Post on 14-Apr-2017
Printing:A Positron Lifetime Spectrometer
M. Salahuddin | H. Jaeger | Department of Physics, Miami University, Oxford, OH, USA
Abstract
Data analysis
Results
Conclusion
Reference
A spectrometer for measuring positron lifetimes in solids is
described. It consists of a pair of BaF2 scintillators coupled to
photomultiplier tubes and a fast-slow coincidence system
composed of standard NIM modules. The instrumental resolution
curve as determined with a 60Co source is well-described by a
Gaussian with a FWHM of approx. 300ππ . We apply this apparatus
setup to the positron lifetime experiment of metal and polymer
samples. We used this apparatus to determine the lifetime of
positrons in commercial grade Al and PTFE (Teflon) at room
temperature with a 22Na positron source enclosed in Kapton foil.
Both samples show a lifetime component in the 350β 450ππ range.
While the PTFE sample has a long component near 2ππ , the Al
sample has a short (approximately 200ππ ) as well a long
component (> 2ππ ).
Experimental setup
Positronium formation
π· π‘ =
π=1
π+1
π΄ππβ(π‘βπ‘0)ππ
π· π‘ = π΄1πβ(π‘βπ‘0)π1 π1 = ππ΅ = Bulk lifetime
π· π‘ =
π=1
π+1π΄π2ππ₯π β
π‘ β π‘0 βπ2
4ππππ
1 β ππππ
2ππβπ‘ β π‘0π
+ π΅πΊ
ππππ2 =1
π π(π‘) β π·(π‘) 2
π2π = π β π β 1
πΉππ»π = 2π ln 2
= π΄πβπ‘βπ‘0π
2
+ π΅πΊGaussian with background
Parameters values for 60Co
Fundamental equations:
πβ + π+ β πΎ + πΎ
FWHM (ππ ) 318 Β± 2
π‘0 (πβπππππ π’πππ‘π ) 1074.32 Β± 0.03
Background 10.9 Β± 4
Time Calibration 18.65ππ /πβ
Reduced ππππ2 1.00
π (ππ ) 188.34 Β± 0.9
π΄ (πππ’ππ‘π ) 45976.40 Β± 220Matter-anti-matter reaction
πΎ β πβ + π+ Pair production
π β π + π+ + ππ π½+ decay
π β π + πβ + ππ π½β decay π + πβ β π + ππ Electron Capture decay
Application
The positron lifetime technique is the most sensitive and also the only
defect-specific one. It is nicely applicable to atomic and molecular free
volumes and holes in polymers, solids.
βππ(π‘)
ππ₯=
π
π΄ππππβπ‘ππ
β’ Intensities, π΄π β proportional with vacancy concentration
β’ Lifetimes ππ β characteristic value for each vacancy type
typically 100-300ps (bulk solid, vacancies) 1-2ns (large
voids, positronium)
Typical lifetime spectrum
π‘0 (πβπππππ π’πππ‘π ) 1075.53
Background (π΅πΊ)(πππ’ππ‘π )
9.06
π΄ (πππ’ππ‘π ) 10103.2
FWHM (ππ ) 284.65
Time Calibration 18.63ππ /πβ
Parameters values for 207Bi
Decay scheme of 22Na
πΏ+ = π·+π
π·+ β πβ1/2
Metals: πΏ+ = 100ππSemiconductor: πΏ+ = 200ππ
Diffusion trapping model
FWHM (ππ ) 283.8 Β± 2
π1 (ππ ) 170.97 Β± 11
π΄1 (πππ’ππ‘π ) 16848.28 Β± 983.2
π2 (ππ ) 347.02 Β± 15
π΄2 (πππ’ππ‘π ) 7942.84 Β± 1308
π3 (ππ ) 2133.71 Β± 410
π΄3(πππ’ππ‘π ) 43.05 Β± 13
πΌ1 (%) 50.28 Β± 6
πΌ2 (%) 48.11 Β± 9
πΌ3 (%) 1.60 Β± 0.6
Parameters values for Al
The figure on the left side shows the decay
scheme of 60Co and 207Bi respectively
Lederer, C. M., Hollander, J. M., and Perlman, I.,
Table of isotopes(1967).
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3388 (2006)
3. Djourelov, N., and Misheva, M. Source correction in positron annihilation lifetime spectroscopy. J. Phys.
Condens. Matter, 8(12), 2081(1996).
4. Monge, M. A., and Del Rio, J., Position annihilation in Kapton source-supporting foils. J. Phys. Condens. Matter,
6(13), 2643(1994)
β’ We use a positron lifetime spectrometer that is efficient for defect detection in metals and
their alloys.
β’ The performance of the setup has been tested by 207Bi radioactive isotope.
β’ We measure the lifetime of commercial grade Al which has longer lifetime than the bulk
lifetime 165ππ [3 β 5]. However, after annealing process the lifetime gets closer to bulk
lifetime.
β’ We measure the lifetime of Teflon shows comparatively longer component of lifetime due to
positronium formation, hence the graph for Teflon is shallower than Al.