Measurement of the polarized 3He neutron absorption cross section

A three-body force is a force that is not present in a system of two objects but manifests in a three-body system, that is to say the two body forces are insufficient to describe the system. For neutron-rich (exotic) nuclei the stability becomes increasing critical to the three-body nuclear (3N) forces. Also calculations of the deuteron-tritium fusion reaction, intensively studied for power generation, require 3N forces. Although models for the 3N forces exist and give a rough estimate of the relative sizes and spin dependence of three- and higher-body effects compared to two-nucleon forces, more precise experimental data on systems with few nucleons is needed to determine the relative strengths of these forces. To better constrain the spin-dependent parts of the 3N force, data are required on the spin-dependent scattering of three- and four-body systems with precision at the sub-percent level. We have been working to determine the n-3He incoherent scattering cross section in experiments at FRM2 and upcoming at SNS. These values however are dependent on the polarized 3He neutron absorption cross section, σp, which has only been weakly experimentally or theoretically bound, further knowledge of this cross section in itself adds to the experimental constraints on theory of three-body forces. In order to determine σp absolutely we propose to measure the transmission of a polarized 3He cell while performing a precision NMR measurement of absolute 3He polarization.

三体力（three-body force）是指不存在于两体系统中、仅在三体系统中显现的作用力，换言之，仅依靠两体力无法完整描述该系统。对于富中子（奇异）原子核而言，其稳定性对三体核（3N）力的依赖程度愈发显著。针对被广泛研究以用于发电的氘-氚聚变反应的计算，同样需要引入三体核力。尽管现有的三体核力模型已可粗略估算相较于两核子力的三体及更高体效应的相对尺度与自旋依赖性，但仍需获取更多少核子系统的高精度实验数据，以确定此类作用力的相对强度。为更严格地约束三体核力的自旋相关部分，需要获取亚百分比精度的三体及四体系统自旋相关散射数据。我们已在FRM2装置上开展实验，并计划在SNS上进行后续实验，以测定中子与氦3的非相干散射截面。然而该数值依赖于极化氦3的中子吸收截面σ_p，目前仅通过实验或理论对其给出了较弱的约束，而对该截面的进一步认知本身即可为三体力理论提供更多实验约束条件。为绝对测定σ_p，我们提出将通过测量极化氦3池的透射率，同时开展氦3绝对极化度的高精度核磁共振（NMR）测量。