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Electrolytic preparation of actinide filaments for laser resonance ionization spectroscopy
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Nuclear Instruments and Methods in Physics Research A 334 (1993) 93-95 North-Holland
Electrolytic preparation of actinide filaments for laser resonance ionization spectroscopy H. Wendeler
a,
R. Deissenberger
a,
FA. Urban
b,
Institut für Kernchemie, Universität Mainz, W-6500 Mainz, Germany b Institut für Physik, Universität Mainz, W-6500 Mainz, Germany
N. Trautmann
a
and G. Herrmann
a
For the determination of trace amounts of actinide elements by resonance ionization mass spectroscopy, atomic beams of these elements are produced by evaporation from rhenium substrates . The actinides are deposited by electrolysis as hydroxides in a 2 mm spot on a rhenium foil and covered electrolytically with a thin layer of rhenium or platinum . During heating of such a sandwich reduction occurs to the metallic state while diffusing through the overplated layers and the actinides are evaporated as atoms from the surface. With lead-doped platinum, efficient release is observed already at 1700°C in contrast to rhenium where 2100°C is required and a higher background is produced .
1. Introduction Long-lived isotopes of transuranium elements such as and 238-242Pu are present in our environment as a result of global fallout from nuclear weapons tests and nuclear and satellite accidents. For their detection a-particle spectroscopy is mainly used, e.g . ref. [1]. This approach has certain disadvantages: (i) the detection limit depends on the half-life of the nuclide, (ii) one of the mentioned plutonium isotopes, 241 Pu, has only a tiny a-decay branch, and (iii) two other isotopes, 239pu and 24° Pu, can hardly be resolved because of very similar a-particle energies. Mass spectrometry is an alternative approach but may require laborious chemical purifications to avoid isobaric interference, e.g ., between 238Pu and 238U. Hence, resonance ionization mass spectroscopy [2,3], RIMS, has recently been developed for the determination of neptunium [4] and plutonium [5] in low concentrations . The method is based on a stepwise excitation and ionization of actinide atoms with resonant laser light followed by timeof-flight mass spectrometry of the accelerated ions . This combination of two powerful techniques ensures high elemental and isotopic selectivity. The experimental setup [5] for RIMS consists of three tunable dye lasers which are pumped simultaneously by two copper vapour lasers. The dye laser beams are deflected into the time-of-flight spectrometer . Here, ions are produced by interaction of the pulsed laser beams with an atomic beam evaporated from a hot sandwich filament . The ions are accelerated by electric fields and detected by channel plates at the end of the 237Np
flight pass . The spectrometer has a mass resolution of
M/A M = 1500 [5].
The atomic beam is produced by heating a sandwich arrangement of the actinide element deposited on a rhenium foil and covered with a thin rhenium or platinum layer. The actinide is electrodeposited as hydroxide on the rhenium substrate in a 2 mm spot, required in order to achieve a small interaction zone between atomic and laser beams. The overplating layer is subsequently electrodeposited . During heating of such a sandwich filament the actinide oxide is reduced to the metallic state while diffusing through the overplated metal and an atomic beam is evaporated from the surface. Since electrodeposition is often used in the preparation of thin, uniform layers of actinide elements [6-8] to be applied as targets or as counting samples we report here briefly on our procedure and on the evaporation of plutonium from such sandwich filaments. 2. Experimental Some modifications of a conventional electrodeposition cell were made because of the small spot diameter, as shown in fig. 1 . A glass funnel contains the solution and confines the area to be plated to 2 mm diameter . A platinum wire serves as anode and a titanium block as cathode. The rhenium backing foil, a 17 x 2.5 mm 2 ribbon, 25 wm thick, is fixed on this block by a polyethylene cap. The distance between anode and cathode is 4 mm . Before electrolysis the rhenium ribbon is treated with 6M HCl at 80°C, washed
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
IV . RADIOACTIVE TARGETS
94
H. Wendeler et al. / Electrolytic preparation of actinide filaments E=4 24MeV 120 ~
80
4o
r
Eo=515MeV
-4cmFig. l. Cell for the preparation of actinide sandwich filaments by electrodeposition of actmide samples and overplating a metal layer . with Millipore water and acetone and heated in vacuum to 2000°C for 30 min. Prior to electrodeposition the actinide solution is fumed with concentrated H2SO4 to complete dryness in order to destroy colloidal and polymeric species. The residue is dissolved in 3 ml of an electrolyte solution containing 0.6 g (NH 4)Z SO 4 and adjusted to pH 2.5 with 1M H2SO4 . Electrodeposition of the actinide hydroxides is carried out with a current density of 3.2 A em -2 (total current I = 100 mA) at 10 V and a plating time of 1 h with water cooling of the whole setup. After finishing the deposition of the actinide hydroxide a thin rhenium layer is overplated [8] by adding 0.6 mg of rhenium as NH,Re0 4 dissolved in 300 wl of 0.1M HCl and applying a current density of 2.4 Acm -2 (I = 75 mA) at 10 V and 60°C for 1 h. An alternative to rhenium as covering layer is platinum [9,10] . In order to decrease the electron work function the platinum is poisoned with lead . Platinum is applied as dinitritosulfato-platinous acid ; 0.48 mg of Pt in this form and 10 l.Lg of Pb 2+ are added to 300 wl of 1 .5M HCl and electrolysis is carried out with a current density of 3.2 A cm -2 (I= 100 rnA) at 10 V and 60°C for 1 h. Before interrupting the current NH 40H is added to ensure that the deposited material is not redissolved . 3. Results The yield with electrolytic deposition of actinides is about 85%, as determined by a-particle spectroscopy . In fig. 2, a-particle spectra of Z39Pu sandwich samples with rhenium or platinum coatings are shown. Due to the absorption of a-particles in the coatings the spectra are spread out and their maxima are shifted to lower energies than the a-particle energy of Z39P u of
Energy/MeV
Fig. 2. a-Particle spectrum of a Z39Pu sample after overplating (a) rhenium, 2.0 Wm, and (b) platinum, 1 .0 wm thick . Ep = 5.15 MeV (average of three mayor groups), which appears as an absorption edge in the spectra. From this energy shift the thickness of the overplated layer can be estimated. The average energy loss per wm of path length is calculated from Bethe's formula [11] to be 0.44 McVwm- ' for rhenium and 0.45 McVP.m -1 for platinum . Table 1 shows the thicknesses for overplatings prepared at different plating temperatures . The thickness increases with increasing temperature . Beyond about 2 ltm thickness the spectra often show a distinct peak at the unperturbed a-particle energy, indicating incomplete coverage . Microscopic inspection of such coatings shows cracks and flakes at the surfaces. The actinides evaporate partly as oxides from such sandwiches, which are therefore not suitable for RIMS . The coating thickness recommended for RIMS of plutonium is 0.7-2 .0 ~Lm. Table 1 Thicknesses of electroplated metal coatings on Z'1Pu deposits as deduced from absorption shifts in a-particle spectra Coating metal
Rhenium Platinum
Electroplating temperature [°C] 55 80 25 45 80
Thickness of coating [Wml 0.9-1 .4 1.8-2.0 0.1-0.2 0.4-0.8 l .1-1 .3
95
H. Wendeler et al. /Electrolytic preparation of actmidefilaments 4. Conclusion
200 2250°C
Electrodeposition can be applied to the production
150
of small spots of actinide elements on rhenium backings . Platinum doped with lead is superior to rhenium
2150'C 2100°C
as the overplated metal for the production of atomic
beams by heating as required in resonance ionization mass spectroscopy .
10
15
20
25
30
35
40
Evaporation time /min
239Pu from a rhenium backing overFig. 3 . Evaporation of plated with a rhenium layer, 2.0 Win thick: ion count rate in the RIMS setup plotted versus evaporation time at stepwise increased temperatures .
Acknowledgements The authors would like to thank A. Niihler for her
help
during the experiments and the Bundesmini-
sterium für Umwelt, Reaktorsicherheit and NaturFig. 3 shows the temperature-dependent evaporation of 239Pu in vacuum from a sandwich overplated
schutz for financial support.
with rhenium. The ion count rate in the RIMS setup is
plotted versus time of evaporation while the heating temperature is increased stepwise as shown. As can be
seen efficient evaporation of atomic plutonium from the sandwich starts at a temperature of 2100°C . How-
ever, surface ionization of contaminants leads to an
increased background at this temperature. With leaddoped platinum overplatings much lower temperatures
are sufficient for evaporation, as shown in fig. 4. Here, evaporation
of
plutonium atoms starts
already
at
efficiency
as
1670°C and is completed within 10 min at somewhat higher
temperatures
with
the
same
achieved with rhenium at much higher temperatures, namely more than 90% .
Eviporotion time/ min
Fig. 4. Evaporation of 239Pu from a rhenium backing overplated with a platinum layer, 0.7 win thick: ion count rate in the RIMS setup plotted versus evaporation time at stepwise increased temperatures.
References [1] P. Peuser, H. Gabelmann, M. Lerch, B. Sohnius, N. Trautmann, M. Weber, G. Herrmann, H.O . Denschlag, W. Ruster and J. Bonn, in : Methods of Low-Level Counting and Spectrometry (International Atomic Energy Agency, Wien, 1981) p. 257. [2] G.S . Hurst and C. Grey Morgan (eds.), Resonance Ionization Spectroscopy 1986, Institute of Physics Conf . Set. no. 84 (Institute of Physics, Bristol, 1987).
[3] V.S . Letokhov, Laser Photoiomzation Spectroscopy (Academic Press, London, 1987). [4] P. Sattelberger, R. Deissenberger, G. Herrmann, J. Riegel, H. Rimke, N. Trautmann, F. Ames and H.-J. Kluge, Institute of Physics Conf. Ser. no. 114 (Institute of Physics, Bristol, 1991) sect. 5, p. 239. [5] W. Ruster, F. Ames, H.-J. Kluge, E.-W. Otten, D. Rehklau, F. Scheerer, G. Herrmann, C. Mühleck, J. Riegel, H. Rimke, P. Sattelberger and N. Trautmann, Nucl . Instr. and Meth . A281 (1989) 547. [61 R.F . Mitchell, Anal . Chem . 32 (1960) 326 . [7] N. Trautmann and H. Folger, Nucl . Instr. and Meth . A282 (1989) 102. [8] R.E . Perrin, G.W. Knobeloch, V.M . Armijo and D.W. Efurd, Int. J. Mass Spectrom . Ion Process . 64 (1985) 17. [9] R.E . Perrin, G.W. Knobeloch, V.M. Armijo and D.W. Efurd, Los Alamos National Laboratory Report LA10013-MS (1984) . [10] M. Attrep, F.R. Roensch, R. Aguilar and J. FabrykaMartin, Radiochim. Acta 57 (1992) 15 . [11] P. Marmier and E. Sheldon, Physics of Nuclei and Particles (Academic Press, New York, 1969) p. 156.
IV . RADIOACTIVE TARGETS
Nuclear Instruments and Methods in Physics Research A 334 (1993) 93-95 North-Holland
Electrolytic preparation of actinide filaments for laser resonance ionization spectroscopy H. Wendeler
a,
R. Deissenberger
a,
FA. Urban
b,
Institut für Kernchemie, Universität Mainz, W-6500 Mainz, Germany b Institut für Physik, Universität Mainz, W-6500 Mainz, Germany
N. Trautmann
a
and G. Herrmann
a
For the determination of trace amounts of actinide elements by resonance ionization mass spectroscopy, atomic beams of these elements are produced by evaporation from rhenium substrates . The actinides are deposited by electrolysis as hydroxides in a 2 mm spot on a rhenium foil and covered electrolytically with a thin layer of rhenium or platinum . During heating of such a sandwich reduction occurs to the metallic state while diffusing through the overplated layers and the actinides are evaporated as atoms from the surface. With lead-doped platinum, efficient release is observed already at 1700°C in contrast to rhenium where 2100°C is required and a higher background is produced .
1. Introduction Long-lived isotopes of transuranium elements such as and 238-242Pu are present in our environment as a result of global fallout from nuclear weapons tests and nuclear and satellite accidents. For their detection a-particle spectroscopy is mainly used, e.g . ref. [1]. This approach has certain disadvantages: (i) the detection limit depends on the half-life of the nuclide, (ii) one of the mentioned plutonium isotopes, 241 Pu, has only a tiny a-decay branch, and (iii) two other isotopes, 239pu and 24° Pu, can hardly be resolved because of very similar a-particle energies. Mass spectrometry is an alternative approach but may require laborious chemical purifications to avoid isobaric interference, e.g ., between 238Pu and 238U. Hence, resonance ionization mass spectroscopy [2,3], RIMS, has recently been developed for the determination of neptunium [4] and plutonium [5] in low concentrations . The method is based on a stepwise excitation and ionization of actinide atoms with resonant laser light followed by timeof-flight mass spectrometry of the accelerated ions . This combination of two powerful techniques ensures high elemental and isotopic selectivity. The experimental setup [5] for RIMS consists of three tunable dye lasers which are pumped simultaneously by two copper vapour lasers. The dye laser beams are deflected into the time-of-flight spectrometer . Here, ions are produced by interaction of the pulsed laser beams with an atomic beam evaporated from a hot sandwich filament . The ions are accelerated by electric fields and detected by channel plates at the end of the 237Np
flight pass . The spectrometer has a mass resolution of
M/A M = 1500 [5].
The atomic beam is produced by heating a sandwich arrangement of the actinide element deposited on a rhenium foil and covered with a thin rhenium or platinum layer. The actinide is electrodeposited as hydroxide on the rhenium substrate in a 2 mm spot, required in order to achieve a small interaction zone between atomic and laser beams. The overplating layer is subsequently electrodeposited . During heating of such a sandwich filament the actinide oxide is reduced to the metallic state while diffusing through the overplated metal and an atomic beam is evaporated from the surface. Since electrodeposition is often used in the preparation of thin, uniform layers of actinide elements [6-8] to be applied as targets or as counting samples we report here briefly on our procedure and on the evaporation of plutonium from such sandwich filaments. 2. Experimental Some modifications of a conventional electrodeposition cell were made because of the small spot diameter, as shown in fig. 1 . A glass funnel contains the solution and confines the area to be plated to 2 mm diameter . A platinum wire serves as anode and a titanium block as cathode. The rhenium backing foil, a 17 x 2.5 mm 2 ribbon, 25 wm thick, is fixed on this block by a polyethylene cap. The distance between anode and cathode is 4 mm . Before electrolysis the rhenium ribbon is treated with 6M HCl at 80°C, washed
0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
IV . RADIOACTIVE TARGETS
94
H. Wendeler et al. / Electrolytic preparation of actinide filaments E=4 24MeV 120 ~
80
4o
r
Eo=515MeV
-4cmFig. l. Cell for the preparation of actinide sandwich filaments by electrodeposition of actmide samples and overplating a metal layer . with Millipore water and acetone and heated in vacuum to 2000°C for 30 min. Prior to electrodeposition the actinide solution is fumed with concentrated H2SO4 to complete dryness in order to destroy colloidal and polymeric species. The residue is dissolved in 3 ml of an electrolyte solution containing 0.6 g (NH 4)Z SO 4 and adjusted to pH 2.5 with 1M H2SO4 . Electrodeposition of the actinide hydroxides is carried out with a current density of 3.2 A em -2 (total current I = 100 mA) at 10 V and a plating time of 1 h with water cooling of the whole setup. After finishing the deposition of the actinide hydroxide a thin rhenium layer is overplated [8] by adding 0.6 mg of rhenium as NH,Re0 4 dissolved in 300 wl of 0.1M HCl and applying a current density of 2.4 Acm -2 (I = 75 mA) at 10 V and 60°C for 1 h. An alternative to rhenium as covering layer is platinum [9,10] . In order to decrease the electron work function the platinum is poisoned with lead . Platinum is applied as dinitritosulfato-platinous acid ; 0.48 mg of Pt in this form and 10 l.Lg of Pb 2+ are added to 300 wl of 1 .5M HCl and electrolysis is carried out with a current density of 3.2 A cm -2 (I= 100 rnA) at 10 V and 60°C for 1 h. Before interrupting the current NH 40H is added to ensure that the deposited material is not redissolved . 3. Results The yield with electrolytic deposition of actinides is about 85%, as determined by a-particle spectroscopy . In fig. 2, a-particle spectra of Z39Pu sandwich samples with rhenium or platinum coatings are shown. Due to the absorption of a-particles in the coatings the spectra are spread out and their maxima are shifted to lower energies than the a-particle energy of Z39P u of
Energy/MeV
Fig. 2. a-Particle spectrum of a Z39Pu sample after overplating (a) rhenium, 2.0 Wm, and (b) platinum, 1 .0 wm thick . Ep = 5.15 MeV (average of three mayor groups), which appears as an absorption edge in the spectra. From this energy shift the thickness of the overplated layer can be estimated. The average energy loss per wm of path length is calculated from Bethe's formula [11] to be 0.44 McVwm- ' for rhenium and 0.45 McVP.m -1 for platinum . Table 1 shows the thicknesses for overplatings prepared at different plating temperatures . The thickness increases with increasing temperature . Beyond about 2 ltm thickness the spectra often show a distinct peak at the unperturbed a-particle energy, indicating incomplete coverage . Microscopic inspection of such coatings shows cracks and flakes at the surfaces. The actinides evaporate partly as oxides from such sandwiches, which are therefore not suitable for RIMS . The coating thickness recommended for RIMS of plutonium is 0.7-2 .0 ~Lm. Table 1 Thicknesses of electroplated metal coatings on Z'1Pu deposits as deduced from absorption shifts in a-particle spectra Coating metal
Rhenium Platinum
Electroplating temperature [°C] 55 80 25 45 80
Thickness of coating [Wml 0.9-1 .4 1.8-2.0 0.1-0.2 0.4-0.8 l .1-1 .3
95
H. Wendeler et al. /Electrolytic preparation of actmidefilaments 4. Conclusion
200 2250°C
Electrodeposition can be applied to the production
150
of small spots of actinide elements on rhenium backings . Platinum doped with lead is superior to rhenium
2150'C 2100°C
as the overplated metal for the production of atomic
beams by heating as required in resonance ionization mass spectroscopy .
10
15
20
25
30
35
40
Evaporation time /min
239Pu from a rhenium backing overFig. 3 . Evaporation of plated with a rhenium layer, 2.0 Win thick: ion count rate in the RIMS setup plotted versus evaporation time at stepwise increased temperatures .
Acknowledgements The authors would like to thank A. Niihler for her
help
during the experiments and the Bundesmini-
sterium für Umwelt, Reaktorsicherheit and NaturFig. 3 shows the temperature-dependent evaporation of 239Pu in vacuum from a sandwich overplated
schutz for financial support.
with rhenium. The ion count rate in the RIMS setup is
plotted versus time of evaporation while the heating temperature is increased stepwise as shown. As can be
seen efficient evaporation of atomic plutonium from the sandwich starts at a temperature of 2100°C . How-
ever, surface ionization of contaminants leads to an
increased background at this temperature. With leaddoped platinum overplatings much lower temperatures
are sufficient for evaporation, as shown in fig. 4. Here, evaporation
of
plutonium atoms starts
already
at
efficiency
as
1670°C and is completed within 10 min at somewhat higher
temperatures
with
the
same
achieved with rhenium at much higher temperatures, namely more than 90% .
Eviporotion time/ min
Fig. 4. Evaporation of 239Pu from a rhenium backing overplated with a platinum layer, 0.7 win thick: ion count rate in the RIMS setup plotted versus evaporation time at stepwise increased temperatures.
References [1] P. Peuser, H. Gabelmann, M. Lerch, B. Sohnius, N. Trautmann, M. Weber, G. Herrmann, H.O . Denschlag, W. Ruster and J. Bonn, in : Methods of Low-Level Counting and Spectrometry (International Atomic Energy Agency, Wien, 1981) p. 257. [2] G.S . Hurst and C. Grey Morgan (eds.), Resonance Ionization Spectroscopy 1986, Institute of Physics Conf . Set. no. 84 (Institute of Physics, Bristol, 1987).
[3] V.S . Letokhov, Laser Photoiomzation Spectroscopy (Academic Press, London, 1987). [4] P. Sattelberger, R. Deissenberger, G. Herrmann, J. Riegel, H. Rimke, N. Trautmann, F. Ames and H.-J. Kluge, Institute of Physics Conf. Ser. no. 114 (Institute of Physics, Bristol, 1991) sect. 5, p. 239. [5] W. Ruster, F. Ames, H.-J. Kluge, E.-W. Otten, D. Rehklau, F. Scheerer, G. Herrmann, C. Mühleck, J. Riegel, H. Rimke, P. Sattelberger and N. Trautmann, Nucl . Instr. and Meth . A281 (1989) 547. [61 R.F . Mitchell, Anal . Chem . 32 (1960) 326 . [7] N. Trautmann and H. Folger, Nucl . Instr. and Meth . A282 (1989) 102. [8] R.E . Perrin, G.W. Knobeloch, V.M . Armijo and D.W. Efurd, Int. J. Mass Spectrom . Ion Process . 64 (1985) 17. [9] R.E . Perrin, G.W. Knobeloch, V.M. Armijo and D.W. Efurd, Los Alamos National Laboratory Report LA10013-MS (1984) . [10] M. Attrep, F.R. Roensch, R. Aguilar and J. FabrykaMartin, Radiochim. Acta 57 (1992) 15 . [11] P. Marmier and E. Sheldon, Physics of Nuclei and Particles (Academic Press, New York, 1969) p. 156.
IV . RADIOACTIVE TARGETS