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Preparation of targets by electrodeposition for heavy element studies
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
Preparation of targets by electrodeposition for heavy element studies . a, N. Trautmanna K. Eberhardta,*, M. Sch.adelb, E. Schimpfb, P. Thorle a
Universitat . Mainz, Institut fur . Kernchemie, Fritz-Strassmann-Weg 2, D-55128 Mainz, Germany b Gesellschaft fur . Schwerionenforschung, Planckstrasse 1, D-64291 Darmstadt, Germany
Abstract For heavy element studies at GSI, lanthanide and actinide targets have been prepared by molecular plating. The deposition occurs from an isopropanolic solution at 1000–1200 V with current densities of a few mA/cm2. Several lanthanide targets have been prepared for test experiments. With natGd deposited on a 10 mm thick Be backing foil a target density of 1100 mg/cm2 could be achieved. Gd-targets were used for the production of a-emitting isotopes of Os, the homologue of hassium (Hs; Z ¼ 108), in order to develop a chemical separation procedure for Hs. 248Cm targets with densities up to 730 mg/cm2 have been produced for recent experiments to investigate the chemical behaviour of Hs. Here, a rotating wheel system with a multi-target device has been applied enabling higher beam intensities, compared to a stationary target. The targets were irradiated with a pulsed 26Mg5+ beam applying beam currents up to 6.6 mAelectr. An a-spectroscopic investigation of the irradiated Cm-targets showed that the Cm-material is not evenly distributed over the entire target area. Very often, for heavy element investigations, chemical separation procedures are required to ensure high purity of the deposited actinide materials. r 2003 Elsevier B.V. All rights reserved. PACS: 81.15.P; 84.50.D; 25.70 Keywords: Electrodeposition; Lanthanides; Actinides; Uranium; Curium
1. Introduction The chemical properties of the heaviest elements (transactinide elements; Z > 103) have been a matter of considerable interest in recent years. Modern atomic and molecular calculations for these elements predict a large influence of relativistic effects on the sequence of their valence shell *Corresponding author. Tel.: +49-6131-3925846; fax: +496131-3924488. E-mail address: [email protected] (K. Eberhardt).
electrons and thus on their chemical properties. By comparing experimentally the chemical behaviour of the heaviest elements in the gas phase or in aqueous solution with that of their lighter homologues, deviations from known trends in the periodic table due to relativistic effects can be recognized. Transactinide elements are produced in heavy-ion (HI)-reactions and decay predominantly by a-particle emission or spontaneous fission (SF). Reactions with actinide targets are used to synthesize the most neutron rich isotopes of the heaviest elements with half-lives in the order of seconds. Due to the low-production
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.407
ARTICLE IN PRESS K. Eberhardt et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
cross-sections in the order of nanobarn or even picobarn, high beam intensities and target densities in the order of mg/cm2 have to be applied. In the last 2 years, a number of lanthanide and actinide targets have been prepared for use at the heavy-ion accelerator UNILAC of the Gesellschaft fur . Schwerionenforschung (GSI) in Darmstadt for experiments to investigate the chemical behaviour of the heaviest elements. In order to prevent excessive heating of a stationary target with high beam currents, a multi-target device mounted on a rotating wheel has recently been developed at GSI. Here, a complete target wheel consists of three banana-shaped segments. Since the heavier actinide elements like curium (Cm) and californium (Cf) are available only in limited amounts, the target preparation technique should give high deposition yields. Easy and complete recovery of the target material is another prerequisite. The electrodeposition technique is well suited for the preparation of lanthanide and actinide targets on metallic and non-metallic backing materials with deposition yields approaching 100% [1–14]. In this paper the electrolytical deposition of lanthanide elements, uranium (U) and Cm, respectively, on thin titanium (Ti) or beryllium (Be) backings from organic solutions is described. This technique is referred to as ‘‘Molecular Plating’’ [10–14]. Chemical procedures for recycling and purification of target materials as well as the application of such targets in heavy element research are briefly discussed.
2. Experimental For molecular plating the lanthanide or actinide compound, typically the nitrate, is dissolved in a small volume (5–10 ml) of nitric acid and the aqueous phase is mixed with a surplus of an organic solvent (E14 ml), usually isopropanol. Under these conditions no electrolytic dissociation occurs by applying an electric current and probably the same compound as originally dissolved in the aqueous phase is deposited [10]. Fig. 1 shows a schematic view of the cell used for molecular plating. A funnel made of polyether– etherketone (PEEK) confines the area to be plated
209
Fig. 1. Schematic view of the cell used for molecular plating. A funnel made of polyether-etherketone (PEEK) confines the area to be plated and acts as a solution container with a volume of E15 ml. The backing foil is pre-mounted into an aluminium frame and sealed to the funnel by means of Viton O-ring (not shown here, see Fig. 2b) and a Ti-block that serves as the cathode. A Rh-wire is used as anode material.
and acts as a solution container with a volume of about 15 ml.The backing foil with an active target area of 1.9 cm2 is pre-mounted into an aluminium (Al) frame and sealed to the funnel by means of a Viton O-ring. A Ti- block serves as the cathode. A rhodium (Rh) wire is used as anode material. Molecular plating is carried out by applying a voltage of 1000–1200 V, yielding a current density in the order of several mA/cm2 under the described conditions. Normally, the deposition is performed on commercially available self-supporting foils with thicknesses ranging from 5–50 mm. The foils should be pinhole-free and pre-cleaned with perchlorethylene, dilute nitric acid, water and isopropanol prior to use. To avoid any cross contamination, for each actinide isotope a separate deposition cell is used. Fig. 2 shows a photograph of the dismounted deposition cell as used for the production of targets for the new rotating wheel assembly at GSI. Also shown here is a backing-foil (Be), mounted onto an Al-frame together with its
ARTICLE IN PRESS K. Eberhardt et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
210
Viton-seal. The choice of a suited backing material is a crucial point in many HI-reaction studies. Very often a material with a low Z-number like Be ðZ ¼ 4Þ is used in order to prevent the production of interfering transfer products obtained in reactions with the beam. For the same reason, Rh ðZ ¼ 45Þ instead of platinum (Pt, Z ¼ 78) has been employed as anode material to prevent contamination of the target with a high-Z material. This may result in the formation of, e.g., a-emitting polonium (Po) isotopes which can hinder the detection of the transactinide element under investigation [9].
Fig. 2. Photograph of the deposition cell used for the production of targets for the new rotating wheel target assembly at GSI. A PEEK-funnel (see Fig. 2a) confines the area to be plated and forms a solution container with an inner volume of 15 ml. The backing foil is pre-mounted onto an Alframe that fits into the Ti cathode block and sealed to the funnel by means of a Viton-ring (see Fig. 2b). Three units form a complete target wheel (see also Fig. 4a).
Table 1 comprises the experimental conditions for the electrodeposition of several lanthanide elements, U and Cm on Ti or Be-backings. All these targets were made at the Institut fur . Kernchemie in the last 2 years for experiments with the GSI rotating wheel target arrangement. Furthermore, three 248Cm targets with densities up to 730 mg/cm2 (T1–T3, see Table 1) were produced for a pioneering experiment to investigate the chemical behaviour of Hs in the gas phase [15]. Another 248Cm target (T4) -doped with 152Gd for on-line production of carrier-free amounts of aemitting Os-isotopes—has very recently been applied at GSI in a second Hs-experiment [16]. The deposition yield was either determined by aparticle measurement of the target layer in a low geometry counter in the case of the Cm-targets or by neutron activation analysis of the supernatant solution for the other elements. For this, aliquots of the plating solution before and after the deposition process have been irradiated at the TRIGA Mainz research reactor. Deposition yields of 90% or higher and target densities up to 1100 mg/cm2 were obtained for various lanthanide elements and also for U (see Table 1). The yield for Cm under these conditions was up to 65% and did not increase with plating time or voltage. For the deposition of a mixture of Cm and Gd it was found, that a stepwise increase of the voltage from 1000 to 1200 V after 10 min gives the highest
Table 1 Experimental conditions for the deposition of various lanthanide elements, uranium and curium from isopropanolic solution Isotope
Backing/thickness (mm)
Voltage (V)
Plating time (min)
Target density (mg/cm2)
Ce (nat) Nd (nat) Gd(nat) Gd-152 Dy (nat) Er (nat) Yb (nat)
Ti/5 Ti/5 Be/10 Be/10 Ti/5 Ti/5 Ti/6
1200 1200 1200 1200 1200 1200 1200
90 90 90 90 90 90 90
800 800 1100 800 800 800 300
U(nat)/Nd(nat)
Be/10
1200
90
800
Cm-248 (T1) Cm-248 (T2) Cm-248 (T3) Cm-248/Gd-152
Be/15 Be/15 Be/15 Be/15
1200 1200 1200 1000–1200
40 40 40 45
240 730 690 500
(T1–T4)
(T4)
Targets used in experiments to investigate the chemistry of Hs. See text for details.
ARTICLE IN PRESS K. Eberhardt et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
deposition yield. In all cases the composition of the plating solution was the same: the nitrate compound was dissolved in 5–10 ml 0.1 N nitric acid and mixed with 14 ml of isopropanol. Very often, prior to deposition, chemical separation procedures are required to ensure high purity of the target material. This is of special importance in many HI-experiments, because the cross-section of impurities such as Pt or lead (Pb) with the beam is much higher compared to the one with the target material itself. Furthermore, traces of Be present after a recovery process prevent an effective deposition in molecular plating and therefore Be must be separated. The trivalent actinides were purified by ion-exchange procedures, as shown in Fig. 3. Be is separated on an anion-exchange column (AIX; BioRad AG1 8; 4 150 mm) operated at room temperature by evaporating the solution to dryness and dissolving it in 2 ml of a 1 N HNO3/90% methanol mixture. The solution is then transferred to the AIXcolumn and the column is washed with 4 2 ml of the methanolic solution. Under these conditions, Be is eluted completely, as could be shown with 7Be. Subsequently, the trivalent actinides are eluted with 5 2 ml 1 N HNO3. For the separation of lead and for the purification of the actinides, a cation-exchange column (CIX; Dowex 50WX8; 4 150 mm at 55 C) is used. The CIX has to be pre-treated with pure water, 8 N HCl and 0.5 N HCl-solution at least five times. A solution of the actinides in 0.5 N HCl is transferred to the CIXcolumn and the column is washed with 8 2 ml 0.5 N HCl. Then Pb is eluted with 15 ml 1.5 N HCl, whereas the trivalent actinides remain on the column. In a subsequent step, the actinides are eluted with 15 ml 8 N HCl. For further purification of the trivalent actinides and for a complete conversion of the chloride into the nitrate form, the separation procedure should be repeated with a smaller clean-up column (CIX; Dowex 50WX8; 3 50 mm) and by using nitric acid instead of hydrochloric acid. The isolated actinide fraction is then evaporated to dryness, dissolved in 5–10 ml 0.1 N HNO3 and mixed with isopropanol for the molecular plating procedure. A homogeneity study by a-particle counting of the 248Cm layers of targets T1–T3 (see Table 1) has
211
Actinide-solution in 90%methanol
AIX 1 N HNO3
AG1X8
Actinides (III)
4x150 mm Room temperature
Beryllium
in 0.5 N HCl
1.5 N HCI
CIX
Lead
1. 50WX8 8 N HCI
4x150 mm
Actinides (III)
2. T=55˚ C
Fig. 3. Separation scheme for the purification of trivalent actinides from beryllium and lead.
been carried out in such a way that after irradiation with an intense 26Mg5+-beam (total dose: 1 1018 ions in 64 h of beam time with beam currents up to 6.6 mAelect.) the target wheel has been dismantled and the active target area in each of the three targets has been covered with a mask containing a hole of 6 mm (the maximum width of the Cm-layer). The target was placed in a lowgeometry counter and a series of a-particle spectra were recorded. By moving the mask along the central axis of the target, the entire target area was divided into seven circular sub-areas. From the analysis of the corresponding a-particle spectra the
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K. Eberhardt et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
(local) target density was determined. Fig. 4a shows a picture of the target wheel after irradiation. The dark spots indicate the areas where the Mg-beam hit the target. Fig. 4b shows schematically the sub-division of the target area into seven circular areas for a-particle measurements. Table 2 summarizes the results of the homogeneity test for the three targets. It could be shown that the target material is not evenly distributed over the entire target area of 1.9 cm2. Deviations in target density from the average value of up to 20% were observed. These deviations may result from insufficient electrical contact between the backing foil and the cathode. The deposition cell is going to be re-designed to ensure proper electrical contact between the backing and the cathode over the entire target area to improve the uniformity of the target layer.
3. Applications The 248Cm-targets T1–T3 (see Table 1) have been used at GSI in a pioneering experiment to investigate the chemical behaviour of hassium (Hs, Z ¼ 108) [15]. Hs-nuclides with half-lives in the order of 10 s were produced in the reaction 248 Cm(26Mg;5,4n)269,270Hs. The Hs isotopes were converted on-line to volatile oxides (most probably HsO4 [15]) by interaction with a dry He/O2 gas mixture, and rapidly transported with the carrier gas to a detection system consisting of a longitudinal array of 12 pairs of PIN diodes suitable to detect the energies of a-particles and fragments from spontaneous fission resulting from the decay of 269,270Hs and its daughter nuclei. A negative temperature gradient from 20 to 170 C was established along the array to render the adsorption position (-temperature) of HsO4. From this, the adsorption enthalpy (volatility) of HsO4 was derived. Within a total beam time of 64 h seven correlated decay chains were registered. It was found that HsO4 has a lower volatility than OsO4, its lighter homologue. This result indicates that Hs is a member of group 8 of the periodic table [15]. Very recently, a second experiment was performed also at GSI. Here, the chemical reaction of HsO4 (and the corresponding Os-compound) with a NaOH-surface was studied [16]. One of the pure 248Cm-targets was replaced by a target doped with 152Gd for the on-line production of a-emitting Os-isotopes. So far, the data analysis has not yet been completed. Gas-phase studies of even heavier elements than Hs are also planned [17]. A challenging experiment to look into the chemistry of element 112 produced in the reaction 238 U(48Ca;1,2n)284,285112 is scheduled at GSI for February 2003. For this, a number of U-targets
Fig. 4. (a) Picture of a target wheel with three 248Cm targets after irradiation with a total dose of 1 1018 26Mg5+ ions. The dark spots indicate the areas where the Mg beam hit the Cm layer. (b) Shows schematically how the target area was divided into seven circular sub-areas for successive a-particle measurements. Table 2 Uniformity of the Target
T1 T2 T3
248
Cm layers produced by electrodeposition as deduced from a-particle measurements
Target density per segment (mg/cm2) 1
2
3
4
5
6
7
Average
219 821 766
239 757 685
243 637 586
238 576 551
236 694 627
247 845 798
249 780 831
239 730 692
Deviations in layer density from the average value of up to 20% occur over a target area of 1.9 cm2.
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with densities of 1–1.5 mg/cm2 on Be-backings must be prepared.
Acknowledgements The authors are indebted to the European Commission Joint Research Center, Institute for Transuranium Elements, Karlsruhe, for long-term storage of an intense 252Cf source and the subsequent chemical separation of the 248Cm target material. One of us (N.T.) acknowledges financial support from the Gesellschaft fur . Schwerionenforschung, Darmstadt.
References [1] P.G. Hansen, J. Inorg. Nucl. Chem. 12 (1959) 30. [2] E.J. Evans, R.W. Lougheed, M.S. Coops, R.W. Hoff, E.K. Hulet, Nucl. Instr. and Meth. 102 (1972) 389. [3] W. van der Eijk, W. Oldenhof, W. Zehner, Nucl. Instr. and Meth. 112 (1973) 343.
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[4] N. Trautmann, H. Folger, Nucl. Instr. and Meth. A 282 (1989) 102. [5] P.A.L. Da Cruz, R. Poldena, Nucl. Instr. and Meth. A 286 (1990) 453. [6] A. Becerril-Vilchis, A. Cortes, F. Dayras, J. de Sanoit, Nucl. Instr. and Meth. A 369 (1996) 613. [7] C. Ingelbrecht, A. Moens, R. Eykens, A. Dean, Nucl. Instr. and Meth. A 397 (1997) 34. [8] S. Bajo, J. Eikenberg, J. Radioanal. Nucl. Chem. 242 (1999) 745. . [9] K. Eberhardt, P. Thorle, A. N.ahler, N. Trautmann, in: J.L. Duggan, I.L. Morgan (Eds.), AIP Conference Proceedings, Vol. 576, 2001, pp. 1144–1147. [10] W. Parker, R. Falk, Nucl. Instr. and Meth. 16 (1962) 355. [11] W. Parker, H. Bildstein, N. Getoff, Nucl. Instr. and Meth. 26 (1964) 55. [12] W. Parker, H. Bildstein, N. Getoff, H. Fischer-Colbrie, H. Regal, Nucl. Instr. and Meth. 26 (1964) 61. [13] N. Getoff, H. Bildstein, Nucl. Instr. and Meth. 36 (1965) 173. [14] B.W. Filippone, M. Wahlgren, Nucl. Instr. and Meth. A 243 (1986) 41. [15] Ch.E. Dullmann, . et al., Nature 418 (2002) 859. [16] A. von Zweidorf, Private communication, 2002. [17] M. Sch.adel, J. Nucl. Radiochem. Sci. 3 (2002) 113.
Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
Preparation of targets by electrodeposition for heavy element studies . a, N. Trautmanna K. Eberhardta,*, M. Sch.adelb, E. Schimpfb, P. Thorle a
Universitat . Mainz, Institut fur . Kernchemie, Fritz-Strassmann-Weg 2, D-55128 Mainz, Germany b Gesellschaft fur . Schwerionenforschung, Planckstrasse 1, D-64291 Darmstadt, Germany
Abstract For heavy element studies at GSI, lanthanide and actinide targets have been prepared by molecular plating. The deposition occurs from an isopropanolic solution at 1000–1200 V with current densities of a few mA/cm2. Several lanthanide targets have been prepared for test experiments. With natGd deposited on a 10 mm thick Be backing foil a target density of 1100 mg/cm2 could be achieved. Gd-targets were used for the production of a-emitting isotopes of Os, the homologue of hassium (Hs; Z ¼ 108), in order to develop a chemical separation procedure for Hs. 248Cm targets with densities up to 730 mg/cm2 have been produced for recent experiments to investigate the chemical behaviour of Hs. Here, a rotating wheel system with a multi-target device has been applied enabling higher beam intensities, compared to a stationary target. The targets were irradiated with a pulsed 26Mg5+ beam applying beam currents up to 6.6 mAelectr. An a-spectroscopic investigation of the irradiated Cm-targets showed that the Cm-material is not evenly distributed over the entire target area. Very often, for heavy element investigations, chemical separation procedures are required to ensure high purity of the deposited actinide materials. r 2003 Elsevier B.V. All rights reserved. PACS: 81.15.P; 84.50.D; 25.70 Keywords: Electrodeposition; Lanthanides; Actinides; Uranium; Curium
1. Introduction The chemical properties of the heaviest elements (transactinide elements; Z > 103) have been a matter of considerable interest in recent years. Modern atomic and molecular calculations for these elements predict a large influence of relativistic effects on the sequence of their valence shell *Corresponding author. Tel.: +49-6131-3925846; fax: +496131-3924488. E-mail address: [email protected] (K. Eberhardt).
electrons and thus on their chemical properties. By comparing experimentally the chemical behaviour of the heaviest elements in the gas phase or in aqueous solution with that of their lighter homologues, deviations from known trends in the periodic table due to relativistic effects can be recognized. Transactinide elements are produced in heavy-ion (HI)-reactions and decay predominantly by a-particle emission or spontaneous fission (SF). Reactions with actinide targets are used to synthesize the most neutron rich isotopes of the heaviest elements with half-lives in the order of seconds. Due to the low-production
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.407
ARTICLE IN PRESS K. Eberhardt et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
cross-sections in the order of nanobarn or even picobarn, high beam intensities and target densities in the order of mg/cm2 have to be applied. In the last 2 years, a number of lanthanide and actinide targets have been prepared for use at the heavy-ion accelerator UNILAC of the Gesellschaft fur . Schwerionenforschung (GSI) in Darmstadt for experiments to investigate the chemical behaviour of the heaviest elements. In order to prevent excessive heating of a stationary target with high beam currents, a multi-target device mounted on a rotating wheel has recently been developed at GSI. Here, a complete target wheel consists of three banana-shaped segments. Since the heavier actinide elements like curium (Cm) and californium (Cf) are available only in limited amounts, the target preparation technique should give high deposition yields. Easy and complete recovery of the target material is another prerequisite. The electrodeposition technique is well suited for the preparation of lanthanide and actinide targets on metallic and non-metallic backing materials with deposition yields approaching 100% [1–14]. In this paper the electrolytical deposition of lanthanide elements, uranium (U) and Cm, respectively, on thin titanium (Ti) or beryllium (Be) backings from organic solutions is described. This technique is referred to as ‘‘Molecular Plating’’ [10–14]. Chemical procedures for recycling and purification of target materials as well as the application of such targets in heavy element research are briefly discussed.
2. Experimental For molecular plating the lanthanide or actinide compound, typically the nitrate, is dissolved in a small volume (5–10 ml) of nitric acid and the aqueous phase is mixed with a surplus of an organic solvent (E14 ml), usually isopropanol. Under these conditions no electrolytic dissociation occurs by applying an electric current and probably the same compound as originally dissolved in the aqueous phase is deposited [10]. Fig. 1 shows a schematic view of the cell used for molecular plating. A funnel made of polyether– etherketone (PEEK) confines the area to be plated
209
Fig. 1. Schematic view of the cell used for molecular plating. A funnel made of polyether-etherketone (PEEK) confines the area to be plated and acts as a solution container with a volume of E15 ml. The backing foil is pre-mounted into an aluminium frame and sealed to the funnel by means of Viton O-ring (not shown here, see Fig. 2b) and a Ti-block that serves as the cathode. A Rh-wire is used as anode material.
and acts as a solution container with a volume of about 15 ml.The backing foil with an active target area of 1.9 cm2 is pre-mounted into an aluminium (Al) frame and sealed to the funnel by means of a Viton O-ring. A Ti- block serves as the cathode. A rhodium (Rh) wire is used as anode material. Molecular plating is carried out by applying a voltage of 1000–1200 V, yielding a current density in the order of several mA/cm2 under the described conditions. Normally, the deposition is performed on commercially available self-supporting foils with thicknesses ranging from 5–50 mm. The foils should be pinhole-free and pre-cleaned with perchlorethylene, dilute nitric acid, water and isopropanol prior to use. To avoid any cross contamination, for each actinide isotope a separate deposition cell is used. Fig. 2 shows a photograph of the dismounted deposition cell as used for the production of targets for the new rotating wheel assembly at GSI. Also shown here is a backing-foil (Be), mounted onto an Al-frame together with its
ARTICLE IN PRESS K. Eberhardt et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
210
Viton-seal. The choice of a suited backing material is a crucial point in many HI-reaction studies. Very often a material with a low Z-number like Be ðZ ¼ 4Þ is used in order to prevent the production of interfering transfer products obtained in reactions with the beam. For the same reason, Rh ðZ ¼ 45Þ instead of platinum (Pt, Z ¼ 78) has been employed as anode material to prevent contamination of the target with a high-Z material. This may result in the formation of, e.g., a-emitting polonium (Po) isotopes which can hinder the detection of the transactinide element under investigation [9].
Fig. 2. Photograph of the deposition cell used for the production of targets for the new rotating wheel target assembly at GSI. A PEEK-funnel (see Fig. 2a) confines the area to be plated and forms a solution container with an inner volume of 15 ml. The backing foil is pre-mounted onto an Alframe that fits into the Ti cathode block and sealed to the funnel by means of a Viton-ring (see Fig. 2b). Three units form a complete target wheel (see also Fig. 4a).
Table 1 comprises the experimental conditions for the electrodeposition of several lanthanide elements, U and Cm on Ti or Be-backings. All these targets were made at the Institut fur . Kernchemie in the last 2 years for experiments with the GSI rotating wheel target arrangement. Furthermore, three 248Cm targets with densities up to 730 mg/cm2 (T1–T3, see Table 1) were produced for a pioneering experiment to investigate the chemical behaviour of Hs in the gas phase [15]. Another 248Cm target (T4) -doped with 152Gd for on-line production of carrier-free amounts of aemitting Os-isotopes—has very recently been applied at GSI in a second Hs-experiment [16]. The deposition yield was either determined by aparticle measurement of the target layer in a low geometry counter in the case of the Cm-targets or by neutron activation analysis of the supernatant solution for the other elements. For this, aliquots of the plating solution before and after the deposition process have been irradiated at the TRIGA Mainz research reactor. Deposition yields of 90% or higher and target densities up to 1100 mg/cm2 were obtained for various lanthanide elements and also for U (see Table 1). The yield for Cm under these conditions was up to 65% and did not increase with plating time or voltage. For the deposition of a mixture of Cm and Gd it was found, that a stepwise increase of the voltage from 1000 to 1200 V after 10 min gives the highest
Table 1 Experimental conditions for the deposition of various lanthanide elements, uranium and curium from isopropanolic solution Isotope
Backing/thickness (mm)
Voltage (V)
Plating time (min)
Target density (mg/cm2)
Ce (nat) Nd (nat) Gd(nat) Gd-152 Dy (nat) Er (nat) Yb (nat)
Ti/5 Ti/5 Be/10 Be/10 Ti/5 Ti/5 Ti/6
1200 1200 1200 1200 1200 1200 1200
90 90 90 90 90 90 90
800 800 1100 800 800 800 300
U(nat)/Nd(nat)
Be/10
1200
90
800
Cm-248 (T1) Cm-248 (T2) Cm-248 (T3) Cm-248/Gd-152
Be/15 Be/15 Be/15 Be/15
1200 1200 1200 1000–1200
40 40 40 45
240 730 690 500
(T1–T4)
(T4)
Targets used in experiments to investigate the chemistry of Hs. See text for details.
ARTICLE IN PRESS K. Eberhardt et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 208–213
deposition yield. In all cases the composition of the plating solution was the same: the nitrate compound was dissolved in 5–10 ml 0.1 N nitric acid and mixed with 14 ml of isopropanol. Very often, prior to deposition, chemical separation procedures are required to ensure high purity of the target material. This is of special importance in many HI-experiments, because the cross-section of impurities such as Pt or lead (Pb) with the beam is much higher compared to the one with the target material itself. Furthermore, traces of Be present after a recovery process prevent an effective deposition in molecular plating and therefore Be must be separated. The trivalent actinides were purified by ion-exchange procedures, as shown in Fig. 3. Be is separated on an anion-exchange column (AIX; BioRad AG1 8; 4 150 mm) operated at room temperature by evaporating the solution to dryness and dissolving it in 2 ml of a 1 N HNO3/90% methanol mixture. The solution is then transferred to the AIXcolumn and the column is washed with 4 2 ml of the methanolic solution. Under these conditions, Be is eluted completely, as could be shown with 7Be. Subsequently, the trivalent actinides are eluted with 5 2 ml 1 N HNO3. For the separation of lead and for the purification of the actinides, a cation-exchange column (CIX; Dowex 50WX8; 4 150 mm at 55 C) is used. The CIX has to be pre-treated with pure water, 8 N HCl and 0.5 N HCl-solution at least five times. A solution of the actinides in 0.5 N HCl is transferred to the CIXcolumn and the column is washed with 8 2 ml 0.5 N HCl. Then Pb is eluted with 15 ml 1.5 N HCl, whereas the trivalent actinides remain on the column. In a subsequent step, the actinides are eluted with 15 ml 8 N HCl. For further purification of the trivalent actinides and for a complete conversion of the chloride into the nitrate form, the separation procedure should be repeated with a smaller clean-up column (CIX; Dowex 50WX8; 3 50 mm) and by using nitric acid instead of hydrochloric acid. The isolated actinide fraction is then evaporated to dryness, dissolved in 5–10 ml 0.1 N HNO3 and mixed with isopropanol for the molecular plating procedure. A homogeneity study by a-particle counting of the 248Cm layers of targets T1–T3 (see Table 1) has
211
Actinide-solution in 90%methanol
AIX 1 N HNO3
AG1X8
Actinides (III)
4x150 mm Room temperature
Beryllium
in 0.5 N HCl
1.5 N HCI
CIX
Lead
1. 50WX8 8 N HCI
4x150 mm
Actinides (III)
2. T=55˚ C
Fig. 3. Separation scheme for the purification of trivalent actinides from beryllium and lead.
been carried out in such a way that after irradiation with an intense 26Mg5+-beam (total dose: 1 1018 ions in 64 h of beam time with beam currents up to 6.6 mAelect.) the target wheel has been dismantled and the active target area in each of the three targets has been covered with a mask containing a hole of 6 mm (the maximum width of the Cm-layer). The target was placed in a lowgeometry counter and a series of a-particle spectra were recorded. By moving the mask along the central axis of the target, the entire target area was divided into seven circular sub-areas. From the analysis of the corresponding a-particle spectra the
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(local) target density was determined. Fig. 4a shows a picture of the target wheel after irradiation. The dark spots indicate the areas where the Mg-beam hit the target. Fig. 4b shows schematically the sub-division of the target area into seven circular areas for a-particle measurements. Table 2 summarizes the results of the homogeneity test for the three targets. It could be shown that the target material is not evenly distributed over the entire target area of 1.9 cm2. Deviations in target density from the average value of up to 20% were observed. These deviations may result from insufficient electrical contact between the backing foil and the cathode. The deposition cell is going to be re-designed to ensure proper electrical contact between the backing and the cathode over the entire target area to improve the uniformity of the target layer.
3. Applications The 248Cm-targets T1–T3 (see Table 1) have been used at GSI in a pioneering experiment to investigate the chemical behaviour of hassium (Hs, Z ¼ 108) [15]. Hs-nuclides with half-lives in the order of 10 s were produced in the reaction 248 Cm(26Mg;5,4n)269,270Hs. The Hs isotopes were converted on-line to volatile oxides (most probably HsO4 [15]) by interaction with a dry He/O2 gas mixture, and rapidly transported with the carrier gas to a detection system consisting of a longitudinal array of 12 pairs of PIN diodes suitable to detect the energies of a-particles and fragments from spontaneous fission resulting from the decay of 269,270Hs and its daughter nuclei. A negative temperature gradient from 20 to 170 C was established along the array to render the adsorption position (-temperature) of HsO4. From this, the adsorption enthalpy (volatility) of HsO4 was derived. Within a total beam time of 64 h seven correlated decay chains were registered. It was found that HsO4 has a lower volatility than OsO4, its lighter homologue. This result indicates that Hs is a member of group 8 of the periodic table [15]. Very recently, a second experiment was performed also at GSI. Here, the chemical reaction of HsO4 (and the corresponding Os-compound) with a NaOH-surface was studied [16]. One of the pure 248Cm-targets was replaced by a target doped with 152Gd for the on-line production of a-emitting Os-isotopes. So far, the data analysis has not yet been completed. Gas-phase studies of even heavier elements than Hs are also planned [17]. A challenging experiment to look into the chemistry of element 112 produced in the reaction 238 U(48Ca;1,2n)284,285112 is scheduled at GSI for February 2003. For this, a number of U-targets
Fig. 4. (a) Picture of a target wheel with three 248Cm targets after irradiation with a total dose of 1 1018 26Mg5+ ions. The dark spots indicate the areas where the Mg beam hit the Cm layer. (b) Shows schematically how the target area was divided into seven circular sub-areas for successive a-particle measurements. Table 2 Uniformity of the Target
T1 T2 T3
248
Cm layers produced by electrodeposition as deduced from a-particle measurements
Target density per segment (mg/cm2) 1
2
3
4
5
6
7
Average
219 821 766
239 757 685
243 637 586
238 576 551
236 694 627
247 845 798
249 780 831
239 730 692
Deviations in layer density from the average value of up to 20% occur over a target area of 1.9 cm2.
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with densities of 1–1.5 mg/cm2 on Be-backings must be prepared.
Acknowledgements The authors are indebted to the European Commission Joint Research Center, Institute for Transuranium Elements, Karlsruhe, for long-term storage of an intense 252Cf source and the subsequent chemical separation of the 248Cm target material. One of us (N.T.) acknowledges financial support from the Gesellschaft fur . Schwerionenforschung, Darmstadt.
References [1] P.G. Hansen, J. Inorg. Nucl. Chem. 12 (1959) 30. [2] E.J. Evans, R.W. Lougheed, M.S. Coops, R.W. Hoff, E.K. Hulet, Nucl. Instr. and Meth. 102 (1972) 389. [3] W. van der Eijk, W. Oldenhof, W. Zehner, Nucl. Instr. and Meth. 112 (1973) 343.
213
[4] N. Trautmann, H. Folger, Nucl. Instr. and Meth. A 282 (1989) 102. [5] P.A.L. Da Cruz, R. Poldena, Nucl. Instr. and Meth. A 286 (1990) 453. [6] A. Becerril-Vilchis, A. Cortes, F. Dayras, J. de Sanoit, Nucl. Instr. and Meth. A 369 (1996) 613. [7] C. Ingelbrecht, A. Moens, R. Eykens, A. Dean, Nucl. Instr. and Meth. A 397 (1997) 34. [8] S. Bajo, J. Eikenberg, J. Radioanal. Nucl. Chem. 242 (1999) 745. . [9] K. Eberhardt, P. Thorle, A. N.ahler, N. Trautmann, in: J.L. Duggan, I.L. Morgan (Eds.), AIP Conference Proceedings, Vol. 576, 2001, pp. 1144–1147. [10] W. Parker, R. Falk, Nucl. Instr. and Meth. 16 (1962) 355. [11] W. Parker, H. Bildstein, N. Getoff, Nucl. Instr. and Meth. 26 (1964) 55. [12] W. Parker, H. Bildstein, N. Getoff, H. Fischer-Colbrie, H. Regal, Nucl. Instr. and Meth. 26 (1964) 61. [13] N. Getoff, H. Bildstein, Nucl. Instr. and Meth. 36 (1965) 173. [14] B.W. Filippone, M. Wahlgren, Nucl. Instr. and Meth. A 243 (1986) 41. [15] Ch.E. Dullmann, . et al., Nature 418 (2002) 859. [16] A. von Zweidorf, Private communication, 2002. [17] M. Sch.adel, J. Nucl. Radiochem. Sci. 3 (2002) 113.