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Exploring Beta-Decay: Direct Transitions to Continuum States in Nuclei

This study investigates beta-decay processes, particularly focusing on direct transitions to continuum states in unstable nuclei like 6He and 11Li. It examines phenomena such as beta-delayed deuterons, the implications of resonance states in daughter nuclei, and the R-matrix formalism's application in modeling these complex decay patterns. Notable findings include new resonance states in 12C and the behavior of halo nuclei concerning direct decay mechanisms. Through experimental data from various collaborations, this work advances our understanding of nuclear decay and its potential connections to broader nuclear theories.

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Exploring Beta-Decay: Direct Transitions to Continuum States in Nuclei

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  1. Beta-decay directly to continuum K Riisager Dept. of Physics and Astronomy Aarhus University

  2. Beta-delayed deuterons • Qβd = 3007 keV – S2nJonson, Riisager, NPA693 (01) 77 • 6He, low branch several exp: Raabe et al,PRC80 (09) 054307 theory: directly to continuum states • 11Li, again pointing to direct transitions Raabe et al, PRL 101 (08) 212502

  3. Why continuum transitions ? • Alternative: through resonances in daughter • obvious for narrow peaks, not for broad features • may give “unphysical/unnatural” interpretation • I.e.: when do resonances “cover everything” ? • Non-resonant continuum states, cf. Berggren (NPA109 (68) 265 etc) Need to define (fit-)procedures carefully

  4. ..the world according to R-matrix • Inner and outer space • refs: Lane and Thomas, RMP 30 (58) 257 • Descouvemont and Baye, RPP 73 (10) 036301 • Complete basis internally • can describe direct reactions • “no scattering” Wigner, Eisenbud, PR72 (47) 29 R-matrix levels ≠ resonances • Adapted to β-decay by Barker • employed here for A=8,12 www.am.qub.ac.uk 8Be 2+ resonance at 3 MeV plays a key role

  5. Example: 8B decay • Decade-long discussion on interpretation, e.g. Barker, Aust.J.Phys. 42 (89) 25 - Bhattacharya, Adelberger, PRC65 (02) 055502 • “Intruder” 2+ below/above 16 MeV doublet ? • New data from JYFL (and KVI) Poster: T. Roger

  6. 8B fits – preliminary results 1 16 MeV 3 MeV Spectrum corrected for phase space and penetrability

  7. 8B fits – preliminary results 2 Fits with 3 MeV resonance, the 16 MeV doublet and one extra 2+ level (not yet satisfactory description).

  8. Example: 12N and 12B decay 12N 12B Data from JYFL and KVI 12C 8Be 2a

  9. Results  12N 12B Phys. Lett. B 678 (2009) 459

  10. Models including max. three unbound states Sum spectra (KVI) 12N components (JYFL) c2/df 281 15.9 44.8 3.24 6.06 18.4 12N 8Be peak 8Be excited states 12B

  11. Four unbound states Two 0+, two 2+ c2/df = 1.65 Three 0+, one 2+ c2/df = 1.24

  12. JYFL data, Dalitz plots Detailed analysis: at 10.5-11.7 MeV 2+/tot ≈ 0.3 above 12.7 MeV 2+ dominates C.Aa. Diget et al, PRC80 (09) 034316

  13. Three 0+, two 2+ states c2/df = 1.21 3 0+, 1 2+ 2 0+, 2 2+ Phys. Rev. C 81, 024303 (2010)

  14. A=12 summary • Two (new) resonances in 12C: • 0+ at 11.2(3) MeV, Γ = 1.5(6) MeV • 2+ at 11.1(3) MeV, Γ = 1.4(4) MeV • Higher lying 0+ and 2+ strength • position depends on channel radius • width/BGT values unrealistic (100 MeV/50, 1 MeV/5 – only room for BGT of 1 from sum rule for T=0) • Direct decay to continuum ?! • acceptable fits with “R-matrix continuum” BGT = 0.6

  15. A (very) simple model • Beta-decay gives Oβ|i> -- a Gaussian/Yukawa • Final two-body state with no interaction, i.e. by construction: decay only to continuum • Fits: “normal” resonances…. E/ħω E/Sn

  16. Final comments • Not a new discovery (reaction exp, low+high E, radiative capture) • Most likely (?) not just light nuclei • Part of the GTGR ? ! (for some nuclei) • Pronounced effects for halo nuclei F + GT • Technically more complex calculations (? due to coexistence with decays to resonances)

  17. Many thanks to: collaborators in experiments at ISOLDE JYFL KVI my close coworkers Hans Fynbo Solveig Hyldegaard ** Aksel Jensen Oliver Kirsebom Special acknowledgment to Fred Barker

  18. R-matrix Phys. Rev. C 81, 024303 (2010)

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