  It is shown that rough inner walls of a trap of ultracold neutrons can be
coated with a superfluid helium film much thicker than the depth of penetration
of ultracold neutrons into helium. This coating should reduce the rate of loss
of ultracold neutrons caused by absorption in the walls of the trap by orders
of magnitude. It is demonstrated that triangular roughness is more efficient
than rectangular for the reduction of the rate of loss of ultracold neutrons.
Triangular roughness is more easily implemented technically and such
diffraction gratings are fabricated industrially. Other methods are proposed to
increase the thickness of the protective helium film.
