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Lithium induced nuclear reactions and the production of negative lithium ions
NUCLEAR
INSTRUMENTS
AND METHODS
61 (i968) I I 5 - I I 6 ;
© NORTH-HOLLAND
PUBLISHING
CO.
L I T H I U M I N D U C E D N U C L E A R R E A C T I O N S AND T H E P R O D U C T I O N OF N E G A T I V E L I T H I U M I O N S * R. MIDDLETON, C. T. A D A M S and K. B E T H G E t
Tandem Accelerator Laboratory, Physics Department, University of Pennsylvania, Philadelphia, Pennsylvania, U.S.A. Received 25 January 1968 Lithium induced nuclear reactions in the energy range available with electrostatic accelerators provide a useful tool in nuclear structure studies. An easy method is described for obtaining large
currents of negative charged lithium ions for tandem Van de G raaff accelerators.
Recent advances in nuclear structure theory and experiment suggest that the two-hole four-particle and four-hole four-particle states may not be uncommon. Such states frequently cannot be directly excited by reactions involving the transfer of one or two nucleons but can be excited by reactions transferring four nucleons. The simplest and most promising reactions sal:isfying the latter requirement are the (TLi,t) and (6 Li,d) reactions. Since the binding energy of the alpha-particle in 6Li and 7Li is only 1.47 and 2.47 MeV respectively, it seems likely that both reactions will proceed directly at intermediate energies. This appears to be confirmed experimentally in the case of the (TLi,t) reaction by recent measurements made at Heidelberg 1) and this laboratoryZ). The mechanism of the (6Li,d) reaction, however, remains obscure and in any event its utility may be limited by the three-body break-upS). For the study of light and intermediate mass nuclei the most suitable lithium energy is in the range 12 to about 28 MeV. Such energies are just within the range. and most conveniently obtainable from the present generation of tandem accelerators provided that a convenient source of negative ions is available. Production of negative lithium ions: As far as we are aware, negative lithium ion sources have been under development at Heidelberg, Saclay and in this laboratory. Bethge and collaborators at Heidelberg have developed a direct extraction system using a Penning type source4). Negative lithium beams of about 1 /~A were obtained enabling an analyzed beam of about 0.2/~A of Li + ÷ + to be obtained through an H V E C ENtandem. At Saclay a filament type ion source has been developed to produce intense beams of positive lithium ionsS). The intention was to form negative ions by charge exchange in propane. The final outcome of this work is not known. The approach in this laboratory was similar to that at Saclay with the exception that charge exchange was performed in lithium vapor. At exchange voltages in the range 30 to 40 kV negative
lithium beams of up to 3/zA were obtained and analyzed beams of Li + + + in excess of 1/zA. The reliability of the three systems briefly described was not high and all suffered to some extent from the many problems associated with operating an ion source on lithium vapor. To obviate these difficulties it was recently decided to try making negative lithium ions using the method first described by Gentner and Hortig6). In this method, the element whose negative ion is required is leaked in gaseous form into the adder canal and then bombarded with 40 to 50 kV heavy ions such as krypton. Although the mechanism for the formation of the negative ion is not well understood it is generally believed to be one of dissociation of a molecule. Prior to this work, the method had been restricted to elements available in gaseous form. This restriction has been removed by the introduction of the optical bench source, a detailed description of which can be found in a recent University of Pennsylvania laboratory report by Middleton and AdamsT). The optical bench construction allows the normal adder canal to be replaced with a canal-boat combination, both of which are indirectly heated. Thus, a metal or compound placed in the boat can be vaporized and leaked into the adder canal. The optical bench system was originally developed for the production of intense H e - beams by charge exchange in lithium vapor. To adapt the system for producing L i - by the method of Gentner and Hortig, it was necessary only to change the source gas. Several gases have been tried and the observed L i - current in microampere are presented in table 1 as a function of exchange voltage. It is noteworthy that although the best yield of L i was obtained from argon, hydrogen may in many respects be preferable. Certainly the life of the duo-
115
* Work supported by the National Science Foundation. t On leave of absence from the University of Heidelberg,
Germany.
116
R. MIDDLETON et al. TABLE 1
Exchange voltage (kV)
Hydrogen
Neon
Argon
Krypton
36 40 45 50
0.8 0.96 1.25 1.50
0.15 0.25 0.20
1.2 1.5 1.7 2.1
0.8 1.1 1.2
p l a s m a t r o n filament is longer a n d less erosion of the intermediate electrode occurs. The measurements we have made give little or no indication of the m e c h a n i s m responsible for the formation of negative lithium ions. Since it is k n o w n that the percentage of Li/ molecules in lithium vapor increases with temperatureS), it seems likely that it m a y be due to the dissociation of a diatomic molecule. This belief is strengthened by the fact that an a t t e m p t to make negative m a g n e s i u m ions by a similar method was unsuccessful and m a g n e s i u m vapor is t h o u g h t to be entirely monoatomic9).
The method described provides an extremely convenient source of negative 6Li and 7Li ions. It would also appear to have wide application for p r o d u c i n g other negative ions species. References 1) K. Bethge, K. Meier-Ewert, K. Pfeiffer and R. Bock, Phys. Letters 24B (1967) 663. 2) R. Middleton, B. Rosner, D. Pullen and L. Polsky, Phys. Rev. Letters 20 (1968) 118. 3) K. Meier-Ewert, K. Bethge and K. O. Pfeiffer, Nuclear Physics, to be published; K. Bethge and K. Meier-Ewert, Phys. Rev. Letters 18 0967) 1010. 4) E. Heinicke, K. Bethge and H. Baumann, Nucl. lnstr, and Meth. 58 (1968) 125. 2) B. Delaunay-Olkowski, L. Bianchi, J. P. Meurgues and J. Petres, J. Phys. 24 (1963) 1012. 6) W. Gentner and G. Hortig, Z. Physik 172 (1963) 353. 7) R. Middleton and C. T. Adams, Lab. Rep. Univ. of Pennsylvania (1967). 8) An. N. Nesmeyanov, Vapour pressure o f the elements (Acad. Press, N.Y. 1963). 9) G. Herzberg, Molecular spectra and molecular structure (Van Nostrand, Princeton, 1961).
INSTRUMENTS
AND METHODS
61 (i968) I I 5 - I I 6 ;
© NORTH-HOLLAND
PUBLISHING
CO.
L I T H I U M I N D U C E D N U C L E A R R E A C T I O N S AND T H E P R O D U C T I O N OF N E G A T I V E L I T H I U M I O N S * R. MIDDLETON, C. T. A D A M S and K. B E T H G E t
Tandem Accelerator Laboratory, Physics Department, University of Pennsylvania, Philadelphia, Pennsylvania, U.S.A. Received 25 January 1968 Lithium induced nuclear reactions in the energy range available with electrostatic accelerators provide a useful tool in nuclear structure studies. An easy method is described for obtaining large
currents of negative charged lithium ions for tandem Van de G raaff accelerators.
Recent advances in nuclear structure theory and experiment suggest that the two-hole four-particle and four-hole four-particle states may not be uncommon. Such states frequently cannot be directly excited by reactions involving the transfer of one or two nucleons but can be excited by reactions transferring four nucleons. The simplest and most promising reactions sal:isfying the latter requirement are the (TLi,t) and (6 Li,d) reactions. Since the binding energy of the alpha-particle in 6Li and 7Li is only 1.47 and 2.47 MeV respectively, it seems likely that both reactions will proceed directly at intermediate energies. This appears to be confirmed experimentally in the case of the (TLi,t) reaction by recent measurements made at Heidelberg 1) and this laboratoryZ). The mechanism of the (6Li,d) reaction, however, remains obscure and in any event its utility may be limited by the three-body break-upS). For the study of light and intermediate mass nuclei the most suitable lithium energy is in the range 12 to about 28 MeV. Such energies are just within the range. and most conveniently obtainable from the present generation of tandem accelerators provided that a convenient source of negative ions is available. Production of negative lithium ions: As far as we are aware, negative lithium ion sources have been under development at Heidelberg, Saclay and in this laboratory. Bethge and collaborators at Heidelberg have developed a direct extraction system using a Penning type source4). Negative lithium beams of about 1 /~A were obtained enabling an analyzed beam of about 0.2/~A of Li + ÷ + to be obtained through an H V E C ENtandem. At Saclay a filament type ion source has been developed to produce intense beams of positive lithium ionsS). The intention was to form negative ions by charge exchange in propane. The final outcome of this work is not known. The approach in this laboratory was similar to that at Saclay with the exception that charge exchange was performed in lithium vapor. At exchange voltages in the range 30 to 40 kV negative
lithium beams of up to 3/zA were obtained and analyzed beams of Li + + + in excess of 1/zA. The reliability of the three systems briefly described was not high and all suffered to some extent from the many problems associated with operating an ion source on lithium vapor. To obviate these difficulties it was recently decided to try making negative lithium ions using the method first described by Gentner and Hortig6). In this method, the element whose negative ion is required is leaked in gaseous form into the adder canal and then bombarded with 40 to 50 kV heavy ions such as krypton. Although the mechanism for the formation of the negative ion is not well understood it is generally believed to be one of dissociation of a molecule. Prior to this work, the method had been restricted to elements available in gaseous form. This restriction has been removed by the introduction of the optical bench source, a detailed description of which can be found in a recent University of Pennsylvania laboratory report by Middleton and AdamsT). The optical bench construction allows the normal adder canal to be replaced with a canal-boat combination, both of which are indirectly heated. Thus, a metal or compound placed in the boat can be vaporized and leaked into the adder canal. The optical bench system was originally developed for the production of intense H e - beams by charge exchange in lithium vapor. To adapt the system for producing L i - by the method of Gentner and Hortig, it was necessary only to change the source gas. Several gases have been tried and the observed L i - current in microampere are presented in table 1 as a function of exchange voltage. It is noteworthy that although the best yield of L i was obtained from argon, hydrogen may in many respects be preferable. Certainly the life of the duo-
115
* Work supported by the National Science Foundation. t On leave of absence from the University of Heidelberg,
Germany.
116
R. MIDDLETON et al. TABLE 1
Exchange voltage (kV)
Hydrogen
Neon
Argon
Krypton
36 40 45 50
0.8 0.96 1.25 1.50
0.15 0.25 0.20
1.2 1.5 1.7 2.1
0.8 1.1 1.2
p l a s m a t r o n filament is longer a n d less erosion of the intermediate electrode occurs. The measurements we have made give little or no indication of the m e c h a n i s m responsible for the formation of negative lithium ions. Since it is k n o w n that the percentage of Li/ molecules in lithium vapor increases with temperatureS), it seems likely that it m a y be due to the dissociation of a diatomic molecule. This belief is strengthened by the fact that an a t t e m p t to make negative m a g n e s i u m ions by a similar method was unsuccessful and m a g n e s i u m vapor is t h o u g h t to be entirely monoatomic9).
The method described provides an extremely convenient source of negative 6Li and 7Li ions. It would also appear to have wide application for p r o d u c i n g other negative ions species. References 1) K. Bethge, K. Meier-Ewert, K. Pfeiffer and R. Bock, Phys. Letters 24B (1967) 663. 2) R. Middleton, B. Rosner, D. Pullen and L. Polsky, Phys. Rev. Letters 20 (1968) 118. 3) K. Meier-Ewert, K. Bethge and K. O. Pfeiffer, Nuclear Physics, to be published; K. Bethge and K. Meier-Ewert, Phys. Rev. Letters 18 0967) 1010. 4) E. Heinicke, K. Bethge and H. Baumann, Nucl. lnstr, and Meth. 58 (1968) 125. 2) B. Delaunay-Olkowski, L. Bianchi, J. P. Meurgues and J. Petres, J. Phys. 24 (1963) 1012. 6) W. Gentner and G. Hortig, Z. Physik 172 (1963) 353. 7) R. Middleton and C. T. Adams, Lab. Rep. Univ. of Pennsylvania (1967). 8) An. N. Nesmeyanov, Vapour pressure o f the elements (Acad. Press, N.Y. 1963). 9) G. Herzberg, Molecular spectra and molecular structure (Van Nostrand, Princeton, 1961).