HALO PHYSICS Ian J. Thompson University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom.

3rd April 2000RNB52 Why Study Haloes? zSee prominent single-particle states zSee pairing outside nuclear surface yin two-neutron halo ground states yin two-neutron continuum via breakup yin two-proton decay via tunnelling zSee bound states in classically forbidden regions.

3rd April 2000RNB53 Progress with Better Experiments and Theories  Knowledge of haloes comes from nuclear reactions and  -decay. z Nuclear reactions need to be suitable and accurate for halo nucleons. Need to allow: ylarge size of wave functions ystrong (non-perturbative) couplings yfinal-state interactions from resonances zWhat should we learn from new kinds of experiments?

3rd April 2000RNB54 Reaction Cross Sections and Sizes zOriginal identification of haloes zRadii were fitted with Optical Limit Glauber yThese radii inaccurate just for halo nuclei: yNeed few-body Glauber reaction models; yNew radii are larger. The reaction cross section is less with few- body model, so a larger size fits the  R data.

3rd April 2000RNB55 Momentum Distributions  Serber model  breakup shows initial Fermi momenta, strongly dependent on halo l -value. zBut reaction dynamics change this: yScattering broadens transverse momenta;  Shadowing narrows momenta of l >0 states; yFinal-state resonances narrow momenta of light particles zExperiments should confirm these mechanisms?

3rd April 2000RNB56 Elastic Scattering zDepends on yFolded potential from densities yPolarisation potential from breakup channels zHalo breakup effects  folding changes. zConfirm 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.

3rd April 2000RNB57 RECENT EXPERIMENTS zTransfer reactions y (p,d) or (d,t) probes single-particle structure  Particle-  coincidences from Stripping yprobes particle correlation with excited core zCoincident Coulomb Breakup yprobes response of halo to Coulomb excitation to low-energy continuum

3rd April 2000RNB58 Transfer Reactions to resolved final states. zOne-nucleon transfers, eg (p,d)  shape shows l -value of orbital ymagnitude gives spectroscopic factor zTwo-neutron transfers, eg (p,t) yMagnitude depends on s-wave pairing in halo yOnly relative magnitudes reliably modelled. zFull analysis requires multi-step calculations; yCan we see the intermediate steps experimentally?

3rd April 2000RNB59 Particle stripping +  -rays  Remove one nucleon and look for  -decays of the residual nuclei. zLarger cross sections than transfers at higher beam energies. zSee particle correlations with excited core states. zCan remove particles from `inside the halo’ Stripping cross sections for one-neutron removal from 11 Be, in coincidence with  -decays from 10 Be*. Halo as well as core neutrons are removed.

3rd April 2000RNB510 Complete Breakup zDiffraction dissociation  elastic breakup: all fragments survive with target in g.s. zMain part of Coulomb breakup, exciting the halo to the low-energy continuum zSensitive to residual correlations eg nn virtual state, and n-core resonances

3rd April 2000RNB511 FUTURE EXPERIMENTS zPolarised Beams zNear-barrier fusion zTwo-proton decay

3rd April 2000RNB512 Polarised Beams zFragmentation beams are very probably already polarised (non-random spin distributions) zAligned beams (if nuclear spin  1) give scattering asymmetries for stripping, depending on single- particle amplitudes. Tensor analysing powers for 17 C stripping as function of s-wave amplitude, for two gs spin choices.

3rd April 2000RNB513 Near-barrier Fusion zHalo neutrons should affect fusion: y  Increase fusion, from neutron flow; y  Decrease complete fusion, from breakup; y  Increase fusion, from molecular states. zSo: need experiments + good theories! ySome experiments already performed with 6 He and 9 Be, but theoretical interpretations are still unclear. yTheory (eg. CDCC) is easier with a one-neutron halo.

3rd April 2000RNB514 Two-proton Decay zTwo-proton radioactivity is not via point diproton; zNeed three-body models with pairing in exterior zPrediction: 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

3rd April 2000RNB515 CONCLUSIONS zWith the nuclear halo we see strong pairing effects even outside the nucleus. zNew non-perturbative theories allow the proper interpretation of both old and new experiments. zProposed new experiments will reveal more pairing structure and pairing dynamics.