arXiv subjects

L. R. Gasques

Publications and source records attributed to L. R. Gasques.

17 recordsLinked to original sources

arXiv2025

Crust composition and the Shallow Heat Source in KS 1731-260

The presence of a strong shallow heat source of unknown origin in accreting neutron star crusts has been inferred by analyzing X-ray observations of their cooling in quiescence. We model the cooling of KS 1731-260 using realistic crust compositions and nuclear heating and cooling sources from detailed nuclear reaction network calculations. We find that the required strength of the shallow heat source in KS 1731-260 is reduced by more than a factor of 3 compared to previous analysis, a 5-sigma difference that alleviates the need for exotic solutions. Our analysis also suggests the existence of an impure nuclear pasta layer in the inner crust of KS 1731-260 though future observations will provide more stringent constraints. In addition, we obtain constraints on the dominant surface burning modes of KS 1731-260 over its history.

astro-ph.HE↗

arXiv2023

Impact of Pycnonuclear Fusion Uncertainties on the Cooling of Accreting Neutron Star Crusts

The observation of X-rays during quiescence from transiently accreting neutron stars provides unique clues about the nature of dense matter. This, however, requires extensive modeling of the crusts and matching the results to observations. The pycnonuclear fusion reaction rates implemented in these models are theoretically calculated by extending phenomenological expressions and have large uncertainties spanning many orders of magnitude. We present the first sensitivity studies of these pycnonuclear fusion reactions in realistic network calculations. We also couple the reaction network with the thermal evolution code dStar to further study their impact on the neutron star cooling curves in quiescence. Varying the pycnonuclear fusion reaction rates alters the depth at which nuclear heat is deposited although the total heating remains constant. The enhancement of the pycnonuclear fusion reaction rates leads to an overall shallower deposition of nuclear heat. The impurity factors are also altered depending on the type of ashes deposited on the crust. These total changes correspond to a variation of up to 9 eV in the modeled cooling curves. While this is not sufficient to explain the shallow heat source, it is comparable to the observational uncertainties and can still be important for modeling the neutron star crust.

astro-ph.HE↗

arXiv2019

Systematical study of optical potential strengths in reactions involving strongly, weakly bound and exotic nuclei on $^{120}$Sn

We present new experimental angular distributions for the elastic scattering of $^6$Li$+^{120}$Sn at three bombarding energies. We include these data in a wide systematic involving the elastic scattering of $^{4,6}$He, $^7$Li, $^9$Be, $^{10}$B and $^{16,18}$O projectiles on the same target at energies around the respective Coulomb barriers. Considering this data set, we report on optical model analyses based on the double-folding Sao Paulo Potential. Within this approach, we study the sensitivity of the data fit to different models for the nuclear matter densities and to variations in the optical potential strengths.

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arXiv2019

Constraining the $^{12}$C+$^{12}$C astrophysical S-factors with the $^{12}$C+$^{13}$C measurements at very low energies

We use an underground counting lab with an extremely low background to perform an activity measurement for the $^{12}$C+$^{13}$C system with energies down to $E\rm_{c.m.}$=2.323 MeV, at which the $^{12}$C($^{13}$C,$p$)$^{24}$Na cross section is found to be 0.22(7) nb. The $^{12}$C+$^{13}$C fusion cross section is derived with a statistical model calibrated using experimental data. Our new result of the $^{12}$C+$^{13}$C fusion cross section is the first decisive evidence in the carbon isotope systems which rules out the existence of the astrophysical S-factor maximum predicted by the phenomenological hindrance model, while confirming the rising trend of the S-factor towards lower energies predicted by other models, such as CC-M3Y+Rep, DC-TDHF, KNS, SPP and ESW. After normalizing the model predictions with our data, a more reliable upper limit is established for the $^{12}$C+$^{12}$C fusion cross sections at stellar energies.

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arXiv2019

Elastic scattering, inelastic excitation, and 1n pick-up transfer cross sections for $^{10}$B+$^{120}$Sn at energies near the Coulomb barrier

The $^{10}$B+$^{120}$Sn reaction has been systematically studied at laboratory energies around the Coulomb barrier: E$_{\rm LAB}=$31.5, 33.5, 35.0, and 37.5 MeV. Cross sections for the elastic scattering and some reaction processes have been measured: excitation to the $1^+$ state of $^{10}$B; excitation to the $2^+$ and $3^-$ states of $^{120}$Sn; and the one-neutron pick-up transfer $^{120}$Sn($^{10}$B,$^{11}$B)$^{119}$Sn. Coupled reaction channel (CRC) calculations have been performed in the context of the double-folding São Paulo potential. The theoretical calculations result in a good overall description of the experimental angular distributions. The effect on the theoretical elastic-scattering angular distributions of couplings to the inelastic and transfer states (through the CRC calculations) and to the continuum states (through continuum-discretized coupled-channels calculations) has been investigated.

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arXiv2018

Nuclear Reactions in the Crusts of Accreting Neutron Stars

X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion, and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density is 2E12 g/cm^3 using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond 2E12 g/cm^3. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A >= 102 nuclei, where the inner crust remains impure with an impurity parameter of Qimp~20 due to the N = 82 shell closure. In agreement with previous work we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. This work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.

astro-ph.HE↗

arXiv2017

Measurement of 1323 and 1487 keV resonances in 15N(α, γ)19F with the recoil separator ERNA

The origin of fluorine is a widely debated issue. Nevertheless, the ^{15}N(α,γ)^{19}F reaction is a common feature among the various production channels so far proposed. Its reaction rate at relevant temperatures is determined by a number of narrow resonances together with the DC component and the tails of the two broad resonances at E_{c.m.} = 1323 and 1487 keV. Measurement through the direct detection of the 19F recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a 15N beam impinging onto a 4He windowless gas target. The observed yield of the resonances at Ec.m. = 1323 and 1487 keV is used to determine their widths in the α and γ channels. We show that a direct measurement of the cross section of the ^{15}N(α,γ)^{19}F reaction can be successfully obtained with the Recoil Separator ERNA, and the widths Γ_γ and Γ_α of the two broad resonances have been determined. While a fair agreement is found with earlier determination of the widths of the 1487 keV resonance, a significant difference is found for the 1323 keV resonance Γ_α . The revision of the widths of the two more relevant broad resonances in the 15N(α,γ)19F reaction presented in this work is the first step toward a more firm determination of the reaction rate. At present, the residual uncertainty at the temperatures of the ^{19}F stellar nucleosynthesis is dominated by the uncertainties affecting the Direct Capture component and the 364 keV narrow resonance, both so far investigated only through indirect experiments.

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arXiv2011

Investigation of the role of shell structure in quasi-fission mass distributions

Systematic measurements of mass-ratio distributions for fission following collisions of $^{48}$Ti projectiles with even-even target nuclei from $^{144}$Sm to $^{208}$Pb have been made at sub-barrier energies. They show the presence of quasifission, and depend strongly on target nucleus deformation and the fissility of the composite nucleus. A new framework to analyse systematic mass-ratio measurements allows direct comparison with the trends expected from shell structure, independent of assumptions or fits. This indicates that quasi-fission mass distributions show trends consistent with low energy mass-asymmetric fission of the same actinide elements.

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arXiv2010

A simple analytic model for astrophysical S-factors

We propose a physically transparent analytic model of astrophysical S-factors as a function of a center-of-mass energy E of colliding nuclei (below and above the Coulomb barrier) for non-resonant fusion reactions. For any given reaction, the S(E)-model contains four parameters [two of which approximate the barrier potential, U(r)]. They are easily interpolated along many reactions involving isotopes of the same elements; they give accurate practical expressions for S(E) with only several input parameters for many reactions. The model reproduces the suppression of S(E) at low energies (of astrophysical importance) due to the shape of the low-r wing of U(r). The model can be used to reconstruct U(r) from computed or measured S(E). For illustration, we parameterize our recent calculations of S(E) (using the Sao Paulo potential and the barrier penetration formalism) for 946 reactions involving stable and unstable isotopes of C, O, Ne, and Mg (with 9 parameters for all reactions involving many isotopes of the same elements, e.g., C+O). In addition, we analyze astrophysically important 12C+12C reaction, compare theoretical models with experimental data, and discuss the problem of interpolating reliably known S(E) values to low energies (E <= 2-3 MeV).

astro-ph.HE↗

arXiv2010

Astrophysical S-factors for fusion reactions involving C, O, Ne and Mg isotopes

Using the Sao Paulo potential and the barrier penetration formalism we have calculated the astrophysical factor S(E) for 946 fusion reactions involving stable and neutron-rich isotopes of C, O, Ne, and Mg for center-of-mass energies E varying from 2 MeV to 18-30 MeV (covering the range below and above the Coulomb barrier). We have parameterized the energy dependence S(E) by an accurate universal 9-parameter analytic expression and present tables of fit parameters for all the reactions. We also discuss the reduced 3-parameter version of our fit which is highly accurate at energies below the Coulomb barrier, and outline the procedure for calculating the reaction rates. The results can be easily converted to thermonuclear or pycnonuclear reaction rates to simulate various nuclear burning phenomena, in particular, stellar burning at high temperatures and nucleosynthesis in high density environments.

astro-ph.SR↗

arXiv2008

Suppression of complete fusion due to breakup in the reactions $^{10,11}$B + $^{209}$Bi

Above-barrier cross sections of $α$-active heavy reaction products, as well as fission, were measured for the reactions of $^{10,11}$B with $^{209}$Bi. Detailed analysis showed that the heavy products include components from incomplete fusion as well as complete fusion (CF), but fission originates almost exclusively from CF. Compared with fusion calculations without breakup, the CF cross sections are suppressed by 15% for $^{10}$B and 7% for $^{11}$B. A consistent and systematic variation of the suppression of CF for reactions of the weakly bound nuclei $^{6,7}$Li, $^{9}$Be, $^{10,11}$B on targets of $^{208}$Pb and $^{209}$Bi is found as a function of the breakup threshold energy.

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arXiv2007

Effects of nuclear molecular configurations on the astrophysical S-factor for $^{16}$O + $^{16}$O

The impact of nuclear molecular configurations on the astrophysical S-factor for $^{16}$O + $^{16}$O is investigated within the realistic two-center shell model based on Woods-Saxon potentials. These molecular effects refer to the formation of a neck between the interacting nuclei and the radial dependent collective mass parameter. It is demonstrated that the former is crucial to explain the current experimental data with high accuracy and without any free parameter, whilst in addition the latter predicts a pronounced maximum in the S-factor. In contrast to very recent results by Jiang et al., the S-factor does not decline towards extremely low values as energy decreases.

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arXiv2007

Relating breakup and incomplete fusion of weakly-bound nuclei through a classical trajectory model with stochastic breakup

A classical dynamical model that treats break-up stochastically is presented for low energy reactions of weakly-bound nuclei. The three-dimensional model allows a consistent calculation of breakup, incomplete and complete fusion cross sections. The model is assessed by comparing the breakup observables with CDCC quantum mechanical predictions, which are found to be in reasonable agreement. Through the model, it is demonstrated that the breakup probability of the projectile as a function of its distance from the target is of primary importance for understanding complete and incomplete fusion at energies near the Coulomb barrier.

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arXiv2006

Fusion reactions in multicomponent dense matter

We analyze thermonuclear and pycnonuclear fusion reactions in dense matter containing atomic nuclei of different types. We extend a phenomenological expression for the reaction rate, proposed recently by Gasques et al. (2005) for the one-component plasma of nuclei, to the multi-component plasma. The expression contains several fit parameters which we adjust to reproduce the best microscopic calculations available in the literature. Furthermore, we show that pycnonuclear burning is drastically affected by an (unknown) structure of the multi-component matter (a regular lattice, a uniform mix, etc.). We apply the results to study nuclear burning in a carbon_12-oxygen_16 mixture. In this context we present new calculations of the astrophysical S-factors for carbon-oxygen and oxygen-oxygen fusion reactions. We show that the presence of a CO lattice can strongly suppress carbon ignition in white dwarf cores and neutron star crusts at densities > 3e9 g cm^{-3} and temperatures T<1e8 K.

astro-ph↗

arXiv2005

Nuclear fusion in dense matter: Reaction rate and carbon burning

In this paper we analyze the nuclear fusion rate between equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S-factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate which covers all cases. The parameters of this formula can be varied, taking into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the ^{12}C+^{12}C fusion reaction. This reaction is very important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovae, and in accreting neutron stars. The S-factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S-factor using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities rho > 10^9 g/cc.

astro-ph↗

arXiv2002

A parameter-free optical potential for the heavy-ion elastic scattering proces

Thirty elastic scattering angular distributions for seven heavy-ion systems, in wide energy ranges, have been studied with the aim of systematizing the optical potential, real and imaginary parts, in a global way. The framework is: i) an extensive systematization of nuclear densities, ii) the energy dependence of the bare potential accounted by a model based on the nonlocal nature of the interaction, and iii) the real and imaginary parts of the optical potential assumed to have the same radial shape.

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arXiv2002

Toward a global description of the nucleus-nucleus interaction

Extensive systematization of theoretical and experimental nuclear densities and of optical potential strengths exctracted from heavy-ion elastic scattering data analyses at low and intermediate energies are presented.The energy-dependence of the nuclear potential is accounted for within a model based on the nonlocal nature of the interaction.The systematics indicate that the heavy-ion nuclear potential can be described in a simple global way through a double-folding shape,which basically depends only on the density of nucleons of the partners in the collision.The poissibility of extracting information about the nucleon-nucleon interaction from the heavy-ion potential is investigated.

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