Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, USA and National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA and University of Tennessee, Knoxville, Tennessee 37996, USA.
National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
Phys Rev Lett. 2013 Nov 27;111(22):222501. doi: 10.1103/PhysRevLett.111.222501.
The most precise determination of the neutron lifetime using the beam method was completed in 2005 and reported a result of τ(n)=(886.3±1.2[stat]±3.2[syst]) s. The dominant uncertainties were attributed to the absolute determination of the fluence of the neutron beam (2.7 s). The fluence was measured with a neutron monitor that counted the neutron-induced charged particles from absorption in a thin, well-characterized 6Li deposit. The detection efficiency of the monitor was calculated from the areal density of the deposit, the detector solid angle, and the evaluated nuclear data file, ENDF/B-VI 6Li(n,t)4He thermal neutron cross section. In the current work, we measure the detection efficiency of the same monitor used in the neutron lifetime measurement with a second, totally absorbing neutron detector. This direct approach does not rely on the 6Li(n,t)4He cross section or any other nuclear data. The detection efficiency is consistent with the value used in 2005 but is measured with a precision of 0.057%, which represents a fivefold improvement in the uncertainty. We verify the temporal stability of the neutron monitor through ancillary measurements, allowing us to apply the measured neutron monitor efficiency to the lifetime result from the 2005 experiment. The updated lifetime is τ(n)=(887.7±1.2[stat]±1.9[syst]) s.
使用束流方法对中子寿命进行的最精确测定是在 2005 年完成的，并报告了τ(n)=(886.3±1.2[stat]±3.2[syst])s 的结果。主要的不确定度归因于中子束通量的绝对测定（2.7s）。通量是用中子监测器测量的，该监测器通过在薄而特征明显的 6Li 沉积物中吸收中子来计数中子诱导的带电粒子。监测器的探测效率是从沉积物的面密度、探测器的立体角和评估的核数据文件 ENDF/B-VI 6Li(n,t)4He 热中子截面计算得出的。在当前的工作中，我们使用第二个完全吸收的中子探测器来测量用于中子寿命测量的相同监测器的探测效率。这种直接的方法不依赖于 6Li(n,t)4He 截面或任何其他核数据。探测效率与 2005 年使用的值一致，但测量精度为 0.057%，这代表不确定性提高了五倍。我们通过辅助测量验证了中子监测器的时间稳定性，从而使我们能够将测量到的中子监测器效率应用于 2005 年实验的寿命结果。更新后的寿命为 τ(n)=(887.7±1.2[stat]±1.9[syst])s。
