Francois de Oliveira Santos

Observed abundances of Z ∼ 40 elements in metal-poor stars vary from star to star, indicating that the rapid and slow neutron capture processes may not contribute alone to the synthesis of elements beyond iron. The weak r-process was proposed to produce Z ∼ 40 elements in a subset of old stars. Thought to occur in the ν-driven ejecta of a core-collapse supernova, (α, xn) reactions would drive the nuclear flow toward heavier masses at T = 2−5 GK. However, current comparisons between modeled and observed yields do not bring satisfactory insights into the stellar environment, mainly due to the uncertainties of the nuclear physics inputs where the dispersion in a given reaction rate often exceeds 1 order of magnitude. Involved rates are calculated with the statistical model where the choice of an α-optical-model potential (αOMP) leads to such a poor precision. The first experiment on ⁸⁷Rb(α, xn) reactions at weak r-process energies is reported here. Total inclusive cross sections were assessed at E _c.m. = 8.1−13 MeV (3.7−7.6 GK) with the active target MUlti-Sampling Ionization Chamber. With an N = 50 seed nucleus, the measured values agree with statistical model estimates using the αOMP Atomki-V2. A reevaluated reaction rate was incorporated into new nucleosynthesis calculations, focusing on ν-driven ejecta conditions known to be sensitive to this specific rate. These conditions were found to fail to reproduce the lighter heavy element abundances in metal-poor stars.

Observed abundances of Z ~ 40 elements in metal-poor stars vary from star to star, indicating that the rapid and slow neutron capture processes may not contribute alone to the synthesis of elements beyond iron. The weak r-process was proposed to produce Z ~ 40 elements in a subset of old stars. Thought to occur in the ν-driven ejecta of a core-collapse supernova, (α, xn) reactions would drive the nuclear flow toward heavier masses at T = 2-5 GK. However, current comparisons between modelled and observed yields do not bring satisfactory insights into the stellar environment, mainly due to the uncertainties of the nuclear physics inputs where the dispersion in a given reaction rate often exceeds one order of magnitude. Involved rates are calculated with the statistical model where the choice of an α-optical-model potential (αOMP) leads to such a poor precision. The first experiment on 87Rb(α, xn) reactions at weak r-process energies is reported here. Total inclusive cross sections were assessed at Ec.m. = 8.1 - 13 MeV (3.7 - 7.6 GK) with the active target MUlti-Sampling Ionization Chamber (MUSIC). With a N = 50 seed nucleus, the measured values agree with statistical model estimates using the αOMP Atomki-V2. A re-evaluated reaction rate was incorporated into new nucleosynthesis calculations, focusing on ν-driven ejecta conditions known to be sensitive to this specific rate. These conditions were found to fail to reproduce the lighter-heavy element abundances in metal-poor stars.

Classical novae are thermonuclear explosions in stellar binary systems, and important sources of ²⁶Al and ²²Na. While γ rays from the decay of the former radioisotope have been observed throughout the Galaxy, ²²Na remains untraceable. Its half-life (2.6 yr) would allow the observation of its 1.275 MeV γ-ray line from a cosmic source. However, the prediction of such an observation requires good knowledge of its nucleosynthesis. The ²²Na(p, γ)²³Mg reaction remains the only source of large uncertainty about the amount of ²²Na ejected. Its rate is dominated by a single resonance on the short-lived state at 7785.0(7) keV in ²³Mg. Here, we propose a combined analysis of particle-particle correlations and velocity-difference profiles to measure femtosecond nuclear lifetimes. The application of this method to the study of the ²³Mg states, places strong limits on the amount of ²²Na produced in novae and constrains its detectability with future space-borne observatories.

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetricN/Zratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10⁻²¹ s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of²⁸O and²⁷O through their decay into²⁴O and four and three neutrons, respectively. The²⁸O nucleus is of particular interest as, with theZ = 8 andN = 20 magic numbers^1,2, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both²⁷O and²⁸O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of²⁸O from a²⁹F beam is consistent with it not exhibiting a closedN = 20 shell structure.

Simulations of explosive nucleosynthesis in novae predict the production of 22Na, a key astronomical observable to constrain nova models. Its gamma-ray line at 1.275 MeV has not yet been observed by the gamma-ray space telescopes. The 20Ne/22Ne ratio in presolar grains, a possible tool to identify nova grains, also depends on 22Na produced. Uncertainties on its yield in classical novae currently originate from the rate of the 22Na(p, γ)23Mg reaction. At peak novae temperatures, this reaction is dominated by a resonance at ER=0.204 MeV, corresponding to the E x =7.785 MeV excited state in 23Mg. The resonance strengths measured so far disagree by one order of magnitude. An experiment has been performed at GANIL to measure the lifetime and the proton branching ratio of this key state, with a femtosecond resolution for the former. The reactions populating states in 23Mg have been studied with a high resolution detection set-up, i.e. the particle VAMOS, SPIDER and gamma tracking AGATA spectrometers, allowing the measurements of lifetimes and proton branchings. We present here a comparison between experimental results and shell-model calculations, that allowed us to assign the spin and parity of the key state. Rather small values obtained for reduced M1 matrix elements, M(M1) ≲ 0.5 µ N , and proton spectroscopic factors, C 2 S p <10−2, seem to be beyond the accuracy of the shell model. With the reevaluated 22Na(p, γ)23Mg rate, the 22Na detectability limit and its observation frequency from novae are found promising for the future space telescopes.

Classical novae are thermonuclear explosions in stellar binary systems, and important sources of ²⁶Al and ²²Na^1-3. While γ rays from the decay of the former radioisotope have been observed through-out the Galaxy, ²²Na remains untraceable. The half-life of ²²Na (2.6 yr⁴) would allow the observation of its 1.275 MeV γ-ray line from a cosmic source. However, the prediction of such an observation requires good knowledge of the nuclear reactions involved in the production and destruction of this nucleus⁵. The ²²Na(p, γ)²³Mg⁵ reaction remains the only source of large uncertainty about the amount of ²²Na ejected. Its rate is dominated by a single reso- nance on the short-lived state at 7785.0(7) keV in ²³Mg⁶. In the present work, a combined analysis of particle-particle correlations and velocity-difference profiles is proposed to measure femtosecond nuclear lifetimes. The application of this novel method to the study of the ²³Mg states, combining magnetic and highly-segmented tracking γ-ray spectrometers, places strong limits on the amount of ²²Na produced in novae, explains its non-observation to date in γ rays (flux < 2.5×10⁻⁴ ph.cm⁻²s^{−1 7}), and constrains its detectability with future space-borne observatories.