977 resultados para U-shaped slots


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本文首先回顾了重离子物理学的发展简史,并介绍了当前的一些关于重离子反应的理论方法,同时对于现在核物理的研究热点,热核的特性进行了概括性的叙述.由于裂变是作者工作的重点,因此在文章中,对裂变反应的理论也进行了介绍.为了能够清楚全面的说明我们的工作,本文对于实验装置和探测器也给出了详细的描述.最后,文章对25 MeVu Ar+Bi反应中的裂变进行了讨论.在这些讨论中,我们分析了反应中与裂变相关的数据,得到了碎片的质量,动能分布。同时运用与碎片相关联的4He粒子能谱提取了核温度和裂前裂后的4He粒子多重性,并利用这一数据计算了不同初始激发能下的裂点激发能和裂变时标.

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重离子核反应机制的研究一直是人们关注的课题。用25MeV/u 40Ar的束流进行多种反应机制共存及其对碎片的贡献的研究,有助于我们更清楚地了解反应机制的演化和对放射性束流产额的预言。 实验中用25MeV/u40Ar的束流轰击115In、58Ni、27Al,这时对前方向6°、8.5°的碎片测量,发现它们的平行动量分布宽度满足Goldhaber关系式,碎裂部分提取的约化平行动量分布宽度σ0=54MeV/c,这比弹核碎裂部分的宽度要窄,说明转移反应比弹核碎裂受到相空间的严格限制更严格。转移反应主要是对弹核附近的碎片有贡献,当类弹碎片质量与弹核质量相差较大时,主要是弹核碎裂的贡献。对于弹核碎裂产物的最可几能量的提取,在实验能谱中拟合出碎片的最可几能量,对于接近弹核的碎片,我们利用外推法来确定碎裂部分的最可几能量,同时用实验提取的σ0值,拟合出碎裂能谱,从而将弹核碎裂和转移反应的贡献区分开来。 通过碎片同位素的测量,我们认碎片的同位素分布与靶核有一定的依赖关系,当靶核的N/Z值增加时,产物的N/Z也增加。弹核40Ar的N/Z为1.22,靶核的N/Z均大于1,所以产物N/Z的变化意味着系统由非平衡向平衡的演化,且在这一能区,重离子反应既有低能下一体耗散的特点,又有高能下核子-核子碰撞的特点,多种反应机制共存并竞争。

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利用气体探测器和CsI望远镜探测器测量了30MeV/u 40Ar+natAg反应中出射的轻粒子、中等质量碎片和重碎片。中能区轻粒子和中等质量碎片的发射成分与角度和碰撞参数有关。在前角区粒子和碎片主要来自于弹核碎裂,其平行动量分布宽度满足Goldhaber公式。提取的动量分布约化宽度σ0≈76MeV/c;在后角区轻粒子和中等质量碎片主要来自于类靶平衡热核的蒸发。利用单源模型能很好地拟合其发射能谱,由α粒子能谱提取的核温度约为4.2MeV;同时利用He/Li同位素比的方法得到的核温度约为4.6MeV。

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中等质量碎片(IMF)的发射是中能重离子碰撞的一大特点,且随着轰击能量的升高其产额迅速增加。通过测量30MeV/u 40Ar+natAg反应中出射的中等质量碎片,研究了高激发热核的发射机制、发射时标和时空演化规律。 对反应中发射碎片能谱的运动源拟合表明,前角区(11°—22°)中等质量碎片(IMF)来自于三种成分:类靶源、类弹源和中速源成分,其中类弹和中速成分占主导,关联测量的IMF能谱拟合得到,一个来自于类弹源而另一个来自于中速源成分的事件占有相当大的比例。 利用小相对角度内的两碎片关联测量,研究高激发核衰变中中等质量碎片的发射时标和寿命。IMF发射时间随能量的变化很大,从低能碎片的250 fm/c到高能粒子的100 fm/c,表明在此能量下,反应中出射的IMF主要来自于相继两体衰变。通过与其它实验的比较可知,随着束流能量的升高,IMF发射时间由相继两体衰变向多重碎裂过渡。 IMF时空演化研究表明,发射空间的大小对IMF关联函数的影响主要来自于发射源的核物质密度而几乎不依赖于发射源的质量数。

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本论文中重点研究了30MeV/u 40Ar+58Ni,64Ni和115In反应中中等质量碎片(IMF)的发射机制。实验中,测量了实验室系5°~140°角度范围内出射碎片的能谱和角分布。对前角区出射的IMF(3≤Z≤13)实现了同位素鉴别,对中后角区出射的碎片在低探测阈(<2MeV/u )的前提下实现了直到Z~30的元素鉴别。 用运动源模型对不同角度下出射的碎片能谱进行了分析和讨论,并结合角分布特征定性地研究了碎片的三个发射源。通过对各源的贡献随角度以及出射碎片电荷数Z的演化,观察到:类弹源主要发射的是那些前角区出射的、接近束流速度的高能碎片;中等速度源的发射是中角度区出射碎片和前角区低能碎片的主要来源;后角区出射的碎片则主要来自于类熔合源的发射。并观察到相对于类熔合源非平衡源更容易发射较轻的碎片。 通过对前角区出射IMF(3≤Z≤13)的能谱和同位素分布的分析,确定了那些基本保持束流速度的碎片主要来自于弹核碎裂过程。用各种模型对实验同位素分布进行了拟合,发现Sümmerer等人给出的经验公式和abrasion-ablation模型均能比较满意地拟合实验同位素分布的宽度和峰位。同时也观察到abrasion-ablation模型计算对奇Z元素的同位素分布能给出较好的拟合,但对偶Z元素的同位素分布,计算结果与实验值相比出现向丰中子方向的系统性偏移(~lamu)。另外,还着重研究了这些产物的靶核相关性问题。通过系统性分析以及同位旋相关的量子分子动力学(IQMD)模型计算,得出了弹核碎裂产物的靶核相关性是源于靶核表面中子与质子分布的不同和平均场及核子核子相互作用的同位旋效应相关。并且,通过计算还指出了靶核中子皮的厚度对于用弹核碎裂方法产生丰中子同位素的重要性。 通过用统计模型拟合后角区出射碎片的电荷分布,指出了这些碎片主要来自于非完全熔合过程中形成的复合系统的统计发射。 实验中观察到30MeV/u 40Ar轰击Ni和In靶在中角区出射的碎片的电荷分布有不同的特征。相对来说,前者更服从power-law,后者则倾向于服从指数规律。结合核态方程和QMD模型计算分析得出,在30MeV/u 的40Ar引起的反应中,对于弾靶质量接近对称的碰撞对所形成的系统,在一定的碰撞条件下可能已进入spinodal区,而非对称碰撞对的碰撞中压缩能还难以使得系统在膨胀时进入力学不稳定区。从而对观察到的实验现象进行了说明,并认为30MeV/u 40Ar轰击与其质量接近的靶核的反应中出射的碎片中已有相当数量的动力学发射成分的贡献。 利用实验中在前角区出射的接近束流速度的碎片的同位素产额提取了类弹源的温度参数。观察到直接由实验同位素产额比得到的表观核温度与所选择的同位素组合有较强烈的依赖关系,而与靶核和实验室探测角基本无关。利用M.B.Tsang等人给出的修正方法,提取了经边馈效应修正后的发射源温度。得到该温度值与反应靶、探测角以及用于提出温度的同位素组合没有明显的依赖关系,均为~4MeV。并用abrasion-ablation模型计算进行了讨论,得到在假定能级密度参数的倒数k=10MeV时,该温度值与abrasion-ablation模型计算是一致的。

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Self-assembled behavior of rod-terminally tethered three-armed star-shaped coil block copolymer melts was studied by applying self-consistent-field lattice techniques in three-dimensional (3D) space. Similar to rod-coil diblock copolymers, five morphologies were observed, i.e., lamellar, perforated lamellar, gyroidlike, cylindrical and sphericallike structures, while the distribution of the morphologies in the phase diagram was dramatically changed with respect to that Of rod-coil diblock copolymers.

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The morphology of a H-shaped block copolymer (poly(ethylene glycol) backbone and polystyrene branches (PS)(2)PEG(PS)(2)) in a thin film has been investigated. A peculiar square lamella that has a phase-separated microdomain at its surface is obtained after spin coating. The experimental temperature plays a critical role in the lamellar formation. The copolymer first self-assembles into square lamellar micelles with an incomplete crystalline core due to the crystallizability of PEG.

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The formation of ring-shaped structures in an H-shaped block copolymer [a poly(ethylene glycol) backbone with polystyrene branches, i.e., (PS)(2)PEG(PS)(2)] thin film was investigated when it was annealed in saturated PEG-selective acetonitrile vapor. Our results clearly indicate that ring formation is determined by the initial morphology of the spin-coated film, the solvent vapor selectivity and the environmental temperature of the solvent-annealing process. Only the films with the initial core-shell cylindrical structure in strongly PEG-selective acetonitrile vapor could form the ring-shaped structures.

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The amphiphilic PEG1 500-b-EM AP-b-PEG1 500 (EM PAP) triblock copolymer of poly(ethylene glycol) (PEG) and emeraldine aniline-pentamer (EM AP) in its concentrated solution can self-assemble into a special shape like "sandglass", as observed by transmission electron microscopy (TEM), field emission scanning electron microscopy (ESEM) and atomic force microscopy (AFM). This "sandglass"- shaped assembly is composed of several "rods" aggregated in the middle, with every "rod" being about 8 VLrn in length and 300 nm in diameter.

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Self-assembled behavior of T-shaped rod-coil block copolymer melts is studied by applying self-consistent-field lattice techniques in three-dimensional space. Compared with rod-coil diblock copolymers with the anchor point positioned at one end, the copolymers with the anchor point at the middle of the rod exhibit significantly different phase behaviors. When the rod volume fraction is low, the steric hindrance of the lateral coils prevents the rods stacking into strip or micelle as that in rod-coil diblock copolymers. The competition between interfacial energy and entropy results in the formation of lamellar structures and the increasing thickness of the lamellar layer with increasing rod volume fraction.

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By using a combinatorial screening method based on the self-consistent field theory (SCFT) for polymer systems, the micro-phase morphologies of the H-shaped (AC)B(CA) ternary block copolymer system are studied in three-dimensional (3D) space. By systematically varying the volume fractions of the components A, B, and C, six triangle phase diagrams of this H-shaped (AC)B(CA) ternary block copolymer system with equal interaction energies among the three components are constructed from the weaker segregation regime to the strong segregation regime, In this study, thirteen 3D micro-phase morphologies for this H-shaped ternary block copolymer system are identified to be stable and seven 3D microphase morphologies are found to be metastable.