to be 1020 photons s1 m2.
2.4. Bleaching treatment/UV irradiation
A hydrogen peroxide solution (6% (g/v)) adjusted to pH 10 (measured by using a digital pH meter, Omega make), by adding ammonia water, was used for the bleaching treatment. No acceler- ators or stabilizers were used in this treatment. Instead, the bleaching solution was freshly replaced after every 20 min. Bleach- ing was carried out by immersing a bundle in the bleaching solu- tion at a ratio of hair to solution 1:75. After a known interval of time (a total of 1, 3, and 5 h) the bundle was washed with plenty of distilled water and air-dried. The 5 h bleached hair was then subjected to UV irradiation for a total time of 50–500 h.
2.5. Positron lifetime measurements
The PALS set up used was a standard fast–fast coincidence cir- cuit with BaF2 scintillators, coupled to photomultiplier tubes of type XP2020/Q having quartz window. The spectrometer had a time resolution of 220 ps. The experimental and procedural details of this technique may be found in Refs. [10,17].Thesource-sample sandwich geometry was used for positron lifetime measurements. One hair bundle was placed on either side of the positron source and the combination was held together tightly by a pair of clampa- ble aluminum plates that had circular holes at the center in line with the source spot (about 2 mm in diameter, 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
M.N. Chandrashekara, C. Ranganathaiah/Journal of Photochemistry and Photobiology B: Biology 101 (2010) 28