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Acceleration of cluster ions to MeV energies
Nuclear Instruments North-Holland
and Methods
in Physics Research
B 88 (1994) 6-9
NIIIMIB
Beam Interactions with Materials&Atoms
Acceleration Ch. Tomaschko
of cluster ions to MeV energies *, Ch. Schoppmann,
D. Brand1 and H. Voit
Physikalisches Institut der Universitiit Erlangen - Niimberg, D 91058 Erlangen, Germany
Gold and C,, cluster ions have been accelerated to MeV energies with the EN tandem accelerator at Erlangen. Negatively charged cluster ions were produced in the sputter source and accelerated through both stages of the accelerator. The identification
of the MeV cluster ions was performed by time-of-flight and energy measurements.
1. Introduction A very large energy density and a strong compression of matter is obtained within a sample if the sample is bombarded with MeV cluster ions. As a result, interesting processes are expected to occur. The study of these processes demands a variety of MeV cluster ions with different features. This contribution deals with the acceleration of Au, and C,, clusters to MeV energies with a tandem accelerator [1,2].
2. Experimental 2.1. Acceleration
of the cluster ions
and C& ions were proNegatively charged Au, duced with a conventional Cs-sputter source and injected into the accelerator through a 20” deflection magnet. A first acceleration takes place between the low energy end (at ground potential) and the terminal in the middle of the accelerator (terminal voltage U, = + 1 to +5 MV). The cluster ions pass a gas channel in the terminal and become neutralized or positively charged. A certain percentage of the cluster ions survive the collisions with the gas molecules without being fragmentated. Subsequently they are accelerated toward ground potential. A fraction of these ions leaves the accelerator, having gained MeV energies. 2.2. Identification
of Au:
cluster
ions by time-of-fight
measurements
the
The accelerated cluster ions were identified behind accelerator exit by means of their time-of-flight
* Corresponding author. 0168-583X/94/$07.00 0 1994 - Elsevier SSDI 0168-583X(93)E0911-Y
(TOF). Fig. 1 shows the experimental setup used for these measurements. Intact cluster ions as well as cluster fragments pass through a thin foil and produce secondary electrons, which are accelerated onto a micro channel plate (MCP) detector. The detector delivers the start signal for the TOF measurement. It is reasonable to assume that intact clusters break apart within the foil. Most probably they decay into atomic ions. These ions hit the stop detector (two MCPs or a Si surface barrier detector), which delivers the stop signal. Fig. 2 shows TOF spectra obtained if Au; cluster ions are injected into the accelerator. The terminal voltage was set to U, = n MV. In this way all intact Au: cluster ions should have the same velocity as the Au: ion. Thus they should show up at the position of the Au, + ion in the corresponding spectra. A close inspection of Fig. 2 shows, that there are no peaks with exactly the Au: peak position. There exist, however, peaks in the Au,, Au; and Au; spectra (the latter is rather tiny) which are slightly shifted to larger flight times with respect to the Au: position. These peaks must be attributed to intact Au: cluster ions. The observed TOF shifts are most probably due to two different effects: (i) the energy loss of Au,f cluster ions is somewhat larger than the energy loss of Au: ions having the same velocity, and (ii) the energy loss of the fragmentation products in the foil (i.e. atomic Au: ions) increases with increasing ion energy in the energy range in question [3]. Indeed the observed shifts increase with increasing n of the original intact cluster. The fact that Au: peaks show up in the spectra if Au; ions are injected does, of course, not mean that intact Au: clusters have reached the start detector. It only tells, that intact Au: clusters have left the accelerator exit. All other peaks in the spectra of Fig. 2 are due to fragment ions (atomic or molecular ions) or neutrals. The most probable candidates for the ionic species are indicated in the figure.
Science B.V. All rights reserved
7
Ch. Tomaschkoet al. /Nucl. Instr. and Meth. in Phys.Res. B 88 (1994) 6-9
..____X___
__._
^-____-_
cluster ion
Fig. 1. Schematics of the setup for time-of-flight and energy measurements. G: grid for acceleration of the electrons onto the micro channel plates (Dl), D2: stop-detector (MCPs or alternatively Si-detector), F: mylar foil ( = 10 pg cm-*).
500 AUs* 0
2.3. Identification of Au,f cluster ions by correlated measurements of cluster energy and TOF
In an additional experiment both energy and TOF of the accelerated ions were measured simult~eously,
100
300
500
700 TOF
fns)
Fig. 2. Time-of-flight spectra (a)-(e) obtained, if Au; cluster ions are injected into the accelerator. Note, that the terminal voltage Ur was n MV for the injection of Au; ions.
L
*‘*‘*i*‘*‘*i-‘~~*
0
2
4
6
8
IO
12
14
16
18
energy (MeV) Fig. 3. Energy calibration curve for the .%-detector. The solid line marks the calibration obtained with Auqc ions from the Au; injection. The triangles refer to Au; (Ur = 3 MV), the squares to Au; (Ur = 4.5 MV) injection, respectively.
using a Si-detector as stop detector which delivers the energy signal and the time signal. The energy calibration of the Si-detector was performed with Au?’ (4 = l-3) ions, obtained from Au; ions injected into the accelerator (terminal voltages U, = -I-1.5, -I-3.0 and +5.0 MV). The calibration was performed with the start-detector foil inserted into the ion path. The calibration curve is shown in Fig. 3 (solid line). Subsequently Au; and Au; ions were injected into the accelerator (te~inal voltages U, = + 3.0 and +4.5 MV, respectively). An energy spectrum obtained if Au; ions are injected is shown in Fig. 4a. The spectrum clearly exhibits Auyf ions (q = l-3) resulting from the break up of the Au2 clusters in the terminal. The energy-channel position of these peaks fits nicely on the calibration curve (triangles in Fig. 3). At 3 MeV an additional peak shows up in the energy spectrum of Fig. 4a. It can be explained, if one assumes that an Au: cluster ion leaves the accelerator unfragmentated with the maximum possible energy (6 MeV) and breaks subsequently apart. This happens most probably in the start detector foil. Oniy one of the two fragments reaches the Si-detector (a detector with 10 mm diameter was used) and deposites an energy of roughly 3 MeV (see calibration curve of Fig. 3). The TOF spectrum for the Au, injection is shown in Fig. 4b. If one gates the TOF signals with the energy signals which correspond to one of the peaks in the energy spectrum one obtaines only one TOF peak. A gate with the 3 MeV peak gives the TOF spectrum shown in Fig. 4c. The peak position corresponds to the TOF of a fully accelerated Au: cluster.
8
Ch. To~~schko et al. /Nd.
fnstr. and Meth. in Pisys.Res. B 88 fl994,lcL9
The same investigation was performed for Au; injection and a terminal voltage of 4.5 MV. In the energy spectrum one finds the Au!” peaks (fragment ions with q = l-3) and two additional peaks belonging to the intact Au: ion and the Au; fragment ion. The energy of the latter (see calibration curve) is in agreement with the assumption that only one Au, ion or atom of the cluster reaches the %-detector. Again the TOF spectrum gated with the Au: energy peak exhibits only one TOF peak with the correct flight time for a A$ cluster ion.
1
300
320
340
360
magneticf&d (G) It was tried to identify intact C, clusters by means of the 90” deflection magnet. For this purpose only a Si-detector was used, which was placed behind the 90” analyzing magnet. The detector could be moved to a tilt angle (Y= 3” with respect to the beamline (see Fig. 5, inset). In a first step Au; and Cc0 ions were injected
2250.
1500-
750-
0
LOO
800
1200 CHANNEL
Fig. 5. (a) Events measured with the %-detector positioned behind the analyzing magnet with a tilt angle of a: = 3” with respect to the beam direction (see inset) as a function of the magnetic field. Au; ions were injected into the accelerator (UT = + 5.0 MV). Only events with an energy of 10 MeV were accepted. (b) energy spectrum obtained with the S&detector in the position shown in the inset of (a) for a magnetic field of 375 G at the 90” magnet, if C, ions are injected into the accelerator.
Fig. 4. Spectra obtained, if Au,- is injected into the accelerator. (a) Energy spectrum; (b) time-of-flight spectrum measured without correlation and (c) time-of-flight spectrum measured in correlation with the energy window set on the Au: peak of (a).
into the accelerator. The terminal voltages were +5.0 and + 1.37 MV, respectively. The reason for this particular choice is that Au: and C& ions are deflected through 3” by the 90” magnet if identically the same magnetic field is applied. The energy spectrum obtained for Au; injection is easy to understand. It allows to establish an energy calibration curve. The interpretation of the energy spectrum obtained if C, ions are accelerated is rather complicated. There exists a significant peak with an energy which exceeds the expected C& energy (2.74 MeV) by 0.7 MeV. This peak was tentatively assigned as the C& peak. The energy shift can be explained by the fact, that a larger amount of energy is deposited by the C, projectiles in the Si-detector as electronic excitation (only the electronic energy loss in the Si-detector contributes to the detector signal) than by Auf+ ions which were used to set up the energy calibration.
Ch. Tomaschko et al. /Nucl.
Instr. and Meth. in Phys. Res. B 88 (1994) 6-9
In a second step the %-detector was moved to the tilt angle (Y= 3”. First of all Au; ions were injected. The magnetic field of the analyzing magnet was then slowly increased until the maximum count rate of signals in the energy window of the Au{ peak was observed (see Fig. 5a). This magnetic field was maintained. After injection of C, ions an energy spectrum was obtained which exhibits a single peak with the energy of the assumed C& peak from above. This may be interpreted as an indication that intact C& cluster ions survive the flight time between accelerator and analyzing magnet exit. Due to the relatively poor resolution of the magnet it can, however, not be excluded that heavy C,, fragments were deflected instead of the intact C, molecule. In fact a calculation shows that the analyzing magnet would accept all fragments between C,, and C,,. This uncertainty can be removed by a real TOF measurement performed in the tilted position after the analyzing magnet.
3. Concluding remarks It is shown, that it is possible to accelerate cluster ions like Au, and C, through both stages of a tandem
9
accelerator. It is also shown, that intact C& ions or heavy fragments of C,, with MeV energies exist behind the 90” deflection magnet.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft, Bonn, Germany. The authors would like to thank Y. Le Beyec for organizing the conference at St. Malo in his very own style.
References [l] Ch. Schoppman et al., Nucl. Instr. and Meth. B 82 (1993)
156. [2] S. Della-Negra, A. Brunelle, Y. Le Beyec, J.M. Curaudeau, J.P. Mouffron, B. Waast, P. H%kansson, B.U.R. Sundqvist and E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [3] L.C. Northcliffe and K.F. Shilling, Nucl. Data Tables 1 (1970) 233.
and Methods
in Physics Research
B 88 (1994) 6-9
NIIIMIB
Beam Interactions with Materials&Atoms
Acceleration Ch. Tomaschko
of cluster ions to MeV energies *, Ch. Schoppmann,
D. Brand1 and H. Voit
Physikalisches Institut der Universitiit Erlangen - Niimberg, D 91058 Erlangen, Germany
Gold and C,, cluster ions have been accelerated to MeV energies with the EN tandem accelerator at Erlangen. Negatively charged cluster ions were produced in the sputter source and accelerated through both stages of the accelerator. The identification
of the MeV cluster ions was performed by time-of-flight and energy measurements.
1. Introduction A very large energy density and a strong compression of matter is obtained within a sample if the sample is bombarded with MeV cluster ions. As a result, interesting processes are expected to occur. The study of these processes demands a variety of MeV cluster ions with different features. This contribution deals with the acceleration of Au, and C,, clusters to MeV energies with a tandem accelerator [1,2].
2. Experimental 2.1. Acceleration
of the cluster ions
and C& ions were proNegatively charged Au, duced with a conventional Cs-sputter source and injected into the accelerator through a 20” deflection magnet. A first acceleration takes place between the low energy end (at ground potential) and the terminal in the middle of the accelerator (terminal voltage U, = + 1 to +5 MV). The cluster ions pass a gas channel in the terminal and become neutralized or positively charged. A certain percentage of the cluster ions survive the collisions with the gas molecules without being fragmentated. Subsequently they are accelerated toward ground potential. A fraction of these ions leaves the accelerator, having gained MeV energies. 2.2. Identification
of Au:
cluster
ions by time-of-fight
measurements
the
The accelerated cluster ions were identified behind accelerator exit by means of their time-of-flight
* Corresponding author. 0168-583X/94/$07.00 0 1994 - Elsevier SSDI 0168-583X(93)E0911-Y
(TOF). Fig. 1 shows the experimental setup used for these measurements. Intact cluster ions as well as cluster fragments pass through a thin foil and produce secondary electrons, which are accelerated onto a micro channel plate (MCP) detector. The detector delivers the start signal for the TOF measurement. It is reasonable to assume that intact clusters break apart within the foil. Most probably they decay into atomic ions. These ions hit the stop detector (two MCPs or a Si surface barrier detector), which delivers the stop signal. Fig. 2 shows TOF spectra obtained if Au; cluster ions are injected into the accelerator. The terminal voltage was set to U, = n MV. In this way all intact Au: cluster ions should have the same velocity as the Au: ion. Thus they should show up at the position of the Au, + ion in the corresponding spectra. A close inspection of Fig. 2 shows, that there are no peaks with exactly the Au: peak position. There exist, however, peaks in the Au,, Au; and Au; spectra (the latter is rather tiny) which are slightly shifted to larger flight times with respect to the Au: position. These peaks must be attributed to intact Au: cluster ions. The observed TOF shifts are most probably due to two different effects: (i) the energy loss of Au,f cluster ions is somewhat larger than the energy loss of Au: ions having the same velocity, and (ii) the energy loss of the fragmentation products in the foil (i.e. atomic Au: ions) increases with increasing ion energy in the energy range in question [3]. Indeed the observed shifts increase with increasing n of the original intact cluster. The fact that Au: peaks show up in the spectra if Au; ions are injected does, of course, not mean that intact Au: clusters have reached the start detector. It only tells, that intact Au: clusters have left the accelerator exit. All other peaks in the spectra of Fig. 2 are due to fragment ions (atomic or molecular ions) or neutrals. The most probable candidates for the ionic species are indicated in the figure.
Science B.V. All rights reserved
7
Ch. Tomaschkoet al. /Nucl. Instr. and Meth. in Phys.Res. B 88 (1994) 6-9
..____X___
__._
^-____-_
cluster ion
Fig. 1. Schematics of the setup for time-of-flight and energy measurements. G: grid for acceleration of the electrons onto the micro channel plates (Dl), D2: stop-detector (MCPs or alternatively Si-detector), F: mylar foil ( = 10 pg cm-*).
500 AUs* 0
2.3. Identification of Au,f cluster ions by correlated measurements of cluster energy and TOF
In an additional experiment both energy and TOF of the accelerated ions were measured simult~eously,
100
300
500
700 TOF
fns)
Fig. 2. Time-of-flight spectra (a)-(e) obtained, if Au; cluster ions are injected into the accelerator. Note, that the terminal voltage Ur was n MV for the injection of Au; ions.
L
*‘*‘*i*‘*‘*i-‘~~*
0
2
4
6
8
IO
12
14
16
18
energy (MeV) Fig. 3. Energy calibration curve for the .%-detector. The solid line marks the calibration obtained with Auqc ions from the Au; injection. The triangles refer to Au; (Ur = 3 MV), the squares to Au; (Ur = 4.5 MV) injection, respectively.
using a Si-detector as stop detector which delivers the energy signal and the time signal. The energy calibration of the Si-detector was performed with Au?’ (4 = l-3) ions, obtained from Au; ions injected into the accelerator (terminal voltages U, = -I-1.5, -I-3.0 and +5.0 MV). The calibration was performed with the start-detector foil inserted into the ion path. The calibration curve is shown in Fig. 3 (solid line). Subsequently Au; and Au; ions were injected into the accelerator (te~inal voltages U, = + 3.0 and +4.5 MV, respectively). An energy spectrum obtained if Au; ions are injected is shown in Fig. 4a. The spectrum clearly exhibits Auyf ions (q = l-3) resulting from the break up of the Au2 clusters in the terminal. The energy-channel position of these peaks fits nicely on the calibration curve (triangles in Fig. 3). At 3 MeV an additional peak shows up in the energy spectrum of Fig. 4a. It can be explained, if one assumes that an Au: cluster ion leaves the accelerator unfragmentated with the maximum possible energy (6 MeV) and breaks subsequently apart. This happens most probably in the start detector foil. Oniy one of the two fragments reaches the Si-detector (a detector with 10 mm diameter was used) and deposites an energy of roughly 3 MeV (see calibration curve of Fig. 3). The TOF spectrum for the Au, injection is shown in Fig. 4b. If one gates the TOF signals with the energy signals which correspond to one of the peaks in the energy spectrum one obtaines only one TOF peak. A gate with the 3 MeV peak gives the TOF spectrum shown in Fig. 4c. The peak position corresponds to the TOF of a fully accelerated Au: cluster.
8
Ch. To~~schko et al. /Nd.
fnstr. and Meth. in Pisys.Res. B 88 fl994,lcL9
The same investigation was performed for Au; injection and a terminal voltage of 4.5 MV. In the energy spectrum one finds the Au!” peaks (fragment ions with q = l-3) and two additional peaks belonging to the intact Au: ion and the Au; fragment ion. The energy of the latter (see calibration curve) is in agreement with the assumption that only one Au, ion or atom of the cluster reaches the %-detector. Again the TOF spectrum gated with the Au: energy peak exhibits only one TOF peak with the correct flight time for a A$ cluster ion.
1
300
320
340
360
magneticf&d (G) It was tried to identify intact C, clusters by means of the 90” deflection magnet. For this purpose only a Si-detector was used, which was placed behind the 90” analyzing magnet. The detector could be moved to a tilt angle (Y= 3” with respect to the beamline (see Fig. 5, inset). In a first step Au; and Cc0 ions were injected
2250.
1500-
750-
0
LOO
800
1200 CHANNEL
Fig. 5. (a) Events measured with the %-detector positioned behind the analyzing magnet with a tilt angle of a: = 3” with respect to the beam direction (see inset) as a function of the magnetic field. Au; ions were injected into the accelerator (UT = + 5.0 MV). Only events with an energy of 10 MeV were accepted. (b) energy spectrum obtained with the S&detector in the position shown in the inset of (a) for a magnetic field of 375 G at the 90” magnet, if C, ions are injected into the accelerator.
Fig. 4. Spectra obtained, if Au,- is injected into the accelerator. (a) Energy spectrum; (b) time-of-flight spectrum measured without correlation and (c) time-of-flight spectrum measured in correlation with the energy window set on the Au: peak of (a).
into the accelerator. The terminal voltages were +5.0 and + 1.37 MV, respectively. The reason for this particular choice is that Au: and C& ions are deflected through 3” by the 90” magnet if identically the same magnetic field is applied. The energy spectrum obtained for Au; injection is easy to understand. It allows to establish an energy calibration curve. The interpretation of the energy spectrum obtained if C, ions are accelerated is rather complicated. There exists a significant peak with an energy which exceeds the expected C& energy (2.74 MeV) by 0.7 MeV. This peak was tentatively assigned as the C& peak. The energy shift can be explained by the fact, that a larger amount of energy is deposited by the C, projectiles in the Si-detector as electronic excitation (only the electronic energy loss in the Si-detector contributes to the detector signal) than by Auf+ ions which were used to set up the energy calibration.
Ch. Tomaschko et al. /Nucl.
Instr. and Meth. in Phys. Res. B 88 (1994) 6-9
In a second step the %-detector was moved to the tilt angle (Y= 3”. First of all Au; ions were injected. The magnetic field of the analyzing magnet was then slowly increased until the maximum count rate of signals in the energy window of the Au{ peak was observed (see Fig. 5a). This magnetic field was maintained. After injection of C, ions an energy spectrum was obtained which exhibits a single peak with the energy of the assumed C& peak from above. This may be interpreted as an indication that intact C& cluster ions survive the flight time between accelerator and analyzing magnet exit. Due to the relatively poor resolution of the magnet it can, however, not be excluded that heavy C,, fragments were deflected instead of the intact C, molecule. In fact a calculation shows that the analyzing magnet would accept all fragments between C,, and C,,. This uncertainty can be removed by a real TOF measurement performed in the tilted position after the analyzing magnet.
3. Concluding remarks It is shown, that it is possible to accelerate cluster ions like Au, and C, through both stages of a tandem
9
accelerator. It is also shown, that intact C& ions or heavy fragments of C,, with MeV energies exist behind the 90” deflection magnet.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft, Bonn, Germany. The authors would like to thank Y. Le Beyec for organizing the conference at St. Malo in his very own style.
References [l] Ch. Schoppman et al., Nucl. Instr. and Meth. B 82 (1993)
156. [2] S. Della-Negra, A. Brunelle, Y. Le Beyec, J.M. Curaudeau, J.P. Mouffron, B. Waast, P. H%kansson, B.U.R. Sundqvist and E. Parilis, Nucl. Instr. and Meth. B 74 (1993) 453. [3] L.C. Northcliffe and K.F. Shilling, Nucl. Data Tables 1 (1970) 233.