用磁俘获中子测量中子寿命
Measuring the Neutron Lifetime Using Magnetically Trapped Neutrons
摘要：
中子β衰变寿命在理解标准模型框架内的弱相互作用和大爆炸核合成中4He原始丰度的理论预测中起着重要作用。在以前的工作中，我们成功地证明了超冷中子（UCN）在保守势磁阱中的捕获。美国国家标准与技术研究所中子研究中心（NCNR）对该装置的重大升级即将完成。在我们的方法中，一束0.89 nm的中子入射到超流4He目标内的最小场区域的Ioffe型磁阱。一小部分中子在氦中向下散射到能量小于200 neV，而那些处于适当自旋状态的中子则被捕获。在低于200 mK的温度下，氦的低声子密度抑制了逆过程，使中子不受干扰地旅行。当中子衰变时，高能电子电离氦，产生闪烁光，用光电倍增管检测。由于场强和阱体积的显著增加，导致捕获的中子增加了20倍，因此以前装置的统计局限性将得到缓解。
英文摘要：
The neutron beta-decay lifetime plays an important role both in understanding weak interactions within the framework of the Standard Model and in theoretical predictions of the primordial abundance of 4He in Big Bang Nucleosynthesis. In previous work, we successfully demonstrated the trapping of ultracold neutrons (UCN) in a conservative potential magnetic trap. A major upgrade of the apparatus is nearing completion at the National Institute of Standards and Technology Center for Neutron Research (NCNR). In our approach, a beam of 0.89 nm neutrons is incident on a superfluid 4He target within the minimum field region of an Ioffe-type magnetic trap. A fraction of the neutrons is downscattered in the helium to energies <200 neV, and those in the appropriate spin state become trapped. The inverse process is suppressed by the low phonon density of helium at temperatures less than 200 mK, allowing the neutron to travel undisturbed. When the neutron decays the energetic electron ionizes the helium, producing scintillation light that is detected using photomultiplier tubes. Statistical limitations of the previous apparatus will be alleviated by significant increases in field strength and trap volume resulting in twenty times more trapped neutrons.
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