Positron lifetimes and mechanical properties of gamma-irradiated ultra high molecular weight polyethylene Y. Kobayashi, M. Yamawaki, T. Oka, S Saiki, H. F. M. Mohamed, K. Hattori and Y. Watanabe National Institute of Advanced Industrial Science and Technology, Japan Atomic Energy Agency, Faculty of Science, Minia University Toyo Seiko Inc. Japan & Egypt PPC10

Conventional PALS is not strictly nondestructive! 22 Na source and sample start scintillator stop PMT PALS system two specimens and the 22Na source Specimen #1 Specimen #2 & the source PALS is in principle non-destructive. But conventional PALS requires cutting out of the samples and is destructive!

New method for PALS for potential onsite inspection A novel method that does not require cutting out of the sample and is potentially applicable to onsite material inspection was developed by Yamawaki et al. (PPC10 and Jap. J. Appl. Phys., 50, (2011)) without anti-coincidence with anti-coincidence positron lifetime spectra of a steel strip

Purpose of the study Glassy polymers Seek the possibility of using PALS for onsite detection of polymer degradation due to gamma-irradiation Schmidt and Maurer reported excellent correlations between the mechanical property and o-Ps lifetimes in polymers long time ago

Experiments Sample : ultra high molecular weight polyethylene (UHMWPE), gamma-irradiated at different doses Gamma-irradiation: with a cobalt-60 source at Japan Atomic Energy Agency Tensile test PALS : conventional setup and sandwich geometry using two specimens ・ three component analysis ・ without source correction ・ time resolution : 280 ps fwhm UHMWPE

Tensile tests and mechanical properties Stress-strain curves for UHMWPE before and after gamma-irradiation Sample size 1.2 mm x 20 mm x mm Instrument STA-1225 (ORIENTEC, Japan) Tensile speed 20 mm/min Tensile test

Variations of tensile strength and elongation at break as a function of gamma-ray dose Both tensile strength and elongation at break are reduced with increasing gamma-ray dose due to degradation.

Variations of o-Ps lifetime and intensity with gamma-ray dose 1 2 The o-Ps intensity is reduced at low doses and tends to level off at higher doses. The o-Ps lifetime is shortened with increasing gamma-ray dose. The o-Ps intensity is reduced at low doses and tends to level off at higher doses. The o-Ps lifetime is shortened with increasing gamma-ray dose.

Effect of gamma-irradiation on o-Ps intensity in LDPE and HDPE Y. Kobayashi et al, Radiat. Phys. Chem., 50, 589 (1997) LDPE HDPE

Positron mobilities in gamma-irradiated LDPE and HDPE ・ Both LDPE and HDPE have very large positron mobilties ・ The mobilities get smaller after gamma-irradiation, which indicates positron trapping by defects produced by the irradiation ・ Both LDPE and HDPE have very large positron mobilties ・ The mobilities get smaller after gamma-irradiation, which indicates positron trapping by defects produced by the irradiation W. Zheng, Y. Kobayashi et al, Radiat. Phys. Chem., 51, 269 (1998)

Fast positron track and Ps formation Drawing by S. V. Stepanov 1 2

Ps formation by highly mobile positrons １０－４０ ｎｍ Ps E thermalized e + e-e- in the presence of an electric field in the presence of a positron trap positron blob

Reduction of Ps formation by irradiation In non-irradiated UHMWPE about 1/3 Ps is accounted for by mobile positrons re-entering the blob to recombine with an electron, similarly to LDPE and HDPE. The rapid reduction of Ps formation at low doses is attributed to the trapping of mobile positrons by defects produced by the irradiation, prohibiting the re-entering of the positrons into the blob. 1

Ps lifetime and mechanical properties chain scissions by gamma-rays densification degradation in mechanical properties o-Ps lifetime Elongation, tensile stress

Conclusions Positronium formation in UHMWPE is significantly suppressed by low dose gamma-ray irradiation in the way similar to LDPE and HDPE. This is reasonably attributed to the trapping of mobile positrons, otherwise re-entering the blob to form positronium by recombination with one of the intra-blob electrons. With increasing the gamma-ray dose both tensile stress and elongation at break of UHMWPE were substantially reduced. With the reduction in mechanical properties, ortho-positronium lifetime was shortened. On site monitoring of degradation of UHMWPE due to radiations may be possible by applying new PALS.