Showing papers by "Carlos A. Bertulani published in 2013"

Beyond the neutron drip line: The unbound oxygen isotopes O-25 and O-26

Abstract: The very neutron-rich oxygen isotopes O-25 and O-26 are investigated experimentally and theoretically. The unbound states are populated in an experiment performed at the R3B-LAND setup at GSI via proton-knockout reactions from F-26 and F-27 at relativistic energies around 442 and 414 MeV/nucleon, respectively. From the kinematically complete measurement of the decay into O-24 plus one or two neutrons, the O-25 ground-state energy and width are determined, and upper limits for the O-26 ground-state energy and lifetime are extracted. In addition, the results provide indications for an excited state in O-26 at around 4 MeV. The experimental findings are compared to theoretical shell-model calculations based on chiral two- and three-nucleon (3N) forces, including for the first time residual 3N forces, which are shown to be amplified as valence neutrons are added.

Quasifree ( p,2p) and ( p,pn) reactions with unstable nuclei

Abstract: We study ($p,2p$) and ($p,pn$) reactions at proton energies in the range of 100 MeV--1 GeV. Our purpose is to explore the most sensitive observables in unpolarized reactions with inverse kinematics involving radioactive nuclei. We formulate a model based on the eikonal theory to describe total cross sections and momentum distributions of the recoiled residual nucleus. The model is similar to the one adopted for knockout reactions with heavy ions. We show that momentum distributions are sensitive to the angular momentum of the ejected nucleon which can be used as an spectroscopic tool. The total cross sections are sensitive to the nucleon separation energies and to multiple scattering effects. Our calculations also indicate that a beam energy around 500 MeV/nucleon has a smaller dependence on the anisotropy of the nucleon-nucleon elastic scattering.

Updated evidence of the Trojan horse particle invariance for the 2 H( d , p ) 3 H reaction

Abstract: The Trojan Horse nucleus invariance for the binary d(d,p)t reaction was tested using the quasi free $^2$H($^6$Li, pt)$^4$He and $^2$H($^3$He,pt)H reactions after $^6$Li and $^3$He break-up, respectively. The astrophysical S(E)-factor for the d(d,p)t binary process was then extracted in the framework of the Plane Wave Approximation applied to the two different break-up schemes. The obtained results are compared with direct data as well as with previous indirect investigations. The very good agreement confirms the applicability of the Plane Wave Approximation and suggests the independence of binary indirect cross section on the chosen Trojan Horse nucleus also for the present case.

Big bang nucleosynthesis with a non-maxwellian distribution

Abstract: The abundances of light elements based on the big bang nucleosynthesis model are calculated using the Tsallis non-extensive statistics. The impact of the variation of the non-extensive parameter q from the unity value is compared to observations and to the abundance yields from the standard big bang model. We find large differences between the reaction rates and the abundance of light elements calculated with the extensive and the non-extensive statistics. We found that the observations are consistent with a non-extensive parameter , indicating that a large deviation from the Boltzmann-Gibbs statistics (q = 1) is highly unlikely.

Role of pairing in the description of giant monopole resonances

Abstract: We compare the results obtained in the framework of the quasiparticle random-phase approximation on top of a Hartree-Fock-Bogoliubov with the most recent experiments on giant monopole resonances in Pb, Sn, Zr, Sm, Mo, and Cd. Our calculations are fully self-consistent and the density dependence of pairing interactions is, for the first time in this framework, properly taken into account. In the particle-hole (ph) channel we employ different Skyrme functionals (SLy5, SkM* and Skxs20) while in the particle-particle (pp) channel we make use of density-dependent contact interactions. We introduce in the pp channel the recently proposed contact interactions which take into account the neutron-proton asymmetry. We find that no single parametrization is able to reproduce with sufficient accuracy all the nuclei. Since about two-thirds of the nuclei under investigation are better explained with a soft parametrization, this tends to suggest that the currently accepted picture for the incompressibility might require modifications.

Determining the 7 Li( n , γ ) cross section via Coulomb dissociation of 8 Li

Abstract: The applicability of Coulomb dissociation reactions to determine the cross section for the inverse neutron capture reaction was explored using the reaction ${}^{8}$Li($\ensuremath{\gamma}$,$n$)${}^{7}$Li. A 69.5 MeV/nucleon ${}^{8}$Li beam was incident on a Pb target, and the outgoing neutron and ${}^{7}$Li nucleus were measured in coincidence. The deduced ($n$,$\ensuremath{\gamma}$) excitation function is consistent with data for the direct capture reaction ${}^{7}$Li($n$,$\ensuremath{\gamma}$)${}^{8}$Li and with low-energy effective field theory calculations.

Microscopic in-medium nucleon-nucleon cross sections with improved Pauli blocking effects

Abstract: The investigation of the effective nucleon-nucleon (NN) interaction in dense hadronic matter is a topic of fundamental importance for nuclear reactions at intermediate energies (20 Elab 300 MeV/nucleon) and for nuclear structure in general. The relevant literature is very vast. Reference [1 ]i s justarepresentativeexampleofthetraditionalmicroscopicapproach where two-nucleon correlations in nuclear systems are introduced through the G-matrix. Moreover, the effective NN interaction is the main ingredient of microscopic predictions of the nuclear equation of state (EOS) and thus impacts the properties of compact stars. Dense hadronic matter can also be createdinthelaboratoryinenergeticheavy-ion(HI)collisions. Simulations of HI collisions are typically based on transport equations and describe the evolution of a nonequilibrium system of strongly interacting hadrons undergoing two-body collisions in the presence of a mean field. The BoltzmannUehling-Uhlenbeck equation [2,3] and quantum molecular dynamics [4], along with their relativistic counterparts [5‐7], have been typically employed to describe intermediate-energy HI reactions. In-medium two-body cross sections are therefore an important component of such simulations. In direct reactions at intermediate energies the NN cross sections are often used as input to obtain quantum refractive and diffractive effects, replacing the role of optical potentials commonly used in low energy reactions [8]. Examples such as knockout (stripping and diffraction dissociation) reactions, elastic scattering, charge exchange, and excitation of giant resonances are often carried out using reaction mechanisms based on the construction of scattering matrices built from the underlying NN scattering. Reaction calculations at intermediate to high energy are often conducted within the framework of the Glauber approximation [9] and have been a frequent tool for testing nuclear models and constraining nuclear sizes. In fact, the description of complex nuclear reactions at intermediate energies based on individual NN collisions hasalongtradition.Intheframework oftheGlauber model, the reaction cross section is written in terms of the “thickness function,” which is the product of the averaged NN cross section and the overlap integral of the target and projectile local densities. In-medium NN cross sections have been calculated with a variety of methods. In semiphenomenological approaches, one makes the assumption that the transition matrix in the medium is approximately the same as the one in vacuum and that medium effects come in only through the use of effective masses in the phase-space factor [10‐12]. Then, the in-medium cross section is scaled (relative to its value in vacuum) as the square of the ratio of the (reduced) masses. Phenomenological formulas, such as the one in Ref. [13], have been developed for practical purposes and combine the energy dependence of empirical free-space NN cross sections with the density dependence of some microscopic models. Microscopic predictions based on a medium-modified collisionmatrixwerereported,forinstance,inRef.[14],where Dirac-Brueckner-Hartree-Fock (DBHF) medium effects were applied to obtain a medium-modified K-matrix, and in Ref. [15], where the predictions are based on the BruecknerHartree-Fock scheme together with the Paris potential. More recentmicroscopiccalculationsappliedDBHFmediumeffects

Nuclear Astrophysics from View Point of Few-Body Problems

Abstract: Few-body systems provide very useful tools to solve different problems for nuclear astrophysics. This is the case of indirect techniques, developed to overcome some of the limits of direct measurements at astrophysical energies. Here the Coulomb dissociation, the asymptotic normalization coefficient and the Trojan Horse method are discussed.

Coulomb distortion and medium corrections in nucleon-removal reactions

Abstract: Background: One-nucleon removal reactions at or above the Fermi energy are important tools to explore the single-particle structure of exotic nuclei. Experimental data must be compared with calculations to extract structure information, evaluate correlation effects in nuclei, or determine reaction rates for nuclear astrophysics. However, there is insufficient knowledge to calculate the cross sections for these reactions accurately.Purpose: We evaluate the contributions of the final-state interaction (FSI) and of the medium modifications of the nucleon-nucleon interactions and obtain the shapes and magnitudes of the momentum distributions. Such effects have been often neglected in the literature.Method: Calculations for reactions at energies of 35--1000 MeV/nucleon are reported and compared to published data. For consistency, the state-of-the-art eikonal method for stripping and diffraction dissociation is used.Results: We find that the two effects are important and their relative contributions vary with the energy and with the atomic and mass number of the projectile involved.Conclusions: These two often neglected effects modify considerably the one-nucleon-removal cross sections. As expected, the effects are largest at lower energies, around 50 MeV/nucleon, and on heavy targets.

Nuclear astrophysics with radioactive ions at FAIR

Abstract: The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process beta-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.

Medium effects in direct reactions

Abstract: We discuss medium corrections of the nucleon-nucleon (NN) cross sections and their influence on direct reactions at intermediate energies 50 MeV/nucleon. The results obtained with free NN cross sections are compared with those obtained with a geometrical treatment of Pauli-blocking and Dirac-Bruecker methods. We show that medium corrections may lead to sizable modifications for collisions at intermediate energies and that they are more pronounced in reactions involving weakly bound nuclei.

Nucleosynthesis: What Direct Reactions Can Do for It?

Abstract: where E is the center of mass energy, λ = √ h/2mE is the reduced wavelength and ` = 0, 1, 2, · · · . The cross section is proportional to πλ, the area of the quantum wave. Each part of the wave corresponds to different impact parameters having different probabilities for fusion. As the impact parameter increases, so does the angular momentum, hence the reason for the 2`+ 1 term. P`(E) is the probability that fusion occurs at a given impact parameter, or angular momentum. Sometimes, for a better visualization, or for extrapolation to low energies, one uses the concept of astrophysical S-factor, redefining the cross section as

Recent Results for the Effects of Distortion in the Inter-Cluster Motion in Light Nuclei and Application to Nuclear Astrophysics

Abstract: Deuteron induced quasi-free scattering and reactions have been extensively investigated in the past few decades. This was done not only for the study of the nuclear structure and processes but also for the important astrophysical implication (Trojan Horse Method, THM). In particular the width of the neutron momentum distribution in deuteron will be studied as a function of the transferred momentum. THM applications will also be discussed because the momentum distribution of the spectator particle inside the Trojan horse nucleus is a necessary input for this method. The impact of the width variation on the extraction of the astrophysical S(E)-factor is discussed as well as the relevance of the s and d wave component in the deuteron wave function.