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HIGH-RESOLUTION STUDY OF Kα X-RAY
SPECTRA INDUCED IN MID-Z ATOMS BY LIGHT HEAVY IONS
J.-C. Dousse, M. Carlen, M. Gasser, J. Kern, B. Perny, C. Rhême, P. Rymuza, Z. Sujkowski
To cite this version:
J.-C. Dousse, M. Carlen, M. Gasser, J. Kern, B. Perny, et al.. HIGH-RESOLUTION STUDY OF Kα
X-RAY SPECTRA INDUCED IN MID-Z ATOMS BY LIGHT HEAVY IONS. Journal de Physique
Colloques, 1987, 48 (C9), pp.C9-259-C9-262. �10.1051/jphyscol:1987944�. �jpa-00227362�
JOURNAL DE PHYSIQUE
Colloque C9, suppl6ment au n012, Tome 48, dgcembre 1987
HIGH-RESOLUTION STUDY OF Ka X-RAY SPECTRA INDUCED IN MID-Z ATOMS BY LIGHT HEAVY IONS
J.-C. DOUSSE, M. CARLEN, M. GASSER, J. KERN, B. PERNY, C. R H ~ M E , P. RYMUZA* and Z. SUJKOWSKI*
P h y s i c s D e p a r t m e n t o f t h e U n i v e r s i t y , CH-1700 F r i b o u r g , S w i t z e r l a n d
* ~ n s t i t u t e for N u c l e a r S t u d i e s , PL-05-400 S w i e r k , Poland
Rlsum6.- Les spectres de rayons X Ka du molybdene ( 2 = 4 2 ) , , palladium (Z=46) et holmium (2-67) bombard& par des ions de 4He (6.5 MeV/amu) et de 1 6 0 (5.5 MeV/amu) ont bt6 mesur6s au moyen d'un spectrometre A cristal incurve de transmission du type DuMond install6 aupres du cyclotron A Bnergie variable du SIN (Villigen, Suisse). Grace a la resolution &levee du spectrometre et A sa bonne sensibilit6, les lignes principales Kal et Ka2 et leurs satellites K a l L n = 1 $ 2 * 3 et KazLn=l?z ont pu Btre observes et rbsolus pour la premiere fois dans des atomes mi-lourds. Les knergies, largeurs et intensit& relatives des lignes observees sont commentOes dans l'article.
Abstract.- The Ka x-ray spectra of molybdenum (Z=42), palladium (Z=46) and holmium (2=67) bombarded with 6.5 MeV/amu He and 5.5 MeV/amu 1 6 0 ions were recorded with an on-line bent crystal spectrometer in DuMond geometry installed at the SIN variable energy cyclotron (Villigen, Switzerland). Thanks to the high resolution and
the good sensitivity of the instrument, KalLn=O* l P 2 * and K a ~ L ~ = ~ , i p ~ principal and satellite lines were observed and resolved for the
first time for atoms in the mid-Z region. The energies, line-widths and relative yields were determined and are discussed in the paper.
Multiple inner-shell ionization of atoms bombarded by heavy ions has been a subject of increasing interest during the last decade. The interest is stirred by some fundamental problems concerning the mechanism of the reaction itself as well as by a number of possible applications in astrophysics, plasma physics, surface physics or nuclear physics. The observation of enhanced satellite structure in heavy-ion-induced x-ray spectra has proved to be a very powerful tool for studying multiple inner-shell ionization phenomena. This has led during the last years to a large number of such high-resolution x-ray measurements 11-51. Experimental data, however, are limited practically to light elements (Z<30). We have started recently .a series of experiments with the purpose of studying the multiple inner-shell ionization process induced by interactions of "light"
heavy-ions with atoms in the mid-Z region. This paper presents some preliminary results of that study (1).
The crystal spectrometer 161 used for these measurements is of the transmission type and has a bending radius of 315 cm. The instrument, installed at the SIN variable energy cyclotron in Villigen, Switzerland, enables us to measure x-rays in the range of 15 to 100 keV with an energy resolution comparable to the natural line-width. This high resolution gives a clear distinction between the Kal and Kaz lines and their individual L-satellites (Fig. 1). The (1) Work supported in part by SIN and by the Swiss National Science Foundation
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987944
JOURNAL DE PHYSIQUE
to the natural line-width 21.5 21.4 21.3 21.2 21.1 21.0 20.9 20.8 ,f the x-rays to be spectrometer is of the
BRAGG ANGLE [degrees]-- DuMond slit-geometry type.
6.75" 6.80" 6.85" 6.90' 6.95" In this geometry, a narrow
--ENERGY [KeVl observed.
he
(110)reflecting planes of a 1.5 Fig.1.- High resolution crystal mm thick quartz crystal spectrometer spectra of palladium plate are used for the
K a x-rays induced by 4He and 1 6 0 ions. diffraction of the x-rays.
In the He-spectrum, the line on the The peak reflectivity of left, drawn at an arbitrary energy the crystal is enhanced by position, is the result of the folding about a factor 4 by of the instrumental response with the applying a high-frequency natural Lorentzian shape of the K a i alternating electric field
line. across it 171. Spectra are
collected by step-scanning over the desired angular region. Normalization of the data at each angular position is provided by subtracting the beam-uncorrelated background and dividing the resulting x-ray counts by the integrated x-ray peak measured with a Ge(Li) detector-monitor. The monitor can account, to a good approximation, for the beam fluctuations in intensity as well as in position. An optical interferometer permits us to determine wavelength ratios with a precision of about 1 to 3 ppm in the angular range of the K x-rays. Depending on the used target, different radioactive sources, placed 30 cm behind the Slit, are used for the energy calibration and the determination of the instrumental line shape. This arrangement allows the calibration line to be measured under the same conditions as for the in-beam measurements.Self-supporting metallic foils of natural molybdenum.
palladium and holmium, with thicknesses of 25 mg/cmz for the 4 H e and 12.5 mg/cmz for the 1 6 0 beam, were used as targets. The 4He+ and
i 6 0 4 + ions, with, respectively 28 and 112 MeV initial energy, were
delivered from the SIN variable energy cyclotron. After passing the slit is placed on the focal circle of the crystal in a fixed position and serves as the effective source of radiation. The target, placed behind the slit, is seen by the crystal through the slit. This geometry results in a minimum absorption of the x-rays in the target. For photon energies in the 20-30 keV range, the luminosity of the instrument is then enhanced by about a factor 2, although the solid angle
-
I-U
4-
o CV
a --. -
3 -E 5
2-0 U
I -
7-
6 - under which the target is
seen by the crystal is smaller than in the standard DuMond geometry.
C 4 With the slit geometry, the
M s
-
measurements sensitive to are position lessF
3-z drifts of the target and
the line shape is not affected by thermal target
1
-
deformations. Moreover, theslit width can be adapted Ka,Lo '
fi 1
!r
i t I
Ka,L1 K ~ , L 'J
L-/JL.J[
K ~ ~ L " ( + K ~ ~ ~ ) He-Pd
Ka2L"
.'
17.0 eV
--
5.3 mg/cm2 thick Havar entrance window of the target chamber, the energies are degraded down to 27 MeV and 99 MeV, respectively and the projectiles are totally ionized. Taking into account self-absorption, stopping power in the target and energy dependence of K-shell ionization cross sections, we obtain effective mean energies for the production of the x-rays of about 6.5 MeV/nucleon for 4He and 5.5 MeV/nucleon for the 1 6 0 . The 4He beam current was 4 CIA and the 1 6 0
beam current was about 150 nA.
Due to multiple inner-shell ionization, the K x-ray spectra of atoms bombarded by heavy ions exhibit a composite structure of principal and satellite lines. These latter correspond to various configurations of the "spectator" vacancies in the inner shells. The energy shifts of the K x-ray lines due to each additional L-shell vacancy are larger than the natural line widths so that the K x-ray spectra can be resolved into individual L-shell satellite lines. On the contrary, the energy shifts due to additional M-shell vacancies are smaller than the natural line widths so that the M-satellite lines cannot be resolved and their contribution has to be extracted from the broadened principal and L-satellite lines. Three features of the observed spectra are of particular interest in that study: the energy of the individual lines, their relative intensity and width.
The energies determined experimentally can be compared to the results of theoretical calculations based on relativistic Dirac-Fock computer codes 181, allowing therefore to check the reliability of such calculations for different electron-hole configurations. From the relative intensities, we can test and develop models for multiple inner-shell ionization. The width of the observed lines is an interesting parameter too, because from the &omparison of the latter with the theoretical width obtained by folding the natural line shape with the instrumental response, it is expected that some features of the unresolved M-satellite lines can be extracted.
Our measurement program is in progress. By now, Ka spectra of good quality have been taken on molybdenum and palladium with both 4He and 1 6 0 beams. These data have been analysed and a first paper on molybdenum is in press [9]. A preliminary measurement with a target of holmium was also performed to test the capability of the spectrometer to observe satellite lines in higher-Z atoms. In order to get the largest possible intensities for the L-satellite lines, the beam energy has to verify the "velocity matching condition"
[v(projectile)=v(L-shell electron)]. For holmium, this condition corresponds to a beam-energy of about 17 MeV/nucleon, i.e. 272 MeV for the 1 6 0 beam. The highest energy the Philips SIN cyclotron can produce when accelerating 160-ions is presently only 112 MeV with a maximal intensity of about 200 nA. For these reasons, only first- order L-satellite lines could be observed so far with holmium.
The observed energies of the Kal and Ka2 principal lines of molybdenum, palladium and holmium have been compared with the reference values quoted by Bearden [lo]
.
For the 4He spectra, the agreement is quite satisfactory, the relative differences being smaller than 30 ppm and thus consistent with our experimental uncertainties. The energy differences between satellite and principal lines have been compared with the theoretical values given by the computer code of Desclaux C81. For the 4He induced spectra of both molybdenum and palladium, a very good agreement is obtained. In the1 6 0 induced spectra, the principal and satellite lines are much
broader and slightly asymmetric. The fitted centroids of these lines are shifted by about 5 to 6 eV towards highcr energies relative to those in the 4He spectra. These energy shifts and the asymmetric broadening of the line result from the additional multiple M-shell vacancies.
Using the statistical procedure described by F.P. Larkins [Ill, the initial L-vacancy distributions have been calculated from
C9-262 JOURNAL DE PHYSIQUE
the observed satellite yield ratios KLo/KLI/KL2/KL3. The effect of the additional M-shell ionization on the calculated distributions is noticeable but not very pronounced. Anyway, with or without the M-shell ionization correction, the deduced initial L-shell vacancy distributions are not compatible with a binomial distribution, neither for molybdenum nor for palladium. The explanation of our data requires therefore improvements of the model describing the process, the contribution of L-electron capture by the bare projectile to the total ionization cross section being probably not negligible in that case.
Another interesting feature of the measured spectra is the width of the principal and satellite lines. The expected line-width contributions come from the natural atomic line-width, instrumental resolution, unresolved shifts due to outer-shell multiple ionization and Doppler broadening. The latter was studied carefully (see Appendix of [9]). A Monte Carlo computer code was written to calculate the line broadenings and the energy shifts introduced by the Doppler effect. The calculated broadenings are negligible ( < 0.2 eV) for both molybdenum and palladium targets with the 28 MeV 4He beam while a small width enhancement of about 1.5
-
2 eV is expected with the 112 MeV 1 6 0 beam. In the 4 H e induced spectra, the observed widths are in good agreement with the line-width obtained by folding the natural x-ray shape with the instrumental response function(see Fig. 1). In the 1 6 0 induced spectra, the observed widths are much larger than expected. These increased widths can be ascribed to unresolved KLnMm satellites and multiplet lines arising from states of various angular momentum couplings of the electrons or vacancies.Furthermore, the K a i x-ray lines of molybdenum, palladium and holmium targets observed with proton and helium beams showed nearly equal widths for the two beams, which is reasonnable, the Doppler broadening and the M-shell contribution being both very small as well for protons as for alpha-particles. These test-measurements have also demonstrated that there is no significant influence of the target thickness on the observed line widths.
References
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- - - .
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