Measurement of theNe18(α,p0)Na21Reaction Cross Section in the

Abstract

The18 Ne(α,p) 21 Na reaction provides one of the main HCNO-breakout routes into the rp-process in X-ray bursts. The 18 Ne(α,p0) 21 Na reaction cross section has been determined for the first time in the Gamow energy region for peak temperatures T∼2GK by measuring its time-reversal reaction 21 Na(p,α) 18 Ne in inverse kinematics. The astrophysical rate for ground-state to ground-state transitions was found to be a factor of 2 lower than Hauser-Feshbach theoretical predictions. Our reduced rate will affect the physical conditions under which breakout from the HCNO cycles occurs via the 18 Ne(α,p) 21 Na reaction.PACS numbers: 25.60. 26.30.Ca, 25.45.Hi Type I X-ray bursters (XRBs) exhibit brief recurrent bursts of intense X-ray emission and represent a frequent phenomenon in our Galaxy. Recent observations from space-borne X-ray satellites (BeppoSAX, RXTE, Chandra and XMM Newton) have provided a great wealth of data and have marked a new era in X-ray astronomy. Yet, to fully exploit these observations, similar advances in our understanding of the nuclear reactions responsible for the bursts are required. XRBs are driven by a thermonuclear runaway on the surface of a neutron star that accretes H-and He-rich material from a less evolved companion in a semi-detached binary system [1]. Depending on the mass accretion rate, high enough temperatures and densities can be achieved that trigger hydrogen burning through the hot, β-limited CNO cycles (HCNO) and the subsequent ignition of the triple-α process. However, the thermonuclear runaway requires a breakout from the HCNO cycle and the ignition of the rapid-proton capture process (rp-process) at peak temperatures T≃ 1-2 GK. The 18 Ne(α,p) 21 Na reaction is believed to provide the main breakout route at T≥0.8GK and ρ ≥10 5 g/cm 3 [2], but the actual physical conditions at which the breakout occurs depend critically on the accurate knowledge of the 18 Ne(α,p) 21 Na reaction rate. A direct investigation of this important reaction is severely hampered by the low intensity (≤ 10 6 pps) of radioactive 18 Ne beams presently available and by the further complications associated with the use of a 4 He gas target. Thus, the only two direct measurements available to date extend to minimum energies of E cm = 2.0 MeV [3] and E cm = 1.7 MeV [4]. These are still too high compared to the energy region E cm ≤1.5 MeV of interest for HCNO breakout in X-ray bursts.The first theoretical estimates of the 18 Ne(α,p) 21 Na reaction rate [5] were based on sparse experimental information on the level structure of the compound nucleus 22 Mg above the α-particle threshold at 8.14 MeV. Inspection of the structure of the mirror nucleus . Yet, a comparison of all the reaction rates currently available shows discrepancies of up to several orders of magnitude both below and above T ∼1GK (see [9] and references therein). In addition, it remains unclear whether the Hauser-Feshbach statistical calculations provide a reliable estimate of the 18 Ne(α,p) 21 Na rate in the whole temperature region relevant ...

Proton decays from $α$-unbound states in $^{22}$Mg and the $^{18}$Ne($α, p_{0}$)$^{21}$Na cross section

“…This rate used the RatesMC Monte Carlo code [32,33] to estimate the reaction rate with realistic uncertainties. The rate which resulted from these calculations was somewhat reduced compared to the earlier evaluation of Mohr and Matic [29], but in better agreement with the rate derived from the time-reversed experiment of Salter et al [20]. The calculation of the reaction rate also resulted in a relatively well-constrained temperature (0.60(2) GK) at which the 18 Ne(α, p) 21 Na reaction becomes faster than the β + decay of 18 Ne.…”

“…The 18 Ne(α, p) 21 Na reaction, which influences the shape of the rise of the X-ray burst lightcurve [8], has been the focus of extensive study. This includes timereversed measurements of the 21 Na(p, α 0 ) 18 Ne cross section [20] (which will be discussed in more detail in the following paragraphs), direct measurements of the cross section using beams of radioactive 18 Ne ions and gaseous 4 He targets [21][22][23], transfer spectroscopy of 22 Mg states using the 24 Mg(p, t) 22 Mg reaction [24,25], and resonance-scattering 21 Na(p, p) 21 Na measurements in inverse kinematics [26,27].…”

“…Of particular note for the currently reported measurement is the use of time-reversed measurements. Due to the technical difficulty of measuring the 18 Ne(α, p) 21 Na direct reaction requiring both an intense radioactive 18 Ne beam and a helium gas target, Salter et al [20] measured the time-reversed 21 Na(p, α) 18 Ne cross section. This has the advantage of allowing a solid hydrogen-containing target (e.g.…”

“…Mohr and Matic [29] compiled a detailed and valuable summary of existing data related to the 18 Ne(α, p) 21 Na reaction. They make detailed comparisons between three sets of data: the direct measurements of Groombridge et al and Bradfield-Smith et al [21,22], the time-reversed measurement of Salter et al [20] and Sinha et al, a time-reversed measurement only available in an internal report from Argonne National Laboratory [30], and the transfer measurements of Matic et al [24], and Chae et al [25]. They conclude that the cross sections from the direct measurements are problematically high, exhausting or exceeding the theoretical strengths derived by considering the states observed in the transfer measurements, and the cross section from the time-reversed experiment.…”

“…The proton decays from the populated excited 22 Mg states were observed and, with this information, the contribu- tion of proton decays to excited states in 21 Na to the 18 Ne(α, p) 21 Na reaction rate was reassessed. In light of the obtained energy-dependent branching ratio, we reestimate the 18 Ne(α, p) 21 Na reaction rate based on the time-reversed measurement of Salter et al [20] and compare it to other evaluations of the reaction rate based on spectroscopic information on 22 Mg.…”