Isγ-Ray Emission from Novae Affected by Interference Effects in theF18(p,α)

Abstract

The 18 F(p,α) 15 O reaction rate is crucial for constraining model predictions of the γ-ray observable radioisotope 18 F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, 19 Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between J π =3/2 + states near the proton threshold (Sp = 6.411 MeV) and a broad J π =3/2 + state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the 19 F( 3 He,t) 19 Ne reaction. In stark contrast with previous assumptions we find at least 3 resonances between the proton threshold and Ecm=50 keV, all with different angular distributions. None of these are consistent with J π = 3/2 + angular distributions. We find that the main uncertainty now arises from the unknown proton-width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects 18 F production by at least a factor of two in the model considered.PACS numbers: 26.50.+x, 26.30.Ca, 25.55.Kr Novae occur in binary systems where hydrogen-rich material is accreted from a companion star onto a white dwarf, leading to thermonuclear runaway and subsequent ejection of material. Their ejecta is thought to be the main source of 13 C, 15 N and 17 O in the Galaxy [1,2]. The relevant unstable nuclei are accessible to experiments, and consequently, novae are the only explosive environment where the nuclear physics input is almost entirely based on experimental data [3].However, there are a number of outstanding challenges in our understanding of nova explosions [4], one of which is to reproduce the amount of ejected material inferred from infrared and radio observations, which is systematically underestimated by models. An independent way to constrain the ejected masses would be the detection of γ-rays, produced at the explosion stage. When the envelope becomes optically thin, novae are expected to emit γ-rays, dominated by a prominent 511 keV line. Predicted detectability distances of this prompt γ-ray emission (about 2 -3 kpc [2]) strongly depend on the overall amount of 18 F (T 1/2 (β + )=110 mins) left over after the explosion. This is critically influenced by the 18 F(p,α) 15 O reaction. Sensitivity studies of the impact of reaction rates on nova nucleosynthesis suggest that rates should be known to a precision of, at least, 30% [3]. However, this rate is currently poorly understood and considerable experimental and theoretical effort has been focused on determining this rate ([5, 6] and references therein).Until recently, this rate was thought to be dominated by (i) the 3/2 − resonance at E cm = 330 keV, and (ii) the interference of the 3/2 + states, at 8 and 38 keV E cm , with the known, broad 3...

Fusion reactions induced by radioactive beams: the18F(p,α)15O case

“…A good agreement shows up, within the experimental uncertainties. The observed levels, correspond to levels in 19 Ne which are reported in Table 1, taken from [35] or [9]. Although the energy resolution is poorer than in the previous run (mainly due to the poorer angular resolution of the present experimental apparatus), the agreement between the two data sets confirms once again the applicability of the THM to the present reaction.…”

“…Direct data from [5,6] are reported for comparison and normalization purposes. In the lower panel the full dots represents the results for a J π =5/2 − assumption for the same level as discussed in [9]. This uncertainty leads therefore to an uncertainty between the S(E) lower limit (corresponding to J π =5/2 − for the 6460 keV state in Figure 2.…”

“…Several data sets are up-to-now available [5][6][7][8][9][10][11][12][13]. Nevertheless many uncertainties are still present on the lowenergy resonance and its width, thus affecting the determination of the reaction rate at the temperatures relevant for astrophysics and, consequently, the novae nucleosynthesis.…”

Measurement of radiative proton capture onF18and implications for oxygen-neon novae reexamined

“…A Γ p of 7.3±0.6 eV was extracted from proton transfer data [10], however, the population of the 330 keV resonance could have been contaminated by nearby states. 19 Ne studies by Laird et al [17] and Bardayan et al [18], respectively, found discrepancies in the spin and parity assignments for levels close to the proton threshold in 19 Ne. This may have ramifications for the 18 F(p,γ) 19 Ne reaction rate at nova temperatures; the new spin and parity assignments differ from those published previously, leading to changes in the interference effects with stronger, higher lying states, hence altering the capture cross section.…”

Emergence of Simple Patterns in Complex Atomic Nuclei from First Principles

“…This points to the need of further major advances in many-body methods to access a wider range of nuclei and experimental observables, while retaining the predictive power of ab initio methods, which makes them suitable for e.g. targeting short-lived nuclei that are inaccessible by experiment but essential to further modeling, for example, of the dynamics of X-ray bursts and the path of nucleosynthesis (see, e.g., [13,14]). …”