This paper describes the experimental procedure we have developed for producing spherical deuterated polyethylene targets with a continuum of densities in the range 20–60&percent; that of solid (&rgr;solid?1.0 g/cm3). These spheres (termed ’’microsponges’’) are typically 80–300 &mgr;m in diameter and are highly porous networks with average pore sizes of a few microns or smaller. The technique consists of (i) forming solid (i.e., normal density) microspheres by pulverizing and spheroidizing the (CD2)nstarting material, (ii) cross linking the polyethylene chains in a &ggr;‐ray flux, (iii) swelling the cross‐linked network in an appropriate hot solvent such asp‐xylene, and (iv) rapidly extracting the solvent to ’’freeze‐in’’ the expanded structure in the final target. A quantitative model based upon the theory of swollen networks has been employed to correlate the measured densities with the experimental parameters, such as radiation dosage, solvent type, and swelling temperature.