HALO PHYSICS

Ian J. ThompsonUniversity of Surrey,Guildford, Surrey GU2 7XH, United Kingdom.

3rd April 2000 RNB5 2

Why Study Haloes?

See prominent single-particle statesSee 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 -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 few-

body model, so a larger size fits the R data.

The reaction cross section is less with few-body model, so a larger size fits the R 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 particlesExperiments 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?

Red curve from folded potential is much closer to blue curve (core-only scattering) than full three-body result (black line).Blue-green line is core*|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- 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 + -rays

Remove one nucleon and look for -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’

Stripping cross sections for one-neutron removal from 11Be, in coincidence with -decays from 10Be*.Halo as well as core neutrons are removed.

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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 single-particle amplitudes.

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 cluster-nucleus relative motion. 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.