K-SHELL FLUORESCENT YIELDS IN HIGHLY STRIPPED 57 Fe ATOMS

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Submitted on 1 Jan 1987

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K-SHELL FLUORESCENT YIELDS IN HIGHLY STRIPPED 57 Fe ATOMS

W. Phillips, K. Rehm, W. Henning, I. Ahmad, Johannes Schiffer, B. Glagola, T. Wang

To cite this version:

W. Phillips, K. Rehm, W. Henning, I. Ahmad, Johannes Schiffer, et al.. K-SHELL FLUORESCENT

YIELDS IN HIGHLY STRIPPED 57 Fe ATOMS. Journal de Physique Colloques, 1987, 48 (C9),

pp.C9-311-C9-313. �10.1051/jphyscol:1987956�. �jpa-00227374�

JOURNAL DE PHYSIQUE

Colloque C9, supplbment au n012, Tome 48, dgcembre 1987

K-SHELL FLUORESCENT YIELDS IN HIGHLY STRIPPED 5 7 ~ e ATOMS

W . R . PHIL LIPS(^)

,

Physics Division, A r g 0 ~ e National Laboratory, Argonne, IL 60439, U.S.A.

Charge changing effects during transit of a magnetic spectrometer have been used to measure K-shell fluorescent yields W K ( ~ ) in highly stripped 5 7 ~ e atoms of ionicity q. Results from preliminary experiments give average W K (Q) in the configurations existing during the experiments of 0.97(3), 0.54(7), 0.42(6) and 0.41(9) for q=23, 22, 21 and 20 respectively

K-shell fluorescent yields in highly ionized atoms depend on the electron configurations and on inner shell wave functions which have been modified by the removal of other inner shell electrons.

No experiments have been made for high Z ions, for which experimental data of any type hardly exist. Knowledge of the properties of highly stripped ions is needed in several scientific fields, and fluorescent yield measurements can complement energy level measurements in tests of self- consistent field calculation^(^). This note presents a new technique for studies of highly stripped atoms and its application to measurements on 5 7 ~ e ions of different charge states. The 57Fe nucleus has a first excited state at 14.4 keV with a lifetime of 98.1(3) ns in the neutral atom, where(2) the total conversion coefficient is 8.18(11) and the ratio of K- to L-conversion is about 10. In our experiments the change in ionic charge caused by internal conversion decay of the 14.4 keV level during passage of 57Fe ions through a magnetic spectrometer was used to investigate the lifetimes r(q) of the level and the K-shell fluorescent yields w ~ ( q ) as a function of the ionic charge state q.

57Fe ions containing a significant fraction of nuclei in the 14.4 keV state were produced by Coulomb excitation of a 330 MeV 57Fe beam in a 400 ,ugcm-2 Au target. Scattered beams forward of the nuclear grazing angle were magnetically analysed in an Enge split pole spectrometer(3).

The ions entered the spectrometer ~ 3 1 2 ns after leaving the foil, by which time they were to a good approximation all in the 5 7 ~ e nuclear ground state or 14.4 keV level, and in considerably cooled down electronic configurations compared with those present at the exit of the Au foil. Ions traversed the magnetic field in ~ 3 1 0 0 1:s and were detected in the focal plane of the spectrometer in a broad-range gas counte/which measured their positions in the focal plane and their energies.

Essentially all the events for scattering angles O L less than 45' were scattered 57Fe ions. Figure 1 shows position spectra at OL = 35" and 45" obtained by gating on narrow regions of the position- energy planes. Large peaks are seen corresponding to 57Fe ions which underwent no charge change during transit of the magnetic field, together with events in between the peaks. These (')permanent address : Department of Physics, University of Manchester, GB-Manchester M13 9PL.

Great-Britain

("permanent address : GSI, Planckstrasse 1. D-6100 Darmstadt. F.R.G.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987956

C9-312 JOURNAL DE PHYSIQUE

correspond to charge changes Aq=+l when an electron vacancy produced by internal conversion decay of the 14.4 keV level is filled by photon emission, and Aq=+2 when vacancies are filled with accompanying Auger electron emission.

The difficulty of identifying whether an event arose from single or double charge change, and hence from which charge state q the nuclear decay took place, prevented all the events in between the main peaks from being useful. However, the nearly field-free gap in the split-pole spectrometer gives rise to subsidiary peaks which can be identified as due to Aq=+l or Aq=+2 charge changing decays in known charge states q in the gap. For example the first peak to the right of the unchanged q=24 peak on Figure 1 arises from a A q = l event in the gap in charge state q=23; the second peak to the right from a Aq=2 event in charge state q=22. There is only one subsidiary peak near the undisturbed q=25 peak. This is because there are no Aq=2 events from the q=23 charge state. On entry to the gap, w30 ns after leaving the foil, there is only one electron outside of the filled K-shell and no Auger emission can take place following K-vacancy formation.

10"

r

Z 3

s

CHANNEL NUMBER

Figure 1: Position spectra of scattered 57Fe ions at two angles of scattering.

The intensities of the subsidiary peaks were used to give the K-shell fluourescent yields. The average w ~ ( q ) in the various configurations occupied by the electrons in the ions of different q as they traverse the gap are given directly by the ratios N1/(N1

+

errors of less than 4% in the values of w ~ ( q ) . Figure 2 shows the values obtained for q=20-23 together with the experimental value for neutral 57Fe atoms(4).

Figure 2: Average K-shell fluorescent yields in 5 7 ~ e ions as they transit the magnet gap in dif- ferent charge states.

The subsidiary peaks also contain information on the lifetimes ~ ( q ) . The decay- of the 14.4 keV level is predominantly by magnetic dipole t r a n s i t i ~ n s ( ~ ) . The total conversion coefficients in the different ions, and hence the lifetimes ~ ( q ) , are thus to a good approximation measures of the total electron densities at the nucleus(5), other data of interest in stripped ions. However although there is clear evidence that ~ ( 2 5 ) is much longer than ~ ( 2 4 ) tfie statistical accuracy of the present data is not high, and meaningful statements on ~ ( q ) await improved experiments.

In summary we have outlined a new technique for studying physical processes in highly stripped ions. The technique utilised charge changing phenomena which occur during nuclear delays in ions in transit through magnetic fields. The technique has been applied to the study of inner shell electrons in highly stripped 5 7 ~ e ions and has provided new measurements of K-shell fluorescent yields in these systems.

References

1 W.R. Johnson and K.T. Cheng, in Atomic Inner Shell Physics; Plenum Press, New York and London, 1985, p.3

2 T.W. Burrows and M.R. Bhat, Nucl. Data Sheets 47 (1986) 1 3 J.E. Spencer and H.A. Enge, Nucl. Inst. Meth. 49 (1967) 181

4 R.W. Fink, R.C. Jopson, H. Mark and C.D. Swift, Rev. Mod. Phys. (1966) 513 5 U. RafF, K. Alder and G. Baur, Helv. Phys. Acta 45 (1972) 427