Nuclear Physics A 834 (2010) 679c–681c www.elsevier.com/locate/nuclphysa
α-Induced Reaction Rates for Accreting Compact Stars W. P. Tana ∗ , S. Almaraza , A. Aprahamiana , B. Buchera , J. G¨orresa , H. Macha , M. Wieschera , C. Bruneb Z. Heinenb , and T. Masseyb a
Department of Physics, University of Notre Dame, Notre Dame, Indiana, USA
b
Department of Physics and Astronomy, Ohio University, Athens, Ohio, USA
Studies have shown that alpha-induced nuclear reactions in the hot CNO cycles and on the waiting points of the rp-process play a principal role in energy production and nucleosynthesis on the surface of mass-accreting white dwarfs and neutron stars. This paper will present recent measurements at Notre Dame on the rate of 15 O(α,γ) and new experiments for (α,p) reactions on the waiting point nuclei of 14 O and 26 Si. Preliminary results and astrophysical implications will be discussed. 1. Introduction Explosive phenomena on the surface of accreting compact stars, such as novae and X-ray bursts, are understood as thermonuclear explosions[1,2]. In such nuclear burning processes the two waiting points of 14 O and 15 O in the hot CNO cycle play a critical role in regulating the flow between the hot CNO cycle and the rp-process. When the temperature and density are high enough, the α-capture reactions will overwhelm the β + decays of 14 O and 15 O, leading to the breakout from the hot CNO cycle and the subsequent energetic burning along the rp-process as shown in Fig. 1. Well measured reaction rates of 15 O(α, γ) and 14 O(α,p) will help us understand the trigger conditions of such explosive burning. In the early phase of the rp-process burning, the reaction flow could also be impeded by β + -unstable even-even waiting point nuclei on the rp-process, such as 22 Mg, 26 Si, 30 S, and 34 Ar. The (α,p) reaction rates of these nuclei, which are very sensitive to the temperature, could have significant impact in the early burning process, such as on the light curves of X-ray bursts [3]. 2.
15
O(α, γ)
Direct measurement of the 15 O(α, γ) rate has not been possible yet due to the insufficient O beam intensities presently available. Measuring the properties of the resonance levels just above the α-decay threshold in 19 Ne that dominate the rate, including energies, spins, and partial decay widths, is therefore the focus for the study of the 15 O(α, γ) rate. The first successful measurement on the γ widths or lifetimes of the states in 19 Ne near α-threshold 15
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Supported by the National Science Foundation under grant No. PHY01-40324 and the Joint Institute for Nuclear Astrophysics, NSF-PFC under grant No. PHY02-16783. 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.123
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W.P. Tan et al. / Nuclear Physics A 834 (2010) 679c–681c
C (6)
3 4 5 6 7 8 3α process
N
Figure 1. Nuclear reaction flow chart in the region of the CNO cycle and along the αpprocess path.
15O(α,γ)19Ne Breakout Domination
Density (g/cm3)
Z
K (19) Ar (18) Cl (17) 21 22 S (16) P (15) 17 18 19 20 Si (14) Al (13) 15 16 Mg (12) Na (11) 13 14 Ne (10) F (9) 11 12 hot CNO cycle O (8) N (7) 9 10
XRB Novae
15O β-decay
Domination
0.2
0.3
0.5 0.7 T (GK)
1
2
Figure 2. Experimental rate of 15 O(α, γ) (shaded band) in the temperature-density diagram under the astrophysical scenario.
was done with the improved Doppler shift attenuation method at Notre Dame [4]. Similar results were later obtained independently at TRIUMF [5]. However, the α-decay widths or branching ratios of these levels, in particular the one at 4.03 MeV, are more difficult to measure. Many attempts have been made in the past to measure the relative α-decay widths directly [6,7]. But the only successful laboratory measurement of the α decay branching ratios for the near threshold states was done at Notre Dame[8]. An experimental rate for 15 O(α,γ)19 Ne[8] is then proposed as demonstrated in Fig. 2 under the scenario of astrophysical environments. 3.
14 14
O(α,p) and
26
Si(α,p)
O(α,p), the first reaction link of the (α,p) sequence leading to one of the breakout routes from the CNO cycle to rp-process, has been studied with various approaches. The time-inverse 17 F(p,α)14 O reaction has been studied at Argonne National Laboratory[9] and Oak Ridge National Laboratory (ORNL)[10]. The 14 O(α,p1 )17 F* branch was also measured using inelastic scattering 17 F(p,p’) at CERN and ORNL[10]. Even the direct measurement of the 14 O(α,p) rate has already been conducted at RIKEN[11]. However, there are significant discrepancies between the results of these approaches. Recent result using 14 C(6,7 Li,2,3 H) reactions and the ANC approach[12] differs significantly from the direct measurement. An independent approach is clearly needed to clarify the issue. 26 Si(α,p), one link of the (α,p) reaction sequence on the rp-process waiting points, has been little studied. No direct measurement is possible yet at current radioactive beam facilities. The levels adopted in the compilation for 30 S are just slightly above proton decay threshold. Recently updated level scheme up to about 7 MeV [13] is still well below the α decay threshold of about 9.3 MeV. We studied both reactions at the same time using bunched 15 MeV 3 He beam produced at the FN tandem accelerator of the University of Notre Dame and bombarded on the SiO and Si targets, respectively. The relevant excited states in 18 Ne and 30 S were populated
W.P. Tan et al. / Nuclear Physics A 834 (2010) 679c–681c
Figure 3. Preliminary neutron TOF spectra for reactions of O(3 He,n) after background subtraction (left plot).
28
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Si(3 He,n) (right plot) and
16
via (3 He,n). An array of liquid scintillator detectors was positioned in the forward angles between 11-38 degrees to detect neutrons with time-of-flight (TOF) and pulse-shapediscrimination techniques. A Silicon detector array was used in the backward angles to detect the decaying protons and alphas in coincidence with neutrons. Fig. 3 shows the preliminary neutron TOF spectra in one typical neutron detector for the 28 Si(3 He,n) reaction (right plot) and 16 O(3 He,n) reaction after background subtraction (left plot). Proton decays from 18 Ne* and 30 S* to the ground and excited states of 17 F and 29 P are observed as well as two proton decays and alpha decays. Further data analysis is ongoing to extract the decay branching ratios, in particular the decay channels to the first excited state in 17 F, in order to solve the discrepancies in previous measurements for the 14 O(α,p) rate. With observed new levels in 30 S we hope to provide a first experimental constraint on the 26 Si(α,p) rate. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
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