Charge state dependence of K X-ray production in thin targets by multicharged silicon ions

Nuclear Instruments and Methods in Physics Research B42 (1989) 485-490 North-Holland, Amsterdam

485

CHARGE STATE DEPENDENCE OF K X-RAY PRODUCTION IN THIN TARGETS BY MULTICHARGED SILICON IONS F.D. MCDANIEL and J.L. DUGGAN Department of Physics and Center for Materials Characterization, University of North Texas, Denton, Texas 76203, USA

G. LAPICKI Department of Physics, East Carolina University, Greenville, North Carolina 27858, USA

P.D. MILLER Physics Division, Oak Ridge National Luboratoq,

Oak Ridge, Tennessee 37830, USA

Typical data for target K-shell X-ray production cross sections in different charges states q are reviewed. Charge-state dependence is revisited and a small, shallow minimum in the cross section as a function of q, at q = 10 or 11 for Si ions, is noted. A number of mechanisms that are considered show a monotonic decrease in cross sections with the decreasing q, which is contrary to the observed trend. We postulate that the increase of K-shell X-ray cross sections, as q becomes smaller, can be understood in terms of a larger ionization probability into L- and higher shells by a greater number of the projectile’s electrons, which widens the excitation channels for the K-shell and, hence enhances the K-shell X-ray production cross section with the decreasing q.

1. Introduction

2. Experimental procedures

Coulomb interactions of highly-charged ions with atomic systems can result in target inner-shell electrons being excited into higher bound states [l], ejected into the projectile’s continuum [2], captured into bound states of the projectile or ionized into the target’s continuum [3,4]. The ionization mechanisms may be characterized by two parameters which separate their regions of applicability: the ratio of ion velocity to target electron and ratio of ion atomic number to velocity, vi/v,,, target atomic number, 2,/Z,. Direct ionization (DI) into continuum states is dominant when Z,/Z, % 1 and vl/vZK B 1 (fast asymmetric collision) [5,6]. Electron capture from the target atom to the ion may be important when Z,/Z, < 1 and v~/v~~ - 1 if vacancies exist on the ion [3,4,7]. For slow, symmetric collisions where Z,/Z, - 1 and v~/v~~ << 1, electron promotion due to the formation of quasi-molecules is possible [8]. This paper surveys our typical data [4] for target K-shell X-ray production cross sections for 1.86 MeV/u $iq+ ions in different charge states, q. X-ray production cross sections are extracted from the measurements for a range of target thicknesses. For these measurements, both direct ionization of the target atom and electron capture to a bound state of the Si ion are appreciable and have been extracted from the data [4]. We will discuss the effects of L-shell electrons on the $Si ion.

Silicon ions in different charge states were obtained from the 6.5 MV EN tandem van de Graaff accelerator at Oak Ridge National Laboratory, Oak Ridge, Tennessee. The wide range of Si ion charge states from 7 to 14 was produced by post acceleration stripping of the ion in N, gas or carbon foil strippers placed in the beam line and selected by magnetic analysis. The Si ions were focused and collimated to a beam spot size of - 0.5 mm on the target. Fig. 1 shows the foil-wheel scattering chamber used in these measurements. The chamber holds up to 24 targets on a rotating wheel which permits the targets to be changed rapidly. In the present experiment this was important, since both X rays and scattered ions were detected for each target for a number of different target thicknesses and ion charge states. The thin foil targets were mounted at 45” to the incident beam direction. The transmitted ion beam was collected after the chamber in a Faraday cup with a 90-V electron suppressor. This integrated charge was used to monitor the progress in the experiment. Target K-shell X rays were detected at 90” to the incident beam direction with an Ortec 7000 series Si(Li) detector. The detector has a 7.62 pm thick Be window and is covered with a 3.81 km thick Mylar foil to attenuate scattered ions, low-energy X rays and bremsstrahlung radiation. The energy resolution of the Si(Li)

0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

F.D. MeDame

486

et al. / K X-ray production

Vvewmg and Target Loadmg Port

in thin targets

24 Positlon Tat-get Carrying Wheel

Rotary Motlon Feed-Through

Surface-Barrier Detectors

Beam

Mylar

AXE

+

To Faraday

CUP

Fo11

Fig. 1. Scattering chamber with a wheel of target foils.

detector

was measured to be 175 eV (at 5.94 keV) with an 55Fe source. The Si(Li) detector X-ray efficiency was determined with a number of activity-calibrated X-ray and -r-ray radioactive sources. Ions scattered from the thin foil targets were simultaneously detected with silicon surface barrier detectors at 20 O, 30 O, or 45 o as shown in fig. 1. The solid angles of the silicon surface barrier detectors were determined with a calibrated ‘41 Cm source. Absolute X-ray production cross sections were obtained by normalizing the X-ray yields to the number of scattered ions as discussed elsewhere [9]. Elemental targets of **Ti, &.I, and ,,Ge [ranging in thickness 4-100 pg/cm2 (Ti), l-500 pg/cm2 (Cu) and 4.8-100 pg/cm2 (Ge)] were made by vacuum evaporation of the element on 5-20 pg/cm2 thick carbon backings. The thinnest targets were used to determine X-ray production cross sections while the thicker targets were used to study the target-thickness dependence of the X-ray yields and to obtain initial fit parameters during data analysis. The thinner targets were more affected by contamination from high-Z elements which have L- and M-shell X rays near the same energies as the elements of interest. Steps were taken during target preparation to reduce the presence of the contamination as described elsewhere [lo].

3. Results and discussion Some data in this paper have been presented earlier [4] in the extraction of direct ionization and electron capture contributions to ionization and the comparison

to first Born [ll-141 and ECPSSR [6,7,15] theories. Other aspects of these data will be discussed in regards to electron screening effects on ionization.

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THICKNESS

I

,,,I

I

1

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I

100

I

I

1 00

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Fig. 2. Effective titanium cross sections for target K-shell X-ray production by fiSi’r+ ions in different charge states 4 as a function of titanium target thickness. The solid squares, half-filled circles, and open circles denote measurements made with ions with zero-, one-, or two-K’-shell vacancies, respectively. The solid circles represent measurements made for ions with two electrons that are in a metastable state. See text for discussion.

F. D. McDaniel et al. / K X-ray production in thin targets

Fig. 2 shows measured values of the effective titanium to cross sections that cross sections, uW, comparable would be obtained from averages over thickness which are weighted by the fractions of the :$i ion beam with zero-, one-, or two-K’-shell vacancies. The primary 28Si7+ ion beam was prepared by post-acceleration 14 stripping and magnetic analysis to produce charge states 7-14. After passing through approximately 100 ug/cm’ of titanium, all initial ion charge states approached an equilibrium charge state distribution as seen as fig. 2. These experimental values are nearly independent of incident charge states for q = 7-11. For very thin targets, the cross sections are enhanced by a factor of about 14 for fully stripped ions (q = 14) and by a factor of about 7 for ions with one K’-shell vacancy (q = 13). These increases are due to electron capture from the K-shell of the target to the K’-shell of the ion. A smaller enhancement of a factor of 2.5 (or - 30% of the enhancement for ions with one K’-shell vacancy) is attributed to a 1~2s metastable state with a K’-shell vacancy in the incident He-like, :i Si ion beam. This metastable state is formed when the $Si ion beam passes through the post-acceleration foil stripper [16-181. By subtracting the q = 7-11 X-ray production cross sections from the q = 14 cross sections and the q = 13 cross sections for the thinnest targets, one can determine the contribution to target X-ray production from electron capture of an electron from the target K-shell to the K’-shell of the ion, for two or one K’-shell vacancies, respectively. The q = 7-11 cross sections represent direct ionization plus electron capture to vacancies in L’-, M’-, and highershells of the ion. The direct ionization and electron capture contributions to target K-shell ionization have been presented earlier [4] and have been compared to first Born [ll-14) and ECPSSR [6,7,15] theories. The first Born calculations were found to overpredict the K-shell data by l-2 orders of magnitude while the ECPSSR calculations provided good agreement [4]. Similar results have been found for electron capture from the L-shell of the target to the K’-shell ion the ion [19,20] and from the M-shell of the target to the K/-shell of the ion [20,21]. The titanium K-shell X-ray production cross sections for the thinnest targets (4.0 kg/cm2) are plotted as a function of :tSi’ion charge state q in fig. 3. The q = 10 cross section is somewhat lower in value than the q = 7 cross section although still within uncertainties, which are about the size of the data points. It is also difficult to decipher a trend on the basis of two data points. Figs. 4 and 5 show K-shell X-ray production cross section, obtained for the thinnest copper targets (1 ug/cm2) and germanium targets (4.8 ug/cm2), plotted as a function of $i ion charge state q. The trends appeared more evident for copper and germanium as for titanium, in fig. 3. The K-shell X-ray production cross sections were found to decrease slightly as charge

487

6

7

8

9

10

11

CHARGE

12

13

14

q

STATE

Fig. 3. Titanium K-shell the thinnest

X-ray production cross sections for target (4.0 pgg/cm’) plotted as a function of the $iq+ ion charge state q.

state q increases from 7 to 11. This minimum in the X-ray cross section at q - 10 or 11 was small but consistent for the three target elements studied. The reason for it was not clear. A number of possibilities are considered heretofore. Electron capture from the K-shell of the target to the L’-shell of the ion could not be the reason. As the ion

1.86

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.

*

I

I

,

,

I

I

7

8

9

10

11

12

CHARGE

STATE

1

I

13

14

q

Fig. 4. Copper K-shell X-ray production cross sections thinnest target (1.0 pgg/cm2) plotted as a function fiSi4+ ion charge state q.

for the of the

F.D. McDaniel et al. / K X-ray production in thin targets

1.86

f$iq+

MeVlu

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! 1

P 1

I

I

s

I

I

1

I

7

8

9

10

11

12

13

14

CHARGE

STATE

q

Fig. 5. Germanium K-shell X-ray production cross sections for the thinnest target (4.8 pg/cm2) plotted as a function of the :iSiq+ ion charge state 4.

charge state q increases from 7-11, the number of L’-shell vacancies on the ion increase from 3 to 7. An increased number of L’-shell vacancies on the ion would allow increased capture from the target K-shell to the L/-shell of the ion increasing the number of target K-shell vacancies and hence, K-shell X rays. Also, as the charge state of ion increases from q = 7 to 11, the higher shells can be ionized, electron capture from the L-shell of the target to the L’-, M’shells of the ion may open target excitation channels which would increase the target K-shell vacancy production and X-ray yields. This effect, however, points in the wrong direction. Ion electron screening also could not explain the trends of figs. 3-5. As the ion charge state is increased from 7 to 11, the number of electrons on the ion is reduced. The ion becomes more like a bare nucleus as q goes from 7 to 11. The less screened ion should increase target K-shell vacancy production which would not account for the minimum seen in our data for q - 10 or 11. One might check whether this minimum could be a data analysis problem since we normalize the X-ray yields to the Rutherford scattered ions. These heavy ions cannot be considered to be bare nuclei which the Rutherford cross section calculation assumes. These ions have three (q = 11) to seven (q = 7) electrons attached to them and should be considered to be screened. If one uses a screened or reduced 2, in the data analysis, the corrected Rutherford cross section is increased as q increases from 7 to 11 which increases the X-ray production cross section and again does not explain the

shallow minimum for projectiles with just a few electrons. Target fluorescence yield effects also could not be the culprit. As the charge state of the ion is increased from 7 to 11, multiple ionization of the target may increase. An increase in the target multiple ionization would increase the fluorescence yield and would cause an increase in the target K-shell X-ray production cross section which disagrees with what is observed experimentally. Recoil effects on the target are expected to have no effect as ion charge state changes from 7 to 11 since the ion is at the same energy and same mass to within a small fraction of 1%. Finally, we consider a mechanism that could enhance the K-shell vacancy production with increasing number of electrons on the projectile. Excitation of the target K-shell electrons might occur if the target L, M, and or higher shells were to be effectively ionized in a simultaneous collision. The ionization by the ion, contrary to observed trend, decreases as q changes from 11 to 7 because fewer L vacancies would be available for electron capture from the target. However, the ionization by the projectile’s electrons would increase with the decreasing q since the electron ionization cross sections are proportional to a larger number of electrons as q goes from 11 to 7. While each electron associated with a 52-MeV **Si has an energy E, = E,m,/M, = 52000 keV/(28 X 1836) = 1 keV, it is energetically capable of ionizing L, M, N-subshells of **Ti, L,, L,, MN, N-subshells of z&u, and M and N subshells of ,,Ge. The masses of the electron and Si are in atomic units 1 and 28 X 1836, respectively. We have calculated the probability P of ionizing these subshells as P = us/( N,~Tu&) where N, is the number of electrons in the subshell and u2s = n2/Zzs is the subshell radius in terms of the Slater screened Z,, and the principal quantum number n. For es we used electron ionization cross sections from the Lotz formula [22]. The product of P times aE, which is the excitation cross section from K to S subshell [23], gives the cross section for K-shell vacancy production through excitation into the S-subshell that is emptied by each electron with the probability P. Upon summing over all subshells and multiplying by the K-shell X-ray fluorescence yield, we find that calculated ukx is enhanced per each projectile’s eIectron by 8%, 7%, and 3% for titanium, copper, and germanium, respectively. Thus, with increasing number of the projectile’s electrons, the excitation from the target K-shell becomes more probable into a larger manifold of empty states created by more electrons of the ion with a smaller q. This would explain the trends, which we have noticed in our data. A four-electron difference in q translates for zzTi and &u into about 30% effect whereas, in 3zGe it results only in a 10% increase in K-shell production

F. D. McDaniel et al. / K X-ray production in thin targets

cross sections with the decreasing q for q I 11. While the titanium data (see fig. 3) are too few to support our quantitative prediction, the copper (fig. 4) and germanium (fig. 5) data confirm and conform with our calculations - a 30% enhancement is seen in r&u and practically no effect is observed in ,,Ge, where 10% changes are obscured by the experimental scatter of the data. Work supported in part by the National Science Foundation Division of Materials Science, the Department of Defense (Defense University Research Instrumentation Program), the Defense Nuclear Agency Division of Materials Research, the Office of Naval Research Electronics Division. The Robert A. Welch Foundation, The State of Texas Coordinating Board Advanced Technology Research Program and Energy Research in Applications Program, Texas Instruments Incorporated, and the University of North Texas Organized Research Fund. Research at Oak Ridge National Laboratory was sponsored by the US Department of Energy, Division of Chemical Sciences, under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Travel to Oak Ridge National Laboratory for F.D. McDaniel and J.L. Duggan supported in part by Oak Ridge Associated Universities.

[4] [5] [6] [7] [8]

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X-rayproduction in thin targets [20] R. Mehta, J.L. Duggan, F.D. McDaniel, M.C. Andrews, G. Lapicki, P.D. Miller, L.A. Raybum and A.R. Zander, Phys. Rev. A28 (1983) 2722. [21] M.C. Andrews, F.D. McDaniel, J.L. Duggan, P.D. Miller, P.L. Pepmiller, H.F. Krause, T.M. Rosseel, L.A. Raybum, R. Mehta and G. Lapicki, Phys. Rev. A36 (1987) 3699. [22] W. Lotz, Z. Physik 232 (1970) 101. [23] G. Lapicki, to be published. Excitation cross sections are calculated using the ECPSCR theory for direct ionization to target’s continuum (see ref. [6]);

0:: = 2[ 43 AE,,)

- (I:‘( A&,,)]/(

rr’%),

where 0:’ are evaluated at effective values of Ok, corresponding to the transitions into an excited S level with the minimum and maximum energy transfers as dictated by the position of this level and its width W,.