The astrophysical rate of O(α,γ)Ne via recoil-decay tagging and its implications for nova nucleosynthesis

The O(α,γ)Ne reaction is one of two known routes for breakout from the hot CNO cycles into the rp process. Its astrophysical rate depends on the decay properties of excited states in Ne lying just above the O + α threshold. We have measured the α-decay branching ratios for these states using the p(Ne,t)Ne reaction at 43 MeV/u. Combining our branching ratio measurements with previous determinations of the radiative widths of these states, we calculate the astrophysical rate of O(α, γ)Ne. Using this reaction rate, we perform hydrodynamic calculations of nova outbursts and conclude that no significant breakout from the hot CNO cycles into the rp process occurs in classical novae via O(α, γ)Ne. Novae are thermonuclear runaways powered by the accretion of hydrogenrich material from a stellar companion onto the surface of a white dwarf in a close binary system. Energy production and nucleosynthesis in such stellar sites are in general determined by the CNO cycles, with contributions from the NeNa and MgAl cycles [1]. The possibility of breakout from the hot CNO cycles into the rp process has been extensively discussed in previous work, in particular under the high temperature and density conditions attained in the most massive ONe nova outbursts. Several reactions have been suggested as pathways for this breakout [2], but only two are currently thought to be possibilities: O(α,γ)Ne and Ne(α,p)Na. In astrophysical environments the O(α,γ)Ne reaction proceeds predominantly through resonances lying just above the O + α threshold at 3.529 MeV in Ne. For nova conditions in particular, the reaction rate is determined by the α width Γα of the 4.033 MeV, 3/2 state, owing both to its close proximity to the O + α threshold and its low centrifugal barrier to α capture. Direct measurements of the low energy cross section, which require highintensity radioactive O beams, are planned. For states in Ne lying at excitation energies relevant to novae and accreting neutron stars, only the αand γ-decay channels are open, as the proton and neutron separation energies are 6.4 and 11.6 MeV [3] respectively. Hence, by populating these states and observing the subsequent α and γ decays, one can deduce the branching ratio Bα ≡ Γα/Γ. If Γγ is also known, one can then calculate Γα and thereby the contribution of each state to the resonant rate of O(α,γ)Ne. Efforts of this kind to detect α particles from the decay of Ne states populated via transfer reactions were made [4, 5], but these experiments were insufficiently sensitive to measure Bα for the critical 4.033 MeV state, which was expected to be of order 10 [6]. In an experiment at the Kernfysisch Versneller Instituut [7], we have succeeded in obtaining branching ratio data at this level of sensitivity. Populating the important states via the Ne(p, t)Ne reaction in inverse kinematics with a Ne beam energy of 43 MeV/u, we detected either Ne recoils or their O αdecay products in coincidence with tritons in the Big-Bite Spectrometer (BBS) [8]. The large momentum acceptance of the BBS (∆p/p = 19%) allowed detection of either Ne recoils or O decay products along with tritons emitted backward in the center of mass system. Positioning the BBS at 0 maximized the yield to the 4.033 MeV, 3/2 state in Ne. This state, whose dominant shell-model configuration is (sd)(1p) [9], was selectively populated by an l = 0, two-neutron transfer from the 3/2 ground state of Ne. Position measurements in two vertical drift chambers (VDCs) [10] allowed reconstruction of the triton trajectories. Excitation energies of the Ne residues were determined from the kinetic energies and scattering angles of the triton ejectiles. The γ decays of states in Ne were observed as Ne-triton coincidences in the BBS, whereas α decays were identified from O-triton coincidences. Recoils and decay products were detected and stopped just in front of the VDCs by fast-plastic/slow-plastic phoswich detectors [11] that provided energy loss and total energy information. A separate array of phoswich detectors was used to identify tritons after they passed through the VDCs. Timing relative to the cyclotron radio frequency signal was also employed for unambiguous particle identification. An Al plate prevented many of the heavy ions copiously produced by projectile fragmentation reactions of the Ne beam in the (CH2)n target from reaching the VDCs. The spatial extent of the heavy-ion phoswich array was sufficient to guarantee 100% geometric efficiency for detection of Ne