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On producing negative ion beams of magnesium and calcium in a cesium sputter source
NUCLEAR
INSTRUMENTS
AND
METHODS
141
(1977) 3 7 3 - 3 7 5 :
:~
NORTH-HOLLAND
PUBLISHING
CO.
ON P R O D U C I N G NEGATIVE I O N BEAMS OF M A G N E S I U M AND C A L C I U M IN A C E S I U M S P U T T E R SOURCE* R. M I D D L E T O N
Physics Department, UniversiO, ~[' Pennsylvania, Philadelphia, Pennsylvania 19174, U.S.A. Received 24 N o v e m b e r 1976 Moderately intense negative ion b e a m s o f m a g n e s i u m and calcium hydrides have been generated by cesium beam sputtering o f metallic cones while being sprayed with a m m o n i a gas. T h e o p t i m u m yields for M g H - and MgH~- were 0.5 and 1.65 u A respectively and for C a l l a n d CaH~" 0.6 IrA in both cases.
Interest in the study of heavy ion induced nuclear reactions has increased dramatically in recent years, particularly in laboratories having tandem accelerators. Although electrostatic technology has improved greatly, voltage limitations confine present nuclear interests to ions with mass <60. Of particular interest are reactions initiated by the magnesium and calcium i s o t o p e s - t h e former because of its highly deformed shape and the latter not only because it has a doubly closed shell but also since its heaviest isotope, 48Ca, is exceptionally neutron rich. Unfortunately the electron affinity of both elements is known to be < 0 t) and this compounds the difficulty of generating intense negative ion beams suitable for acceleration in a tandem. The first fairly intense beam of calcium negative ions was produced by Kutschera and Korschinek 2) using the method of Gentner and Hortig3). In their case the adder canal contained calcium vapor and the primary beam was h e l i u m - the best currents obtained were about 0.6 pA. Since then several other laboratories have accelerated calcium using the same method but usually with lower currents and at least one 4) has used the same technique to produce a weak beam of magnesium negative ions. It would seem highly likely that magnesium and calcium negative ions are formed in a metastable state. During a recent survey of negative ions produced by cesium beam sputtering, magnesium and calcium were studied in some detail. In this work a source similar to that shown in fig. 4 of ref. 5 was used. The negative ion beam emerging from the cone was restricted in divergence to + 3 ° and then focussed with an einzel lens, having a magnification of 3.5, onto a set of slits. The latter, which for current measuring purposes were set at +3.8 mm, formed the object slits for a 30 cm double focussing 90 ° analyzing magnet. The * W o r k supported by a National Science F o u n d a t i o n Grant.
magnesium and calcium sputter cones were turned from metal bar to the dimensions shown in fig. 1. In both cases no peaks of significant intensity were observed that could be positively identified as corresponding to elemental negative ions. This came as no surprise since it was not expected that the cesium sputter process would generate metastable negative ions which probably require a two step excitation process. Spectra were then recorded while hydrogen gas was being sprayed onto the inner surfaces of the sputter cones - the partial pressure of hydrogen in the source was gradually increased from about i 0 - 6 to 2 x 10- 5 tort. No peaks with an intensity greater than a few nano amps. could be identified as corresponding to magnesium and calcium hydrides. This result came as a considerable surprise since it is known that MgH has a fairly large electron affinity, 1.08 eV 6). Since this technique had been used very successfully to form the hydrides of many other elements (e.g. 0.4 IrA of 56Fell) it was concluded that the interaction between H 2 and Mg and Ca must be very weak and it was decided to try a cone made out of magnesium hydride. Such a cone was made by pressing magnesium hydride powder to the dimensions of fig. 1 - a little methylmethacrylate was added as a binder. After several hours of operation with this cone, during which the
16 mm
i
/
[ iOImm IOmm~
Fig. I. Showing the dimensions o f the m a g n e s i u m and calcium sputter cones which were turned from metal bar.
374
a. MIDDLETON
vacuum improved from about 5 x 10- 5 to 2 x 10 -6 tort, no positive identification of M g H - could be made. It is thought likely that the magnesium hydride was decomposing. As part of the systematic investigation oxygen gas was also sprayed onto the metallic magnesium and calcium cones to enhance the already observed oxide beams. The MgO beam increased to about I yA and the CaO beam to 0.35 y A at partial pressures in the range (5-8) x l 0 - 6 t o r r . Both of these beams are usuable in a tandem accelerator but the increased mass and consequent energy reduction at the terminal renders both inferior to an elemental or hydride beam. Recently, during an attempt to reaffirm some previous measurements on the formation of N H ions, ammonia gas was first directed onto a titanium cone and then onto a calcium cone. The latter not only gave a stronger N H - beam but the spectrum exhibited two strong peaks which were subsequently identified to be 4°Call and 4°Call3. A portion of a spectrum obtained with an ammonia partial pressure of 7 x 10 -6 tort is shown in fig. 2. At this pressure the intensities of the 4°Call and *°Call3 peaks are roughly equal and are about 0.6yA. Positive identification of these ions has not been achieved by acceleration through a tandem but is reasonably sure since peaks were observed corresponding to a4CaH and 44CaH3 close to the correct isotopic ratio. Also evident in fig. 2 are relatively weak peaks corresponding to 4°CaHz, the natural hydride of calcium, CaN,
C a N H and CaNH2, the latter coinciding and indistinguishable from CaO. Variation of the flow rate of ammonia revealed that at low pressures Calla was strongest, at about 7 x 10- 6 torr both Call and Call 3 attained maximum strength and at higher pressures both peaks declined but with Call being the stronger. The source was operated for several hours with an ammonia partial pressure of about 7 x l 0 - O t o r r . Operation remained extremely steady and the ammonia did not appear to adversely affect the efficiency of the porous tungsten ionizer. The intensity of the N H beam was about 0.3 IrA at 7 x l0 -6 torr and increased to about 0.5/tA at 2 x 10 - s torr. The negative hydrogen beam was particularly intense and increased from 10/~A at low pressures to in excess of 20/~A at 2 x 10 -5 torr. Similar measurements have been made with the metallic magnesium cone and a portion of a spectrum recorded at a partial pressure of about 6 x 10- o torr is shown in fig. 3. The general features are remarkably similar to those of calcium with the exception that the intensity of the triple hydride beam, 1.65 yA, was about three times that of the single hydride. Increasing the flow of ammonia had little effect on the strength of the MgH beam but resulted in a steady decline in MgH 3 up to a pressure of about 2x 10 -5 torr when both beams began to decline. As for calcium positive identification by acceleration has not been made but seems reasonably certain since the intensities of the
2.0
NH2+O CoH
24MQH3
:k v
0.5
:k
1.5 --
lZ I.~
z
CN
I Z
Z
o W
>
n,, E) lad
Ca NH2-1.-CaO
1.0
o 0.25
O,.i-NH2
W
W Z
b I
"i' I/ll;,c]\{ /
Z
0.5-
Ni
I?;
Fig. 2. A p o r t i o n o f a n e g a t i v e i o n s p e c t r u m o b t a i n e d w h i l e ammonia gas was being sprayed on a metallic calcium cone; t h e p a r t i a l p r e s s u r e o f a m m o n i a in t h e s o u r c e w a s 7 × 10 - 6 t o r t .
Fig. 3. P a r t o f t h e n e g a t i v e ion s p e c t r u m f r o m m a g n e s i u m p l u s a m m o n i a g a s ; t h e p a r t i a l p r e s s u r e o f a m m o n i a w a s 6 x 10 - 6 t o r r .
P R O D U C I N G N E G A T I V E ION BEAMS p e a k s c o r r e s p o n d i n g to masses 27, 28, 29 p r e d o m i n a n t l y Z 4 M g- H 3, 2 S M g H 3 a n d 2 6 M g H 3 are closely in a g r e e m e n t w i t h the i s o t o p i c ratios. T h e intensities o f H- and NH- both increased with increasing ammonia flow a n d m a x i m i z e d at a b o u t 2 × 10 5 torr, the c u r r e n t s w e r e 23 a n d 0.7 p A respectively.
References ~) w . c .
Lineberger, Int. Conf. on Heat,y ion sources, IEEE
375
Trans. Nucl. Sci. NS-23 (1976) 934. 2) W. Kutschera and G. Korschinek, Proc. 2nd Int. Conf. on Ion sources (eds. F. ViehbOck, H. Winter and M. Bruck; IAEA, Vienna, 1972) p. 908. a) W. Gentner and G. Hortig, Z. Physik 172 (1963) 353. 4) T. Lund, University of Rochester, private communication. s) R. Middleton, Int. Conf. on Heavy ion sources, IEEE Trans. Nucl. Sci. NS-23 (1976) 1098. 6) B. Steiner, Case studies in atomic collision physics (ed. E. W. McDaniel; North-Holland Publ. Co., Amsterdam, 1972) vol. 7, p. 536.
INSTRUMENTS
AND
METHODS
141
(1977) 3 7 3 - 3 7 5 :
:~
NORTH-HOLLAND
PUBLISHING
CO.
ON P R O D U C I N G NEGATIVE I O N BEAMS OF M A G N E S I U M AND C A L C I U M IN A C E S I U M S P U T T E R SOURCE* R. M I D D L E T O N
Physics Department, UniversiO, ~[' Pennsylvania, Philadelphia, Pennsylvania 19174, U.S.A. Received 24 N o v e m b e r 1976 Moderately intense negative ion b e a m s o f m a g n e s i u m and calcium hydrides have been generated by cesium beam sputtering o f metallic cones while being sprayed with a m m o n i a gas. T h e o p t i m u m yields for M g H - and MgH~- were 0.5 and 1.65 u A respectively and for C a l l a n d CaH~" 0.6 IrA in both cases.
Interest in the study of heavy ion induced nuclear reactions has increased dramatically in recent years, particularly in laboratories having tandem accelerators. Although electrostatic technology has improved greatly, voltage limitations confine present nuclear interests to ions with mass <60. Of particular interest are reactions initiated by the magnesium and calcium i s o t o p e s - t h e former because of its highly deformed shape and the latter not only because it has a doubly closed shell but also since its heaviest isotope, 48Ca, is exceptionally neutron rich. Unfortunately the electron affinity of both elements is known to be < 0 t) and this compounds the difficulty of generating intense negative ion beams suitable for acceleration in a tandem. The first fairly intense beam of calcium negative ions was produced by Kutschera and Korschinek 2) using the method of Gentner and Hortig3). In their case the adder canal contained calcium vapor and the primary beam was h e l i u m - the best currents obtained were about 0.6 pA. Since then several other laboratories have accelerated calcium using the same method but usually with lower currents and at least one 4) has used the same technique to produce a weak beam of magnesium negative ions. It would seem highly likely that magnesium and calcium negative ions are formed in a metastable state. During a recent survey of negative ions produced by cesium beam sputtering, magnesium and calcium were studied in some detail. In this work a source similar to that shown in fig. 4 of ref. 5 was used. The negative ion beam emerging from the cone was restricted in divergence to + 3 ° and then focussed with an einzel lens, having a magnification of 3.5, onto a set of slits. The latter, which for current measuring purposes were set at +3.8 mm, formed the object slits for a 30 cm double focussing 90 ° analyzing magnet. The * W o r k supported by a National Science F o u n d a t i o n Grant.
magnesium and calcium sputter cones were turned from metal bar to the dimensions shown in fig. 1. In both cases no peaks of significant intensity were observed that could be positively identified as corresponding to elemental negative ions. This came as no surprise since it was not expected that the cesium sputter process would generate metastable negative ions which probably require a two step excitation process. Spectra were then recorded while hydrogen gas was being sprayed onto the inner surfaces of the sputter cones - the partial pressure of hydrogen in the source was gradually increased from about i 0 - 6 to 2 x 10- 5 tort. No peaks with an intensity greater than a few nano amps. could be identified as corresponding to magnesium and calcium hydrides. This result came as a considerable surprise since it is known that MgH has a fairly large electron affinity, 1.08 eV 6). Since this technique had been used very successfully to form the hydrides of many other elements (e.g. 0.4 IrA of 56Fell) it was concluded that the interaction between H 2 and Mg and Ca must be very weak and it was decided to try a cone made out of magnesium hydride. Such a cone was made by pressing magnesium hydride powder to the dimensions of fig. 1 - a little methylmethacrylate was added as a binder. After several hours of operation with this cone, during which the
16 mm
i
/
[ iOImm IOmm~
Fig. I. Showing the dimensions o f the m a g n e s i u m and calcium sputter cones which were turned from metal bar.
374
a. MIDDLETON
vacuum improved from about 5 x 10- 5 to 2 x 10 -6 tort, no positive identification of M g H - could be made. It is thought likely that the magnesium hydride was decomposing. As part of the systematic investigation oxygen gas was also sprayed onto the metallic magnesium and calcium cones to enhance the already observed oxide beams. The MgO beam increased to about I yA and the CaO beam to 0.35 y A at partial pressures in the range (5-8) x l 0 - 6 t o r r . Both of these beams are usuable in a tandem accelerator but the increased mass and consequent energy reduction at the terminal renders both inferior to an elemental or hydride beam. Recently, during an attempt to reaffirm some previous measurements on the formation of N H ions, ammonia gas was first directed onto a titanium cone and then onto a calcium cone. The latter not only gave a stronger N H - beam but the spectrum exhibited two strong peaks which were subsequently identified to be 4°Call and 4°Call3. A portion of a spectrum obtained with an ammonia partial pressure of 7 x 10 -6 tort is shown in fig. 2. At this pressure the intensities of the 4°Call and *°Call3 peaks are roughly equal and are about 0.6yA. Positive identification of these ions has not been achieved by acceleration through a tandem but is reasonably sure since peaks were observed corresponding to a4CaH and 44CaH3 close to the correct isotopic ratio. Also evident in fig. 2 are relatively weak peaks corresponding to 4°CaHz, the natural hydride of calcium, CaN,
C a N H and CaNH2, the latter coinciding and indistinguishable from CaO. Variation of the flow rate of ammonia revealed that at low pressures Calla was strongest, at about 7 x 10- 6 torr both Call and Call 3 attained maximum strength and at higher pressures both peaks declined but with Call being the stronger. The source was operated for several hours with an ammonia partial pressure of about 7 x l 0 - O t o r r . Operation remained extremely steady and the ammonia did not appear to adversely affect the efficiency of the porous tungsten ionizer. The intensity of the N H beam was about 0.3 IrA at 7 x l0 -6 torr and increased to about 0.5/tA at 2 x 10 - s torr. The negative hydrogen beam was particularly intense and increased from 10/~A at low pressures to in excess of 20/~A at 2 x 10 -5 torr. Similar measurements have been made with the metallic magnesium cone and a portion of a spectrum recorded at a partial pressure of about 6 x 10- o torr is shown in fig. 3. The general features are remarkably similar to those of calcium with the exception that the intensity of the triple hydride beam, 1.65 yA, was about three times that of the single hydride. Increasing the flow of ammonia had little effect on the strength of the MgH beam but resulted in a steady decline in MgH 3 up to a pressure of about 2x 10 -5 torr when both beams began to decline. As for calcium positive identification by acceleration has not been made but seems reasonably certain since the intensities of the
2.0
NH2+O CoH
24MQH3
:k v
0.5
:k
1.5 --
lZ I.~
z
CN
I Z
Z
o W
>
n,, E) lad
Ca NH2-1.-CaO
1.0
o 0.25
O,.i-NH2
W
W Z
b I
"i' I/ll;,c]\{ /
Z
0.5-
Ni
I?;
Fig. 2. A p o r t i o n o f a n e g a t i v e i o n s p e c t r u m o b t a i n e d w h i l e ammonia gas was being sprayed on a metallic calcium cone; t h e p a r t i a l p r e s s u r e o f a m m o n i a in t h e s o u r c e w a s 7 × 10 - 6 t o r t .
Fig. 3. P a r t o f t h e n e g a t i v e ion s p e c t r u m f r o m m a g n e s i u m p l u s a m m o n i a g a s ; t h e p a r t i a l p r e s s u r e o f a m m o n i a w a s 6 x 10 - 6 t o r r .
P R O D U C I N G N E G A T I V E ION BEAMS p e a k s c o r r e s p o n d i n g to masses 27, 28, 29 p r e d o m i n a n t l y Z 4 M g- H 3, 2 S M g H 3 a n d 2 6 M g H 3 are closely in a g r e e m e n t w i t h the i s o t o p i c ratios. T h e intensities o f H- and NH- both increased with increasing ammonia flow a n d m a x i m i z e d at a b o u t 2 × 10 5 torr, the c u r r e n t s w e r e 23 a n d 0.7 p A respectively.
References ~) w . c .
Lineberger, Int. Conf. on Heat,y ion sources, IEEE
375
Trans. Nucl. Sci. NS-23 (1976) 934. 2) W. Kutschera and G. Korschinek, Proc. 2nd Int. Conf. on Ion sources (eds. F. ViehbOck, H. Winter and M. Bruck; IAEA, Vienna, 1972) p. 908. a) W. Gentner and G. Hortig, Z. Physik 172 (1963) 353. 4) T. Lund, University of Rochester, private communication. s) R. Middleton, Int. Conf. on Heavy ion sources, IEEE Trans. Nucl. Sci. NS-23 (1976) 1098. 6) B. Steiner, Case studies in atomic collision physics (ed. E. W. McDaniel; North-Holland Publ. Co., Amsterdam, 1972) vol. 7, p. 536.