~(40)Ar+~(238)U实验介绍

主要介绍了利用兰州重离子加速器提供的270 MeV的40Ar离子束轰击238U靶,通过熔合蒸发反应进行试合成Z=110附近的新同位素的实验情况。分析了目前关于超重核研究的现状并描述了这次实验的目的、可行性分析、实验装置以及实验过程等。本次实验仍然用氦喷嘴技术对产物进行传输,并用一套具有数对探测器组的转轮收集探测系统对产物进行收集和测量。

单次分子镀法制备部分La系及~(238)U靶的实验研究

采用单次分子镀方法研究了异丙醇-硝酸体系中电流密度、分子镀持续时间及两极间距离对镀层性能和电沉积效率的影响,确定了制备La,Sm,Eu,Gd,Tb靶及238U靶的最佳工艺条件。因制备的靶不同,电流密度一般介于2—8mA/cm2之间,两极间最优距离为3cm,分子镀1h,用分光光度法测定各靶的沉积效率均高于85%。利用扫描电子显微镜(SEM)对部分靶的表面形貌分析后发现靶面结构均匀致密。目前制得的Gd靶和Tb靶已用于中国科学院近代物理研究所加速器SFC低能核化学终端上,利用19F束流轰击,分别产生了Ta和W的短寿命同位素,从而成功完成了Db(Z=105)及Sg(Z=106)的模型试验。

~(23)Si + ~(238)U非完全熔合裂变与跟随裂变测量

用~(28)Si束流在15.0 Mev/A和21.7 Mev/A两个能量轰击~(238)U靶，裂变碎片由六片PSAD来探测，六片PSAD分为两组，前角组覆盖角度为5°～ 74 °，后角组覆盖角度为-60 °～ -168。在这能区出现了非完全线性动量转移现象，线性动量转移分布是双峰结构，小线性动量转移部分对应于跟随裂变，大线性动量转移部分对应于非完全熔合裂变。用线性动量转移将它们区分开来，得到了跟随裂变截应的相对激发函数。用简单的引导粒子模型对非完全熔合裂变线性动量转移最可几值进行了估算，并与21.7Mev/A的实验进行了比较。用跟随裂变线性动量转移最可几值修正后对其类靶核激发能进行了估算，用单体耗散能对其进行了解释。在这能区，考虑了线性动量转移修正后，总动能仍符合Vio/a系统化公式。由于轻粒子蒸发使裂变碎片产生出反应平面角，得到出反应平面角分布的标准偏差与激发能的关系。发现对Zcn> 100的重系统，有额列轻粒子发射现象

Element enrichment and U-series isotopic characteristics of the hydrothermal sulfides at Jade site in the Okinawa Trough

The geochemical and U-series isotopic characteristics of hydrothermal sulfide samples from the Jade site (127A degrees 04.5'E, 27A degrees 15'N, water depth 1300-1450 m) at Jade site in the Okinawa Trough were analyzed. In the hydrothermal sulfide samples bearing sulfate (samples HOK1 and HOK2), the LREEs are relatively enriched. All the hydrothermal sulfide samples except HOK1 belong to Zn-rich hydrothermal sulfide. In comparison with Zn-rich hydrothermal sulfides from other fields, the contents of Zn, Pb, Ag, Cd, Au and Hg are higher, the contents of Fe, Al, Cr, Co, Ni, Sr, Te, Cs, Ti and U lower, and the Pb-210 radioactivity ratios and Pb-210/Pb ratios very low. In the hydrothermal sulfide mainly composed of sphalerite, the correlations between rare elements Hf and U, and Hf and Mn as well as that between dispersive elements Ga and Zn, are strongly positive; also the contents of Au and Ag are related to Fe-sulfide, because the low temperature promotes enrichment of Au and Ag. Meanwhile, the positive correlations between Fe and Bi and between Zn and Cd are not affected by the change of mineral assemblage. Based on the Pb-210/Pb ratios of hydrothermal sulfide samples (3.99x10(-5)-5.42x10(-5)), their U isotopic composition (U-238 content 1.15-2.53 ppm, U-238 activity 1.07-1.87 dpm/g, U-234 activity 1.15-2.09 dpm/g and U-234/U-238 ratio 1.07-1.14) and their Th-232 and Th-230 contents are at base level, and the chronological age of hydrothermal sulfide at Jade site in the Okinawa Trough is between 200 and 2000 yr.

中国北方风成堆积的风化作用与环境变迁—U-Th、~(10)Be及元素地球化学的研究

The surface of the Earth is continuously undergoing changes as a result of weathering-erosion, plate tectonics and volcanic processes. Continental weathering-erosion with its complex rock-water interactions is the central process of global biochemical cycling of elements, and affects the long-term ocean atmosphere budget of carbon dioxide both through the consumption of carbonic acid during silicate weathering and through changes in the weathering and burial rates of organic carbon. Rates of the weathering-erosion depend on a variety of factors, in particular rock properties and chemical composition, climate (especially rainfall), structure, and elevation. They are quite variable on a regional scale. Thus, environmental changes in a region could be indicated by the history of weathering-erosion in the region. Recent attention has focused on increased silicate weathering of tectonically uplifted areas in the India-Asia collision zone as a possible cause for falling atmospheric CO_2 levels in the Cenozoic era. The wind blown dust deposits in the Loess Plateau is derived from the arid and semiarid regions in northwestern China, in turn, where the deposits have been derived from the Qinghai-Xizang Plateau and the high mountains around. Therefore, geochemistry of the wind blown loess-paleosol and red clay sequences may provide insight both to paleoenvironmental changes on the Loess Plateau, and to the uplift and weathering-erosion histories of the Qinghai-Xizang Plateau. In this paper, uranium-thorium series nuclides and cosmogenic ~(10)Be have been employed as tracers of weathering intensities and histories of the dust sediments in the Loess Plateau. Major elements, such as Na, Al, Fe etc., are also used to estimate degree of chemical alteration of the dust sediments and to rebuild the history of weathering on the Loess Plateau. First of all, using a low-level HPGe γ-ray detector, we measured U and Th series nuclides in 170 loess and paleosol samples from five sites in the Loess Plateau, going back 2.6 Ma. The results show that ~(238)U activities are disequilibrium with its daughter nuclide ~(230)Th in young loess-paleosol sequence, indicating that weathering was happened both in dust deposition site and in dust source regions. Using concentrations of ~(238)U and ~(232)Th in the samples, we estimated the amounts of ~(238)U leached out of from paleosols due to weathering. Further, based on analyses of ~(230)Th in paleosols deposited in the past ca. 140 ka, we determined when the paleosols weathered in the source regions. We conclude that most of the weathering in the dust-source regions may have occurred during the interglacials before dust deposition.

CSR的辐射屏蔽设计(英文)

CSR(cooling storage ring)按计划将于2005年底建成调束,届时从~(12)C到~(238)U的重离子将可以分别被加速到900和400MeV的能量。HIRFL(兰州重离子加速器Heavy Ion Research Facility in Lanzhou)将用作CSR的注入器。为了CSR的屏蔽设计,本文利用现有的实验数据计算了由于束流损失产生的中子及其能谱、角分布,同时也估算了屏蔽体外表面的中子剂量、环境中子剂量及天空返照中子剂量。在源项计算中使用了400MeV/u~(12)C+Cu反应的中子产额、能谱、角分布的实验数据。计算表明,CSR对环境剂量影响最大的是天空返照中子。

用1-苯基-3-甲基-4-苯甲酰基-5-吡唑啉酮从硝酸中萃取痕量钍

用1-苯基-3-甲基-4-苯甲酰基-5-吡唑啉酮(PMBP)为萃取剂,~(234)Th作示踪剂,完成了在硝酸介质中痕量钍溶剂萃取行为的研究。在~(234)Th萃取效率与酸度、萃取剂浓度、平衡时间等依赖关系的条件实验的基础上,获得了萃取钍的最佳条件。使用改进的PMBP萃取钍的流程,从~(18)O离子辐照过的铀靶中分离钍,钍样品的γ射线单谱显示绝大部分反应产物和大量铀的去除是满意的。

143Pm高自旋态和重丰中子新核素的合成及衰变性质研究

本论文从内容上可以分为两大部分：第一部分：对近球形核~(143)Pm高自旋态进行的在束γ研究。这一部分是本文的重点。利用能量为82和90 MeV的~(19)F束流，通过融合蒸发反应~(128)Te (~(19)F,4nγ) ~(143)Pm布局~(143)Pm的高自旋态。利用联合在束装置(JIBGE)的10套带BGO反康的HPGe探测器进行了标准在束γ谱学测量，包括γ射线的激发函数、γ单谱、γ-γ-t符合谱以及DCO测量。在已有工作的基础上，建立了~(143)Pm激发能高达10.5 MeV，自旋约为(61/2)h的高自旋态能级结构。观测到了28条新的跃迁能级和48条新的γ射线。对两个已知的同质异能态寿命进行了提取，并在8 MeV激发能附近进行了高自旋态同质异能态的搜索。以~(142)Nd，~(144)Sm为核实，用零级弱耦合模型对~(143)Pm的晕态能级结构进行了定量的解释。计算表明，直到激发能Ex = 6.77 MeV，自旋宇称J~π = (37/2~+)的晕态能级都可以用弱耦合模型进行很好的解释。但是，对于更高激发能的能级，组态情况要复杂的多，有出现N = 82中子闭壳打破的可能。同时，利用大基壳模型OXBASH程序对其能级结构进行了计算和讨论，其结果支持弱耦合模型的假设。第二部分：利用中能重离子的多核子转移反应，(~(186)W-2p + 2n)，(~(238)U-2p + 2n)对丰中子核~(186)Hf和~(238)Th进行了合成和鉴别。测量它们的半衰期分别为(2.6 ± 1.2) min和 (9.4 ± 2.0) min，与用质子一中子准粒子随机相近似方法的预言值是相符的。同时，对β延发裂变先驱核~(230)Ac进行了实验研究。观测到了它的两个裂变事件，得到~(230)Acβ延发裂变几率为(1.19 ± 0.85) * 10~(-8)。从而使~(230)Ac成为世界上第一种被确认了的基态β延发裂变先驱核。

近垒及垒下能区重离子熔合-裂变反应机制的研究

本工作用固体云母核径迹探测器测量了质心能量为73.7至93.4MeV的~(16)O + ~(238)U和能量为57.9至76.0MeV的~(12)C + ~(237)Np系统熔合裂变碎片角分布和截面；首次用双栅位置灵敏雪崩计数器[DGAC]测量了质心能量为46.5至56.2 MeV的~(11)B + ~(238)U和能量为58.9至76.0 MeV的~(12)C + ~(237)Np及84.0MeV的~(16)O + ~(238)U系统熔合裂变碎片角关联和截面。研究结果表明：使用二种测量方法得到的熔合裂变截面和裂变碎片角分布是一致的，对于三个反应系统熔合裂变截面用考虑靶核形变的耦合道理论能得到合理的描述；而从计算熔合截面中提取穿透系数，从有限力程转动液滴模型中提取有效转动惯量，用鞍点过渡态理论计算裂变碎片角分布，发现裂变碎片角分布各向异性异常；随入射能量减少，各向异性并不是单调地变化，而是呈现异常的“鼓包”；裂变碎片各向异性还有明显的入射道效应，在近垒和垒上能区~(12)C + ~(237)Np和~(11)B + ~(238)U系统裂变碎片角分布可以用鞍点过渡态理论描述，没有发现异常；而对于~(16)O + ~(238)U系统裂变碎片角分布都有大于理论的异常，而且在垒上低能端异性度随能量的变化走向不同于以前测量的结果。虽然入射道效应证明了前平衡裂变的理论预言，但对于垒下能区，碎片异性度的异常依然存在，表明仅用前平衡裂变理论解释裂变碎片角分布的异常是不够的。为了澄清转移裂变对角分布异常的贡献，采用了角关联技术成功地区分出复合核裂变和转移裂变，实现了对纯复合核裂变碎片角分布各向异性的研究。在被研究的能区内，转移裂变占总裂变截面10%以下，而且它的各向异性度比复合核裂变的要小，扣除转移裂变的贡献使裂变碎片异性度提高5%，因此角分布异常不可能起源于转移裂变的贡献

中能重离子反应中靶碎片的质量分布研究

本文利用核化学方法研究了中能重离子反应机制。在135MeV/u~(12)C + Fe反应中鉴别了39个放射性核素，并得到了这些靶余核的生成截面，通过电荷分布假设导出了该反应中靶余核的质量分布。结果表明，中能重离子与非裂变靶核相互作用中靶余核质量分布的形状相似，随着入射能的增加，质量分布曲线变得更加平缓。同时，在低质量区（A < 30）反应产物的产额明显增加，且产物有偏向丰中子一侧的趋势。利用熔合碎裂统计模型和级联两体统计模型对实验结果进行了拟合，两种理论模型都能较好地符合实验数据的总体趋势，但考虑了多重碎裂衰变模式的熔合碎裂统计模型的计算结果再现了A < 30质量区产额的回升现象，而级联两体模型更好地符合了蒸发余核的质量分布，预示着在很高入射能的情况下，多重碎裂成为靶余核形成的主要反应机制之一。在20MeV/u~(40)Ar引起~(238)U的裂变反应中测到了101个核素的生成截面，通过高斯电荷分布曲线拟合得到了各同量民位素的独立产额以及各质量链的质量产额。从电荷分布曲线中可以看出，裂变碎片来源于两种不同的反应机制，即丰中子产物来自高激发核的裂变过程，而缺中子产物来自低能裂变过程。质量分布曲线在裂变质量区（80 < A < 140）有一很宽的对称裂变峰而在近靶区的重靶余核也能以很高的截面通过周边碰撞产生

Production of new superheavy Z=108-114 nuclei with U-238, Pu-244, and Cm-248,Cm-250 targets

Within the framework of the dinuclear system (DNS) model, production cross sections of new superheavy nuclei with charged numbers Z=108-114 are analyzed systematically. Possible combinations based on the actinide nuclides U-238, Pu-244, and Cm-248,Cm-250 with the optimal excitation energies and evaporation channels are pointed out to synthesize new isotopes that lie between the nuclides produced in the cold fusion reactions and the Ca-48-induced fusion reactions experimentally, which are feasible to be constructed experimentally. It is found that the production cross sections of superheavy nuclei decrease drastically with the charged numbers of compound nuclei. Larger mass asymmetries of the entrance channels enhance the cross sections in 2n-5n channels.

Production cross-section of Ra-230 in the reaction of 60 MeV/u O-18 ions with U-238

A thick natural uranium target was bombarded with a 60 MeV/u O-18 beam. The neutron-rich isotope Ra-230 as the target residue was produced through the multinucleon transfer reaction (U-238-4p-4n). The barium and radium fraction as BaCl2 precipitate were radiochemically separated first from the mixture of uranium and reaction products. Then, the radium fraction was separated from BaCl2 precipitate by using cation exchange technique. The gamma-ray spectra of the Ra fraction were measured using an HPGe detector. The production cross sections of Ra-230 were obtained by a combination of the radiochemical separation technique and off-line gamma-ray spectroscopy. The cross section of Ra-230 has been determined to be 66 +/- 20 mu b.

Production cross section of Th-236 in the interaction of U-238 with 60 MeV/u O-18 ions

The neutron-rich target-like isotope Th-236 has been produced in the U-238-2p multinucleon transfer reaction between a 60 MeV/u O-18 beam and natural U-238 targets. The activities of thorium were determined after radiochemical separation of Th from the mixture of uranium and reaction products. The Th-236 isotope was identified by the characteristic gamma-rays of 642.2, 687.6 and 229.6 keV. The production cross section of Th-236 was determined to be 250 +/- 50 mu b.

Production cross-section of Th-236 in the interaction of U-238 with 60MeV/u O-18 Ions

The neutron-rich target-like isotope Th-236 was produced in U-238-2p multinucleon transfer reaction between a 60MeV/u O-18 beam and nature U-238 targets. The thorium activities were radiochemically separated from the mixture of uranium and reaction products. The isotope Th-236 was identified by 642.2keV, 687.6keV and 229.6keV characteristic gamma-rays. The production cross section of Th-236 has been determined to be 250 +/- 50 mu b.