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