Probing correlations by use of two-nucleon removal

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• Probing correlations by use of two-nucleon
  removal
• Methods of many-body systems mean field theories
  and beyond - March 20 - 22, 2006, RIKEN, Saitama,
  Japan.

Jeff Tostevin Department of Physics School of
Electronics and Physical Sciences University of
Surrey, UK
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Question that arose at RIBF meeting was ..
Can one observe experimentally the
correlations of pairs of nucleons in exotic
nuclei by using suitable nuclear reactions
(with fast secondary beams - RIBF) ?

I will consider the direct 2N knockout
reaction mechanism will show specific test
cases and some first applications and that
results show sensitivity to pair correlations.
Quenching of calculated strength is a common
feature in comparisons of structure calculations
(e.g. the shell model) with experiment. What are
the expectations for 2N removal?
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Asymmetric nuclei two Fermi surfaces
32Ar ? 31Ar
22O ? 21O
Z8 N14 Sn6.8 MeV Sp23 MeV
Z18 N14 Sn22 MeV Sp2.4 MeV
A.Gade et al., Phys. Rev. Lett. 93 (2004), 042501
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Two nucleon knockout restricted reaction set
Z
54Ti
34Ar
32Ar
44S
2p from neutron rich
30S
28S
52Ca
42Si
28P
26P
34Si
26Si
24Si
34Al
32Al
2n from neutron deficient
28Mg
32Mg
30Mg
32Na
30Na
28Na
30Ne
26Ne
28Ne
N
5
Two-nucleon removal at 80 - 100 MeV/u
9Be
fast spectator
c
Experiments are inclusive (with respect to the
target final states). Core final state measured
using coincident gamma rays.
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Structure need nucleon overlaps
Spectroscopic factor/strength
In two-nucleon case there are (in general)
several coherent 2N configurations the
two-nucleon motions are correlated
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Reaction drills out a cylindrical volume at
surface
Cross section will be sensitive to the spatial
localisations of pairs of nucleons near the
surface No spin selection rule (for S0 versus
S1 pairs) from the reaction mechanism What can
we learn of the 2N wave function and 2-body
correlations from this sampled volume?
z
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Good sd-shell test cases
D. Bazin et al., PRL 91 (2003) 012501
K. Yoneda et al., PRC submitted three cases.
28Mg (Z12, N 16) ? 26Ne
26Si (Z14, N 12) ? 24Si
?
?
and also 30S and 34Ar
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Spectroscopic strengths independent particles
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Uncorrelated 28Mg ? 26Ne(0,2,4), 82.3 MeV/u
uncorrelated d5/22
Sigma (mb)
summed 2
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Radial localisation 28Mg ? 26Ne as
2
1
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Antisymmetrized 28Mg ? 26Ne as
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Antisymmd 28Mg ? 26Ne(0,2,4), 82.3 MeV/u
antisymmetrized d5/22
summed 2
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Correlations in the shell model wave function
28Mg (Z12, N 16) ? 26Ne(0)
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Role of correlations
28Mg ?26Ne(0, 2, 4 ) 82.3 MeV/u
uncorrelated d5/22
antisymmd d5/22
correlated (SM)
Sigma (mb)
summed 2
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Knockout cross sections correlated SM case
28Mg ?26Ne(0, 2, 4 , 22) 82.3 MeV/u
Sigma (mb)
1
2
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Two-neutron removal g.s. branching fractions
correlated
uncorrelated
Sigma (0) / Sigma(inclusive)
26Si
34Ar
30S
K. Yoneda et al., Phys Rev C, submitted
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Importance of diffractive terms
28Mg ?26Ne(0, 2, 4 ,22) 82.3 MeV/u
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Two-nucleon removal suppression - Rs(2N)
Rs (2N)
Preliminary
34Ar
54Ti(gs)
-2p
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Summary
At fragmentation energies (gt50 MeV/u)
reaction theory is rather accurate, allowing one
to extract quantitative structure information and
test structure model predictions. Limited
two neutron/proton knockout data - but these
reveal sensitivity to correlations in the 2N wave
functions (in both S0 and S1 configurations)
and effects of pairing in active 2N
configurations. Direct 2N knockout
reaction mechanism can be very clean and
selective need for more test cases and
applications. Data sets (5 cases) are
consistent with a suppression of 2N strength
relative to the shell model 0.50(5). This
compares with a typical 1N removal suppression of
order 0.6 0.7 for well-bound nucleons.