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A new technique of target preparation
NUCLEAR
A N D M E T H O D S 87 (197o) 3 1 1 - 3 1 2 ;
INSTRUMENTS
© NORTH-HOLLAND
PUBLISHING
CO
A NEW T E C H N I Q U E OF TARGET PREPARATION T. SUZUKI, S. BABA, H. U M E Z A W A and H. A M A N O
Japan Atomic Energy Research Institute, Tokai, Ibaraki, Japan Received 19 May 1970 A technique for preparing targets of various thicknesses ranging from 0.1 mg/cm~ to 60 mg/cm 2 was developed by centrifuging powder suspended in liquid paraffin. The uniformity was investigated by measuring the penetrability of a collimated fl-ray beam. The local fluctuation in thickness from the average value was found to be less than + 3% except at the very edge of each target.
In the study of nuclear reactions, preparation of the targets with sufficient solidity and appropriate and uniform thickness is indispensable. In cases where the target materials are available in metallic form, various methods ~-6) are easily applied to satisfy a variety of requirements. However, for elements that are available practically in the powder form, the suspension technique using a suitable liquid is a powerful method. Although various methods of depositing target materials from suspensions have been devised2'7-11), each method has disadvantages such as limitations in the obtainable thickness, uniformity, and purity, or the need of a large excess of the material. These disadvantages were successfully eliminated by the pres-
~36mm~
12°.~.z'
I[
,
Fig. 1. Centrifuge tube made of aluminum: (1) top-half; (2) bottom-half; (3) polyethylene film ring, 0.2 mm thick; (4) target holder with backing foil; (5) polyethylene film, 0.1 mm thick; (6) bottom disc.
ently developed technique. The method consists of suspensions of the powder material in liquid paraffin and centrifuge. It is applicable to all materials in the powder form, regardless of compounds or elements, which satisfy the following conditions: (a) chemically stable, (b) insoluble in liquid paraffin and in ethyl acetate, (c) not hygroscopic nor deliquescent, and (d) able to be ground finely without any chemical change. Procedure: Fig. 1 shows a bottom-removable centrifuge tube made of aluminum. On the bottom disc a polyethylene film, a backing foil supported by an aluminum ring, and a polyethylene film ring were piled up in this order, and then the whole assembly was fixed to the edge of the cylinder with screws. The polyethylene films work as packing material. Metallic foil or Mylar film was used as backing. The top-half of the cylinder is also removable so that liquid paraffin is easily pipetted out after the powder has settled down. The target-holder attached with a backing foil is shown in fig. 2. The powder of a suitable amount, 10% excess of the needed amount, was put into a beaker. Less than 1 ml of ethyl acetate as wetting agent and an appropriate amount of liquid paraffin (density 0.855) were added. Up to 50 mg of the powder could be suspended in 1 ml of liquid paraffin. The beaker was warmed to reduce the viscosity of liquid paraffin and also to evaporate out the added ethyl acetate. Then the beaker was put in a water bath of an ultrasonic vibrator. When uniform dispersion is not attained and the suspended powder looks coagulated, the beaker should be shaken carefully till the localization disappears. The obtained suspension of the powder in liquid paraffin was poured into the centrifuge tube (fig. 1) and was charged in a centrifuge. These procedures ought to be performed as soon as possible because a considerable amount of powder settles down as time passes. Centrifuge had to be carried out for 30 to 60 min. with turn of 3000 rpm. After centrifuge, about 3 ml of ethyl acetate was poured into the tube with a capillary pipette. Liquid paraffin floating on the ethyl acetate
311
'~L20i ~-36mm ~I~
312
T. SUZUKI et al.
r,
Fig. 2. Target holder made of aluminum: (1) aluminum ring; (2) backing foil, sticked to the aluminum ring. layer was removed by repeated washing with ethyl acetate. In the case o f a very thin target, a mixture o f equal amounts o f ethyl acetate and liquid paraffin should be used instead, to avoid the coagulation o f particles. Washing with ethyl acetate was repeated several times. The deposit was allowed to stand for 5 to 10 min in air and dried in a v a c u u m dryer at 130°C for 20 to 50 h till a constant weight was attained. Results: The technique was applied to target prep-
aration for various cross-section measurements o f nuclear reactions induced by neutrons t2) and charged particles'a). Rare earth elements and uranium were o f our interest, which are hardly obtained in the metallic f o r m free f r o m the surface oxidation. The needed thickness ranged from 0.1 to 60 mg/cmZ; this requirement was thoroughly satisfied by the present method. In table 1 is given a list o f targets which have been prepared in our laboratory. Uniformities o f U 3 0 stargets were tested by measuring the absorption rate o f the collimated //-ray beam f r o m 9°Sr-9°Y source. It was found that the deviation from the average thickness was less than 3% except in the edge portion for each target. The strength o f the targets was sufficient for the usual handling. They bore up against several hours transportation by car. After the b o m b a r d m e n t with 0.1-/~A p r o t o n beam for 20 h in a cyclotron, no sign o f change was observed. This strength is t h o u g h t to be caused by liquid paraffin remaining in the deposit and serving as binder o f fine particles. The a m o u n t o f remaining liquid paraffin was found negligibly small (=< 0.2% o f the deposit) by measuring the difference between weights of each deposit before and after heating it at 700°C for 2 h.
References
TABLE 1 A list of targets prepared. Material
Thickness (mg/cm2)
Nuclear reaction studied
Backing material (thickness,/tin)
U3Os 150SmzOz 15~Sm203 154Sm~O3 151Eu~O3 12~Te 143Nd~O3 149Sm203 147Sm203
20-60 6 10 2-3 0.2 0.3 0.2 0.14).3 0.2
(p,f) (p,t) (p,t) (p,t) (n,~) (n,ct) (n,~) (n,~) (n,~)
A1 (20) Mylar (8) Mylar (8) Mylar (8) A1 (.20) A1 (20) AI (20) A1 (20) AI (20)
t) M. L. Smith, AERE-R-5097 (1965). 2) L. Yaffe, Ann. Rev. Nucl. Sci. 12 (1962) 153. 3) H. P. H/inni, Helv. Phys. Acta 33 (1960) 987. 4) S. H. Maxman, Nucl. Instr. and Meth. 50 (1967) 53. 5) L. Westgaard and S. Bjornholm, Nucl. Instr. and Meth. 42 (1966) 77. 6) L. D. F. Allen, LA 2769 t1962). 7) S. Bjornholm, PH. Dam, H. Nordby and N. O. Roy Poulsen, Nucl. Instr. and Meth. 5 (1959) 196. s) D. J. Carswell and J. Milsted, J. Nucl. Energy 4 (1957) 51; L. N. Blumberg, P. Stein and J. C. Gursky, LA 2711 (1962). 9) j. B. Natowitz, F. Pement and R. L. Wolke, Rev. Sci. Instr. 37 (1966) 121. 10) N. S. Wall and J. W. Irvine, Jr., Rev. Sci. Instr. 24 (1953) 1146. 11) G. Fodor and B. L. Cohen, Rev. Sci. Instr. 31 (1960) 73. 12) K. Okamoto, Nucl. Phys. A141 ('1970) 193. 13) H. Umezawa and S. Baba, to be published.
A N D M E T H O D S 87 (197o) 3 1 1 - 3 1 2 ;
INSTRUMENTS
© NORTH-HOLLAND
PUBLISHING
CO
A NEW T E C H N I Q U E OF TARGET PREPARATION T. SUZUKI, S. BABA, H. U M E Z A W A and H. A M A N O
Japan Atomic Energy Research Institute, Tokai, Ibaraki, Japan Received 19 May 1970 A technique for preparing targets of various thicknesses ranging from 0.1 mg/cm~ to 60 mg/cm 2 was developed by centrifuging powder suspended in liquid paraffin. The uniformity was investigated by measuring the penetrability of a collimated fl-ray beam. The local fluctuation in thickness from the average value was found to be less than + 3% except at the very edge of each target.
In the study of nuclear reactions, preparation of the targets with sufficient solidity and appropriate and uniform thickness is indispensable. In cases where the target materials are available in metallic form, various methods ~-6) are easily applied to satisfy a variety of requirements. However, for elements that are available practically in the powder form, the suspension technique using a suitable liquid is a powerful method. Although various methods of depositing target materials from suspensions have been devised2'7-11), each method has disadvantages such as limitations in the obtainable thickness, uniformity, and purity, or the need of a large excess of the material. These disadvantages were successfully eliminated by the pres-
~36mm~
12°.~.z'
I[
,
Fig. 1. Centrifuge tube made of aluminum: (1) top-half; (2) bottom-half; (3) polyethylene film ring, 0.2 mm thick; (4) target holder with backing foil; (5) polyethylene film, 0.1 mm thick; (6) bottom disc.
ently developed technique. The method consists of suspensions of the powder material in liquid paraffin and centrifuge. It is applicable to all materials in the powder form, regardless of compounds or elements, which satisfy the following conditions: (a) chemically stable, (b) insoluble in liquid paraffin and in ethyl acetate, (c) not hygroscopic nor deliquescent, and (d) able to be ground finely without any chemical change. Procedure: Fig. 1 shows a bottom-removable centrifuge tube made of aluminum. On the bottom disc a polyethylene film, a backing foil supported by an aluminum ring, and a polyethylene film ring were piled up in this order, and then the whole assembly was fixed to the edge of the cylinder with screws. The polyethylene films work as packing material. Metallic foil or Mylar film was used as backing. The top-half of the cylinder is also removable so that liquid paraffin is easily pipetted out after the powder has settled down. The target-holder attached with a backing foil is shown in fig. 2. The powder of a suitable amount, 10% excess of the needed amount, was put into a beaker. Less than 1 ml of ethyl acetate as wetting agent and an appropriate amount of liquid paraffin (density 0.855) were added. Up to 50 mg of the powder could be suspended in 1 ml of liquid paraffin. The beaker was warmed to reduce the viscosity of liquid paraffin and also to evaporate out the added ethyl acetate. Then the beaker was put in a water bath of an ultrasonic vibrator. When uniform dispersion is not attained and the suspended powder looks coagulated, the beaker should be shaken carefully till the localization disappears. The obtained suspension of the powder in liquid paraffin was poured into the centrifuge tube (fig. 1) and was charged in a centrifuge. These procedures ought to be performed as soon as possible because a considerable amount of powder settles down as time passes. Centrifuge had to be carried out for 30 to 60 min. with turn of 3000 rpm. After centrifuge, about 3 ml of ethyl acetate was poured into the tube with a capillary pipette. Liquid paraffin floating on the ethyl acetate
311
'~L20i ~-36mm ~I~
312
T. SUZUKI et al.
r,
Fig. 2. Target holder made of aluminum: (1) aluminum ring; (2) backing foil, sticked to the aluminum ring. layer was removed by repeated washing with ethyl acetate. In the case o f a very thin target, a mixture o f equal amounts o f ethyl acetate and liquid paraffin should be used instead, to avoid the coagulation o f particles. Washing with ethyl acetate was repeated several times. The deposit was allowed to stand for 5 to 10 min in air and dried in a v a c u u m dryer at 130°C for 20 to 50 h till a constant weight was attained. Results: The technique was applied to target prep-
aration for various cross-section measurements o f nuclear reactions induced by neutrons t2) and charged particles'a). Rare earth elements and uranium were o f our interest, which are hardly obtained in the metallic f o r m free f r o m the surface oxidation. The needed thickness ranged from 0.1 to 60 mg/cmZ; this requirement was thoroughly satisfied by the present method. In table 1 is given a list o f targets which have been prepared in our laboratory. Uniformities o f U 3 0 stargets were tested by measuring the absorption rate o f the collimated //-ray beam f r o m 9°Sr-9°Y source. It was found that the deviation from the average thickness was less than 3% except in the edge portion for each target. The strength o f the targets was sufficient for the usual handling. They bore up against several hours transportation by car. After the b o m b a r d m e n t with 0.1-/~A p r o t o n beam for 20 h in a cyclotron, no sign o f change was observed. This strength is t h o u g h t to be caused by liquid paraffin remaining in the deposit and serving as binder o f fine particles. The a m o u n t o f remaining liquid paraffin was found negligibly small (=< 0.2% o f the deposit) by measuring the difference between weights of each deposit before and after heating it at 700°C for 2 h.
References
TABLE 1 A list of targets prepared. Material
Thickness (mg/cm2)
Nuclear reaction studied
Backing material (thickness,/tin)
U3Os 150SmzOz 15~Sm203 154Sm~O3 151Eu~O3 12~Te 143Nd~O3 149Sm203 147Sm203
20-60 6 10 2-3 0.2 0.3 0.2 0.14).3 0.2
(p,f) (p,t) (p,t) (p,t) (n,~) (n,ct) (n,~) (n,~) (n,~)
A1 (20) Mylar (8) Mylar (8) Mylar (8) A1 (.20) A1 (20) AI (20) A1 (20) AI (20)
t) M. L. Smith, AERE-R-5097 (1965). 2) L. Yaffe, Ann. Rev. Nucl. Sci. 12 (1962) 153. 3) H. P. H/inni, Helv. Phys. Acta 33 (1960) 987. 4) S. H. Maxman, Nucl. Instr. and Meth. 50 (1967) 53. 5) L. Westgaard and S. Bjornholm, Nucl. Instr. and Meth. 42 (1966) 77. 6) L. D. F. Allen, LA 2769 t1962). 7) S. Bjornholm, PH. Dam, H. Nordby and N. O. Roy Poulsen, Nucl. Instr. and Meth. 5 (1959) 196. s) D. J. Carswell and J. Milsted, J. Nucl. Energy 4 (1957) 51; L. N. Blumberg, P. Stein and J. C. Gursky, LA 2711 (1962). 9) j. B. Natowitz, F. Pement and R. L. Wolke, Rev. Sci. Instr. 37 (1966) 121. 10) N. S. Wall and J. W. Irvine, Jr., Rev. Sci. Instr. 24 (1953) 1146. 11) G. Fodor and B. L. Cohen, Rev. Sci. Instr. 31 (1960) 73. 12) K. Okamoto, Nucl. Phys. A141 ('1970) 193. 13) H. Umezawa and S. Baba, to be published.