Development of a method for positron annihilation lifetime measurement in thin polyethylene films using a Na-22 source

Yamawaki, Masato*; Uesugi, Naoya*; Oka, Toshitaka   ; Nagasawa, Naotsugu*; Ando, Hirokazu*; O'Rourke, B. E.*; Kobayashi, Yoshinori*

Positron annihilation lifetime measurements were performed on polyethylene films with thickness of 15m - 2000m using a Na-22 positron source enclosed in a Kapton film. For thin films, some positrons will pass through the film and annihilate behind it. Using a single film in a commercial anti-coincidence system, by placing an annealed stainless steel (SUS304) cover behind the sample, it is possible to sufficiently measure the long lifetime ortho-positronium (o-Ps) component even in thin films. Additionally, calculated intensities of the o-Ps component determined from the estimated film transmittance agreed well with the measured values. Furthermore, by applying this method to uniaxially stretched UHMWPE, we were able to observe structural changes owing to the stretching consistent with shorter measured o-Ps lifetime and increased o-Ps intensity.