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