deposited at the center
of a 1.5 cm2 Kapton foil of 0.0127 mm thick). The thickness of the hair sample in the sandwich was about 2.5 mm, such as to ensure stopping of all the positrons inside the sample. A typical spectrum accumulation time was 2 h with a 17 lCi 22Na positron source, with more than 106 counts under each spectrum. The instrumental time resolution and source correction terms were ob- tained from the lifetime spectrum of a well-annealed (defects free) aluminum by using the program RESOLUTION [38].Thelifetime spectra in hair samples so acquired were analyzed into three