No abstract

No 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 ...

An experiment to measure the 18 F(p,α) 15 O cross section at low centre of mass energies has been performed with the TRIUMF-UK Detector Array (TUDA) facility at TRIUMF. Measurements of the 18 F(p,α) 15 O reaction rate at four separate centre of mass energies; 665 keV, 430 keV, 330 keV and 250 keV, were carried out. The main goal of the experiment was the measurement of the 18 F(p,α) 15 O cross section between 250 keV and 300 keV to help constrain the contribution to the reaction rate from nearby states and determine the effect of interference on the overall 18 F(p,α) 15 O reaction rate from the proposed low lying 3/2 + states with the higher 664.7 keV 3/2 + state. A high intensity 18 F beam (∼5x10 6 pps) was used to bombard a 31.6±1.9 µg.cm −2 polyethylene target within the TUDA scattering chamber at the four centre of mass energies. Four highly segmented silicon strip detector arrays were used for coincident detection of the reaction products. Preliminary results will be presented.10th Symposium on Nuclei in the Cosmos