The correlation functions and emission time scales for light particles and light fragments in 40Ar+197Au reactions at 25 MeV/nucleon

Two-particle correlation functions were measured for the emission of protons, deuterons, tritons, α, and lithium fragment at small relative momenta in ⁴⁰Ar-induced reactions on ¹⁹⁷Au at 25 MeV/nucleon. Based on three-body trajectory calculations the emission time scales for the particles were extracted from p-d, d-d, t-t, and lithium-lithium correlation functions. The mean emission time was found to decrease with increasing sum of the kinetic energies of the particle in a correlated pairs, indicating the emission of more energetic particles at earlier stages of the reaction than at later stages. The time scale for the emission of lithium fragments emission suggests that sequential binary disassembly occurs in ⁴⁰Ar+¹⁹⁷Au reaction at MeV/nucleon. Mean emission temperature of 3.5_−0.8^+1.3 and 3.6±0.4 MeV were also extracted from the relative populations of the excited and ground states of the respective emitted nuclides ⁸Be and ⁴He.     

Directed collective flow and azimuthal distributions in 36Ar + 27Al collisions from 55 to 95 MeV/u

A 4π charged particle detector array with a low velocity threshold has been used to detect the products from reactions induced by ³⁶Ar on ²⁷Al at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Two methods were used for sorting these events with respect to their impact parameters and three methods were compared to determine the reaction plane. The transverse momentum analysis has been found to be the best method to extract the direction of the reaction plane for this system and for the experimental set-up used here. The energy of vanishing flow for central collisions has been found to be around 90–95 MeV/u. The azimuthal distributions of mid-rapidity particles exhibit a preferential in-plane emission and no squeeze-out effect.     

Inclusive excitation-energy distributions of hot nuclei from 44 MeV/nucleon Ar- and 32 MeV/nucleon Kr-induced reactions

Inclusive neutron multiplicity distributions were measured by means of 4π liquid-scintillator detectors for Ar and Kr-induced reactions at 44 MeV/nucleon and 32 MeV/nucleon, respectively. For all the systems studied, the observed distributions exhibit a bump structure at large multiplicity, corresponding to highly dissipative collisions. For Ar-induced reactions, the excitation energies necessary to explain the most probable neutron multiplicity associated with these dissipative collisions are estimated, the correspondence between excitation energy and neutron multiplicity being calculated in the framework of the statistical model. The so-obtained values of excitation energies, which are systematically lower than those predicted using the massive-transfer picture, are discussed.     

Isospin dependence of the isotopic distributions from 36Ar and 40Ar fragmentation at about 60 MeV/nucleon

Fragments produced in the reactions of 69 MeV/nucleon ³⁶Ar and 55 MeV/nucleon ⁴⁰Ar on a Be target have been measured experimentally. The isotopic distributions from fragmentation reaction for both projectiles are compared with modified statistical abrasion-ablation model calculations. A strong isospin effect exhibits in the fragment distributions produced in ³⁶Ar and ⁴⁰Ar fragmentation. Experimental evidence for the disappearance of the isospin effect in the isotopic distributions produced by projectile fragmentation is observed with the increase of the violence of nuclear reaction. Received: 15 November 2000 / Accepted: 11 April 2001     

核核碰撞中不稳定轻核的方位角各向异性发射

利用符合的粒子粒子关联测量，研究了２５ＭｅＶ／ｕ^４０Ａｒ＋^１９７Ａｕ反应中不稳定轻核的在平面发射和出平面发射．对于中速的不稳定轻核，观测到在平面发射几率增大，这表明该反应系统中存在旋转效应．该现象随碰撞参数增大更为强烈，而随着不稳定核激发态能量的升高稍有变弱     

Components of collective flow and azimuthal distribution in40Ar+27Al collision with BUU method

The in-plane, out-of-plane and longitudinal components of the collective flow are extracted from the BUU simulation for⁴⁰Ar+²⁷Al collision in intermediate energy domain. It is found that the in-plane collective flow changes from a negative value to a positive value around 85 MeV/u. The flow angles are estimated by the in-plane and longitudinal components of the collective flow, and the rotational behavior in addition to the inplane flow is also found by the asymmetry of the azimuthal distribution in the calculation.     

Azimuthal Correlations of Fragments with Light Charged Particles in Reaction of 25MeV/u 40Ar+115In

The large angle correlations for in-plane and out-of-plane have been measured for the pairs of the fragments and light charged particles (LCPs) in the reaction of 25MeV/u ⁴⁰Ar+¹¹⁵In. The azimuthal correlation functions and the azimuthal asymmetry factors were obtained. The azimuthal correlation functions of all pairs between fragments and a particles are of minimum value in φ=90°plane. It indicates that the LCPs and fragments formed in the reaction exhibit an enhanced emission in the reaction plane due to collective rotation-like effect induced by attractive mean field. The more heavier the masses of the coincident LCPs and fragments are, the more stronger the left-right asymmetries of the coincident particles with respect to the beam direction in the reaction plane are, the more preferential the particle emission to the direction opposite to the coincident reaction products is. Along with the increase of the mass of the coincident LCPs and fragments, the influences of the sequential decay and particle final state interactions on the azimuthal correlation functions of the correlated pairs in the φ=0° plane decrease and vanish at last, the collective rotation-like effect is enhanced, and the azimuthal asymmetries increase.     

Impact parameter selected nuclear temperatures of hot nuclei from excited state populations for 40Ar+197 Au reactions at E/A=25MeV

Nuclear temperatures extracted from excited state populations were measured as a function of linear momentum transfer (LMT) for ⁴⁰Ar+¹⁹⁷ Au reactions at 25MeV/nucleon. The emission temperatures increased slightly with increasing linear momentum transfer or decreasing impact parameter. Taking into account the corrections of detection efficiency and sequential feeding from higher-lying states, a temperature of T ≈ 4MeV was deduced for central collisions. For peripheral collisions the extracted temperatures increased with the energy of the particles.     

30MeV/u40Ar+159Tb反应中中等质量碎片的发射时标与发射机制

报道30MeV/u⁴⁰Ar+¹⁵⁹Tb反应中碎片-碎片关联函数的实验结果．利用三体弹道模型从关联函数提取了中等质量碎片发射时间．中等质量碎片的平均发射时间随碎片能量而变化，从低能时的约500fm/c下降至高能时的约100fm/c．中等质量碎片发射时间随束流能量的升高而下降，表明随着束流能量的升高中等质量碎片发射机制逐渐从相继两体衰变向多重碎裂发射过渡．对于⁴⁰Ar+¹⁵⁹Tb反应，此过渡能区在35—45MeV/u之间 关键词：     

Origin of projectile-like fragments from40Ar +68Zn collisions between 15 and 35 MeV/nucleon: Direct transfer of nucleons and projectile fragmentation

The projectile-like fragments emitted in the⁴⁰Ar +⁶⁸Zn reaction performed at 14.6, 19.6, 27.6 and 35 MeV/nucleon are studied. Their energy spectra and angular distributions have been measured. Velocities, widths of the linear momentum distributions and cross sections have been deduced. The results are discussedi) in terms of transfer of a few nucleons and analysed with a diffractional model. They are consistent, for stripping reactions, with a direct transfer of nucleons and a target excitation yielding multiparticlemultihole configurations,ii) In terms of projectile fragmentation-like process with a modified abrasion model taking into account the energy separation of the participant nucleons. The projectile fragmentation-like process appears above 20 MeV/nucleon and strongly competes with transfer of nucleons at 35 MeV/nucleon. The evolution of the mechanisms with incident energy is discussed on the basis of the reduced widths of the linear momentum distributions and on those of velocities and cross sections.     

Few nucleon transfer versus fragmentation in the production of projectile-like fragments in the40Ar+68Zn reaction at 27.6 MeV/nucleon

The isotope distributions, momentum width distributions, and velocities of the projectile-like fragments in the⁴⁰Ar on⁶⁸Zn reaction have been measured at 27.6 MeV/nucleon incident energy. The results show the existence of a fragmentation process well described in the framework of the high energy fragmentation model. However, important contributions from direct nucleon transfer and damped collisions are present.     

25MeV/u 40Ar+115In反应碎片和轻带电粒子的方位角关联

在２５ＭｅＶ／ｕ４０Ａｒ＋１１５Ｉｎ反应的在平面和出平面大角度关联测量中,提取了碎片和α粒子之间的方位角关联函数和方位角非对称性因子．φ＝９０时,方位角关联函数呈现为最小值,表明在集体类转动效应影响下,反应产生的轻带电粒子和碎片优先在反应平面内发射．随着关联对质量的增加,在平面关联粒子的发射相对于束流轴不对称性逐渐增大．随着关联对质量的增加,相继衰变和粒子末态相互作用对φ＝０的关联粒子对方位角关联函数值的影响逐渐降低并直至消失,同时集体转动效应增强,方位角非对称性增加．     

Observation of composite nuclei at very high temperatures

Analysis of data collected with a 19.5 MeV/A⁴⁰Ar beam on a target of²⁷Al shows strong evidences for the formation of very excited composite nuclei. Incomplete linear momentum transfer remains small and the excitation energies per nucleon can be estimated to be close to 5 MeV.     

Prompt light particle emission in the 36Ar + 58Ni reaction at 95 MeV/nucleon

The prompt component at intermediate velocity of light charged particles is investigated. An improved coalescence model coupled to the intra-nuclear cascade code ISABEL is used to obtain light complex particle energy spectra and multiplicities as a function of impact parameter. The results are compared with experimental data from the ³⁶Ar + ⁵⁸Ni experiment at 95 MeV/nucleon, performed with the INDRA 4π detection system. The calculated prompt component is found to rather well reproduce proton spectra. For complex light charged particles the calculated components well populate the high energy part of spectra. Prompt emission can therefore explain the large transverse energies experimentally observed at mid-rapidity. Received: 27 July 2000 / Accepted: 20 November 2000     

Intermediate mass fragment emission in Boltzmann master equation theory of pre-equilibrium reactions

A coalescence model originally proposed to account for the emission of nucleons and light particles in nuclear reactions is extended to account for intermediate mass fragment (IMF) emission. This theory uses a set of Boltzmann master equations to evaluate the time evolution of the occupation probability of nucleon states in the course of the de-excitation cascade during which the IMF are emitted. The internal structure of the IMF and the depletion of nucleon states due to IMF emission are explicitly considered. The theoretical predictions reproduce satisfactorily the experimental multiplicity spectra measured in central collisions of³⁶Ar ions with¹⁹⁷Au at incident energies of 35 MeV/nucleon.     

Local equilibration and diffusive phenomena in40Ar+27Al and14N+27Al heavy ion reactions

Extensive analysis of the energy dissipation and nucleon exchange has been done with heavier systems. Here we choose the two light systems⁴⁰Ar+²⁷Al (E _lab=340 MeV) and¹⁴N+²⁷Al (E _lab=100 MeV) in order to study the correlation of the energy dissipation with the variance of the charge distributions as a function of total kinetic energy loss bins. Considerable energy damping is found to occur in the approach phase which cannot be explained by a simple Fokker-Planck diffusion model. Indeed a model which interprets the collision as a local equilibration followed by diffusive phenomena is more appropriate to fit the data.     

Impact parameter dependence of high-energy gamma-ray production in argon induced reaction at 85 MeV/nucleon

The double-differential cross sections for high-energy γ-rays were measured for collisions of ³⁶Ar on C, Al, Cu, Ag, Tb, and Au at 85 MeV/nucleon. The system ³⁶Ar+ ²⁷Al has been studied in more detail in an exclusive experiment where the charged-particle multiplicity was measured in coincidence with high-energy γ-rays. A clear correlation between the hardness of the γ-spectra and the overlap distance of the two ions is observed. This correlation is interpreted as due to the spatial dependence of the Fermi momentum of the nucleons.     

Diffusion coefficients in reactions of heavy ions with deep inelastic nucleon transfer

Angular distributions of deep inelastic nucleon transfers in the ⁴⁰Ar (288 MeV) + ²³²Th reaction are calculated. Comparison of the theoretical results and the experimental data shows that in the Fokker-Planck equations that were used, the coefficients of charged diffusion cannot be assumed to be time-independent.     

态布居测热核的核温度及其与碰撞参数的关系

测量了２５ＭｅＶ／ｕ^４０Ａｒ＋¹⁹⁷Ａｕ反应中两裂片符合下的小角α-α关联．由α-α关联函数提取了⁸Ｂｅ核的相对态布居和热核的核温度．实验测得发射温度随碰撞参数而稍有变化，从中心碰撞的４ＭｅＶ变化至周边碰撞的３.８ＭｅＶ．在周边碰撞中，观察到发射温度随着粒子能量的增加而升高     

Light charged particle emission in40Ar induced reactions on68Zn at 14.6, 19.6 and 35 MeV/nucleon

The light charged particles emitted in the⁴⁰Ar+⁶⁸Zn reaction performed at 14.6, 19.6 and 35 MeV/nucleon have been studied inclusively. The energy spectra have been analysed in terms of preequilibrium emission, moving source and coalescence models. A complete coherence between the present data and those obtained separately from heavy fragment studies is achieved. These data are consistent with the onset of projectile fragmentation between 20 and 35 MeV/nucleon correlated with the formation of a highly excited region of the nuclear system inducing light particle emission. The other sources of light particles can be interpreted as statistical and sequential decay of the quasi projectile.