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 UniversityToyo Seiko Inc.

Japan & Egypt

PPC10

Conventional PALS is not strictly nondestructive!

22Na sourceand sample

start

scintillator

stopPMT

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, 086301 (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

Time / ns0 5 10 15 20 25

Nor

mal

ized

cou

nts

100

101

102

103

104

105

106

UMPE

ortho-positronium (3, I3)

UHMWPE

Tensile tests and mechanical properties

Stress-strain curves for UHMWPE before and after gamma-irradiation

Sample size1.2 mm x 20 mm x 0.2-0.3 mm

InstrumentSTA-1225 (ORIENTEC, Japan)

Tensile speed20 mm/min

Tensile test

0 kGy

Strain (%)0 100 200 300 400

Stre

ss (M

Pa)

0

20

40

60Specimen 1Specimen 2Specimen 3

Kept in air

1000 kGy

Strain (%)0 100 200 300 400

Stre

ss (M

Pa)

0

20

40

60Specimen 1Specimen 2Specimen 3Specimen 4

Kept in air

Variations of tensile strength and elongation at break as a function of gamma-ray dose

Dose (kGy)0 200 400 600 800 1000 1200

Tens

ile s

tren

gth

(MPa

)

0

20

40

60AirVacuum

Dose (kGy)0 200 400 600 800 1000 1200

Elon

gatio

n at

bre

ak (%

)0

100

200

300

400 AirVacuum

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

Dose (kGy)0 200 400 600 800 1000 1200

o-Ps

inte

nsity

I 3 (%

)

0

5

10

15

20

25

AirVacuum

Dose (kGy)0 200 400 600 800 1000 1200

o-Ps

life

time

3 (ns

)2.0

2.2

2.4

2.6

AirVacuum

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.

Effect of gamma-irradiation on o-Ps intensity in LDPE and HDPE

Y. Kobayashi et al, Radiat. Phys. Chem., 50, 589 (1997)

LDPE

LDPE

HDPE HDPE

1

2

1

2

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

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

１０－４０ｎｍ

Ps

E

thermalized 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.

Dose (kGy)0 200 400 600 800 1000 1200

o-Ps

inte

nsity

I 3 (%

)

0

5

10

15

20

25

AirVacuum

1

Ps lifetime and mechanical properties

chain scissions by gamma-rays

densification degradation in mechanical propertieso-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 .