奇异核的实验研究

总结了兰州放射性束实验小组在兰州放射性次级束流线上近几年来利用产生的放射性束流,轰击Si或C等靶子,测量它的反应总截面.并利用经验公式将这些结果归一到相同的能量和靶子,与其相邻的核相比较,发现了⁹C,¹¹Be,¹⁴Be,⁸B,¹⁴B 和¹²N等核素的反应总截面值有奇异增大的现象.利用微观的Glauber模型进行了计算,对有奇异结构的核采用核芯加价核子的密度分布形式,理论计算和实验结果符合得很好,可以给出奇异核的弥散的密度分布.     

推广的Glauber理论及其在晕核散射中的应用

介绍了推广到晕核散射的Glauber理论,并用其研究晕核¹⁴Be的散射问题.弹核的密度分布分别采用谐振子密度分布和相对论平均场理论计算得到具有两个晕中子结构的密度分布,对晕核模型的多重积分采用蒙特卡洛数值积分方法.计算了不同能量下¹⁴Be,¹²Be与靶核¹²C散射的反应截面,并与实验结果进行比较,¹⁴Be的两个中子采用具有晕中子密度分布的理论计算与实验符合较好,而采用不具有晕中子密度分布的结果与实验值相差较大.     

中能8B和9C与Si反应总截面的测量

能量为75MeV/u的¹²C初级束轰击2mm厚的初级Be靶,并利用RIBLL从弹核碎片中分离出54.2MeV/u质子滴线核束8B和61.1MeV/u的⁹C,再轰击Si靶,用透射法测量了它们与Si的反应总截面σ_R.并应用Glauber模型进行理论计算,分析结果表明⁸B和⁹C都可能具有质子晕结构.     

轻丰中子核反应中子集团产生截面的理论研究

采用Boltzmann-Langevin方程研究了能量为35MeV/u的¹⁴Be, ⁸He,⁶He,¹¹Li,¹⁷B,¹¹Be,¹⁹C与¹²C靶的反应,计算了产生中子集团的截面, 发现^14,Be与¹²C靶反应产生⁴n的截面与实验值符合得很好. 通过这几个入射核与¹²C靶形成中子集团截面的对比, 发现核的晕中子越多产生中子集团的截面越大, 晕中子数相同时, 质量数越大产生中子集团的截面越大.中子集团可能主要来自晕核子.     

中能区丰中子核11Li的反应总截面的测量

利用HIRFL提供的50MeV/u的¹³C束流轰击Be靶, 通过RIBLL选择出放射性核素¹¹Li. 实验采用透射法测量了25—45MeV/u的¹¹Li在²⁸Si靶上的反应总截面. 采用双参数Gauss密度分布形式, 利用Glauber模型很好地拟合了高能和中能区的¹¹Li实验数据, 并从密度分布中提取了核的物质均方根半径.     

中子分布弥散度对反应总截面的影响

在引入库仑场修正的Glauber模型基础上,区分入射弹核和靶核中的中子、质子,并考虑有限力程相互作用,发展了一个计算核反应总截面的微观修正模型.如果引入一个核内中子分布弥散度随中子分离能的变化关系,则能较好地解释奇异核(如⁸He,¹¹Li,¹¹Be)反应总截面的反常增加.同时,该修正模型还能在低能到高能的范围内较好地拟会稳定核的反应总截面.     

晕核反应总截面和密度分布

利用考虑了量子修正、库仑修正、核子–核子碰撞同位旋效应和假定有效原子核密度分布后得到的改进的Glauber理论,计算了晕核与稳定核反应总截面,研究了晕核结构对反应总截面的影响.结果发现对于¹¹Be,¹⁴Be和¹¹Li等入射核,必须考虑它们的晕核结构和利用自由的核子–核子碰撞截面才能得到与实验符合的反应截面,并可依据反应总截面来确定晕核的密度分布和均方半径等信息.     

丰中子核8He在Si靶上的反应总截面的测量

利用HIRFL 50MeV/u ¹³C束流在Be靶上碎裂,RIBLL选择出丰中子放射性次级束流⁸He,实验测量了25—40MeV/u ⁸He在²⁸Si靶上的反应总截面.采用双参数HO密度分布形式,通过微观Glauber模型拟合⁸He实验数据,发现⁸He具有扩展的中子密度分布.实验结果与Warner反应总截面实验和Alkhazov弹性散射实验结果较好地符合.     

197Au中子辐射俘获过程中γ射线强度函数与能谱研究

文章假定在复合核首次和级联γ退激过程中,除包含巨偶极共振模式外,还存在⁶He,⁶Li,⁶Be,⁷Li和⁷Be等粒子集团的激发态的退激过程,并据此建立了包括这些过程的γ射线强度函数,计算了入射中子能量在0.01—3MeV能区的中子辐射俘获反应:¹⁹⁷Au(n,γ)的反应截面和γ能谱,得到了与实验数据较好符合的结果.特别是,很好地解释了γ能谱中5.5MeV之后的反常突起.     

中重核中子辐射俘获γ射线强度函数研究

假设退激过程中除包含巨偶极共振模式外还存在⁶He,⁶Li,⁶Be,⁷Li和⁷Be等粒子集团激发态的退激过程.本文对这一物理假定作了进一步分析.利用据此建立的包括这些过程在内的γ射线强度函数,计算了在核素⁹³Nb,¹⁸¹Ta和天然元素Ag上入射中子能量在0.01—5MeV能区以及在核素¹⁹⁷Au上入射中子能量在0.01—10MeV能区的中子辐射俘获反应截面和γ能谱,得到了与实验数据较好符合的结果.特别是可以较好地解释γ能谱中5.5MeV之后的反常突起.这表明,上述物理假定适用于中重核中子辐射俘获反应.     

THE MASSIVE TRANSFER IN 12C+197Au REACTION

In this paper, we present energy spectra and angular distributions for the α-par-ticles emitted in the reaction of ¹²C+¹⁹⁷Au at 64 and 67 MeV and for α, Be and B emittedin the reaction at 71.5 MeV. The most-probable energles of the variousproducts emittedin this reaction decrease.with decreasing incident energy. The angular distributions peak near grazing angle.The peak position of the angular distributions shift from--80°to--120°while the projec-tile energy decreases from 71.5 MeV to 64 MeV. These all show the features of the transferreaction. The ⁶Li product also has been measured and the ⁸Be particle emitted from thisreaction was detected at 90°to the beam direction using α-α coincident technique at 71.5MeV. The coross section of the ³Be is only few percent at the same angle.     

Probing configuration mixing in 12Be with Gamow-Teller transition strengths

We present a novel technique for studying the quenching of shell gaps in exotic isotopes. The method is based on extracting Gamow-Teller (ΔL=0, ΔS=1) transition strengths [B(GT)] to low-lying states from charge-exchange reactions at intermediate beam energies. These Gamow-Teller strengths are very sensitive to configuration mixing between cross-shell orbitals, and this technique thus provides an important complement to other tools currently used to study cross-shell mixing. This work focuses on the N=8 shell gap. We populated the ground and 2.24 MeV 0+ states in 12Be using the 12B(1+) (7Li, 7Be) reaction at 80 MeV/u in inverse kinematics. Using the ground-state B(GT) value from β-decay measurements (0.184±0.007) as a calibration, the B(GT) for the transition to the second 0+ state was determined to be 0.214±0.051. Comparing the extracted Gamow-Teller strengths with shell-model calculations, it was determined that the wave functions of the first and second 0+ states in 12Be are composed of 25±5% and 60±5% (0s)4(0p)8 configurations, respectively.     

INFLUENCE OF THE NEUTRON NUMBER OF TARGET NUCLEUS ON CROSS SECTIONS OF PARTICLES EMISSION IN 12C+112,124Sn

The energy spectra and angular distributions of Li, Be, B particles emitted from the ¹²C+¹¹²Sn and ¹²C+¹²⁴Sn reactions at E_L=70.0MeV are measured. The angular distributions of the Li, Be, and B particles are found to be peaked around the grazing angle, without any forward peaked component.
The experimental results also showed that the cross section for emission is larger in ¹²C+¹¹²Sn reaction than that in ¹²C+¹²⁴Sn reaction, in contrast with the cross sections for Li, Be and B emissions. The indicates the effect of the neutron number of the target nucleus on the emission probability.     

Extended Glauber Theory and Its Application in Halo Nucleus Scattering

We introduced an extended Glauber theory for a halo nucleus scattering, where the halo nucleons and the nuclear core were treated separately. Expressions of reaction and interaction cross sections of the halo nucleus scattering were derived. We took the halo structure of the projectile nucleus into account and adopted an improved optical limit approximation. In the framework of the extended Glauber theory, we studied the reaction cross section for the halo nucleus ¹⁴Be scattering on a target ¹²C. For comparison, the reaction cross sections of ¹²Be+¹²C were calculated as well. The density distribution of target ¹²C is taken from experiments, and those of the projectiles ¹²Be and ¹⁴Be were obtained by two methods. One is that the harmonic oscillator wave functions for ¹²Be and ¹⁴Be are used. The length of harmonic oscillator is adjusted to reproduce the reaction cross section of ¹²Be+¹²C at the high energy E=790MeV/u . The density distribution of ¹⁴Be was also calculated self-consistently in the relativistic mean field (RMF) theory, with a long tail wave functions for the two neutrons in ¹⁴Be. It was found that the calculated reaction cross sections for ¹²Be+¹²C at E=790MeV/u and E=56.5MeV/u were in good agreement with the experimental data no matter harmonic oscillator or RMF wave functions were used. In contrast, the experiments of the reaction cross sections for ¹⁴Be+ ¹²C could only be reproduced when the wave functions of two 2s_1/2 neutrons spreaded over with a long tail. It comes to a conclusion that two outside neutrons in ¹⁴Be form a halo structure.     

Measurement of the total reaction cross section for the mirror nuclei ~(12)N and ~(12)B

The mirror nuclei 12N and 12B are separated by the Radioactive Ion Beam Line in Lanzhou(RIBLL) at HIRFL from the breakup of 78.6 MeV/u 14N on a Be target.The total reaction cross-sections of 12N at 34.9 MeV/u and 12B at 54.4 MeV/u on a Si target have been measured by using the transmission method.Assuming 12N consists of a 11C core plus one halo proton,the excitation function of 12N and 12B on a Si target and a C target were calculated with the Glauber model.It can fit the experimental data very well.The ch...     

Study of the <Superscript>12</Superscript>C (<Superscript>8</Superscript>B, <Superscript>7</Superscript>Be)<Emphasis Type="Italic">X</Emphasis> knockout reaction at intermediate energies

The breakup reactions of ⁸B on a ¹²C target at 142, 285, 790, and 936MeV/nucleon have been studied. One-proton-removal cross sections, leading to the production of ⁷Be fragments in the ground and first excited states (at 0.429MeV), and the longitudinal momentum distributions of the ⁷Be fragments are obtained in the Eikonal approximation of the Glauber Model. The results of the calculations including the contribution of the ⁷Be to the ground and first excited states of ⁸B are compared with the available experimental data. One-proton-removal cross section for the ¹²C(⁸B, ⁷Be)X knockout reaction at 142, 285, 790, and 936 MeV/nucleon energy has been calculated. ⁸B and ⁷Be cross sections and momentum distribution are in a good agreement with available data.     

Mechanism of the 12C(11B,15N)8Be reaction and 8Be + 15N optical-model potential

Angular distributions of the ¹²C( ¹¹B, ¹⁵N) ⁸Be reaction were measured at the energy E_lab(¹¹B) = 49 MeV for the transitions to the ground and 2.94 MeV (2⁺) excited state of ⁸Be and to the ground and 5.270 MeV (5/2⁺) + 5.299 MeV (1/2⁺), 6.324 MeV (3/2^-), 7.155 MeV (5/2⁺) + 7.301 MeV (3/2⁺), 7.567 MeV (7/2⁺) excited states of ¹⁵N. The data were analyzed by the coupled-reaction-channel method. The elastic, inelastic scattering and one- and two-step transfers were included in the coupling scheme. The data of the ¹²C( ¹¹B, ⁸Be) ¹⁵N reaction at E_cm = 9.4-17.8 MeV known from the literature, were also included in the analysis. The mechanism of the ¹²C( ¹¹B, ¹⁵N) ⁸Be reaction and the optical-model potential parameters for the ¹⁵N + ⁸Be channel were deduced. The energy dependence of the optical-model parameters for the ¹⁵N + ⁸Be channel was obtained.     

The astrophysical S-factor of the reaction 7Be (p, γ)8B in the direct capture model

The astrophysical S-factor for the reaction ⁷Be(p, γ)⁸B up to an energy of 2 MeV (c.m.) and the capture cross section of ⁷Li(n,γ)⁸Li up to 1 MeV (c.m.) are calculated using the Direct Capture model (DC). Both calculations are in good agreement with experimental data.