Two-nucleon structures in the most exotic nuclei

今天，核物理学家通过使用专用的实验室加速器，可以制造出中子和质子组合异常的奇异原子核。虽然在我们地球上的实验室中很少产生，而且产生的时间也很短，但这些奇异的原子核是合成宇宙中较重元素的原料，在核天体物理学中具有相当重要的意义。计算和测量它们的新性质、结构和稳定性是一项科学挑战。一个非常成功适用的核结构理论是壳层模型。最初是一个涉及独立核子运动的模型，现代壳模型结构计算也能够包括（其他独立的）核子对之间的剩余相互作用的影响，使用再现大量已被充分研究的原子核的测量质量、电荷半径和低空激发态的力。这些结果还对奇异核进行了详细的预测，其中核子对的运动现在是相关的，并且在能量上有利于两个核子“配对”到总自旋为零的状态。测试和提高这些全新原子核的理论预测的准确性是本研究项目将有助于实现的一个长期目标。这些量子力学壳层模型状态的探测（光谱学），单个核子和非常短寿命的外来核中的核子对，可以使用合适的直接核反应来实现，只探测一个或两个核子，而其余的，理想情况下，不受干扰。双核子敲除、双核子转移和双核子衰变都是对核子对波函数敏感的过程，尽管它们在不同的自旋态和空间区域对波函数分量的敏感性有所不同。拟议的研究汇集了壳模型结构和核反应专家开始澄清这种敏感性。我们建议研究不同的直接反应在多大程度上可以用来探测相关的核子运动和配对效应，包括当一种核子大量过剩时可能出现的任何新的核子配对效应：例如，中子非常丰富的原子核中的双中子对。这些研究将在这样的系统中进行：(a)去除一对紧密结合的类核子-就像从富含中子的核中去除两个质子一样-这是一个相当干净的直接过程；(b)去除一对非常弱结合的类核子-就像从富含中子的核中去除两个中子一样-这将涉及直接和间接（两步）反应路径。这项研究有望指导未来的实验建议，以检验理论预测。

Today nuclear physicists are able to produce exotic nuclei with very abnormal neutron and proton combinations by using dedicated, laboratory-based accelerators. Although produced only rarely and fleetingly in our earth-based laboratories, these exotic nuclei are the raw materials for the synthesis of the heavier elements in the Universe and are of considerable importance in nuclear astrophysics. It is a scientific challenge both to calculate and measure their novel properties, structures and stability. One very successful applicable nuclear structure theory is the shell model. Originally a model involving independent nucleon motions, modern shell-model structure calculations are also able to include the effects of residual interactions between the pairs of (otherwise independent) nucleons, using forces that reproduce the measured masses, charge radii and low-lying excited states of a large number of well studied nuclei. These result in detailed predictions also for the exotic nuclei in which the motions of pairs of nucleons are now correlated and where it is energetically favourable for two nucleons to 'pair' to states with total spin of zero. Testing and improving the accuracy of the theoretical predictions for these completely new nuclei is a long term goal to which this research project will contribute. The interrogation (spectroscopy) of these quantum mechanical shell model states, of individual nucleons and of pairs of nucleons in very short-lived exotic nuclei, can be achieved using suitable direct nuclear reactions that probe just one or two nucleons leaving the rest, ideally, undisturbed. Two-nucleon knockout, two-nucleon transfer and two-nucleon decay are all processes that are sensitive to the wave function of pairs of nucleons, although they are expected to have somewhat different sensitivities to the wave function components in different spin states and spatial regions. The proposed research brings together shell model structure and nuclear reactions experts to begin to clarify such sensitivities. We propose to study to what extent different direct reactions could be used to probe correlated nucleon motions and pairing effects, including any novel nucleon pairing effects that may arise when there is a large excess of one type of nucleon: e.g. di-neutron pairs in very neutron-rich nuclei. These studies will be carried out for systems in which one: (a) removes a pair of like-nucleons that are tightly bound - as occurs in two-proton removal from a neutron rich nucleus - which is a rather clean direct process, and (b) removes a pair of like nucleons that are very weakly bound - as occurs in two-neutron removal from a neutron-rich nucleus - and which will involve both direct and indirect (two-step) reaction paths. The research is expected to guide future experimental proposals to test the theoretical predictions.