氦膜可大大增加超冷中子在材料阱中的储存时间
Helium film may greatly increase the storage time of ultracold neutrons in material traps
摘要：
我们提出了一种方法，以增加中子存储时间和其寿命测量的精度至少十倍。超冷中子（UCN）在物质阱中的储存现在提供了最精确的中子寿命测量，并用于许多其他实验。这些测量的精度受到UCN与陷阱壁的相互作用的限制。我们发现，覆盖陷阱壁与液氦可以大大减少UCN从材料陷阱的损失。$^4$他根本不吸收中子。由于范德华吸引力，超流体He以$\sim 10$ nm厚的薄膜覆盖陷阱壁。然而，平壁上的He膜太薄，无法保护UCN免受陷阱材料内的吸收。通过结合范德华吸引力与毛细管效应，我们表明，表面粗糙度可能会增加该膜的厚度远远超过中子穿透深度$\sim 33$ nm。使用液态He进行UCN存储需要低温$T<0.5$ K以避免中子与He蒸气相互作用，而由于与表面波相互作用而导致的中子损失很小，并且可以使用其线性温度依赖性来解释。
英文摘要：
We propose a method to increase both the neutron storage time and the precision of its lifetime measurements by at least tenfold. The storage of ultracold neutrons (UCN) in material traps now provides the most accurate measurements of neutron lifetime and is used in many other experiments. The precision of these measurements is limited by the interaction of UCN with the trap walls. We show that covering trap walls with liquid helium may strongly decrease the UCN losses from material traps. $^4$He does not absorb neutrons at all. Superfluid He covers the trap walls as a thin film, $\sim 10$ nm thick, due to the van der Waals attraction. However, this He film on a flat wall is too thin to protect the UCN from their absorption inside a trap material. By combining the van der Waals attraction with capillary effects we show that surface roughness may increase the thickness of this film much beyond the neutron penetration depth $\sim 33$nm. Using liquid He for UCN storage requires low temperature $T<0.5$ K to avoid neutron interaction with He vapor, while the neutron losses because of the interaction with surface waves are small and can be accounted for using their linear temperature dependence.
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