I. The properties of hot Ca-like fragments from the 40Ca + 40Ca reaction at 35AMeV

The creation of hot Ca-like fragments was investigated in the ⁴⁰Ca + ⁴⁰Ca reaction at 35 AMeV. Using the AMPHORA 4π detector system, the primary projectile-like fragment was reconstructed and its properties were determined. Both primary and secondary distributions are compared with the predictions of a Monte Carlo code describing a heavy-ion collision as a two-step process. Some of the nucleons which are identified as participants in the first step are transferred in the second step to these final states, which correspond on the average to the maximum value of entropy (thermodynamic probability). The model allows for competition between mean-field effects and nucleon-nucleon interactions in the overlap zone of the interacting nuclei. The analysis presented here suggests a thermalized source picture of the decay of the projectile-like fragment. The validity of the reconstruction procedure for projectile-like fragments is discussed. Received: 12 March 2001 / Accepted: 20 June 2001     

136Xe+198Pt多核子转移反应机制的理论研究

如何生成丰中子重核是目前原子核物理领域的热点问题之一,多核子转移反应是生成丰中子重核的一种可行途径.本工作采用时间相关的Hartree-Fock(TDHF)方法研究了136Xe+198Pt在Ec.m.=645 MeV的多核子转移反应.对动力学过程中的核子转移、能量耗散、形变核的朝向效应等进行了细致的分析,并结合统计衰变...     

Track detector study of fission of bismuth nuclei induced by He, radioactive He and Li ions

The cross-sections for fission of ²⁰⁹Bi induced by secondary beams of ⁶He by ⁴He and ⁷Li were measured. For the registration of fission fragments polyterephtalate plastic and muscovite mica detectors were used. In the case of the reaction of helium ions with bismuth the cross-section of the fission of ²⁰⁹Bi induced by ⁶He are significantly higher than the corresponding -particle-induced fission cross-section.     

30MeV/u 40Ar+natAg反应中中等质量碎片的发射时标

采用强度干涉技术,研究了30MeV/u ⁴⁰Ar+^angAg反应中IMF-IMF关联的实验结果.利用三体弹道模型从约化速度关联函数提取了中等质量碎片的发射时间.中等质量碎片的平均发射时间随其能量而变化,从低能时的约300fm/c下降到高能时的约100fm/c.扣除类弹碎片的贡献后,得到此反应发射中途中等质量碎片的平均发射时间约为250fm/c,表明此反应发射的中等质皇碎片主要来自于相继两体衰变.     

THE COMPETITION BETWEEN TRANSFER AND FRAGMENTATION PROCESSES IN 20Ne (14.7,19.2MeV/u) INDUCED REACTION

Inclusive momentum spectra of projectile-like fragments are decomposed to several components including a fragmentation contribution and a transfer contribution in the ²⁰Ne(14.7,19.2MeV/u) induced reaction.The competition between these two reaction mechanisms with increasing beam energy is studied.     

Do we really understand nuclear reactions within thick targets using GeV-hadrons?

Three experimental evidences are presented to show that, using classical concepts of physics, we are unable to understand the interaction of 72 GeV ⁴⁰Ar (and 44 GeV ¹²C) and their relativistic secondary fragments in thick Cu-targets. The first observation is connected with fragmentation products of mass A in the range of 10A40, in particular to the production of more-than-calculated yields of ²⁴Na. The reason for this unexpected behavior is due to some enhanced interaction properties for relativistic secondary fragments. Moreover, due to the observation of constant classical total interaction cross sections for primary and secondary hadrons, this enforces a reduced production of spallation products being close to the target, as is in fact experimentally observed (the second evidence). The third experimental observation is a more-than-linear increase in the production of secondary neutrons for reactions induced by 44 GeV ¹²C in thick Pb- and Cu-targets as compared to 12 GeV ¹²C. This can be understood within the context of the first two experimental evidences. These effects are experimentally confirmed above about 40 GeV for hadronic primaries, however, they may even extend down to 1 GeV primary hadron energy.

Results are discussed within the context of a new theoretical approach.     

缺中子核素101In低位同核异能态的首次观测

在兰州重离子加速器冷却储存环（HIRFL-CSR）上,用初级束流112Sn35+轰击了靶厚约10 mm的Be靶,产生了101In的基态和低位同核异能态。这些实验产生的碎片每25 s经过放射性束流线RIBLL2的筛选后注入到实验环CSRe中,利用飞行时间探测器测量离子在CSRe中的回旋周期。在此次实验中,磁场晃动会导致离子在环内的循环周期发生改变,传统的离子鉴别方法难以完成大部分离子的鉴别。通过发展和运用单次注入离子鉴别这一新的离子鉴别方法,有效地消除了磁场晃动对于离子鉴别的影响,并清楚地将101In基态和低位同核异能态鉴别出来,从而首次在实验中观测到101In的低位同核异能态。实验得到的激发能与理论外推值在112 keV的误差范围内一致,其低位同核异能态的寿命大于200 μs。Isochronous mass spectrometry has been applied to 112Sn projectile fragments at the HIRFL-CSR facility in Lanzhou. To produce short-lived nuclei of interest, we used projectile fragmentation of 112Sn35+ primary beams in a~10 mm thick 9Be production target. The fragments were selected and analyzed by RIBLL2 and injected into the experimental storage ring(CSRe) every 25 s. To measure revolution times of stored ions,we used a Time-Of-Flight detector installed in CSRe. A new particle identification method was developed to distinguish ions on the measured revolution time spectrum for each injection. Based on this method, the shifts of the revolution time due to instable dipole magnet fields can be corrected and the ground and isomeric states of 101In have been well-resolved. The measured excitation energy is consistent with the theoretical value in the error range of 112 keV. The lifetime of the isomeric states of 101In is more than 200 μs.     

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.     

Decay Analysis of Ge Isotopes Formed in Reactions Induced by Tightly and Loosely Bound Projectiles

The dynamical cluster-decay model (DCM) is employed to investigate the decay of ^68,70Ge^* compound nuclei formed respectively via tightly (⁴He) and loosely (⁶He) bound projectiles, using ⁶⁴Zn target. The study is carried out over a wide energy range (E_c.m.~5 MeV to 16 MeV) by including the quadrupole deformations (β2i) and optimum orientations (θ_i^opt) of the decaying fragments. The fusion cross-sections, obtained by adding various evaporation channels show nice agreement with the experimental data for ⁴He+⁶⁴Zn reaction. The contribution from competing compound inelastic scattering channel is also analyzed particularly for ⁶⁸Ge^* nucleus at above barrier energies. On the other hand, the decrement in the fusion cross-sections of ⁷⁰Ge^* nuclear system is addressed by presuming that ⁶⁵Zn ER is formed via two different modes:(i) the αn evaporation of ⁷⁰Ge^* nucleus, and (ii) 1n-evaporation of ⁶⁶Zn^* nuclear system, formed via breakup and 2n-transfer channels due to halo structure of the ⁶He projectile. Besides this, the suppression in 2np evaporation cross-sections suggests the contribution of another breakup and transfer process of ⁶He i.e. ⁴He+⁶⁴Zn. The contribution of breakup+transfer channels for ⁶He+⁶⁴Zn reaction is duly addressed by applying relevant energy corrections due to the breakup of " ⁶He" projectile into 2n and ⁴He. In addition to this, the barrier lowering, angular momentum and energy dependence effects are also explored in view of the dynamics of chosen reactions.     

Production of fragments in He+Au reaction at 12.7 GeV

The emission of fragments (A≥16) from reaction of ⁴He with ¹⁹⁷Au has been studied at bombarding energy of 12.7 GeV. Policarbonate Makrofol in sandwich form was used as a detector. Fragments produced in the experiment were identified and an event by event model-free analysis was performed. The decay channels ending with one, two or more (three, four) fragments were detected. A classification scheme based on the multiplicity (M_H) of heavy fragments (A≥40) was used in order to define multifragmentation (M_H = 0), spallation (M_H = 1) and fission (M_H = 2) events. The various experimental features of the fragmentation process are discussed.     

Some contributions from SSNTD towards nuclear science: from multifragmentation towards accelerator driven systems (ADS)

Three examples are chosen to show the importance of SSNTD as one of the essential tools in nuclear science:

(1) Multifragmentation into more than two heavy reaction products: Starting with the observation of three heavy reaction products in the interaction of relativistic protons or 414 MeV ⁴⁰Ar with actinides in the early 1960s, up to the observation of five heavy reaction products in the interaction of 2400 MeV ²³⁸U with uranium, SSNTD had a leading role in this research.

(2) In the search for superheavy elements (SHE: Z around 114): Many different techniques are used. However, SSNTD are exclusively decisive in the possible observation of SHE within the heavy element component of galactic cosmic rays.

(3) Accelerator driven systems: They are increasingly important in the discussion of energy producing nuclear power stations and in the corresponding ability to transmute long-lived poisonous radioactive materials (above all plutonium) into shorter lived fission fragments or stable nuclides. SSNTDs play an important role in the determination of the energy dependent neutron fluence in small volumes (≈cm³) or in the exact beam profile determinations of the primary proton beams.

This contribution ends with an outlook into possible future fields of physics research: With the advent of a new generation of relativistic heavy ion accelerators, such as the NUCLOTRON at the JINR in Dubna, Russia, and RHIC in Brookhaven in the United States, one can continue to study (and finally confirm or disprove) all phenomena mentioned already, as well as additional controversial phenomena, such as “enhanced nuclear cross-sections over short distances”, called colloquially “anomalons”. Again SSNDT can be used in at least a twofold manner as an important tool: (a) the enhanced neutron production with ¹²C ions or heavier ions in thick targets at energies above approximately 50 GeV and (b) the reduced “mean-free-path” of secondary fragments produced by the same heavy and energetic ions.     

利用18O(6Li,d)22Ne反应研究22Ne Eα=470 keV共振态的性质

慢速中子俘获过程（s过程）是合成比铁重元素的重要途径之一。22Ne（α,n）25Mg反应是大质量AGB星中s过程主要的中子源,其中的22Ne主要通过14N（α,γ）18F（β+）18O（α,γ）22Ne反应链合成。该反应链中关键反应18O（α,γ）22Ne在天体物理感兴趣能区的截面非常低,其天体反应率主要来自于22Ne α分离阈附近低能共振态的贡献,但目前相关能级的共振参数严重缺失。在HI-13串列加速器的Q3D磁谱仪上,通过测量18O（6Li,d）22Ne反应的角分布,利用DWBA分析确定了22Ne分离阈附近共振能级E_α=470 keV的自旋宇称为0+,为后续计算18O（α,γ）22Ne的天体反应率打下了基础。About a half of the abundances of elements heavier than iron comes from the so-called slowneutron capture process (s-process) in Asymptotic Giant Branch (AGB) stars, with the 22Ne(α, n)25Mg reaction as one of the main neutron sources. In the beginning phase of AGB thermal pulse, 22Ne is produced by the 14N(α, γ)18F(β+)18O(α, γ)22Ne reaction sequence, in which the 18O(α, γ)22Ne reaction plays a key role. While the reaction rate of the 18O(α, γ)22Ne is mainly affected by several resonant states lying closely to the α threshold in 22Ne, up to now, the relevant 22Ne parameters are fragmentary in the energy region corresponding to the typical temperatures of s-process. The direct measurement of the 18O(α, γ)22Ne reaction rate is extremely difficult due to the very low cross section. In this work, we investigated the 22Ne resonant states via the 18O(6Li, d)22Ne reaction at the Beijing HI-13 tandem accelerator of China Institute of Atomic Energy. Based on the DWBA analysis, preliminary results showed that the spin-parity of 22Ne E_α=470 keV resonant states was assigned as 0+, which would make contributions to subsequent calculation for the reaction rate of the 18O(α, γ)22Ne.     

中能核反应中反应平面的确定

介绍并分析了中高能核反应过程中与反应平面相关的可观测量 ,以及确定核反应平面的各种方法 .这些方法包括裂变碎片方位角测量、类弹碎片测量、动量流球形张量、横向动量的方向性、方位角关联和Fourier谐波分析等 . Observables that related to the nuclear reaction plane in intermediate and relativistic energy heavy ion reactions are discussed. Reaction plane determination for intermediate and relativistic energy heavy ion reactions is presented. The methods include fission fragments measurement, projectile-like fragments measurement, kinetic flow-tensor, transverse momentum directivity, azimuthal correlation, Fourier series and etc.     

Magnitude and alignment of transferred angular momentum in both quasi and deeply inelastic scattering

In-plane and out-of-plane angular correlations of fission fragments in coincidence with projectile-like fragments have been measured for the ⁸⁶Kr + ²³⁸U reaction at 730 MeV. The dependence of the magnitude and alignment of the angular momentum transferred to the fissioning heavy reaction product has been determined. Both quantities decrease strongly with decreasing energy loss in the quasi elastic region, in agreement with the predictions of a transport model.     

恒星氦燃烧关键反应12C(α,γ)16O天体物理S因子及其反应率

恒星氦燃烧阶段3α反应和12C（α,γ）16O反应相互竞争,两者的反应率共同决定了氦燃烧结束后12C与16O的丰度比,该比值是大质量恒星后继演化以及伴随的元素核合成过程的初始条件。目前,氦燃烧12C（α,γ）16O反应起始T₉=0.2处,天体物理模型要求的反应率的精确度要低于10%,然而尚未有实验或理论给出满足要求的结果。最为直接和可靠地获取12C（α,γ）16O反应率的方法,就是尽可能往低能区测量其天体物理S因子,然后通过理论外推到感兴趣的能区。为此基于经典的R-矩阵理论,建立了适用于低能核反应的多道、多能级的约化R-矩阵理论来拟合几乎所有可用的16O系统的实验数据。配合使用协方差统计和误差传播理论,拟合外推得到了客观的、内部自恰的和唯一性好的12C（α,γ）16O反应天体物理S因子。总的外推S因子STOT（0.3 MeV）=162.7±7.3 keV·b,理论上首次给出达到恒星演化与元素核合成模型的最低要求的S因子。基于计算给出的全能区的S因子,数值积分给出了温度位于0.04 6 T₉ 6 10的12C（α,γ）16O天体物理反应率。在T₉=0.2处,推荐的反应率为（7.83 ±0.35）×10-15 cm3mol-1s-1。During stellar helium burning, the rates of 3α and the 12C(α,γ)16O reaction, in competition with one another, determine the relative abundances of 12C and 16O in a massive star. The abundance ratio is the beginning condition of the following nucleosynthesis and star evolution of massive stars, which are extremely sensitive to the rate of 12C(α,γ)16O reaction at T₉=0.2. The most direct and trustworthy way to obtain the reaction rate of the 12C(α,γ)16O reaction is to measure the S factor for that reaction to as low energy as possible, and to extrapolate to energies of astrophysical interest. Based on a new multilevel and multichannel reduced R-matrix theory for applications in nuclear astrophysics, we have obtained an accurate and self-consistent astrophysical S factor of 12C(α,γ)16O, by a global fitting for almost all available experimental data of 16O system, with the coordination of covariance statistics and error-propagation theory. The extrapolated S factor of 12C(α,γ)16O was obtained with a recommended value STOT (0.3 MeV)=162.7±7.3 keV·b. And the reaction rates of 12C(α,γ)16O for stellar temperatures between 0.04 6 T₉ 6 10 are provided. At T₉=0.2, the reaction rate is (7.83 ±0.35)×10-15 cm3mol-1s-1, where stellar helium burning occurs.     

Dynamical fluctuations in fragmentation reactions in a Boltzmann–Langevin approach

Based on the isospin-dependent Boltzmann–Langevin model, the dynamical fluctuations in the fragmentation reaction of ¹¹²Sn+¹¹²Sn are investigated. The quadrupole moment and octupole moment with zero magnetic quantum number have large fluctuations in the early time of the collisions. The dynamical fluctuations in momentum space show a strong dependence on the incident energy. The effects of using different fluctuations on the fragment cross sections are also studied in the fragmentation reactions. The results by using Q₂₀ + Q₃₀ fluctuation have a better agreement with the experimental data. Calculations using Q₂₀ + Q₃₀ fluctuation produce more proton-rich and neutron-rich nuclei than those using Q₂₀ fluctuation only. Besides, the difference between the production cross sections of fragments calculated by using Q₂₀ and Q₂₀ + Q₃₀ fluctuations is larger in the vicinity of the projectile. These results present that the dynamical fluctuations may affect the whole dynamical process of fragmentation reactions including the production of fragments, due to the nonlinear nature of the Boltzmann–Langevin equation.     

Heavy fragment production in He+Th reactions at 0.65–12.7 GeV

The processes of production of heavy fragments in the fission mass region in the interaction of 0.65, 1.74, 5.1, 8.8 and 12.7 GeV ⁴He with ²³²Th have been analyzed using the polycarbonate track detector Makrofol in the form of a sandwich. The decay channels ending with one, two, or more (three, four) fragments were detected. We have analyzed the events in which at least one heavy fragment (Z>20) was registered. Fragments produced in the experiment were identified and an event-by-event model-free analysis was performed in order to separate different production mechanisms. We have identified the events produced in fission, deep spallation and fragmentation processes. The cross-sections and experimental features have been determined for the various reaction mechanisms, and their variation as a function of the incident energy has been investigated.     

Production of cold CN molecules by photodissociating ICN precursors in brute-force field

We theoretically investigate the production of cold CN molecules by photodissociating ICN precursors in a brute-force field. The energy shifts and adiabatic orientation of the rotational ICN precursors are first investigated as a function of the external field strength. The dynamical photofragmentation of ICN precursors is numerically simulated for cases with and without orienting field. The CN products are compared in terms of their velocity distributions. A small portion of the CN fragments are recoiled to near zero speed in the lab frame by appropriately selecting the photo energy for dissociation. With a precursor ICN molecular beam of ～ 1.5 K in rotational temperature, the production of low speed CN fragments can be improved by more than 5 times when an orienting electrical field of 100 k V/cm is present. The corresponding production rate for decelerated fragments with speeds ≤ 50 m/s is simulated to be about ～2.1×10~(-4) and CN number densities of 10~(8) –10~(10) cm~(-3) can be reached with precursor ICN densities of ～10~(12) –10~(14) cm~(-3) from supersonic expansion.     

The reaction mechanism in the system132Xe+56Fe at 5.73 MeV/u: Evidence for a new type of strongly damped collisions

We present data for the¹³²Xe+⁵⁶Fe-system at 5.73 MeV/u laboratory energy. Due to inverted kinematics, where Xe is the projectile, we were able to measure energy spectra as well as angular distributions for reaction products with 20≦Z≦60 with unitZ-resolution; i.e. target- and projectile-like fragments have been investigated. The reaction shows a well focussed quasielastic component, where charge transfer from the light to the heavy collision partner dominates. This apparent tendency towards more asymmetric fragmentation is explained by a potential energy surface which favours such charge transfer in order to minimize the asymmetry energy of the liquid drop. The strongly damped component which constitutes the major part of the reaction cross section exhibits characteristics of a fusion-fission reaction with typical fission fragment kinetic energies and 1/sinΘ _c.m. angular distributions. The maximum cross section is found for the symmetric fragmentation, no clear indication is observed for a diffusion process leading to target- and projectile-like fragments. Our data are difficult to reconcile either with the standard diffusion models or with an equilibrated compound nucleus fission picture. We tentatively conclude that an essential part of the fully damped cross section originates from partial waves for which the compound nucleus has no fission barrier.     

Multifragmentation induced by GeV He beams

Multifragmentation of ^natAg and ¹⁹⁷Au induced by 7 and 14.4 GeV ⁴He has been investigated by using CR-39 plastic track detectors. Since our experimental technique enables the determination of the multiplicity of intermediate mass fragments (IMFs) with uncertainty of only a few percent, the results obtained pointed out the inconsistency in the data regarding the quantity reported by other authors. We have also studied the behavior of characteristics of IMFs dependent on the reaction parameters (incident energy, atomic number of the target). From this analysis, we made some conclusions about the reaction mechanism of multifragmentation.