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Y. Kobayashi, M. Yamawaki, T. Oka, S Saiki, H. F. M. Mohamed, K. Hattori and Y. Watanabe

Positron lifetimes and mechanical properties of gamma-irradiated ultra high molecular weight polyethylene. PPC10. 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,

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Y. Kobayashi, M. Yamawaki, T. Oka, S Saiki, H. F. M. Mohamed, K. Hattori and Y. Watanabe

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  1. Positron lifetimes and mechanical properties of gamma-irradiated ultra high molecular weight polyethylene PPC10 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

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

  3. New method for PALS for potential onsite inspection without anti-coincidence with anti-coincidence positron lifetime spectra of a steel strip A novel method that does not require cutting out of the sample and is potentially applicable to onsite material inspection wasdeveloped by Yamawaki et al. (PPC10 and Jap. J. Appl. Phys., 50, 086301 (2011))

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

  5. Experiments UHMWPE • 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

  6. Tensile tests and mechanical properties Tensile test Sample size 1.2 mm x 20 mm x 0.2-0.3 mm Instrument STA-1225 (ORIENTEC, Japan) Tensile speed 20 mm/min Stress-strain curves for UHMWPE before and after gamma-irradiation

  7. 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.

  8. 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.

  9. Effect of gamma-irradiation on o-Ps intensity in LDPE and HDPE LDPE 1 1 2 LDPE HDPE HDPE 2 Y. Kobayashi et al, Radiat. Phys. Chem., 50, 589 (1997)

  10. Positron mobilities in gamma-irradiated LDPE and HDPE W. Zheng, Y. Kobayashi et al, Radiat. Phys. Chem., 51, 269 (1998) ・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

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

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

  13. 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

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

  15. 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 .

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