Isospin fractionation in the nucleon emissions and fragment emissions in the intermediate energy heavy ion collisions
Abstract The degree of isospin fractionation is measured by ( N / Z ) n /( N / Z ) N imf , where ( N / Z ) n and ( N / Z ) N imf are the saturated neutron–proton ratio of nucleon emissions (gas phase) and that of fragment emissions (liquid phase) in heavy ion collision at intermediate energy. The calculated results by using the isospin-dependent quantum molecular dynamics model show that the degree of isospin fractionation is sensitive to the neutron–proton ratio of colliding system but insensitive to the difference between the neutron–proton ratio of target and that of projectile. In particular, the degree of isospin fractionation sensitively depends on the symmetry potential. However, its dependences on the isospin dependent in-medium nucleon–nucleon cross section and momentum dependent interaction are rather weak. The nucleon emission (gas phase) mainly determines the dynamical behavior of the degree of isospin fractionation in the isospin fractionation process, compared to the effect of fragment emission. In this case, we propose that ( N / Z ) n /( N / Z ) N imf or ( N / Z ) n can be directly compared with the experimental data so that the information about symmetry potential can be obtained.
Proton differential elliptic flow and the isospin dependence of the nuclear equation of state
Within an isospin-dependent transport model for nuclear reactions involving neutron-rich nuclei, we study the first-order direct transverse flow of protons and their second-order differential elliptic flow as a function of transverse momentum. It is found that the differential elliptic flow of midrapidity protons, especially at high transverse momenta, is much more sensitive to the isospin dependence of the nuclear equation of state than the direct flow. Origins of these different sensitivities and their implications to the experimental determination of the isospin dependence of the nuclear equation of state by using neutron-rich heavy-ion collisions at intermediate energies are discussed. The isospin dependence of the nuclear equation of state ~EOS! is one of the most important but very poorly known properties of neutron-rich matter @1#. Its determination in laboratory-controlled experiments has profound implications for the study of the structure and evolution of many astrophysical objects @2#. Nuclear reactions induced by stable neutron-rich nuclei and/or radioactive beams provide a means to extract useful information about the isospin dependence of the nuclear EOS and to explore novel phenomena in nuclear matter at extreme isospin asymmetries. A number of dedicated experiments to study the isospin dependence of the nuclear EOS have been performed/planned at several available radioactive beam facilities and the future Rare Isotope Accelerator @3#. For these experiments to be fruitful it is important to first understand well the role of the isospin degree of freedom in nuclear reaction dynamics. Moreover, theoretical predictions on the sensitivity of experimental observables to the isospin dependence of the nuclear EOS are useful for explaining available data and planning new experiments. Recently, several useful observables have been identified in neutron-rich heavy-ion collisions at intermediate energies. These include the isospin fractionation @ 4‐1 0#, neutron to proton ratio of heavy residues @11# or projectilelike fragments @7,12#, and the neutron-proton differential flow @13#. Most of these observables make use of the relative multiplicities and kinetic energies of mirror nuclei. Information about the isospin dependence of the nuclear EOS can also be obtained from studying free nucleons as neutrons and protons have the opposite symmetry potentials in nuclear media. In a recent work, one of the present authors has shown that the neutron-proton differential flow is a rather useful probe of the isospin dependence of the nuclear EOS. The method utilizes constructively both the isospin fractionation and the nuclear collective flow as well as their sensitivities to the nuclear EOS. It, however, requires measuring the neutron and proton collective flows simultaneously. Experimentally, it is easier to identify charged particles and measure their momenta accurately, thus observables using charged particles only are more useful. In this work, within an isospin-dependent transport model for nuclear reactions involving neutron-rich nuclei, we explore the sensitivity of proton collective flow to the isospin dependence of the nuclear EOS in heavy-ion collisions at intermediate energies. We examine both the first-order transverse flow and the second-order differential elliptic flow as functions of transverse momenta. It is found that the differential elliptic flow of midrapidity protons, especially at high transverse momenta, is very sensitive to the isospin dependence of the nuclear EOS. This sensitivity is much stronger than that found in the first-order transverse collective flow around the projectile and target rapidity. At present, nuclear many-body theories predict vastly different isospin dependence of the nuclear EOS depending on both the calculation techniques and the bare two-body and/or three-body interactions employed, see, e.g., Refs. @14 ‐17#. Various theoretical studies ~e.g., Refs. @18,19#! have shown that the energy per nucleon e(r,d) in nuclear matter of density r and isospin-asymmetry parameter d defined as d[~r n2r p!/~r n1r p! ~1!
Coexistence of Virulence and Antibiotic Resistance Genes in Pasteurella multocida Isolated from Diseased Rabbits
Pasteurella multocida (P. multocida)is considered a predominant pathogenic bacterial agent causing respiratory manifestations (snuffles) in rabbits with considerable economic losses and unfavorable prognosis in Egypt. A few recent P. multocida strains exhibit resistance to the most commonly used antibiotics in the veterinary field. Therefore, the present study was carried out to investigate the prevalence of both virulence and antibiotic resistance genes among P. multocida isolated from diseased rabbits in Sharkia Governorate, Egypt. Only 10 out of 100 tested rabbits` nasal swabs were finally confirmed positive for P. multocida of serogroup A by polymerase chain reaction (PCR). Antibiotic susceptibility testing of the recovered isolates revealed that they were all multidrug resistant (MDR) with a predominance of resistance to amoxicillin, ampicillin, amoxicillin/clavulanic acid, neomycin and tetracycline (100% each), followed by kanamycin and streptomycin (90% each). All the recovered isolates were further subjected to PCR screening of some common virulence and antibiotic resistance genes of interest . With the exception of toxA gene, the other virulence associated genes (ptfA, Omp87 and nanB) were found among all the examined isolates. Totally, all MDR P. multocida isolates contained at least one antibiotic resistance gene with aphA1 being the most prevalent (100%), followed by ermX gene (40%). Antibiotic resistance genotyping demonstrated the presence of multiple antibiotic resistance genes among majority of the isolates (40%) with only one isolate harboring 4 genes encoding identical resistance phenotypes. Evidentially, all MDR P. multocida isolates possessed at least 3 virulence genes accompanied by the attendance of antibiotic resistance genes. These findings evidenced that rabbits are potential sources of pathogenic P. multocida strains harboring virulence and antibiotic resistance genes. Therefore, it is evident that there is an urgent need for the judicious use of antibiotics in rabbits` treatment systems to successfully mitigate the propagation of drug resistance across P. multocida species.