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Differential ionization of Ar by positron and electron impact
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 261 (2007) 892–895 www.elsevier.com/locate/nimb
Differential ionization of Ar by positron and electron impact O.G. de Lucio *, R.D. DuBois, J. Gavin Department of Physics, University of Missouri-Rolla, Rolla, MO 65409, USA Available online 28 March 2007
Abstract Coincidences between recoil ions–ejected electrons, and recoil ions–scattered projectiles were used to study the kinematics of electron and positron impact ionization. Differential studies, recently performed at the University of Missouri-Rolla, for 500 eV electron and positron impact on Ar are presented. The positron and electron beams are produced by means of a 22Na source and an electron gun respectively. The target is a simple gas jet. An electrostatic energy analyzer and a position sensitive detector measure the post-collision projectile energies and scattering angles. A channeltron counts recoil ions which are extracted by means of an electric field. In the opposite direction, a channelplate detector records low energy electrons emerging from the collision volume. Ó 2007 Elsevier B.V. All rights reserved. PACS: 34.85.+x; 34.80. i Keywords: Electron; Positron; Ionization
1. Introduction Studies of the inelastic atomic processes have occupied an important place since the early days of atomic physics, and yet some fundamental questions remain open to discussion. Of particular interest is the description of the time evolution of the Coulomb forces involved in many-body interactions. This kind of process can become extremely difficult for processes in which multiple ionization is concerned, where simple physical magnitudes such as the mass or the charge of the projectile play an important role. It is known that at high impact velocities first order theories such as the Born approximation predict a dependence of the ionization cross section that goes as the square of the charge of the projectile. Consequently for single ionization by positrons and electrons, the cross sections are expected to be the same. For double ionization, experiments point towards significant differences [1] between ionization due to electron impact and positron impact. Such differences have been described in terms of different processes like *
Corresponding author. Tel.: +1 573 341 6791; fax: +1 573 341 4715. E-mail address: [email protected] (O.G. de Lucio).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.03.026
shake-off or two step ionization, momentum transfer to the target, changes of the projectile trajectory, inner-shell contributions or post-collision interactions. Mainly because of the many bodies interacting through the long-range Coulomb force it is still uncertain how these processes or their magnitudes affect the dynamics of the ionizing interaction. This requires measurements as a function of each one of the variables involved, in order to test the theoretical predictions. In this context, differential information represents a better and more accurate tool. In this work we present differential data for ionization of argon induced by 500 eV positron and electron impact. This continues with the work that the group has been developing during the last years, adding some improvements to the experimental apparatus as well as to the analysis techniques employed previously. 2. Experimental The positron beam is produced by means of a 22Na radioactive source, by extraction and secondary emission from a tungsten moderator located right in front of the source. The beam is brought into the interaction region
O.G. de Lucio et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 892–895
by means of an electrostatic transport system consisting of an array of lenses and deflectors. A detailed description of this experimental setup can be found in [2–6]. Once the beam is in the interaction region it collides with an Ar target, which is a simple gas jet. After the beam interacts with the target it will induce some ionization, the recoil ions and ejected electrons are extracted by means of an uniform electric field which is created by applying a ±2.5 V on the metallic plates in front of those two detectors (see Fig. 1). Before the ions and ejected electrons reach their detectors they must go through a couple of grids which are used to produce a ‘‘field free’’ region. Under this configuration and because of the geometry of the system, it is possible to detect electrons ejected in a cone normal to the beam direction. Some improvements have been made in this region with respect to the previous works. First the recoil detector was changed from a channelplate position sensitive detector (PSD) to a channeltron, and also a channelplate has been added to detect the ejected electrons. Projectile–emitted electron coincidences give the option to distinguish between binary and recoil interactions. After the beam exits the impact region it goes through a vertical slit, which is used to define a scattering plane, and then into an electrostatic energy spectrometer, which focuses projectiles with a particular energy loss without affecting the scattering angle. After this process the beam goes through a grid and then into a projectile PSD. The PSD allows measurements of the energy loss (horizontal axis) and scattering angle (vertical axis) of the projectiles by their position on the detector. In this way two dimensional spectra can be generated by time differences between signals on the PSD anodes. Data acquisition is done by means of a TDC and a computer. Scattered projectiles and ejected electrons are mea-
893
sured in coincidence with recoil ions so it is possible to distinguish between different degrees of ionization by analyzing their time of flight. This supplies information about the kinematics for single and multiple ionization processes.
3. Analysis By imposing conditions on the raw data it is possible to present the information in different fashions. For example, one can extract information on the projectiles and ejected electrons which correspond only to certain degree of ionization, preserving information about the energy loss and scattering angle. At present some information about the electron emission direction is also available. The degrees of ionization are defined by setting windows around the peaks in the TOF spectra. Then 2D spectra are generated for the projectile and ejected electrons PSD. Since these spectra contain backgrounds due to random interactions it is necessary to generate a background spectrum and then subtract this from the spectrum being analyzed. Because the beam is not a point but a distribution it was necessary to acquire the beam energy profile in order to use later in the analysis. This implies that a position on the projectile detector corresponds to an energy/scattering angle distribution weighted by the beam energy profile. In order to extract differential information as a function of the projectile energy loss and scattering angle, the following procedure was employed: first the 2D background subtracted ejected electron spectrum was generated for single ionization. Next, by generating windows in this spectrum, the forward and backward scattering directions are defined. Then, by relating this set of conditions with the projectile single ionization information it was possible to
Projectile PSD
Electron PSD
Beam Extraction region
Aperture
Recoil Detector
Spectrometer
Fig. 1. Experimental apparatus. A two dimensional projectile spectrum is shown on the right side. The beam position is indicated by the solid circle.
894
O.G. de Lucio et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 892–895
identify projectile energy loss and scattering angle associated with the binary or recoil lobes. For our geometry, the projectiles that scatter down with respect to the original beam direction, generate a binary lobe that is directed towards the electron detector and a recoil lobe that is directed away. For projectiles that scatter up, the directions of the binary and recoil lobes are away and toward the electron detector, respectively. From simulations of the extraction region employing the SIMION computer program [7] it was pointed out that although the recoil lobe is directed away from the electron PSD, some of the ejected electrons belonging to that lobe could still be recorded if their energy was low enough such that the recoil extraction field is able to turn them around. This also happens for low energy binary electrons. If this happens the data exhibits ‘‘false’’ events coming from the turned around electrons. The projectile spectra generated was used to extract information on the intensities of both the binary and recoil lobes as a function of the projectile scattering angle for a range of energy losses. In order to make comparisons between electron and positron impact, the data were normalized to the total number of events associated with single ionization in each case. 4. Results Figs. 2(a) and (b) illustrate to the projectile spectra associated with binary and recoil events respectively for electron impact. These are obtained from coincidences with electrons emitted in the forward directions and backward direction, respectively. As can be seen, projectile scattering associated with binary interactions has more intensity when the scattering is small; as the scattering increases the intensity decreases uniformly, following a ridge. It can be noticed also that there is a low intensity structure above the main distribution. This is associated with low energy recoil electrons, as discussed above. Projectile scattering associated with recoil events exhibit a structure that has also more intensity on the small scattering region, but in this case the intensity decreases suddenly. The structure below zero scattering angle corresponds to events coming from turned around binary electrons. The distributions in Fig. 2 are examined more closely by their relative intensities for the binary and recoil structures, as displayed in Figs. 3(a) and (b) respectively. Only real events are shown. Angles are measured with respect to zero scattering as defined by the position of the beam on the projectile detector. Based upon our normalization, the binary intensity is larger for electron impact, but it decreases faster with scattering angle, while for the positron binary intensity remains constant for small scattering angles, and then starts to fall rapidly. For electron impact, binary intensity decreases monotonically. Because of this, the relative binary intensity for positron impact appears to be enhanced for larger scat-
Fig. 2. Electron impact two dimensional spectrum for projectiles in coincidence with ejected electrons associated with the (a) binary lobe and (b) recoil lobe. The beam position is shown as a circle. The regions marked as ‘‘false’’ events correspond to projectiles that are associated with turned around electrons. The same scale is used in both plots for positions and intensities. The actual data has been smoothed for display purposes.
tering angles. In contrast, within uncertainties the recoil lobes appear to be identical in shape, although electron impact generates more intensity. 5. Summary Results for differential measurements of ionization induced by positron and electron impact have been presented. Although it is necessary to deconvolute all the experimental parameters in order to quantify how they affect the raw data, noticeable differences associated with projectile charge are observed. Additional improvements are in progress and future studies are planned to study projectile charge effects and
O.G. de Lucio et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 892–895
Recoil
Binary -2
10
-3
Relative Intensity
10
895
Positron Electron -8
-6
-4
-2
0
0
2
4
6
8
Projectile Scattering Angle (deg) Fig. 3. Relative intensity for projectile scattering resulting from (a) binary and (b) recoil events for both positron and electron impact. Data has been normalized in each case with respect to the total single ionization. Each point corresponds to an integral for a range of scattering angles and energy losses.
how it influences the different ionization mechanisms. Use of the experimental method employed here can also be extended to multiple electron transitions, e.g. kinematic analysis of double and triple ionization. Acknowledgement This work was supported by the National Science Foundation. References [1] H. Bluhme, H. Knudsen, J.P. Merrison, K.A. Nielsen, J. Phys. B 32 (1999) 5835.
[2] R.D. DuBois, C. Doudna, C. Lloyd, M. Kahveci, Kh. Khayyat, Y. Zhou, D.H. Madison, J. Phys. B 34 (2001) L783. [3] R.D. DuBois, Kh. Khayyat, C. Doudna, C. Lloyd, Nucl. Instr. and Meth. B 192 (2002) 63. [4] A.C.F. Santos, A. Hasan, T. Yates, R.D. DuBois, Phys. Rev. A 67 (2003) 052708. [5] A.C.F. Santos, A. Hasan, R.D. DuBois, Phys. Rev. A 69 (2004) 032706. [6] R.D. DuBois, A.C.F. Santos, M.A. Thomason, J. Gavin, Nucl. Instr. and Meth. B 241 (2005) 19. [7] D.A. Dahl, ‘SIMION 3D Version 7.0 User’s Manual’, Report INEEL95/0403, Idaho National Engineering and Environmental Laboratory, Idaho (2000).
Nuclear Instruments and Methods in Physics Research B 261 (2007) 892–895 www.elsevier.com/locate/nimb
Differential ionization of Ar by positron and electron impact O.G. de Lucio *, R.D. DuBois, J. Gavin Department of Physics, University of Missouri-Rolla, Rolla, MO 65409, USA Available online 28 March 2007
Abstract Coincidences between recoil ions–ejected electrons, and recoil ions–scattered projectiles were used to study the kinematics of electron and positron impact ionization. Differential studies, recently performed at the University of Missouri-Rolla, for 500 eV electron and positron impact on Ar are presented. The positron and electron beams are produced by means of a 22Na source and an electron gun respectively. The target is a simple gas jet. An electrostatic energy analyzer and a position sensitive detector measure the post-collision projectile energies and scattering angles. A channeltron counts recoil ions which are extracted by means of an electric field. In the opposite direction, a channelplate detector records low energy electrons emerging from the collision volume. Ó 2007 Elsevier B.V. All rights reserved. PACS: 34.85.+x; 34.80. i Keywords: Electron; Positron; Ionization
1. Introduction Studies of the inelastic atomic processes have occupied an important place since the early days of atomic physics, and yet some fundamental questions remain open to discussion. Of particular interest is the description of the time evolution of the Coulomb forces involved in many-body interactions. This kind of process can become extremely difficult for processes in which multiple ionization is concerned, where simple physical magnitudes such as the mass or the charge of the projectile play an important role. It is known that at high impact velocities first order theories such as the Born approximation predict a dependence of the ionization cross section that goes as the square of the charge of the projectile. Consequently for single ionization by positrons and electrons, the cross sections are expected to be the same. For double ionization, experiments point towards significant differences [1] between ionization due to electron impact and positron impact. Such differences have been described in terms of different processes like *
Corresponding author. Tel.: +1 573 341 6791; fax: +1 573 341 4715. E-mail address: [email protected] (O.G. de Lucio).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.03.026
shake-off or two step ionization, momentum transfer to the target, changes of the projectile trajectory, inner-shell contributions or post-collision interactions. Mainly because of the many bodies interacting through the long-range Coulomb force it is still uncertain how these processes or their magnitudes affect the dynamics of the ionizing interaction. This requires measurements as a function of each one of the variables involved, in order to test the theoretical predictions. In this context, differential information represents a better and more accurate tool. In this work we present differential data for ionization of argon induced by 500 eV positron and electron impact. This continues with the work that the group has been developing during the last years, adding some improvements to the experimental apparatus as well as to the analysis techniques employed previously. 2. Experimental The positron beam is produced by means of a 22Na radioactive source, by extraction and secondary emission from a tungsten moderator located right in front of the source. The beam is brought into the interaction region
O.G. de Lucio et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 892–895
by means of an electrostatic transport system consisting of an array of lenses and deflectors. A detailed description of this experimental setup can be found in [2–6]. Once the beam is in the interaction region it collides with an Ar target, which is a simple gas jet. After the beam interacts with the target it will induce some ionization, the recoil ions and ejected electrons are extracted by means of an uniform electric field which is created by applying a ±2.5 V on the metallic plates in front of those two detectors (see Fig. 1). Before the ions and ejected electrons reach their detectors they must go through a couple of grids which are used to produce a ‘‘field free’’ region. Under this configuration and because of the geometry of the system, it is possible to detect electrons ejected in a cone normal to the beam direction. Some improvements have been made in this region with respect to the previous works. First the recoil detector was changed from a channelplate position sensitive detector (PSD) to a channeltron, and also a channelplate has been added to detect the ejected electrons. Projectile–emitted electron coincidences give the option to distinguish between binary and recoil interactions. After the beam exits the impact region it goes through a vertical slit, which is used to define a scattering plane, and then into an electrostatic energy spectrometer, which focuses projectiles with a particular energy loss without affecting the scattering angle. After this process the beam goes through a grid and then into a projectile PSD. The PSD allows measurements of the energy loss (horizontal axis) and scattering angle (vertical axis) of the projectiles by their position on the detector. In this way two dimensional spectra can be generated by time differences between signals on the PSD anodes. Data acquisition is done by means of a TDC and a computer. Scattered projectiles and ejected electrons are mea-
893
sured in coincidence with recoil ions so it is possible to distinguish between different degrees of ionization by analyzing their time of flight. This supplies information about the kinematics for single and multiple ionization processes.
3. Analysis By imposing conditions on the raw data it is possible to present the information in different fashions. For example, one can extract information on the projectiles and ejected electrons which correspond only to certain degree of ionization, preserving information about the energy loss and scattering angle. At present some information about the electron emission direction is also available. The degrees of ionization are defined by setting windows around the peaks in the TOF spectra. Then 2D spectra are generated for the projectile and ejected electrons PSD. Since these spectra contain backgrounds due to random interactions it is necessary to generate a background spectrum and then subtract this from the spectrum being analyzed. Because the beam is not a point but a distribution it was necessary to acquire the beam energy profile in order to use later in the analysis. This implies that a position on the projectile detector corresponds to an energy/scattering angle distribution weighted by the beam energy profile. In order to extract differential information as a function of the projectile energy loss and scattering angle, the following procedure was employed: first the 2D background subtracted ejected electron spectrum was generated for single ionization. Next, by generating windows in this spectrum, the forward and backward scattering directions are defined. Then, by relating this set of conditions with the projectile single ionization information it was possible to
Projectile PSD
Electron PSD
Beam Extraction region
Aperture
Recoil Detector
Spectrometer
Fig. 1. Experimental apparatus. A two dimensional projectile spectrum is shown on the right side. The beam position is indicated by the solid circle.
894
O.G. de Lucio et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 892–895
identify projectile energy loss and scattering angle associated with the binary or recoil lobes. For our geometry, the projectiles that scatter down with respect to the original beam direction, generate a binary lobe that is directed towards the electron detector and a recoil lobe that is directed away. For projectiles that scatter up, the directions of the binary and recoil lobes are away and toward the electron detector, respectively. From simulations of the extraction region employing the SIMION computer program [7] it was pointed out that although the recoil lobe is directed away from the electron PSD, some of the ejected electrons belonging to that lobe could still be recorded if their energy was low enough such that the recoil extraction field is able to turn them around. This also happens for low energy binary electrons. If this happens the data exhibits ‘‘false’’ events coming from the turned around electrons. The projectile spectra generated was used to extract information on the intensities of both the binary and recoil lobes as a function of the projectile scattering angle for a range of energy losses. In order to make comparisons between electron and positron impact, the data were normalized to the total number of events associated with single ionization in each case. 4. Results Figs. 2(a) and (b) illustrate to the projectile spectra associated with binary and recoil events respectively for electron impact. These are obtained from coincidences with electrons emitted in the forward directions and backward direction, respectively. As can be seen, projectile scattering associated with binary interactions has more intensity when the scattering is small; as the scattering increases the intensity decreases uniformly, following a ridge. It can be noticed also that there is a low intensity structure above the main distribution. This is associated with low energy recoil electrons, as discussed above. Projectile scattering associated with recoil events exhibit a structure that has also more intensity on the small scattering region, but in this case the intensity decreases suddenly. The structure below zero scattering angle corresponds to events coming from turned around binary electrons. The distributions in Fig. 2 are examined more closely by their relative intensities for the binary and recoil structures, as displayed in Figs. 3(a) and (b) respectively. Only real events are shown. Angles are measured with respect to zero scattering as defined by the position of the beam on the projectile detector. Based upon our normalization, the binary intensity is larger for electron impact, but it decreases faster with scattering angle, while for the positron binary intensity remains constant for small scattering angles, and then starts to fall rapidly. For electron impact, binary intensity decreases monotonically. Because of this, the relative binary intensity for positron impact appears to be enhanced for larger scat-
Fig. 2. Electron impact two dimensional spectrum for projectiles in coincidence with ejected electrons associated with the (a) binary lobe and (b) recoil lobe. The beam position is shown as a circle. The regions marked as ‘‘false’’ events correspond to projectiles that are associated with turned around electrons. The same scale is used in both plots for positions and intensities. The actual data has been smoothed for display purposes.
tering angles. In contrast, within uncertainties the recoil lobes appear to be identical in shape, although electron impact generates more intensity. 5. Summary Results for differential measurements of ionization induced by positron and electron impact have been presented. Although it is necessary to deconvolute all the experimental parameters in order to quantify how they affect the raw data, noticeable differences associated with projectile charge are observed. Additional improvements are in progress and future studies are planned to study projectile charge effects and
O.G. de Lucio et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 892–895
Recoil
Binary -2
10
-3
Relative Intensity
10
895
Positron Electron -8
-6
-4
-2
0
0
2
4
6
8
Projectile Scattering Angle (deg) Fig. 3. Relative intensity for projectile scattering resulting from (a) binary and (b) recoil events for both positron and electron impact. Data has been normalized in each case with respect to the total single ionization. Each point corresponds to an integral for a range of scattering angles and energy losses.
how it influences the different ionization mechanisms. Use of the experimental method employed here can also be extended to multiple electron transitions, e.g. kinematic analysis of double and triple ionization. Acknowledgement This work was supported by the National Science Foundation. References [1] H. Bluhme, H. Knudsen, J.P. Merrison, K.A. Nielsen, J. Phys. B 32 (1999) 5835.
[2] R.D. DuBois, C. Doudna, C. Lloyd, M. Kahveci, Kh. Khayyat, Y. Zhou, D.H. Madison, J. Phys. B 34 (2001) L783. [3] R.D. DuBois, Kh. Khayyat, C. Doudna, C. Lloyd, Nucl. Instr. and Meth. B 192 (2002) 63. [4] A.C.F. Santos, A. Hasan, T. Yates, R.D. DuBois, Phys. Rev. A 67 (2003) 052708. [5] A.C.F. Santos, A. Hasan, R.D. DuBois, Phys. Rev. A 69 (2004) 032706. [6] R.D. DuBois, A.C.F. Santos, M.A. Thomason, J. Gavin, Nucl. Instr. and Meth. B 241 (2005) 19. [7] D.A. Dahl, ‘SIMION 3D Version 7.0 User’s Manual’, Report INEEL95/0403, Idaho National Engineering and Environmental Laboratory, Idaho (2000).