25MeV/u 40Ar+115In反应中前中角区碎片发射研究

从25MeV/u 40Ar+115In反应前中角区出射碎片的能谱、角分布和Z电荷分布出发,讨论了碎片发射机制随出射角和核电荷数Z的渐进变化;用改进的量子分子动力学(MQMD)模型研究了碎片角分布及Z电荷分布,理论计算和实验值整体上符合较好,但在前角区MQMD模型低估了碎片的产额,在中角区对于Z接近弹核的碎片,理论计算比实验值偏高;碎片产物的角分布及Z电荷分布还与统计模型GEMINI进行了比较,发现在前角区平衡蒸发的成分所占比例很小,中角区所占的比例有所增加,但仍然是较小的比例.随着碎片核电荷数Z的减小,平衡核的蒸发成分逐渐减少,非平衡的中速成分逐渐占主导地位.     

25MeV/u 40)Ar+115In,58Ni,27Al反应中前中角区碎片角分布和元素Z分布

测量了25MeV/u ⁴⁰Ar+¹¹⁵In,⁵⁸Ni,²⁷A1反应前中角区出射碎片的角分布和元素Z分布.用改进的量子分子动力学(MQMD)模型研究了碎片的角分布和Z分布.理论计算值和实验值整体上符合得很好,但在前角区,MQMD模型低估了碎片的产额,在中角区对于Z接近弹核的碎片,理论计算值比实验值偏高.碎片产物的角分布和Z分布还与统计蒸发模型GEMINI进行了比较,结果表明,在前角区平衡蒸发成份所占的比例很小,中角区所占的比例有所增加,但仍然是较小的比例.同时发现平衡蒸发成份随着出射碎片核电荷数Z的减小而逐渐减少.     

25MeV／u ^40Ar＋^115In,^58Ni,^27A1反应中前中角区碎片角分？…

测量了２５ＭｅＶ／ｕ＾４０Ａｒ＋＾１１５Ｉｎ,＾５８Ｎｉ,＾２７Ａ１反应前中角区出射碎片的角分布和元素Ｚ分布,用改进的量子分子动力学（ＭＱＭＤ）模型研究了碎片的角分布和Ｚ分布,理论计算值和实验值整体上符合得很好,但在前角区,ＭＱＭＤ模型低估了碎片的产额,在中角区对于Ｚ接近弹的碎片,理论计算值比实验值偏高,碎了碎片的产额,在中角区对于Ｚ接近弹核的碎片,理论计算值比实验值偏高,碎片产物的角分布和Ｚ分布     

294MeV 20Ne+159Tb反应发射中等质量碎片的研究

在294MeV ²⁰Ne轰击¹⁵⁹Tb靶的反应中,用△E—E望远镜测量了10°—150°发射的中等质量碎片(290°)这些能谱是钟罩型,峰位接近于出射道的库仑位垒.用运动源模型分析了这些能谱,结果说明这些碎片来自于共同的源—非完全熔合所形成的复合核.用统计模型GEMINI程序拟合了碎片的电荷分布和角分布,计算值和实验值很好的符合证实了中等质量碎片来源于复合核非对称的两体衰变.     

25MeV/u 40Ar+159Tb反应中等质量碎片的热核蒸发

在20°至155°范围内测量了25MeV/u的⁴⁰Ar轰击¹⁵⁹Tb靶产生的单举中等质量碎片(3≤Z≤9)能谱.后角区中等质量碎片能谱具有明显的统计蒸发特性,不同元素的能谱可用相同参数的单个运动源模型很好地拟合.拟合提取的源速度和核温度等源参数与非完全熔合所形成热核的反冲速度和核温度近似一致,表明这些不同的中等质量碎片共同起源于热核的统计蒸发.使用统计程序GEMINI计算了碎片角分布和电荷分布.     

16O+238U垒下熔合裂变碎片角分布

利用云母核径迹探测器测量了轰击能量从73.7—93.4MeV的¹⁶O+²³⁸U反应的裂变截面和碎片角分布.用考虑了核静态形变的Wong模型很好地重现了裂变激发函数,由此抽取了复合核的自旋分布.同时,利用裂变鞍点过渡态理论以抽取的自旋分布计算了裂变碎片角分布,表明碎片各向异性实验值大于理论值.此外实验揭示出,在低能区碎片各向异性随入射能量变化的走向不同于以前的测量结果.     

30MeV/u 40Ar+58,64Ni和115In反应中中等质量碎片的角分布

在5°—140°范围内测量了30MeV/u ⁴⁰Ar束流轰击不同靶核(⁵⁸Ni、⁶⁴Ni、¹¹⁵In)后出射的中等质量碎片(IMF,Z=3—19)的角分布.用指数分布函数dσ/dΩ=N·exp(–θ/α)对出射碎片角分布分角区做了拟合.对3个反应系统分别提取出与相互作用时间有关的衰减因子α和与发射源强度有关的物理量N,讨论了α与N在不同角区与反应系统和出射IMF电荷数的关系.考察了角分布与同位旋自由度和反应系统对称性的关系.     

中子诱发232Th裂变初始碎片质量及动能分布Monte-Carlo研究

随着第四代反应堆以及先进核能利用系统的发展,对中子核数据提出了高精度、多核素、宽能区的新要求.目前,中国核数据评价库(CENDL库)中相关核裂变的数据较缺失,不足以满足当前核能发展的需求.因此,建立面向中子核数据需求的可靠计算方法和工具变得极为重要.本文基于Monte-Carlo方法建立了裂变碎片质量动能计算模型,研究了中低能中子诱发²³²Th(n,f)反应发射中子前裂变碎片的分布特性.对于裂变碎片质量分布,本模型计算结果与实验值最大偏差约1%,与GEF, TALYS程序计算结果 (与实验值最大偏差约2%)相比具有一定优势.对于发射中子前裂变碎片动能分布,本模型计算结果与实验数据一致.结果表明,所发展的计算模型能够较好地预测²³²Th(n,f)反应发射中子前裂变碎片数据,为中子诱发锕系核裂变反应计算提供一种新思路.     

60 A GeV16O原子核在核乳胶中的电磁离解

对60 A GeV¹⁶O在原子核乳胶中的电磁离解现象首次进行了高统计的研究,得到电磁离解截面随束流能量的增加而增加,射弹碎片电荷分布和200 A GeV¹⁶O在 乳胶中电磁离解的电荷分布一致,但电荷为3≤Z≤5的射弹碎片的发射概率低于200 A GeV能 区,这些特点和Weiszacker和Williams的经典电磁理论模型计算结果一致.60 A GeV¹⁶ O电磁离解下各反应道出现的概率和200 A GeV能区结果基本一致,但同14.6 关键词： 电磁离解 射弹碎片 碎裂     

16O+232Th垒下全熔合裂变

利用裂变碎片的折叠角分布,从实验上实现了全熔合裂变和转移跟随裂变两种成份的区分.在此基础上测量了质心系能量72.61至80.11MeV ¹⁶O+²³²Th全熔合裂变截面和碎片角分布.包含靶核静态形变效应的耦合道模型计算与实验激发曲线一致.然而,裂变统计理论无法解释实验上观察到的全熔合裂变碎片角分布.而鞍点模型与断点模型的理论预言有较明显的差别.     

Angular and Z Distributions of the Fragments at Forward and Intermediate Angles in the Reaction of 25MeV/u 40Ar+115In,58Ni,27Al

The angular & Z distributions of foe fragments emitted in the forward and intermediary angles are measured at the reaction of 25 MeV/u ⁴⁰Ar+¹¹⁵In,⁵⁸Ni,²⁷Al. Their characteristics are investigated based on the model of Modified Qantum Molecular Dynamics (MQMD). Generally, the theoretical calculations are in good agreement with the experiment data. But in the forward angles the yield of the fragments is underestimated by MQMD model while in the case of the intermediate angle region, the calculation is higher than the experiment data in some degree for the fragments whose charge numbers are in the vicinity of the proiectile. The angular & charge distributions of the fragments are also compared with the statistical model of GEMINI. The results turn out to be that a small proportion of the statistical evaporation component exists in the forward angles while in the intermediate angle region, this component increases to some extent. However, its ratio is still small. It is found that the equilibrium evaporation component decreases gradually when the Z of the fragments becomes smaller.     

Origin of complex fragments from32S+natAg reaction at 37.5 A·MeV

Fragment emission from collisions of³²S with^natAg at 37.5 A·MeV has been studied with the 4π multi-detector AMPHORA. Production of intermediate mass and heavy fragments as well as of light charged particles has been measured. The total charged particle multiplicity and polar angular distributions have been used to select various classes of collisions. Analysis of angular and energy distributions of fragments and light particles in central collisions indicates the formation of a hot source (excitation energy of≈4.4 A·MeV) with an additional contribution from a preequilibrium process at more forward angles. Azimuthal angle correlations of He-Li, Li-Li, B-B, and C-C pairs have been used as a tool to study the origin of complex fragments. Data at backward angles are well described by considering a thermalized emitter with an angular momentum around 70? and a fragment emission time of the order of 200 fm/c. A microscopic approach of BNV type confirms these emission times and angular momenta indicating the persistence of an incomplete fusion process responsible for the emission of complex fragments at backward angles.     

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.     

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.     

Evidence of incomplete relaxation in the reaction Ag+40Ar at 288 and 340 MeV bombarding energies

The particles emitted in the reaction induced by ⁴⁰Ar on natural Ag at 288 and 340 MeV bombarding energy have been studied. The fragments have been identified in atomic number, their kinetic energy distribution and their angular distributions have been measured. The kinetic energy spectra show two components: a high-energy component related to the beam energy, or “quasi-elastic” component, and a low kinetic energy component, close to the Coulomb energy called “relaxed” component. The relaxed component is present at all angles and for all particles. The quasi-elastic component is present close to the grazing angle for atomic numbers close to that of the projectile. The relaxed cross section increases with atomic number for Z > 9. The increase in cross section is sharper for the lower bombarding energy. The angular distributions are forward peaked, in excess of $\text{1}\text{sin}\text{θ}$ for all the measured atomic numbers. The forward peaking is larger for particles close in Z to the projectile. The results are interpreted in terms of characteristic times associated with a short-lived intermediate complex. The cross sections and angular distributions are satisfactorily reproduced on the basis of a model accounting for a diffusion process occuring along the mass asymmetry coordinate of the intermediate complex.     

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.     

Preequilibrium α-particle emission in heavy-ion reactions

The α-particle emission following ¹⁴N-induced reactions on various heavy targets at 85–115 MeV has been studied. Cross sections of heavy residual nuclei produced after α-emission were measured in the case of the ²⁰⁹Bi target and were found to be close to the angle-integrated cross sections of α-particles, indicating that the α-emission mainly takes place in a binary process. The measured angular distributions of α-particles are pronouncedly forward-peaked, while the energy spectra are always characterized by the Maxwellian distribution even at forward angles and reproduced excellently by the statistical evaporation formula when nuclear temperature is treated as a free parameter. The resultant value of the temperature is high (4–6 MeV) at forward angles and decreases monotonically with increasing emission angles.The energy and angular distributions of protons, deuterons and tritons were also measured in the ¹⁸¹Ta + ¹⁴N reaction at 115 MeV. The results are similar to those of α-particles. In particular, nuclear temperatures turned out nearly equal to each other, being consistent with the hot-spot interpretation for the relevant preequilibrium light-particle emission.     

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.     

Intermediate mass fragments emitted in the reaction Ag+14N at 100, 160 and 250 MeV bombarding energy

The fragments produced in the reaction between a ¹⁴N beam of various energies and a natural Ag target have been studied. The atomic numbers of the fragments have been identified up to Z = 17 by means of a E-ΔE counter telescope. The cross sections, the kinetic energy distributions as well as the angular distributions have been measured for each atomic number. The kinetic energy distributions show two components: a high-energy component (quasi-elastic), prevailing at angles close to the grazing angles and for atomic numbers close to Z = 7, and a low-energy component (relaxed), at energies close to the Coulomb repulsion energy, present at all angles and for all the Z. A detailed study of the relaxed components of the kinetic energy seems to account for both them means and the widths of these distributions on a purely statistical basis. The cross sections of the relaxed components appear to be quite large at low Z and to decrease rapidly to a fairly constant value in the region of 10 ≦ Z ≦ 17. A marked even-odd alternation in the cross sections is observed. The angular distributions are strongly forward peaked for Z < 7. For Z > 7 the forward peaking decreases rapidly until, for Z > 13, the limiting form 1/sinθ is attained. Evidence for the existence of a diffusion process along the mass asymmetry coordinate is discussed.     

Measurement of Angular Cross Correlation of the Cross Section Fluctuation in Dissipative Collision 27Al+27Al

The excitation function was measured from θ_lab=10.4° to 57.4° in dissipative collision ²⁷Al+²⁷Al with incident energy ranging from 114 to 123 MeV in step of 200 keV. The Angular-cross Correlation Functions (ACFs) of cross section fluctuation within large angle region were obtained through experiment for the first time. It is found that the angular coherent width is at least 40°. It is interesting that the ACFs is not simple bell shape but shows strong asymmetry. The over-all behavior is that ACF varies more quickly for the backward angles. Itis also noticed that the cross section fluctuation in the forward angle region (θ_cm<70°) shows much stronger angular coherence than in the backward angle region (θ_cm>70°). This angular dependence may be attributed to the coherent rotation effect of intermediate dinuclear system.