Charge Exchange Spectra of Hydrogenic and He-like Iron Page: 8 of 25
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with the electron beam turned off once the desired ion charge balance has been attained.
The ions are still confined (though less densely) within the trap region, where they collide
with neutral nitrogen molecules, undergo CX, and emit photons. Our measurements record
the net result of all relevant CX processes, whether from single or multi-electron transfer,
radiative decays, or autoionization, as manifested by their spectra.
Two high-purity Ge detectors with energy resolutions of ~ 250 eV and ~ 370 eV
(FWHM at 7 keV), were used to collect spectra. The signal-processing lower level discrimi-
nators were set at 5 and 4 key, respectively, to exclude unnecessary events and prevent event
pile-up. All the results we present were obtained with the higher-resolution detector, but the
second detector with its lower energy threshold was helpful in identifying trap contaminants.
Because the detector resolution was insufficient to directly separate the spectra of
Fe XXVI and Fe XXV, data were collected in two measurements using different electron
beam energies. The low-energy run (L) used Ebeam = 9.2 keV, and the two high-energy runs
(I and J) were at 17.2 keV. For comparison, the ionization potentials of Li-like Fe23+, He-like
Fe24+, and H-like Fe25+ are 2.046, 8.828, and 9.278 keV, respectively (see Table 1). During
run L most of the trapped ions were He-like, with a small fraction of H-like. The observed
CX spectrum was therefore a nearly pure He-like spectrum. (The Li-like CX spectrum lies
below 2 keV, well below the lower level discriminator setting.) In runs I and J (17.2 keV),
the trap contained significant fractions of He-like, H-like, and bare ions, with a roughly 2:1
ratio of Fe+25 and Fe26+, resulting in a mixed CX spectrum of He-like Fe XXV and H-like
Fe XXVI lines.
In the 31-hour L run, ions were electrostatically trapped and ionized for 3.5 seconds (the
beam-on phase), followed by 2.5 seconds of magnetic trapping (the beam-off CX phase). In
the I and J runs (18 and 20 hours) the beam-on phase lasted 4.5 seconds. The beam current
was ~140 mA in all cases, with a trap electric potential of 300 V for run L and 100 V for I
and J. The difference in trap potentials was inadvertent and results in only a small difference
in effective ion-neutral collision energies. Based on past measurements of ion energies as a
function of trapping parameters (Beiersdorfer et al. 1996b), we estimate the average ion
energy in both cases to be very roughly 10 eV amu-1: between 5 and 20 eV amu-1 for runs
I and J, and approximately double that for run L. Although the Fe XXV CX spectrum was
therefore collected under two different conditions, its weak dependence on collision energy,
as explained in 2, means that the results from run L can be applied to runs I and J with
negligible error, as was confirmed during data analysis.
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Wargelin, B J; Beiersdorfer, P; Neill, P A; Olson, R E & Scofield, J H. Charge Exchange Spectra of Hydrogenic and He-like Iron, article, April 27, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc879558/m1/8/: accessed May 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.