Investigations of Exotic Nuclei with Large Arrays

地球上几乎所有的元素都是在星星的核心形成的。核反应不断地将轻元素转化为重元素，但这一过程发生的确切机制和天体物理学位置仍不清楚。之所以出现这种情况，是因为关于核强力的确切性质有许多不明确之处，核强力负责将质子和中子结合在原子中心。当代核物理学的一个主要目标是揭示这种力的全面统一描述，以便我们能够理解所有核物质的起源和演化。 地球上存在的绝大多数元素都是稳定核，形成了所谓的核稳定谷。这些稳定核在诺贝尔奖获得者M。Goppert-Mayers和J.H.詹森在20世纪40年代末。这个模型已经成功地解释了许多实验观测，包括所谓幻数的存在，这是一组特殊的质子和中子数（2，8，20，28，82和126），它给核物质带来了额外的稳定性，类似于化学中惰性气体的电子构型。几十年来，壳层模型一直是我们理解原子核结构的支柱。近年来，对中子数大于质子数的原子核的实验研究揭示了这些奇异原子核的新幻数。挑战在于理解为什么幻数会随着质子和中子的不同组合而变化，并成功预测其他可能只存在几分之一秒的奇异核是否可以在远离稳定状态的情况下形成。壳层结构的演化可能导致核稳定性理解的新前沿。 为了开发新的模型来描述远离稳定性的核物质结构、幻数的演化以及恒星中元素的产生，科学家需要详细了解核质量、寿命和衰变概率。 实验核科学，如本建议中所描述的，负责收集远离稳定核的关键信息。特别是，大量的锡原子核是一个很好的研究案例，因为它们都有50个质子的幻数，而许多锡同位素的中子数超过82的幻数时远不稳定。拟议的实验将主要在不列颠哥伦比亚省温哥华的TRIUMF进行。不稳定的原子核是在世界上最大的回旋加速器上就地产生的。然后用大型最先进的探测器研究这些原子核，这些探测器能够观察原子核衰变时发出的辐射。从这些实验中提取的信息为核物质提供了线索，并为核天体物理学提供了有关宇宙中元素创造的基本信息。

Almost every element on earth was formed at the core of a star. Nuclear reactions continually convert light elements into heavier ones, but the exact mechanism and astrophysical site where this takes place remain unclear. This situation arises because there are many ambiguities concerning the exact nature of the nuclear strong force, which is responsible for binding the protons and neutrons together at the centre of an atom. A primary goal of contemporary nuclear physics is to reveal a comprehensive unified description of this force so that we can understand the origin and evolution of all nuclear matter. The vast majority of elements existing on earth are stable nuclei, forming the so-called valley of nuclear stability. These stable nuclei are well described in the nuclear shell model developed by Nobel laureates M. Goppert-Mayers and J. H. Jensen in the late 1940s. This model has successfully explained many experimental observations, including the existence of the so-called magic numbers, a special set of proton and neutron numbers (2, 8, 20, 28, 82 and 126) that brings extra stability to nuclear matter, similar to the electron configurations of the noble gases in chemistry. For decades, the shell model has been the backbone to our understanding of nuclear structure. In recent years, experimental investigations of nuclei with larger numbers of neutrons compared to protons have revealed new magic numbers in these exotic nuclei. The challenge is to understand why exactly the magic numbers shift with different combinations of protons and neutrons, and to successfully predict whether other more exotic nuclei that might only exist for fractions of a second can be formed far from stability. The evolution of shell structure could lead to a new frontier of nuclear stability comprehension. In order to develop new models that will describe the structure of nuclear matter far from stability, the evolution of magic numbers, and the creation of elements in stars, scientists need detailed knowledge of nuclear masses, lifetimes, and decay probabilities. Experimental nuclear science, such as that described in this proposal, is responsible for the critical gathering of information for nuclei situated far from stability. In particular, the multitude of tin nuclei makes an excellent case study because they all have a magic number of 50 protons, and many tin isotopes exist far from stability with neutron numbers beyond the magic number of 82. The proposed experiments will take place mainly at TRIUMF in Vancouver, BC. Unstable nuclei are created on-site with the world’s largest cyclotron. These nuclei are then investigated with large state-of-the-art detectors that are able to observe the radiation emitted when the nuclei decay. The information extracted from these experiments provides clues into the nuclear matter and produces essential information for nuclear astrophysics about the creation of elements in the universe.