A simple positron lifetime spectrometer for a magnetically guided
Mihail P. Petkov,
and Kelvin G. Lynn
Department of Physics, Washington State University, Pullman, Washington 99164
͑Received 14 May 2001; accepted for publication 9 October 2001͒
We present a new, simple, and inexpensive positron lifetime spectrometer intended for the
depth-resolved characterization of thin ﬁlms and buried interfaces. The spectrometer operates on a
conventional magnetically guided positron beam with energies ranging from 1 to ϳ50 keV. Given
is a detailed description of the performance of the apparatus, built on thorough experimental
investigations and computer simulations. A timing resolution of 350Ϯ13ps at
full-width-at-half-maximum was obtained. The count rate for thin ﬁlms ͑low positron energies͒ was
of the order of 1000 s
. A maximum peak-to-background ratioϾ10
, aiding the measurements of
long-living ͑10–100 ns͒ positronium in voids, was achieved by constant beam rate reduction and by
beam chopping. Examples are presented for measured lifetimes in well-characterized systems.
© 2002 American Institute of Physics. ͓DOI: 10.1063/1.1424905͔
Positron annihilation lifetime spectroscopy ͑PALS͒ is a
well-established tool for open-volume defect
The positron-electron annihilation rate
͑inverse lifetime͒, determined by the overlap of the positron
and the electron wave functions, depends on the size of the
open volume, which can be uniquely identiﬁed for less than
ten missing atoms.
The lifetime of positrons in solids var-
ies from about 95 ps ͑defect-free Ni, Ref. 6͒ to 500 ps ͑large
vacancy clusters in Si Ref. 4͒. In some materials, a positron
may bond with an electron creating a positronium atom ͑Ps͒.
Although the lifetime of ortho-Ps ͑o-Ps,
state͒ in vacuum
is 142 ns, it is reduced when the Ps atom is localized in a
void. The amount of the reduction depends on the void size.
The existing PALS systems can be separated into two
groups, according to their application—for investigation of
bulk or thin-ﬁlm samples. The most common system for bulk
studies utilizes the nuclear relaxation photon, accompanying
the positron emission from the
Na radioactive source, to
start the lifetime clock.
This principle is used by both vari-
eties, the ‘‘fast-slow’’ coincidence system, utilizing timing
and energy information, and the ‘‘fast-fast’’ coincidence sys-
tem, simpliﬁed by eliminating the energy-resolving electron-
ics. Excellent timing resolution ͓full-width-at-half-maximum
͑FWHM͒ of ϳ150 ps͔ and a large ratio of peak to back-
ground ͑1300͒ were obtained.
These schemes are not appli-
cable for thin-ﬁlm research, since the positrons from the
source ͑with ϳMeV energy͒ are implanted at large depths
͑0.1–1 mm͒. A lifetime apparatus has also been constructed
for high-energy positron beam.
There, the start signal of the
lifetime measurement was taken from positrons ﬂying
through a thin scintillator prior to impinging on the sample.
The most advanced low-energy PALS systems, appli-
cable for studying thin ﬁlms, are sophisticated machines.
They incorporate several bunching stages with time- and
space-dependent electric ﬁelds, used to improve the time fo-
cus of a prepulsed beam. The positron source can be a high-
intensity pulsed LINAC beam,
or a radioactive nuclide
the low-energy positrons of which are converted to
a pulsed beam. These beam-based spectrometers have depth-
resolving capabilities and achieve excellent time resolution
(FWHMϭ240ps Ref. 12͒. Despite their superior qualities,
however, their budget and degree of complexity makes them
unaffordable for small laboratories ͑e.g., universities͒.
Simpler systems were built by using secondary elec-
trons, emitted upon positron entering a solid, to start the
Nico et al.
utilized the electrons pro-
duced at a positron remoderation stage to achieve satisfac-
tory performance for measurements of long o-Ps lifetimes.
Lynn et al.
used secondary electrons emitted from the
sample upon positron injection, and obtained the timing reso-
lution FWHM of about 600 ps. All of these spectrometers
were built for electrostatically guided positron beams, and
are inapplicable in magnetic ﬁelds, where E ϫB effects in-
ﬂuence the particle trajectories. Moderate success in using
secondary-electrons-based spectrometer in a magnetically
guided beam was achieved by Kong,
being able to measure
positron lifetimes with some success.
In this article, we describe a low-budget simple positron
lifetime spectrometer designed for a conventional magneti-
cally guided positron beam. To start the lifetime clock, the
spectrometer uses the secondary electrons emitted upon the
positron injection into the sample. Experimental tests and
computer simulations are carried out for the thorough inves-
tigation of the performance of the lifetime setup.
A schematic drawing of the lifetime apparatus is shown
in Fig. 1. A constant-rate beam of monoenergetic ͑1–50 keV͒
Present address: Department of Physics and Astronomy, The University of
Western Ontario, London, Ontario N6A 3K7, Canada.
Present address: Jet Propulsion Laboratory, California Institute of Technol-
ogy, 4800 Oak Grove Drive, Pasadena, CA 91109.
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 73, NUMBER 1 JANUARY 2002
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