The thickness of the helium film

Abstract

A description is given of an optical method for the determination of the thickness of the helium II film. Linearly polarized light is reflected from a stainless steel mirror, the upper part of which is coated with a layer of barium stearate one molecule thick and the lower part with a similar layer three molecules thick. The reflected light passes through a mica compensating plate and a nicol prism. Adjustment of the mica plate and nicol gives equality of illumination on the ‘1’ and ‘3’ areas. If now a film of helium II covers the mirror the nicol must be rotated to restore equality of illumination. The rotation is a measure of the thickness of the helium film, the relation between the two quantities being calculated in terms of the angle of incidence and the optical constants of liquid helium, barium stearate and stainless steel. The observed thickness at any given height above the liquid helium was found to be nearly independent of the temperature between 1⋅1 and 2⋅18° K but then decreased rapidly to zero at the λ -point. In the formula d = k / h 1/ z for the thickness d at height h cm. for any given temperature, the value of z varies from 3⋅5 at 1⋅1° K to 2⋅5 at 2⋅1° K. The thickness at 1 cm. and 1⋅5° K is provisionally given as 1⋅9 x 10 -6 cm.

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

“…If one denotes d 0 ≡ d He (h = 1cm), and the units of height h is also given in cm, the latest measurement [51] using He oscillation method suggests d 0 = 28.5nm and 1/n ≈ 1/3.5 = 0.286. This reasonably agrees with earlier measurements using He oscillations [46] and optical methods [47,54]. The latter suggests [54] slightly different values d 0 = 30nm and n ≈ 2 − 3.…”

“…The experiments [46][47][48][49][50][51] confirmed Eq. ( 26), but the values of d 0 and of the exponent 1/n slightly vary depending on the measurement method.…”

“…The study of the profile of superfluid helium film on the vertical walls, i.e. the dependence of He film thickness d He on the height z above liquid helium level, turned out to be a non-trivial problem, both experimentally [46][47][48][49][50][51] and theoretically [49][50][51][52].…”

“…Before ending this section we note that the surface roughness and the corresponding increase of the effective He film thickness due to capillary effects may explain the discrepancy of its experimental values determined by different methods [46][47][48][49][50][51]. Indeed, the microbalance weight method [48] measures the total He weight.…”

“…Indeed, the microbalance weight method [48] measures the total He weight. This He weight includes the filled cavities on the surface, which are not detected by optical methods [47,54]. Hence, the weight method gives a thicker He film on a rough surface [48].…”