Breakout from the hot CNO cycle via the18Ne(α,p)21Nareaction. II. Extend

Abstract

We have extended the experiment reported in Bradfield-Smith et al. ͓Phys. Rev. C 59, 3402 ͑1999͔͒ concerning the 18 Ne(␣,p) 21 Na reaction, which is of importance to nuclear astrophysics. This study covered an energy region ϳ1.7-2.9 MeV in the center of mass frame of the 18 Neϩ␣ system. A change in the detector geometry resulted in an increase in the detection efficiency and significantly reduced the proton background that hindered the previous measurement. A direct measurement of the energy loss of the 18 Ne beam, as it passed through He gas, was undertaken to reduce a major source of uncertainty in the determination of the stellar reaction rate. Eight states have been identified in the compound nucleus, 22 Mg. These resonances were used to calculate an enhanced stellar reaction rate which shows good agreement with theoretical predictions ͓Görres, Wiescher, and Thielemann, Phys. Rev. C 51, 392 ͑1995͔͒ at and above a temperature of 1.5 GK. The impact of this enhanced stellar reaction rate upon the nucleosynthesis that occurs during the peak phase of an x-ray burst has been investigated.

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

“…2 and 3). The origin of these large discrepancies is not clear but we note that the resonance widths used in [4,18] imply unphysically large alpha-decay partial widths. Similarly, the rate in [9] is subject to large uncertainties in the choice of resonance strength values, which are not determined experimentally.…”

“…As shown in Figure 3, our reaction rate agrees well with the overall energy dependence of the HF gs rate but is typically a factor of ∼2-3 lower in the whole temperature region (T=1.0-2.4 GK) that corresponds to our measured energy range. However, a comparison with the rate from the direct measurement of Groombridge et al [4] reveals significant discrep- ancies both in energy dependence and magnitude, with a disagreement of up to a factor of ≃ 25 at T=2.4GK. Our rate is also between a factor of about 10-40 lower than the rate deduced using an indirect approach by Matić et al [9] for 0.9 GK≤T≤2.4 GK.…”

“…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.…”

“…In addition, the Rutherford elastic scattering of 21 Ne off 12 C at E lab =5.357 MeV/A was used as a fourth calibration point for the CD quadrants only. Good events, corresponding to 4 He and 18 Ne ions detected in coincidence, were extracted from the raw data by imposing appropriate conditions of two-body co-planarity, identification of 4 He and 18 Ne kinematics loci, and total energy reconstruction. An example of the measured and simulated α-particle kinematics loci for the 21 Na(p,α) 18 Ne reaction at E beam = 5.476 MeV/A is shown in Figure 1.…”

“…Heavy ions and α particles were detected in coincidence using an array of Double Sided Silicon Strip Detectors (DSSSDs). Detection of 4 He ions was primarily achieved with a ∆E-E telescope consisting of two MSL [11] type S2 detectors (65 µm and 500 µm thick, respectively, each segmented into 48 annular front strips and 16 rear sectors) covering a laboratory angular range of θ α = 7 o − 19 o (the maximum emission angle for α particles was less than 20…”