Precision Measurements with Polarized Cold Neutron Beams and 3He

使用偏振冷中子束和 3He 进行精密测量

• 批准号: 0244972
• 负责人: Timothy Chupp
• 金额: $ 66万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-05-15 至 2006-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0244972&HistoricalAwards=false
• 关键词: Precision; Measurements; Polarized; Cold; Neutron

The neutron is a sub--atomic particle thatcomprises more than half of the matter in theworld. Within the nuclei of most atoms, theneutron remains stable, but when freed from thenucleus, it is unstable. Free neutrons are thusan important tool for study ofsub--atomic physics, because the decay andinteractions of free neutrons reveal theinteractions of its constituents and decayproducts. Neutrons also have spin, the quantummechanical property of the most fundamentalpieces of matter that distinguishes two states calledspin--up and spin--down. Spin is responsiblefor the nuclear magnetism exploited, for example,in NMR and MRI. The spin states also affect thedecay and interactions of neutrons, and so thecontrol of neutron spin becomes useful for moredetailed study of sub--atomic interactions. Inthe proposed research, free neutrons produced bya particle accelerator at Los Alamos NationalLaboratory and at the nuclear reactor at NISTwill be used in experiments that control theneutron spin to reveal weak interactions andpossible new physics beyond the Standard Model that summarizes known elementaryparticle interactions.The neutron spins in a beam of neutron will be manipulated byselecting predominantly one spin state with aspin filter based on laser polarized $^3$He. Theneutron spin can be reversed usingmagnetic fields that couple to the nuclearmagnetic moment. Very precise measurement of theneutron polarization ({\it i.e.} the excessfraction of the selected spin state) and precisespin reversal are required to mitigate falseeffects that may arise. The techniques of$^3$He polarization, the spin filter, and spinflipping will be greatly improved as we undertakethese experiments.The objective of the Los Alamos experiment ismeasurement of the weak interaction effects onthe absorption of neutrons by hydrogen. The weakinteractionis characterized by absorption and emission ofparticles that have a handedness that correlatesthe spin orientation with the direction of motion.This handedness allows observation of the weakinteraction amidst the much stronger interactionsof neutrons and protons. Definitive characterization of the weakinteraction between the neutron and proton iscrucial to fully understanding the forces thatbind the nucleus. The experiment at NIST measuresthe dependence of neutron decays on the spinstate. In a dedicated detector, neutron decaysare observed by simultaneously detecting the proton andelectron. Thethird particle emitted, the neutrino, is too elusive todetect with high efficiency. A dependence ofthe electron's and proton's relative motion on the neutron spinstate would reveal interactions that are notsymmetric under reversal of time, a specialsignature that could reveal new physics beyondthe Standard Model, physics that mayexplain the origin of matter in the Big Bang. The advancedtechniques of spin state selection, precisionpolarimetry, and spin flipping will be applied toa planned experiment with the objective of preciselymeasuring the handedness of neutron decays. Whencombined with other neutron decay measurementsthese measurements will also probe physics beyond theStandard Model.This work probes deep intellectual questionsabout the most fundamentalpieces of matter. The aim is to collect datathat will help complete the picture of elementaryparticles and their interactions. The techniqueshave much broader impact. Laser polarized$^3$He and $^{129}$Xe used in related experimentsare now used in biomedical research, materialsscience, and may be applied in advances ofquantum computation. This research is a remarkabletraining ground for undergraduate and graduatestudents. The technical challenges combined withthe deep intellectual issues provide motivationand develop technical skills. Several undergraduates willgain research experience working along with graduate students andsenior researchers. Graduate students emergebroadly capable and move on to prepare forfaculty positions as well as interdisciplinaryresearch. This work has also led to developmentof new courses for non--physics majors and to aset of public lectures on Nuclear Magnets.The theme for the public isthat, in the context of probing fundamentalproblems of physics -- exciting in their ownright, the hardest problems produce the mostinnovative solutions with spin--offs unimaginableat the outset. Atomic clocks, enhanced MRI, andprobing the origin of matter all follow from thecontrol of nuclear magnetism.

中子是一种亚原子粒子，构成了世界上一半以上的物质。在大多数原子的原子核内，中子保持稳定，但当从中子脱离原子核时，它就不稳定。因此，自由中子是研究亚原子物理的重要工具，因为自由中子的衰变和相互作用揭示了其成分和衰变产物的相互作用。中子还具有自旋，这是物质最基本的量子力学特性，它区分了称为自旋向上和自旋向下的两种状态。自旋负责核磁共振和核磁共振等领域所利用的核磁性。自旋态也会影响中子的衰变和相互作用，因此中子自旋的控制对于亚原子相互作用的更详细研究变得有用。在拟议的研究中，由洛斯阿拉莫斯国家实验室的粒子加速器和 NIST 的核反应堆产生的自由中子将用于控制中子自旋的实验，以揭示弱相互作用和超出总结已知基本粒子相互作用的标准模型的可能的新物理。中子束中的中子自旋将通过基于以下公式的 aspin 滤波器主要选择一个自旋状态来操纵： 激光偏振$^3$He。使用与核磁矩耦合的磁场可以反转中子自旋。需要非常精确地测量中子极化（即所选自旋态的过量部分）和精确的自旋反转，以减轻可能出现的错误效应。随着我们进行这些实验，$^3$He极化、自旋过滤器和自旋翻转技术将得到极大的改进。洛斯阿拉莫斯实验的目的是测量弱相互作用对氢吸收中子的影响。弱相互作用的特征是粒子的吸收和发射，这些粒子具有将自旋方向与运动方向相关联的旋向性。这种旋向性允许在中子和质子的更强相互作用中观察到弱相互作用。中子和质子之间弱相互作用的明确表征对于充分理解束缚原子核的力至关重要。 NIST 的实验测量了中子衰变对自旋态的依赖性。在专用探测器中，通过同时探测质子和电子来观察中子衰变。发射的第三种粒子是中微子，它太难以捉摸，无法高效检测。电子和质子的相对运动对中子自旋态的依赖将揭示时间反转下不对称的相互作用，这是一个特殊的特征，可以揭示超出标准模型的新物理，可以解释大爆炸中物质的起源。自旋态选择、精密偏振和自旋翻转等先进技术将应用于计划中的实验，以精确测量中子衰变的旋向性。当与其他中子衰变测量相结合时，这些测量还将探索标准模型之外的物理学。这项工作探讨了有关物质最基本部分的深层知识问题。目的是收集有助于完整了解基本粒子及其相互作用的数据。这些技术具有更广泛的影响。相关实验中使用的激光偏振$^3$He和$^{129}$Xe现在已用于生物医学研究、材料科学，并可能应用于量子计算的进步。这项研究对于本科生和研究生来说是一个非凡的训练场。技术挑战与深刻的智力问题相结合提供了动力并发展了技术技能。 一些本科生将获得与研究生和高级研究人员一起工作的研究经验。研究生表现出广泛的能力，并继续为教师职位和跨学科研究做好准备。这项工作还导致了针对非物理专业的新课程的开发，以及一系列关于核磁体的公开讲座。面向公众的主题是，在探索物理学的基本问题的背景下——它们本身就令人兴奋，最困难的问题会产生最具创新性的解决方案，其附带效果在一开始是难以想象的。原子钟、增强型核磁共振成像以及探测物质起源都源于​​核磁的控制。