A slow positron flux generator using well annealed polycrystalline tungsten moderators and a Na{sup {minus}22} positron source was developed for microstructural characterization of thin insulator (ceramics and polymeric) films. A 250 micro-c source, deposited on a 2.55 {times} 10 {sup {minus}4} cm thick aluminized mylar film, is sandwiched between two 0.0127 cm thick, 2.54 {times} 2.54 cm tungsten strips. Two 2.54 {times} 2.54 cm test insulator films, whose thicknesses may range from 0.00127 to 0.0127 cm, insulate the two tungsten moderator strips from the aluminized mylar source holder. A potential difference of 10 to 100 volts, depending on the test insulator film thickness, is applied between the tungsten strips and the source holder. Thermalized positrons diffusing out of the moderator strips are attracted to the source holder when it is at a negative potential. These positrons have to drift through the test insulator films in order to reach the source holder. On the other hand, they are prevented from entering the test films when the source holder is at a positive potential. The difference between the positron lifetime spectrum with the source at + or {minus} volts is thus expected to provide the test film lifetime spectrum.