REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02B908 (2014)
Photo-ionization of aluminum in a hot cavity for the selective production of exotic species projecta) D. Scarpa,1,b) L. Makhathini,2 A. Tomaselli,3 D. Grassi,4 S. Corradetti,1 M. Manzolaro,1 J. Vasquez,1 M. Calderolla,1 M. Rossignoli,1 A. Monetti,1 A. Andrighetto,1 and G. Prete1 1
INFN-Laboratori Nazionali di Legnaro, Viale dell’Università 2, Legnaro (PD), Italy iThemba LABS, Cape Town, South Africa 3 Dipartimento di Ingegneria Elettronica, Università di Pavia, Via Ferrata 1, Pavia, Italy 4 Dipartimento di Chimica Generale, Università di Pavia, Via Taramelli 12, Pavia, Italy 2
(Presented 12 September 2013; received 4 September 2013; accepted 10 October 2013; published online 13 November 2013) SPES (Selective Production of Exotic Species) is an Isotope Separation On-Line (ISOL) based accelerator facility that will be built in the Legnaro-Istituto Nazionale di Fisica Nucleare (INFN) Laboratory (Italy), intended to provide intense neutron-rich radioactive ion beams obtained by protoninduced fission of a uranium carbide (UCx) target. Besides this main target material, silicon carbide (SiC) will be the first to be used to deliver p-rich beams. This target will also validate the functionality of the SPES facility with aluminum beam as result of impinging SiC target with proton beam. In the past, off line studies on laser photoionization of aluminum have been performed in Pavia Spectroscopy Laboratory and in Laboratori Nazionali di Legnaro; a XeCl excimer laser was installed in order to test the laser ionization in the SPES hot cavity. With the new Wien filter installed a better characterization of the ionization process in terms of efficiency was performed and results are discussed. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4828722] I. INTRODUCTION
The definitive uranium carbide (UCx) target that will be used in SPES1, 2 (Selective Production of Exotic Species) project for the production of n-rich isotopes, will be preceded by a silicon carbide (SiC) target, in order to calibrate and validate the functionality of the whole facility. Impinging SiC target with protons, one of the elements coming out from nuclear reaction is aluminum with its isotopes.3 Isobaric contamination of produced aluminum could be due to surface ionized elements, i.e., magnesium and sodium. In order to provide an aluminum ion beam, among various types of ionization techniques, the main effectiveness is expected with laser photoionization. Off line studies on laser photoionization have been made at Pavia Spectroscopy Laboratory, and tests have been performed on the SPES off-line front-end in Legnaro National Laboratory of INFN, in which the SPES facility is under construction. In this work results on laser photoionization and extraction capability of aluminum using the SPES off-line front-end system (Figure 1) will be presented. II. EXPERIMENTAL SETUP
As described in Ref. 4, we performed ionization of aluminum atoms using the LPX200 XeCl excimer laser by Lambda Phisyk, charged with XeCl gas, lasing around 308 nm wavelength. The new Wien filter inserted in the ion path can separate ions of interest according to their masses, in this case a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) [email protected]
0034-6748/2014/85(2)/02B908/3/$30.00
27, from the background signal due to other species ionized. Aluminum is loaded in a tantalum oven heated up to 2000 ◦ C and directly connected to the SPES transfer line and the hot cavity. After evaporation of atoms, they are transferred to the hot cavity where they interact with laser radiation and are ionized. Laser delivery is the same as in Ref. 4 with the telescope adjusted to the new distance taking into account the length added by the Wien filter. After ionization, a high voltage extraction system extracts and accelerates ions up to 25 kV. In the beam line, electrostatic quadrupoles are used to optimize the beam shape, before entering the Wien filter. The ions after the mass selection are collected by a Faraday cup (FC). As in Ref. 4 due to the collinearity of the ion beam and the laser beam, represented in Figure 2, the FC must be placed off-center. In this case the displacement of the Faraday cup is in the vertical direction, because in the horizontal one works the mass selection principle of the Wien filter. The ion beam can be centered into the Faraday cup using the same principle as in Ref. 4 with electrostatic vertical deflector action. Closing the slits to increase the effective mass resolution for the Wien filter also acts as spatial filter for the laser beam which has to pass through to reach the ion source. To avoid this inconvenience and to deliver the maximum energy of the laser beam to the ion source, the regular slits were changed with the new slits that have an “L” shape (as shown in Figure 3). This slits configuration allows the maximum energy of the laser to go through and also to be able to close the slits to the minimum position possible in order to do a perfect mass scan.
85, 02B908-1
© 2013 AIP Publishing LLC
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Scarpa et al.
FIG. 1. SPES off-line front-end lab. Excimer laser (orange) and into the cage the Wien filter (blue).
In Figure 3 it is possible to see the Faraday cup partially covered by the slits to have mass separation effect by the Wien filter, the ions enter in the cup coming along the observer of the figure, while the laser beam came just below the Faraday cup.
Rev. Sci. Instrum. 85, 02B908 (2014)
FIG. 3. Faraday cup covered by “L” filter slits.
B. Mass scanning inactive, mass tuned to aluminum
In this setup the Wien filter is active but the magnet current is tuned to perform a mass selection tuned to aluminum mass, filtering all other ions. The current collected to the FC is directly affected by the laser parameters like repetition rate (Figure 5).
III. RESULTS
The collected current in the FC after the mass selector, which is tuned to the mass of interest, represents the aluminum atoms ionized by laser action and other ionization processes. Several measurements were performed in two different setup:
A. Mass scanning active
In this setup the Wien filter is active and the magnet current is ramped up to perform a mass scanning during the ions collection. Current scans from 0 A to 120 A and corresponding mass scans from 0 (no mass selection effect) to 190 amu. Slits of 2 mm in front of Faraday cup during the measurement are open; this value is the best compromise between mass resolution (expected below 1 amu) and current obtained at the Faraday cup. Laser is either active or non-active during the scan. The mass scan provides a full characterization of the ion species coming out from the ions source. Several peaks were identified by data analysis, and the aluminum peak was identified at the expected mass. As expected during the laser-on mass scanning an enhancement of the solo aluminum peak was recorded, otherwise, in the laser off scanning no increment was observed (Figure 4).
FIG. 2. SPES front-end with laser and current path.
IV. CONSIDERATIONS
Numerous data were recorded in several measurement conditions and all results can lead to several considerations:
r Laser ionization of aluminum, as suggested in Ref. 4, was confirmed by the new Laboratori Nazionali di Legnaro (LNL) measurements set improved with mass selection by Wien filter. r Mass scan consecutive measurements underline that while average current decreases in time due to the pauperization of the loaded materials, aluminum peak behavior can be reversed increasing laser repetition rate or pulse energy, as shown in Fig. 5 with rep rate increasing from 20 to 80 Hz. r Some residual ionization of aluminum is present even if the laser is off, this can be explained by charge exchange between species present in the hot cavity
FIG. 4. Mass scan data on aluminum peak where Lon and Loff represent Laser-on and Laser-off, respectively.
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Rev. Sci. Instrum. 85, 02B908 (2014)
function of the front-end system using other kind of ion-source as (Surface Ion Source) SIS5 or (Plasma Ion Source) PIS6, 7 just using the FC in regular position and the “lower” part of the slits as can be figured out in Figure 3. V. CONCLUSIONS
FIG. 5. Current data on filtered aluminum peak varying laser rep rate.
(mainly K, Ce, Rb as background due to previous measurements in the same hot cavity) ionized by surface ionization. r In spite of the mass selection an efficiency measurement of the aluminum ionization process cannot be performed due to the instability of the laser. It cannot ensure a stable operation during a long time integration measurement. r In some measurements campaign under laser action in the mass scanning data, tantalum peak appears, this contribute can be due to laser ablation on hot cavity tube when some misalignments of the laser beam are present. r Because the slits of the Wien filter can totally overlap each other, the “L” slits configurations allow regular
Primary target for the SPES project is to validate the front-end system with all the available ionization techniques: surface, plasma, and laser. In Legnaro INFN National Laboratory, laser photoionization was obtained with excimer XeCl laser, covering a possible ionization path for aluminum. The results in the measured ion beam current allow to conclude that under laser action an ionization process takes place in the SPES hot cavity. The new Wien filter mass separator allows the authors to better define the aluminum current contribute certifying the laser action onto aluminum atoms. ACKNOWLEDGMENTS
Authors want to thank Professor P. Benetti for his precious support and advices, and also D. Stracener, I. Moore, Lyu, and M. Lollo. 1 A.
Andrighetto, C. M. Antonucci, S. Cevolani, C. Petrovich, and M. S. Leitner, Eur. Phys. J. A 30, 591 (2006). 2 Andrighetto et al., Nucl. Instrum. Methods Phys. Res. B 266, 4257 (2008). 3 M. Barbui et al., Nucl. Instrum. Methods Phys. Res. B 266, 4289 (2008). 4 D. Scarpa et al., Rev. Sci. Instrum. 83, 02B317 (2012). 5 See http://www.lnl.infn.it/~spes_target/ENG/sis.php for information about SIS. 6 See http://www.lnl.infn.it/~spes_target/ENG/pis.php for information about PIS. 7 M. Manzolaro, M. Manente, D. Curreli, J. Vasquez, J. Montano, A. Andrighetto, D. Scarpa, G. Meneghetti, and D. Pavarin, Rev. Sci. Instrum. 83, 02A907 (2012).
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
Photo-ionization of aluminum in a hot cavity for the selective production of exotic species projecta) D. Scarpa,1,b) L. Makhathini,2 A. Tomaselli,3 D. Grassi,4 S. Corradetti,1 M. Manzolaro,1 J. Vasquez,1 M. Calderolla,1 M. Rossignoli,1 A. Monetti,1 A. Andrighetto,1 and G. Prete1 1
INFN-Laboratori Nazionali di Legnaro, Viale dell’Università 2, Legnaro (PD), Italy iThemba LABS, Cape Town, South Africa 3 Dipartimento di Ingegneria Elettronica, Università di Pavia, Via Ferrata 1, Pavia, Italy 4 Dipartimento di Chimica Generale, Università di Pavia, Via Taramelli 12, Pavia, Italy 2
(Presented 12 September 2013; received 4 September 2013; accepted 10 October 2013; published online 13 November 2013) SPES (Selective Production of Exotic Species) is an Isotope Separation On-Line (ISOL) based accelerator facility that will be built in the Legnaro-Istituto Nazionale di Fisica Nucleare (INFN) Laboratory (Italy), intended to provide intense neutron-rich radioactive ion beams obtained by protoninduced fission of a uranium carbide (UCx) target. Besides this main target material, silicon carbide (SiC) will be the first to be used to deliver p-rich beams. This target will also validate the functionality of the SPES facility with aluminum beam as result of impinging SiC target with proton beam. In the past, off line studies on laser photoionization of aluminum have been performed in Pavia Spectroscopy Laboratory and in Laboratori Nazionali di Legnaro; a XeCl excimer laser was installed in order to test the laser ionization in the SPES hot cavity. With the new Wien filter installed a better characterization of the ionization process in terms of efficiency was performed and results are discussed. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4828722] I. INTRODUCTION
The definitive uranium carbide (UCx) target that will be used in SPES1, 2 (Selective Production of Exotic Species) project for the production of n-rich isotopes, will be preceded by a silicon carbide (SiC) target, in order to calibrate and validate the functionality of the whole facility. Impinging SiC target with protons, one of the elements coming out from nuclear reaction is aluminum with its isotopes.3 Isobaric contamination of produced aluminum could be due to surface ionized elements, i.e., magnesium and sodium. In order to provide an aluminum ion beam, among various types of ionization techniques, the main effectiveness is expected with laser photoionization. Off line studies on laser photoionization have been made at Pavia Spectroscopy Laboratory, and tests have been performed on the SPES off-line front-end in Legnaro National Laboratory of INFN, in which the SPES facility is under construction. In this work results on laser photoionization and extraction capability of aluminum using the SPES off-line front-end system (Figure 1) will be presented. II. EXPERIMENTAL SETUP
As described in Ref. 4, we performed ionization of aluminum atoms using the LPX200 XeCl excimer laser by Lambda Phisyk, charged with XeCl gas, lasing around 308 nm wavelength. The new Wien filter inserted in the ion path can separate ions of interest according to their masses, in this case a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) [email protected]
0034-6748/2014/85(2)/02B908/3/$30.00
27, from the background signal due to other species ionized. Aluminum is loaded in a tantalum oven heated up to 2000 ◦ C and directly connected to the SPES transfer line and the hot cavity. After evaporation of atoms, they are transferred to the hot cavity where they interact with laser radiation and are ionized. Laser delivery is the same as in Ref. 4 with the telescope adjusted to the new distance taking into account the length added by the Wien filter. After ionization, a high voltage extraction system extracts and accelerates ions up to 25 kV. In the beam line, electrostatic quadrupoles are used to optimize the beam shape, before entering the Wien filter. The ions after the mass selection are collected by a Faraday cup (FC). As in Ref. 4 due to the collinearity of the ion beam and the laser beam, represented in Figure 2, the FC must be placed off-center. In this case the displacement of the Faraday cup is in the vertical direction, because in the horizontal one works the mass selection principle of the Wien filter. The ion beam can be centered into the Faraday cup using the same principle as in Ref. 4 with electrostatic vertical deflector action. Closing the slits to increase the effective mass resolution for the Wien filter also acts as spatial filter for the laser beam which has to pass through to reach the ion source. To avoid this inconvenience and to deliver the maximum energy of the laser beam to the ion source, the regular slits were changed with the new slits that have an “L” shape (as shown in Figure 3). This slits configuration allows the maximum energy of the laser to go through and also to be able to close the slits to the minimum position possible in order to do a perfect mass scan.
85, 02B908-1
© 2013 AIP Publishing LLC
02B908-2
Scarpa et al.
FIG. 1. SPES off-line front-end lab. Excimer laser (orange) and into the cage the Wien filter (blue).
In Figure 3 it is possible to see the Faraday cup partially covered by the slits to have mass separation effect by the Wien filter, the ions enter in the cup coming along the observer of the figure, while the laser beam came just below the Faraday cup.
Rev. Sci. Instrum. 85, 02B908 (2014)
FIG. 3. Faraday cup covered by “L” filter slits.
B. Mass scanning inactive, mass tuned to aluminum
In this setup the Wien filter is active but the magnet current is tuned to perform a mass selection tuned to aluminum mass, filtering all other ions. The current collected to the FC is directly affected by the laser parameters like repetition rate (Figure 5).
III. RESULTS
The collected current in the FC after the mass selector, which is tuned to the mass of interest, represents the aluminum atoms ionized by laser action and other ionization processes. Several measurements were performed in two different setup:
A. Mass scanning active
In this setup the Wien filter is active and the magnet current is ramped up to perform a mass scanning during the ions collection. Current scans from 0 A to 120 A and corresponding mass scans from 0 (no mass selection effect) to 190 amu. Slits of 2 mm in front of Faraday cup during the measurement are open; this value is the best compromise between mass resolution (expected below 1 amu) and current obtained at the Faraday cup. Laser is either active or non-active during the scan. The mass scan provides a full characterization of the ion species coming out from the ions source. Several peaks were identified by data analysis, and the aluminum peak was identified at the expected mass. As expected during the laser-on mass scanning an enhancement of the solo aluminum peak was recorded, otherwise, in the laser off scanning no increment was observed (Figure 4).
FIG. 2. SPES front-end with laser and current path.
IV. CONSIDERATIONS
Numerous data were recorded in several measurement conditions and all results can lead to several considerations:
r Laser ionization of aluminum, as suggested in Ref. 4, was confirmed by the new Laboratori Nazionali di Legnaro (LNL) measurements set improved with mass selection by Wien filter. r Mass scan consecutive measurements underline that while average current decreases in time due to the pauperization of the loaded materials, aluminum peak behavior can be reversed increasing laser repetition rate or pulse energy, as shown in Fig. 5 with rep rate increasing from 20 to 80 Hz. r Some residual ionization of aluminum is present even if the laser is off, this can be explained by charge exchange between species present in the hot cavity
FIG. 4. Mass scan data on aluminum peak where Lon and Loff represent Laser-on and Laser-off, respectively.
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Rev. Sci. Instrum. 85, 02B908 (2014)
function of the front-end system using other kind of ion-source as (Surface Ion Source) SIS5 or (Plasma Ion Source) PIS6, 7 just using the FC in regular position and the “lower” part of the slits as can be figured out in Figure 3. V. CONCLUSIONS
FIG. 5. Current data on filtered aluminum peak varying laser rep rate.
(mainly K, Ce, Rb as background due to previous measurements in the same hot cavity) ionized by surface ionization. r In spite of the mass selection an efficiency measurement of the aluminum ionization process cannot be performed due to the instability of the laser. It cannot ensure a stable operation during a long time integration measurement. r In some measurements campaign under laser action in the mass scanning data, tantalum peak appears, this contribute can be due to laser ablation on hot cavity tube when some misalignments of the laser beam are present. r Because the slits of the Wien filter can totally overlap each other, the “L” slits configurations allow regular
Primary target for the SPES project is to validate the front-end system with all the available ionization techniques: surface, plasma, and laser. In Legnaro INFN National Laboratory, laser photoionization was obtained with excimer XeCl laser, covering a possible ionization path for aluminum. The results in the measured ion beam current allow to conclude that under laser action an ionization process takes place in the SPES hot cavity. The new Wien filter mass separator allows the authors to better define the aluminum current contribute certifying the laser action onto aluminum atoms. ACKNOWLEDGMENTS
Authors want to thank Professor P. Benetti for his precious support and advices, and also D. Stracener, I. Moore, Lyu, and M. Lollo. 1 A.
Andrighetto, C. M. Antonucci, S. Cevolani, C. Petrovich, and M. S. Leitner, Eur. Phys. J. A 30, 591 (2006). 2 Andrighetto et al., Nucl. Instrum. Methods Phys. Res. B 266, 4257 (2008). 3 M. Barbui et al., Nucl. Instrum. Methods Phys. Res. B 266, 4289 (2008). 4 D. Scarpa et al., Rev. Sci. Instrum. 83, 02B317 (2012). 5 See http://www.lnl.infn.it/~spes_target/ENG/sis.php for information about SIS. 6 See http://www.lnl.infn.it/~spes_target/ENG/pis.php for information about PIS. 7 M. Manzolaro, M. Manente, D. Curreli, J. Vasquez, J. Montano, A. Andrighetto, D. Scarpa, G. Meneghetti, and D. Pavarin, Rev. Sci. Instrum. 83, 02A907 (2012).
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
