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Preparation of the thicker americium targets by molecular plating
Applied Radiation and Isotopes 54 (2001) 741–744
Technical note
Preparation of the thicker americium targets by molecular plating Qin Zhi*, Guo Junsheng, Gan Zaiguo Institute of Modern Physics, Chinese Academy of Science, Lanzhou 730000 People’s Republic of China Received 21 August 2000; received in revised form 22 September 2000; accepted 24 October 2000
Abstract Electrodeposition of americium from the mixture of isopropyl alcohol and dilute nitric acid was studied. The Am targets with thickness between 600 mg/cm2 and 1.2 mg/cm2 had been prepared on thin aluminum foil (7 mm) by molecular plating at one time. # 2001 Elsevier Science Ltd. All rights reserved.
241,243
Keywords: Americium targets; Molecular plating
1. Intruduction The preparation of heavy actinide targets is of importance for production of transuranium elements by heavy-ion fusion evaporation reactions. It is difficult to prepare this kind of target because of the high aradioactivity of the actinide target material. These materials are usually very expensive because of limited availability. Such a target consists of a layer of a pure actinide compound deposited on a thin metal foil. The deposit should be uniform, adherent to the substructure. When actinide targets are used in very intense heavy-ion beams, as in attempts to make new transuranium radioisotopes, special attention must be paid to the problem of the radiation and thermal stability of the target. Frequently used target preparation techniques are electrospraying, electromagnetic isotope separation and electrodeposition. Among these methods, electrodeposition is the preferred method for preparing targets of the actinide elements. Its deposition efficiency is very high and the equipment needed for preparing the deposits is simple and can easily be handled in glove box or hoods.
*Corresponding author. Tel.: 86-931-8275924; fax: 86-9318272100. E-mail address: [email protected] (Q. Zhi).
The molecular plating method allows rapid preparation of a thin and extremely uniform deposit, especially necessary for the accurate activity measurement of aemitting nuclides (Shinohara and Kohno, 1989; Getoff and Bildstein, 1965). Shinohara and Bildstein rapidly prepared high-resolution sources for a-ray spectrometry of actinides in spent fuel by electrodeposition from isopropyl alcohol solution (Shinohara and Bildstein, 1965). On the other hand, the molecular plating method had also been used for the preparation of relatively thick targets (Trautmann et al., 1982; Evans et al., 1972; Mullen and Aumann, 1975; Aumann and Muller, 1974). Trautmann et al. prepared 248Cm targets from of isopropyl alcohol and dilute nitric acid (Trautmann et al., 1982). Mullen et al. electrodeposited Th, U, Np, Pu, Am, Cm, Cf from organic solutions onto different backing materials in the range from 10 mg/cm2 up to 2 mg/cm2 (Evans et al., 1972) . Experience had shown that for all the systems investigated, the following three experimental conditions were very advantageous for producing uniform and adhering films: (1) the use of very small volumes of the organic solvent, namely 1–2 ml; (2) the application of relatively high current densities between 2–6 mA/cm2; (3) the layers were very adherent as long as not more than 100 mg/cm2 of the elements were precipitated at
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 6 7 - 5
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Q. Zhi et al. / Applied Radiation and Isotopes 54 (2001) 741–744
one time. To obtain thicker layers, the backing with the layers was calcinated in a muffle furnace and the deposition procedure repeated several times. In this way, layers of between 0.5 and 2 mg/cm2 could be obtained. For example, an americium target with a thickness of 550 mg/cm2 had been produced by repeated electrodeposition of layers of approximately 85 mg/cm2 and heating the foils at 5508C six times. The electrodeposition techniques described in this paper were used to produce short-lived transuranium nuclei. In order to increase the yield of the expected radioisotopes in heavy-ion fusion-evaporation reactions, thicker americium targets are required. By using the procedures given in this paper, 241,243Am targets on thin aluminum foil with thickness in the range of 600 mg/ cm2–1.2 mg/cm2 were obtained by a single molecular plating cycle.
2. Experimental Because americium isotopes have extremely high toxicity and high specific activity, the maximum permissible body burdens or atmospheric concentrations of americium are very small, namely 1.5 10ÿ2 mg or 2 10ÿ12 mCi/ml (Seaborg and Loveland, 1985), all experiments were carried out in a glove-box and under the most rigid precautions. 2.1. Preparation of
241,243
Am stock solution
Enriched 241,243Am was purchased from the China National Nuclear Corporation and State Scientific Center of Russia, respectively. Americium dioxide AmO2 was obtained in the form of a dark brown powder. AmO2 dissolves readily in hydrochloric acid with the evolution of chlorine, and in nitric acid with the evolution of oxygen. We found differences between two kinds of americium dioxides in dissolution behavior. Americium dioxide 241 AmO2 (8 mg) dissolved completely in 5 ml of 0.16 N nitric acid, and the solution of Am(NO3)3 producing a pink colored stock solution of 241Am, with a concentration of 1.6 mg/ml. Americium dioxide 243AmO2 (10 mg) did not dissolve in 5 ml of 0.16 N nitric acid after standing overnight, leaving a residue of dark brown powder at the bottom of the tube. The solution was heated to dryness to remove nitric acid, and the residue was dissolved in 3 ml of hot concentrated hydrochloric acid yielding a solution orange–yellow in color. Since americium nitrate was needed in the following molecular plating method, the solution of AmCl3 was heated to dryness, and the residue dissolved in 2 ml of 0.16 N nitric
acid to yield a pink stock solution of concentration of 5 mg/ml. 2.2. Measurement of standards
241
Am and
241
Am, with a
243
Am reference
As 241Am, T1=2 432 yr, has a characteristic g-ray of energy of 59.5 keV with a branch ratio of 35.9%, and 243 Am, T1=2 7073 yr, a characteristic g-ray of energy of 74.7 keV with a branch ratio of 68%, it was convenient to measure the g-activities of the sample to determine the amount of americium present. It is not necessary to make a thin layer of actinide on platinum or stainless steel disk by electrodeposition to determine a-activity. Furthermore, self-absorption effects would be serious when measuring a-activity for thicker target layers. An aliquot of 241Am or 243Am stock solution was transferred to a small plastic tube, and the concentration of the solution was assayed by measuring g-activity with HPGe detector. The g-spectra was obtained using a GMX-40220 detector, multi-channel analyzer MCA919 and a personal computer. The energy resolution was 2.2 keV for 1332 keV g-ray of 60Co. It was demonstrated that only the g-ray of 59.5 keV could be observed in the sample of 241Am, and other g-activities which could contaminate the sample were not found. The same sample was measured with a silicon surface barrier detector in another experiment, and no unexpected aactivity was found. With the 243Am solution, we observed g-rays of 74.7, 99.6, 103.8, 106.1, 117.6, 177.6 and 228.2 keV, and also found the 59.5 keV g-ray contributed by 241Am. The isotopic composition of 243 Am sample was determined to be 243Am: 99.3%, 241 Am: 0.7%, this was consistent with the assay by the supplier. 2.3. Electrodeposition The molecular plating was carried out in the cell shown schematically in Fig. 1. The electrodeposition cell was made of Pyrex glass and consisted of a cylinder of 18 mm i.d. and 80 mm length, and a jacket for cooling the electrolyte solution. This cell was attached to a stainless steel base. A rubber ‘‘O’’ ring of 10 mm inner diameter was used to define the area of deposition. To stir the solution, a platinum wire (anode, 1 mm diameter) was rotated continuously by a motor. After washing with acetone to remove any grease, an aluminum foil (cathode, 7 mm thickness) was put between the cell and the stainless-steel base as the backing material for electrodeposition. For determining the optimum condition of the electrodeposition, the deposition behavior of 241Am and 243Am on an aluminum foil were investigated in the present experiment. The cell was filled with 15 ml isopropyl alcohol, an aliquot of the 241Am or 243Am
Q. Zhi et al. / Applied Radiation and Isotopes 54 (2001) 741–744
743
Fig. 2. The efficiency of deposition of americium by molecular plating method as a function of the duration of electrodeposition. The circle represents the efficiency of deposition of 241Am, and triangle represents that of 243Am. Fig. 1. Sketch scheme of the molecular plating. 1. Pyrex cell body; 2. ‘‘O’’ ring; 3. Aluminium foil with thickness of 7 mm; 4. Stainless steel base; 5. Platinum wire.
stocking solution. According to the suggestion of Shinohara and Kohno, the highest yield of over 98% was obtained in the case of current density of 3–5 mA/ cm2 at the potential of 400–800 V. A typical electrolysis, which was carried out with a current of 5 mA/cm2 at the voltage of 500 V, and the distance between cathode and anode was 3 cm, lasted for 1 h. The efficiency of deposition of americium by molecular plating method was investigated as a function of duration of electrodeposition as shown in Fig. 2. The efficiency of americium was determined by measuring the activities of the characteristic g-ray of 100 ml of the solution before, during and after the electrodeposition with g spectroscopy. One can find that the main part (ca. 85%) of 241Am or (ca 70%) of 243Am had already deposited within 5 min. For plating the remaining fraction a total time of 1 h was necessary. It was further found that the efficiency of electrodeposition of 241Am was different from that of 243Am during molecular plating, this discrepancy might have originated from the solubility of 241AmO2 and 243AmO2 as mentioned above. The total plating efficiencies of 241,243Am reached up to 98% and 94%, respectively, under the conditions described above. It was found that at a constant potential of 540 V, the current density remained constant during the period of electrodeposition. After electrodeposition, the electrolysis solution in the cell was poured into a receptacle from which any remaining Am could be recovered later. The layer on the aluminum foil was rinsed with 5 ml of isopropyl
alcohol, and then placed in a muffle furnace at 5008C for 15 min. Since americium is electrodeposited as hydrolytic species of americium which has a chemical formula of Am(OH)3nH2O, it is necessary to convert it in to an oxide form as AmO2 which has a good thermal stability under 10008C. The color of the layer was dark-brown. The quality of the layers was almost equally good. The films were not flaky and adhered strongly to the backing material. Finally, each target film was mounted into an appropriate holder made of plastic, and stored in a closed desiccator filled with anhydrous calcium chloride, which was periodically monitored. To obtain the thicker americium target, the volume of 241Am or 243Am stock solution added into the plating cell was increased generally from 100 to 600 ml. The target thickness was also determined by g-spectrometry. Target films of 241 Am with thickness of 622 to 910 mg/cm2 and for 243 Am 1.0–1.2 mg/cm2 were from the mixture of isopropyl alcohol and dilute nitric acid by the single molecular plating method. The previous procedure for preparation of thicker actinide targets by molecular plating has been simplified. The 241Am targets were irradiated by low-energy heavyion beam with intensity of the order of mA to produce transuranium nuclei. It was demonstrated that the thermal stability and radiation stability of the target obtained in this experiment were completely satisfactory.
Acknowledgements This work was supported by National Natural Science Foundation under Contract No.19775053, the
744
Q. Zhi et al. / Applied Radiation and Isotopes 54 (2001) 741–744
Foundation of the Chinese Academy of Sciences for the researcher returned from abroad, and Major State Basic Research Development Program under Contract No. G200007740. References Aumann, D.C., Mullen, G., 1974. Nucl. Instr. and Meth. 115, 75.
Evans, J.E., Lougheed, R.W., Coops, M.S., et al., 1972. Nucl. Instr. and Meth. 102, 389. Getoff, N., Bildstein, H., 1965. Nucl. Instr. and Meth. 36, 173. Mullen, G., Aumann, D.C., 1975. Nucl. Instr. and Meth. 128, 425. Seaborg, G.T., Loveland, W.D., 1985. Treaties on Heavy-ion Science, Plenum press, New York, 255–330. Shinohara, N., Kohno, N., 1989. Appl. Radiat. Isot. 40 (1), 41. Trautmann, N., Heimann, R., Gembalies-Datz, D., et al., 1982. GSI report 81–2, 186.
Technical note
Preparation of the thicker americium targets by molecular plating Qin Zhi*, Guo Junsheng, Gan Zaiguo Institute of Modern Physics, Chinese Academy of Science, Lanzhou 730000 People’s Republic of China Received 21 August 2000; received in revised form 22 September 2000; accepted 24 October 2000
Abstract Electrodeposition of americium from the mixture of isopropyl alcohol and dilute nitric acid was studied. The Am targets with thickness between 600 mg/cm2 and 1.2 mg/cm2 had been prepared on thin aluminum foil (7 mm) by molecular plating at one time. # 2001 Elsevier Science Ltd. All rights reserved.
241,243
Keywords: Americium targets; Molecular plating
1. Intruduction The preparation of heavy actinide targets is of importance for production of transuranium elements by heavy-ion fusion evaporation reactions. It is difficult to prepare this kind of target because of the high aradioactivity of the actinide target material. These materials are usually very expensive because of limited availability. Such a target consists of a layer of a pure actinide compound deposited on a thin metal foil. The deposit should be uniform, adherent to the substructure. When actinide targets are used in very intense heavy-ion beams, as in attempts to make new transuranium radioisotopes, special attention must be paid to the problem of the radiation and thermal stability of the target. Frequently used target preparation techniques are electrospraying, electromagnetic isotope separation and electrodeposition. Among these methods, electrodeposition is the preferred method for preparing targets of the actinide elements. Its deposition efficiency is very high and the equipment needed for preparing the deposits is simple and can easily be handled in glove box or hoods.
*Corresponding author. Tel.: 86-931-8275924; fax: 86-9318272100. E-mail address: [email protected] (Q. Zhi).
The molecular plating method allows rapid preparation of a thin and extremely uniform deposit, especially necessary for the accurate activity measurement of aemitting nuclides (Shinohara and Kohno, 1989; Getoff and Bildstein, 1965). Shinohara and Bildstein rapidly prepared high-resolution sources for a-ray spectrometry of actinides in spent fuel by electrodeposition from isopropyl alcohol solution (Shinohara and Bildstein, 1965). On the other hand, the molecular plating method had also been used for the preparation of relatively thick targets (Trautmann et al., 1982; Evans et al., 1972; Mullen and Aumann, 1975; Aumann and Muller, 1974). Trautmann et al. prepared 248Cm targets from of isopropyl alcohol and dilute nitric acid (Trautmann et al., 1982). Mullen et al. electrodeposited Th, U, Np, Pu, Am, Cm, Cf from organic solutions onto different backing materials in the range from 10 mg/cm2 up to 2 mg/cm2 (Evans et al., 1972) . Experience had shown that for all the systems investigated, the following three experimental conditions were very advantageous for producing uniform and adhering films: (1) the use of very small volumes of the organic solvent, namely 1–2 ml; (2) the application of relatively high current densities between 2–6 mA/cm2; (3) the layers were very adherent as long as not more than 100 mg/cm2 of the elements were precipitated at
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 6 7 - 5
742
Q. Zhi et al. / Applied Radiation and Isotopes 54 (2001) 741–744
one time. To obtain thicker layers, the backing with the layers was calcinated in a muffle furnace and the deposition procedure repeated several times. In this way, layers of between 0.5 and 2 mg/cm2 could be obtained. For example, an americium target with a thickness of 550 mg/cm2 had been produced by repeated electrodeposition of layers of approximately 85 mg/cm2 and heating the foils at 5508C six times. The electrodeposition techniques described in this paper were used to produce short-lived transuranium nuclei. In order to increase the yield of the expected radioisotopes in heavy-ion fusion-evaporation reactions, thicker americium targets are required. By using the procedures given in this paper, 241,243Am targets on thin aluminum foil with thickness in the range of 600 mg/ cm2–1.2 mg/cm2 were obtained by a single molecular plating cycle.
2. Experimental Because americium isotopes have extremely high toxicity and high specific activity, the maximum permissible body burdens or atmospheric concentrations of americium are very small, namely 1.5 10ÿ2 mg or 2 10ÿ12 mCi/ml (Seaborg and Loveland, 1985), all experiments were carried out in a glove-box and under the most rigid precautions. 2.1. Preparation of
241,243
Am stock solution
Enriched 241,243Am was purchased from the China National Nuclear Corporation and State Scientific Center of Russia, respectively. Americium dioxide AmO2 was obtained in the form of a dark brown powder. AmO2 dissolves readily in hydrochloric acid with the evolution of chlorine, and in nitric acid with the evolution of oxygen. We found differences between two kinds of americium dioxides in dissolution behavior. Americium dioxide 241 AmO2 (8 mg) dissolved completely in 5 ml of 0.16 N nitric acid, and the solution of Am(NO3)3 producing a pink colored stock solution of 241Am, with a concentration of 1.6 mg/ml. Americium dioxide 243AmO2 (10 mg) did not dissolve in 5 ml of 0.16 N nitric acid after standing overnight, leaving a residue of dark brown powder at the bottom of the tube. The solution was heated to dryness to remove nitric acid, and the residue was dissolved in 3 ml of hot concentrated hydrochloric acid yielding a solution orange–yellow in color. Since americium nitrate was needed in the following molecular plating method, the solution of AmCl3 was heated to dryness, and the residue dissolved in 2 ml of 0.16 N nitric
acid to yield a pink stock solution of concentration of 5 mg/ml. 2.2. Measurement of standards
241
Am and
241
Am, with a
243
Am reference
As 241Am, T1=2 432 yr, has a characteristic g-ray of energy of 59.5 keV with a branch ratio of 35.9%, and 243 Am, T1=2 7073 yr, a characteristic g-ray of energy of 74.7 keV with a branch ratio of 68%, it was convenient to measure the g-activities of the sample to determine the amount of americium present. It is not necessary to make a thin layer of actinide on platinum or stainless steel disk by electrodeposition to determine a-activity. Furthermore, self-absorption effects would be serious when measuring a-activity for thicker target layers. An aliquot of 241Am or 243Am stock solution was transferred to a small plastic tube, and the concentration of the solution was assayed by measuring g-activity with HPGe detector. The g-spectra was obtained using a GMX-40220 detector, multi-channel analyzer MCA919 and a personal computer. The energy resolution was 2.2 keV for 1332 keV g-ray of 60Co. It was demonstrated that only the g-ray of 59.5 keV could be observed in the sample of 241Am, and other g-activities which could contaminate the sample were not found. The same sample was measured with a silicon surface barrier detector in another experiment, and no unexpected aactivity was found. With the 243Am solution, we observed g-rays of 74.7, 99.6, 103.8, 106.1, 117.6, 177.6 and 228.2 keV, and also found the 59.5 keV g-ray contributed by 241Am. The isotopic composition of 243 Am sample was determined to be 243Am: 99.3%, 241 Am: 0.7%, this was consistent with the assay by the supplier. 2.3. Electrodeposition The molecular plating was carried out in the cell shown schematically in Fig. 1. The electrodeposition cell was made of Pyrex glass and consisted of a cylinder of 18 mm i.d. and 80 mm length, and a jacket for cooling the electrolyte solution. This cell was attached to a stainless steel base. A rubber ‘‘O’’ ring of 10 mm inner diameter was used to define the area of deposition. To stir the solution, a platinum wire (anode, 1 mm diameter) was rotated continuously by a motor. After washing with acetone to remove any grease, an aluminum foil (cathode, 7 mm thickness) was put between the cell and the stainless-steel base as the backing material for electrodeposition. For determining the optimum condition of the electrodeposition, the deposition behavior of 241Am and 243Am on an aluminum foil were investigated in the present experiment. The cell was filled with 15 ml isopropyl alcohol, an aliquot of the 241Am or 243Am
Q. Zhi et al. / Applied Radiation and Isotopes 54 (2001) 741–744
743
Fig. 2. The efficiency of deposition of americium by molecular plating method as a function of the duration of electrodeposition. The circle represents the efficiency of deposition of 241Am, and triangle represents that of 243Am. Fig. 1. Sketch scheme of the molecular plating. 1. Pyrex cell body; 2. ‘‘O’’ ring; 3. Aluminium foil with thickness of 7 mm; 4. Stainless steel base; 5. Platinum wire.
stocking solution. According to the suggestion of Shinohara and Kohno, the highest yield of over 98% was obtained in the case of current density of 3–5 mA/ cm2 at the potential of 400–800 V. A typical electrolysis, which was carried out with a current of 5 mA/cm2 at the voltage of 500 V, and the distance between cathode and anode was 3 cm, lasted for 1 h. The efficiency of deposition of americium by molecular plating method was investigated as a function of duration of electrodeposition as shown in Fig. 2. The efficiency of americium was determined by measuring the activities of the characteristic g-ray of 100 ml of the solution before, during and after the electrodeposition with g spectroscopy. One can find that the main part (ca. 85%) of 241Am or (ca 70%) of 243Am had already deposited within 5 min. For plating the remaining fraction a total time of 1 h was necessary. It was further found that the efficiency of electrodeposition of 241Am was different from that of 243Am during molecular plating, this discrepancy might have originated from the solubility of 241AmO2 and 243AmO2 as mentioned above. The total plating efficiencies of 241,243Am reached up to 98% and 94%, respectively, under the conditions described above. It was found that at a constant potential of 540 V, the current density remained constant during the period of electrodeposition. After electrodeposition, the electrolysis solution in the cell was poured into a receptacle from which any remaining Am could be recovered later. The layer on the aluminum foil was rinsed with 5 ml of isopropyl
alcohol, and then placed in a muffle furnace at 5008C for 15 min. Since americium is electrodeposited as hydrolytic species of americium which has a chemical formula of Am(OH)3nH2O, it is necessary to convert it in to an oxide form as AmO2 which has a good thermal stability under 10008C. The color of the layer was dark-brown. The quality of the layers was almost equally good. The films were not flaky and adhered strongly to the backing material. Finally, each target film was mounted into an appropriate holder made of plastic, and stored in a closed desiccator filled with anhydrous calcium chloride, which was periodically monitored. To obtain the thicker americium target, the volume of 241Am or 243Am stock solution added into the plating cell was increased generally from 100 to 600 ml. The target thickness was also determined by g-spectrometry. Target films of 241 Am with thickness of 622 to 910 mg/cm2 and for 243 Am 1.0–1.2 mg/cm2 were from the mixture of isopropyl alcohol and dilute nitric acid by the single molecular plating method. The previous procedure for preparation of thicker actinide targets by molecular plating has been simplified. The 241Am targets were irradiated by low-energy heavyion beam with intensity of the order of mA to produce transuranium nuclei. It was demonstrated that the thermal stability and radiation stability of the target obtained in this experiment were completely satisfactory.
Acknowledgements This work was supported by National Natural Science Foundation under Contract No.19775053, the
744
Q. Zhi et al. / Applied Radiation and Isotopes 54 (2001) 741–744
Foundation of the Chinese Academy of Sciences for the researcher returned from abroad, and Major State Basic Research Development Program under Contract No. G200007740. References Aumann, D.C., Mullen, G., 1974. Nucl. Instr. and Meth. 115, 75.
Evans, J.E., Lougheed, R.W., Coops, M.S., et al., 1972. Nucl. Instr. and Meth. 102, 389. Getoff, N., Bildstein, H., 1965. Nucl. Instr. and Meth. 36, 173. Mullen, G., Aumann, D.C., 1975. Nucl. Instr. and Meth. 128, 425. Seaborg, G.T., Loveland, W.D., 1985. Treaties on Heavy-ion Science, Plenum press, New York, 255–330. Shinohara, N., Kohno, N., 1989. Appl. Radiat. Isot. 40 (1), 41. Trautmann, N., Heimann, R., Gembalies-Datz, D., et al., 1982. GSI report 81–2, 186.