Study of the Halo Nucleus 6 He using the 6 Li(   ) 6 He Reaction Derek Branford - Edinburgh University for the A2-Collaboration MAMI-B Mainz

Although most halo nucleus studies have been made using secondary beams of radioactive ions, it was noted quite early in their study that a few halos can be accessed by photonuclear reactions (e.g. 6 Li(   ) 6 He, 11 B(   ) 11 Be and 17 O(   ) 17 F). What are the advantages? The use of a photoproduction reaction provides an alternative method of studying halo nuclei using the very precise e/m probe. Initial (ISI) and final state interactions (FSI) are relatively small. Photopion reactions also allow excited state halos to be accessed, which is generally not possible using secondary beams of radioactive halo ions as they decay to their ground states before reaching the downstream target.

DWBA Models Experimental results are usually compared to DWBA calculations. From the theory it can be seen that the method is potentially very sensitive to the halo nucleon wavefunction. For the 6 Li(  + ) 6 He reaction the g.s. wf of 6 Li is well known from electron scattering measurements. Also at a gamma energy of ~200MeV the outgoing  + -particles are sufficiently low in energy to be reasonably well described by plane waves (small FSI).

DWBA Results Compared to Published 6 Li(   ) 6 He Reaction Data DWBA calculations made for the 6 Li  +  6 He reaction compared to the Bremsstrahlung end- point data of Shaw et al. Phys. Rev. C (1991) Note cross section label on this published figure should have read nb/sr. Solid line – Woods-Saxon wavefunctions (Halo). Dashed line – Harmonic Oscillator wavefunctions (No Halo).

More Recent DWBA Calculations for the 6 Li(   ) 6 He Reaction Calculations of Karatiglides at E  = 170 MeV also show larger cross section at large angles due to the halo. (Private Communication 1998). Confirmed at E  = 200 MeV by Young (Surrey PhD 2004) Conclusion – Halo gives rise to enhanced cross section at large angles!! Large angle cross section sensitive to halo nucleon(s) wavefunction.

The New Mainz Measurement of the 6 Li(  + ) 6 He Reaction at 200MeV The plan was to make a new measurement using tagged photons at the 850 MeV Mainz microtron MAMI-B. However, the tagger has a resolution of 2 Mev and the 1 st excited (J  = 2 + ) state of 6 He is at 1.8 MeV. Hence, we used a Tagging Microscope in addition to the Main Tagger to cover the range MeV. Resolution ~500 keV. To detect the  + -particles we required high resolution, large solid angle detectors. Decided to build a new array of Ge detectors (Ge6-Array) for detecting  + -particles.

The Ge6 - Array

Schematic of Geometry Used to Study the Halo Nucleus 6 He using the 6 Li(   ) 6 He Reaction Double Sided Strip Detectors: Tigre 10cm x 10cm 128 x128 strips BB2s 2.5cm x 2.5cm 24 x 24 strips Amplifiers: HpGe TFA Pulse length 400ns Strips Edinburgh RAL 2-3  s  + -particle identification:  E vs E bananas Afterpulses    e  s

Tests with  + beams at PSI Resolution equal to  + beam resolution of ~1 MeV MeV _________________

Run period was 6 days on 6 Li(  + ) 6 He Calibrations and X-section normalisation based on p(  + )n reaction Strip detectors were all BB2s Main tagger resolution ~2 MeV Tagger microscope ~ MeV had nominal resolution ~400 keV Time resolution ~4 ns Ge resolution measured with  s 15 keV Edinburgh 30 keV Mainz

Particle Identification and Position Measurements using DSSSDs x – y distribution on Ge frontface

 E vs E for Particles that Penetrate through Detector #1

Pion Calibrations These were obtained by analyzing the p(  + )n data obtained with the CH 2 target.

Missing Energy Spectra for p(  + )n Reaction Resolution with Main Tagger ~ 2 MeV Resolution with Tagging Microscope ~1.4 MeV ?

Missing Energy Results from the 6 Li(  + ) 6 He Measurement Average E   200 MeV  100 O  150 O

Angular Distribution Results for the 6 Li(  + ) 6 He Reaction at E  = 200 Mev Blue – Present Data Red – Shaw et al. Phys. Rev. C (1991) Green – Shoda et al. Phys. Lett. B (1981) Solid Black Line -- Young Surrey PhD Thesis (2004) Assumes a Halo!!!

Angular Distribution for the 6 He 2 + Excited State at 1.8 MeV Red squares – Present data Black triangles – Shoda et al. Phys. Lett. B (1981) CK Curve – Cohen and Kurath Calcn. presented in Shoda et al. Sask C Curve – Bergstrom Phys. C (1980 ) Large angle cross sections comparable to g.s. results Suggests 1 st excited state also a halo state.

Conclusions and Future Work Our backward angle cross section results approximately a factor of 5 larger than the 1 previous measurement of Shaw et al. Would like to assume new results more reliable. Experimental results larger than present theory. Could imply (i) Halo more pronounced than thought, or (ii) Theory needs to be improved (e.g. to include two body currents). Plans for the future include a new high statistics, higher resolution measurement of 6 Li(   ) 6 He at MAX-lab, Lund Sweden (2005/2006). New calculations from Surrey University for the g.s. and 1 st excited state. Study of the 11 B(  + ) 11 Be reaction to investigate the halo nucleus 11 Be.

Estimation of Losses due to Hadronic Interactions in the Ge Deduced from the p(  + )n Results These results were used when comparing data from the Li target to those from the CH 2 target

Cluster Models of 6 He