论文标题:氘化金属靶中D（d，p）T反应电子屏蔽效应温度相关性的实验研究
Temperature Dependence of Electron Screening in D(d,p) T Reaction for Deuterated Metals
论文作者
论文导师 柳卫平；C.Rolfs,论文学位 博士,论文专业 粒子物理与原子核物理
论文单位 中国原子能科学研究院,点击次数 102,论文页数 93页File Size5858k
2005-07-01论文网 http://www.lw23.com/lunwen_455777887/
electron screening; temperature effects; Debye model; secondary beam;angular distribution
带电粒子热核反应截面是研究恒星演化进程中核能的产生和核素的合成所必需的关键核物理输入量。恒星中的热核反应发生在远低于库伦势垒的能区,通常的实验很难达到。为了研究该能区的带电粒子反应截面,上世纪80年代晚期以来,人们建立了一些特殊的装置和方法,使测量不断向低能区延伸,测量了一系列同位素靶轻核反应的截面。这些测量清楚地揭示出电子屏蔽效应的存在及其随能量变化的趋势。近年来,德国柏林大学和鲁尔大学的两个实验小组分别研究了一系列氘化金属、半导体、绝缘体靶中D(d,p)T反应的电子屏蔽效应。实验结果表明,氘化金属靶中电子屏蔽效应呈现异常的增大。通过对金属中的电子作准自由近似,这种现象可以用德拜模型解释。但德拜模型是一种经典的模型,没有考虑量子效应,必须经过进一步的实验验证。从德拜模型的表达式可以看出,电子屏蔽势与温度的平方根成反比。因此,进行温度相关性的研究是十分必要的。 为此,我们在德国鲁尔大学DTL实验室的100kV加速器上测量了T=20-340℃范围内氘化Co、Ta和Pt靶中D(d,p)T反应的电子屏蔽效应,得到了电子屏蔽势随温度升高而逐渐减小的结果,与德拜模型的预言相符。然而,对于一系列元素周期表中第三、四族以及镧系元素的氘化靶(在常温下氘在这些元素中的溶度很高,即它们对氘具有很强吸附能力),在T=200℃时测量的D(d,p)T反应电子屏蔽势却比常温下的显著增大。在常温下,由于氘在这些介质中的溶度很高,注入层中形成的类氘化物使金属性趋于消失,导致电子屏蔽效应很弱。当T=200℃时,氘在注入层中的溶度急剧下降,金属性得以恢复,因而电子屏蔽效应增强。这一现象也可以用德拜模型解释,并利用完全无关的霍尔系数方法就电子浓度信息进行了验证。 这项工作第一次从实验上研究了核反应截面的温度相关性。并对即将开展的相关α衰变实验进行了讨论。目前,实验数据处理中存在两方面的不确定性:一是
The experimental and theoretical studies of the electron screening effects in charged-particle induced reactions at energies of astrophysical interest are systematically reviewed, and the theoretical discussions on the electrostatic screening effects are also summarized briefly.The electron screening in D(d, p)T was studied previously for deuterated metals, insulators, and semiconductors. As compared to measurements performed with a gaseous D_2 target (U_e = 25 eV), a large screening was observed in the metals, while a small screening was found for the insulators and semiconductors. An exception was found for the metals of groups 3 and 4 of the periodic table and the lanthanides, which showed a small screening; this is related to their high hydrogen solubility y. For the metals with high U_e values, the solubilities were small (a few percent) leaving the metallic character of the samples essentially unchanged. An explanation of the large screening was suggested by the plasma screening of Debye applied to the quasi-free metallic electrons.For a deep understanding of the electron screening effects, we cooperated with the astrophysics group of Ruhr University to study the electron screening in the D(d, p)T reaction, using the 100kV accelerator of the Dynamitron-Tandem- Laboratorium, for the deuterated metal Pt at a sample temperature T = 20℃ to 340℃, and for Ta and Co at T = 20℃ and 200℃. The enhanced electron screening decreases with increasing temperature, where the data agree with the plasma model of Debye applied to the quasi-free metallic electrons. The data represent the first observation of a temperature dependence of a nuclear cross section. We also measured the screening effect for the deuterated metal Ti (an element of group 4 of the periodic table) at T = -10℃ to 200℃: above 50℃ the hydrogen solubility dropped to values far below one and alarge screening effect became observable. Similarly, all metals of groups 3 and 4 and the lanthanides showed a solubility of a few percent at T = 200°C (compared to T = 20°C) and a large screening became also observable. Within the Debye model the deduced number of valence electrons per metallic atom agrees with the corresponding number from the Hall coefficient, for all metals investigated. As another application of the Debye model, the possible enhancement of the a-decay rate in a metallic environment is discussed.
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