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The energy dependence of an optical potential which reasonably fits all of the existing experimental data on 16 O + 28 Si elastic scattering in the energy range of E = 54.7–215.2 MeV is studied. The real part of the potential found for this system shows a transition from a proximity to a double-folding type of the potential as a result of quenching of the Pauli principle effects with an increase of the incident energy. The imaginary part shows a transition from a surface-transparent to a strongly absorptive type of the potential.

The cross sections for the reactions 208 Pb(p, pn) 207m Pb, 208 Pb(p, p2n) 206m Pb have been measured for 24, 28, 36, 44 and 52 MeV incident protons. The experimental results are shown to be consistent with the clean knockout mechanism for the (p, pn) reaction and with knockout of one neutron followed by evaporation of another for the (p, p2n) reaction. We deduce a theoretical formula for the cross section for these reactions.

Generic early-warning signals such as increased autocorrelation and variance have been demonstrated in time-series of systems with alternative stable states approaching a critical transition. However, lag times for the detection of such leading indicators are typically long. Here, we show that increased spatial correlation may serve as a more powerful early-warning signal in systems consisting of many coupled units. We first show why from the universal phenomenon of critical slowing down, spatial correlation should be expected to increase in the vicinity of bifurcations. Subsequently, we explore the applicability of this idea in spatially explicit ecosystem models that can have alternative attractors. The analysis reveals that as a control parameter slowly pushes the system towards the threshold, spatial correlation between neighboring cells tends to increase well before the transition. We show that such increase in spatial correlation represents a better early-warning signal than indicators derived from time-series provided that there is sufficient spatial heterogeneity and connectivity in the system.

We discuss a method to isolate the part of the effective diffuseness in heavy ion reactions associated with collective zero-point fluctuations of the surface.

The channel-non-orthogonality correction to the heavy-ion transfer polarization potential is discussed.

We introduce an operational measure for the enhancement of the fusion cross section at sub-barrier energies in terms of an asymptotic energy shift ΔE . It is shown that ΔE has a continuously growing trend with the size of the system. This trend is explained in terms of neck formation using the liquid-drop model. Deviations from this trend are attributed to strong coupling to specific channels.

We calculate the effect of the neck degree of freedom on subbarrier fusion reaction cross sections. The potential energy and mass tensor are calculated on the basis of a two-dimensional representation of the liquid drop model. The WKB and Hill-Wheeler approximations are used to calculate the tunneling amplitudes for motion along the direction of the neck size degree of freedom. The resulting cross section for collisions between two identical Ni isotopes show a marked enhancement at low energies compared with the standard one-dimensional radial penetration calculations.

We find that the real part of the optical potential for the /sup 16/O+ /sup 28/Si system extracted from the elastic scattering data in the 55--215 MeV laboratory energy range undergoes a transition from a Brueckner to a folding-type shape. These changes agree with the theoretically expected reduction in the effects of the Pauli principle with increasing incident energy and suggest that reference potentials incorporating these effects could be an improved starting point to perform an optical model analysis of low energy scattering data.

We investigate the effects on the elastic S matrix of the coupling to ..cap alpha..-transfer channels. On the basis of a separable approximation for the propagator in the transfer channel, we obtain simple expressions for the elastic S matrix which include effects of the coupling to all orders. These expressions are used to study the elastic scattering of /sup 16/O+/sup 28/Si at 180/sup 0/. It is shown that a standard two-step distorted-wave Born approximation treatment of the coupling is unable to explain the order of magnitude of the data. On the other hand, the inclusion of high-order processes leads to a qualitative agreement with the data. This modification in the results was traced down to the appearance of a pole in the elastic S matrix near the real axis. The S matrix calculated here was shown to be consistent with others obtained phenomenologically from fits to the available data. We suggest that the apparent importance of ..cap alpha..-transfer effects in elastic scattering implies that such processes should be relevant in other nuclear reactions involving n..cap alpha.. nuclei

We discuss different aspects which could influence temperatures deduced from experimental isotopic yields in the multifragmentation process. It is shown that fluctuations due to the finite size of the system and distortions due to the decay of hot primary fragments conspire to blur the temperature determination in multifragmentation reactions. These facts suggest that caloric curves obtained through isotope thermometers, which were taken as evidence for a first-order phase transition in nuclear matter, should be investigated very carefully., 9 pages, 7 figures

It is suggested that the de-averaged 180\ifmmode^\circ\else\textdegree\fi{} excitation function of $^{16}\mathrm{O}$ + $^{28}\mathrm{Si}$, recently considered by Frahn and Kaufmann, is strongly model dependent. Within a multistep $\ensuremath{\alpha}$-transfer description of the back-angle anomaly, we obtain a de-averaged 180\ifmmode^\circ\else\textdegree\fi{} excitation function that exhibits a more regular gross structure.

We use the liquid drop model to calculate the cross section for neck formation in a heavy-ion collision and show that for the recently measured /sup 58/Ni+/sup 124/Sn case this cross section is strongly related to the sum of the fusion and deep-inelastic cross sections. We note that the observation of deep-inelastic collisions at sub-Coulomb barrier energies may be classically understood by the effective barrier lowering obtained when the neck degree of freedom is considered.

Three different methods for deriving heavy ion polarization potentials are critically compared. It is argued that their possible applications depend strongly on whether they reproduce the elastic wave function or just the elastic phase shifts.

Medidas das seções de choque de fusão, de processos fortemente amortecidos e de espalhamento elástico foram realizadas para os sistemas 16,17,18O + 10,11B e 19F + 9Be no intervalo de energia de bombardeio compreendido entre 22 ELAB64 Mev. São apresentadas evidências de que processos binários fortemente amortecidos observados nestes sistemas originam-se preferencialmente de um processo de fusão-fissão e não de um mecanismo de orbiting dinuclear. A importância relativa do processo de fusão-fissão nestes sistemas muito leves é comprovada pelos resultados experimentais, que indicam a ocorrência de um processo de fissão de um núcleo composto estatisticamente equilibrado, assim como por cálculos de modelos teóricos. Os números atômicos e os de massa dos produtos de reação e dos resíduos de evaporação foram identificados usando-se duas câmaras de ionização e um sistema de tempo de voo. Cross sections for fusion, strongly energy-damped processes and elastic scattering were measured for the 16 ,17, 18O + 10 11B and 19F + 9Be systems in the energy range 22 ELAB 64 HeV. Evidence that the fully energy-damped binary products observed in these reactions originate from a fusion-fission process, rather than through a deep-inelastic orbiting mechanism, is presented. The relative importance of the fusion-fission process in these very light systems is supported by experimental results which points towards the fission of a statistically equilibrated compound nucleus and also by model calculations. Charges and masses of the reaction products and evaporation residues were indentified using two ionization chamber and a time-of-flight system.

Foram medidas funções de excitação para a fusão completa dos sistemas 16O+46,50Ti em energias abaixo e em torno da barreira Coulombiana. A seção de choque de fusão foi obtida utilizando-se os métodos da espectroscopia em linha e fora da linha através da soma das seções de choque dos canais de decaimento do núcleo composto formado, observados experimen- talmente. As vantagens e limitações do método são discutidas em detalhes. As funções de excitação da fusão dos sistemas citados foram analisados utilizando-se modelos semiclássicos de penetração de barreira. Através desta análise determinamos o raio e a altura da barreira de fusão para estes sistemas. Os valores obtidos concordam com valores da literatura na mesma região de massa. A seção de choque de fusão calculada pelo modelo de penetração de barreira unidimensional utilizando potenciais que descrevem a interação entre íons pesados, é subestimada em energias abaixo da barreira Coulombiana. A introdução da vibração de ponto zero da superfície dos núcleos interagentes, apesar de produzir um aumento da seção de choque em energias sub- Coulombianas, não consegue reproduzir os dados de maneira satisfatória, pois prevê uma diferença isotópica nas funções de excitação que não e observada experimentalmente. As previsões do modelo estatístico para o decaimento do núcleo composto (programa CASCA- DE) apresentam uma concordância satisfatória para os canais de decaimento mais intensos. Excitation functions for complete fusion of the systems 16O + 46,50Ti, with energies near and below the Coulomb barrier, were measured. With the use of the in-beam and out of beam spectroscopy, the formation of the compound nucleus was experimentally detected. The fusi- on cross section was then attained by the sum of all observed compound nucleus decay chan- nels. The limitation and advantages of measurements methods are discussed. Theoretical analysis of the experimental results using the semi-classical barrier penetration model allowed us to obtain the fusion barrier height and radius for the studied systems. These values are in good agreement with others reported for this mass range. Using the unidimensionaL barrier penetration model with different nuclear potentials, descri- bing the heavy ion interactions gave theoretical fusion cross section values systematically smaller than our measured values in the energy region below the Coulomb barrier. The introduction of the nuclear surface zero point vibrations enhances the theoreticaL fusion cross sections in the sub-Coulomb region, but simultaneously introduces an isotopic difference in the fusion excitation functions that is not observed experimentally. The statistical model predictions for the compound nucleus decay (calculated by the CASCADE program) show reasonable agreement for the more intense decay channels.

Estudamos o fenômeno de correlação entre dois bosons idênticos (efeito Hanbury Brown-Twiss) no caso de fontes em rápida expansão. Partimos da hipótese que, em colisões hadrônicas a altas energias, forma-se um plasma de quarks e gluons, o qual se expande de acordo com as equações do Modelo Hidrodinâmico de Landau. Ao atingir a temperatura crítica, o fluido sofre uma transição de fase de plasma para hadrons usuais, os quais são então emitidos. Estudamos a dependência da função de correlação de duas partículas em algumas grandezas, como por exemplo, no momento transversal médio das partículas finais e na massa do estado de plasma inicialmente formado. Analisamos também a compatibilidade dos cálculos efetuados com alguns dados experimentais disponíveis. Mostramos que a consideração de expansão da fonte emissora introduz efeitos cinemáticos importantes no comportamento da função de correlação. Assim sendo, os resultados usualmente obtidos com a hipótese de fontes estáticas devem ser completamente modificados no caso de fontes que se expandem rapidamente, como aquelas formadas em colisões hadrônicas a altas energias. The phenomenon of correlation among two identical bosons (Hanbury Brown-Twiss effect) is studied for rapidly expanding sources. We consider the hypothesis of a quark-gluon plasma formation in high energy hadronic collisions, which expands according to Landau\'s Hydrodynamical Model. When the fluid reaches the critical temperature it undergoes a phase transition from plasma to ordinary hadrons t which are then emitted. We study the dependence of the two-particle correlation function on some quantities, such as the average transverse momentum of final particles and the mass of the initially formed plasma. Vile also analyse the compatibility of our resulte with some available experimental data. We show that the source expansion causes important kinematical effects Oh the behavior of the correlation function. Being so, the results usually obtained by means of static sources, should be completely changed in case of rapidly expanding ones, such as the sources formed in high energy hadronic collisions.

Componentes de buraco de nêutrons nos isótopos ANTPOT 99, 101, 103 Ru foram investigadas atrav6s de reações (d,t), induzidas por dêuterons de 15,5 MeV em ANTPOT 100Ru e 16 MeV em ANTPOT 102 Ru e ANTPOT 101 Ru. Os grupos de trítons emergentes, analisados em momento por um espectrógrafo magnético, foram detectados em placas de emulsão nuclear. Resolução em energia melhor que 8 keV permitiu a identificação de 14, 36 e 46 níveis, até as energias de excitação de 1,4; 2,1 e 2,5 MeV, respectivamente, nos ANTPOT 99, 101, 103 Ru. Os momentos angulares orbitais transferidos, l, e as intensidades espectroscópicas foram obtidos a partir da comparação entre distribuições angulares experimentais, medidas cm ângulos judiriosamente escolhidos entre 8º e 46º, e as previstas pela Aproximação de Born com Ondas Distorcidas. A análise das distribuições das intensidades espectroscópicas, associadas a cada l, nos três isótopos revela um padrão de semelhança, embora haja um deslocamento das maiores intensidades espectroscópicas para menores energias, quando se eleva o número de nêutrons, que indica, juntamente com um aumento da densidade de níveis, crescente deformação nuclear. Especial atenção é dirigida a transições para estados de baixa energia de excitação com l = 3 e l = 1, associadas a orbitais não pertencentes à camada de valência, cuja descrição é discutida em termos de um modelo quase-partícula-rotor prolato, não-rígido, com o tratamento completo da interação de Coriolis, e do Modelo de Bósons e Férmions Interatuantes. Neutron-hole components in 99, 101, 103 Ru Isotopes were Investigated by (d,t) reactions at incident deuteron energies of 15.5 MeV and 16 MeV on, respectively, 100 Ru and 102,104 Ru. Outgoing triton groups were momentum analyzed by a magnetic spectrograph and dctectcd in nuclear emulsion plates with an energy resolution better than 8 keV. A total of 14, 36 and 46 leveis up to 1.4, 2.1 and 2.5 MeV excitation energy were identified, respectively, 99, 101, 103 Ru. The transferred orbital angular momenta, l, and the spectroscopic strengths were obtained by con1paring experimental angular distributions, measured at carefully chosen scattering angles between 8º and 46º, with Distorted Wave Born Approximation predictions. The analysis of the spectroscopic strength distributions corresponding to each l-value reveals a similar pattern among the three isotopes, although there is a shift of the highest strengths towards low energy, for increasing neutron number, indicating increasing deform at ion. Special attention is drawn to transitions to low-lying states with l=3 and l=1 character, associated with the next major shell, whose description is discussed in terms of a quasiparticle-prolate non-rigid rotor model with the Coriolis effect fully treated, and the Interacting Boson-Fermion Model.