丰中子奇异核17B的实验研究

在兰州重离子加速器国家实验室(HIRFL)放射性次级束流线(RIBLL)上，用束流透射法测量了丰中子奇异核17B与C靶反应的总截面.假定17B具有15B(核芯)＋2n结构，采用Gauss＋HO形式的密度分布和零力程Glauber模型进行计算的结果可以很好地拟合实验数据，并得出17B的密度分布有一个很大的弥散，表明17B是双中子晕核.

The properties of halo structure for B-17

The total reaction cross section (1724 +/- 93 mb) of B-17 at the energy of 43.7 A MeV on C target has been measured by using the transmission method at the Radioactive Ion Beam Line in Lanzhou (RIBLL). Assuming B-17 consists of a core B-15 plus two halo neutrons, the total cross section of B-17 on C target was calculated with the zero-range Glauber model, where double Gaussian density distributions and Gaussian plus HO density distributions were used. It can fit the experimental data very well. The characteristic of halo structure for B-17 was found with a large diffusion of the neutrons density distribution.

Configuration of the valence neutrons of B-17

The reaction cross section of B-17 on C-12 target at (43.7 +/- 2.4) MeV/u has been measured at the Radioactive Ion Beam Line in Lanzhou (RIBLL). The root-mean-square matter radius (R-rms) was deduced to be (2.92 +/- 0.10) fm, while the R-rms of the core and the valence neutron distribution are 2.28 fm and 5.98 fm respectively. Assuming a "core plus 2n" structure in B-17, the mixed configuration of (2s(1/2)) and (1d(5/2)) of the valence neutrons is studied and the s-wave spectroscopic factor is found to be (80 +/- 21)%.

Studies on the exotic structure of Al-23

The longitudinal momentum distribution (P-//) of fragments after one-proton removal from Al-23 and reaction cross sections (sigma(R)) for Al-23,Al-24 on carbon target at 74A MeV have been measured simultaneously. An enhancement in sigma(R) is observed for Al-23 compaxed with Al-24. The full width at half maximum of the P-// distribution for Mg-22 fragments has been determined to be 232 +/- 28 MeV/c. Analysis of P-// using the Few-Body Glauber Model indicates a dominant d-wave configuration for the valence proton in the ground state of Al-23. The exotic structure in Al-23 is discussed.

Experimental Study on the Exotic Structure of N-12 in RIBLL

We measured the total reaction cross sections of N-12 in Si at 36.2 MeV/u. using Radioactive Ion Beam Line in Lanzhou (RIBLL) with a new method. The reaction target was installed at the intermediate focusing point T1 at RIBLL. This scheme allows us to identify particles before and after the reaction target unambiguously. The total reaction cross section (1760 +/- 78mb) of N-12 in Si is obtained. Assuming that N-12 consists of a core C-11 plus one halo proton, the excitation function of N-12 on the Si and C targets is calculated with the Glauber model and the Fermi-Fermi density distributions. It can fit the experimental data very well. A large diffusion of the protons density distribution supports the halo structure for N-12.

丰质子奇异轻核反应总截面测量

核反应总截面是表征原子核反应基本特征的一个基本量，从实验测得的核反应总截面中可以得到有关核反应、核结构和核内核子分布的信息。在由放射性束流所产生奇的结构与各种反应机制研究中，反应总截面的测量更是具有特殊的重要性，具有奇异晕核结构的核的一个典型的物理现象就是其反应总截面要比稳定核大很多，Tanihata等人最早就是通过对放射性束流的相互作用截面的测量发现了具有奇异结构的核，即中子晕核。这次实验结果得出的跟放射性奇异核性质等有关的一些有趣现象，为放射性束核物理的研究注入了新的活力。我们采用能量为80MeV/u的初级~(20)Ne束轰击3mm厚的初级Be靶，在RIBLL上测量了由初级靶上产生碎裂反应所得到的次级~(12)N, ~(17)F和~(17)Ne等质子滴线核在硅靶上的中能核反应总截面，从而进一步补充了现有的中能区奇异核反应总截面测量的空白，并对现有的数据进行了检验。由于~(12)N, ~(17)F和~(17)Ne都是有争议的可能具有奇异质子结构（质子晕或质子皮）的核，测量它们在中能区的核反应总截面，可以对上述说法的成立与否提供实验上的验证。在与相邻同位素核的反应截面测量结果的比较中，发现实验得出的这些奇异核的截面值都偏大，而且由相对论平均场（RMF）计算得到的它们的质子密度分布明显比中子分布松散的多，因此推断它们可能具有奇异质子分布结构。并且基于库仑修正后的微观Glauber模型的理论计算出这些奇异核反应截面的结果同我们得出的实验数据吻合得很好。为了更精确地进行奇异核反应总截面的测量，研制了两种双维位置灵敏平行板雪崩电离室（PPAC），可以放在待测硅靶前，用来选择小于一定入射角的次级束粒子，有助于得到更准确的结果。该PPAC的有效探测面积为50 * 50mm~2，分别采用电荷分除法和延迟线读出的方法确定入射粒子的位置，并具有结构简单、价格便宜的特点。用三组分α源(5.155Mev~(239)Pu, 5.486 Mev~(241)Am, 5.806Mev~(244)Cm) 对探测器进行了测试，由阴极电荷分除读出法得到的位置分辨（FWHM）约为1.5mm。

同中子素核9C、8B、7Be和6Li在中能区与28Si反应总截面的测量

放射性束流(RIB)装置拓广了实验核物理在同位旋(T_z)自由度上从稳定核直到滴线核的广袤空间。通常，位于β-稳定线及其附近的核，N／Z在1－1.5范围变化，其分离能E_s无论对于质子还是中子，总是在6-8 MeV之间；对于远离稳定线的非稳定核，N／Z可在0.5-4范围变化，如~9C的N／Z = 0.5，~(10)He的N／Z = 4，而且分离能E_s是在0-40 MeV之间变化的，开展对这些远离β-稳定线非稳定核性质、结构的研究是目前核物理的前沿之一。核反应总截面σ_R是表征原子核性质特征的一个基本物理量，从实验测得的核反应总截面中可以得到有关核结构和核内核子分布的信息。在由放射性束流所产生奇异核的结构与各种反应机制研究中，反应总截面的测量更是有其特殊的重要性，具有奇异核结构如晕核的一个典型的物理特征就是其反应总截面要比稳定核大得多，Tanihata等人最早就是通过对放射性束流的相互作用截面的测量发现了具有奇异结构的核，即中子晕核。由于反应总截面的测量对探测器的要求不高，而且数据分析过程相对较为简单，因而反应总截面的测量已经成为放射性束核物理的研究的一个非常重要实验手段。中子晕核以及中子皮核的发现促使人们去寻找质子晕核和质子皮核，由于最后一个质子的结合能非常小只有136，keV，并且有较大的电四极矩，因而使得~8B成为质子晕的最大热门候选核，有关~8B是否具有质子晕核结构的问题，许多实验科学家得出了相互矛盾的回答；而目前有关另外一个质子晕候选核~9C的实验数据非常少，目前还没有人从实验上对~9C是否为质子晕核这一问题进行肯定或否定的回答。因此非常有必要测量~9C和~8B的反应总截面。对反应总截面进行研究的一个非常有用理论就是Glauber模型，该模型考虑了库伦效应的多次散射理论。它是一种基于自由核子-核子(N-N)碰撞的与核物质密度相关的理论，因而能够从实验测量到的反应总截面中提取核物质分布的信息。该理论对中低能区的反应总截面描述却有一个缺憾：理论值比实验值都要小。本论文主要描述了利用透射法测量了中能区同中子素核~9C、~8B、~7Be及~6Li与~(28)Si的反应总截面，并介绍了重离子碰撞以及描述重离子性质的几种常用理论。在论文里对实验测量得到的结果进行了理论分析，这些理论包括半经验的Shen公式、Glauber模型、BUU模型以及SHF理论。如果将~9C和~8B当成具有正常核结构来处理，半经验的Shen公式和Glauber模型(HO密度分布)的理论计算值总是比实验值要小得多；对于Glauber模型的理论计算值和实验值的差异，Ozawa等定义了一个差值因子d，方德清等人对轻核系统的d值进行了详细的分析。一般认为，正常核的d值在20％以内，而对于具有晕或皮奇异结构的核，其d值则超过30％，甚至可达50％，因此可根据一个核的d值是否超过30％而且比相邻核的d值明显大这种半经验的方法来判断一个核是否具有奇异结构；利用d值的分析结果，我们认为：~9C和~8B都具有奇异核结构；对于BUU模型用同样的方法引进差值因子d值，对于~9C和~8B有相同的结论。用SHF理论计算得到B和C同位素的密度分布结果显示，~9C和~8B的密度分布比相邻的同位素扩展都要大得多。为减小Glauber模型计算的反应截面与实验值的差别，本论文还对Glauber模型的输入密度形式进行了修改，在原单一HO分布基础上加一个高斯分布的尾巴，并对丰质子的同中子素核~9C、~8B、~7Be及~6Li与~(28)Si靶以及~(12)C和丰中子的C同位素核~(13-16)C与~(12)C靶的反应截面重新进行了计算，结果显示在中能区的计算值比原来单一密度分布的计算结构有明显改善。

Radiative capture of nucleons at astrophysical energies with single-particle states

Radiative capture of nucleons at energies of astrophysical interest is one of the most important processes for nucleosynthesis. The nucleon capture can occur either by a compound nucleus reaction or by a direct process. The compound reaction cross sections are usually very small, especially for light nuclei. The direct capture proceeds either via the formation of a single-particle resonance or a non-resonant capture process. In this work we calculate radiative capture cross sections and astrophysical S-factors for nuclei in the mass region A < 20 using single-particle states. We carefully discuss the parameter fitting procedure adopted in the simplified two-body treatment of the capture process. Then we produce a detailed list of cases for which the model works well. Useful quantities, such as spectroscopic factors and asymptotic normalization coefficients, are obtained and compared to published data. (C) 2010 Elsevier Inc. All rights reserved.

Indirect study of (11)B(p,alpha(0))(8)Be and (10)B(p,alpha)(7)Be reactions at astrophysical energies by means of the Trojan Horse Method: recent results

Nuclear (p,alpha) reactions destroying the so-called ""light-elements"" lithium, beryllium and boron have been largely studied in the past mainly because their role in understanding some astrophysical phenomena, i.e. mixing-phenomena occurring in young F-G stars [1]. Such mechanisms transport the surface material down to the region close to the nuclear destruction zone, where typical temperatures of the order of similar to 10(6) K are reached. The corresponding Gamow energy E(0)=1.22 (Z(x)(2)Z(X)(2)T(6)(2))(1/3) [2] is about similar to 10 keV if one considers the ""boron-case"" and replaces in the previous formula Z(x) = 1, Z(X) = 5 and T(6) = 5. Direct measurements of the two (11)B(p,alpha(0))(8)Be and (10)B(p,alpha)(7)Be reactions in correspondence of this energy region are difficult to perform mainly because the combined effects of Coulomb barrier penetrability and electron screening [3]. The indirect method of the Trojan Horse (THM) [4-6] allows one to extract the two-body reaction cross section of interest for astrophysics without the extrapolation-procedures. Due to the THM formalism, the extracted indirect data have to be normalized to the available direct ones at higher energies thus implying that the method is a complementary tool in solving some still open questions for both nuclear and astrophysical issues [7-12].

Limitation of double folding potentials to simulate the polarization in reactions involving halo nuclei

In this work, we investigate the limitation of the use of strength coefficients on double folding potentials to study the presence of the threshold anomaly in the elastic scattering of halo nuclei at near barrier energies. For this purpose, elastic angular distributions and reaction cross sections for the He-6 on Bi-209 are studied. (c) 2008 Elsevier B.V. All rights reserved.

Elastic scattering of a proton-halo nucleus: (8)B+(58)Ni

The elastic channel of the (8)B + (58)Ni system has been measured at energies around the Coulomb barrier. An optical potential fi to the experimental angular distributions is obtained. The total reaction cross section consistent with the obtained potential is reported and possible deviations from normal behaviour are discussed.

Transfer reactions in the investigation of light-nuclei nucleosynthesis

Cross sections for the (6)Li(p,gamma)(7)Be, (7)Li(n,gamma)(8)Li (8)Li(n,gamma)(9)Li and (8)Li(p,gamma)(9)Be capture reactions have been investigated in the framework of the potential model. The main ingredients of the potential model are the potentials used to generate the continuum and bound-state wave functions and spectroscopic factors of the corresponding bound systems. The spectroscopic factors for the (7)Li circle times n=(8)Li(gs), (8)Li circle times n=(9)Li(gs) bound systems were obtained from a FR-DWBA analysis of neutron transfer reactions induced by (8)Li radioactive beam on a (9)Be target, while spetroscopic factor for the (8)Li circle times n=(9)Be(gs) bound system were obained from a proton transfer reaction. From the obtained capture reaction cross section, reaction rate for the (8)Li(n,gamma)(9)Li and (8)Li(p,gamma)(9)Be direct neutron and proton capture were determined and compared with other experimental and calculated values.

Efeito túnel multidimensional em reações nucleares

Pós-graduação em Física - IFT

Investigation of the nuclear matter density distributions of the exotic 12 Be, 14 Be and 8 B nuclei by elastic proton scattering in inverse kinematics

The proton-nucleus elastic scattering at intermediate energies is a well-established method for the investigation of the nuclear matter distribution in stable nuclei and was recently applied also for the investigation of radioactive nuclei using the method of inverse kinematics. In the current experiment, the differential cross sections for proton elastic scattering on the isotopes $^{7,9,10,11,12,14}$Be and $^8$B were measured. The experiment was performed using the fragment separator at GSI, Darmstadt to produce the radioactive beams. The main part of the experimental setup was the time projection ionization chamber IKAR which was simultaneously used as hydrogen target and a detector for the recoil protons. Auxiliary detectors for projectile tracking and isotope identification were also installed. As results from the experiment, the absolute differential cross sections d$sigma$/d$t$ as a function of the four momentum transfer $t$ were obtained. In this work the differential cross sections for elastic p-$^{12}$Be, p-$^{14}$Be and p-$^{8}$B scattering at low $t$ ($t leq$~0.05~(GeV/c)$^2$) are presented. The measured cross sections were analyzed within the Glauber multiple-scattering theory using different density parameterizations, and the nuclear matter density distributions and radii of the investigated isotopes were determined. The analysis of the differential cross section for the isotope $^{14}$Be shows that a good description of the experimental data is obtained when density distributions consisting of separate core and halo components are used. The determined {it rms} matter radius is $3.11 pm 0.04 pm 0.13$~fm. In the case of the $^{12}$Be nucleus the results showed an extended matter distribution as well. For this nucleus a matter radius of $2.82 pm 0.03 pm 0.12$~fm was determined. An interesting result is that the free $^{12}$Be nucleus behaves differently from the core of $^{14}$Be and is much more extended than it. The data were also compared with theoretical densities calculated within the FMD and the few-body models. In the case of $^{14}$Be, the calculated cross sections describe the experimental data well while, in the case of $^{12}$Be there are discrepancies in the region of high momentum transfer. Preliminary experimental results for the isotope $^8$B are also presented. An extended matter distribution was obtained (though much more compact as compared to the neutron halos). A proton halo structure was observed for the first time with the proton elastic scattering method. The deduced matter radius is $2.60pm 0.02pm 0.26$~fm. The data were compared with microscopic calculations in the frame of the FMD model and reasonable agreement was observed. The results obtained in the present analysis are in most cases consistent with the previous experimental studies of the same isotopes with different experimental methods (total interaction and reaction cross section measurements, momentum distribution measurements). For future investigation of the structure of exotic nuclei a universal detector system EXL is being developed. It will be installed at the NESR at the future FAIR facility where higher intensity beams of radioactive ions are expected. The usage of storage ring techniques provides high luminosity and low background experimental conditions. Results from the feasibility studies of the EXL detector setup, performed at the present ESR storage ring, are presented.

Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms

Predicted 20 years ago, positron binding to neutral atoms has not yet been observed experimentally. A scheme is proposed to detect positron-atom bound states by colliding Rydberg positronium (Ps) with neutral atoms. Estimates of the charge-transfer reaction cross section are obtained using the first Born approximation for a selection of neutral atom targets and a wide range of incident Ps energies and principal quantum numbers. We also estimate the corresponding Ps ionization cross section. The accuracy of the calculations is tested by comparison with earlier predictions for charge transfer in Ps collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps beam suitable for producing positron-atom bound states and estimate signal rates based on the calculated cross sections and realistic experimental parameters. We conclude that the proposed methodology is capable of producing such states and of testing theoretical predictions of their binding energies.