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Ag foils by electron and positron impact
Nuclear Instruments and Methods in Physics Research B68 (1992) 491-494 North-Holland
HON B
Beam Interactions with Materials&Atoms
L-shell ionization in Au/Ag foils by electron and positron impact a, Ingo Tobehn a and Rainer Hippler b Strahlenzentrum der Universität Giessen, Leihgesterner Weg 217, D-6.300 Giessen, Germany b Fakultdt für Physik, Universität Bielefeld, Universitätsstrasse 25, D-4800 Bielefeld 1, Germany
Hans Schneider
Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30-70 keV were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum in Giessen. The experimental equipment will be described shortly and the corresponding results will be presented . These results agree well with independently performed plane wave Born (PWBA) calculations accounting for electron exchange, and the deceleration or acceleration of the projectiles in the nuclear field of the target atom . 1 . Introduction The interaction of an incident projectile with a target atom may give rise to a variety of different elastic and inelastic scattering processes . Ionization of, for example, inner atomic shells is of fundamental importance for our understanding of collision dynamics [1] . For a quantitative analysis, the cross sections for impact ionization are required . In this paper we present experimental results for K- and L-shell ionization of atoms by electron and positron impact, respectively, at incident energies close to ionization threshold. We -ompare our experimental results to calculations per-
I
formed in PWBA with the Coulomb effect causing the acceleration or deceleration of the acident projectiles in the nuclear field of the target atom, and the exchange effect taken into account. This will yield a fair explanation for the measured difference of inner-shell ionization by electron and positron impact [2-4]. 2. Experimental setup The positron experiments were performed at the low-energy positron source TEPOS installed at the 65 MeV pulsed linear electron accelerator (linac) of the
AF
10
\~r ~M Wiiiiii
0IFmi\
Fig . 1 . (a) Vertical cross section of the target chamber. Target support (1), Pb collimator (2), 50 Wm thick mylar window (3), Si(Li) detector (4), 1/10 mm thick Al window (5), additional NaI(TI)-detector (6). (b) Horizontal cross section of the target chamber. Target support (1), high-voltage connection (2) . 0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved
IX. MISCELLANEOUS
492
H. Schneideret al. /L-shell ionization in AulAgfoils
Strahlenzentrum Giessen; details of this source have been described previously [5-8]. After moderation, the emerging slow positrons are accelerated to at most 2 keV and focussed onto the entrance of a solenoid transport system . At the presented experiments the positrons or electrons enter then a vacuum chamber, containing the target support with the target foil. Most of the electron measurements are carried out with an electron gun, placed in the solenoid system at a distance of about 4 m from the target chamber. The kinetic energy of the impinging positrons (or electrons) can be varied by a suitable chosen electrical potential at which the target foil is held, The target foil is hit under 45' with respect to the beam direction; X-rays emitted by the target had to pass through a 50 gm mylar window and were registered perpendicular to the incident beam by a Si(Li) detector (fig . [). The measurements were performed with different target foils. K-shell ionization [4] was investigated at TEPOS for silver and copper targets; the thickness of these foils ranged from 100 to 520 IWg/cm 2. In this case the X-rays resulting from K-shell ionization were normalized to the simultaneously recorded X-rays from the silver L-shell. Such a normalization procedure is useful as long as the cross section ratio aK laK for L-shell ionization by electron and positron impact equals unity (n- - =o.+ ); it circumvents the problems which otherwise were to arise from a normalization to the incident beam current. L-shell ionization was investigated with gold/ silver-multilayer targets; the X-rays resulting from Au L-shell ionizatio:: (fig. 2) were normalized to the simultaneously recorded X-rays fiom the silver L-shell. Such a normalization procedure is useful also here as long as the L-shell ionization crosssection is furtheron independent of the projectile energy and charge. The L ,-subshell has a comparatively small binding energy of 3.35 keV only; for Ag L-shell ionization, we
are, hence, far away from the threshold regime where both, the Coulomb (C) and the exchange effect (Ex), must be taken into account. In fact, our calculations do show that at these relatively large incident energies (8
This implies that the silver L-shell data can in fact serve he . :, as a reference to which the silver K-shell and the gold L-shell data can be normalized. L-shell ionization was investigated for the Au Lsubshells . In this case we performed the experiments with Au/Ag multilayer targets. Different 0 targets were 0 used consisting of multilayers of Au 400 A/Ag 125 A, Au 800 A/Ag 1800 A on a carbon backing and Au 1450 A/Ag 3500 A. 3. Results and discussion In fig. 3 we display, for K-shell ionization of silver, the calculated cross section ratio o-K /o-K for electron (PWBA-C-Ex) relative to positron (PWBA-C) impact. At large incident energies, the predicted ratio is close to unity, as is also expected from (uncorrected) PWBA calculations. With decreasing energy, the cross section ratio OK /O-K' obtained from uncorrected PWBA-Ex and PWBA calculations for electron and positron impact, respectively, gradually decreases [9]. This is due to electron exchange becoming more important at low velocities, yielding a smaller cross section for electron compared to positron impact. For example, at E0 /1= 2 uncorrected PWBA-Ex/PWBA calculations [9] predict aK /o.K _ 0.67. By our simple model only a shallow 2000!
3600
1500
2800
c
(b) :
1750
3200 2400
c
1250
Û 1000 750
Û 2000 1600
500
1200
250
800
0 energy [keV]
energy [keV]
Fig. 2. X-ray spectra from 45 keV (a) positron impact and (b) electron impact . The peak numbers correspond to : (1) Au M, (2) Ag L, (3) Au L 1 , (4) Au La, (5) Pb La, (6) Au Lß, (7) Pb Lß, (8) Au Ly 1 , (9) Au Ly2Ae, (10) Ag Ka, 01) Ag Kß .
H. Schneider et aL
minimum of oK /o' = 0.96 is predicted around E0/1 12 . In this regime, the PWBA-C calculation is in fair agreement with previous experimental results [10-11] and also with a recent calculation by Hock [12] . Towards the K-shell ionization threshold, a pronounced increase of the cross section ratio is predicted by the presented model. This result is in reasonable agreement with recent measurements at TEPOS by Ebel et al . [4]. For L-shell ionization, our experimental results as well as our PWBA-C-Ex calculations stress again the importance of the projectile-target nucleus interaction (Coulomb effect). The measured cross section ratio o- -L /o' , extracted from the measured ratios o(Au La)/o(Ag L) for electron and positron impact is displayed in fig. 4; the theoretical ratios were obtained from corresponding PWBA-C and PWBA-C-Ex calculations for positron and electron impact, respectively. As before for K-shell ionization, the measured ratio increases with decreasing projectile energy ; this result is in fair agreement with our PWBA-C calculation. This means that the cross section ratio o -/o+ increases with decreasing °nergy of the projectiles, because electrons are accelerated whereas positrons are slowed down in the nuclear field of the target. This effect overcompensates the exchange effect. The cross section ratio would otherwise increase with increasing incident projectile energy . This result is in agreement with measurements on the K-shell as mentioned above [4]. Somewhat surprising is the large difference to previous experimental results of Lennard et al . [13],
Il ionization in Au /Ag feils
493
b w b
energy [kev]
of the Au L-shell as a Fig. 4. Cross section ratio function of the projectile energy. (+)Present results (1450 A Au; 3500 Â Ag), (0) Present results (800 f1 Au ; 1800 A Ag), (v)Resultsfrom Lennardet al . [13], (-)PresentPBWA-C-Ex calculations. which display an opposite energy dependence. At present, the origin of this discrepancy is unknown and should deserve further investigations. Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG), Bonn-Bad Godesberg, Germany.
10o
b
4 .0
2.0 0 .0 1
0.0
I
5.0
10 .0
15 .0
20.0
Eo / I
Fig. 3. Cross section ratio o,- /or' for K-shell ionization of silver by electron and positron impact vs incident energy Eo in units of the K-shell binding energy 1. The predictions from the PWBA-C model (solid line) are compared with uncorrected PWBA calculations [9] (dashed line) and with a calculation by Hock [12] (dash-dotted line). Also shown are the experimental results of Ebel et al. [4] forAg (0)andCu (0), Itoet al . [10] (4), andSchultzet at . [11] (v). IX . MISCELLANEOUS
494
H. Schneideret al. / L-shell ionization in AulAgfoils
References [1] C.J . Powell, Rev . Mod . Phys., 48 (1976) 33; and in: Electron Impact Ionization, eds. T.D. Märk and G.H. Dunn (Springer, Berlin, 1986) chap . 6 . [2) H. Schneider, I. Tobehn and R . Hippler, Phys. Lett. A156 (1991) 303 . [3) R. Hippler, Phys. Lett. A144 (1990) 81 . [4] F. Ebel, W . Faust, C. Hahn, M . Rückert, H . Schneider, A. Singe and I . Tobehn, Phys . Lett. A140 (1989) 114 . [5] F. Ebel, W . Faust, C. Hahn, S. Langer and H. Schneider, Appl. Phyß . A44 (1987) 119. [6] F. Ebel, W . Faust, C. Hahn, S . Langer, M. Rückert, H. Schneider, A. Singe and I . Tobehn, Nucl. Instr. and Meth. A272 (1988) 626 . [7] F. Ebel, W . Faust, C. Hahn, M . Rückert, H. Schneider,
[8] [9] [10] [11] [12] [13]
A . Singe and I. Tobehn, Nucl. Instr. and Meth . B50 (1990) 328. W . Faust, C. Hahn, M. Rückert, H. Schneider, A. Singe and I. Tobehn, Nucl . Instr. and Meth. B56/57 (1991) 575 . R . Hippler and W . Jitschin, Z. Phys . A307 (1982) 287 . S. Ito, S . Shimizu, T. Kawaratani, and K . Kubota, Phys. Rev. A22 (1980) 407 . P .J . Schultz and J .L. Campbell, Phys. Lett . A112 (1985) 316. G. Hock, in: 11th. Int. Conf. on PEAC, Electronic and Atomic Collisions, Book of Abstracts, eds. K . Takanayagi and N . Oda (North-Holland, Amsterdam, 1979) p . 974. W.N . Lennard, P.J. Schultz, G .R. Massoumi and L.R . Logan, Phys. Rev. Lett. 61 (1988) 2428 .
HON B
Beam Interactions with Materials&Atoms
L-shell ionization in Au/Ag foils by electron and positron impact a, Ingo Tobehn a and Rainer Hippler b Strahlenzentrum der Universität Giessen, Leihgesterner Weg 217, D-6.300 Giessen, Germany b Fakultdt für Physik, Universität Bielefeld, Universitätsstrasse 25, D-4800 Bielefeld 1, Germany
Hans Schneider
Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30-70 keV were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum in Giessen. The experimental equipment will be described shortly and the corresponding results will be presented . These results agree well with independently performed plane wave Born (PWBA) calculations accounting for electron exchange, and the deceleration or acceleration of the projectiles in the nuclear field of the target atom . 1 . Introduction The interaction of an incident projectile with a target atom may give rise to a variety of different elastic and inelastic scattering processes . Ionization of, for example, inner atomic shells is of fundamental importance for our understanding of collision dynamics [1] . For a quantitative analysis, the cross sections for impact ionization are required . In this paper we present experimental results for K- and L-shell ionization of atoms by electron and positron impact, respectively, at incident energies close to ionization threshold. We -ompare our experimental results to calculations per-
I
formed in PWBA with the Coulomb effect causing the acceleration or deceleration of the acident projectiles in the nuclear field of the target atom, and the exchange effect taken into account. This will yield a fair explanation for the measured difference of inner-shell ionization by electron and positron impact [2-4]. 2. Experimental setup The positron experiments were performed at the low-energy positron source TEPOS installed at the 65 MeV pulsed linear electron accelerator (linac) of the
AF
10
\~r ~M Wiiiiii
0IFmi\
Fig . 1 . (a) Vertical cross section of the target chamber. Target support (1), Pb collimator (2), 50 Wm thick mylar window (3), Si(Li) detector (4), 1/10 mm thick Al window (5), additional NaI(TI)-detector (6). (b) Horizontal cross section of the target chamber. Target support (1), high-voltage connection (2) . 0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved
IX. MISCELLANEOUS
492
H. Schneideret al. /L-shell ionization in AulAgfoils
Strahlenzentrum Giessen; details of this source have been described previously [5-8]. After moderation, the emerging slow positrons are accelerated to at most 2 keV and focussed onto the entrance of a solenoid transport system . At the presented experiments the positrons or electrons enter then a vacuum chamber, containing the target support with the target foil. Most of the electron measurements are carried out with an electron gun, placed in the solenoid system at a distance of about 4 m from the target chamber. The kinetic energy of the impinging positrons (or electrons) can be varied by a suitable chosen electrical potential at which the target foil is held, The target foil is hit under 45' with respect to the beam direction; X-rays emitted by the target had to pass through a 50 gm mylar window and were registered perpendicular to the incident beam by a Si(Li) detector (fig . [). The measurements were performed with different target foils. K-shell ionization [4] was investigated at TEPOS for silver and copper targets; the thickness of these foils ranged from 100 to 520 IWg/cm 2. In this case the X-rays resulting from K-shell ionization were normalized to the simultaneously recorded X-rays from the silver L-shell. Such a normalization procedure is useful as long as the cross section ratio aK laK for L-shell ionization by electron and positron impact equals unity (n- - =o.+ ); it circumvents the problems which otherwise were to arise from a normalization to the incident beam current. L-shell ionization was investigated with gold/ silver-multilayer targets; the X-rays resulting from Au L-shell ionizatio:: (fig. 2) were normalized to the simultaneously recorded X-rays fiom the silver L-shell. Such a normalization procedure is useful also here as long as the L-shell ionization crosssection is furtheron independent of the projectile energy and charge. The L ,-subshell has a comparatively small binding energy of 3.35 keV only; for Ag L-shell ionization, we
are, hence, far away from the threshold regime where both, the Coulomb (C) and the exchange effect (Ex), must be taken into account. In fact, our calculations do show that at these relatively large incident energies (8
This implies that the silver L-shell data can in fact serve he . :, as a reference to which the silver K-shell and the gold L-shell data can be normalized. L-shell ionization was investigated for the Au Lsubshells . In this case we performed the experiments with Au/Ag multilayer targets. Different 0 targets were 0 used consisting of multilayers of Au 400 A/Ag 125 A, Au 800 A/Ag 1800 A on a carbon backing and Au 1450 A/Ag 3500 A. 3. Results and discussion In fig. 3 we display, for K-shell ionization of silver, the calculated cross section ratio o-K /o-K for electron (PWBA-C-Ex) relative to positron (PWBA-C) impact. At large incident energies, the predicted ratio is close to unity, as is also expected from (uncorrected) PWBA calculations. With decreasing energy, the cross section ratio OK /O-K' obtained from uncorrected PWBA-Ex and PWBA calculations for electron and positron impact, respectively, gradually decreases [9]. This is due to electron exchange becoming more important at low velocities, yielding a smaller cross section for electron compared to positron impact. For example, at E0 /1= 2 uncorrected PWBA-Ex/PWBA calculations [9] predict aK /o.K _ 0.67. By our simple model only a shallow 2000!
3600
1500
2800
c
(b) :
1750
3200 2400
c
1250
Û 1000 750
Û 2000 1600
500
1200
250
800
0 energy [keV]
energy [keV]
Fig. 2. X-ray spectra from 45 keV (a) positron impact and (b) electron impact . The peak numbers correspond to : (1) Au M, (2) Ag L, (3) Au L 1 , (4) Au La, (5) Pb La, (6) Au Lß, (7) Pb Lß, (8) Au Ly 1 , (9) Au Ly2Ae, (10) Ag Ka, 01) Ag Kß .
H. Schneider et aL
minimum of oK /o' = 0.96 is predicted around E0/1 12 . In this regime, the PWBA-C calculation is in fair agreement with previous experimental results [10-11] and also with a recent calculation by Hock [12] . Towards the K-shell ionization threshold, a pronounced increase of the cross section ratio is predicted by the presented model. This result is in reasonable agreement with recent measurements at TEPOS by Ebel et al . [4]. For L-shell ionization, our experimental results as well as our PWBA-C-Ex calculations stress again the importance of the projectile-target nucleus interaction (Coulomb effect). The measured cross section ratio o- -L /o' , extracted from the measured ratios o(Au La)/o(Ag L) for electron and positron impact is displayed in fig. 4; the theoretical ratios were obtained from corresponding PWBA-C and PWBA-C-Ex calculations for positron and electron impact, respectively. As before for K-shell ionization, the measured ratio increases with decreasing projectile energy ; this result is in fair agreement with our PWBA-C calculation. This means that the cross section ratio o -/o+ increases with decreasing °nergy of the projectiles, because electrons are accelerated whereas positrons are slowed down in the nuclear field of the target. This effect overcompensates the exchange effect. The cross section ratio would otherwise increase with increasing incident projectile energy . This result is in agreement with measurements on the K-shell as mentioned above [4]. Somewhat surprising is the large difference to previous experimental results of Lennard et al . [13],
Il ionization in Au /Ag feils
493
b w b
energy [kev]
of the Au L-shell as a Fig. 4. Cross section ratio function of the projectile energy. (+)Present results (1450 A Au; 3500 Â Ag), (0) Present results (800 f1 Au ; 1800 A Ag), (v)Resultsfrom Lennardet al . [13], (-)PresentPBWA-C-Ex calculations. which display an opposite energy dependence. At present, the origin of this discrepancy is unknown and should deserve further investigations. Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG), Bonn-Bad Godesberg, Germany.
10o
b
4 .0
2.0 0 .0 1
0.0
I
5.0
10 .0
15 .0
20.0
Eo / I
Fig. 3. Cross section ratio o,- /or' for K-shell ionization of silver by electron and positron impact vs incident energy Eo in units of the K-shell binding energy 1. The predictions from the PWBA-C model (solid line) are compared with uncorrected PWBA calculations [9] (dashed line) and with a calculation by Hock [12] (dash-dotted line). Also shown are the experimental results of Ebel et al. [4] forAg (0)andCu (0), Itoet al . [10] (4), andSchultzet at . [11] (v). IX . MISCELLANEOUS
494
H. Schneideret al. / L-shell ionization in AulAgfoils
References [1] C.J . Powell, Rev . Mod . Phys., 48 (1976) 33; and in: Electron Impact Ionization, eds. T.D. Märk and G.H. Dunn (Springer, Berlin, 1986) chap . 6 . [2) H. Schneider, I. Tobehn and R . Hippler, Phys. Lett. A156 (1991) 303 . [3) R. Hippler, Phys. Lett. A144 (1990) 81 . [4] F. Ebel, W . Faust, C. Hahn, M . Rückert, H . Schneider, A. Singe and I . Tobehn, Phys . Lett. A140 (1989) 114 . [5] F. Ebel, W . Faust, C. Hahn, S. Langer and H. Schneider, Appl. Phyß . A44 (1987) 119. [6] F. Ebel, W . Faust, C. Hahn, S . Langer, M. Rückert, H. Schneider, A. Singe and I . Tobehn, Nucl. Instr. and Meth. A272 (1988) 626 . [7] F. Ebel, W . Faust, C. Hahn, M . Rückert, H. Schneider,
[8] [9] [10] [11] [12] [13]
A . Singe and I. Tobehn, Nucl. Instr. and Meth . B50 (1990) 328. W . Faust, C. Hahn, M. Rückert, H. Schneider, A. Singe and I. Tobehn, Nucl . Instr. and Meth. B56/57 (1991) 575 . R . Hippler and W . Jitschin, Z. Phys . A307 (1982) 287 . S. Ito, S . Shimizu, T. Kawaratani, and K . Kubota, Phys. Rev. A22 (1980) 407 . P .J . Schultz and J .L. Campbell, Phys. Lett . A112 (1985) 316. G. Hock, in: 11th. Int. Conf. on PEAC, Electronic and Atomic Collisions, Book of Abstracts, eds. K . Takanayagi and N . Oda (North-Holland, Amsterdam, 1979) p . 974. W.N . Lennard, P.J. Schultz, G .R. Massoumi and L.R . Logan, Phys. Rev. Lett. 61 (1988) 2428 .