Transfer Reactions with Halo Nuclei

1
Transfer Reactions with Halo Nuclei

• Barry Davids, TRIUMF
• ECT
• 2 Nov 2006

2
S17(0) Remaining Issues

• Cyburt, Davids, and Jennings examined theoretical
  and experimental situation in 2004
• Extrapolation is model-dependent
• Even below 400 keV, GCM cluster model of
  Descouvemont and potential model based on 7Li n
  scattering lengths differ by 7

3
Extrapolation
4
The Data
5
Concordance?

• Using a pole model, fit radiative capture data
  below 425 keV
• Allows data to determine shape, consistent with
  cluster and potential models
• Junghans et alia result 21.4 0.7 eV b
• All other radiative capture 16.3 2.4 eV b
• Transfer reaction ANC determinations 17.3
  1.8 eV b and 17.6 1.7 eV b

6
Mirror ANCs

• Timofeyuk, Johnson, and Mukhamedzhanov have shown
  that charge symmetry implies a relation between
  the ANCs of 1-nucleon overlap integrals in light
  mirror nuclei
• Charge symmetry implies relation between widths
  of narrow proton resonances and ANCs of analog
  neutron bound states
• Tested by Texas A M group for 8B-8Li system
• Ground state agreement excellent
• 1 1st excited state shows 2.5s discrepancy
  between theory and experiments (Texas A M and
  Seattle)

7
The Experiment

• Measure ANCs of the valence neutron in 8Li via
  the elastic scattering/transfer reaction
  7Li(8Li,7Li)8Li at 11 and 13 MeV
• Interference between elastic scattering and
  neutron transfer produces characteristic
  oscillations in differential cross section
• Amplitudes of maxima and minima yield ANC

8
Calculations

• DWBA calculations performed with FRESCO by
  Natasha Timofeyuk and Sam Wright
• 8Li 7Li Optical potentials from Becchetti (14
  MeV 8Li on 9Be, modified to be appropriate for
  7Li), two from Potthast (energy-dependent global
  fit to combined 6Li6Li and 7Li7Li data from
  5-40 MeV)
• 7Li n binding potentials taken from Esbensen
  Bertsch and from Davids and Typel

9
Calculations by Sam Wright
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Advantages of the Method

• Identical initial and final states gt single
  vertex is involved
• Statistical precision greater (compared with
  distinct initial and final states)
• Single optical model potential needed
• Elastic scattering measured simultaneously
• More than one beam energy allows evaluation of
  remnant term in DWBA amplitude
• Absolute normalization of cross section enters
  only as a higher-order effect in ANC determination

11
Experimental Setup
12
Target, Beam, Detectors

• Two annular, segmented Si detectors
• 25 µg cm-2 7LiF target
• LEDA detector covers lab angles from 35-61
• S2 detector covers 5-15 in the lab
• 7Li cm angular coverage from 10-30 and 70-122
• 8Li beam intensities of 2-4 ? 107 s-1

13
Online Spectrum from S2 Detector
14
Ground state structure of 9Li (N6 new closed
shell?)
R. Kanungo et al.
9Li(d,t)8Li
E 1.7A MeV
8Liex2
8Ligs
8Liex1
PRELIMINARY ONLINE SPECTRUM
Q-value for d(9Li,t)8Li MeV
15
11Li Transfer Studies

• 11Li is the most celebrated halo nucleus but
  isnt well understood because of its soft
  Borromean structure
• In particular, the correlation between two halo
  neutrons is insufficiently studied experimentally
• Two-neutron transfer reactions are known to be
  the best tool for studying pair correlations of
  nucleons in nuclei
• TRIUMF, for the first time in the world, can
  provide a low energy beam of 11Li with sufficient
  intensity for such studies.

16
11Li Halo Wave Function

• Admixture of 2s1/2 and 1p1/2 waves dominate the
  halo wave function
• Change of shell structure in nuclei far from the
  stability line? How about other waves such as
  2d5/2 and other higher orbitals? --gt pairing near
  the continuum
• The spectroscopic factor of (2s1/2)2 would
  reflect the strength of other components
• Unfortunately, but interestingly, 10Li is not
  bound
• The single particle structure of halo neutrons is
  difficult to study. s1/2 does not make clear
  resonance state.
• Measurements of neither the fragment momentum
  distribution nor the single particle transfer
  reactions (p,d) and (d,p) have provided
  conclusive results

17
Cross Section Calculations by Ian Thompson
direct two-neutron transfer only
including two-step transfer
1. Bertsch-Esbensen, 2. Thompson-Zhukov, 3.
Yabana (No three body correlation)
1.6 A MeV 11Li(p, t)
18
Correlation of Neutrons in Halos

• Interesting suggestion from three body
  calculation
• Mixing of di-neutron and cigar -type
  configurations in 6He

19
Recent Density Correlation Studies
rn-n
r2n
rc
rc-2nrcr2n
ltr1r2gt
20
Three Methods

• HBT interferometry measurement
• Fragmentation, fusion of core
• Electromagnetic dissociation
• Matter and charge radii

21
11Li result
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ISAC_at_TRIUMF
ISAC II
ISAC I
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Too Low Beam Energy?

• 1.6A MeV is appropriate for the study.
• The effect of Coulomb barrier is extremely small
  for halo neutrons.1.6A MeV is much higher than
  the separation energy (180 keV) .
• Energy-momentum matching is not bad because of
  the narrow internal momentum distribution of the
  halo neutrons.
• 6A MeV is conventional transfer reaction energy
  and thus analysis tools were well developed.

25
MAYA
K
active zone
zone of amplification and detection
wall of Si CsI
gassiplex
26
11Li(p,t)9Li at TRIUMF

• The first run is planned at the end of November
  2006
• We expect 5000 (p,t) reactions to ground state of
  9Li
• Reactions populating the excited state of 9Li are
  also expected
• Will measure other channels such as (p,d)
• Be ready for data (Ian)

27
Acknowledgements

• 7Li(8Li,7Li)8Li Derek Howell (M.Sc. Student,
  Simon Fraser University)
• d(9Li,t)8Li Rituparna Kanungo (TRIUMF)
• p(11Li,t)9Li Isao Tanihata (TRIUMF) and Hervé
  Savajols (GANIL)

28
Kinematics of p(11Li, 9Li)t
1.6A MeV
29
Typical events
20 CsI Array
C4H10 gas
20 Si Array
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Differences between 6He and 11Li

• 11Li is much less bound than 6He
• 6He mostly 1p3/2 wave
• 11Li has mixed waves of 1p1/2, 2s1/2,
• The core of 11Li (9Li) may be much softer than
  that of 6He (4He)

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6He results
33
HBT measurement
Rn-n 5.9 1.2 fm
Rn-n 6.6 1.5 fm
Rn-n 5.4 1.0 fm
34
EMD 11LI (70A MeV)Pb -gt9Linn
1MeV
E(9Li-n)
Nakamura et al. 2006
1MeV
E(9Li-n)
35
He and Li Radii
Li
He
36
RMS radii and the configuration
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MAYA
MAYA is essentially an ionization chamber, where
the gas plays also the role of reaction target.
It could be used with H2, d2, C4H10, between 0-2
atm.
the projectile makes reaction with a nucleus of
the gas.
there are two little drift chambers before MAYA,
to monitor the beam.
the light scattered particles do not stop inside,
and go forward to a wall made of 20 CsI
detectors, where they are stopped, and identified.
cathode
wall of CsI detectors
anode amplification area.
segmented cathode
the recoil product leaves enough energy to induce
an image of its trajectory in the plane of the
segmented cathode.
we measure the drift time up to each
amplification wire. The angle of the reaction
plane is calculated with these times.
F
tn
t1