Study of the Halo Nucleus 6He using the 6Li(g,p+)6He 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. 6Li(g,p+)6He, 11B(g,p+)11Be and 17O(g,p-)17F). 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 6Li(g,p+)6He reaction the g.s. wf of 6Li is well known from electron scattering measurements. Also at a gamma energy of ~200MeV the outgoing p+-particles are sufficiently low in energy to be reasonably well described by plane waves (small FSI).
DWBA Results Compared to Published 6Li(g,p+)6He Reaction Data DWBA calculations made for the 6Li(g,p+)6He reaction compared to the Bremsstrahlung end-point data of Shaw et al. Phys. Rev. C43 1800 (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 6Li(g,p+)6He Reaction Calculations of Karatiglides at Eg = 170 MeV also show larger cross section at large angles due to the halo. (Private Communication 1998). Confirmed at Eg = 200 MeVby 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 6Li(g,p+)6He 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 1st excited (Jp = 2+) state of 6He is at 1.8 MeV. • Hence, we used a Tagging Microscope in addition to the Main Tagger to cover the range 170 -220 MeV. Resolution ~500 keV. • To detect the p+-particles we required high resolution, large solid angle detectors. • Decided to build a new array of Ge detectors (Ge6-Array) for detecting • p+-particles.
Schematic of Geometry Used to Study the Halo Nucleus 6He using the 6Li(g,p+)6He 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 ms p+-particle identification: DE vs E bananas Afterpulses m+ -> e+ 2.2 ms
Tests with p+ beams at PSI _________________ MeV Resolution equal to p+ beam resolution of ~1 MeV
Ge6-Array_Run@Mainz Run period was 6 days on 6Li(g,p+)6He Calibrations and X-section normalisation based on p(g,p+)n reaction Strip detectors were all BB2s Main tagger resolution ~2 MeV Tagger microscope ~170 -220 MeV had nominal resolution ~400 keV Time resolution ~4 ns Ge resolution measured with gs 15 keV Edinburgh 30 keV Mainz
Particle Identification and Position Measurements using DSSSDs x – y distribution on Ge frontface
Pion Calibrations These were obtained by analyzing the p(g,p+)n data obtained with the CH2 target.
Missing Energy Spectra for p(g,p+)n Reaction Resolution with Main Tagger ~ 2 MeV Resolution with Tagging Microscope ~1.4 MeV ?
Missing Energy Results from the 6Li(g,p+)6HeMeasurement Average Eg = 200 MeV Qp = 100OQp=150O
Angular Distribution Results for the 6Li(g,p+)6He Reaction at Eg = 200 Mev Blue – Present Data Red – Shaw et al. Phys. Rev. C43 1800 (1991) Green – Shoda et al. Phys. Lett. B101 124 (1981) Solid Black Line -- Young Surrey PhD Thesis (2004) Assumes a Halo!!!
Angular Distribution for the 6He 2+ Excited State at 1.8 MeV Red squares – Present data Black triangles – Shoda et al. Phys. Lett. B101 124 (1981) CK Curve – Cohen and Kurath Calcn. presented in Shoda et al. Sask C Curve – Bergstrom Phys. C21 2496 (1980) Large angle cross sections comparable to g.s. results Suggests 1st 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 6Li(g,p+)6He at MAX-lab, Lund Sweden (2005/2006). • New calculations from Surrey University for the g.s. and 1st excited state. • Study of the 11B(g,p+)11Be reaction to investigate the halo nucleus 11Be.
Estimation of Losses due to Hadronic Interactions in the Ge Deduced from the p(g,p+)n Results These results were used when comparing data from the Li target to those from the CH2 target