912-1999)
Nobel prize chemistry 1951
(co-)discovered 10 transuranic elements
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Cold fusion (GSI – SHIP)
209Bi(54Cr,1n)262Bh (1981)209Bi(58Fe,1n)266Mt (1982)208Pb(58Fe,1n)265Hs (1984)208Pb(62Ni,1n)269Ds (1994)209Bi(64Ni,1n)272Rg (1994)208Pb(70Zn,1n)277112 (1996)209Bi(70Zn,1n)278113 (RIKEN, 2003, σ=55 fb)
identification through
correlated α decay chains
cold fusion:
compound nucleus at very
low excitation energy
only one neutron evaporated
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Hot fusion (Dubna)
244Pu(48Ca,3n)289114 (1999)248Cm(48Ca,3n)292116 (2000)243Am(48Ca,3n)288115 → 284113 (2003)249Cf(48Ca,3n)294118 (2005)
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Spectroscopy of heavy elements
Experimental limits in SHE synthesis reached ?
Alternative approach:
probe the orbital structure of nuclei around Z=102, N=152
cross sections are significantly higher
perform spectroscopy
in this deformed region same active orbitals as in spherical SHE
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Ionisation
chamber
Recoil decay tagging
Beam
Filter
γ detectors
Recoils
Identification :
ToF, recoil energy,
characteristic decay
Silicon
DSSD
∆E, T E, T
Eγ, T
ToF, ∆E-E
recoil
identified
associate
with γ
α decay: Eα, T
same pixel
T ≈ T½
isotope identified
Pin diodes:
electrons Planar Ge
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Rotating 254No
Gammasphere + FMA @ Argonne208Pb(48Ca,2n)254No
P.Reiter et al., PRL 82, 509 (1999)
Jurogam+RITU @ Jyväskylä
S.Eeckhaudt et al.,
Eur.Phys.J. A26, 227 (2005)
254No can spin up to 22 ħ
without fissioning !
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Rotational bands in odd nuclei
205Tl(48Ca,2n)251Md
σ ~ 760 nb
A. Chatillon et al.,
PRL 98, 132503 (2007)
209Bi(48Ca,2n)255Lr
σ ~ 300 nb
S. Ketelhut et al.,
Submitted to PRL
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• Odd-proton orbitals in 251Md / 255Lr
• B(M1)/B(E2) depends on (gK-gR)/Q0
gK ~ 0.7 Mainly E2[514] 7-
2
7 -
2
7 +
2
[633] 7+
2gK ~ 1.3 Mainly M1
1 -
2
[521] 1-
2a ~ 0.9:
gK ~ -0.55
Mainly E2
Electromagnetic properties
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Internal conversion
Low-energy transitions in
heavy nuclei are highly
converted.
→ electron spectroscopy
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Decay spectroscopy
without requirement to detect
prompt γ (or electrons),
much higher beam currents
can be used.
low count rates after filter.
direct access to single-particle
states after α (or isomer) decay
in odd nuclei.