R Bock and W R Hering (Eds) Amsterdam: North-Holland 1970 pp xii + 821 price £12.60 This volume contains the proceedings of the conference at Heidelberg in July 1969. As well as contributed papers and invited review papers, the conference included a number of 'invited reports' in which the contributor summarized current work in the field at his own laboratory.

Nuclear Reactions Induced by Heavy Ions

The $^{18}\mathrm{F}(p,\ensuremath{\alpha})^{15}\mathrm{O}$ reaction is key to determining the $^{18}\mathrm{F}$ abundance in classical novae. However, the cross section for this reaction has large uncertainties at low energies largely caused by interference effects. Here, we resolve a longstanding issue with unknown spin-parities of subthreshold states in $^{19}\mathrm{Ne}$ that reduces these uncertainties. The $^{20}\mathrm{Ne}(^{3}\mathrm{He},^{4}\mathrm{He})^{19}\mathrm{Ne}$ neutron pick-up reaction was used to populate $^{19}\mathrm{Ne}$ excited states, focusing on the energy region of astrophysical interest $(\ensuremath{\approx}6--7 \mathrm{MeV})$. The experiment was performed at the Triangle Universities Nuclear Laboratory using the high resolution Enge split-pole magnetic spectrograph. Spins and parities were found for states in the astrophysical energy range. In particular, the state at 6.133 MeV $({E}_{r}^{\text{c.m.}}=\ensuremath{-}277 \mathrm{keV})$ was found to have spin and parity of $3/{2}^{+}$ and we confirm the existence of an unresolved doublet close to 6.288 MeV $({E}_{r}^{\text{c.m.}}=\ensuremath{-}120 \mathrm{keV})$ with ${J}^{\ensuremath{\pi}}=1/{2}^{+}$ and a high-spin state. Using these results, we demonstrate a significant factor of two decrease in the reaction rate uncertainties at nova temperatures. 

/pdf/spin-parities-of-subthreshold-resonances-in-the-mml-math-942mr0uc.pdf

Spin-parities of subthreshold resonances in the 
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 The 6Li(16O,19Ne*)3H multi-nucleon transfer-reaction experiment was performed to populate the highly excited states in 19Ne. The subsequent decay particles, 4He or proton from the 19Ne resonant states, were detected in coincidence with the recoil 3H. The excitation-energy spectra of 19Ne were reconstructed by using the detected proton or 4He and the deduced 18F or 15O, respectively. The broad resonance at about 7.85 MeV (1/2+) was observed with partial decay widths updating the previously reported values, which may have significant impact on the destruction of 18F in astrophysical processes. Several resonances up to very high excitation energies have been identified with large α-clustering strength, which confirm the formation of the cluster structure with one-hole configuration in light nuclei, and encourage further systematic studies of cluster structure in 19Ne.

New measurement of the 4He and proton decays from the resonant states in 19Ne

When we think of clustering in nuclear physics, the astrophysical importance within light nuclei and structural manifestations with classical analogs immediately come to mind. 4 He, also known as the alpha particle, is the most abundant nucleus in the Universe, being quite tightly bound for its mass, with a first excited state of over 20 MeV. The nature of the alpha particle places it in a unique position within nuclear astrophysics and structure (including geometry). The plurality of energy release from stellar hydrogen fusion—whether quiescent or explosive—comes from the conversion of hydrogen to helium. Within more complex nuclei, the alpha particles are continuously arranged, leading to fascinating phenomena such as excited rotational bands, Borromean ring ground states, and linear structures . Nuclei with an equal and even number of protons and neutrons are colloquially referred to as “alpha conjugate nuclei,” where such special properties are the most pronounced and easiest to spot. However, when a single nucleon or a pair of nucleons is added to the system, alpha clustering not only remains evident but it may also be enhanced. Excited states with large alpha partial widths are a signature of clustering behavior, and these states can have a profound effect on the reaction rates in astrophysical systems when the excitation energy aligns with the so-called Gamow energy—the preferential thermal energy to statistically overcome the Coulomb barrier. In this article, we will consider in detail the specific ramifications of alpha clustering in selected scenarios for both nuclear astrophysics and topology. In particular, we discussed the astrophysical reactions of 7 Li ( α , γ ), 7 Be+ α , 11 C ( α , p ), and 30 S ( α , p ), where α -clusters may increase the reaction rates from 10% to an order of magnitude; large α resonances make the astrophysical rate of 18 F ( p , α ) quite uncertain. We also focused on the α rotational bands of both positive and negative parities of 11 B and 11 C, and finally on the strongest evidence for the linear-chain cluster state observed in 14 C. 

pdf/alpha-clustering-in-nuclear-astrophysics-and-topology-gto1qwf9ne.pdf

Alpha clustering in nuclear astrophysics and topology

The metallicity of a star affects its evolution in a variety of ways, changing stellar radii, luminosities, lifetimes, and remnant properties. In this work, we use the population synthesis code binary_c to study how metallicity affects novae in the context of binary stellar evolution. We compute a 16-point grid of metallicities ranging from $Z=10^{-4}$ to 0.03, presenting distributions of nova white dwarf masses, accretion rates, delay-times, and initial system properties at the two extremes of our 16-point metallicity grid. We find a clear anti-correlation between metallicity and the number of novae produced, with the number of novae at $Z=0.03$ roughly half that at $Z=10^{-4}$. The white dwarf mass distribution has a strong systematic variation with metallicity, while the shape of the accretion rate distribution is relatively insensitive. We compute a current nova rate of approximately 33 novae per year for the Milky Way, a result consistent with observational estimates relying on extra-Galactic novae but an under-prediction relative to observational estimates relying on Galactic novae. However, the shape of our predicted Galactic white dwarf mass distribution differs significantly to existing observationally derived distributions, likely due to our underlying physical assumptions. In M31, we compute a current nova rate of approximately 36 novae per year, under-predicting the most recent observational estimate of $65^{+15}_{-16}$. Finally, we conclude that when making predictions about currently observable nova rates in spiral galaxies, or stellar environments where star formation has ceased in the distant past, metallicity can likely be considered of secondary importance compared to uncertainties in binary stellar evolution.

The impact of metallicity on nova populations

Classical novae result from thermonuclear explosions producing several $\gamma$-ray emitters which are prime targets for satellites observing in the MeV range. The early 511 keV gamma-ray emission depends critically on the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate which, despite many experimental and theoretical efforts, still remains uncertain. One of the main uncertainties in the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate is the contribution in the Gamow window of interference between sub-threshold $^{19}$Ne states and known broad states at higher energies. Therefore the goal of this work is to clarify the existence and the nature of these sub-threshold states. States in the $^{19}$Ne compound nucleus were studied at the Tandem-ALTO facility using the $^{19}$F($^3$He,t)$^{19}$Ne charge exchange reaction. Tritons were detected with an Enge Split-pole spectrometer while decaying protons or $\alpha$-particles from unbound $^{19}$Ne states were collected, in coincidence, with a double-sided silicon strip detector array. Angular correlations were extracted and constraints on the spin and parity of decaying states established. The coincidence yield at $E_x$ = 6.29 MeV was observed to be high spin, supporting the conclusion that it is indeed a doublet consisting of high spin and low spin components. Evidence for a broad, low spin state was observed around 6 MeV. Branching ratios were extracted for several states above the proton threshold and were found to be consistent with the literature. R-matrix calculations show the relative contribution of sub-threshold states to the astrophysically important energy region above the proton threshold. The levels schemes of $^{19}$Ne and $^{19}$F are still not sufficiently well known and further studies of the analogue assignments are needed. The tentative broad state at 6 MeV may only play a role if the reduced proton width is large.

Sub-threshold states in $^{19}$Ne relevant to $^{18}$F(p,$\alpha$)$^{15}$O

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Sub-threshold states in $^{19}$Ne relevant to $^{18}$F(p,$\alpha$)$^{15}$O