- Email: [email protected]
Preparation of Pd-based intermetallic targets for high intensity irradiations
Nuclear Instruments and Methods in Physics Research A 691 (2012) 5–9
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Preparation of Pd-based intermetallic targets for high intensity irradiations a,b a ¨ ¨ I. Usoltsev a,b,n, R. Eichler a,b, R. Dressler a, D. Piguet a, D. Wittwer a,b, A. Turler , R. Brutsch , c c c E.A. Olsen , J.P. Omtvedt , A. Semchenkov a
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland University of Bern, CH-3012 Bern, Switzerland c University of Oslo, 0316 Oslo, Norway b
a r t i c l e i n f o
abstract
Article history: Received 27 March 2012 Received in revised form 20 June 2012 Accepted 26 June 2012 Available online 7 July 2012
A new method of intermetallic target preparation is described. Based on the molecular plating technique followed by ‘‘coupled reduction’’, this method allows producing stable and homogeneous metallic targets for high intensity irradiations. In the first step, the target material is electroplated on a noble metal surface, ensuring homogeneous distribution of the desired element on the target backing. In the second step, the foil with the plated material is heated in a hydrogen flow. Due to the formation of an intermetallic compound with the noble metal support, reduction of the target material with hydrogen at high temperatures becomes thermodynamically possible. Nitrates of all six investigated elements (U, Th, Am, Gd, Nd, and Eu) were electroplated with excellent yields on Pd backing foils by the molecular plating technique and completely reduced in hydrogen atmosphere. A homogeneous distribution of the target material over the whole thickness of the Pd foil was observed suggesting a pronounced diffusion of the reduced metals into the backing material already during the reduction process. A first test irradiation experiment with a thin 3.5 mm U/Pd intermetallic target is described. & 2012 Elsevier B.V. All rights reserved.
Keywords: Target Electrodeposition Lanthanides Actinides Diffusion Intermetallic
1. Introduction The stability of the target during the irradiation is a crucial factor for performing successful experiments in modern radiochemistry and physics, especially when intense heavy ion beams are applied. The chemical characterization of isotopes of transactinide elements [1] is a good example of an experimental work, where well prepared targets are prerequisite for success in the month-long experiments. The heavy-ion beam deposits a significant part of its energy when passing through a target. An excessive heating could in turn damage the integrity of the target and jeopardize an experiment. In order to avoid an extreme heating of stationary targets rotating target wheels were developed. Gas- or water-cooled targets can withstand beam intensities of a few particle mA [2–4], which is a huge step forward in comparison to stationary targets. However, recent developments of accelerator and ion source techniques promise an order of magnitude higher beam intensity allowing higher production rates. Therefore, further progress in target preparation is required.
n Corresponding author at: Paul Scherrer Institut, OFLB/105, CH-5232 Villigen PSI, Switzerland Tel.: þ 41 56 3105771; fax: þ 41 56 3104435. E-mail address: [email protected] (I. Usoltsev).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.06.060
Four crucial constituent properties which define overall target stability can be highlighted: thermal conductivity, electrical conductivity, chemical, and mechanical stability. Poor electrical conductivity and mechanical stability account for target material loss during irradiation, while deficient thermal conductivity could simply cause a meltdown of the target in use. Chemical stability refers to possible reactions between a foil and a target material under irradiation conditions e.g. oxidation of the backing by the target material. The molecular plating technique (MP) is typically used for depositing the desired target element on a backing material. In most cases rare and expensive actinide targets are prepared this way [5–9]. The desired element is deposited in the form of an undefined mixture of different compounds such as nitrates, oxynitrates, oxyhydrates, oxides and hydroxides [10,11]. The actual chemical composition of an electroplated material does not play a significant role though, since under irradiation conditions this compound mixture will be most probably transformed into oxides. One of the main advantages of MP is the efficient production of highly homogeneous and adhesive layers of the above mentioned compounds on a backing material. However, the poor thermal and electrical conductivity of the obtained layer significantly decreases the overall target stability during irradiation. The chemical stability of plated targets is also uncertain and depends on the combination of the target material and the backing.
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Due to the high thermodynamic stability expressed by the highly negative standard formation enthalpies of actinide/lanthanide oxides, these desired target elements cannot be reduced to the metal just by heating up the respective oxide in a hydrogen atmosphere. High vacuum evaporation and pure metal electrodeposition from organic media are among the options, but cannot be used in most cases with the costly actinide materials because of low efficiencies [12]. Introduction of a noble metal into the system could facilitate the reduction of an actinide/lanthanide oxide in a hydrogen atmosphere, due to the formation of thermodynamically very stable intermetallic compounds. The possibility of such a reaction for a whole variety of different combinations of oxides and noble metals was demonstrated in numerous publications (for a review see [13]). Depending on the desired composition of the intermetallic compound, powders of an oxide and a noble metal are mixed in a fixed ratio and heated in a flow of pure hydrogen [14]. The chemical reaction of a ‘‘coupled reduction’’ process of an actinide (Ac) oxide in contact with a noble metal (Me) can be written as follows [15]: xMe þ Acx Oy þyH2 2xAcMe þyH2 O: Here we propose Pd-based intermetallic targets which are expected to be superior to the widely used solely electroplated targets. Intermetallic materials should reveal higher thermal and electrical conductivity combined with chemical and mechanical stability. In this paper we report an approach towards preparation of such targets.
2. Experimental Layers of lanthanide/actinide material with 0.73 mg cm 2 (calculated as pure metal) thickness were deposited from nitrate solutions in isopropanol by the MP technique on a previously polished and sonicated Pd foil (thickness 25 mm) using a electrodeposition cell shown in Fig. 1. In case of thin 2 mm or 3 mm foils, the foils were only rinsed with acetone and ethanol prior to electrodeposition. The uniform MP parameters used for all the elements and all types of backings are given in Table 1. The applied voltage was varied from step to step in order to keep the current density within the limits given in Table 1. In the first test experiments natGd, natEu, and natNd were plated and reduced on a 25 mm Pd backing. The homogeneity of the deposition was examined by optical microscopy using a laboratory microscope (Bresser light microscope with DinoEye Eyepiece Camera). Subsequently, the foil with the plated material was placed on a tantalum block serving as heat susceptor, which in turn was placed in a tubular quartz reaction chamber for subsequent heating in a 100 ml/min flow of pure hydrogen.
Fig. 1. Scheme of the cell for electrodeposition.
Table 1 Uniform parameters of the molecular plating technique. Deposition thickness Solvent Backing Area deposited Current Potential The distance between two electrodes Overall deposition time Temperature Anode
0.73 mg cm 2 Isopropanol Pd foil 0.38 cm2 0.8–2.1 mA cm 2 500–800 V 1 cm Performed in 5 consecutive steps. Each step 50 min long 25o C Platinum spiral wire
The susceptor was heated by RF induction using a NovaStars 3LW (Ameritherm Inc.) source. Thus, the deposited lanthanide/ actinide material was reduced to the metal in a ‘‘coupled reduction’’ process. The temperature of the tantalum susceptor was measured with an infrared thermometer (Optoelektronik, Portable Radiation Thermometer, IR-AH3SU). The targets prepared on 2 mm and 3 mm backing foils were placed on an alumina support and were heated by a standard resistance oven. In this case the temperature was monitored by a K-type thermocouple. 241Am was used as tracer in all the experiments with natural Eu as carrier; activities of the final 241Am/Pd products did not exceed 300 Bq. 238U and 232Th due to low activity were used carrier free. Th stock solution was prepared from a nitrate salt (Sigma-Aldrich) and used without further purification. Deposition yields of these three isotopes (241Am, 238 U and 232Th) were quantified by alpha-particle and gamma-ray spectrometry in conjunction with acquisition and analysis systems based on Canberra’s Genie2ks. In case of natural isotopic mixtures of Nd, Gd, and Eu, deposition yields were estimated by weighing (Mettler Toledo AG285 analytical balance). 152Eu and 241Am tracers as well as natural Eu as carrier were used in experiments with attenuation of gamma lines in 25 mm Pd backing foil. The structure of the foil surface of all three Eu/Pd, Nd/Pd, and Gd/Pd products on the 25 mm Pd foils were examined by scanning electron microscopy (SEM). Energy dispersive X-ray spectrometry (EDS) was used to characterize the chemical composition of the product. SEM analyses were carried out with a Zeiss DSM 962 having a hairpin cathode made of tungsten and operating at 30 kV. EDS analysis was performed with the Noran System SIX and a pioneer detector with Norvars window.
3. Results and discussion As expected, the MP method allowed for high deposition yields for all studied target materials (see Table 2). Optical microscopy revealed that the plated material was evenly distributed on the backing material if the plating procedure was performed in five consecutive steps at low current density (see Fig. 2). The most suitable temperature for an efficient and complete reduction of the studied actinide/lanthanide species on a 25 mm Pd foil appeared to be 11001 C. By lowering this temperature to 10001 C, the reaction rate decreased considerably for all examined elements. The optimum reduction temperature was shown to be independent of the target element, but only on the used noble metal (the backing material). Therefore, for all examined elements the same reduction temperature of 11001 C was used. The minimum reduction time depends on the amount of plated material. In our experiments with target metal thicknesses of 0.73 mg cm 2, 10 min was observed to be sufficient for the complete reduction of all elements tested so far. Since the oxide
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Table 2 Molecular plating efficiency for investigated isotopes. Coverage factor k¼ 1. nat
Gd Nd nat Eu 238 U 232 Th 241 Am nat
85 710% 90 710% 85 710% 98 71% 99 71% 95 71%
Fig. 4. EDX overview spectrum of the Eu/Pd target. The diffusion of the reduced Eu metal into the depth of the Pd foil explains the weak Eu fluorescence.
Fig. 2. Optical microscopy images of the surface of the Pd foil after (A) Th and (B) Gd electrodeposition. Scale bar is 100 mm.
Fig. 5. 241Am/Pd target alpha-particle spectra before (A) and after (B) coupled reduction (100 ml/min H2 at 11001 C, 60 min).
depth probed by EDX. A pronounced diffusion of lanthanide/ actinide metal into the noble metal foil after reduction was observed in a series of experiments with radioactive isotopes. Gamma spectrometric measurements (i) as well as alpha-particle spectroscopy (ii) have been performed from both sides of the produced intermetallic foils:
Fig. 3. SEM picture of an Eu/Pd target produced by the molecular plating technique followed by reduction in hydrogen.
remains on the surface whereas the reduced metal diffuses into the Pd-backing, it was possible to control the completeness of the reaction visually through the transparent quartz tube. Candoluminescence of investigated metal oxides accounts for bright glowing, easily distinguishable on a 25 mm Pd backing. The enhancement of the visibility of the Pd grain boundaries on the SEM image (Fig. 3), is an indirect proof that Eu was reduced and diffused into the Pd foil and that the plated oxide layer has disappeared (for comparison see Fig. 2). Moreover, as one can see from the EDX analysis (Fig. 4), the intensity of the oxygen line is at the threshold of sensitivity of EDX, which means that the surface of the Pd foil is free of macroscopic amounts of Eu oxide. The observed weak Eu fluorescence is explained by diffusion of the reduced metal into the depth of the Pd foil, which is substantially decreasing the amount of Eu atoms within the
(i) The 59.54 keV gamma line of 241Am is significantly attenuated by the Pd foil, while for the 121.78 keV gamma line of 152Eu line an attenuation is negligible and cannot be detected within the limits of uncertainty (less than 1%). After the complete reduction of the plated material the 241Am 59.54 keV line intensity increased by about 10%. Since the back side of the foil was facing a gamma detector the intensity growth is explained by penetration of 241Am into the Pd foil, which leads to a decrease of attenuation of the low energetic line. (ii) Alpha spectra shown in Fig. 5 illustrate the diffusion as a dramatic change in activity distribution before and after reduction. An almost continuous alpha energy distribution in the spectrum taken after complete reduction of the target material is explained by broad 241Am distribution in the depth of the backing material. After only 7.5 min of reduction in H2 at 11101 C, significant Am alpha activity (0.1% of the total amount) was detected on the back side of the 25 mm Pd foil. Note that the 5.48 MeV alpha particles are completely absorbed by 25 mm Pd and no alpha activity was detected on the back side of the foil before the 241
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Fig. 6. 241 Am/Pd target alpha-particle spectrum after complete reduction in hydrogen and additional heating in He for 241Am equilibration within the depth of the foil. Full energetic 5.48 MeV alpha peak reappeared due to 241Am oxidation.
Fig. 7. 232Th/Pd target alpha-particle spectra before (A) and after (B) coupled reduction (100 ml/min H2 at 11001 C, 15 min).
reduction. After 2.5 h of additional heating at 12001 C in He of the already reduced product, the 241Am tracer was almost equally distributed across the entire 25 mm Pd foil. It is important to mention, that the reduction process is reversible, which means that heating the foil in a noble gas with a small admixture of oxygen in the system can ‘re-extract’ the reduced actinide and lanthanide metal to the surface due to oxide formation. This phenomenon is illustrated in Fig. 6. The helium gas used for creating an inert atmosphere for the activity equilibration experiment contained about 1 ppmv of oxygen admixture. This amount of oxygen was sufficient to initiate the surface oxidation and hence a reversible re-enrichment process of the oxide on the surface of the target. As a result, full energetic peak of 5.48 MeV alpha-particles reappeared in the spectrum in contrast to the fully reduced product (Fig. 5B), where no peak at 5.48 MeV was observed. Additional heating in hydrogen atmosphere reduced the new oxide layer again completely, which in turn lead to complete equilibration of the activity within the target. Similar results in terms of reduction and diffusion into the backing material were obtained for 238U and 232Th. Because of 238 U nitrate was prepared from the pure metal, only two alpha peaks (attributed to 238U and 234U) were detected in the alphaparticle spectra before and after the reduction. While in case of 232 Th, not only 228Th, but also 224Ra (half-life 3.66 d) was electroplated on the Pd backing. 224Ra compounds were not reduced in a coupled reduction process and remained intact on the surface, which was reflected in the alpha-particle spectrum (Fig. 7). Alpha-particle measurement (A) was performed in the same day as 232Th was electroplated, and only 24 h separated (A) and (B) measurements.
Broad peaks of 232Th and 228Th are accompanied by sharp peaks of 224Ra and its short-lived daughter nuclides 220Rn, 216Po, and 212Bi. The low intensity peak at 4.7 MeV most likely corresponds to 230Th. The target material for heavy ion irradiations should be deposited on the backing in an as thin as possible layer, since nuclear fusion cross sections are strongly dependent on the incident energy of the ion beam on the target material. Despite the possibility to plate an additional amount of the target material on the other side or even on the same side of the prepared intermetallic target, the pronounced diffusion of the reduced metal into the backing material prevents the use of thick e.g. 25 mm noble metal foils. However, an even distribution of the target material within the backing material appears not to be a big problem using thin e.g. 2–3 mm metal foils as backing materials. In experiments with thin 2 mm and 3 mm Pd foils, the reduction temperature of 11001 C was found to be unacceptably high due to disintegration of the foil shortly after introduction of hydrogen into the system. Thus the reduction temperature was lowered to 9001 C, which significantly increased the reduction time. A pinhole free 3.5 mm U/Pd target was produced after 3 h reduction time in a hydrogen flow of 50 ml/min. In case of using a 2 mm Pd foil we faced serious difficulties in producing pinhole free targets. Apparently an accumulated tension in the foil after the rolling process is released upon heating, destroying the initial integrity of the foil. Although improved by lowering the temperature, the problem was not completely solved so far. From a thermal and mechanical stability point of view Rh backing is superior to Pd. At almost the same density, Rh has a much higher melting point and is much easier to handle than Pd. The only serious drawback of Rh is that it is not as chemically reactive as Pd. Temperatures of at least 1300–14001 C are required for the reduction to be completed within a reasonable time period. Furthermore, at temperatures close to 11001 C, the Rh foil is affected by hydrogen induced embrittlement processes, which decreases the mechanical stability of the foil considerably. Attempts have been made to decrease the reduction temperature of U on Rh backing by evaporating 1 mm Pd layer on the backing prior to U deposition. Unfortunately, this approach did not alleviate the problem, mainly because of the intermetallic reaction between Pd and Rh, which are most likely forming a chemically inert alloy before the reduction is taking place. We suggest in this case increasing the chemical activity of the Rh foil by doping it with Pt, which is known as a protecting agent against hydrogen embrittlement in tantalum [16]. First irradiation experiments with intermetallic targets have been carried out to test the target handling and its irradiation behavior. Therefore, at the Oslo Cyclotron Laboratory, University of Oslo, Norway a 3.5 mm 238U/Pd target was irradiated at the MC35 Scanditronix cyclotron using a 0.5 mA proton beam with a cyclotron energy of 30 MeV. The target was mounted free-standing on a water cooled copper ring holder. It was irradiated at vacuum conditions for several hours. The handling turned out to be absolutely unproblematic. Optical inspection after the irradiation did not reveal any significant damage or oxidation.
4. Conclusions It was shown, that intermetallic targets can be prepared by reducing electroplated lanthanide/actinide materials on Pd foils of different thicknesses. Because of the diffusion of a reduced metal into the backing material, only thin (few mm) backing materials can be successfully used for actinide target preparation. A thickness of 3 mm was found to be a lower limit for the production of
I. Usoltsev et al. / Nuclear Instruments and Methods in Physics Research A 691 (2012) 5–9
pinhole free Pd based targets. High intensity heavy ion irradiation tests of the prepared intermetallic targets are envisaged.
Acknowledgments This research project was supported by Swiss National Science Foundation Grant 200020_126639. References [1] R. Eichler, et al., Nature 447 (2007) 72. [2] K. Eberhardt, et al., Nuclear Instruments and Methods A 521 (2004) 208. [3] A. Yoshida, et al., Nuclear Instruments and Methods A 521 (2004) 65.
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[4] B. Lommel, et al., Nuclear Instruments and Methods A 590 (2008) 141. [5] John P. Greene, Irshad Ahmad, Nuclear Instruments and Methods A 590 (2008) 131. [6] Lina Zhang, et al., Nuclear Instruments and Methods B 267 (2009) 3480. [7] A. Vascon, et al., Nuclear Instruments and Methods A 655 (2011) 72. [8] N. Trautmann, H. Folger, Nuclear Instruments and Methods A 282 (1989) 102. [9] W. Parker, et al., Nuclear Instruments and Methods 26 (1964) 61. [10] D. Liebe, et al., Nuclear Instruments and Methods A 590 (2008) 145. [11] N. Getoff, et al., Nuclear Instruments and Methods 46 (1967) 305. [12] Anna Stolarz, et al., Nuclear Instruments and Methods A 613 (2010) 351. [13] H. Kleykamp, Journal of Nuclear Materials 201 (1993) 193. [14] B. Erdmann, C. Keller, Journal of Solid State Chemistry 7 (1973) 40. ¨ [15] S. Mobius, L. Hellwig, C. Keller, Journal of Less-Common Metals 121 (1986) 43. [16] W. Ensinger, et al., Nuclear Instruments and Methods B, 2011, doi:10.1016/ j.nimb.2011.01.119.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Preparation of Pd-based intermetallic targets for high intensity irradiations a,b a ¨ ¨ I. Usoltsev a,b,n, R. Eichler a,b, R. Dressler a, D. Piguet a, D. Wittwer a,b, A. Turler , R. Brutsch , c c c E.A. Olsen , J.P. Omtvedt , A. Semchenkov a
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland University of Bern, CH-3012 Bern, Switzerland c University of Oslo, 0316 Oslo, Norway b
a r t i c l e i n f o
abstract
Article history: Received 27 March 2012 Received in revised form 20 June 2012 Accepted 26 June 2012 Available online 7 July 2012
A new method of intermetallic target preparation is described. Based on the molecular plating technique followed by ‘‘coupled reduction’’, this method allows producing stable and homogeneous metallic targets for high intensity irradiations. In the first step, the target material is electroplated on a noble metal surface, ensuring homogeneous distribution of the desired element on the target backing. In the second step, the foil with the plated material is heated in a hydrogen flow. Due to the formation of an intermetallic compound with the noble metal support, reduction of the target material with hydrogen at high temperatures becomes thermodynamically possible. Nitrates of all six investigated elements (U, Th, Am, Gd, Nd, and Eu) were electroplated with excellent yields on Pd backing foils by the molecular plating technique and completely reduced in hydrogen atmosphere. A homogeneous distribution of the target material over the whole thickness of the Pd foil was observed suggesting a pronounced diffusion of the reduced metals into the backing material already during the reduction process. A first test irradiation experiment with a thin 3.5 mm U/Pd intermetallic target is described. & 2012 Elsevier B.V. All rights reserved.
Keywords: Target Electrodeposition Lanthanides Actinides Diffusion Intermetallic
1. Introduction The stability of the target during the irradiation is a crucial factor for performing successful experiments in modern radiochemistry and physics, especially when intense heavy ion beams are applied. The chemical characterization of isotopes of transactinide elements [1] is a good example of an experimental work, where well prepared targets are prerequisite for success in the month-long experiments. The heavy-ion beam deposits a significant part of its energy when passing through a target. An excessive heating could in turn damage the integrity of the target and jeopardize an experiment. In order to avoid an extreme heating of stationary targets rotating target wheels were developed. Gas- or water-cooled targets can withstand beam intensities of a few particle mA [2–4], which is a huge step forward in comparison to stationary targets. However, recent developments of accelerator and ion source techniques promise an order of magnitude higher beam intensity allowing higher production rates. Therefore, further progress in target preparation is required.
n Corresponding author at: Paul Scherrer Institut, OFLB/105, CH-5232 Villigen PSI, Switzerland Tel.: þ 41 56 3105771; fax: þ 41 56 3104435. E-mail address: [email protected] (I. Usoltsev).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.06.060
Four crucial constituent properties which define overall target stability can be highlighted: thermal conductivity, electrical conductivity, chemical, and mechanical stability. Poor electrical conductivity and mechanical stability account for target material loss during irradiation, while deficient thermal conductivity could simply cause a meltdown of the target in use. Chemical stability refers to possible reactions between a foil and a target material under irradiation conditions e.g. oxidation of the backing by the target material. The molecular plating technique (MP) is typically used for depositing the desired target element on a backing material. In most cases rare and expensive actinide targets are prepared this way [5–9]. The desired element is deposited in the form of an undefined mixture of different compounds such as nitrates, oxynitrates, oxyhydrates, oxides and hydroxides [10,11]. The actual chemical composition of an electroplated material does not play a significant role though, since under irradiation conditions this compound mixture will be most probably transformed into oxides. One of the main advantages of MP is the efficient production of highly homogeneous and adhesive layers of the above mentioned compounds on a backing material. However, the poor thermal and electrical conductivity of the obtained layer significantly decreases the overall target stability during irradiation. The chemical stability of plated targets is also uncertain and depends on the combination of the target material and the backing.
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Due to the high thermodynamic stability expressed by the highly negative standard formation enthalpies of actinide/lanthanide oxides, these desired target elements cannot be reduced to the metal just by heating up the respective oxide in a hydrogen atmosphere. High vacuum evaporation and pure metal electrodeposition from organic media are among the options, but cannot be used in most cases with the costly actinide materials because of low efficiencies [12]. Introduction of a noble metal into the system could facilitate the reduction of an actinide/lanthanide oxide in a hydrogen atmosphere, due to the formation of thermodynamically very stable intermetallic compounds. The possibility of such a reaction for a whole variety of different combinations of oxides and noble metals was demonstrated in numerous publications (for a review see [13]). Depending on the desired composition of the intermetallic compound, powders of an oxide and a noble metal are mixed in a fixed ratio and heated in a flow of pure hydrogen [14]. The chemical reaction of a ‘‘coupled reduction’’ process of an actinide (Ac) oxide in contact with a noble metal (Me) can be written as follows [15]: xMe þ Acx Oy þyH2 2xAcMe þyH2 O: Here we propose Pd-based intermetallic targets which are expected to be superior to the widely used solely electroplated targets. Intermetallic materials should reveal higher thermal and electrical conductivity combined with chemical and mechanical stability. In this paper we report an approach towards preparation of such targets.
2. Experimental Layers of lanthanide/actinide material with 0.73 mg cm 2 (calculated as pure metal) thickness were deposited from nitrate solutions in isopropanol by the MP technique on a previously polished and sonicated Pd foil (thickness 25 mm) using a electrodeposition cell shown in Fig. 1. In case of thin 2 mm or 3 mm foils, the foils were only rinsed with acetone and ethanol prior to electrodeposition. The uniform MP parameters used for all the elements and all types of backings are given in Table 1. The applied voltage was varied from step to step in order to keep the current density within the limits given in Table 1. In the first test experiments natGd, natEu, and natNd were plated and reduced on a 25 mm Pd backing. The homogeneity of the deposition was examined by optical microscopy using a laboratory microscope (Bresser light microscope with DinoEye Eyepiece Camera). Subsequently, the foil with the plated material was placed on a tantalum block serving as heat susceptor, which in turn was placed in a tubular quartz reaction chamber for subsequent heating in a 100 ml/min flow of pure hydrogen.
Fig. 1. Scheme of the cell for electrodeposition.
Table 1 Uniform parameters of the molecular plating technique. Deposition thickness Solvent Backing Area deposited Current Potential The distance between two electrodes Overall deposition time Temperature Anode
0.73 mg cm 2 Isopropanol Pd foil 0.38 cm2 0.8–2.1 mA cm 2 500–800 V 1 cm Performed in 5 consecutive steps. Each step 50 min long 25o C Platinum spiral wire
The susceptor was heated by RF induction using a NovaStars 3LW (Ameritherm Inc.) source. Thus, the deposited lanthanide/ actinide material was reduced to the metal in a ‘‘coupled reduction’’ process. The temperature of the tantalum susceptor was measured with an infrared thermometer (Optoelektronik, Portable Radiation Thermometer, IR-AH3SU). The targets prepared on 2 mm and 3 mm backing foils were placed on an alumina support and were heated by a standard resistance oven. In this case the temperature was monitored by a K-type thermocouple. 241Am was used as tracer in all the experiments with natural Eu as carrier; activities of the final 241Am/Pd products did not exceed 300 Bq. 238U and 232Th due to low activity were used carrier free. Th stock solution was prepared from a nitrate salt (Sigma-Aldrich) and used without further purification. Deposition yields of these three isotopes (241Am, 238 U and 232Th) were quantified by alpha-particle and gamma-ray spectrometry in conjunction with acquisition and analysis systems based on Canberra’s Genie2ks. In case of natural isotopic mixtures of Nd, Gd, and Eu, deposition yields were estimated by weighing (Mettler Toledo AG285 analytical balance). 152Eu and 241Am tracers as well as natural Eu as carrier were used in experiments with attenuation of gamma lines in 25 mm Pd backing foil. The structure of the foil surface of all three Eu/Pd, Nd/Pd, and Gd/Pd products on the 25 mm Pd foils were examined by scanning electron microscopy (SEM). Energy dispersive X-ray spectrometry (EDS) was used to characterize the chemical composition of the product. SEM analyses were carried out with a Zeiss DSM 962 having a hairpin cathode made of tungsten and operating at 30 kV. EDS analysis was performed with the Noran System SIX and a pioneer detector with Norvars window.
3. Results and discussion As expected, the MP method allowed for high deposition yields for all studied target materials (see Table 2). Optical microscopy revealed that the plated material was evenly distributed on the backing material if the plating procedure was performed in five consecutive steps at low current density (see Fig. 2). The most suitable temperature for an efficient and complete reduction of the studied actinide/lanthanide species on a 25 mm Pd foil appeared to be 11001 C. By lowering this temperature to 10001 C, the reaction rate decreased considerably for all examined elements. The optimum reduction temperature was shown to be independent of the target element, but only on the used noble metal (the backing material). Therefore, for all examined elements the same reduction temperature of 11001 C was used. The minimum reduction time depends on the amount of plated material. In our experiments with target metal thicknesses of 0.73 mg cm 2, 10 min was observed to be sufficient for the complete reduction of all elements tested so far. Since the oxide
I. Usoltsev et al. / Nuclear Instruments and Methods in Physics Research A 691 (2012) 5–9
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Table 2 Molecular plating efficiency for investigated isotopes. Coverage factor k¼ 1. nat
Gd Nd nat Eu 238 U 232 Th 241 Am nat
85 710% 90 710% 85 710% 98 71% 99 71% 95 71%
Fig. 4. EDX overview spectrum of the Eu/Pd target. The diffusion of the reduced Eu metal into the depth of the Pd foil explains the weak Eu fluorescence.
Fig. 2. Optical microscopy images of the surface of the Pd foil after (A) Th and (B) Gd electrodeposition. Scale bar is 100 mm.
Fig. 5. 241Am/Pd target alpha-particle spectra before (A) and after (B) coupled reduction (100 ml/min H2 at 11001 C, 60 min).
depth probed by EDX. A pronounced diffusion of lanthanide/ actinide metal into the noble metal foil after reduction was observed in a series of experiments with radioactive isotopes. Gamma spectrometric measurements (i) as well as alpha-particle spectroscopy (ii) have been performed from both sides of the produced intermetallic foils:
Fig. 3. SEM picture of an Eu/Pd target produced by the molecular plating technique followed by reduction in hydrogen.
remains on the surface whereas the reduced metal diffuses into the Pd-backing, it was possible to control the completeness of the reaction visually through the transparent quartz tube. Candoluminescence of investigated metal oxides accounts for bright glowing, easily distinguishable on a 25 mm Pd backing. The enhancement of the visibility of the Pd grain boundaries on the SEM image (Fig. 3), is an indirect proof that Eu was reduced and diffused into the Pd foil and that the plated oxide layer has disappeared (for comparison see Fig. 2). Moreover, as one can see from the EDX analysis (Fig. 4), the intensity of the oxygen line is at the threshold of sensitivity of EDX, which means that the surface of the Pd foil is free of macroscopic amounts of Eu oxide. The observed weak Eu fluorescence is explained by diffusion of the reduced metal into the depth of the Pd foil, which is substantially decreasing the amount of Eu atoms within the
(i) The 59.54 keV gamma line of 241Am is significantly attenuated by the Pd foil, while for the 121.78 keV gamma line of 152Eu line an attenuation is negligible and cannot be detected within the limits of uncertainty (less than 1%). After the complete reduction of the plated material the 241Am 59.54 keV line intensity increased by about 10%. Since the back side of the foil was facing a gamma detector the intensity growth is explained by penetration of 241Am into the Pd foil, which leads to a decrease of attenuation of the low energetic line. (ii) Alpha spectra shown in Fig. 5 illustrate the diffusion as a dramatic change in activity distribution before and after reduction. An almost continuous alpha energy distribution in the spectrum taken after complete reduction of the target material is explained by broad 241Am distribution in the depth of the backing material. After only 7.5 min of reduction in H2 at 11101 C, significant Am alpha activity (0.1% of the total amount) was detected on the back side of the 25 mm Pd foil. Note that the 5.48 MeV alpha particles are completely absorbed by 25 mm Pd and no alpha activity was detected on the back side of the foil before the 241
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Fig. 6. 241 Am/Pd target alpha-particle spectrum after complete reduction in hydrogen and additional heating in He for 241Am equilibration within the depth of the foil. Full energetic 5.48 MeV alpha peak reappeared due to 241Am oxidation.
Fig. 7. 232Th/Pd target alpha-particle spectra before (A) and after (B) coupled reduction (100 ml/min H2 at 11001 C, 15 min).
reduction. After 2.5 h of additional heating at 12001 C in He of the already reduced product, the 241Am tracer was almost equally distributed across the entire 25 mm Pd foil. It is important to mention, that the reduction process is reversible, which means that heating the foil in a noble gas with a small admixture of oxygen in the system can ‘re-extract’ the reduced actinide and lanthanide metal to the surface due to oxide formation. This phenomenon is illustrated in Fig. 6. The helium gas used for creating an inert atmosphere for the activity equilibration experiment contained about 1 ppmv of oxygen admixture. This amount of oxygen was sufficient to initiate the surface oxidation and hence a reversible re-enrichment process of the oxide on the surface of the target. As a result, full energetic peak of 5.48 MeV alpha-particles reappeared in the spectrum in contrast to the fully reduced product (Fig. 5B), where no peak at 5.48 MeV was observed. Additional heating in hydrogen atmosphere reduced the new oxide layer again completely, which in turn lead to complete equilibration of the activity within the target. Similar results in terms of reduction and diffusion into the backing material were obtained for 238U and 232Th. Because of 238 U nitrate was prepared from the pure metal, only two alpha peaks (attributed to 238U and 234U) were detected in the alphaparticle spectra before and after the reduction. While in case of 232 Th, not only 228Th, but also 224Ra (half-life 3.66 d) was electroplated on the Pd backing. 224Ra compounds were not reduced in a coupled reduction process and remained intact on the surface, which was reflected in the alpha-particle spectrum (Fig. 7). Alpha-particle measurement (A) was performed in the same day as 232Th was electroplated, and only 24 h separated (A) and (B) measurements.
Broad peaks of 232Th and 228Th are accompanied by sharp peaks of 224Ra and its short-lived daughter nuclides 220Rn, 216Po, and 212Bi. The low intensity peak at 4.7 MeV most likely corresponds to 230Th. The target material for heavy ion irradiations should be deposited on the backing in an as thin as possible layer, since nuclear fusion cross sections are strongly dependent on the incident energy of the ion beam on the target material. Despite the possibility to plate an additional amount of the target material on the other side or even on the same side of the prepared intermetallic target, the pronounced diffusion of the reduced metal into the backing material prevents the use of thick e.g. 25 mm noble metal foils. However, an even distribution of the target material within the backing material appears not to be a big problem using thin e.g. 2–3 mm metal foils as backing materials. In experiments with thin 2 mm and 3 mm Pd foils, the reduction temperature of 11001 C was found to be unacceptably high due to disintegration of the foil shortly after introduction of hydrogen into the system. Thus the reduction temperature was lowered to 9001 C, which significantly increased the reduction time. A pinhole free 3.5 mm U/Pd target was produced after 3 h reduction time in a hydrogen flow of 50 ml/min. In case of using a 2 mm Pd foil we faced serious difficulties in producing pinhole free targets. Apparently an accumulated tension in the foil after the rolling process is released upon heating, destroying the initial integrity of the foil. Although improved by lowering the temperature, the problem was not completely solved so far. From a thermal and mechanical stability point of view Rh backing is superior to Pd. At almost the same density, Rh has a much higher melting point and is much easier to handle than Pd. The only serious drawback of Rh is that it is not as chemically reactive as Pd. Temperatures of at least 1300–14001 C are required for the reduction to be completed within a reasonable time period. Furthermore, at temperatures close to 11001 C, the Rh foil is affected by hydrogen induced embrittlement processes, which decreases the mechanical stability of the foil considerably. Attempts have been made to decrease the reduction temperature of U on Rh backing by evaporating 1 mm Pd layer on the backing prior to U deposition. Unfortunately, this approach did not alleviate the problem, mainly because of the intermetallic reaction between Pd and Rh, which are most likely forming a chemically inert alloy before the reduction is taking place. We suggest in this case increasing the chemical activity of the Rh foil by doping it with Pt, which is known as a protecting agent against hydrogen embrittlement in tantalum [16]. First irradiation experiments with intermetallic targets have been carried out to test the target handling and its irradiation behavior. Therefore, at the Oslo Cyclotron Laboratory, University of Oslo, Norway a 3.5 mm 238U/Pd target was irradiated at the MC35 Scanditronix cyclotron using a 0.5 mA proton beam with a cyclotron energy of 30 MeV. The target was mounted free-standing on a water cooled copper ring holder. It was irradiated at vacuum conditions for several hours. The handling turned out to be absolutely unproblematic. Optical inspection after the irradiation did not reveal any significant damage or oxidation.
4. Conclusions It was shown, that intermetallic targets can be prepared by reducing electroplated lanthanide/actinide materials on Pd foils of different thicknesses. Because of the diffusion of a reduced metal into the backing material, only thin (few mm) backing materials can be successfully used for actinide target preparation. A thickness of 3 mm was found to be a lower limit for the production of
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pinhole free Pd based targets. High intensity heavy ion irradiation tests of the prepared intermetallic targets are envisaged.
Acknowledgments This research project was supported by Swiss National Science Foundation Grant 200020_126639. References [1] R. Eichler, et al., Nature 447 (2007) 72. [2] K. Eberhardt, et al., Nuclear Instruments and Methods A 521 (2004) 208. [3] A. Yoshida, et al., Nuclear Instruments and Methods A 521 (2004) 65.
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