Marielle Chartier, Roy Lemmon (Visiting Senior Research Fellow), Jonathan Taylor (PhD student)

The key science objectives of this research programme are to investigate the nature of the nuclear wavefunction, in particular how single-particle structure and correlations evolve, in nuclei with large proton-neutron asymmetry.

Large isospin variations in exotic nuclei are predicted to modify the nuclear mean-field picture and also the long-range and short-range correlations. The spin-orbit interaction reduces in magnitude as the nuclear surface evolves towards diffuse neutron matter. The role of certain residual interactions becomes enhanced with proton-neutron asymmetry, in particular the monopole interaction that acts between proton-neutron spin-orbit partners. The combination of these effects will mean that new shell gaps and magic numbers will appear, such as N=14, 16 instead of N=8, 20. Another consequence is that the occupancies will become isospin dependent. Such an effect has recently been predicted theoretically using sophisticated many-body techniques such as Self-Consistent Green's Function theory. There is also some limited experimental evidence that this is the case. Experimentally, the most precise probes of single-particle motion, and deviations from this picture, are direct reactions that add or remove one particle at a time, thereby exciting a minimal number of nucleonic degrees of freedom. In particular such reactions allow the determination of spectroscopic factors, quantities that reflect the degree to which the wavefunction of the removed (or added) nucleon is a single-particle wavefunction.

Our current research programme uses direct reactions to investigate experimentally the effects of the isospin dependence of the mean field and the long- and short-range correlations in exotic nuclei. Energy levels, orbital angular momentum values and spectroscopic factors in light-to-medium heavy nuclei are measured. Three types of direct reactions, all performed in inverse kinematics, are used: one- or two-nucleon transfer reactions (~10 AMeV), one-nucleon removal (50-500 AMeV) and quasifree hadronic scattering (~500 AMeV). All three types of reaction build on our recent initiatives and on-going research that we lead or are strongly involved in at GANIL/SPIRAL (France), GSI (Germany) and RIKEN (Japan), and constitute our main long-term programme. All will lead to our future research at the FAIR facility (R3B and EXL at NUSTAR) and at the SPIRAL2 facility (GASPARD and ACTAR).

The aim of our research is to investigate two physics topics: the evolution of single-particle level ordering and the modification of the occupancies with isospin.

• We exploit the proven methods of transfer and one-nucleon removal to investigate the first topic. One current area is investigations of the region around the new shell closures at N=14,16. We have recently led an experimental programme to make extensive measurements of the light neutron-rich sd-shell nuclei O, F and Ne using one-neutron removal at 50 AMeV and transfer reactions at GANIL/SPIRAL at 10 AMeV [1-5]. Another current area is studies of the changing shell structure across the Ni isotopic chain from N=28 to N=40 with a recent measurement of one-nucleon-removal from 56-72Ni at 500 AMeV at GSI [6-7]. We also recently pursued complementary studies, with the one-nucleon removal technique, in light neutron-rich and proton-rich nuclei, at GSI with the ALADIN/LAND set-up.
• To address the second topic, the method will be to measure spectroscopic factors for the removal of a single nucleon from the same nucleus using transfer, one-nucleon removal and quasifree scattering reactions. The application of quasifree scattering to exotic nuclei is crucial to this topic, given its potentially greater sensitivity to the wavefunction in the nuclear interior. A pilot UK-led experiment was recently conducted at GSI using the ALADIN/LAND set-up to study the reaction 12C(p,2p)11B at 400 AMeV. Follow-up measurements with light neutron-rich and proton-rich nuclei in the Be-Ne region were recently made at GSI. Taken together with our measurements of transfer and one-nucleon removal reactions on the same nuclei at GANIL, we will then have a complete set of data to extract absolute spectroscopic factors and occupancies.

This research is carried out in collaboration with STFC Daresbury Laboratory, The Universities of Surrey, Paisley and Birmingham in the UK, GANIL, IPN-Orsay and LPC-Caen in France, and the R³B Collaboration. Theoretical support comes from the Surrey group.

1. W. N. Catford, C. N. Timis, R. C. Lemmon, M. Labiche, N. A. Orr, B. Fernández-Domínguez, R. Chapman, M. Freer, M. Chartier, H. Savajols, M. Rejmund, N. L. Achouri, N. Amzal, N. I. Ashwood, T. D. Baldwin, M. Burns, L. Caballero, J. M. Casadjian, N. Curtis, G. de France, W. Gelletly, X. Liang, S. D. Pain,V. P. E. Pucknell, B. Rubio, O. Sorlin, K. Spohr, Ch. Theisen, and D. D. Warner,
   Migration of nuclear shell gaps studied in the d(²⁴Ne,pγ)²⁵Ne reaction
   Physical Review Letters 104, 192501 (2010).
2. M. Labiche, W.N. Catford, R.C. Lemmon, C.N. Timis, N.A. Orr et al,
   TIARA: a large solid angle silicon array for direct reaction studies with radioactive beams
   Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 614, 439 (2010).
3. A.Ramus, N.L.Achouri, H.Al Falou, N.I.Ashwood, D.Beaumel, Y.Blumenfeld, S.M.Brown, W.N.Catford, R.Chapman, M.Chartier, N.Curtis, F.Delaunay, B.Fernandez-Dominguez, C.Force, G.De France, S.Franchoo, J.Guillot, D.Gupta, P.Haigh, F.Hammache, M.Labiche, V.Lapoux, R.C.Lemmon, F.Marechal, B.Martin, X.Mougeot, B.Mouginot, L.Nalpas, A.Navin, N.A.Orr, N.Patterson, B.Pietras, E.C.Pollacco, A.Le Prince, M.Rejmund, J.A.Scarpaci, N.De Sereville, I.Stefan, O.Sorlin, J.S.Thomas, G.L.Wilson,
   Study of the ²⁰O(d, t) reaction with the TIARA-MUST2-VAMOS-EXOGAM setup
   Int.J.Mod.Phys. E18, 2056 (2009).
4. B.Fernandez-Dominguez, R.C.Lemmon, C.Timis, M.Labiche, W.N.Catford, M.Chartier, N.I.Ashwood, N.Amzal, T.D.Baldwin, M.Burns, L.Caballero, R.Chapman, N.Curtis, G.de France, M.Freer, W.Gelletly, X.Liang, N.A.Orr, S.D.Pain, V.P.E.Pucknell, M.Rejmund, B.Rubio, H.Savajols, O.Sorlin, K.Spohr, C.Thiesen, D.D.Warner,
   Single particle structure of exotic nuclei with transfer reactions
   Progress in Particle and Nucear Physics 59, 389 (2007)
5. B. Pietras, PhD Thesis, University of Liverpool (2009). Spectroscopy of neutron-rich Oxygen and Fluorine Nuclei via single-neutron knockout reactions.
6. K. Mahata, S. Paschalis, T. Aumann, M. Chartier, B. Fernandez-Dominguez, M. Labiche, R. Lemmon, et al.,
   Proceedings of the International Conference on Frontiers in Nuclear Structure, Astrophysics, and Reactions, Crete, GREECE, 2007.
7. S. Paschalis, PhD Thesis, University of Liverpool (2008). Relativistic One-Nucleon Removal Reactions.