1

Why do we study heavy elements ?

or

What do we want in Dubna ?

3 Feb 2009

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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

3

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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The voyage to the heaviest elements

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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

2

2

1a

b−=ε

If B(ε) -B(ε=0)> 0: gain in energy with deformation

→ fission

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8

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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Shell effects and deformation

harmonic oscillator potential

deformed Woods-Saxon potential

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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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Nuclear Physics experiment of a different kind

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Mike, Eniwetok, Marshall Islands, Nov. 1st, 1952

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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

16

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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Approaching the island of stability ?

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Deformation of SHE

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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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RITU – JUROGAM at Jyväskylä

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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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Isomer tagging

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K Isomers

delayed ER-electron

coincidences in DSSD

delayed ER-γ

coincidences in DSSD

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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.

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GABRIELA @ VASSILISSA

intense beams (~1pµA)

radioactive targets

long beam times

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207Pb(48Ca,2n)253No→249Fm

A. Lopez-Martens et al., PRC 74, 044303 (2006)

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Comparison with theory