Construction of a storage device for ultra-cold neutrons (UCN) using superconducting magnets in order to measure the lifetime of the free neutron

中子寿命n是基本自然常数之一，在物理学的许多领域起着重要的作用。对于粒子物理学来说，它是中子衰变中伴随相关关系的基本量之一，即衰变产物的方向和自旋与中子的螺旋度/自旋有关。其中一个相关性与中子寿命一起使我们能够分别测试Cabibbo- kobayashi - maskawa矩阵和Cabibbo模型的统一性。此外，n在原始核合成中起着至关重要的作用，对原始氦丰度有很大的影响。因此它影响了宇宙学的标准模型。近几十年来，人们对中子寿命进行了多次测量，但实际值仍存在很大的不确定性；最近的实验结果与粒子数据组采用的世界平均水平相差6个百分点以上。为了重新测量n，我们想要将超冷，因此非常慢的中子（UCN）存储在超导线圈产生的磁性壁的陷阱中。通过这种方式，可以避免在存储期间UCN与材料壁的碰撞以及这些壁反射造成的损失。在存储体积的大约。750 dm3可储存大量中子，使统计不确定度在短时间内达到∆n< 0.1 s。中子寿命将通过实时记录衰变质子的时间分布和不同储存时间后存活的中子数来测量。该实验最基本的特性可能是对系统效应的处理以及良好的统计数据：几乎所有的存储参数都可以改变，以深入研究这些效应。我们期望总实验不确定度小于0.1秒。

The neutron lifetime n is one of the elementary natural constants and plays an important role in many fields of physics. For particle physics it is one of the fundamental quantities along with the correlations in neutron decay, namely the directions and spin of the decay products in reference to the helicity/spin of the neutron. One of these correlations together with the neutron lifetime enable us to test the unitarity of the Cabibbo-Kobayashi-Maskawa matrix and the Cabibbo model respectively. Furthermore, n plays a vital role in primordial nucleosynthesis having a big influence on the primordial helium abundance. Hence it influences the standard model of cosmology. The neutron lifetime has been measured repeatedly in the last decades, but there is still great uncertainty about the real value; the result of the most recent experiment deviated by more than 6  from the world average adopted by the Particle Data Group. To remeasure n, we want to store ultra-cold, thus very slow, neutrons (UCN) in a trap with magnetic walls created by superconducting coils. In this way collisions of the UCN with the material walls during the storage period are avoided and with them losses that result from reflections at these walls. In the storage volume of approx. 750 dm3 a big number of neutrons may be stored so that a statistical uncertainty of ∆n< 0.1 s is reached within short time. The neutron lifetime will be measured by recording both, the time distribution of the -decay protons in real-time and the number of neutrons surviving after different storage times. Probably the most essential properties of the proposed experiment are the handles on systematic effects together with good statistics: almost all of the storage parameters can be varied to study those effects in depth. We expect a total experimental uncertainty of less than 0.1 s.