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L X-ray emission from fast highly charged Cu ions in collisions with gaseous targets: Saturation effect in excitation and transfer
ARTICLE IN PRESS
Radiation Physics and Chemistry 75 (2006) 1704–1706 www.elsevier.com/locate/radphyschem
L X-ray emission from fast highly charged Cu ions in collisions with gaseous targets: Saturation effect in excitation and transfer Ajay Kumara, D. Misraa, U. Kadhanea, A.H. Kelkara, B.B. Dhalb, L.C. Tribedia, a
Tata Institute of Fundamental Research, Colaba, Mumbai-400 005, India b School of Physics, University of Melbourne, Victoria 3010, Australia Accepted 27 July 2005
Abstract We have measured L X-ray production cross sections for highly charged 156 MeV Be-like Cu ions in collisions with gaseous targets of H2, Ne, Ar, Kr and Xe. In the present collision systems, measured projectile L X-ray intensity is contributed by the excitation as well as electron transfer processes. The projectile L X-ray production cross sections are found to increase initially and then saturate with increasing target atomic number. The charge state dependence of projectile L X-ray production cross sections have been measured with Kr target. r 2006 Elsevier Ltd. All rights reserved. Keywords: Highly charged ions; X-rays; Excitation; Electron transfer; Saturation effect
1. Introduction The X-ray emission studies in ion–atom collisions provide a testing ground of the various models which are used to explain the observed X-ray emission from comets or in other astrophysical objects (Lisse et al., 1996). The ionization, excitation and charge transfer are the main processes in the inelastic ion–atom collision and their magnitude of the cross sections depends upon the velocity combination of projectile and the active target electron. Only a few measurements have been done on the K shell excitation cross sections for He-like ions with selected ion–atom combinations at limited energies (Tiwari et al., 1998). The projectile energy dependence for the K shell excitation cross section of Corresponding author. Tel.: +91 22 22804545x2465; fax: +91 22 2280 4610/4611. E-mail address: [email protected] (L.C. Tribedi).
He-like Si and S ions with velocities(vp) between 8 and 12.5 au in collisions with several gaseous targets have been studied by Tiwari et al. (1998). An accurate measurement of the excitation cross sections of velocity 23 au Ar16+ ions and multiple processes, which contribute in the excitation, have been separately identified using high-resolution crystal spectrometer by Vernhet et al. (1997). Interesting feature of all these measurements is the observed saturation in the excitation cross sections as the target atomic number (Zt) increases. The first-Born approximation predicted Zt2 dependence and failed to explain the observed saturation effect in the excitation cross section. The values based on Schwinger variational principle (SVP) were found in good agreement with the experimental data in the low as well as high-energy regime. The continuum distorted wave (CDW) calculations work better only at high energy and breaks down at relatively low energy (Tiwari et al., 1998). Same saturation has also been observed in the
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.07.029
ARTICLE IN PRESS A. Kumar et al. / Radiation Physics and Chemistry 75 (2006) 1704–1706
projectile electron loss cross sections and capture cross sections as target atomic number increases (Melo et al., 1999; Sigaud et al., 1997). In the present work, we have measured the L X-ray production cross sections for 156 MeV Be-like Cu (Cu25+) ions colliding with gaseous targets of H2, Ne, Ar, Kr and Xe. The projectile L X-rays have been measured by varying the number of projectile L shell vacancy (1–7) in collisions with Kr target. The observed projectile L X-rays intensity is contributed by excitation and electron transfer processes. 2. Experimental details The 156 MeV C12+ ion beam was obtained from the BARC-TIFR pelletron accelerator facility at Mumbai, India. A carbon post stripper (10–30 mg/cm2) poststripper has been used to get the higher charge states of the projectile. Gaseous targets of H2, Ne, Ar, Kr and Xe have been used in the present work. Cu25+ on Ne 400
n =3-2
1705
The well-collimated ion beam was made to interact at right angle with an effusive jet (mounted on the top flange of the scattering chamber) of the target gas emanating from a capillary. The gas pressure in the capillary was continuously monitored; and controlled using a capacitance manometer and a solenoid valve. Target pressure was kept quite low such that single collision condition was satisfied. The X-rays were detected using a Si(Li) detector (FWHM 160 eV at 5.89 keV). The Si(Li) detector was kept outside the chamber and the chamber pressure during the measurement was 106 mbar. Typical L X-ray spectrum of 156 MeV Cu25+ ions with Ne and Xe targets are shown in Fig. 1(a) and (b), respectively; and of 156 Cu21+ with Kr in Fig. 1(c). The X-ray peak due to n ¼ 3 ! 2 transition is well separated and n ¼ 4 ! 2 transition is slightly resolved. Rest bunch of peaks at high-energy side is resulted due to n ¼ 44 ! 2 transitions (Fig. 1(a) and (b)). The L X-ray production cross sections (in cm2/ atom) were deduced by measuring the X-ray counts under the peak corrected by the number of target atoms, number of interacting projectile ions, solid angle subtended by the detector and detector efficiency for the measured X-ray energy.
n =4-2
3. Result and discussion 200 n =>4-2 (a)
Counts
Cu25+ on Xe 400
n =3-2
n =4-2
200
The measured total L X-ray production cross sections (sx ) of 156 MeV Cu25+ ions as a function of target atomic number are presented in Fig. 2. The error in the measured cross sections is 24%. It is interesting to note that the L X-ray production cross sections saturate as target atomic number (Zt) increases. The first Born approximation predicts Zt2 behavior and is far from the reality (Fig. 2). The L shell contains three subshells of
n =>4-2 (b)
Expt. 103
2
Zt
Cu21+ on Kr σx (Mb)
300
n =3-2
200
n =>3-2
102
101
100 (c)
100 1.0
1.5 X-ray energy (keV)
2.0 0 q+
Fig. 1. Typical L X-ray spectrum of 156 MeV Cu ions in collisions with (a) Ne, (b) Xe and (c) Kr targets. The ordinate represents counts per channel and one channel corresponds to 10 eV.
10
20 30 40 Target atomic number
50
60
Fig. 2. Measured total L X-ray production cross sections (sx ) of 156 MeV/u Cu25+ ions vs. target atomic number. Solid line is Zt2 dependence normalized at Zt ¼ 1.
ARTICLE IN PRESS A. Kumar et al. / Radiation Physics and Chemistry 75 (2006) 1704–1706
1706
projectile L shell vacancy increases. It is because of the captured—and excited—electrons get more room for deexcitation as L shell vacancy increases. In the present case, each of L shell vacancy contributes 22 Mb in the L X-ray production cross sections. The sxn¼3!2 =sxtotal and sxn¼43!2 =sxtotal is 0.8 and 0.2, respectively, and independent of charge states.
140 120
σx (Mb)
100 80 60 40
4. Conclusion
20
The L X-ray production cross sections have been measured for 156 MeV Cu25+ ions with H2, Ne, Ar, Kr and Xe targets. The measured L X-ray production, contributed by excitation and electron transfer, saturate as target atomic number increases. The charge state dependence of projectile L X-ray production cross sections have been measured with Kr target.
20
21
22 23 24 Projectile charge state
25
26
Fig. 3. L X-ray production cross sections (sx ) for 156 MeV Cuq+ ions in collisions with Kr as a function of charge state.
different binding energies and two electrons are present in each of K and L1 subshell of incoming Cu25+ ions. The observed L X-rays originate due to: (a) excitation of L1 electrons to higher states (n ¼ X3) and subsequent de-excitation, and (b) following electron capture to n ¼ X3 and subsequent de-excitation. There are other second- and higher-order processes such as captureionisation, excitation-ionisation, double excitation, etc. Which (Vernhet et al., 1997) can give rise to the production of L X-rays. The extraction of single electron excitation cross sections from the observed L X-ray yield needs the correction due to the capture-ionization (CI), the fraction of metastable state and cascade processes. These corrections are not easy to make in the present collision system. Therefore, we have not attempted to compare our result with the theoretical values of SVP and CDW calculations for single electron excitation cross section. The total L X-ray production cross sections of projectile have been measured by changing the L shell vacancy (1–7) with Kr target. The L X-ray peak position increases towards higher-energy side as projectile charge state increases. This systematic dependence is due to the increased effective nuclear charge of the incident ion. As the projectile charge state increases peaks become sharper (FWHM decreases). For the charge states 20 and 21, all the peaks are not well distinguishable (see Fig. 1(c)). As seen in Fig. 3, the measured L X-ray production cross section shows linear enhancement as
Acknowledgment Authors are thankful to the pelletron accelerator staff at TIFR, Mumbai for their skillful operation of the machine.
References Lisse, C.M., Dennerl, K., Englhauser, J., Harden, M., Marshall, F.E., Mumma, M.J., Petre, R., Pye, J.P., Ricketts, M.J., Schmitt, J., Tru¨mper, J., West, R.G., 1996. Discovery of X-ray and extreme ultraviolet emission from comet C/Hyakutake 1996 B2. Science 274, 205–209. Melo, W.S., Sant’Anna, M.M., Santos, A.C.F., Sigaud, G.M., Montenegro, E.C., 1999. Electron loss and single and double capture of C3+ and O5+ ions in collisions with noble gases. Phys. Rev. A 60, 1124–1134. Sigaud, G.M., Joras, F.S., Santos, A.C.F., Montenegro, E.C., Sant’Anna, M.M., Melo, W.S., 1997. Saturation effects in projectile electron loss. Nucl. Instrum. Methods B 132, 312–315. Tiwari, U., Saha, A.K., Tribedi, L.C., Kurup, M.B., Tandon, P.N., Gulyas, L., 1998. Saturation effect in the excitation of heliumlike Si projectiles in the intermediate velocity range. Phys. Rev. A 58, 4494–4500 and references therein. Vernhet, D., Adoui., L., Rozet, J.P., Wohrer, K., Chetioui, A., Cassimi, A., Grandin, J.P., Ramillion, J.M., Cornille, M., Stephan, C., 1997. Multielectron processes in heavy ion–atom collisions at intermediate velocity. Phys. Rev. Lett. 79, 3625–3628.
Radiation Physics and Chemistry 75 (2006) 1704–1706 www.elsevier.com/locate/radphyschem
L X-ray emission from fast highly charged Cu ions in collisions with gaseous targets: Saturation effect in excitation and transfer Ajay Kumara, D. Misraa, U. Kadhanea, A.H. Kelkara, B.B. Dhalb, L.C. Tribedia, a
Tata Institute of Fundamental Research, Colaba, Mumbai-400 005, India b School of Physics, University of Melbourne, Victoria 3010, Australia Accepted 27 July 2005
Abstract We have measured L X-ray production cross sections for highly charged 156 MeV Be-like Cu ions in collisions with gaseous targets of H2, Ne, Ar, Kr and Xe. In the present collision systems, measured projectile L X-ray intensity is contributed by the excitation as well as electron transfer processes. The projectile L X-ray production cross sections are found to increase initially and then saturate with increasing target atomic number. The charge state dependence of projectile L X-ray production cross sections have been measured with Kr target. r 2006 Elsevier Ltd. All rights reserved. Keywords: Highly charged ions; X-rays; Excitation; Electron transfer; Saturation effect
1. Introduction The X-ray emission studies in ion–atom collisions provide a testing ground of the various models which are used to explain the observed X-ray emission from comets or in other astrophysical objects (Lisse et al., 1996). The ionization, excitation and charge transfer are the main processes in the inelastic ion–atom collision and their magnitude of the cross sections depends upon the velocity combination of projectile and the active target electron. Only a few measurements have been done on the K shell excitation cross sections for He-like ions with selected ion–atom combinations at limited energies (Tiwari et al., 1998). The projectile energy dependence for the K shell excitation cross section of Corresponding author. Tel.: +91 22 22804545x2465; fax: +91 22 2280 4610/4611. E-mail address: [email protected] (L.C. Tribedi).
He-like Si and S ions with velocities(vp) between 8 and 12.5 au in collisions with several gaseous targets have been studied by Tiwari et al. (1998). An accurate measurement of the excitation cross sections of velocity 23 au Ar16+ ions and multiple processes, which contribute in the excitation, have been separately identified using high-resolution crystal spectrometer by Vernhet et al. (1997). Interesting feature of all these measurements is the observed saturation in the excitation cross sections as the target atomic number (Zt) increases. The first-Born approximation predicted Zt2 dependence and failed to explain the observed saturation effect in the excitation cross section. The values based on Schwinger variational principle (SVP) were found in good agreement with the experimental data in the low as well as high-energy regime. The continuum distorted wave (CDW) calculations work better only at high energy and breaks down at relatively low energy (Tiwari et al., 1998). Same saturation has also been observed in the
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.07.029
ARTICLE IN PRESS A. Kumar et al. / Radiation Physics and Chemistry 75 (2006) 1704–1706
projectile electron loss cross sections and capture cross sections as target atomic number increases (Melo et al., 1999; Sigaud et al., 1997). In the present work, we have measured the L X-ray production cross sections for 156 MeV Be-like Cu (Cu25+) ions colliding with gaseous targets of H2, Ne, Ar, Kr and Xe. The projectile L X-rays have been measured by varying the number of projectile L shell vacancy (1–7) in collisions with Kr target. The observed projectile L X-rays intensity is contributed by excitation and electron transfer processes. 2. Experimental details The 156 MeV C12+ ion beam was obtained from the BARC-TIFR pelletron accelerator facility at Mumbai, India. A carbon post stripper (10–30 mg/cm2) poststripper has been used to get the higher charge states of the projectile. Gaseous targets of H2, Ne, Ar, Kr and Xe have been used in the present work. Cu25+ on Ne 400
n =3-2
1705
The well-collimated ion beam was made to interact at right angle with an effusive jet (mounted on the top flange of the scattering chamber) of the target gas emanating from a capillary. The gas pressure in the capillary was continuously monitored; and controlled using a capacitance manometer and a solenoid valve. Target pressure was kept quite low such that single collision condition was satisfied. The X-rays were detected using a Si(Li) detector (FWHM 160 eV at 5.89 keV). The Si(Li) detector was kept outside the chamber and the chamber pressure during the measurement was 106 mbar. Typical L X-ray spectrum of 156 MeV Cu25+ ions with Ne and Xe targets are shown in Fig. 1(a) and (b), respectively; and of 156 Cu21+ with Kr in Fig. 1(c). The X-ray peak due to n ¼ 3 ! 2 transition is well separated and n ¼ 4 ! 2 transition is slightly resolved. Rest bunch of peaks at high-energy side is resulted due to n ¼ 44 ! 2 transitions (Fig. 1(a) and (b)). The L X-ray production cross sections (in cm2/ atom) were deduced by measuring the X-ray counts under the peak corrected by the number of target atoms, number of interacting projectile ions, solid angle subtended by the detector and detector efficiency for the measured X-ray energy.
n =4-2
3. Result and discussion 200 n =>4-2 (a)
Counts
Cu25+ on Xe 400
n =3-2
n =4-2
200
The measured total L X-ray production cross sections (sx ) of 156 MeV Cu25+ ions as a function of target atomic number are presented in Fig. 2. The error in the measured cross sections is 24%. It is interesting to note that the L X-ray production cross sections saturate as target atomic number (Zt) increases. The first Born approximation predicts Zt2 behavior and is far from the reality (Fig. 2). The L shell contains three subshells of
n =>4-2 (b)
Expt. 103
2
Zt
Cu21+ on Kr σx (Mb)
300
n =3-2
200
n =>3-2
102
101
100 (c)
100 1.0
1.5 X-ray energy (keV)
2.0 0 q+
Fig. 1. Typical L X-ray spectrum of 156 MeV Cu ions in collisions with (a) Ne, (b) Xe and (c) Kr targets. The ordinate represents counts per channel and one channel corresponds to 10 eV.
10
20 30 40 Target atomic number
50
60
Fig. 2. Measured total L X-ray production cross sections (sx ) of 156 MeV/u Cu25+ ions vs. target atomic number. Solid line is Zt2 dependence normalized at Zt ¼ 1.
ARTICLE IN PRESS A. Kumar et al. / Radiation Physics and Chemistry 75 (2006) 1704–1706
1706
projectile L shell vacancy increases. It is because of the captured—and excited—electrons get more room for deexcitation as L shell vacancy increases. In the present case, each of L shell vacancy contributes 22 Mb in the L X-ray production cross sections. The sxn¼3!2 =sxtotal and sxn¼43!2 =sxtotal is 0.8 and 0.2, respectively, and independent of charge states.
140 120
σx (Mb)
100 80 60 40
4. Conclusion
20
The L X-ray production cross sections have been measured for 156 MeV Cu25+ ions with H2, Ne, Ar, Kr and Xe targets. The measured L X-ray production, contributed by excitation and electron transfer, saturate as target atomic number increases. The charge state dependence of projectile L X-ray production cross sections have been measured with Kr target.
20
21
22 23 24 Projectile charge state
25
26
Fig. 3. L X-ray production cross sections (sx ) for 156 MeV Cuq+ ions in collisions with Kr as a function of charge state.
different binding energies and two electrons are present in each of K and L1 subshell of incoming Cu25+ ions. The observed L X-rays originate due to: (a) excitation of L1 electrons to higher states (n ¼ X3) and subsequent de-excitation, and (b) following electron capture to n ¼ X3 and subsequent de-excitation. There are other second- and higher-order processes such as captureionisation, excitation-ionisation, double excitation, etc. Which (Vernhet et al., 1997) can give rise to the production of L X-rays. The extraction of single electron excitation cross sections from the observed L X-ray yield needs the correction due to the capture-ionization (CI), the fraction of metastable state and cascade processes. These corrections are not easy to make in the present collision system. Therefore, we have not attempted to compare our result with the theoretical values of SVP and CDW calculations for single electron excitation cross section. The total L X-ray production cross sections of projectile have been measured by changing the L shell vacancy (1–7) with Kr target. The L X-ray peak position increases towards higher-energy side as projectile charge state increases. This systematic dependence is due to the increased effective nuclear charge of the incident ion. As the projectile charge state increases peaks become sharper (FWHM decreases). For the charge states 20 and 21, all the peaks are not well distinguishable (see Fig. 1(c)). As seen in Fig. 3, the measured L X-ray production cross section shows linear enhancement as
Acknowledgment Authors are thankful to the pelletron accelerator staff at TIFR, Mumbai for their skillful operation of the machine.
References Lisse, C.M., Dennerl, K., Englhauser, J., Harden, M., Marshall, F.E., Mumma, M.J., Petre, R., Pye, J.P., Ricketts, M.J., Schmitt, J., Tru¨mper, J., West, R.G., 1996. Discovery of X-ray and extreme ultraviolet emission from comet C/Hyakutake 1996 B2. Science 274, 205–209. Melo, W.S., Sant’Anna, M.M., Santos, A.C.F., Sigaud, G.M., Montenegro, E.C., 1999. Electron loss and single and double capture of C3+ and O5+ ions in collisions with noble gases. Phys. Rev. A 60, 1124–1134. Sigaud, G.M., Joras, F.S., Santos, A.C.F., Montenegro, E.C., Sant’Anna, M.M., Melo, W.S., 1997. Saturation effects in projectile electron loss. Nucl. Instrum. Methods B 132, 312–315. Tiwari, U., Saha, A.K., Tribedi, L.C., Kurup, M.B., Tandon, P.N., Gulyas, L., 1998. Saturation effect in the excitation of heliumlike Si projectiles in the intermediate velocity range. Phys. Rev. A 58, 4494–4500 and references therein. Vernhet, D., Adoui., L., Rozet, J.P., Wohrer, K., Chetioui, A., Cassimi, A., Grandin, J.P., Ramillion, J.M., Cornille, M., Stephan, C., 1997. Multielectron processes in heavy ion–atom collisions at intermediate velocity. Phys. Rev. Lett. 79, 3625–3628.