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Printing:A Positron Lifetime Spectrometer
M. Salahuddin | H. Jaeger | Department of Physics, Miami University, Oxford, OH, USA
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. 300𝑝𝑠. 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 350– 450𝑝𝑠 range.
While the PTFE sample has a long component near 2𝑛𝑠, the Al
sample has a short (approximately 200𝑝𝑠 ) as well a long
component (> 2𝑛𝑠).
𝐷 𝑡 =
𝐷 𝑡 = 𝐴1𝑒−(𝑡−𝑡0)𝜏1 𝜏1 = 𝜏𝐵 = Bulk lifetime
𝐷 𝑡 =
𝑡 − 𝑡0 −𝜎2
1 − 𝑒𝑟𝑓𝜎
2𝜏𝑖−𝑡 − 𝑡0𝜎
𝜈 𝑂(𝑡) − 𝐷(𝑡) 2
𝜎2𝜈 = 𝑁 − 𝑛 − 1
𝐹𝑊𝐻𝑀 = 2𝜎 ln 2
+ 𝐵𝐺Gaussian with background
Parameters values for 60Co
𝑒− + 𝑒+ → 𝛾 + 𝛾
FWHM (𝑝𝑠) 318 ± 2
𝑡0 (𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑢𝑛𝑖𝑡𝑠) 1074.32 ± 0.03
Background 10.9 ± 4
Time Calibration 18.65𝑝𝑠/𝑐ℎ
Reduced 𝜒𝑟𝑒𝑑2 1.00
𝜏 (𝑝𝑠) 188.34 ± 0.9
𝐴 (𝑐𝑜𝑢𝑛𝑡𝑠) 45976.40 ± 220Matter-anti-matter reaction
𝛾 → 𝑒− + 𝑒+ Pair production
𝑝 → 𝑛 + 𝑒+ + 𝜈𝑒 𝛽+ decay
𝑛 → 𝑝 + 𝑒− + 𝜈𝑒 𝛽− decay 𝑝 + 𝑒− → 𝑛 + 𝜈𝑒 Electron Capture decay
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.
• Intensities, 𝐴𝑖 – proportional with vacancy concentration
• Lifetimes 𝜏𝑖 – characteristic value for each vacancy type
typically 100-300ps (bulk solid, vacancies) 1-2ns (large
Typical lifetime spectrum
𝑡0 (𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑢𝑛𝑖𝑡𝑠) 1075.53
𝐴 (𝑐𝑜𝑢𝑛𝑡𝑠) 10103.2
FWHM (𝑝𝑠) 284.65
Time Calibration 18.63𝑝𝑠/𝑐ℎ
Parameters values for 207Bi
Decay scheme of 22Na
𝐿+ = 𝐷+𝜏
𝐷+ ∝ 𝑇−1/2
Metals: 𝐿+ = 100𝑛𝑚Semiconductor: 𝐿+ = 200𝑛𝑚
Diffusion trapping model
FWHM (𝑝𝑠) 283.8 ± 2
𝜏1 (𝑝𝑠) 170.97 ± 11
𝐴1 (𝑐𝑜𝑢𝑛𝑡𝑠) 16848.28 ± 983.2
𝜏2 (𝑝𝑠) 347.02 ± 15
𝐴2 (𝑐𝑜𝑢𝑛𝑡𝑠) 7942.84 ± 1308
𝜏3 (𝑝𝑠) 2133.71 ± 410
𝐴3(𝑐𝑜𝑢𝑛𝑡𝑠) 43.05 ± 13
𝐼1 (%) 50.28 ± 6
𝐼2 (%) 48.11 ± 9
𝐼3 (%) 1.60 ± 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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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,
• We use a positron lifetime spectrometer that is efficient for defect detection in metals and
• The performance of the setup has been tested by 207Bi radioactive isotope.
• We measure the lifetime of commercial grade Al which has longer lifetime than the bulk
lifetime 165𝑝𝑠[3 − 5]. However, after annealing process the lifetime gets closer to bulk
• 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.