Collaborative Research: Preliminary Design of BL3, A New Neutron Lifetime Experiment Using the Beam Method

中子是普通物质的基本组成部分，地球的大部分质量来自中子。然而，当中子从稳定的原子核中释放出来时，它就会衰变为质子、电子和反中微子，平均寿命约为15分钟。中子衰变在早期宇宙中发挥了重要作用：它决定了在大爆炸后最初几分钟内形成的轻元素（氢、氦、锂、铍）及其同位素的相对丰度。由于它的简单性，中子衰变是研究亚原子物理中最基本的力，特别是弱核力细节的理想系统。这些研究提高了我们对自然的理解，并可能为尚未发现的新的基本物理现象提供线索。单个实验测得的中子平均寿命的不确定度小于1秒，但目前两种主要的实验方法——束流法和超冷中子储存法的结果相差超过8秒。解决这一矛盾是一个非常重要的问题。该奖项支持下一代束流中子寿命实验BL3的初步设计。它将采用新的、强大的技术特征来实现解决差异的目标，并提供一个可靠的测量中子寿命的不确定度远低于一秒。本项目为培养本科生、研究生和博士后中子科学的方法和理论提供了良好的机会，这些方法和理论适用于世界各地现有和新兴中子源的物理、化学、材料科学和生物学的各种科学研究。本课题的工作范围是用束流法测量中子寿命的新实验装置BL3的初步设计。它在概念上是相似的，并且改进了以前的束流中子寿命实验。它将采用一个更大的超导磁体来容纳大面积的中子束，并将纳入许多技术改进，例如更高的计数统计，在捕获区域更均匀的磁场；大型、分段、超薄窗口硅质子探测器；还有精密的中子飞行时间光谱仪。BL3有两个科学目标：1)研究和测试可能导致束流和超冷中子储存实验之间8.4 s （4 sigma）差异的束流方法中的系统效应；2)将中子寿命束法的总不确定度降低到0.3 s以下。中子寿命的数值在核物理学、粒子物理学、天体物理学和宇宙学中具有重要的意义。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The neutron is a basic building block of ordinary matter and most of the Earth's mass comes from neutrons. However, when freed from a stable atomic nucleus the neutron decays into a proton, electron, and antineutrino with a mean lifetime of about fifteen minutes. Neutron decay played an important role in the early universe: it determined the relative abundances of light elements (hydrogen, helium, lithium, beryllium) and their isotopes that were formed in the first minutes after the Big Bang. Due to its simplicity, neutron decay is an ideal system for studying details of the most basic forces of subatomic physics, in particular the weak nuclear force. Such studies improve our understanding of nature and may provide hints of new fundamental physical phenomena yet to be discovered. The neutron mean lifetime has been measured with an uncertainty of less than one second by individual experiments, but results from the two main experimental methods, the beam method and the ultracold neutron storage method, currently disagree by more than eight seconds. Resolving this discrepancy is a matter of great importance. This award supports the preliminary design of a next-generation beam neutron lifetime experiment called BL3. It will employ new, powerful technical features to enable its goals of resolving the discrepancy and providing a reliable measurement of the neutron lifetime to well below one second of uncertainty. This project provides an excellent opportunity to train undergraduates, graduate students, and postdocs in the methods and theory of neutron science which are applicable to diverse scientific studies in physics, chemistry, materials science, and biology at existing and emerging neutron sources around the world.The scope of this project is a preliminary design of the BL3 apparatus, a new experiment to measure the neutron lifetime using the beam method. It is similar in concept and improves upon previous beam neutron lifetime experiments. It will employ a significantly larger superconducting magnet to accommodate a large area neutron beam and will incorporate many technical improvements, such as much higher counting statistics, a more uniform magnetic field in the trapping region; a large, segmented, ultrathin window silicon proton detector; and a sophisticated neutron time of flight spectrometer. BL3 has two scientific goals: 1) to investigate and test systematic effects in the beam method that may contribute to the 8.4 s (4 sigma) discrepancy between the beam and ultracold neutron storge experiments; and 2) reduce the total uncertainty of the neutron lifetime beam method to less than 0.3 s. The value of the neutron lifetime has important consequences in nuclear physics, particle physics, astrophysics, and cosmology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.