POSITRON annihilation investigations of defects in crystals have shown that for sufficiently high defect concentrations (typically above about 10−6) all positrons become trapped in the defects before annihilation, thus changing the characteristics of the annihilation process. For example, trapping of positrons may result in the increase in the positron lifetime, a narrowing of the 2-γ angular correlation distribution, and a reduction in the Doppler broadening of the annihilation line. Vacancies in metals1,2, deformation effects in metals3, and defects (F-centres or cation vacancies) in ionic crystals4 have all now been studied by this technique. The trapping of positronium (Ps) in defects in quartz5 and ice6 has also been investigated. Here we report the study of another crystal defect, the void, a small vacancy of diameter few tens of Ångstroms. In practice voids may contain gas atoms and may be disturbed either in a random fashion or arranged in a macro lattice7.