Superfluid helium film may greatly increase the storage time of ultracold neutrons in material traps

On the possibility of a significant increase in the storage time of ultracold neutrons in traps coated with a liquid helium film

“…The triangular roughness of the wall shown in Fig. 1 has some significant advantages compared to the rectangular profile shown and considered in [44] (Fig. 6).…”

“…Thus, h R l R and such roughness will be stable against mechanical load. This property is an advantage of the proposed triangular surface relief compared to the rectangular relief considered in [36], where to reduce the absorption of neutrons by the wall, roughness satisfying the relation h R > l R , i.e., a very brittle wall relief, was required. It is noteworthy that diffraction gratings with the period l R ≈ 4 µm and depth h R ≈ 0.2 µm are already actively used for the scattering of UCNs [45,46].…”

“…We recently assumed [36] that the thickness of the helium film on the rough wall of the trap of UCNs increases because of capillary effects. The roughness of the wall usually increases the loss of neutrons by a factor of 2 − 3 through absorption inside the walls of the trap because this roughness makes the average repulsive potential of the walls smoother, so that the wavefunction of the neutron penetrates deeper into the wall [20,21].…”

Improving of ultracold neutron traps coated with liquid helium using capillarity and electric field

“…of the neutron wave function 𝜓 inside He is not sufficient to strongly reduce the neutron losses on the trap walls. A more accurate calculation of the neutron wave function near a solid wall covered with liquid helium [43] increases the estimate of 𝜓 (0) by ∼ 30% as compared to Eq. ( 1) for relevant UCN kinetic energy 𝐸 < 0.8𝑉 He 0 , making the problem of too thin 4 He film even more serious.…”

“…( 1) for relevant UCN kinetic energy 𝐸 < 0.8𝑉 He 0 , making the problem of too thin 4 He film even more serious. The required thickness of helium film for a good protection of UCN is 𝑑 ≤ 𝑑 * He = 100 nm [43]. An idea [38] of using a rotating He vessel for UCN storage to increase He thickness on side walls has a drawback that the rotating liquid generates additional bulk and surface excitations, leading to inelastic neutron scattering.…”