Symmetry-restored two-centre self-consistent approach to fission with arbitrary deformations, orientations, and distance of fragments

具有任意变形、方向和碎片距离的对称性恢复的两中心自洽裂变方法

• 批准号: ST/W005832/1
• 负责人: Jacek Dobaczewski
• 金额: $ 25.77万
• 依托单位: University of York
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FW005832%2F1
• 关键词: Symmetry; restored; two; centre; self

The vision of this proposal is to bring into the physics of nuclear fission the most advanced theoretical ideas and computation. Since the discovery of fission almost eighty years ago, a wealth of experimental data has been accumulated. This has been accompanied by the development of an efficient phenomenology and microscopy of spontaneous and induced fission. However, almost all of these studies rely on assuming the adiabaticity and/or thermalisation of fission. Is the energy sufficiently low and time sufficiently long for these assumptions to hold? This project has the ambition to implement theoretical modelling of fission that will deliver definite answers to these challenges. From the outside, fission looks like a simple process where a single drop of matter splits into two or more smaller drops. However, this is very misleading: a huge conceptual gap exists between the splitting of liquid drops and nuclear fission. Briefly, during the fission process, one composite quantum system splits into two or more composite quantum systems, and all properties of such a process crucially depend on quantum physics, which is not the case for the classical liquid drop. Here, nucleons move in correlated quantum orbitals that evolve into correlated quantum orbitals within the fission fragments. Altogether, in fission we find all the beauty and difficulty of a mesoscopic system. The fission happens in the border region between classical and quantal, large and small, and slow and fast phenomena. This is why it is so challenging and consequently provides an important subject of fundamental science research.Currently, the methodology used for describing induced fission at varying excitation energies is in a very rudimentary state. The standard framework, dating all the way back to the pioneering work of Bohr&Wheeler in 1939, is that of a hot thermalized compound nucleus, which is created after resonant neutron capture. However, applying this concept to the other mechanisms of creating pre-fission states is not really the right way to proceed. Indeed, after the beta-decay of a precursor system, photon absorption, Coulomb excitation by a passing charge, or particle transfer, the nucleus ends up in a fairly well determined intermediate "doorway" state, which then fissions. Depending on the excitation energy and fission time scale, such an intermediate state may or may not have enough time to thermalize, and then the very concept of a compound nucleus becomes highly questionable.A full research programme to challenge the main paradigms of the theory of fission would require a substantial investment in the workforce and resources. The current proposal aims to deliver one fundamental computational element of such a programme, namely, the computer code that would allow for solving the nuclear DFT equations within the symmetry-restored two-centre self-consistent approach to fission with arbitrary deformations, orientations, and distance of fragments. The main challenge here is to perform efficient fission calculations using novel nonlocal density functionals, which are in the centre of current developments of the nuclear DFT. Using this tool, in the future, we will be able to build the doorway states explicitly, by employing the high-energy vibrational limit of the time-dependent density functional theory (DFT), and then to follow the fission with coupling to such vibrations included. This full programme will redefine future research of fission. We cannot anymore rely on the old ideas and simplifications. At present, not a single prediction of fission lifetimes beyond the adiabatic limit exists. To show that this is actually possible to obtain, would be in itself an important breakthrough in research. To show that without adiabaticity or thermalisation the results are fundamentally and qualitatively different, would lead to an entirely new approach to future research.

本提案的愿景是将最先进的理论思想和计算引入核裂变物理学。自从大约80年前发现裂变以来，积累了大量的实验数据。这一直伴随着自发和诱导裂变的有效现象学和显微镜的发展。然而，几乎所有这些研究都依赖于假设裂变的绝热和/或热化。能量是否足够低，时间是否足够长使这些假设成立？这个项目的目标是实现裂变的理论建模，为这些挑战提供明确的答案。从表面上看，裂变看起来像一个简单的过程，即一滴物质分裂成两个或更多的小液滴。然而，这是非常误导人的：液滴分裂和核裂变之间存在着巨大的概念差距。简而言之，在裂变过程中，一个复合量子系统分裂成两个或两个以上的复合量子系统，而这一过程的所有性质都取决于量子物理，而经典的液滴却不是这样。在这里，核子在相关的量子轨道中运动，这些轨道在裂变碎片中演变成相关的量子轨道。总而言之，在裂变中我们发现了介观系统的所有美丽和困难。裂变发生在经典现象与量子现象、大现象与小现象、慢现象与快现象之间的边界区域。这就是为什么它是如此具有挑战性，并因此提供了一个重要的基础科学研究课题。目前，用于描述不同激发能诱导裂变的方法还处于非常初级的状态。标准的框架，可以追溯到1939年boh&wheeler的开创性工作，是一个热的热化复合核，它是在共振中子捕获后产生的。然而，将这一概念应用于创造裂变前状态的其他机制并不是正确的方法。事实上，在前体系统的β衰变、光子吸收、经过电荷的库仑激发或粒子转移之后，原子核最终处于一个相当确定的中间“门口”状态，然后发生裂变。根据激发能和裂变时间尺度的不同，这种中间态可能有也可能没有足够的时间来热化，然后复合核的概念就变得非常值得怀疑了。一项挑战裂变理论主要范式的全面研究方案将需要在人力和资源方面进行大量投资。目前的提议旨在提供这样一个程序的一个基本计算元素，即，计算机代码，该代码将允许在对称恢复的双中心自洽方法中求解核DFT方程，该方法具有任意变形，方向和碎片距离。这里的主要挑战是使用新的非局部密度泛函进行有效的裂变计算，这是当前核DFT发展的中心。使用这个工具，在未来，我们将能够明确地建立门口状态，通过使用时间依赖的密度泛函理论（DFT）的高能振动极限，然后跟踪裂变与耦合这些振动包括在内。这个完整的计划将重新定义未来的裂变研究。我们不能再依赖旧观念和简单化了。目前，没有一个预测裂变寿命超过绝热极限的存在。证明这实际上是可能实现的，这本身就是一个重要的研究突破。为了证明没有绝热或热化的结果在本质上和质量上是不同的，将为未来的研究带来一种全新的方法。