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Preparation of targets with the electromagnetic isotope separator in Strasbourg
NUCLEAR
INSTRUMENTS
AND
METHODS
38 (I965) 79-8I; © N O R T H - H O L L A N D
PUBLISHING
CO.
PREPARATION OF TARGETS WITH THE ELECTROMAGNETIC ISOTOPE SEPARATOR I N STRASBOURG S. G O R O D E T Z K Y ,
J. D E N I M A L , C. R I C A U D , B. R A B I N a n d R. A R M B R U S T E R
lnstitut de Recherches Nucl~aires, Strasbourg, France Since September 1963 a n electromagnetic isotope s e p a r a t o r has been operated at the Institute o f Nuclear Research. It is a onestage Bernas type separator. Its m a i n use is the preparation o f targets. T h e isotopes are collected either on metallic foils, or on
self s u p p o r t i n g carbon films. In this latter case a retarding lens is used in order to collect at energies lower t h a n 9 keV on films of 30 to 50 # g / c m 2 thickness.
1. Description of the machine Fig. 1 shows the general lay-out of the separator. The machine is similar to the one at Orsay ~) and was built by the same firm. Several modifications have however been introduced to the separator, since the machine was put into operation; the most important are: z-focusing is achieved and a scanning needle makes it possible to control the beam just before it enters into the analyser.
scanning needle placed in front of the analyser, it is possible, by means of an oscilloscope, to control the type of extraction and the angular spread and orientation of the beam. Fig. 2 shows a block diagram of the scanning system. The tungsten needle oscillates in a vertical plane. The motion of the needle is kept linear with time by means of a suitable cam. On the same axis as the motor and the cam, a continuous rotating potentiometer is mounted which provides a voltage that is also linear with time. This voltage is fed to the horizontal deflecting plates of the oscilloscope while vertical deflection depends on the current to the needle. A momentary short-circuit of the needle gives a measurement of the orientation of the beam relative to the axis of the separator. Fig. 3 shows the distribution of ions corresponding to the three types of extraction.
1.1. SCANNING NEEDLE Rautenbach 2) and mainly Chavet 3) have studied the extraction of ion beams from a concave and a convex meniscus. In the latter case the quality of the image is better than what is obtained by extracting the b e a m - as it is usually d o n e - f r o m a concave meniscus. The distribution of ions in any cross-section of the beam depends on the type of extraction. By use of a
Fig. 1. General lay-out of the separator.
79 II. I O N O P T I C S ,
ETC.
S. G O R O D E T Z K Y et al.
80
lXialltiometer
beam ~ teed ~
2 Fig. 2. Block diagram of the scanning needle.
for the study of nuclear reactions with the 2 and 3 MV Van de Graaff accelerators of the Institute of Nuclear Research, and the 5.5 MV Van de Graaff generator of the Center of Nuclear Research. 2.1. METALLICBACKINGS The metals Ni, AI, Ta are most frequently used as backings, and the usual thickness is 500 pg/cm 2. To avoid destruction of the foils by heating, they are pressed, generally by means of vacuum grease, onto a water cooled brass holder (fig. 5). It is possible without trouble to collect up to 100/~A during several hours. A liquid nitrogen trap, mounted in the collector chamber, prevents the deposition of carbon compounds
(a) (b) (c) Fig. 3. Ionic distribution corresponding to a) concave meniscus; b) plane meniscus; c) convex meniscus.
from the diffusion pumps on the backings. The holder may be oscillated, in order to improve the uniformity of the deposit. Targets containing isotopes of N, Ne, Mg, Ca, K and T1 have been prepared. 14N and 15N targets have been used by Bassompierre 5) and Brochard6); for this latter we collected 1.2 pA during 7 h 45 min and obtained 10 pg/cm 2. 2.2. SELF SUPPORTING CARBON FILM BACKINGS
To avoid destruction of these very fragile backings the beam was slowed down, to energies between 0 and l0 keV, as has been done elsewhere 7-9). The retarding
faradaycage Fig. 4. Top view of a beam with z-focusing. 1.2. Z-FOCUSING This type of focusing is obtained in our separator, by the use of curved emission and extraction electrodes3'*). With this arrangement a source slit of 4 cm length gives rise to an image of 10 mm length, instead of about 8 cm (fig. 4). 2. Colleeting devices The separator is used in the preparation of targets
atuminiumframe
Fig. 5. Collector for metallic backings.
81
P R E P A R A T I O N OF TARGETS TABLE 1 Data for 48Ca targets, a, without z-focusing; b, with z-focusing a
Thickness of the film Collection energy Thickness of the target
b
55 _+ 5 #g/cm 2 4 keV (during 6 h) 2.5 + 0.5/~g/cm2
lens is schematically shown in fig. 6. Plate 1 is biased up to 600 V to prevent secondary electrons from reaching the collector. Cage 2 is grounded and the high voltage is furnished by an auxiliary supply, which is also used for post-acceleration. The carbon films are mounted between two guard plates in order to keep the films flat. Collections are
40 _+ 4 #g/cm2 7.7 keV (during 1 h 45) 7 + 1.4/~g/cm2
made at energies from 2 to 8.3 keV, on films of thicknesses between 30-60/~g/cm 2. The shape of the collector sufficiently compensates the effect of space charge to allow for the retardation of a 600/tA beam down to 0.5 keV. With this device we mainly prepared 48Ca targets, the charge material being natural calcium. The features of the targets have been studied by elastic scattering of protons. As a conclusion the data for two 48Ca targets is given in table 1. References
••\\\\\\•
1) z) 3) 4) 5) 6) \ '~" \ ' ~ , ~ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ '
~
lcm Fig. 6. Retarding lens system. 1, biased plate; 2. earthed plate; 3 and 4, guard plates; 5, carbon film.
7) s)
9) 10)
R. Bernas, Th/~se (Paris, 1954). W. L. Rautenbach, Nucl. Instr. and Meth. 12 (1961) 169. 1. Chavet, Th6se (Paris, 1965). W. G. Cross, Rev. Sci. Instr. 22 (1951) 717. M. von Ardenne, Kernenergie 3 (1962) 1177. G. Bassompierre, Th6se Doctorat (Universit6 de Strasbourg, 1965). F. Brochard, Th6se 3e Cycle (Universit6 de Strasbourg, 1964). G. Sidenius and O. Skilbreid in Electromagnetic Separation of Radioactive Isotopes (ed. M. J. Higatsberger and F. P. Viehb6ck, Springer Verlag, Vienna, 1961) p. 234. I. Bergstrom, F. Brown, J. A. Davies, J. S. Geiger, R. L. Graham and R. Kelly, NucL Instr. and Meth. 21 (1963) 249. j. Uhler, Ark. Fys. 25 (1963) 349.
1|. ION OPTICS, ETC.
INSTRUMENTS
AND
METHODS
38 (I965) 79-8I; © N O R T H - H O L L A N D
PUBLISHING
CO.
PREPARATION OF TARGETS WITH THE ELECTROMAGNETIC ISOTOPE SEPARATOR I N STRASBOURG S. G O R O D E T Z K Y ,
J. D E N I M A L , C. R I C A U D , B. R A B I N a n d R. A R M B R U S T E R
lnstitut de Recherches Nucl~aires, Strasbourg, France Since September 1963 a n electromagnetic isotope s e p a r a t o r has been operated at the Institute o f Nuclear Research. It is a onestage Bernas type separator. Its m a i n use is the preparation o f targets. T h e isotopes are collected either on metallic foils, or on
self s u p p o r t i n g carbon films. In this latter case a retarding lens is used in order to collect at energies lower t h a n 9 keV on films of 30 to 50 # g / c m 2 thickness.
1. Description of the machine Fig. 1 shows the general lay-out of the separator. The machine is similar to the one at Orsay ~) and was built by the same firm. Several modifications have however been introduced to the separator, since the machine was put into operation; the most important are: z-focusing is achieved and a scanning needle makes it possible to control the beam just before it enters into the analyser.
scanning needle placed in front of the analyser, it is possible, by means of an oscilloscope, to control the type of extraction and the angular spread and orientation of the beam. Fig. 2 shows a block diagram of the scanning system. The tungsten needle oscillates in a vertical plane. The motion of the needle is kept linear with time by means of a suitable cam. On the same axis as the motor and the cam, a continuous rotating potentiometer is mounted which provides a voltage that is also linear with time. This voltage is fed to the horizontal deflecting plates of the oscilloscope while vertical deflection depends on the current to the needle. A momentary short-circuit of the needle gives a measurement of the orientation of the beam relative to the axis of the separator. Fig. 3 shows the distribution of ions corresponding to the three types of extraction.
1.1. SCANNING NEEDLE Rautenbach 2) and mainly Chavet 3) have studied the extraction of ion beams from a concave and a convex meniscus. In the latter case the quality of the image is better than what is obtained by extracting the b e a m - as it is usually d o n e - f r o m a concave meniscus. The distribution of ions in any cross-section of the beam depends on the type of extraction. By use of a
Fig. 1. General lay-out of the separator.
79 II. I O N O P T I C S ,
ETC.
S. G O R O D E T Z K Y et al.
80
lXialltiometer
beam ~ teed ~
2 Fig. 2. Block diagram of the scanning needle.
for the study of nuclear reactions with the 2 and 3 MV Van de Graaff accelerators of the Institute of Nuclear Research, and the 5.5 MV Van de Graaff generator of the Center of Nuclear Research. 2.1. METALLICBACKINGS The metals Ni, AI, Ta are most frequently used as backings, and the usual thickness is 500 pg/cm 2. To avoid destruction of the foils by heating, they are pressed, generally by means of vacuum grease, onto a water cooled brass holder (fig. 5). It is possible without trouble to collect up to 100/~A during several hours. A liquid nitrogen trap, mounted in the collector chamber, prevents the deposition of carbon compounds
(a) (b) (c) Fig. 3. Ionic distribution corresponding to a) concave meniscus; b) plane meniscus; c) convex meniscus.
from the diffusion pumps on the backings. The holder may be oscillated, in order to improve the uniformity of the deposit. Targets containing isotopes of N, Ne, Mg, Ca, K and T1 have been prepared. 14N and 15N targets have been used by Bassompierre 5) and Brochard6); for this latter we collected 1.2 pA during 7 h 45 min and obtained 10 pg/cm 2. 2.2. SELF SUPPORTING CARBON FILM BACKINGS
To avoid destruction of these very fragile backings the beam was slowed down, to energies between 0 and l0 keV, as has been done elsewhere 7-9). The retarding
faradaycage Fig. 4. Top view of a beam with z-focusing. 1.2. Z-FOCUSING This type of focusing is obtained in our separator, by the use of curved emission and extraction electrodes3'*). With this arrangement a source slit of 4 cm length gives rise to an image of 10 mm length, instead of about 8 cm (fig. 4). 2. Colleeting devices The separator is used in the preparation of targets
atuminiumframe
Fig. 5. Collector for metallic backings.
81
P R E P A R A T I O N OF TARGETS TABLE 1 Data for 48Ca targets, a, without z-focusing; b, with z-focusing a
Thickness of the film Collection energy Thickness of the target
b
55 _+ 5 #g/cm 2 4 keV (during 6 h) 2.5 + 0.5/~g/cm2
lens is schematically shown in fig. 6. Plate 1 is biased up to 600 V to prevent secondary electrons from reaching the collector. Cage 2 is grounded and the high voltage is furnished by an auxiliary supply, which is also used for post-acceleration. The carbon films are mounted between two guard plates in order to keep the films flat. Collections are
40 _+ 4 #g/cm2 7.7 keV (during 1 h 45) 7 + 1.4/~g/cm2
made at energies from 2 to 8.3 keV, on films of thicknesses between 30-60/~g/cm 2. The shape of the collector sufficiently compensates the effect of space charge to allow for the retardation of a 600/tA beam down to 0.5 keV. With this device we mainly prepared 48Ca targets, the charge material being natural calcium. The features of the targets have been studied by elastic scattering of protons. As a conclusion the data for two 48Ca targets is given in table 1. References
••\\\\\\•
1) z) 3) 4) 5) 6) \ '~" \ ' ~ , ~ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ '
~
lcm Fig. 6. Retarding lens system. 1, biased plate; 2. earthed plate; 3 and 4, guard plates; 5, carbon film.
7) s)
9) 10)
R. Bernas, Th/~se (Paris, 1954). W. L. Rautenbach, Nucl. Instr. and Meth. 12 (1961) 169. 1. Chavet, Th6se (Paris, 1965). W. G. Cross, Rev. Sci. Instr. 22 (1951) 717. M. von Ardenne, Kernenergie 3 (1962) 1177. G. Bassompierre, Th6se Doctorat (Universit6 de Strasbourg, 1965). F. Brochard, Th6se 3e Cycle (Universit6 de Strasbourg, 1964). G. Sidenius and O. Skilbreid in Electromagnetic Separation of Radioactive Isotopes (ed. M. J. Higatsberger and F. P. Viehb6ck, Springer Verlag, Vienna, 1961) p. 234. I. Bergstrom, F. Brown, J. A. Davies, J. S. Geiger, R. L. Graham and R. Kelly, NucL Instr. and Meth. 21 (1963) 249. j. Uhler, Ark. Fys. 25 (1963) 349.
1|. ION OPTICS, ETC.