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A new technique for preparation of targets of powdered materials
NUCLEAR INSTRUMENTS
AND METHODS
145 ( 1 9 7 7 )
409-415
; ©
NORTH-HOLLAND
P U B L I S H I N G CO.
A NEW TECHNIQUE FOR PREPARATION OF TARGETS OF POWDERED MATERIALS ISAO SUGAI
Institute for Nuclear Study, University oJ Tokyo, Tanashi-shi, Tokyo 188, Japan Received 12 April 1977 We have made various targets of powdered materials for nuclear experiments by developing a centrifugal precipitation method, which is particularly useful in the cases of: (a) metals, with high melting point and low vapour pressure, (b) oxides or carbonates which are difficult to be handled by the usual vacuum evaporation technique and (c) some enriched isotopes which are generally very minute in quantity (less than - 1 0 mg). The powdered samples were suspended in liquid paraffin with an ultrasonic vibrator, and then centrifugally precipitated on a thin backing film of Mylar or aluminium. Uniformity in the thicknesses of the targets obtained in this way was measured with an o~-ray thickness gauge and was known to be satisfactory. Inspection for any impurities contained in the materials used or contaminated during the preparation process was carried out by measuring the scattered protons from the targets. No significant impurity was detected larger than 10 - 4 mg/cm 2.
I. Introduction Targets of self-supporting foils are most desirable for nuclear reaction experiments for reasons of purity. There are several methods ~a) in the preparation of self-supporting metal targets, including vacuum evaporation, electroplating, rolling and thermal cracking. In practice, it is hard to produce self-supporting foils of such materials as alkali earth metals and rare-earth metals. Moreover, materials with high melting points, for example Zr, Ta, Re, Os and W etc., are not easy to evaporate even by the method of heating by electron bombardment. In general, such materials as mentioned above are obtainable in the forms of oxides or carbonates, and powder of them can be used for target preparation by the method of making suspensions in some suitable liquid. Gravity precipitation, electrospraying and electrophoresis are used for such suspensions3-S). In these methods, however, there are some disadvantages such as thickness limitations, non-uniformity in thicknesses, contamination by impurities, and weakness for beam bombardment. As an alternative, a method of enforced centrifugal precipitation is proposed to be used for fine grain powder of metal oxides or carbonates suspended in liquid paraffin with an ultrasonic vibrator. The target material can easily be separated from the liquid paraffin with the centrifuge. Hence, it is possible to deposit the material onto the backing more satisfactorily than by the other suspension methods mentioned above. Samples of enriched isotopes are expensive, and usually purchased in small quantities. Therefore,
the yields of the target material are always of great concern. In the centrifugal precipitation method, a small amount of enriched isotope ( ~ 5 mg) can be deposited effectively on the backing surface. Any metallic powder is also able to be evenly deposited on the backing when the grain of the powder is fine enough. The range of target thicknesses obtained by the present method is from 0.1 to 50 mg/cm 2. Relatively thin targets (0.1-5 mg/cm 2) are used for charged particle nuclear reaction experiments. Targets thicker than several mg/cm 2 are used for inbeam ~,-ray experiments and fission and neutron capture measurements. 2. Experimental method 2.1. APPARATUS The apparatus consists essentially of a centrifugal tube, an ultrasonic vibrator (29 kHz, 50 W), a centrifugal separator (Swing type 80 m i x 4 , Max4000 rpm) and a vacuum evaporator. The ultrasonic vibrator is used to obtain uniform suspensions of target materials in liquid paraffin. The structure of the centrifugal tube is shown in fig. 1. It is made of duralumin. At the bottom of the tube a thin backing film with or without target frame is fixed, being used for the preparation of relatively thick or thin targets, respectively. Mylar films are usually used as backings. To protect the deposited target material from breaking in handling, it is better to use a special backing film of Mylar having a ring flame of aluminium
410
I.
~45mm
SUGAI
2.3. SUSPENSION AGENT Liquid paraffin has been found to be most suitable in making uniform suspensions of the target materials. This liquid is a highly refined nonvolatile carbon hydrate oil and is easily diluted with ethyl acetate.
~
!Omm@~
E O
E O
,, 5ram
l!!
it I ~
03 (~)
(~)
Fig. 1. Centrifugal tube made of duralumin: (1) polyethylene film ring (200/lm in thickness), (2) backing film with frame, (3) bottom disc.
foil (200/~m thick) glued to one side of it as shown in fig. 1. A polyethylene ring (200/1m thick) is used as a packing during the centrifugal process. When a frameless backing is used the Mylar film itself plays the role of packing. The centrifugal tube, after being assumbled, is heated in an electric oven kept at about 100°C to remove distortions in the backing film.
2.4. TARGETPREPARATION The target materials which can be treated by the centrifugal precipitation method sould satisfy the following requirements: chemically stable, insoluble and non-reactive in liquid paraffin, indeliquiscent and non-reactive during the pulverization. As an example of the practical methods in the process we describe the case of a neodymium-oxide (143Nd203) target. The desired target thickness was about 2.5 mg/cm 2. For a target with an area of about 3 cm 2 an initial amount of about 7.85 mg of oxide was used. The amount of liquid paraffin was adjusted depending on the amount of target material used. For example, 7.85 mg of Nd203 was suspended in 10 ml of liquid paraffin. The viscosity was decreased by heating with an infrared ray lamp. A homogeneous suspension of neodymiumoxide was obtained with the aid of an ultrasonic vibrator of suitably adjusted frequency. The ultrasonic vibrator was operated for about 30 min. Then the neodymium-oxide became well su2 I
2.2. BACKINGS The backing films should usually be as thin as possible. Such organic films as polyvinyl formal, collodion, polyethylene and Mylar are generally used as backings. On the other hand, aluminium, gold, nickel and carbon foils are also used. In our institute Mylar films are used very often except in some special cases. Mylar is a polymer consisting of carbon, oxygen and hydrogen and has suitable physical and chemical properties, being stable at temperatures up to 150°C, inert to the acids usually employed in target preparation work, easy to handle and strong. Because the Mylar backing is heat treated after being attached to the centrifugal tube as described in the preceding section, the effect of heat treatment on the thickness of the Mylar film was measured with an ~z-ray thickness gauge 6) before and after the heat treatment in an oven. In figs. 2a and 2b, the uniformities in thickness are shown. Changes in the uniformity of thickness due to the heat treatment were found to be negligible (<3%).
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After heat treatment 0.500 mg/cm 2
~
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Before heat treatment 0 ' 4 9 9 mg/cm2
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Fig. 2. (a) Uniformityin thickness of a Mylar backing film before heating. (b) Uniformityin thickness of the Mylar backing film after heating in an oven at about 100°C.
T A R G E T S OF P O W D E R E D M A T E R I A L S
spended, no precipitation being observed even after about 24 h. The homogeneous suspension was subjected to the action of the centrifuge until the oxide was effectively collected on the backing film. The centrifugal separator was operated usually at 3000 rpm for 60 min. The degree of transparency of liquid paraffin showed the efficiency of the precipitation. In the case of unsatisfactory precipitation, the liquid paraffin was cloudy. A due amount of ethylacetate was added for relaxation of the high viscosity when the supernatant liquid paraffin was sucked out of the centrifugal tube. The liquid paraffin remaining on the target was dried off in a vacuum vessel kept at 10 -3 torr and at about 200°C for about 5 h. By this process the colour of the target surface changed from semitransparent to white. A small amount of liquid paraffin is considered to remain in the target as will be discussed in section 3.1. Special care should be taken in detaching the target film from the bottom of the centrifugal tube without breaking it. Originally coarse-grained target material, such as CaCO3, BaCO3, A1203, Sc203 and GeO2, had to be ground down to fine powder in an agate mortar before suspension. 3. Results and discussion Targets in the thickness range of 0.1-50 mg/cm 2 of some sixty different materials have been prepared by the present method. Mylar films (4-10 ~tm) were used as backings except in a few cases of Co, GaI3, LiF, Sb, Se, SrCO 3 and Zr for which 10/.zm aluminium foils were used and for Nd203 6 ~m carbon foils. The metallic powders of W, Zr, Mo and Ta with high melting points, and their oxides (WO3, ZrO2, MoO2 and Ta2 05) could be made into homogeneous targets. The low melting point metals, Ga and In, were transformed into their oxides so as not to be evaporated when they were bombarded with cyclotron beams. For much thicker targets (20-50 mg/cm2), there were some case in which uniform and stable suspensions were not easily obtained. In such cases, the centrifugal process must follow immediately after the ultrasonic mixing. More uniform and better targets could be obtained by the removal of 80% of the supernatant liquid paraffin in the tube and then by addition of an adequate amount of new suspension.
411
When any uniform suspension was difficult to obtain in the procedure described above because of the unequal grain sizes of the powder, a quite fine grain powder of the material could be prepared by the use of a suspension part of the material in ethyl-acetate. 3.1. AMOUNT OF RESIDUAL PARAFFIN
The completed targets prepared by the process described in section 2.4 are considered to contain some residual paraffin. In order to estimate the true value of the thickness of the target material only, the amounts of this residual liquid paraffin must be known. We tried to know this amount by evaporating stepwise the residual paraffin by radiation heating in vacuum and by intermittently weighing the target. The target used for this experiment was natural Nd203 of around 3 mg/cm 2 in thickness. The initial thickness before radiation heating was 2.44 mg/cm 2. The radiation source was a tantalum plate maintained at about 1500°C. The Nd203 target was set at 5 cm above the radiation plate. Radiation heating was done every 20 rain and the thickness of the target was measured with a microbalance before and after the radiation heating. When the colour of the target changed from semi-transparent to white, this being generally the final status of the completed target, the heat treatment was stopped . The thickness after final heat treatment as a normal target was 2.72 mg/cm 2. When the target was heated for a long time in vacuum and all the residual liquid paraffin was evaporated away, it was weighed. The thickness of the target material only was thus known to be 2.50 mg/cm 2. Therefore, the amount of residual liquid paraffin in the target was estimated to be the difference 2.72-2.50=0.22 m g / c m 2 in thickness, or 8.8% of the target material thickness. This amount of the residual liquid paraffin was necessary to keep the strength of the target and seemed to play the role of a binder between the target material and the backing, and also between the grains of the target material. Of course, the amount of the residual liquid paraffin in a target will depend on the thickness of the target and also on the grain size of the target material, being proportional to the target thickness and larger for a smaller grain size. The above mentioned value of 0.22 mg/cm 2 (8.8%) should be considered to be an example showing the order of the amounts of residual liq-
412
]. SUGAI
uid paraffin in completed targets prepared by the present method. Other examples of the amounts of residual paraffin estimated from the recovery ratio of 95% if the fed material, which is described in the next section, are as follows: 9.2% (0.17 mg/cm 2) for a natural Gd203 (relatively coarse grained) target of 1.85 mg/cm 2, 10.9% (0.39 mg/cm 2) for a 158Gd203 (relatively fine grained) target of 3.57 mg/cm 2, and 11.5% (0.37mg/cm 2) for a 1485m203 target of 3.18 mg/cm 2 in thickness. From the above mentioned results, it seems reasonable to consider that the amount of residual paraffin is about 10% of the thickness of the target material for rare earth oxides.
3b shows the uniformity along the direction perpendicular to the former. In this case, the maximum deviation of 2.7% of the central thickness (4.85mg/cm 2) was obtained, corresponding to 131 # g / c m 2 in absolute thickness. Hence, the deposition of target material can be said to be fairly N a t , Gd20;3
on M y l a r
(a)
% ::L
3.2. YIELD OF TARGETMATERIALS Natural rare earth oxide targets of Sm, Eu and Nd of about 2 mg/cm 2 thick were prepared to estimate the yields of the target materials during the preparation process. The recovery ratios, defined as the ratio of the total amount of the target material contained in a completed target to that of the material fed initially into the centrifugal tube, were measured to be 95% for the targets tested. 3.3. THICKNESSUNIFORMITY The uniformity in thickness of the targets prepared by the present method was measured with an o~-ray thickness gauge. Details of the thickness measurement of chemical compounds by this gauge have been reported previously6). Targets of the following compounds were used for the measurements: natural Gd203, PbCO3, CdO and enriched 2°4pbCo3. Mylar backings of about 4 # m in thickness were used in all cases. In the uniformity measurement the target was moved in 1 mm steps, the diameter of the exit slit of the ~z-ray gauge being 1.2 mm. The thicknesses of backing Mylar foils were measured beforehand by weighing and were subtracted from the measured total thicknesses of the targets, being assumed to have uniform thickness distributions. The maximum thickness deviation of Mylar foils of 4 # m thick is about 10 a g / c m 2 as shown in figs. 2a, 2b. So the results of the measured maximum thickness deviations of the target materials shown will contain this value of 10#g/cm 2 due to the Mylar backings. Fig. 3a shows the uniformity in thickness of a natural Gd203 target along one direction, and fig.
"~- 5
-.
i:
-9-8-7-6-5-4-3-2-I ~Distence
0
I 2 3 4 (mrn) ~
5 6
7
8
Nat. Gd203 on Mylar
15 o.~ I
~ 0
~
-
Cenl"re /
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Deviation
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7 8
Distance (mm)~ Fig. 3.(a) U n i f o r m i t y in thickness o f a natural Gd203 target. (b) UniFormity in thickness o f the same target along the direction perpendicular to that in (a).
413
T A R G E T S OF P O W D E R E D M A T E R I A L S
uniform. For the isotopically enriched 2°4PBCO3 target of 2.36 mg/cm 2 in central thickness, the values wer6 3.0°/6 and 72/~g/cm 2, respectively (fig. 4). In general, carbonate target materials like 2°4pbCO3 a r e coarse-grained and have to be ground into fine powder beforehand, otherwise the maximum deviation in thickness is twice as large. The results for a natural CdO target of rather small thickness are shown in fig. 5, where rather large relative deviations are observed because of the thin central thickness, the absolute deviation being rather small. In table 1, the target thicknesses of enriched germanium-oxides estimated by simple weighing with a microbalance, the weihgt of the residual paraffin included, are compared with the values extracted from the absolute values of the elastic scattering cross sections of 52 MeV protons obtained by the method of optical model analysis. The uncertainty in the thickness from the optical model analysis* is about 5%. By taking into account that the thicknesses W by weighing contain those of residual paraffin of about 1096, the DWBA calculations were carried out with the code DWUCK using a Tosbac-3400 computer by Dr. H. Orihara of Tohoku University. Optical parameters derived from proton elastic scattering on 74Ge were in good agreement with those of Becchetti and Greenlees7).
TABLE 1 Comparison of the thicknesses deduced from the scattering of protons and from simple weighing (weight of the residual paraffin included). Enriched isotope
7°GeO 72GeO
74GeO 76GeO
(t4/ O) × ~100% Difference in ratio based on 14/
OpticalWeighing model method (14/) method (O) (mg/cm 2) (mg/cm 2) 1.89 1.28 3.15 1.52
2.10 1.20 3.40 1.65
-
10 7 7 8
results from the two methods can be said to agree with each other except in the case of 72GeO. 3.4. TARGETIMPURITIES The possible sources of impurities are expected to be mainly from SiO2 contamination during the grinding of a coarse grained material into fine powder in an agate mortar, from chlorine contained in ethyl-acetate in minor quantity and from impurities in liquid paraffin. In order to study the above possible sources of target impurities, scattering of 52 MeV protons was measured for the target of Mylar coated with N a t . CdO on Mylar
2°4PbCO~
on
Mylar
1 o=~o
1:
-I 0
_~ Max,mum
-7-6-5-4-3-2-I "
O 2 3 4 5 6 D i s t a n c e (mm)
~
Devlat,on
7 8 9 I0 -
Fig. 4. Uniformity in thickness of an isotropically enriched 2°4pbC03 target.
-8-7-6-5-4-3-2-I
0
2
3 4
Distance (mm)
5 6 -
7
8
F~g. 5. Uniformity in thickness of a thin natural cadmiumoxide target.
414
I. S U ~ A I
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,~
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300
350
400
NUMBER
Fig. 6. Spectrum of protons scattered from a Mylar film of about 0.5 mg/cm 2 in thickness.
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350
Fig. 7. Spectrum of protons scattered from a Mylar backing coated with "first class" liquid paraffin.
400
TARGETS
OF
POWDERED
2000
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_J lxl Z
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target (2.42 mg/cm 2) which was ground for rather a long time in an agate mortar with an initial amount of 10rag and precipitated on a Mylar backing of 0.5 mg/cm 2 in thickness. Any peaks from Si contamination were not observed as shown in fig. 8. Of course, the spectrum in the case of calcium-carbonate ground with a larger initial amount of 50 mg showed no significant difference. An upper limit of 10 -4 mg/cm 2 was calculated for the amount of Si contamination from the experimental spectrum. This is clearly negligible compared with the amounts of 1 and 2 mg/cm 2 for carbon and calcium respectively.
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MATERIALS
i TI
o . "~. I~ Z--.",~ 7O
I00
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25O
300
CHANNEL NUMBER Fig. 8. Spectrum of protons scattered from a calcium-carbonate target, which was ground for a long time in an agate mortar with an initial amount of 10 rag.
"first class and special class" liquid paraffin and natural C a C O 3 ground in an agate mortar (initial amounts being 50 mg and i0 rag). The spectrum of protons scattered from a Mylar backing film of about 0.5 mg/cm 2 in thickness is shown in fig. 6. There is no element detected except for the constituents of the Mylar film. Fig. 7. shows the spectrum of protons scattered from a Mylar backing (0.5 mg/cm 2) coated with "first class" liquid paraffin. We observed no element except for the constituents of Mylar and liquid paraffin. The spectrum in the case of "special class" liquid paraffin had no significant difference from that of the "first class" liquid paraffin shown in fig. 7. Fig. 8. shows the proton spectrum in the case of a very pure (99.999%) natural calcium-carbonate
The author wishes to thank Prof. I. Nonaka, Y. Ishizaki and K. Kaneko for their valuable advice and help, during the development of the techniques described in the present work. Many thanks are also due to Prof. T. Suehiro who helped with the target impurity tests, to Mr. Y. Homma who constructed a series of useful instruments for the experiments, and to Prof. T. Hasegawa for his cooperation and encouragement. The kindness of Drs. G. Madueme and T. Tomita is appreciated for a critical reading of the manuscript. References l) p. R. Kuehn, F. R. O' Donnell and E. H. Kobisk, Nucl. Instr. and Meth. 11)2 (1972) 403. 2) F. J. Karasek, Nucl. Instr. and Meth. 102 (1972) 457. 3) W. Parker, M. de Crofts and K. Sevier, Jr. Nucl. Instr. and Meth. 7 (1960) 22. 4) R. K. Jolly and H. B. White, Jr. Nucl. Instr. and Meth. 97 (1971) 103. 5) V. Verdingh, Nucl. Instr. and Meth. 102 (1972) 431. 6) I. Nonaka and I. Sugai, INS-J-132 (1971) (unpublished). 7) F. D. Becchetti, Jr. and G. W. Greenlees, Phys. Rev. 182 (1969) 1190.
AND METHODS
145 ( 1 9 7 7 )
409-415
; ©
NORTH-HOLLAND
P U B L I S H I N G CO.
A NEW TECHNIQUE FOR PREPARATION OF TARGETS OF POWDERED MATERIALS ISAO SUGAI
Institute for Nuclear Study, University oJ Tokyo, Tanashi-shi, Tokyo 188, Japan Received 12 April 1977 We have made various targets of powdered materials for nuclear experiments by developing a centrifugal precipitation method, which is particularly useful in the cases of: (a) metals, with high melting point and low vapour pressure, (b) oxides or carbonates which are difficult to be handled by the usual vacuum evaporation technique and (c) some enriched isotopes which are generally very minute in quantity (less than - 1 0 mg). The powdered samples were suspended in liquid paraffin with an ultrasonic vibrator, and then centrifugally precipitated on a thin backing film of Mylar or aluminium. Uniformity in the thicknesses of the targets obtained in this way was measured with an o~-ray thickness gauge and was known to be satisfactory. Inspection for any impurities contained in the materials used or contaminated during the preparation process was carried out by measuring the scattered protons from the targets. No significant impurity was detected larger than 10 - 4 mg/cm 2.
I. Introduction Targets of self-supporting foils are most desirable for nuclear reaction experiments for reasons of purity. There are several methods ~a) in the preparation of self-supporting metal targets, including vacuum evaporation, electroplating, rolling and thermal cracking. In practice, it is hard to produce self-supporting foils of such materials as alkali earth metals and rare-earth metals. Moreover, materials with high melting points, for example Zr, Ta, Re, Os and W etc., are not easy to evaporate even by the method of heating by electron bombardment. In general, such materials as mentioned above are obtainable in the forms of oxides or carbonates, and powder of them can be used for target preparation by the method of making suspensions in some suitable liquid. Gravity precipitation, electrospraying and electrophoresis are used for such suspensions3-S). In these methods, however, there are some disadvantages such as thickness limitations, non-uniformity in thicknesses, contamination by impurities, and weakness for beam bombardment. As an alternative, a method of enforced centrifugal precipitation is proposed to be used for fine grain powder of metal oxides or carbonates suspended in liquid paraffin with an ultrasonic vibrator. The target material can easily be separated from the liquid paraffin with the centrifuge. Hence, it is possible to deposit the material onto the backing more satisfactorily than by the other suspension methods mentioned above. Samples of enriched isotopes are expensive, and usually purchased in small quantities. Therefore,
the yields of the target material are always of great concern. In the centrifugal precipitation method, a small amount of enriched isotope ( ~ 5 mg) can be deposited effectively on the backing surface. Any metallic powder is also able to be evenly deposited on the backing when the grain of the powder is fine enough. The range of target thicknesses obtained by the present method is from 0.1 to 50 mg/cm 2. Relatively thin targets (0.1-5 mg/cm 2) are used for charged particle nuclear reaction experiments. Targets thicker than several mg/cm 2 are used for inbeam ~,-ray experiments and fission and neutron capture measurements. 2. Experimental method 2.1. APPARATUS The apparatus consists essentially of a centrifugal tube, an ultrasonic vibrator (29 kHz, 50 W), a centrifugal separator (Swing type 80 m i x 4 , Max4000 rpm) and a vacuum evaporator. The ultrasonic vibrator is used to obtain uniform suspensions of target materials in liquid paraffin. The structure of the centrifugal tube is shown in fig. 1. It is made of duralumin. At the bottom of the tube a thin backing film with or without target frame is fixed, being used for the preparation of relatively thick or thin targets, respectively. Mylar films are usually used as backings. To protect the deposited target material from breaking in handling, it is better to use a special backing film of Mylar having a ring flame of aluminium
410
I.
~45mm
SUGAI
2.3. SUSPENSION AGENT Liquid paraffin has been found to be most suitable in making uniform suspensions of the target materials. This liquid is a highly refined nonvolatile carbon hydrate oil and is easily diluted with ethyl acetate.
~
!Omm@~
E O
E O
,, 5ram
l!!
it I ~
03 (~)
(~)
Fig. 1. Centrifugal tube made of duralumin: (1) polyethylene film ring (200/lm in thickness), (2) backing film with frame, (3) bottom disc.
foil (200/~m thick) glued to one side of it as shown in fig. 1. A polyethylene ring (200/1m thick) is used as a packing during the centrifugal process. When a frameless backing is used the Mylar film itself plays the role of packing. The centrifugal tube, after being assumbled, is heated in an electric oven kept at about 100°C to remove distortions in the backing film.
2.4. TARGETPREPARATION The target materials which can be treated by the centrifugal precipitation method sould satisfy the following requirements: chemically stable, insoluble and non-reactive in liquid paraffin, indeliquiscent and non-reactive during the pulverization. As an example of the practical methods in the process we describe the case of a neodymium-oxide (143Nd203) target. The desired target thickness was about 2.5 mg/cm 2. For a target with an area of about 3 cm 2 an initial amount of about 7.85 mg of oxide was used. The amount of liquid paraffin was adjusted depending on the amount of target material used. For example, 7.85 mg of Nd203 was suspended in 10 ml of liquid paraffin. The viscosity was decreased by heating with an infrared ray lamp. A homogeneous suspension of neodymiumoxide was obtained with the aid of an ultrasonic vibrator of suitably adjusted frequency. The ultrasonic vibrator was operated for about 30 min. Then the neodymium-oxide became well su2 I
2.2. BACKINGS The backing films should usually be as thin as possible. Such organic films as polyvinyl formal, collodion, polyethylene and Mylar are generally used as backings. On the other hand, aluminium, gold, nickel and carbon foils are also used. In our institute Mylar films are used very often except in some special cases. Mylar is a polymer consisting of carbon, oxygen and hydrogen and has suitable physical and chemical properties, being stable at temperatures up to 150°C, inert to the acids usually employed in target preparation work, easy to handle and strong. Because the Mylar backing is heat treated after being attached to the centrifugal tube as described in the preceding section, the effect of heat treatment on the thickness of the Mylar film was measured with an ~z-ray thickness gauge 6) before and after the heat treatment in an oven. In figs. 2a and 2b, the uniformities in thickness are shown. Changes in the uniformity of thickness due to the heat treatment were found to be negligible (<3%).
c o_ .~-
,/ O
q\ \/
,
i.
c] -I ~ ' ~ _2,~,/ -5--10-8
Maximum Deviation 1.8% or 9~.g/cm 2
-6
2,
-4
-2 0 Distance
2 4 (ram)
6
After heat treatment 0.500 mg/cm 2
~
(a)
Before heat treatment 0 ' 4 9 9 mg/cm2
---
-10-8
I -6
8 IO
,_~_ /
(b)
/
MaxLmum Deviation 2 0 % or IO~g/cm 2
I/~
r., /
-2~
]
I { 1 I ] I ; I I -4
-2 0 2 D i s t a n c e (ram)
4
6
8 I0
Fig. 2. (a) Uniformityin thickness of a Mylar backing film before heating. (b) Uniformityin thickness of the Mylar backing film after heating in an oven at about 100°C.
T A R G E T S OF P O W D E R E D M A T E R I A L S
spended, no precipitation being observed even after about 24 h. The homogeneous suspension was subjected to the action of the centrifuge until the oxide was effectively collected on the backing film. The centrifugal separator was operated usually at 3000 rpm for 60 min. The degree of transparency of liquid paraffin showed the efficiency of the precipitation. In the case of unsatisfactory precipitation, the liquid paraffin was cloudy. A due amount of ethylacetate was added for relaxation of the high viscosity when the supernatant liquid paraffin was sucked out of the centrifugal tube. The liquid paraffin remaining on the target was dried off in a vacuum vessel kept at 10 -3 torr and at about 200°C for about 5 h. By this process the colour of the target surface changed from semitransparent to white. A small amount of liquid paraffin is considered to remain in the target as will be discussed in section 3.1. Special care should be taken in detaching the target film from the bottom of the centrifugal tube without breaking it. Originally coarse-grained target material, such as CaCO3, BaCO3, A1203, Sc203 and GeO2, had to be ground down to fine powder in an agate mortar before suspension. 3. Results and discussion Targets in the thickness range of 0.1-50 mg/cm 2 of some sixty different materials have been prepared by the present method. Mylar films (4-10 ~tm) were used as backings except in a few cases of Co, GaI3, LiF, Sb, Se, SrCO 3 and Zr for which 10/.zm aluminium foils were used and for Nd203 6 ~m carbon foils. The metallic powders of W, Zr, Mo and Ta with high melting points, and their oxides (WO3, ZrO2, MoO2 and Ta2 05) could be made into homogeneous targets. The low melting point metals, Ga and In, were transformed into their oxides so as not to be evaporated when they were bombarded with cyclotron beams. For much thicker targets (20-50 mg/cm2), there were some case in which uniform and stable suspensions were not easily obtained. In such cases, the centrifugal process must follow immediately after the ultrasonic mixing. More uniform and better targets could be obtained by the removal of 80% of the supernatant liquid paraffin in the tube and then by addition of an adequate amount of new suspension.
411
When any uniform suspension was difficult to obtain in the procedure described above because of the unequal grain sizes of the powder, a quite fine grain powder of the material could be prepared by the use of a suspension part of the material in ethyl-acetate. 3.1. AMOUNT OF RESIDUAL PARAFFIN
The completed targets prepared by the process described in section 2.4 are considered to contain some residual paraffin. In order to estimate the true value of the thickness of the target material only, the amounts of this residual liquid paraffin must be known. We tried to know this amount by evaporating stepwise the residual paraffin by radiation heating in vacuum and by intermittently weighing the target. The target used for this experiment was natural Nd203 of around 3 mg/cm 2 in thickness. The initial thickness before radiation heating was 2.44 mg/cm 2. The radiation source was a tantalum plate maintained at about 1500°C. The Nd203 target was set at 5 cm above the radiation plate. Radiation heating was done every 20 rain and the thickness of the target was measured with a microbalance before and after the radiation heating. When the colour of the target changed from semi-transparent to white, this being generally the final status of the completed target, the heat treatment was stopped . The thickness after final heat treatment as a normal target was 2.72 mg/cm 2. When the target was heated for a long time in vacuum and all the residual liquid paraffin was evaporated away, it was weighed. The thickness of the target material only was thus known to be 2.50 mg/cm 2. Therefore, the amount of residual liquid paraffin in the target was estimated to be the difference 2.72-2.50=0.22 m g / c m 2 in thickness, or 8.8% of the target material thickness. This amount of the residual liquid paraffin was necessary to keep the strength of the target and seemed to play the role of a binder between the target material and the backing, and also between the grains of the target material. Of course, the amount of the residual liquid paraffin in a target will depend on the thickness of the target and also on the grain size of the target material, being proportional to the target thickness and larger for a smaller grain size. The above mentioned value of 0.22 mg/cm 2 (8.8%) should be considered to be an example showing the order of the amounts of residual liq-
412
]. SUGAI
uid paraffin in completed targets prepared by the present method. Other examples of the amounts of residual paraffin estimated from the recovery ratio of 95% if the fed material, which is described in the next section, are as follows: 9.2% (0.17 mg/cm 2) for a natural Gd203 (relatively coarse grained) target of 1.85 mg/cm 2, 10.9% (0.39 mg/cm 2) for a 158Gd203 (relatively fine grained) target of 3.57 mg/cm 2, and 11.5% (0.37mg/cm 2) for a 1485m203 target of 3.18 mg/cm 2 in thickness. From the above mentioned results, it seems reasonable to consider that the amount of residual paraffin is about 10% of the thickness of the target material for rare earth oxides.
3b shows the uniformity along the direction perpendicular to the former. In this case, the maximum deviation of 2.7% of the central thickness (4.85mg/cm 2) was obtained, corresponding to 131 # g / c m 2 in absolute thickness. Hence, the deposition of target material can be said to be fairly N a t , Gd20;3
on M y l a r
(a)
% ::L
3.2. YIELD OF TARGETMATERIALS Natural rare earth oxide targets of Sm, Eu and Nd of about 2 mg/cm 2 thick were prepared to estimate the yields of the target materials during the preparation process. The recovery ratios, defined as the ratio of the total amount of the target material contained in a completed target to that of the material fed initially into the centrifugal tube, were measured to be 95% for the targets tested. 3.3. THICKNESSUNIFORMITY The uniformity in thickness of the targets prepared by the present method was measured with an o~-ray thickness gauge. Details of the thickness measurement of chemical compounds by this gauge have been reported previously6). Targets of the following compounds were used for the measurements: natural Gd203, PbCO3, CdO and enriched 2°4pbCo3. Mylar backings of about 4 # m in thickness were used in all cases. In the uniformity measurement the target was moved in 1 mm steps, the diameter of the exit slit of the ~z-ray gauge being 1.2 mm. The thicknesses of backing Mylar foils were measured beforehand by weighing and were subtracted from the measured total thicknesses of the targets, being assumed to have uniform thickness distributions. The maximum thickness deviation of Mylar foils of 4 # m thick is about 10 a g / c m 2 as shown in figs. 2a, 2b. So the results of the measured maximum thickness deviations of the target materials shown will contain this value of 10#g/cm 2 due to the Mylar backings. Fig. 3a shows the uniformity in thickness of a natural Gd203 target along one direction, and fig.
"~- 5
-.
i:
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I 2 3 4 (mrn) ~
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7
8
Nat. Gd203 on Mylar
15 o.~ I
~ 0
~
-
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Deviation
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Distance (mm)~ Fig. 3.(a) U n i f o r m i t y in thickness o f a natural Gd203 target. (b) UniFormity in thickness o f the same target along the direction perpendicular to that in (a).
413
T A R G E T S OF P O W D E R E D M A T E R I A L S
uniform. For the isotopically enriched 2°4PBCO3 target of 2.36 mg/cm 2 in central thickness, the values wer6 3.0°/6 and 72/~g/cm 2, respectively (fig. 4). In general, carbonate target materials like 2°4pbCO3 a r e coarse-grained and have to be ground into fine powder beforehand, otherwise the maximum deviation in thickness is twice as large. The results for a natural CdO target of rather small thickness are shown in fig. 5, where rather large relative deviations are observed because of the thin central thickness, the absolute deviation being rather small. In table 1, the target thicknesses of enriched germanium-oxides estimated by simple weighing with a microbalance, the weihgt of the residual paraffin included, are compared with the values extracted from the absolute values of the elastic scattering cross sections of 52 MeV protons obtained by the method of optical model analysis. The uncertainty in the thickness from the optical model analysis* is about 5%. By taking into account that the thicknesses W by weighing contain those of residual paraffin of about 1096, the DWBA calculations were carried out with the code DWUCK using a Tosbac-3400 computer by Dr. H. Orihara of Tohoku University. Optical parameters derived from proton elastic scattering on 74Ge were in good agreement with those of Becchetti and Greenlees7).
TABLE 1 Comparison of the thicknesses deduced from the scattering of protons and from simple weighing (weight of the residual paraffin included). Enriched isotope
7°GeO 72GeO
74GeO 76GeO
(t4/ O) × ~100% Difference in ratio based on 14/
OpticalWeighing model method (14/) method (O) (mg/cm 2) (mg/cm 2) 1.89 1.28 3.15 1.52
2.10 1.20 3.40 1.65
-
10 7 7 8
results from the two methods can be said to agree with each other except in the case of 72GeO. 3.4. TARGETIMPURITIES The possible sources of impurities are expected to be mainly from SiO2 contamination during the grinding of a coarse grained material into fine powder in an agate mortar, from chlorine contained in ethyl-acetate in minor quantity and from impurities in liquid paraffin. In order to study the above possible sources of target impurities, scattering of 52 MeV protons was measured for the target of Mylar coated with N a t . CdO on Mylar
2°4PbCO~
on
Mylar
1 o=~o
1:
-I 0
_~ Max,mum
-7-6-5-4-3-2-I "
O 2 3 4 5 6 D i s t a n c e (mm)
~
Devlat,on
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Fig. 4. Uniformity in thickness of an isotropically enriched 2°4pbC03 target.
-8-7-6-5-4-3-2-I
0
2
3 4
Distance (mm)
5 6 -
7
8
F~g. 5. Uniformity in thickness of a thin natural cadmiumoxide target.
414
I. S U ~ A I
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Fig. 6. Spectrum of protons scattered from a Mylar film of about 0.5 mg/cm 2 in thickness.
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Fig. 7. Spectrum of protons scattered from a Mylar backing coated with "first class" liquid paraffin.
400
TARGETS
OF
POWDERED
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target (2.42 mg/cm 2) which was ground for rather a long time in an agate mortar with an initial amount of 10rag and precipitated on a Mylar backing of 0.5 mg/cm 2 in thickness. Any peaks from Si contamination were not observed as shown in fig. 8. Of course, the spectrum in the case of calcium-carbonate ground with a larger initial amount of 50 mg showed no significant difference. An upper limit of 10 -4 mg/cm 2 was calculated for the amount of Si contamination from the experimental spectrum. This is clearly negligible compared with the amounts of 1 and 2 mg/cm 2 for carbon and calcium respectively.
t2Cgr¢l
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MATERIALS
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CHANNEL NUMBER Fig. 8. Spectrum of protons scattered from a calcium-carbonate target, which was ground for a long time in an agate mortar with an initial amount of 10 rag.
"first class and special class" liquid paraffin and natural C a C O 3 ground in an agate mortar (initial amounts being 50 mg and i0 rag). The spectrum of protons scattered from a Mylar backing film of about 0.5 mg/cm 2 in thickness is shown in fig. 6. There is no element detected except for the constituents of the Mylar film. Fig. 7. shows the spectrum of protons scattered from a Mylar backing (0.5 mg/cm 2) coated with "first class" liquid paraffin. We observed no element except for the constituents of Mylar and liquid paraffin. The spectrum in the case of "special class" liquid paraffin had no significant difference from that of the "first class" liquid paraffin shown in fig. 7. Fig. 8. shows the proton spectrum in the case of a very pure (99.999%) natural calcium-carbonate
The author wishes to thank Prof. I. Nonaka, Y. Ishizaki and K. Kaneko for their valuable advice and help, during the development of the techniques described in the present work. Many thanks are also due to Prof. T. Suehiro who helped with the target impurity tests, to Mr. Y. Homma who constructed a series of useful instruments for the experiments, and to Prof. T. Hasegawa for his cooperation and encouragement. The kindness of Drs. G. Madueme and T. Tomita is appreciated for a critical reading of the manuscript. References l) p. R. Kuehn, F. R. O' Donnell and E. H. Kobisk, Nucl. Instr. and Meth. 11)2 (1972) 403. 2) F. J. Karasek, Nucl. Instr. and Meth. 102 (1972) 457. 3) W. Parker, M. de Crofts and K. Sevier, Jr. Nucl. Instr. and Meth. 7 (1960) 22. 4) R. K. Jolly and H. B. White, Jr. Nucl. Instr. and Meth. 97 (1971) 103. 5) V. Verdingh, Nucl. Instr. and Meth. 102 (1972) 431. 6) I. Nonaka and I. Sugai, INS-J-132 (1971) (unpublished). 7) F. D. Becchetti, Jr. and G. W. Greenlees, Phys. Rev. 182 (1969) 1190.