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Double-electron capture in relativistic U92+ collisions at the ESR gas-jet target
Nuclear Instruments and Methods in Physics Research B 205 (2003) 573–576 www.elsevier.com/locate/nimb
Double-electron capture in relativistic U92þ collisions at the ESR gas-jet target €hlker b, A. Warczak a, H. Beyer b, G. Bednarz a,*, D. Sierpowski a, Th. Sto F. Bosch b, A. Br€ auning-Demian b, H. Br€ auning b, X. Cai c, A. Gumberidze b, S. Hagmann b, C. Kozhuharov b, D. Liesen b, X. Ma c, P.H. Mokler b, A. Muthig b, Z. Stachura d, S. Toleikis b a b
Institute of Physics, Jagiellonian University, 30059 Krakow, Poland Gesellschaft fur Schwerionenforschung, 64291 Darmstadt, Germany c Institute of Modern Physics, 730000 Lanzhou, China d Institute of Nuclear Physics, 30059 Krakow, Poland
Abstract Total cross-sections for radiative single- and double-electron capture are measured in collisions of fast highly charged ions with light target atoms. Cross-sections for non-correlated double capture are in accordance with theoretical predictions. No significant line structure at twice the single K-radiative electron capture (K-REC) photon energy was observed. Angular distributions single K-REC photons associated with single- and double-charge exchange exhibit the same pattern. The corresponding REC lines are almost by a factor of two broader for double-charge exchange than in the single capture case. 2003 Elsevier Science B.V. All rights reserved. PACS: 34.70.+e Keywords: Double PI; Radiative electron capture; Heavy ions; Double-electron capture
1. Introduction Radiative electron capture (REC) of a single electron, in fast collisions of fully stripped high-Z ions with light target atoms, is a dominant chargeexchange process [1]. Here, the fundamental electron–photon interaction mechanisms can be
*
Corresponding author. Fax: +48-126337086. E-mail addresses: [email protected], [email protected] (G. Bednarz).
studied complementary to photoionization experiments when considering REC as time reversal of photoelectric effect. Recently, considerable efforts directed onto electron–photon interaction, went towards details of double photoionization of twoelectron systems. This phenomenon, in particular, deals with very challenging problems of atomic physics where the electron–electron interaction should be taken into account thus entering the area of correlation effects [2]. The main intention of the present experiment was to investigate in more detail processes
0168-583X/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00948-0
574
G. Bednarz et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 573–576
associated with capture of two electrons into bare and fast heavy ions. Measurements of projectile Xrays, associated with double-charge exchange, give access to the investigation of the following radiative processes:
Table 1 RDEC cross-sections [barn] Capture
Experimental value
Non-relativistic 2
RDEC
Double photoionization of the two-electron systems is a key process for the study of correlation effects in atomic multibody systems [2]. Collision experiments with bare ions provide us with the unique possibility to study this effect under time reversal conditions [4,5]. Very recent theoretical consideration of RDEC [6], within a non-relativistic approximation, gives for the corresponding cross-section a very small fraction of REC cross-section which varies between 106 (Z ¼ 18) and 109 (Z ¼ 92). At the jet target of the ESR storage ring several attempts have been performed in order to observe this process in collisions of bare uranium ions with gaseous matter [4]. So far, however, this process has not been confirmed experimentally. We were only allowed to give an upper limit for the RDEC cross-section of about 100 mb [4]. However, it was suggested [6], that in the high-Z region, due to relativistic effects, the corresponding RDEC cross-section should be strongly enhanced with respect to the non-relativistic prediction. Therefore, at high-Z, a scattering of theoretical predictions for this particular cross-section, cov-
Relativistic
6
5 10
<10
Up to 5
ering even six orders of magnitude, requires urgently an experimental verification (Table 1). During the present uranium beam time at the ESR, a new attempt has been undertaken by using a considerably improved experimental setup. The main intention of this experiment was to detect RDEC photons in coincidence with double-charge exchange, the signature of this particular process. For this purpose, projectile photon emission was observed simultaneously at various observation angles. Compared to the former experiment [4], the overall efficiency for photon registration could be enhanced by almost two orders of magnitude.
2. Experiment The experiment was performed at the heavy ion storage ring, ESR, at the GSI in Darmstadt. Bare U92þ ions at an energy of 297 MeV/u have been used in collisions with gaseous Ar-target with a density of about 5 1012 /cm2 . After passing through the target, the ions were charge state analysed in the next ESR bending magnet and collected in a movable position-sensitive multiwire proportional counter. In Fig. 1, the projectile 5
10
+ 91
92+
297 MeV/u U
4
10
counts
• double radiative electron capture – a two-step process in which two uncorrelated electrons are captured in one collision and two photons are emitted, both with the energy of single REC photons. The cross-section for this process can be calculated within the independent electron model [3]. Recently, this process has been observed in our previous experiment with bare uranium ions [4]; • radiative double-electron capture (RDEC) – a one-step process, where the energy and momentum gained by capture of two correlated electrons is converted into one photon with approximately twice the energy of a single REC photon. In analogy to REC, the RDEC can be treated as time inversion of double photoionization.
Theoretical predictions
=> Ar
3
10
2
10
90
1
10
+
0
10
0
10
20
30
40
50
60
70
80
90 100
charge state position [mm] Fig. 1. The particle detector spectrum. Both, 91+ and 90+ charge states are present. Distance between the states is about 80 mm.
G. Bednarz et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 573–576
A sample X-ray spectrum, measured at 90 in coincidence with double-charge exchange, is depicted in Fig. 2. The REC line, with the single K-REC energy, associated with double-electron capture, is clearly observed. However, within the energy region relevant for RDEC, only a few events have been detected and no X-ray line shows up. From the present state of the data analysis we can already conclude that the cross-section for this extremely rare process is below 10 mb. In this way, the upper limit for RDEC was estimated to be significantly lower than measured previously [4]. This is in contradiction to the presently available theoretical predictions [6].
140
Ly
α
120 100
single K-REC energy region
80
Ly β
RDEC energy region
60 40 20 0 0
50 100 150 200 250 300 350 400 450 500 550
keV (lab) Fig. 2. X-ray spectrum (corrected for random events and detector efficiency) measured at 90 in coincidence with doublecharge exchange. A REC line for photons the with single K-REC energy associated with double-electron capture is clearly observed.
297 MeV /u U
92+
=> Ar
differential cross section (arbitrary units)
3. Results
160
counts
charge-state distribution for U92þ ! Ar collisions is shown. The separation between the two neighbouring charge states (91+ and 90+) amounts to about 80 mm. Due to this large separation (the diameter of the ESR beam tube amounts to 250 mm) it was necessary to tune the trajectory of the primary beam out of the centre, in order to find simultaneously both the charge states of interest on the detector. As observed in Fig. 1, the rate of singly down-charged U91þ is over four orders of magnitude larger than that for doubly downcharged U90þ ions. In order to observe efficiently processes related to double capture, the particle detector was placed at the position where no singly down-charged ions could hit the detector. Furtheron, single collision conditions for double-electron capture were tested by measuring the yield of U90þ ions as function of the target density. For registration of X-ray emission, associated with double capture events, an array of germanium detectors was used, which covered observation angles in the range from almost 0 up to 150. The X-ray detectors were triggered by signals from the particle detector, which registered downcharged ions. In the present experiment we improved the efficiency of X-ray detection by about two orders of magnitude, mainly by using a new large area germanium detector with a high efficiency for high energy X-rays.
575
0
20
40
60
80 100 120 140 160 180
deg (lab) Fig. 3. Angular distribution of photons with single K-REC energy associated with double-electron capture. Solid line – full relativistic description for K-REC associated with single-electron capture normalized to the experimental data.
576
G. Bednarz et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 573–576
Simultaneously, the total cross-section for double-electron capture was measured. It amounts to 400 70 mb which is in good agreement with our previous results [4]. In addition, the angular distribution of single K-REC photons, associated with double-charge exchange, was registered. We realise that the corresponding emission pattern (Fig. 3) is similar to that for K-REC photons measured in coincidence with single-electron capture [7]. In this way, a significant improvement of the accuracy of the data presented in [4] was possible. However, the width of the K-REC line associated with capture of two electrons is by a factor of two larger than the width of the K-REC line associated with single-electron capture.
twice the energy of single K-REC photons, can be seen. A few counts collected in the energy region related to the RDEC process allow us to give the upper limit of about 8 mb for the cross-section of this rare correlated process. This is in contradiction to the presently available theoretical prediction [6].
4. Conclusions
References
The corresponding spectra associated with single- and double-electron capture were analysed. A comparison of these X-ray spectra shows several interesting features. First, the double-electron capture spectra are strongly dominated by characteristic lines of the projectile, in contrast to the single-electron capture spectra. In addition, the angular distribution of K-REC photons shows similar structure both for singleand double-capture processes. In the high-energy part of the spectra associated with capture of two electrons, no RDEC line, with
Acknowledgements This work was supported by EU, grant: ECHPRI-CT-1999-00001 and by a Marie Curie Fellowship of the European Community Programme IHP under contract number HPMT-CT-200000197.
[1] Th. Stoehlker, C. Kozhuharov, P.H. Mokler, A. Warczak, Phys. Rev. A 51 (1995) 2098. [2] J.H. McGuire, Electron Correlation Dynamics in Atomic Collisions, Cambridge University Press, 1997. [3] R. Anholt, W.E. Meyerhof, et al., Phys. Rev. A 32 (1985) 3291. [4] G. Bednarz, A. Warczak, et al., Phys. Scr. T 92 (2001) 429. [5] A. Warczak, M. Kucharski, et al., Nucl. Instr. and Meth. B 98 (1995) 303. [6] V.L. Yakhontov, M.Ya Amusia, Phys. Rev. A 55 (1997) 1952. [7] T. St€ ohlker, Th. Ludziejewski, et al., Phys. Rev. Lett. 82 (1999) 3232.
Double-electron capture in relativistic U92þ collisions at the ESR gas-jet target €hlker b, A. Warczak a, H. Beyer b, G. Bednarz a,*, D. Sierpowski a, Th. Sto F. Bosch b, A. Br€ auning-Demian b, H. Br€ auning b, X. Cai c, A. Gumberidze b, S. Hagmann b, C. Kozhuharov b, D. Liesen b, X. Ma c, P.H. Mokler b, A. Muthig b, Z. Stachura d, S. Toleikis b a b
Institute of Physics, Jagiellonian University, 30059 Krakow, Poland Gesellschaft fur Schwerionenforschung, 64291 Darmstadt, Germany c Institute of Modern Physics, 730000 Lanzhou, China d Institute of Nuclear Physics, 30059 Krakow, Poland
Abstract Total cross-sections for radiative single- and double-electron capture are measured in collisions of fast highly charged ions with light target atoms. Cross-sections for non-correlated double capture are in accordance with theoretical predictions. No significant line structure at twice the single K-radiative electron capture (K-REC) photon energy was observed. Angular distributions single K-REC photons associated with single- and double-charge exchange exhibit the same pattern. The corresponding REC lines are almost by a factor of two broader for double-charge exchange than in the single capture case. 2003 Elsevier Science B.V. All rights reserved. PACS: 34.70.+e Keywords: Double PI; Radiative electron capture; Heavy ions; Double-electron capture
1. Introduction Radiative electron capture (REC) of a single electron, in fast collisions of fully stripped high-Z ions with light target atoms, is a dominant chargeexchange process [1]. Here, the fundamental electron–photon interaction mechanisms can be
*
Corresponding author. Fax: +48-126337086. E-mail addresses: [email protected], [email protected] (G. Bednarz).
studied complementary to photoionization experiments when considering REC as time reversal of photoelectric effect. Recently, considerable efforts directed onto electron–photon interaction, went towards details of double photoionization of twoelectron systems. This phenomenon, in particular, deals with very challenging problems of atomic physics where the electron–electron interaction should be taken into account thus entering the area of correlation effects [2]. The main intention of the present experiment was to investigate in more detail processes
0168-583X/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00948-0
574
G. Bednarz et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 573–576
associated with capture of two electrons into bare and fast heavy ions. Measurements of projectile Xrays, associated with double-charge exchange, give access to the investigation of the following radiative processes:
Table 1 RDEC cross-sections [barn] Capture
Experimental value
Non-relativistic 2
RDEC
Double photoionization of the two-electron systems is a key process for the study of correlation effects in atomic multibody systems [2]. Collision experiments with bare ions provide us with the unique possibility to study this effect under time reversal conditions [4,5]. Very recent theoretical consideration of RDEC [6], within a non-relativistic approximation, gives for the corresponding cross-section a very small fraction of REC cross-section which varies between 106 (Z ¼ 18) and 109 (Z ¼ 92). At the jet target of the ESR storage ring several attempts have been performed in order to observe this process in collisions of bare uranium ions with gaseous matter [4]. So far, however, this process has not been confirmed experimentally. We were only allowed to give an upper limit for the RDEC cross-section of about 100 mb [4]. However, it was suggested [6], that in the high-Z region, due to relativistic effects, the corresponding RDEC cross-section should be strongly enhanced with respect to the non-relativistic prediction. Therefore, at high-Z, a scattering of theoretical predictions for this particular cross-section, cov-
Relativistic
6
5 10
<10
Up to 5
ering even six orders of magnitude, requires urgently an experimental verification (Table 1). During the present uranium beam time at the ESR, a new attempt has been undertaken by using a considerably improved experimental setup. The main intention of this experiment was to detect RDEC photons in coincidence with double-charge exchange, the signature of this particular process. For this purpose, projectile photon emission was observed simultaneously at various observation angles. Compared to the former experiment [4], the overall efficiency for photon registration could be enhanced by almost two orders of magnitude.
2. Experiment The experiment was performed at the heavy ion storage ring, ESR, at the GSI in Darmstadt. Bare U92þ ions at an energy of 297 MeV/u have been used in collisions with gaseous Ar-target with a density of about 5 1012 /cm2 . After passing through the target, the ions were charge state analysed in the next ESR bending magnet and collected in a movable position-sensitive multiwire proportional counter. In Fig. 1, the projectile 5
10
+ 91
92+
297 MeV/u U
4
10
counts
• double radiative electron capture – a two-step process in which two uncorrelated electrons are captured in one collision and two photons are emitted, both with the energy of single REC photons. The cross-section for this process can be calculated within the independent electron model [3]. Recently, this process has been observed in our previous experiment with bare uranium ions [4]; • radiative double-electron capture (RDEC) – a one-step process, where the energy and momentum gained by capture of two correlated electrons is converted into one photon with approximately twice the energy of a single REC photon. In analogy to REC, the RDEC can be treated as time inversion of double photoionization.
Theoretical predictions
=> Ar
3
10
2
10
90
1
10
+
0
10
0
10
20
30
40
50
60
70
80
90 100
charge state position [mm] Fig. 1. The particle detector spectrum. Both, 91+ and 90+ charge states are present. Distance between the states is about 80 mm.
G. Bednarz et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 573–576
A sample X-ray spectrum, measured at 90 in coincidence with double-charge exchange, is depicted in Fig. 2. The REC line, with the single K-REC energy, associated with double-electron capture, is clearly observed. However, within the energy region relevant for RDEC, only a few events have been detected and no X-ray line shows up. From the present state of the data analysis we can already conclude that the cross-section for this extremely rare process is below 10 mb. In this way, the upper limit for RDEC was estimated to be significantly lower than measured previously [4]. This is in contradiction to the presently available theoretical predictions [6].
140
Ly
α
120 100
single K-REC energy region
80
Ly β
RDEC energy region
60 40 20 0 0
50 100 150 200 250 300 350 400 450 500 550
keV (lab) Fig. 2. X-ray spectrum (corrected for random events and detector efficiency) measured at 90 in coincidence with doublecharge exchange. A REC line for photons the with single K-REC energy associated with double-electron capture is clearly observed.
297 MeV /u U
92+
=> Ar
differential cross section (arbitrary units)
3. Results
160
counts
charge-state distribution for U92þ ! Ar collisions is shown. The separation between the two neighbouring charge states (91+ and 90+) amounts to about 80 mm. Due to this large separation (the diameter of the ESR beam tube amounts to 250 mm) it was necessary to tune the trajectory of the primary beam out of the centre, in order to find simultaneously both the charge states of interest on the detector. As observed in Fig. 1, the rate of singly down-charged U91þ is over four orders of magnitude larger than that for doubly downcharged U90þ ions. In order to observe efficiently processes related to double capture, the particle detector was placed at the position where no singly down-charged ions could hit the detector. Furtheron, single collision conditions for double-electron capture were tested by measuring the yield of U90þ ions as function of the target density. For registration of X-ray emission, associated with double capture events, an array of germanium detectors was used, which covered observation angles in the range from almost 0 up to 150. The X-ray detectors were triggered by signals from the particle detector, which registered downcharged ions. In the present experiment we improved the efficiency of X-ray detection by about two orders of magnitude, mainly by using a new large area germanium detector with a high efficiency for high energy X-rays.
575
0
20
40
60
80 100 120 140 160 180
deg (lab) Fig. 3. Angular distribution of photons with single K-REC energy associated with double-electron capture. Solid line – full relativistic description for K-REC associated with single-electron capture normalized to the experimental data.
576
G. Bednarz et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 573–576
Simultaneously, the total cross-section for double-electron capture was measured. It amounts to 400 70 mb which is in good agreement with our previous results [4]. In addition, the angular distribution of single K-REC photons, associated with double-charge exchange, was registered. We realise that the corresponding emission pattern (Fig. 3) is similar to that for K-REC photons measured in coincidence with single-electron capture [7]. In this way, a significant improvement of the accuracy of the data presented in [4] was possible. However, the width of the K-REC line associated with capture of two electrons is by a factor of two larger than the width of the K-REC line associated with single-electron capture.
twice the energy of single K-REC photons, can be seen. A few counts collected in the energy region related to the RDEC process allow us to give the upper limit of about 8 mb for the cross-section of this rare correlated process. This is in contradiction to the presently available theoretical prediction [6].
4. Conclusions
References
The corresponding spectra associated with single- and double-electron capture were analysed. A comparison of these X-ray spectra shows several interesting features. First, the double-electron capture spectra are strongly dominated by characteristic lines of the projectile, in contrast to the single-electron capture spectra. In addition, the angular distribution of K-REC photons shows similar structure both for singleand double-capture processes. In the high-energy part of the spectra associated with capture of two electrons, no RDEC line, with
Acknowledgements This work was supported by EU, grant: ECHPRI-CT-1999-00001 and by a Marie Curie Fellowship of the European Community Programme IHP under contract number HPMT-CT-200000197.
[1] Th. Stoehlker, C. Kozhuharov, P.H. Mokler, A. Warczak, Phys. Rev. A 51 (1995) 2098. [2] J.H. McGuire, Electron Correlation Dynamics in Atomic Collisions, Cambridge University Press, 1997. [3] R. Anholt, W.E. Meyerhof, et al., Phys. Rev. A 32 (1985) 3291. [4] G. Bednarz, A. Warczak, et al., Phys. Scr. T 92 (2001) 429. [5] A. Warczak, M. Kucharski, et al., Nucl. Instr. and Meth. B 98 (1995) 303. [6] V.L. Yakhontov, M.Ya Amusia, Phys. Rev. A 55 (1997) 1952. [7] T. St€ ohlker, Th. Ludziejewski, et al., Phys. Rev. Lett. 82 (1999) 3232.