Optical measurements of the density of helium in small bubbles in aluminium films

Some preliminary measurements are reported on the density of helium in bubbles in metal films using optical spectroscopy in the far vacuum ultra-violet. The technique consists of measuring (by transmission through the films) the optical absorption resulting from the transition View the MathML source due to the helium in the bubbles. This transition, in a rarefied gas, occurs at 584 Å (21.2eV) but because of the gas density in the bubbles is observed to be broadened and shifted to higher energy (blue shifted). The measurements imply, for specimens with high gas concentrations (0.5–3 at% He), a gas density in the bubbles greater than 1023 atoms cm−3.