Space-Time Evolution During Emission of Intermediate Mass Fragmentform 40Ar+159Tb Reaction at 30 MeV/u

The emission time of intermediate mass fragments has been studied as afunction of spatial evolution of the source for ⁴⁰Ar+¹⁵⁹Tb reaction at 30 MeV/u.Reduction of the mass number of source has a very small effect on the extractedemission time. For the high-energy fragments with a short emission time ≤100fm/c,a smaller nuclear density would lead to a smaller assignment for, therefore thevalues extracted in the normal nuclear density could be taken as upper limits of realvalues. For the low-energy fragments, their emission times do not vary with size ofthe source and these values could be taken as real values.     

25MeV/u40Ar+197Au反应中高激发热核的时空演化

报道了25MeV/u⁴⁰Ar+¹⁹⁷Au反应中从两粒子相对动量关联函数提取的发射时间随空间大小演化的规律.结果表明，即使发射源质量数减小50%，对计算的关联函数及提取的发射时间影响仍然很小.在短发射时间(τ≤100fm/c)情况下，较小的核物质密度，导致提取的发射时间变小.因此，在正常核物质密度参数下提取的τ值可作为发射时间的上限值.在长发射时间(τ≥300fm/c)情况下发射时间不随空间大小变化，提取的τ即为实际的发射时间. 关键词：     

Origins of Intermediate Mass Fragments for 40Ar+natAg Reaction at 30 MeV/u

The energy spectra of intermediate-mass fragments (IMF) (3≤Z≤16) have been measured from 6° to 110° for ⁴⁰Ar+^natAg reachon at 30 MeV/u. The energy spectra were analyzed by the moving source model. The natUres of the projechle-like source, target-like source and mid-rapidity source as well as the evolution with angle and nuclear charge were discussed. Two intermediate mass fragments, measured by correlation method in forward angles, come from two sources. One comes from projectile-like source, and other one mainly comes from intermediate source respectively.     

Evaporation of Intermediate-Mass Fragments From Hot-Nuclei in Reaction 40Ar+159Tb at 25MeV/u

The inclusive energy spectra of intermediate-mass fragments(IMF)(3≤Z≤9)have been measured in the range from 20°to 155°for ⁴⁰Ar+¹⁵⁹Tb reaction at 25MeV/u.At backward angles,the spectra of IMF show a characteristic of statistic evaporation and can be well fitted by a moving source with same parameters.The obtained source velocity and temperature are in agreement with the corresponding values of hot nuclei produced in an incomplete-fusion process.Therefore,it seems that there IMF are statistically evaporated from a same source.The experimental results have for compared with the predictions of the statistical model using GEMINI code.     

Time Scsle for Emission of Intermediate Mass Fragments in E/A=30MeV 40Ar+natAg Reaction

Fragment-fragment conelation functions have been measured in E/A=30 MeV ⁴⁰Ar+^natAg reaction via the technique of intensity interferometry. Emission times of intermediate mass fragments have been extracted from reduced velocity correlation functions by using the threebody trajectory model. The average emission time of intermediate mass fragments varies with the energy of fragments. It decreases from about 300fm/c for low energy fragments to about 80fm/c for energetic fragments. After subtmcting the contribution of projectile-like fragments, average emission times of mid-rapidity fragments are of the order of 250fm/c for this reaction.It indicates that intermediate mass fragments emitted in this reaction mainly come from sequential binary decays.     

Stuty on the Process of the Fast Neutron Radiative Capture for 40Ca and 208Pb

The pre-equilibrium correction for the nuclear reactions induced by fast neutron is considered as follows: the nuclear reaction processes with the exciton number equal to or larger than five can still be described by the statistical theory of the nuclear reaction. The particle emission processes, in which less than five excitons are involved, are calculated by means of the exciton model and the γ emission of one exciton state is calculated by the direct capture mechanism. For the three-exciton state, only the semidirect capture mechanism, which plays the main role in the researched energy region, is taken into account. The interference effect between the direct and semidirect capture is also considered. The radiative capture cross sections for ⁴⁰Ca and ²⁰⁸Pb in the neutron incident energy region from 3 MeV to 20 MeV are calculated and a better coincidence with the experimental values is obtained. At the same time, the contribution to the (n, γ) reaction cross sections of the γ emissions before and after statistical equilibriums as well as the characteristics of the direct capture, semidirect capture and their interferece terms are discussed.     

Element Distributions of Intermediate Mass Fragments Emitted in 30 MeV/u 40Ar+58,64Ni and 115In Reactions

The element distribuhons of intermediate mass fragments (IMF) emitted over an angular region of 5°—140° have been measured in the reachons of 30MeV/u ⁴⁰Ar+^58,64Ni and ¹¹⁵In. Within 20°—80° range the IMFs with Z=3-24 were observed. The IMF production cross section changes smoothly with its charge number,and the element distribuhon changes continuously when the IMF charge number increases across the charge number of the projectile. There is no strong dependence of the element distribution on the charge number of the projectile in this angular region.The transition nature of the reaction mechanism in the intermediate energy region and the power law of the IMF element distribution,as well as the dependence of element distributions on the threshold of the detectors were discussed.     

The Angular Distributions of intermediate Mass Fragments Emittsd in 30 MeV/u 40Ar+58,64Ni and 115In Reactions

The angular distributions of intermediate mass fragments with Z=3-19 emitted in 30 MeV/u ⁴⁰Ar+^58,64Ni and ¹¹⁵In reactions over an angular range of 5°—140°have been measured.Exponential distribution function: dσ/dΩ=N·exp(-θ/a) was used tO fit the measured angular distributions.We have extracted the decay factor a that can be connected with the interaction time τ, and the factor N that is related to the intensity of the emission source.The relationships of a(Z) with Z and N(Z) with Z for different reaction systems and different angular ranges have been discussed. The dependence of angular distributions on isospin and the symmetry of reaction system have also been discussed.     

中能重离子碰撞中的涨落和中间质量碎片发射的机制

在ＱＭＤ模型的基础上，系统研究了４０Ｃａ＋４０Ｃａ中心碰撞系统的中间质量碎片随能量和时间的演化以及它们的动力学起源．集中讨论了反应机制的跃迁，动力学过程中密度涨落和平均中间质量碎片的关系以及碎裂的时间尺度．发现中间质量碎片随能量的上升和下降伴随着涨落的上升和下降．并预言对所研究的反应，当入射能量为６５ＭｅＶ／ｕ时，彻底解体可能发生．碎裂的时间尺度大约为１４０ｆｍ／ｃ，且随能量的增大，此时间尺度不再减小，达到饱和．     

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.     

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.     

Complete and Incomplete Fusion in the Reaction of 115In with 16O Ions

Angular distributions and differential range distributions have been measured with nuclear chemistry techniques for 20 target residues from the reactions of ¹¹⁵In with 92 and 71MeV ¹⁶O ions.In terms of the increase of impact parameter,the results indicate continuous evolution of the reaction mechanisms from complete fusion,incomplete fusion to direct reaction involving steadily decreasing transfer of mass and momentum.Contribution of incomplete fusion in ¹⁶O+¹¹⁵In reaction is significantly larger than that in ¹⁶O+⁶⁵Cu reaction.     

Investigation of Intermediate Mass Fragments Emitted in the Reaction of 294MeV 20Ne+159Tb

Intermediate mass fragments(2Z≤11) emitted from 294 MeV ²⁰Ne+¹⁵⁹Tb reaction have been measured by △E-E telescope counters at the angle of 10°—150°region.The energy spectra of the fragments at back angle(>90°) have bell shape,and their peaks locate near the Coulomb barrier of exit channel.The analysis of the energy spectra has been done in terms of moving source and indicate that all the fragments are emitted from a common source,the compound nuclei formed in incomplete fusion process.The charge and angular distributions of the fragments originated from the asymmetric binary decay of the formed compound nuclei have been calculated based on statistical GEMINI code,and the results are in well agreement with the experimental data.     

Light Particle and Complex Fragments Emission from 12C Induced Reactions on 58Ni,64Ni at E/A=46.2MeV

Energy spectra of light charged particles and complex fragments emitted in the interaction of 46.2MeV/u ¹²C with different target ⁵⁸Ni,⁶⁴Ni,¹²C,¹⁹⁷Au and ²⁰⁹Bi have been measured by telescopes,consisting of CsI(Tl) and surface barrier detector stacks.These telescopes for light charged particles and complex fragments have good energy resolution and mass identification.The inclusive data have been analysed within a moving source parametrization.The complex fragment yields from different targets have been compared at several angles.The systematic variation of deduced isotope yields ratios with different reaction systems is used to detemine the N/Z degree of freedom equilibration of the system achieved.     

ANALYSIS FOR ENERGY SPECTRA OF LIGHT PARTICLES EMITTED FROM 12C+64Ni REACTION

Some experimental results measured for light emission particles with Z≤2 from ¹²C+⁶⁴Ni reaction at E(¹²C)=69, 56MeV are reported. A comparison between moving source model calculation and proton's spectra have been performed. Extracted parameters of the source with faster velocity are agreement with the systematic values found before. The composite particle energy spectra of α, ³He and D were analyzed by coalescence model.     

Excitation Energy and Temperature of Compound Nuclei in the Reaction of 35MeV/u 40Ar+197Au

The fission fragments in the reaction of ⁴⁰Ar+¹⁹⁷Au have been measured coincidently with light charged particles at 35MeV/u by using large area position sensitive detectors and Si-detectors. The velocities of the fissioning compounds are deduced based on the fragments and the excitation energies of the compounds calculated with a hypothesis of massive transfer and taking into account the influence of pre-equilibrium emission of light particles. As the same time,the temperatures of the compounds are extracted by using the energy spectra of the backward light charged particles measured coincidently with fission fragments. The temperature-excitation energy curve does not present the behaviour predicted by the theories.     

STUDY OF DEEP INELASTIC COLLISION 12C+27Al AT ENERGY 61.8MeV

By using ΔE-E telescope and the time of flight detector, the energy spectra of products between ⁶Li and ¹⁶O were measured for the reaction ¹²C+²⁷Al, at 61.8MeV. The contour plots of differential cross section in c.m. system and the angular distributions of emitted fragments were obtained. The calculated values of fully relaxed energies in deep inelastic collistions agree with the experimental values. The mean interaction time of di-nuclear system was estimated as from 1×10^－21s to 1.4×10^－22s     

Multiplicity characteristics in relativistic 24Mg-nucleus collisions

This work is concerned with the analyses of the shower and gray particle production in 4.5 A GeV/c ²⁴Mg collision with emulsion nuclei. The highest particle production occurs in the region of the low impact parameters. While the multiplicity of the shower particles emitted in the forward direction depends on the projectile mass number and energy, the multiplicity of the backward ones shows a limiting behaviour. The source of the emission of the forward shower particles is completely different from that of the backward ones. The target fragments are produced in a thermalized system of emission.     

Study of Two-Proton Interferometry in 46.7MeV/u 12C Induced Reactions

In this paper,the results of two-proton interferometry obtained in the experiment of 46.7 MeV/u ¹²C+⁵⁸Ni,¹¹⁵In,¹⁹⁷Au have been discussed. The influnce of nuclear force, Couloumb force and Fermi statistics on the correlation function is discussed also. The source radius parameters ro obtained from the experiment increase withincreasing of the target nuclei mass. In the experiment, the p-p correlation function seems to have a strong dependencs on the sum energy of two correlated protons. The ro parameter decreases with increasing of the sum energy, indicating that the high energy protons are mainly from the nonequilibrium emission process.     

Temperatures and Excitation Energies of Hot Nuclei in Reactions of 40Ar+natAg,209Bi at MeV/u

Coincidence measurements of fission fragment and light charged particle have been perfotrmed for the reactions of ⁴⁰Ar+^natAg,²⁰⁹Bi at E/A=25MeV using 4 PPAC and 11 sets of ΔE-E telescopes.Angular correlations of fission fragment were ploted as a function of the folding angle between the two detected fission fragments.The linear momentum transfer distributions were derived by measuring angular correlations.The backward spectra of light particles detected in coincidence with fission fragments having different average〈LMT〉are analyzed with Maxwell distribution.After some corrections the initial temperatures of the hot nuclei are determined from the energy spectra.The excitation energies corresponding to the different average〈LMT〉are obtained considering the reaction Q values and pre-equilibrum emission.In the central collision of the ⁴⁰Ar+^natAg,²⁰⁹Bi reactions,excitation energies are measured to be about 4.2MeV/u,2.4MeV/u and temperatures about 6.1MeV,5.5MeV,respecdvely.In semi-central collision,excitaionen energies are measured to be about 3.5MeV/u,1.9MeV/u and temperatures about 5.8MeV,4.8MeV respectively.