双中子集团结构研究进展
Progress in Study of Dineutron Cluster Structure
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摘要: 回顾了晕核发现以来双中子集团结构的研究进展，分析了可能的发展方向。系统的理论研究表明， 在原子核的表面和低密度核物质中， 空间紧密关联的2n集团的出现是一个普遍的现象。 但在较重原子核表面出现2n集团的机制与核物质或轻晕核中的机制很不一样， 前者是由有限核的平均场造成的尺度效应（size effect），而后者主要是由低密度下对相互作用（或对能隙）的增强造成的。另外，在轻晕核或有限核表面，2n集团的均方根半径与它们到核芯的距离（或背景密度）有关，最小值普遍可以达到2～3 fm，然而在核物质中2n集团最小只能到～5 fm。 实验方面，在重靶上的库仑激发强度， 能够比较准确地给出2n系统到核芯的平均距离。 但到目前为止尚没有有效的实验方法给出基态中两个中子之间的间距， 主要原因是中子发射过程中末态相互作用（共振态或虚粒子态）造成的两步过程的干扰， 这个问题在库仑激发破碎（重靶）或核作用破碎（轻靶）中都出现。“拖出”反应和敲出核芯反应是下一步可以考虑的路径。 双中子的关联测量通常效率比较低，尤其是需要有效排除中子串扰(CT)事件。为此， 需要发展特殊设计的中子关联测量装置， 在提高探测效率的同时，能够通过运动学关系以及其他方法有效排除中子CT信号。 在数据处理阶段，通过反复迭代给出的双中子关联函数， 经验证明是比较有效的关联状态表达方式， 从中可以直接提取出双中子分布均方根半径。 This article outlines the progress in the study of the dineutron structure in various systems. Systematic theoretical investigations reveal that dineutron structure is a general phenomenon appeared at nucleus surface and in low density nuclear matter. But the underline mechanism of forming dineutron clusters at the surface of heavier nuclei is quite different to that for light halo nuclei or at the low density nuclear matter， with the former being basically due to the so called “size effect” and the latter due to the enhanced pairing interaction. It is also realized that the RMS radii of the dineutron cluster at the surface of light halo nuclei or heavier finite nuclei varies with the distance from the center of nuclei (or background density) and may attain a minimum of about 2～3 fm， whereas that in the low density nuclear matter may only attain about 5 fm. From experimental side， Coulomb excitation caused by heavy targets provides a good way to extract the mean distance from the center of the neutron pair to the center of nucleus. But up to now it is still difficult to experimentally determine the distance between the two valence neutrons， due primarily to the final state interactions which lead to two step emission of neutrons via resonances or virtual intermediate states. This problem happens in both Coulomb and nuclear breakup processes. Possible ways to avoid this problem might come from experiments based on “towing mode” or core knockout reactions. Detection of two neutrons in coincidence often suffers from low efficiencies and the need to reject the cross talk events. Therefore it is important to develop specially designed multineutron detection array to achieve high efficiency as well as good cross talk rejection performance using kinematics conditions. For data analysis， it was found that two neutron correlation function generated by iteration method is a good expression of the correlation situation， from which the RMS radii of the two neutron distribution may be deduced.Abstract: This article outlines the progress in the study of the dineutron structure in various systems. Systematic theoretical investigations reveal that dineutron structure is a general phenomenon appeared at nucleus surface and in low density nuclear matter. But the underline mechanism of forming dineutron clusters at the surface of heavier nuclei is quite different to that for light halo nuclei or at the low density nuclear matter， with the former being basically due to the so called “size effect” and the latter due to the enhanced pairing interaction. It is also realized that the RMS radii of the dineutron cluster at the surface of light halo nuclei or heavier finite nuclei varies with the distance from the center of nuclei (or background density) and may attain a minimum of about 2～3 fm， whereas that in the low density nuclear matter may only attain about 5 fm. From experimental side， Coulomb excitation caused by heavy targets provides a good way to extract the mean distance from the center of the neutron pair to the center of nucleus. But up to now it is still difficult to experimentally determine the distance between the two valence neutrons， due primarily to the final state interactions which lead to two step emission of neutrons via resonances or virtual intermediate states. This problem happens in both Coulomb and nuclear breakup processes. Possible ways to avoid this problem might come from experiments based on “towing mode” or core knockout reactions. Detection of two neutrons in coincidence often suffers from low efficiencies and the need to reject the cross talk events. Therefore it is important to develop specially designed multineutron detection array to achieve high efficiency as well as good cross talk rejection performance using kinematics conditions. For data analysis， it was found that two neutron correlation function generated by iteration method is a good expression of the correlation situation， from which the RMS radii of the two neutron distribution may be deduced.
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