Fundamental Neutron Physics

该项目由密歇根大学基础中子物理小组致力于两个基本中子物理测量的研究：1)在NIST中子研究中心（NCNR）使用冷中子束测量中子寿命，精度约为一秒；2)在橡树岭国家实验室（ORNL）的散裂中子源（SNS）使用Nab/abBA/PANDA光谱仪测量β -中微子相关性。这是NSAC最近对该领域的审查建议中确定的五个最优先实验中的两个。该物理学在核物理、粒子物理和天体物理等领域具有广泛的影响。这两项工作都得到了社区的大力合作和支持，并受到了严格审查。密歇根大学的研究小组在这两项工作中都发挥着关键的技术和领导作用。在NIST，我们将对中子寿命的质子阱测量中使用的中子通量监测器进行绝对校准。质子阱装置用于最近的束流寿命测量，其寿命结果为(886.3 +/- 1.2 （stat) +/- 3.2(sys)）秒，其中系统误差主要是与中子通量监测器校准相关的不确定性。通量监测器校准活动包括两个独立的校准，使用NIST的单色中子束和厚靶硼-10或液态氦-3。对于硼-10方法，计算α粒子和伽马射线，并且最近完成了一组测量，证明了内部一致性和精度，可以将系统误差减少到约0.5秒。对于氦-3方法，3He(n,p)3H反应中带电粒子产生的热量是用量热法测量的，并转换为中子通量。由我们小组最初开发的量热计方法已被证明能够在10小时的集成中测量到0.1%的通量。针对液氦-3目标的挑战，密歇根大学已经完成了一些具体的改进，该设备将被转移到NIST，目的是在2013年完成校准活动，并在2014年在新的NIST NGC束流线上进行新的质子阱寿命测量。Nab光谱仪和实验是用来测量SNS上的中微子相关。设计了一种利用质子飞行时间谱法提取a参数来测量质子-电子重合的磁场扩展光谱仪。也有可能从硅探测器的电子能谱中提取菲尔兹干涉b。设想继续使用Nab光谱仪和极化中子（abBA/PANDA）的物理程序具有非常丰富的能力，可以对中子衰变的V-A参数进行限制，从而探测超出标准模型的物理。密歇根大学小组负责中子偏振和偏振测量的所有方面，从Nab开始，通过测量未极化FP-13光束的中子偏振，偏振灵敏度小于0.01%。中子是一种亚原子粒子，占世界物质的一半以上。在大多数原子的原子核中，中子保持稳定，但当它从原子核中释放出来时，它就不稳定了。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。中子也有自旋，这是物质最基本的量子力学特性，区分了自旋向上和自旋向下两种状态。自旋是核磁被利用的原因，例如，在核磁共振和核磁共振中。自旋态也影响中子的衰变和相互作用，因此对中子自旋的控制对亚原子相互作用的更详细的研究是有用的。本项目设想了一个程序，以深入细致地推进中子寿命和中子衰变中电子和中微子动量的相关性的精度和准确性。中子寿命是一个可测量的基本量，从元素的形成到太阳能循环，再到弱相互作用标准模型的结构，都对物理学产生影响。最近的测量结果之间的差异实际上导致了对中子寿命值的置信度降低，使用不同技术的新测量对于解决这些差异至关重要。使用冷中子束测量中子寿命将提供一种不同的方法，而精确测量中子通量对实验至关重要。中子通量将由一个探测器测量，该探测器的灵敏度与探测中子衰变的方式相同，但必须用绝对方法校准。绝对校准将通过测量中子在绝对零度以上2度的低温目标中被氦-3吸收时产生的热量来实现。该校准的目标是提供精确的中子通量测量，并以一秒的精度确定中子寿命。中子衰变中产生的电子与中微子之间的夹角与中子寿命相结合，对弱相互作用的基本强度很敏感。使用极化中子的更广泛的测量可以用来探测粒子物理标准模型之外的东西。一种设计用于测量中子衰变中质子和电子能量的新型光谱仪将首先测量电子-中微子相关，然后用于极化中子。这部作品探讨了关于物质最基本部分的深刻的智力问题。目的是收集数据，以帮助完成基本粒子及其相互作用的图景。这些技术有更广泛的影响。激光极化氦-3用于极化中子，在材料科学和量子信息研究中有应用。本项目是培养本科生、研究生和博士后的重要基地。技术挑战与深刻的智力问题相结合，提供了动力和发展技术技能。本科生将获得与研究生和博士后一起工作的研究经验。研究生具备广泛的能力，并继续为教师或国家实验室职位以及跨学科研究做准备。这项工作还导致了为非物理专业学生开设新课程，以及一系列关于核磁体和中微子的公开讲座，以及向感兴趣的普通大众传播科学的领导作用。在探索物理学基本问题的背景下，最难的问题本身就令人兴奋，产生了最具创新性的解决方案，并带来了最初难以想象的副产品。原子钟，增强的核磁共振成像，以及探测物质起源的实验，都源于对核磁和中子自旋的控制。

This project is devoted to research by the University of Michigan Fundamental Neutron Physics Group on two fundamental-neutron-physics measurements: 1) measurement of the neutron lifetime with precision of approximately one second using a cold-neutron beam at the NIST Center for Neutron Research (NCNR) and 2) measurement of the beta-neutrino correlation with the Nab/abBA/PANDA spectrometer at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL). These are two of the five highest priority experiments identified in recommendations by a recent NSAC review of the field. The physics has broad impact across the fields of nuclear and particle physics and astrophysics. Both efforts enjoy strong collaborations and the support of the community as well as the scrutiny of critical reviews. The University of Michigan group has key technical and leadership roles in both efforts. At NIST, we will perform an absolute calibration of the neutron-flux monitor used in the proton-trap measurement of the neutron lifetime. The proton-trap apparatus was used the most recent beam-lifetime measurement with the result for the lifetime of (886.3 +/- 1.2 (stat) +/- 3.2(sys)) seconds, where the systematic error is dominated by uncertainties related to the neutron-flux monitor calibration. The flux-monitor calibration campaign includes two independent calibrations using a monochromatic neutron beam at NIST and thick targets of Boron-10 or liquid Helium-3. For the Boron-10 approach, alpha particles and gamma rays are counted, and a set of measurements has recently been completed that demonstrated internal consistency and precision that would reduce the systematic error to about 0.5 sec. For the Helium-3 approach, the heat generated by the charged particles in the 3He(n,p)3H reaction is measured calorimetrically and converted to a neutron flux. The calorimeter approach, originally developed by our group, has been shown to be capable of measuring the flux to 0.1 % with 10 hours of integration. A number of improvements specifically related to the challenges of the liquid Helium-3 target have been completed at the University of Michigan, and the device will be moved to NIST with the aim of completing the calibration campaign by 2013 and undertaking the new proton-trap lifetime measurement on the new NIST NGC beam line in 2014.The Nab spectrometer and experiment is to measure the beta-neutrino correlation at SNS. The magnetic-field-expansion spectrometer is designed to measure proton-electron coincidences using proton time-of-flight spectroscopy to extract the a parameter. It may also be possible to extract the Fierz interference b from electron spectroscopy in silicon detectors. The envisioned continuing physics program with the Nab spectrometer and polarized neutrons (abBA/PANDA) is extremely rich with the ability to over constrain V-A parameters for neutron decay and thus probe physics beyond the Standard Model. The University of Michigan group is responsible for all aspects of neutron polarization and polarimetry beginning, for Nab, by measuring the neutron polarization of the unpolarized FP-13 beam with sensitivity to the polarization of less than 0.01%.The neutron is a sub-atomic particle that comprises more than half of the matter in the world. Within the nuclei of most atoms, the neutron remains stable, but when freed from the nucleus, it is unstable. Free neutrons are an important tool for study of sub-atomic physics, because the decay and interactions of free neutrons reveal the interactions of its constituents and decay products. Neutrons also have spin, the quantum mechanical property of the most fundamental pieces of matter that distinguishes two states called spin-up and spin-down. Spin is responsible for the nuclear magnetism exploited, for example, in NMR and MRI. The spin states also affect the decay and interactions of neutrons, and so the control of neutron spin becomes useful for more detailed study of sub-atomic interactions. This project envisions a program to incisively and carefully advance both the precision and the accuracy of the neutron lifetime and the correlation of electron and neutrino momenta in neutron decay. The neutron lifetime is a measured fundamental quantity that impacts physics from the formation of the elements to the solar energy cycle to the structure of the Standard-Model of the weak interaction. Discrepancies among recent measurements have actually led to reduced confidence in the value of the neutron lifetime, and a new measurement using different techniques is essential to resolving the discrepancies. The measurement of the neutron lifetime using a cold neutron beam will provide a different approach, and accurate measurement of the neutron flux is crucial to the experiment. The neutron flux will be measured with a detector that has sensitivity that scales in the same way as the detection of neutron decays, but which must be calibrated by an absolute method. Absolute calibration will be achieved by measuring the heat produced when the neutrons are absorbed by Helium-3 in a cryogenic target at two degrees above absolute zero. The goal of this calibration is to provide an accurate neutron-flux measurement and determine the neutron lifetime with one-second precision.The angle between the electron and neutrino produced in neutron decay is sensitive to the fundamental strength of the weak interaction when combined with the neutron lifetime. A broader class of measurements using polarized neutron can be used to probe beyond the Standard Model of particle physics. A new spectrometer designed to measure both the proton and electron energies in neutron decay will first measure the electron-neutrino correlation and then be used with polarized neutrons.This work probes deep intellectual questions about the most fundamental pieces of matter. The aim is to collect data that will help complete the picture of elementary particles and their interactions. The techniques have much broader impact. Laser polarized Helium-3 used to polarize neutrons has applications to materials science, and quantum information research. This project is a remarkable training ground for undergraduate and graduate students and post doctoral fellows. The technical challenges combined with the deep intellectual issues provide motivation and develop technical skills. Undergraduates will gain research experience working along with graduate and post doctoral fellows. Graduate students emerge broadly capable and move on to prepare for faculty or national lab positions as well as interdisciplinary research. This work has also led to development of new courses for non-physics majors, to a set of public lectures on Nuclear Magnets and Neutrinos and leadership in communicating science to the interested general population. In the context of probing fundamental problems of physics, exciting in its own right, the hardest problems produce the most innovative solutions with spin-offs unimaginable at the outset. Atomic clocks, enhanced MRI, and experiments that probe the origin of matter all follow from the control of nuclear magnetism and neutron spins.