Transcript HALO PHYSICS

HALO PHYSICS
Ian J. Thompson
University of Surrey,
Guildford, Surrey GU2 7XH,
United Kingdom.
Why Study Haloes?
See prominent single-particle states
See pairing outside nuclear surface
in two-neutron halo ground states
in two-neutron continuum via breakup
in two-proton decay via tunnelling
See bound states in classically forbidden
regions.
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Progress with Better
Experiments and Theories
 Knowledge of haloes comes from nuclear
reactions and b-decay.
 Nuclear reactions need to be suitable and
accurate for halo nucleons. Need to allow:
large size of wave functions
strong (non-perturbative) couplings
final-state interactions from resonances
 What should we learn from new kinds of experiments?
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Reaction Cross Sections and
Sizes
Original identification
of haloes
 Radii were fitted with
Optical Limit Glauber
These radii inaccurate just
for halo nuclei:
Need few-body Glauber
reaction models;
New radii are larger.
The reaction cross section is less with fewbody model, so a larger size fits the sR data.
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Momentum Distributions
Serber model  breakup shows initial Fermi
momenta, strongly dependent on halo l-value.
But reaction dynamics change this:
Scattering broadens transverse momenta;
Shadowing narrows momenta of l >0 states;
Final-state resonances narrow momenta of
light particles
Experiments should confirm these mechanisms?
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Elastic Scattering
Depends on
Folded potential from
densities
Polarisation potential from
breakup channels
Halo breakup effects 
folding changes.
Confirm with breakup
measurements?
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Red curve from folded potential is much closer to
blue curve (core-only scattering) than full threebody result (black line).
Blue-green line is score*|F|2, nearly the full result,
where |F|2 is from Fourier transform of halo density.
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RECENT EXPERIMENTS
Transfer reactions
 (p,d) or (d,t) probes single-particle structure
Particle-g coincidences from Stripping
probes particle correlation with excited core
Coincident Coulomb Breakup
probes response of halo to Coulomb
excitation to low-energy continuum
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Transfer Reactions
to resolved final states.
One-nucleon transfers, eg (p,d)
shape shows l-value of orbital
magnitude gives spectroscopic factor
Two-neutron transfers, eg (p,t)
Magnitude depends on s-wave pairing in halo
Only relative magnitudes reliably modelled.
Full analysis requires multi-step calculations;
Can we see the intermediate steps experimentally?
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Particle stripping + g-rays
Stripping cross sections for one-neutron
removal from 11Be,
in coincidence with g-decays from 10Be*.
Halo as well as core neutrons are removed.
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 Remove one nucleon and
look for g-decays of the
residual nuclei.
 Larger cross sections
than transfers at higher
beam energies.
 See particle correlations
with excited core states.
 Can remove particles
from `inside the halo’
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Complete Breakup
Diffraction dissociation  elastic breakup:
all fragments survive with target in g.s.
Main part of Coulomb breakup, exciting
the halo to the low-energy continuum
Sensitive to residual correlations eg nn
virtual state, and n-core resonances
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FUTURE EXPERIMENTS
Polarised Beams
Near-barrier fusion
Two-proton decay
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Polarised Beams
Fragmentation beams are
very probably already
polarised (non-random
spin distributions)
Aligned beams (if nuclear
spin  1) give scattering
asymmetries for stripping,
depending on singleparticle amplitudes.
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Tensor analysing powers for 17C
stripping as function of s-wave
amplitude, for two gs spin choices.
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Near-barrier Fusion
Halo neutrons should affect fusion:
 Increase fusion, from neutron flow;
 Decrease complete fusion, from breakup;
 Increase fusion, from molecular states.
So: need experiments + good theories!
Some experiments already performed with 6He and
9Be, but theoretical interpretations are still unclear.
Theory (eg. CDCC) is easier with a one-neutron halo.
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Two-proton Decay
Two-proton radioactivity
is not via point diproton;
Need three-body models
with pairing in exterior
Prediction: pairing acts
to correlate the protons
to enhance L=0 clusternucleus relative motion.
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Dependence of width on decay energy
for diproton and three-body dynamics
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CONCLUSIONS
With the nuclear halo we see strong
pairing effects even outside the nucleus.
New non-perturbative theories allow the
proper interpretation of both old and new
experiments.
Proposed new experiments will reveal
more pairing structure and pairing
dynamics.
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