Breakout from the Hot-CNO cycle in X-Ray Bursters via the 18 Ne(α,p) reaction International Workshop on Physics of Nuclei at Extremes 26 January 2010 Adam

Breakout from the Hot-CNO cycle in X-Ray Bursters via the 18Ne(α,p) reaction

International Workshop on Physics of Nuclei at Extremes 26 January 2010Adam Tuff, Postgraduate student, Nuclear Physics Group

Department of Physics, the University of YorkYork, United Kingdom


The X-Ray Burster Scenario: High energy stellar scenarios: Red giant/Neutron star binary systems, mass accretion and Thermo-Nuclear Runaway (TNR).

Hot-CNO Cycles & Breakout: Mechanisms through which the HCNO (Hot Carbon-Nitrogen-Oxygen) cycles proceed, competing reactions, breakout paths, Rapid-Proton the (rp)-process and the connotations for energy generation.

18Ne(α,p) 21Na reaction Astrophysical importance, experimental difficulties, using inverse kinematics and radioactive beams and an insight to experimental work at the TRIUMF facility, Vancouver.


The 18Ne(α,p) 21Na reaction is currently considered to be a crucial process governing breakout conditions from the Hot-CNO cycle in X-Ray Bursters (XRBs) under certain stellar conditions – bypasses 15O(α,γ) 19Ne.

Not only dictates further nucleosynthesis from the rp-process in XRB sites as well as energy generation in these environments.

Energy generation in these explosive nuclear scenarios from rp-process versus energy generation from Hot-CNO cycle unknown – reaction cross-sections, resonances, resonance widths & levels in these energy regions still relatively unknown.


A binary star system - comprised of large red giant and a neutron star remnant (figure).

The red giant fills Roche lobe - matter falls into the neutron star accretion disk before surface.

Matter in this region is electron degenerate – temperatures increase as more H and He deposits on surface: leads to thermal instabilities and runaway reactions at above T9 > 0.5 (5x108°K) [2].

Hot CNO cycle leads into rp-process at T9 ≥ 1 – “heavy” element synthesis.


Hot-CNO cycle15O(α,γ)19Ne Breakout

Main generation in Novae and XRBs dominated by Hot-CNO cycle (example in red) for T9 < 0.4 [3].

Nucleosynthesis dictated by “waiting points” – particle capture unfeasible - “wait” for β+-decay.

Competition between (p,γ), (α,p), (α,γ) and (β+ν) reactions.

Primary breakout thought to be through 15O(α,γ)19Ne path (blue) as τ1/2 = 122.2s (T9 > 0.5) [3].


Hot-CNO cycle15O(α,γ)19Ne Breakout18Ne(α,p)21Na Breakout

For T9 > 0.8 (TNR scenario), more energetic α’s – reaction rates increase.18Ne(α,p)21Na bypasses the 15O(α,γ)19Ne reaction (green) – alters nuclear abundance.

Energy generation proceeds through rp-process; proton captures / β+-decays to heavier “proton rich” isotopes.


Figure (a) shows theoretical variation of reaction rate over temperature [8].

Theory predicts change in the 18Ne(α,p) cross-section by nearly four orders of magnitude from between T9 = 0.5 - 1.

Very Sensitive to temperature over this range as well as density (figure (b)).

Figure shows temperature at which competing beta-decay reaction rate matches the corresponding waiting point reaction.


Theoretical reaction rate as function of temperature (normalised to 16O(p,γ)) [8].


Want reaction cross-sections, widths, spin, parity of key resonances.

Hard to produce 18Ne beams of sufficient intensity – half-life (1.67s). Higher energy beams can currently only thoroughly probe energy regions above 10.5MeV/u [Görres et Al., 1995] – corresponds to T9 ≈ 1.5.

Radioactive targets not suitable or practical.Previous work by Bradfield-Smith et al., Sinha et al., Groombridge et al. using direct and time-reversed reactions are in conflict [4], [5], [6].

Energy levels in the “Astrophysically important” region above 8.14MeV (the α-particle threshold) in 22Mg have been identified, however key resonances and their widths, spins etc... have not.


Data from Groombridge et al., Bradfield-Smith et al., and Sinha et al [6].

Cross-sections can be seen to vary by a factor of 50 at 2.5MeV/u in c.m. energy.

Cross-sections taken for regions above the threshold – only extrapolations down to region of interest.


Proton scattering on 21Na forms a 21Na+p system. (22Mg proton threshold = 5.502MeV).

Exptl. spectrum equivalent to excitations in normal kinematics.

Level scheme by Chen et al. [7]

Polyethylene target (CH2)n – proton rich.

Resonances in energy range through target (range from dE/dx): cross-section of detected protons increases: beam does the “stepping”. 5.5

021Na + p


- 21Na interacts with target – scatters proton elastically/inelastically.- Proton detected in segmented ∆E-E detector: get trajectory & energy.- 21Na* γ-decays: TIGRESS detects 332keV γ’s.

HPGe Detector

Si ∆E-E Detector

(CH2)n Target

Beam


CAD design by F Cifarelli of TRIUMF, Vancouver. Image 2 courtesy of Greg Hackman and Douglas Cline.


TIGRESS cluster and BAMBINO target ladder photos by Adam Tuff (UoY) and Greg Hackman (TRIUMF).

Gamma-ray spectra after gating on BGOs, Timestamps, and coincidences. .

Raw spectra after Doppler corrections.

BGO events reject any scattered γ-ray events. Time stamp on particle and γ-ray events included.

Addback now included for single and multiple events. Si ring & sector gates on proton events now accounted allows further b/g reduction.



Inelastic proton spectra (c.m. energy) – run at 3.1MeV/u. . Peak here created by a

large gate on the γ-ray spectra on the 332keV de-excitation.

Tighter gating will allow clean-up of this spectra.

Features will hopefully be more obvious with addition of more data (data taken from 30 minutes of a run).


Elastic & inelastic proton spectra (c.m. energy) – run at 2.85MeV/u. .

Software gates allow isolation of events in coincidence with 332keV gamma-ray event.

Using thick target allows broad scan of energies (scan narrow resonances with thin target).


Confirm gain matching parameters, adjust energies for dead-layer effects. Attainment of comprehensive values for resonance widths, reaction cross-sections, as well as identification of key resonances and associated branching ratios is expected.

REFERENCES & BIBLIOGRAPHY[1] http://www.jca.umbc.edu/press/press.shtml, 27/03/2009[2] C Ruiz, Aspects of Nuclear Phenomena Under Explosive Astrophysical Conditions, PhD thesis, University of Edinburgh, 2003,

p73. [3] AA Chen, Structure of 22Mg and its implications for explosive nucleosynthesis, Physical Review C 63, 065807-1, 2001.[4] Bradfield-Smith, Breakout from the hot CNO cycle via the 18Ne(α,p)21Na reaction, Phys. Rev. C 59, 3402 – 3409, 1999.[5] D Groombridge, Breakout from the Hot-CNO Cycle Via the 18Ne(α,p)21Ne Reaction, Physical Review 66 C 66, 055802, 2002.[6] S Sinha et al., ANL Physics Division Annual Report 04/22 section a.3, 2004.[7] AA Chen, Structure of 22Mg and its implications for explosive nucleosynthesis, Physical Review C 63, 065807-6, 2001.[8] C Iliadis, Nuclear Physics of Stars, Wiley-VCH, Weinheim, 2007.

Many thanks to the Tokyo Institute of Technology, the FGIP and to my host Takashi Nakamura.

http://www.jca.umbc.edu/press/press.shtml


(1) Department of Physics, the University of York, Heslington, York, YO10 5DD, UK(2) TRIUMF, 4004 Westbrook Mall, Vancouver, British Columbia, V6T 2A3, Canada

(3) Department of Astronomy and Physics, Saint Mary's University, Halifax Nova Scotia, B3H 3C3, Canada(4) The School of Physics and Astronomy, The University of Edinburgh, James Clerk Maxwell Building, King's Buildings,

Mayfield Road, Edinburgh, EH9 3JZ. UK(5) Department of Physics, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

(6) Lawrence Livermore National Laboratory , 7000 East Avenue, Livermore, CA 94550, California, USA(7) Department of Physics & Astronomy, University of British Columbia , 6224 Agricultural Road, Vancouver, British

Columbia, V6T 1Z1, Canada(8) Department of Physics, University of Toronto, 60 St. George St., Toronto, M5S 1A7, Ontario, Canada

(9) Department of Physics, University of Liverpool, Liverpool, L69 7ZE, UK(10) The School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK

A.G. Tuff (1), D.G. Jenkins(1), A.P. Robinson(10), C.Aa. Diget(1), O.J. Roberts(1) G. Hackman(2) , R.A.E. Austin(3), K. Chipps(1), S. Fox(1), T. Davinson(4), P.J. Woods(4), G.

Lotay(4), C. Pearson(2), A. Shotter(2), M. Schumaker(5), A.B. Garnsworthy(2), C. Svennson(5), C.Y. Wu(6), ), S. Williams(2), N. Orce(2), D.S. Cross(2), R. Kshetri(2), N. Galinski(7), T.E. Drake(8), R. Kanungo(3), S.J. Rigby(9), M. Jones(9), H. al Falou(3), C.

Sumithrarachi(5), S. Triambak(2), G. Ball(2)


Doppler/non-Doppler corrected γ-ray spectra. .

511keV annihilation events.

332keV peak, non-Doppler corrected – spread across about 40keV.

332keV peak, Doppler corrected to give sharper peak.


A) 4He B) 2H C) Elastic/Inelastic proton D) 511keV random coincidence


S2 and S3 Ring and Segment IDs for Proton events in BAMBINO (E-∆E Si)


S2 and S3 Hit patterns in BAMBINO – Raw events

Central ring at ≈ 16° - useful “visual” tool for angular distributions / check for beam on target – easily modified to look at inelastic events.


Above the CNO cycles lie the NeNa and MgAl cycles.

Breakout from these conditions into the rp-process depend on a) stellar temperatures and b) hydrogen densities in the environment.

Breakout from 19Ne(p,γ)20Na allows bypassing of NeNa cycle via 20Na(p,γ)21Mg – as it can compete with β-decay.

Main importance of study of rp-process is nucleosynthesis. In our case, rp-process is not important in seeding of the ISM – more important as a competing process to HCNO in energy generation in XRBs.


Documents

Breakout from the Hot-CNO cycle in X-Ray Bursters via the 18 Ne(α,p) reaction International Workshop on Physics of Nuclei at Extremes 26 January 2010 Adam