Mass asymmetry dependence of fusion time-scales in11B+237Np and12C,16O,19F+232Th reactions in a dynamical trajectory model

Dynamical trajectory calculations were carried out for the reactions of¹¹B+²³⁷Np and¹²C,¹⁶O and¹⁹F+²³²Th, having mass asymmetries on either side of the Businaro-Gallone critical mass asymmetry α_BG, in order to examine the mass asymmetry dependence of fusion reactions in these systems. The compound nucleus formation times were calculated as a function of the partial wave of the reaction for all the systems. This study brings out that for systems with α<α_BG, the formation times are significantly larger than for α>α_BG, which is caused by the dynamical effects involved in the large scale shape changes taking place in the fusion process as well as due to the interplay between the thermal and the collective motion during the collision process. The calculated time scales are comparable to the experimental values derived from the pre-fission neutron multiplicity measurements.     

Incomplete fusion reactions: Analysis of excitation functions and recoil range distributions in 16O + 51V

Incomplete fusion reactions were investigated by measuring the excitation functions of nine evaporation residues in ¹⁶O + ⁵¹V reaction in the beam energy 4-6 MeV/amu, using the well-known recoil catcher technique and gamma-ray spectrometry. The experimental data were compared with that obtained from Monte Carlo simulation calculations using the PACE2 code. The results indicate the presence of incomplete fusion process in the production of two alpha emission products. This was further confirmed by the measurement of recoil range distribution of these isotopes at 96 MeV beam energy. Calculations of the average angular momentum associated with these products revealed the peripheral nature of these ICF reactions. Received: 20 June 2001 / Accepted: 11 September 2001     

Exploiting barrier distributions to investigate breakup effects in the fusion of 9Be+208Pb

The availability of precisely measured fusion excitation functions have allowed the determination of experimental fusion barrier distributions. This concept is utilised in ⁹Be+²⁰⁸Pb reaction, to reliably predict the expected complete fusion cross-sections. However, the measured cross-sections are found to be only 68% of those predicted. The large cross-sections observed for incomplete fusion products support the interpretation that this suppression of fusion is caused by ⁹Be breaking up into charged fragments before reaching the fusion barrier.     

Quasielastic reactions around the coulomb barrier in 16O+118Sn

Measurement of elastic and quasielastic reaction cross sections were done in ¹⁶O + ¹¹⁸Sn system at two different energies above the barrier. Attempts are being made to understand the results in the framework of coupled reaction channel model.     

Effect of entrance channel parameters on the fusion of two heavy ions: Excitation functions of reaction products in16O +66Zn and37Cl +45Sc reactions

Excitation functions of reaction products formed in¹⁶O +⁶⁶Zn and³⁷Cl +⁴⁵Sc systems, leading to the same compound nucleus,⁸²Sr, were measured using recoil-catcher technique and off-line γ-ray spectrometry. The contribution of non-compound processes like transfer and incomplete fusion (ICF) reactions to the cross-sections of different evaporation residues were delineated by comparing the experimental data with the predictions of Monte Carlo simulation code PACE2. The results show that non-compound processes become a significant fraction of the total reaction cross-section in¹⁶O +⁶⁶ systems in the beam energy range studied, while³⁷Cl +⁴⁵Sc gives mainly compound nucleus products. The mass asymmetry dependence of the fusion and non-compound cross-sections have been analysed in terms of the static fusion model and sum rule model     

Complete and incomplete fusion studies in7Li and16O induced reactions on51V by measurement of excitation functions and recoil ranges

Excitation function and mean projected recoil ranges of nuclei produced in the⁷Li and¹⁶O induced reactions on⁵¹V target were measured by conventional stacked foil and thick-target thick-recoil-catcher technique for bombarding energiesE ≤ 50.0 MeV for⁷Li ions andE ≃ 60.0-96.0 MeV for the¹⁶O ions. The measured recoil ranges are converted to momentum transfer. The momentum transfer information was used to get clues about some aspects of the interaction such as complete and incomplete fusion reaction mechanism which correspond to full and reduced momentum transfer respectively. The measured excitation functions are compared with the calculation based on the statistical model which describes only equilibrium decay of the compound nucleus using the CASCADE code. The comparison of the CASCADE code with the measured excitation functions for the residue radioisotopes⁵¹Cr and⁵⁴Mn for the⁷Li +⁵¹V system indicates the reaction mechanisms is complete fusion of⁷Li with the target nucleus⁵¹V. Similarly the comparison of the CASCADE code with the measured excitation functions of the residue radioisotopes for the system¹⁶O +⁵¹ V indicates that the four reaction mechanisms (i) complete fusion of¹⁶O, (ii) incomplete fusion of¹²C, (iii) incomplete fusion of⁸Be and (iv) incomplete fusion of⁴He respectively with the target might be contributing to reaction cross sections.     

Comparison of cross sections for C + O reactions in the second regime of complete fusion

Kinetic energy spectra, angular distributions, and elemental yield distributions have been measured for the ¹² C + ¹⁶ O, ¹² C + ¹⁸ O and ¹³ C + ¹⁷ O reaction products over an energy range from 2 to 7 times the Coulomb barrier energy. A careful kinematic analysis of the evaporation residues and comparisons with statistical-model calculations show that fusion proceeds with full momentum transfer followed by a statistical decay of the compound nucleus. The competition between complete fusion process and peripheral reactions in the ¹² C + ¹⁶ O system is less important than for the ¹² C + ¹⁸ O and ¹³ C + ¹⁷ O reactions. The unexpectedly high ¹² C + ¹⁶ O complete fusion cross sections are related to the possible occurrence of a superdeformation of the ²⁸ Si compound nucleus.     

Effects of the entrance channel mass asymmetry in fusion reactions

The symmetric and asymmetric fusion reaction systems forming the same compound nuclei 26Al,30Si,38Ar and 170Hf are investigated with the frame of improved isospin dependent quantum molecular dynamics model.The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies,the Coulomb barriers and the fusion cross sections.The calculated fusion cross sections agree quantitatively with the experimental data.The results indicate that compound nucleus formation is favorable for the systems with larger mass asymmetry because of the smaller Coulomb contribution to the fusion barrier.     

A systematic study of the fission and evaporation residue excitation functions in complete fusion reactions with weakly bound stable nuclei

We carry out a systematic study on the fusion-fission and evaporation residue excitation functions for the reactions of ^6,7Li, ⁹Be, ^10,11B + ²⁰⁹Bi and ^6,9Li, ⁹Be + ²⁰⁸Pb, in which the projectiles are loosely bound and have low threshold energies against breakup. The fusion cross sections are calculated by the coupled-channel model. The compound nuclei decay are analyzed with the standard statistical model. The fission and evaporation residue excitation functions are well reproduced by our calculations, proving the validity of the standard statistical model in describing the compound nuclei decay in these characteristic reactions. For the compound nuclei with A=215-220 and Z=86-88, the liquid drop fission barriers need to be scaled by 0.80-1.02 to reproduce the experimental data. And a decreasing trend of the scaling factor with increasing fissionability parameter Z²/(50A) is found.     

熔合反应中入射道质量不对成度效应

The symmetric and asymmetric fusion reaction systems forming the same compound nuclei ^26Al, ^30Si, ^38Ar and ^170Hf are investigated with the frame of improved isospin dependent quantum molecular dynamics model. The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies, the Coulomb barriers and the fusion cross sections. The calculated fusion cross sections agree quantitatively with the experimental data. The results indicate that compound nucleus formation is favorable for the systems with larger mass asymmetry because of the smaller Coulomb contribution to the fusion barrier.     

Cross sections for 6Li production in the reactions 10B + 16O and 12C + 14N at low energies

Absolute cross sections have been measured for the reactions ¹⁰B(¹⁶O, ⁶Li)²⁰Ne and ¹²C(¹⁴N, ⁶Li)²⁰Ne at several energies in the range E_c.m. = 7.5–16.2 MeV, and 13.8–16.6 MeV, respectively. The predictions of Hauser-Feshbach calculations show generally good agreement with the experimental data without parameter variation. The consequences of an angular momentum cutoff in the entrance channel and in the compound nucleus are discussed.     

离子原子碰撞过程中反冲离子飞行时间谱的蒙特卡罗模拟研究

基于蒙特卡罗方法,建立了程序模拟离子与原子碰撞中的反冲离子飞行时间谱.模拟入射离子束的空间密度分布,原子束密度的空间分布,靶原子的初速度分布, 根据反应几率产生不同电荷态反冲离子,并求出反冲离子飞行时间后,对每次模拟事件根据飞行时间进行累计,就得到反冲离子的飞行时间谱.模拟结果与实验(Ar12+-Ar) 测量到的TOF谱进行了分析比较,并进行了定性讨论     

Complete and incomplete fusion in12C+93Nb and16O+89Y reactions

Excitation functions for the evaporation residues for the reactions¹²C+⁹³Nb and¹⁶O+⁸⁹Y in the projectile energy range of 4 to 6.5 MeV/amu have been measured using off-line gamma spectrometry. The excitation functions for neutron(xn), proton(pxn) and one alpha(xn) emission channels are practically similar for both the reactions. However the products formed by two alpha(2xn) emission show much higher cross sections in the¹²C+⁹³Nb than the¹⁶O+⁸⁹Y system. This has been explained in terms of the incomplete fusion process involving transfer of an alpha particle from the projectile to the target in the former case.Authors thank Shri D.C. Ephraim for making the rolled metal foils and the operation crew of PELLETRON facility for their help in carrying out the irradiations. Authors are grateful to Dr. P.R. Natarajan, Head Radiochemistry Division for his keen interest in this work.     

Range Technique in Scattering Medium Using a Needle-Fiber Optical Coherence Tomography System

A needle-fiber optical coherence tomography (OCT) system designed for guiding epidural anesthesia to a target is presented. In the preliminary experiment, a desired target in tofu as a strong scattering medium can be monitored over a sufficient range of several millimeters within a short measuring time. The axial resolution is 21 μm. In order to investigate the ranging performance, the relation between background noise levels and focus-scattering properties in a confocal optical system is numerically studied by use of Monte Carlo technique. Numerical results indicate that an appropriate focal length of the lens attached to the fiber should be used to separate the OCT signal from strong noise in strongly scattered medium.     

Open problems in formation and decay of composite systems in heavy ion reactions

New highly exclusive experiments in the field of formation and decay of composite systems in heavy ion reactions are presented. Dynamical effects are reviewed in the light of recent works on the role of the N/Z asymmetry between projectile and target. The possibility of extracting directly from the experimental data the emission barrier of alpha particles emitted from highly excited nuclei is discussed. Finally, the first experimental evidence of double giant resonance excitation in fusion-evaporation reaction is presented.     

Search for ternary fragmentation in the reaction 856 MeV Mo+V: kinematic probing of intermediate-mass-fragment emissions

A new technique has been applied to coincidence measurements between fission fragments (FF) and intermediate mass fragments (IMF) emitted from the composite system ¹⁴⁹ ₆₅Tb at an excitation energy of 224 MeV. The method permits simultaneous observation of IMF emissions along and normal to the FF separation axes. For the integrated total of 0.10± 0.02 IMF emitted per fission, we find no significant correlation with FF direction, suggesting that IMFs associated with fission reactions are predominantly emitted from the system prior to fission.     

材料中He深度分布演化的Monte Carlo模拟研究

基于He泡生长的迁移-合并机理,用Monte Carlo方法模拟了对材料进行等温退火过程中He深度分布的演化,探讨了不同参数对这一演化的影响.研究表明:材料中He泡的初始浓度和尺寸将影响He深度分布的变化,而退火温度则对演化速率起重要作用但对最终的He深度分布影响较小;随着反应的进行,整个系统的演化是逐渐趋缓的. 关键词： He 深度分布 Monte Carlo模拟     

Static versus energy-dependent nucleus–nucleus potential for description of sub-barrier fusion dynamics of _8~(16)O+_(50)~(112,116,120)Sn reactions

The static and energy-dependent nucleus–nucleus potentials are simultaneously used along with the Wong formula for exploration of fusion dynamics of168O+112,116,12050 Sn reactions. The role of internal structure degrees of freedom of colliding pairs, such as inelastic surface vibrations, are examined within the context of coupled channel calculations performed using the code CCFULL. Theoretical calculations based on the static Woods–Saxon potential along with the one-dimensional Wong formula fail to address the fusion data of168O+112,116,12050 Sn reactions.Such discrepancies can be removed if one uses couplings to internal structure degrees of freedom of colliding nuclei.However, the energy-dependent Woods–Saxon potential model(EDWSP model) accurately describes the sub-barrier fusion enhancement of168O+112,116,12050 Sn reactions. Therefore, in sub-barrier fusion dynamics, energy dependence in the nucleus–nucleus potential governs barrier modification effects in a closely similar way to that of the coupled channel approach.     

Defect clustering in double doped fluorite crystals with Lu and Gd

Abstract

Solid solutions Ca¹ x-^yLu^xGd^y F_2+x+y for 10^?4 ≤ x ≤ 2 × 10^?2 and y=0.0001 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). It has been found that the ITC spectrum from 77 to 420 K is very weak and the main peak is attributed to the relaxation of both Lu³⁺-F^? _x and Gd³⁺F^? _i nn dipoles. No polarizable clusters are present in the temperature range explored here. The EPR spectra show the presence of Gd³⁺ tetragonal and cubic centers due to the local and non local compensation, respectively. The continuous decrease in the molar fraction of Gd³⁺ tetragonal centers together with the low concentration of Lu nn dipoles is an evidence of the existence at these low and intermediate concentrations of large clusters such as the cubo-octahedral hexamer which has been proposed for CaF₂ crystals very highly doped with small trivalent cations.     

Langevin study of neutron emission in the reactions 16O+181Ta and 19F+178Hf

The pre-scission neutrons measured in the reactions 16O+181Ta and 19F+178Hf are studied via a Langevin equation coupled with a statistical decay model.We find that because of the mass asymmetry of different entrance channels,the spin distributions of compound nuclei would be different,consequently,the measured neutrons in these two reactions would also different.This means that the entrance channel will affect the particle emission in the fission process of hot nuclei.