Toward the josephson effect in superfluid helium: How to make 20 nm holes in a 10 nm thick membrane

It was found by electron microscope study that 20 nm diameter holes can be formed in a 10 nm thick self-supported nitrocellulose membrane covered on one side with an island gold film if exposed to the nuclear fragments from a radioactive source. The process of hole formation is a kind of Coulomb explosion. Due to its large atomic number, gold plays a role of an effective source of secondary electrons. However, if the gold thickness is increased, the higher electrical conductivity of the gold film prevents hole formation. This work was initiated in relation to the problem of the Josephson effect in superfluid helium. Our estimates show that there should be a measurable coherent effect for⁴He flow through our membrane with many holes for a temperature close to T, when the coherence length is of order of the hole dimensions.