- Email: [email protected]
Preparation techniques for some exotic nuclear accelerator targets and isotopic sputter-ion source targets
Nuclear Instruments and Methods 200 (1982) 113-120 North-Holland Publishing Company
113
Part V. Miscellaneous methods of target preparation P R E P A R A T I O N T E C H N I Q U E S FOR S O M E EXOTIC NUCLEAR ACCELERATOR TARGETS AND I S O T O P I C S P U T T E R - I O N S O U R C E TARGETS H.J. MAIER Sektion Physik, Universiti~t Miinchen, 8046 Garching, Fed. Rep. Germany
This paper presents a survey of some recent preparative techniques which were developed at the target laboratory of the University of Mianchen. These techniques include the production of sputter-ion source targets of 3°Si and 365, the synthesis of H36S for use in a gas target, high efficiency evaporation-condensation of rare and exotic isotopes, e.g. 48Ca and 248Cm, as well as the rolling of 238U and other reactive metal foils.
1. Sputter targets f o r u s e in i o n s o u r c e s
ARGON SiC2 + 2Mg ~ S i
+ 2MgO + 292.18 kJ
1275
Because of their low consumption of target material, cesium beam type sputter-ion sources are widely applied at the Garching Tandem Laboratory [1]. The so-called backsputter geometry has proved to be especially economic and is therefore utilized for the production of tandem accelerated beams of exotic isotopes [2,3]. Sputter targets appropriate for application in the backsputter geometry are small pellets of spherical or lenticular shape with a mass of approximately 50 mg. The present paper describes the preparation of isotopically enriched sputter targets of 3°Si and 365.
-VIT. CARBON CRUCIBLE REACTION
REACTION MIXTURE, PELLETIZED WITH 1470 MPa
~
2 5 m ~" / ~ j -I Io 10ram
T R E A T M E N T OF R E A C T I O N PRODUCTS
HOT (335K) DILUTE HCI: HOT (355K) AQUA DEST: VACUUM, 1.3 Pa, 295K VACUUM, 1.3 X 10 5 Pa, 1275K
12 hours 1 hour 30 rain 1Stain
1 F'NEGRA'N,BROWN30S'POWDER I COMPACTION IN VACUUM PRESSING TOOL
1.1. 3°Si target
1 3°S,TABLET I VACUUM ELECTRON BEAM MELTING
Enriched 3°Si (95.2%) is available from the Oak Ridge National Laboratory (ORNL) in the chemical form of 3°SIO2. To produce a sputter target pellet of this material, a reduction to the elemental form was necessary. From the literature [4], aluminium is known to be the standard reductant for SiO 2. However, since it is easier to separate MgO rather than A1203 from the reaction products, Mg was selected as the deoxidizing agent. The reduction procedure is outlined in fig. 1. First, a reaction mixture is formed using stoichiometric amounts of SiO 2 and Mg powders with a 5% excess of Mg. A large excess of Mg must be avoided to preclude possible formation of Mg2Si instead of Si. The reaction mixture is pressed under vacuum to form a tablet by applying a pressure of 1470 MPa. The reduction is performed
I 30Si SPUTTER PELLET I
O V E R A L L E F F I C I E N C Y O F PROCEDURE:
70%
TYPICAL 30Si SPUTTER PELLET DATA: 40 50rag, SPHERICALLY SHAPED, ¢ 3-3.5,rrtm
Fig. 1. Sputter pellet preparation of 3°Si.
by heating the reaction tablet in an argon atmosphere to 1275 K for a period of 10 min. A crucible of vitreous carbon is used for this purpose. The reaction products are subjected to a 12 h treatment with dilute HC1 to extract the reaction product MgO. Subsequently the material is carefully washed with hot water, vacuum dried at 1.3 Pa, and heated to about 1275 K in a vacuum
016%5087/82/0000-0000/$02.75 © 1982 North-Holland
v. MISCELLANEOUSMETHODS
114
H.J. Maier / Some exotic nuclear accelerator targets
chamber held at 1.3 × 10 - s Pa. This latter treatment removes possible residues HC1. The result of this procedure is a fine-grained, brown powder of 3°Si, which is processed into the desired sputter pellet by vacuum electron beam melting. The overall yield of the procedure is 70%. A typical 3°Si sputter pellet has a weight of 40-50 mg and a diameter of about 3 to 3.5 mm.
Fe36SPELLET FROM ELEMENTAL 36S M.P. (KI 36B5 392 B.P {K) 7176
1463
DIRECT REACT(ON: Fe + 36 S FU$~EEFe 36S
I ..................... Fe s
I ..................
1.2. ~6S target Because of its high neutron content, 36S is an experimentally interesting nucleus and is frequently used as a projectile in the Garching Tandem Laboratory. Elemental 36S enriched to 81.1% is available at a reasonable price from Rohstoff-Einfuhr, Dtisseldorf, West Germany. Sulfur-36 cannot be used in its elemental form as a target material in a sputter-type ion source because of its low melting point and sublimation temperature. However, since a 32S beam has successfully been produced using a sputter-target machined from commercially available natural FeS [1] a search was made for a suitable method to prepare a sufficient amount of Fe36S with a minimum loss of isotopic material. The phase diagram of the binary iron-sulfur [5] system suggested the fusion of stoichiometric amounts of the elements to form Fe36S. A small deviation from stoichiometry because of possible partial sublimation of the sulfur component during fusion would be expected to result in an alloy of Fe and Fe36S which is thermally stable. Accordingly, the procedure outlined in fig. 2 was chosen. Stoichiometric amounts of powdered Fe (grain size 5/~m) and 36S with a slight excess of Fe were carefully mixed. The mixture was transferred to a vacuum pressing apparatus and compacted into a tablet by applying a pressure of 1470 MPa. Fusion was accomplished by inert electrode arc melting in a purified argon atmosphere. Vacuum electron beam melting is not practicable in this case because it would lead to significant losses of 36S by evaporation. The are melting furnace was originally purchased from Leybold Heraeus G m b H , Hanau, West Germany, but has been modified in our laboratory in order to improve the purity of the argon atmosphere [6]. The operating data are listed in fig. 2. the efficiency of the procedure is approximately 90%.
REACTION D A T A ARGON PRESSURE ARC C U R R E N T : FUSING TIME EFFICIENCY TYPICAL PE LLET
I
I
INERT ELECTRODE ARC M E L T I N G
187 kPa 5 0 - 70A 2 3~ 85 90% 65rag Fe36S, 3mm~
HANDLE A R G O N SUPPLy
FLEXIBLE BELLOWS
TO PUMP WATER COOLED
Cu
ION MIXTURE
Fig. 2. Sputter pellet preparation of Fe36S.
A typical Fe36S pellet has a weight of 65 mg and a diameter of about 3 mm, and provides a 368 beam of 1 /~A for a period of about 100 h.
2. Preparation of H236S for use in gas targets For use as a gas target, elemental 36S must be converted into H2368. It is evident, that a direct reaction of hydrogen and sulfur would be the preferred preparative reaction and should produce a high purity as well as a high yield. However, the free energy of formation of H2S is only -33 k J / m o l as compared with -236 k J / m o l for H20, which can be formed spontaneously from the elements [7]. Therefore, in the case of H2S, it is necessary to use hydrogen in its more reactive atomic form instead of the molecular form to accomplish the desired reaction. A convenient method to produce atomic hydrogen is the dissociation of H 2 molecules in a glow discharge, which leads to a H-abundance of approximately 60% with a mean lifetime of 0.5 s for the individual H-atom [8]. The formation of H2S will take place according to the equation glow discharge
H~
~
2H,
2H -~'-368 ~ H236S. The apparatus used for this procedure is sketched
H.J. Maier / Some exotic nuclear accelerator targets FEED BACK LINE PRESSURECONTROLLER
....... I
I r,-
H' 'N ~
I :::7~
.... s ,.........
/
iOENTROL ~ PRESSURECONTROL VAL
~" --HV
REACTIONMECHANISM GLOW H2 DISCHARGE , 2H ' H2S
L
-
F~-
CATHODE /
/
............
s;:° EtE*:t E
~T~-~---
.............
' ~--~NO-D'~'E..........
J
3. Economic evaporation procedures for exotic isotopes
T~:. . . . . . . . . . ~ Hi (TO TURBO
PYREX20TUBE¢ mm
III LLN COLDTRAR ~J'~'~'~" N2 PYREX
OPERATINGCONDITIONS eH2 H2 THROUGHPUT HV
rIH) = 0.5 s~ H ABUNDANCE~ 60%
=
"J-I L
measuring the pressure in the cold trap after the latter has been warmed.
CAPACITIVE
t
-'-'
l/
2H+S
r
IONIZATION
Iff"
115
i REACTIONSPEED EFFICIENCY TYPICAL QUANTITY DEALT WITH
PROPERTIES of H2S
67 Pa
M.P. 190K
500n cm3/h~r
BP, 214K
150Ov De
10~A I mg S/mln
lm MO~
Fig. 3. Preparation of H2365.
in fig. 3. The reaction chamber is a ring shaped pyrex tube with an inner diameter of 20 mm. The cathodes are aluminum tubes of 40 m m diameter and an area of 100 cm 2 each. The powdered 36S is located in a quartz crucible. A pressure of 66.7 Pa H 2 is maintained in the reaction tube with the aid of a capacitive pressure gauge that is connected to an electronic pressure controller and a piezoelectric control valve. The glow discharge is ignited between one of the cathodes and the anode, which is located at the main pumping orifice. Operations are performed as described in the following procedure. The entire system is pumped to a pressure of 0.0013 Pa with a turbomolecular p u m p that is connected to the system with a 40 m m diameter quick p u m p down port. When the chamber is thoroughly degassed, hydrogen gas is caused to stream through the system by allowing it to enter through the piezoelectric valve while pumping through a bypass cold trap. Then the glow discharge is initiated. The atomic hydrogen reacts immediately with the sulfur powder to form gaseous H368, which is quantitatively condensed in the liquid nitrogen-cooled trap. Operating conditions are listed in fig. 3. The conversion rate is approximately 1 mg 36S per minute with a collection efficiency of approximately 90%. In one typical conversion, 1 × 10 3 mol of H36Sis produced. This corresponds to 22.4 ml S.T.P. The exact quantity is determined by
Since the cost for enriched isotopic material is steadily increasing, economic evaporation procedures have become more and more important. In this section, a vapor deposition assembly is described, which is suitable for preparing relatively thick targets, starting with quantities of enriched isotopes of the order of 1 mg, and which allows the recycling of isotopic material that is wasted during the procedure. A Mathis C H 7 tantalum crucible heater and a tube-shaped graphite crucible (7 m m internal diameter ×25 mm) are used as the evaporation source. The substrate holder is designed as a headpiece located on the top of the heater assembly so that it encloses the space between the crucible and the substrate. This design permits a quantitative collection of the evaporated material on a relatively small area, which is a necessary condition for the successful recycling of vapor-deposited material. A photograph of the whole assembly is shown in fig. 4. The material from which the substrate holder is constructed is chosen according to the chemical nature of the source material to be collected. In the case of actinide fluorides and alkaline earth metals, for example, copper is very convenient because it is not attacked by the solvents which are used to separate the condensed material from the substrate. A tube-shaped evaporation crucible may be referred to as a collimating type evaporation source which generates (in good approximation) a film thickness distribution on a plane surface according to the relation d s ( a ) = d0cosSot. This is outlined in fig. 5 and has been discussed in a more general form in ref. 9. Fig. 5 also shows a measured film thickness distribution which was obtained with the assembly illustrated in fig. 4, using G d F 3 as a test material*. It should be em* This test material was chosen with regard to the preparation of 248CmF3_targets because of the pronounced chemical similarity of Gd and Cm. V. MISCELLANEOUSMETHODS
116
/ Some exotic nuclear accelerator targets
H.J. Maier
i! a
iii~iii~ii
ii:i!ii~iii!iil b :!iii!!!i!
Fig. 4. Sourceassembly for high efficiencyvacuumevaporation condensation. (a) Individual components. (b) Assembled vaporization source.
phasized that 1 mg of source material produces an average film thickness of about 300 # g / c m 2 over a circular area of 8 mm diameter. Targets of 248CMF3 as well as 48Ca metal targets have been prepared successfully with this assembly. In both cases, a complete recycling of the isotopic material, which did not condense on the target area, was achieved. The 248CMF3 film was dissolved from the Cu substrate holder using a mixture of hydrochloric and boric acid and recovered by ion exchange techniques. This procedure was performed by Dr N. Trautmann, Institut ftir Kernchemie, Universit~it Mainz, West Germany.
a)
do
,
,~,
C) 80% 70
Collimatin9 boot Plane substrata ds(°d:do'c°s5 cc
-
60
~ ~ .£ a ~ ~
50 40 30
20 I0 0 400/Jglcm 2
2
4
6
8
10
" ~00%
b}
c
4c E Source ~
/d k 9.Srnrn
GdF3
3O0
Plane Subst rote /
200 50
100
o Meosured for I mg
2
i
i
i
i
4
6
8
10
i
2
1
• S[mini
Evaporont Gdg3
Fig. 5. Vapor collimation data. (a) Definition of thickness distribution. (b) Evaporation geometry.(c) Measured thickness distribution.
In the case of 48Ca, the well-established procedure of gravimetric calcium determination was applied for recycling [10]. After dissolving the metal film in dilute HC1, the 48Ca isotopic material was quantitatively precipitated in the form of 48CAC204 by addition of (NH4)2C20 4 and then converted to 48CAO at 1175 K in vacuum.
4. R o l l i n g o f u r a n i u m t a r g e t s 4.1. Basic considerations
Because of the strong tendency of uranium metal to oxidize, foils must be rolled in a clean, dry argon atmosphere and stored in vacuum at pressures of ~< 1.3 × l0 4 Pa. The equipment is described in section 4.2. Rolling of uranium foils however is also influenced by the physical properties of this element. Uranium exhibits three crystallographic forms designated c~, fl, and % The respective lattice symmetries and related data are listed in table 1. Preparation of thin target foils by cold rolling is only possible with the orthorhombic symmetry or aform. Starting with one of the fl- or y-modifications or a mixture of these will lead to a transformation into the a-modification, caused by the deformation forces. This transformation is coupled with a change of volume and symmetry and thus will necessarily result in the destruction of the foil by the formation of cracks a n d / o r holes. Unfortunately the common procedure of de-
H.J. Maier / Some exotic nuclear accelerator targets
117
Table 1 Crystallographic phases of uranium and thorium metal [2] Modifications of uranium and thorium Lattice symmetry
Temperature Range (K)
Lattice constants (,amX 104)
X-ray density ( g / c m 3)
a
b
5.865
Uranium a: orthorhombic ,8: tetragonal 7: body-centered cubic
<941 941-1047 1047-1405
2.853 10.75 3.534
Thorium a: face-centered cubic ,8: body-centered cubic
< 1673 1673-2023
5.086 4.11
oxidizing uranium metal by vacuum electron beam melting prior to cold-rolling leads to a mixture of the high temperature/3- and y-modifications. This
4.954 5.65
19.07 18.11 18.06
11.724
11.10
is because these are frozen when the electron beam is shut off more or less abruptly and the uranium is cooled from the liquid phase to room tempera-
Uranium
Thorium 1.95 mg Icm 2
2.15mg/cm 2
%
•2!+4
+4
:+I
/"'/~~" "
-2 -4
"+
"
~,
4.83 mg/cm:
Z,.75mg/cm 2 %
,9mrm 765
-
' )
,
;,
"iI
-I0~
89~
Z 86 mg/cm 2
II I015
121314
%
*&
"g -2
"il
--mt 2-1
-
Fig. 6. Homogeneity of rolled uranium and thorium foils. Target thickness was measured at fifteen positions as indicated in the center of the figure. The numbers 1-15 show the scanning sequence. The average of these measurements is referred to as the target thickness. Individual thickness values at the positions measured are presented as their relative deviation from the average value. V. MISCELLANEOUS METHODS
118
H J . Maier / Some exotic nuclear accelerator targets
ture in about 30 s. Therefore it is necessary to perform an additional vacuum annealing of the freshly purified material in order to convert the unwanted crystal forms to the a-modification. This is accomplished by a 10 h treatment of the uranium in a closed molybdenum crucible at 875 K. The effect of the temperature treatment is illustrated in fig. 6 which shows profiles of foils rolled from annealed and unannealed materials. The a-praticle energy loss method was applied for thickness profiling using 8.784 MeV a-particles from 212po. The thickness profiles were obtained by measuring the foil thickness at 15 different positions on one rolled target of 9 X 12 mm 2. The average of these 15 measurements is referred to as the target thickness. In fig. 6 individual thickness values of the 15 measured positions are presented in form of their relative deviation from the average value. It is clearly shown in fig. 6, that the material contained in the 7.72 and 4.83 m g / c m 2 uranium foils was not annealed. Both foils exhibited large inhomogeneities, and it was not possible to prepare 2 m g / c m 2 thick foils from this batch. The 2.15 m g / c m 2 foil was prepared from annealed metal which proved to be sufficiently ductile for a successful rolling. For comparison the thickness profiles of rolled thorium metal foils were measured and found to be much more homogeneous. Thorium has two crystallographic forms which are not very different
(table 1). The high transformation temperature of 1675 K and the similar crystal structure of both forms favor the formation of the a-phase after electron beam melting. Hence no further transformation occurs during rolling and the foils would be expected to be quite homogeneous.
4.2. Equipment Rolling of reactive metals such as uranium, alkali, and alkaline earth metals and rare earth metals must be performed in a clean, dry inert gas atmosphere. Our rolling facility is shown in fig. 7. It consists of a stainless steel glove box equipped with a two-high rolling mill with 9 c m diameter X15 cm work rolls. Minimum foil thicknesses of 300 2000 /~g/cm 2, depending on the specific properties of the material being rolled, are routinely attained using this apparatus and well-known pack-rolling techniques [11]. A dry argon atmospt'ere is maintained in the system by circulating the gas continuously through traps which remove water and oxygen contamination to a level < 1 ppm. Uranium foils rolled in this dry atmosphere maintain their bright, metallic appearance and are completely free of oxygen. A reliable storage facility for reactive metal targets is as important as the preparation equipmerit. Reactive metal targets have been successfully stored at our laboratory for periods of several years in vacuum at a pressure of approximately
Fig. 7. Inert atmosphere rolling apparatus. (a) General view. (b) Internal view with target foil in a stainless steel "'pack".
H.J. Maier / Some exotic nuclear accelerator targets
s2.t2
I/
NEEDLE VALVE FOR FLUSHING AND ROUGHING
T
VACUUM CHAMBER ANGLE VALVE NWSOKF
I
;
TO CRYOPUMP
Fig. 8. Schematic representation ofthe target storage facility
119
1 × 10 .-5 Pa. A schematic d r a w i n g of our storage system is shown in fig. 8. It consists of 12 individual storage c o n t a i n e r s a t t a c h e d to a central v a c u u m c h a m b e r which can be e v a c u a t e d using a 1000 l / s c r y o p u m p . A p p r o p r i a t e valving a n d a 100 1/s b y p a s s t u r b o p u m p for rough p u m p i n g allow the removal a n d r e c o n n e c t i o n of each c o n t a i n e r w i t h o u t losing v a c u u m in the i n d i v i d u a l c o n t a i n e r nor the v a c u u m in the r e m a i n d e r of the system. The v a c u u m storage containers are m a d e of stainless steel a n d have a flat, c i r c u l a r - s h a p e d bott o m of 12 cm d i a m e t e r on which can be screwm o u n t e d up to 30 targets. T h e top p o r t i o n of the c o n t a i n e r is c y l i n d e r - s h a p e d a n d has a height of 5 cm. The b o t t o m p a r t is w e l d - c o n n e c t e d to a P n e u r o p N W 50 K F angle valve. Thus the storage c o n t a i n e r a n d valve form a n o n d i s m o u n t a b l e unit which can be used for target t r a n s p o r t as well as for storage at the p u m p i n g station. The large cross section of the valve p e r m i t s a high p u m p i n g speed in the storage container, that allows a pressure of 1 × 10 s Pa to be achieved. A f t e r s e p a r a t i o n of the c o n t a i n e r from the p u m p i n g manifold, pressure increases slowly to a b o u t 13.3 Pa over a p e r i o d of 24 h; this c o r r e s p o n d s to a leakage rate o f 1.33 × 10 4 Pa 1/s. L o a d i n g a n d u n l o a d i n g of targets is a c c o m p l i s h e d in an inert atmosphere. A small needle valve on top of the storage c o n t a i n e r allows gentle flushing a n d r o u g h - p u m p i n g . A p h o t o g r a p h i c view of the entire storage facility is given in fig, 9. The a u t h o r a p p r e c i a t e s the c o o p e r a t i o n of D r D. Evers in p r o v i d i n g the d a t a from h o m o g e n e i t y tests of the rolled u r a n i u m a n d t h o r i u m metal targets.
References [1] G. Braun-Elwert, J. Huber, G. Korschinek, W. Kutschera, W. Goldstein and R.L. Hershberger, Nucl. Instr. and Meth. 146 (1977) 121. [2] H.J. Maier and W. Kutschera, Nucl. Instr. and Meth. 167 (1979) 91.
Fig, 9. Photographic view of the target storage facility. (a) Individual storage chamber. (b) Assembly of storage chambers on the cryopumped central vacuum chamber. (c).Front view showing the pumping station with the cryopump (center) and the rouging turbomolecular pump (right). V. MISCELLANEOUS METHODS
120
H.J. Maier / Some exotic nuclear accelerator targets
[3] R. Maier, G. Korschinek, P. Spolaore, W. Kutschera, H.J. Maier and W. Goldstein, Nucl. Instr. and Meth. 155 (1978) 55. [4] A.F. Holleman and E. Wiberg, Lehrbuch d. anorganischen chemie (Walter de Gruyter, Berlin, 1976). [5] M. Hansen, Constitution of binary alloys (McGraw-Hill, New York, 1958). [6] H.J. Maier et al., Jahresbericht 1979, Beschleunigerlaboratorium der Universitat und der Technischen Universitat Miinchen.
[7] C.R. Waest, ed., Handbook of chemistry and physic., (CRC, Cleveland, Ohio, 1973-74). [8] K.F. Bonhoeffer, Z. Phys. Chem. 113 (1924) 199. [9] H.J. Maier, IEEE Trans. Nucl. Sci. NS-28 (1981) 1576. [10] I.M. Kolthoff and E.B. Sandell, eds., Textbook of quantitative inorganic analysis (Macmillan, New York, 1948). [11] F.J. Karasek, Nucl. Instr. and Meth. 102 (1972) 457. [12] C. Keller, The chemistry of the transuranium elements (Verlag Chemie, Weinheim, 1971).
113
Part V. Miscellaneous methods of target preparation P R E P A R A T I O N T E C H N I Q U E S FOR S O M E EXOTIC NUCLEAR ACCELERATOR TARGETS AND I S O T O P I C S P U T T E R - I O N S O U R C E TARGETS H.J. MAIER Sektion Physik, Universiti~t Miinchen, 8046 Garching, Fed. Rep. Germany
This paper presents a survey of some recent preparative techniques which were developed at the target laboratory of the University of Mianchen. These techniques include the production of sputter-ion source targets of 3°Si and 365, the synthesis of H36S for use in a gas target, high efficiency evaporation-condensation of rare and exotic isotopes, e.g. 48Ca and 248Cm, as well as the rolling of 238U and other reactive metal foils.
1. Sputter targets f o r u s e in i o n s o u r c e s
ARGON SiC2 + 2Mg ~ S i
+ 2MgO + 292.18 kJ
1275
Because of their low consumption of target material, cesium beam type sputter-ion sources are widely applied at the Garching Tandem Laboratory [1]. The so-called backsputter geometry has proved to be especially economic and is therefore utilized for the production of tandem accelerated beams of exotic isotopes [2,3]. Sputter targets appropriate for application in the backsputter geometry are small pellets of spherical or lenticular shape with a mass of approximately 50 mg. The present paper describes the preparation of isotopically enriched sputter targets of 3°Si and 365.
-VIT. CARBON CRUCIBLE REACTION
REACTION MIXTURE, PELLETIZED WITH 1470 MPa
~
2 5 m ~" / ~ j -I Io 10ram
T R E A T M E N T OF R E A C T I O N PRODUCTS
HOT (335K) DILUTE HCI: HOT (355K) AQUA DEST: VACUUM, 1.3 Pa, 295K VACUUM, 1.3 X 10 5 Pa, 1275K
12 hours 1 hour 30 rain 1Stain
1 F'NEGRA'N,BROWN30S'POWDER I COMPACTION IN VACUUM PRESSING TOOL
1.1. 3°Si target
1 3°S,TABLET I VACUUM ELECTRON BEAM MELTING
Enriched 3°Si (95.2%) is available from the Oak Ridge National Laboratory (ORNL) in the chemical form of 3°SIO2. To produce a sputter target pellet of this material, a reduction to the elemental form was necessary. From the literature [4], aluminium is known to be the standard reductant for SiO 2. However, since it is easier to separate MgO rather than A1203 from the reaction products, Mg was selected as the deoxidizing agent. The reduction procedure is outlined in fig. 1. First, a reaction mixture is formed using stoichiometric amounts of SiO 2 and Mg powders with a 5% excess of Mg. A large excess of Mg must be avoided to preclude possible formation of Mg2Si instead of Si. The reaction mixture is pressed under vacuum to form a tablet by applying a pressure of 1470 MPa. The reduction is performed
I 30Si SPUTTER PELLET I
O V E R A L L E F F I C I E N C Y O F PROCEDURE:
70%
TYPICAL 30Si SPUTTER PELLET DATA: 40 50rag, SPHERICALLY SHAPED, ¢ 3-3.5,rrtm
Fig. 1. Sputter pellet preparation of 3°Si.
by heating the reaction tablet in an argon atmosphere to 1275 K for a period of 10 min. A crucible of vitreous carbon is used for this purpose. The reaction products are subjected to a 12 h treatment with dilute HC1 to extract the reaction product MgO. Subsequently the material is carefully washed with hot water, vacuum dried at 1.3 Pa, and heated to about 1275 K in a vacuum
016%5087/82/0000-0000/$02.75 © 1982 North-Holland
v. MISCELLANEOUSMETHODS
114
H.J. Maier / Some exotic nuclear accelerator targets
chamber held at 1.3 × 10 - s Pa. This latter treatment removes possible residues HC1. The result of this procedure is a fine-grained, brown powder of 3°Si, which is processed into the desired sputter pellet by vacuum electron beam melting. The overall yield of the procedure is 70%. A typical 3°Si sputter pellet has a weight of 40-50 mg and a diameter of about 3 to 3.5 mm.
Fe36SPELLET FROM ELEMENTAL 36S M.P. (KI 36B5 392 B.P {K) 7176
1463
DIRECT REACT(ON: Fe + 36 S FU$~EEFe 36S
I ..................... Fe s
I ..................
1.2. ~6S target Because of its high neutron content, 36S is an experimentally interesting nucleus and is frequently used as a projectile in the Garching Tandem Laboratory. Elemental 36S enriched to 81.1% is available at a reasonable price from Rohstoff-Einfuhr, Dtisseldorf, West Germany. Sulfur-36 cannot be used in its elemental form as a target material in a sputter-type ion source because of its low melting point and sublimation temperature. However, since a 32S beam has successfully been produced using a sputter-target machined from commercially available natural FeS [1] a search was made for a suitable method to prepare a sufficient amount of Fe36S with a minimum loss of isotopic material. The phase diagram of the binary iron-sulfur [5] system suggested the fusion of stoichiometric amounts of the elements to form Fe36S. A small deviation from stoichiometry because of possible partial sublimation of the sulfur component during fusion would be expected to result in an alloy of Fe and Fe36S which is thermally stable. Accordingly, the procedure outlined in fig. 2 was chosen. Stoichiometric amounts of powdered Fe (grain size 5/~m) and 36S with a slight excess of Fe were carefully mixed. The mixture was transferred to a vacuum pressing apparatus and compacted into a tablet by applying a pressure of 1470 MPa. Fusion was accomplished by inert electrode arc melting in a purified argon atmosphere. Vacuum electron beam melting is not practicable in this case because it would lead to significant losses of 36S by evaporation. The are melting furnace was originally purchased from Leybold Heraeus G m b H , Hanau, West Germany, but has been modified in our laboratory in order to improve the purity of the argon atmosphere [6]. The operating data are listed in fig. 2. the efficiency of the procedure is approximately 90%.
REACTION D A T A ARGON PRESSURE ARC C U R R E N T : FUSING TIME EFFICIENCY TYPICAL PE LLET
I
I
INERT ELECTRODE ARC M E L T I N G
187 kPa 5 0 - 70A 2 3~ 85 90% 65rag Fe36S, 3mm~
HANDLE A R G O N SUPPLy
FLEXIBLE BELLOWS
TO PUMP WATER COOLED
Cu
ION MIXTURE
Fig. 2. Sputter pellet preparation of Fe36S.
A typical Fe36S pellet has a weight of 65 mg and a diameter of about 3 mm, and provides a 368 beam of 1 /~A for a period of about 100 h.
2. Preparation of H236S for use in gas targets For use as a gas target, elemental 36S must be converted into H2368. It is evident, that a direct reaction of hydrogen and sulfur would be the preferred preparative reaction and should produce a high purity as well as a high yield. However, the free energy of formation of H2S is only -33 k J / m o l as compared with -236 k J / m o l for H20, which can be formed spontaneously from the elements [7]. Therefore, in the case of H2S, it is necessary to use hydrogen in its more reactive atomic form instead of the molecular form to accomplish the desired reaction. A convenient method to produce atomic hydrogen is the dissociation of H 2 molecules in a glow discharge, which leads to a H-abundance of approximately 60% with a mean lifetime of 0.5 s for the individual H-atom [8]. The formation of H2S will take place according to the equation glow discharge
H~
~
2H,
2H -~'-368 ~ H236S. The apparatus used for this procedure is sketched
H.J. Maier / Some exotic nuclear accelerator targets FEED BACK LINE PRESSURECONTROLLER
....... I
I r,-
H' 'N ~
I :::7~
.... s ,.........
/
iOENTROL ~ PRESSURECONTROL VAL
~" --HV
REACTIONMECHANISM GLOW H2 DISCHARGE , 2H ' H2S
L
-
F~-
CATHODE /
/
............
s;:° EtE*:t E
~T~-~---
.............
' ~--~NO-D'~'E..........
J
3. Economic evaporation procedures for exotic isotopes
T~:. . . . . . . . . . ~ Hi (TO TURBO
PYREX20TUBE¢ mm
III LLN COLDTRAR ~J'~'~'~" N2 PYREX
OPERATINGCONDITIONS eH2 H2 THROUGHPUT HV
rIH) = 0.5 s~ H ABUNDANCE~ 60%
=
"J-I L
measuring the pressure in the cold trap after the latter has been warmed.
CAPACITIVE
t
-'-'
l/
2H+S
r
IONIZATION
Iff"
115
i REACTIONSPEED EFFICIENCY TYPICAL QUANTITY DEALT WITH
PROPERTIES of H2S
67 Pa
M.P. 190K
500n cm3/h~r
BP, 214K
150Ov De
10~A I mg S/mln
lm MO~
Fig. 3. Preparation of H2365.
in fig. 3. The reaction chamber is a ring shaped pyrex tube with an inner diameter of 20 mm. The cathodes are aluminum tubes of 40 m m diameter and an area of 100 cm 2 each. The powdered 36S is located in a quartz crucible. A pressure of 66.7 Pa H 2 is maintained in the reaction tube with the aid of a capacitive pressure gauge that is connected to an electronic pressure controller and a piezoelectric control valve. The glow discharge is ignited between one of the cathodes and the anode, which is located at the main pumping orifice. Operations are performed as described in the following procedure. The entire system is pumped to a pressure of 0.0013 Pa with a turbomolecular p u m p that is connected to the system with a 40 m m diameter quick p u m p down port. When the chamber is thoroughly degassed, hydrogen gas is caused to stream through the system by allowing it to enter through the piezoelectric valve while pumping through a bypass cold trap. Then the glow discharge is initiated. The atomic hydrogen reacts immediately with the sulfur powder to form gaseous H368, which is quantitatively condensed in the liquid nitrogen-cooled trap. Operating conditions are listed in fig. 3. The conversion rate is approximately 1 mg 36S per minute with a collection efficiency of approximately 90%. In one typical conversion, 1 × 10 3 mol of H36Sis produced. This corresponds to 22.4 ml S.T.P. The exact quantity is determined by
Since the cost for enriched isotopic material is steadily increasing, economic evaporation procedures have become more and more important. In this section, a vapor deposition assembly is described, which is suitable for preparing relatively thick targets, starting with quantities of enriched isotopes of the order of 1 mg, and which allows the recycling of isotopic material that is wasted during the procedure. A Mathis C H 7 tantalum crucible heater and a tube-shaped graphite crucible (7 m m internal diameter ×25 mm) are used as the evaporation source. The substrate holder is designed as a headpiece located on the top of the heater assembly so that it encloses the space between the crucible and the substrate. This design permits a quantitative collection of the evaporated material on a relatively small area, which is a necessary condition for the successful recycling of vapor-deposited material. A photograph of the whole assembly is shown in fig. 4. The material from which the substrate holder is constructed is chosen according to the chemical nature of the source material to be collected. In the case of actinide fluorides and alkaline earth metals, for example, copper is very convenient because it is not attacked by the solvents which are used to separate the condensed material from the substrate. A tube-shaped evaporation crucible may be referred to as a collimating type evaporation source which generates (in good approximation) a film thickness distribution on a plane surface according to the relation d s ( a ) = d0cosSot. This is outlined in fig. 5 and has been discussed in a more general form in ref. 9. Fig. 5 also shows a measured film thickness distribution which was obtained with the assembly illustrated in fig. 4, using G d F 3 as a test material*. It should be em* This test material was chosen with regard to the preparation of 248CmF3_targets because of the pronounced chemical similarity of Gd and Cm. V. MISCELLANEOUSMETHODS
116
/ Some exotic nuclear accelerator targets
H.J. Maier
i! a
iii~iii~ii
ii:i!ii~iii!iil b :!iii!!!i!
Fig. 4. Sourceassembly for high efficiencyvacuumevaporation condensation. (a) Individual components. (b) Assembled vaporization source.
phasized that 1 mg of source material produces an average film thickness of about 300 # g / c m 2 over a circular area of 8 mm diameter. Targets of 248CMF3 as well as 48Ca metal targets have been prepared successfully with this assembly. In both cases, a complete recycling of the isotopic material, which did not condense on the target area, was achieved. The 248CMF3 film was dissolved from the Cu substrate holder using a mixture of hydrochloric and boric acid and recovered by ion exchange techniques. This procedure was performed by Dr N. Trautmann, Institut ftir Kernchemie, Universit~it Mainz, West Germany.
a)
do
,
,~,
C) 80% 70
Collimatin9 boot Plane substrata ds(°d:do'c°s5 cc
-
60
~ ~ .£ a ~ ~
50 40 30
20 I0 0 400/Jglcm 2
2
4
6
8
10
" ~00%
b}
c
4c E Source ~
/d k 9.Srnrn
GdF3
3O0
Plane Subst rote /
200 50
100
o Meosured for I mg
2
i
i
i
i
4
6
8
10
i
2
1
• S[mini
Evaporont Gdg3
Fig. 5. Vapor collimation data. (a) Definition of thickness distribution. (b) Evaporation geometry.(c) Measured thickness distribution.
In the case of 48Ca, the well-established procedure of gravimetric calcium determination was applied for recycling [10]. After dissolving the metal film in dilute HC1, the 48Ca isotopic material was quantitatively precipitated in the form of 48CAC204 by addition of (NH4)2C20 4 and then converted to 48CAO at 1175 K in vacuum.
4. R o l l i n g o f u r a n i u m t a r g e t s 4.1. Basic considerations
Because of the strong tendency of uranium metal to oxidize, foils must be rolled in a clean, dry argon atmosphere and stored in vacuum at pressures of ~< 1.3 × l0 4 Pa. The equipment is described in section 4.2. Rolling of uranium foils however is also influenced by the physical properties of this element. Uranium exhibits three crystallographic forms designated c~, fl, and % The respective lattice symmetries and related data are listed in table 1. Preparation of thin target foils by cold rolling is only possible with the orthorhombic symmetry or aform. Starting with one of the fl- or y-modifications or a mixture of these will lead to a transformation into the a-modification, caused by the deformation forces. This transformation is coupled with a change of volume and symmetry and thus will necessarily result in the destruction of the foil by the formation of cracks a n d / o r holes. Unfortunately the common procedure of de-
H.J. Maier / Some exotic nuclear accelerator targets
117
Table 1 Crystallographic phases of uranium and thorium metal [2] Modifications of uranium and thorium Lattice symmetry
Temperature Range (K)
Lattice constants (,amX 104)
X-ray density ( g / c m 3)
a
b
5.865
Uranium a: orthorhombic ,8: tetragonal 7: body-centered cubic
<941 941-1047 1047-1405
2.853 10.75 3.534
Thorium a: face-centered cubic ,8: body-centered cubic
< 1673 1673-2023
5.086 4.11
oxidizing uranium metal by vacuum electron beam melting prior to cold-rolling leads to a mixture of the high temperature/3- and y-modifications. This
4.954 5.65
19.07 18.11 18.06
11.724
11.10
is because these are frozen when the electron beam is shut off more or less abruptly and the uranium is cooled from the liquid phase to room tempera-
Uranium
Thorium 1.95 mg Icm 2
2.15mg/cm 2
%
•2!+4
+4
:+I
/"'/~~" "
-2 -4
"+
"
~,
4.83 mg/cm:
Z,.75mg/cm 2 %
,9mrm 765
-
' )
,
;,
"iI
-I0~
89~
Z 86 mg/cm 2
II I015
121314
%
*&
"g -2
"il
--mt 2-1
-
Fig. 6. Homogeneity of rolled uranium and thorium foils. Target thickness was measured at fifteen positions as indicated in the center of the figure. The numbers 1-15 show the scanning sequence. The average of these measurements is referred to as the target thickness. Individual thickness values at the positions measured are presented as their relative deviation from the average value. V. MISCELLANEOUS METHODS
118
H J . Maier / Some exotic nuclear accelerator targets
ture in about 30 s. Therefore it is necessary to perform an additional vacuum annealing of the freshly purified material in order to convert the unwanted crystal forms to the a-modification. This is accomplished by a 10 h treatment of the uranium in a closed molybdenum crucible at 875 K. The effect of the temperature treatment is illustrated in fig. 6 which shows profiles of foils rolled from annealed and unannealed materials. The a-praticle energy loss method was applied for thickness profiling using 8.784 MeV a-particles from 212po. The thickness profiles were obtained by measuring the foil thickness at 15 different positions on one rolled target of 9 X 12 mm 2. The average of these 15 measurements is referred to as the target thickness. In fig. 6 individual thickness values of the 15 measured positions are presented in form of their relative deviation from the average value. It is clearly shown in fig. 6, that the material contained in the 7.72 and 4.83 m g / c m 2 uranium foils was not annealed. Both foils exhibited large inhomogeneities, and it was not possible to prepare 2 m g / c m 2 thick foils from this batch. The 2.15 m g / c m 2 foil was prepared from annealed metal which proved to be sufficiently ductile for a successful rolling. For comparison the thickness profiles of rolled thorium metal foils were measured and found to be much more homogeneous. Thorium has two crystallographic forms which are not very different
(table 1). The high transformation temperature of 1675 K and the similar crystal structure of both forms favor the formation of the a-phase after electron beam melting. Hence no further transformation occurs during rolling and the foils would be expected to be quite homogeneous.
4.2. Equipment Rolling of reactive metals such as uranium, alkali, and alkaline earth metals and rare earth metals must be performed in a clean, dry inert gas atmosphere. Our rolling facility is shown in fig. 7. It consists of a stainless steel glove box equipped with a two-high rolling mill with 9 c m diameter X15 cm work rolls. Minimum foil thicknesses of 300 2000 /~g/cm 2, depending on the specific properties of the material being rolled, are routinely attained using this apparatus and well-known pack-rolling techniques [11]. A dry argon atmospt'ere is maintained in the system by circulating the gas continuously through traps which remove water and oxygen contamination to a level < 1 ppm. Uranium foils rolled in this dry atmosphere maintain their bright, metallic appearance and are completely free of oxygen. A reliable storage facility for reactive metal targets is as important as the preparation equipmerit. Reactive metal targets have been successfully stored at our laboratory for periods of several years in vacuum at a pressure of approximately
Fig. 7. Inert atmosphere rolling apparatus. (a) General view. (b) Internal view with target foil in a stainless steel "'pack".
H.J. Maier / Some exotic nuclear accelerator targets
s2.t2
I/
NEEDLE VALVE FOR FLUSHING AND ROUGHING
T
VACUUM CHAMBER ANGLE VALVE NWSOKF
I
;
TO CRYOPUMP
Fig. 8. Schematic representation ofthe target storage facility
119
1 × 10 .-5 Pa. A schematic d r a w i n g of our storage system is shown in fig. 8. It consists of 12 individual storage c o n t a i n e r s a t t a c h e d to a central v a c u u m c h a m b e r which can be e v a c u a t e d using a 1000 l / s c r y o p u m p . A p p r o p r i a t e valving a n d a 100 1/s b y p a s s t u r b o p u m p for rough p u m p i n g allow the removal a n d r e c o n n e c t i o n of each c o n t a i n e r w i t h o u t losing v a c u u m in the i n d i v i d u a l c o n t a i n e r nor the v a c u u m in the r e m a i n d e r of the system. The v a c u u m storage containers are m a d e of stainless steel a n d have a flat, c i r c u l a r - s h a p e d bott o m of 12 cm d i a m e t e r on which can be screwm o u n t e d up to 30 targets. T h e top p o r t i o n of the c o n t a i n e r is c y l i n d e r - s h a p e d a n d has a height of 5 cm. The b o t t o m p a r t is w e l d - c o n n e c t e d to a P n e u r o p N W 50 K F angle valve. Thus the storage c o n t a i n e r a n d valve form a n o n d i s m o u n t a b l e unit which can be used for target t r a n s p o r t as well as for storage at the p u m p i n g station. The large cross section of the valve p e r m i t s a high p u m p i n g speed in the storage container, that allows a pressure of 1 × 10 s Pa to be achieved. A f t e r s e p a r a t i o n of the c o n t a i n e r from the p u m p i n g manifold, pressure increases slowly to a b o u t 13.3 Pa over a p e r i o d of 24 h; this c o r r e s p o n d s to a leakage rate o f 1.33 × 10 4 Pa 1/s. L o a d i n g a n d u n l o a d i n g of targets is a c c o m p l i s h e d in an inert atmosphere. A small needle valve on top of the storage c o n t a i n e r allows gentle flushing a n d r o u g h - p u m p i n g . A p h o t o g r a p h i c view of the entire storage facility is given in fig, 9. The a u t h o r a p p r e c i a t e s the c o o p e r a t i o n of D r D. Evers in p r o v i d i n g the d a t a from h o m o g e n e i t y tests of the rolled u r a n i u m a n d t h o r i u m metal targets.
References [1] G. Braun-Elwert, J. Huber, G. Korschinek, W. Kutschera, W. Goldstein and R.L. Hershberger, Nucl. Instr. and Meth. 146 (1977) 121. [2] H.J. Maier and W. Kutschera, Nucl. Instr. and Meth. 167 (1979) 91.
Fig, 9. Photographic view of the target storage facility. (a) Individual storage chamber. (b) Assembly of storage chambers on the cryopumped central vacuum chamber. (c).Front view showing the pumping station with the cryopump (center) and the rouging turbomolecular pump (right). V. MISCELLANEOUS METHODS
120
H.J. Maier / Some exotic nuclear accelerator targets
[3] R. Maier, G. Korschinek, P. Spolaore, W. Kutschera, H.J. Maier and W. Goldstein, Nucl. Instr. and Meth. 155 (1978) 55. [4] A.F. Holleman and E. Wiberg, Lehrbuch d. anorganischen chemie (Walter de Gruyter, Berlin, 1976). [5] M. Hansen, Constitution of binary alloys (McGraw-Hill, New York, 1958). [6] H.J. Maier et al., Jahresbericht 1979, Beschleunigerlaboratorium der Universitat und der Technischen Universitat Miinchen.
[7] C.R. Waest, ed., Handbook of chemistry and physic., (CRC, Cleveland, Ohio, 1973-74). [8] K.F. Bonhoeffer, Z. Phys. Chem. 113 (1924) 199. [9] H.J. Maier, IEEE Trans. Nucl. Sci. NS-28 (1981) 1576. [10] I.M. Kolthoff and E.B. Sandell, eds., Textbook of quantitative inorganic analysis (Macmillan, New York, 1948). [11] F.J. Karasek, Nucl. Instr. and Meth. 102 (1972) 457. [12] C. Keller, The chemistry of the transuranium elements (Verlag Chemie, Weinheim, 1971).