Quantitative molecular plating of large-area 242Pu targetswith improved layer properties

A. Vascon a,b,n, J. Runke c, N. Trautmann a, B. Cremer d, K. Eberhardt a,e, Ch.E. Düllmann a,c,e

a Institute of Nuclear Chemistry, Johannes Gutenberg University Mainz, 55099 Mainz, Germanyb Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germanyc SHE Chemistry Department, GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germanyd European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germanye SHE Chemistry Research Section, Helmholtz Institute Mainz, 55099 Mainz, Germany

H I G H L I G H T S

� Large area (i.e., 42 cm2) 242Pu targets were prepared on Ti-coated Si wafers.� Quantitative deposition yields (495%) were obtained.� Homogeneous 242Pu layers with minimum defectivity were produced.� The targets were prepared for future 242Pu(n, f) cross section measurements.

a r t i c l e i n f o

Article history:Received 22 May 2014Received in revised form23 September 2014Accepted 2 October 2014Available online 12 October 2014

Keywords:242PuLarge-area targetsMolecular platingQuantitative deposition yieldImproved layer propertiesScanning electron microscopy–energydispersive X-ray spectroscopy (SEM–EDX)

a b s t r a c t

For measurements of the neutron-induced fission cross section of 242Pu, large-area (42 cm2) 242Putargets were prepared on Ti-coated Si wafers by means of constant current density molecular plating.Radiochemical separations were performed prior to the platings. Quantitative deposition yields (495%)were determined for all targets by means of alpha-particle spectroscopy. Layer densities in the range of100–150 μg/cm2 were obtained. The homogeneity of the targets was studied by radiographic imaging.

A comparative study between the quality of the layers produced on the Ti-coated Si wafers and thequality of layers grown on normal Ti foils was carried out by applying scanning electron microscopy andenergy dispersive X-ray spectroscopy. Ti-coated Si wafers resulted clearly superior to Ti foils in theproduction of homogeneous 242Pu layers with minimum defectivity.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

To optimize the fast nuclear energy systems (i.e., fast fissionreactors and accelerator-driven systems) needed for transmutationaccurate values of neutron-induced cross sections are required. Inthe range of the fast neutrons (0.1oEno10 MeV) the inaccuracyof presently available experimental data is significant, with twomajor sources of uncertainty being: (i) neutron-induced fissioncross sections of Pu isotopes (especially 242Pu) and minor actinides(Np, Am, Cm), and (ii) inelastic scattering cross sections of fast

neutrons on structural and cooling materials of the reactors (e.g.,Fe, Pb, Na) (Salvatores et al., 2008). In order to reduce theseuncertainties and hence facilitate the development of fast reactors,in 2009 TRAKULA (transmutationsrelevante kernphysikalischeUntersuchungen langlebiger Aktinide, i.e., nuclear physical inves-tigations of long-lived actinides with relevance to transmutation)was established as a joint research project of the German FederalMinistry of Science and Education (BMBF). Since the start of theTRAKULA collaboration, one of the duties of the Institute ofNuclear Chemistry of the Johannes Gutenberg University (JGU)Mainz has been the preparation by means of molecular plating(MP) of large-area (42 cm2) 242Pu targets for future 242Pu(n, f) crosssection measurement at the nELBE source (Klug et al., 2007) of theHelmholtz-Zentrum Dresden-Rossendorf (HZDR), Rossendorf,Germany.

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Applied Radiation and Isotopes

http://dx.doi.org/10.1016/j.apradiso.2014.10.0020969-8043/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author at: Advanced Science Research Center (ASRC), JapanAtomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan.Tel.: þ81 29 282 5490; fax: þ81 29 282 5939.

E-mail address: vascon.alessio@jaea.go.jp (A. Vascon).

Applied Radiation and Isotopes 95 (2015) 36–43

Molecular plating is one of the most successfully appliedtechniques for the production of targets to be used in nuclearscience experiments (Becerril-Vilchis et al., 1996; Trautmann andFolger, 1989; Filippone and Wahlgren, 1986; Evans et al., 1972;Runke et al., 2014). The element of interest is electrodepositedfrom an organic medium either in the constant current or in theconstant voltage mode and layers of acceptable uniformity arequickly produced in quantitative yields. Yet MP produced targetsare frequently defective, characterized by surface cracks, andsometimes with poor structural rigidity—all these being unwantedproperties especially in irradiation experiments (Ali et al., 2010).For these reasons, in the last few years efforts have been under-taken at JGU to significantly increase the efficiency of the MPmethod to yet improve the obtained targets. Several surfaceanalytical techniques have been routinely adopted to fully char-acterize the produced layers (Vascon et al., 2011). Extensiveinvestigations of the molecular plating process have been carriedout to elucidate the basic features of this electrochemical techni-que (Vascon et al., 2012, 2014). Plating variables critically influen-cing layer properties like homogeneity, surface roughness, andmorphology have been identified and this has allowed for theproduction of homogeneous, smooth, and crack-free layers(Vascon et al., 2013a). The improved properties of these layershave also turned out to positively affect their performance innuclear spectroscopy experiments (Vascon et al., 2013b).

The present paper reports on the quantitative MP preparation(495% deposition yield) of large-area 242Pu targets for the fission

cross section measurement of 242Pu at HZDR (Section 2). The paperalso compares the quality of these targets with the quality oftargets produced with a conventional MP approach (Section 3). Itis shown that by exploiting the experience gained so far in the MPdevelopment of optimum layers 242Pu targets with improvedproperties can be prepared.

2. Target production and characterization

2.1. Deposition cell

The cell used for the MPs was already described in detail byVascon et al. (2011). Different materials from the ones reported byVascon et al. (2011) were applied for the plating of Pu. Kel-F(polychlorotrifluoroethylene—PCTFE) and Teflon (polytetrafluor-oethylene—PTFE) were preferred over PEEK (polyether ether ketone)to reduce to a minimum the absorption of Pu inside the cell. The cellwas characterized by a central Kel-F body where the electrolyticsolution was inserted. Water-cooled cylindrical Ti blocks acted aselectrodes of the cell. The cathode and the anode were covered by a400-μm thick Ti-coated Si wafer and a 100-μm thick Pd foil (AlfaAesar), respectively. Each of the two electrodes had a circulardeposition area of 42 cm2. The distance between the electrodeswas 3.5 cm. The Ti-coated Si wafer and the Pd foil were fixed tothe Ti blocks by means of Teflon rings. The electrode assembly wasconnected to the central body of the cell with rectangular Kel-Fframes. The set-up was sealed using Viton O-rings. The water-cooledelectrodes maintained a constant temperature of 16 1C inside the cell.

2.2. Deposition substrates

Mechanically-polished Si wafers coated with 100 nm metallicTi by means of magnetron sputtering were used as depositionbackings for the platings. These wafers are known to be ideally flatsubstrates with very little surface roughness. Typical roughnessvalues, expressed as average root mean square (RMS), are around10 nm—see Vascon et al. (2012) for details. A SEM image of theTi-coated Si wafer used for the experiments is shown in Fig. 1a.

One MP test was also carried out where a 500-μm thick Ti foil(Goodfellow) was used as deposition substrate (see Section 3).Ti foils cut from bigger foils are known to be characterized bylarger surface roughness values (RMS), which are usually in therange of 20–30 nm—see Vascon et al. (2012) for details. A SEMimage of the Ti foil used for the platings is shown in Fig. 1b.

2.3. Dissolution of Pu material and chemical purification

Highly 242Pu-enriched Pu oxide was delivered from Oak RidgeNational Laboratory (ORNL). Its isotopic composition (wt. %) was:0.003% 238Pu, 0.005% 239Pu, 0.022% 240Pu, 0.009% 241Pu, 99.959%242Pu, and 0.002% 244Pu. About 220 mg of the material weredissolved in a Teflon beaker in a mixture of 15 mL of 65% HNO3

(VWR Prolabo) and 5 mL of 48% HF (Merck) while carefully stirringand heating with both a hot plate (at 80 1C) and an infrared (IR) lamp.The complete dissolution of the Pu oxide took about three days. Toremove the unreacted HF 21 mL of a 4.4 M H3BO3 solution wereadded to the Pu solution while continuing to stir and heat. The Puwas precipitated in the form of the hydroxide with 50 mL of 25% NH3

(Fisher Scientific) and later dissolved in 30 mL of 8 M HCl.Due to the presence of 241Pu (T1/2¼14.35 y) in the base material,

the ingrown 241Am had to be separated from the Pu by anionexchange chromatography prior to the platings. Few drops of 65%HNO3 were added to aliquots of the Pu solution to yield Pu ions inan oxidation state of 4þ . The solutions were fed onto a precondi-tioned anion exchange column (Bio Rad AG MP-1 M), which was

Fig. 1. 1000� SEM images of the deposition substrates used to perform the MPexperiments: (a) Ti-coated Si wafer, (b) Ti foil.

A. Vascon et al. / Applied Radiation and Isotopes 95 (2015) 36–43 37

always kept at a temperature of 55 1C. First 241Amwas washed fromthe column with 30 mL of 8 M HCl, and then Pu was eluted with20 mL of 0.5 M HCl. Gamma-ray spectroscopy and α-particlespectroscopy were used to check for the presence of 241Am in thePu eluate. The analyses confirmed a complete removal of 241Am inthe Pu fractions. Alpha-particle spectroscopy was also used todetermine the recovery yield of Pu in the eluate (�100%). For theMP process Pu was converted to the nitrate by evaporation with65% HNO3 and dissolution in 65% HNO3.

2.4. Molecular plating at constant current density

Before each plating, the anode (Pd foil) was cleaned with 6 M HCl(1�3 mL washing), water (3�3 mL washings), and isopropanol(1�3 mL washing). The cathode (Ti-coated Si wafer) underwentthe same washing cycles, but 0.6 M HCl was used to avoid corrosion-induced increase in surface roughness (Vascon et al., 2012). Duringthese cleaning procedures aliquots of the Pu stock solution in 65%HNO3 were evaporated to dryness in a Kel-F beaker by means of aheating plate and an IR lamp. The residue was dissolved in 100 μL of0.1 M HNO3 and the solution was transferred into the plating cell.After washing the Kel-F beaker three times with 1 mL of isopropanol(Fisher Chemical) and transferring the solution into the cell, 217 mLof isopropanol were added. The average concentration of 242Pu insidethe 220 mL solutions was always about 0.1 mM. The solutions werestirred by means of a quartz tip ultrasonic stirrer (Bandelin SonopulsHD 2070) operated at 30% power pulse. MP experiments werecarried out for 3 h by applying a constant current of 26 mA, corres-ponding to a current density of 0.6 mA/cm2.

2.5. Deposition yield measurement

Alpha-particle spectroscopy was used to determine the 242Pudeposition yield. Because of the very large dimensions of thetargets (42 cm2-area 242Pu deposits on 75 cm2 Ti-coated Si wafers)direct counting of the produced samples was not possible at ourinstitute. We had to apply an indirect method: 10 μL of solutionwere taken out of the initial 220 mL before the start and after theend of the plating. These volumes were evaporated to dryness onTa disks using an IR lamp and the obtained drop sources weremeasured with a Si detector connected to an electronic chain witha pre-amplifier, an amplifier, and an analog-to-digital converter.The sources were counted for 30 min in wide-angle geometry. Thecounting rates measured from the evaporated volumes of solutiontaken before the start and after the end of the platings were usedto evaluate the initial and final 242Pu masses in the MP cell,respectively. The deposition yields computed from the mass valuesare reported in Table 1. Error propagation was used to determinethe uncertainties of the data.

The evaluated data clearly show the deposition yield forall eight produced targets to be quantitative (495%) within theerror bars. As required for the neutron-induced fission crosssection experiments, the 242Pu areal densities are in the range of100–150 μg/cm2.

2.6. Radiographic imaging (RI) studies

The homogeneity of the produced 242Pu targets was investi-gated by means of radiographic imaging (RI). A FUJIFILM FLA 7000machine, equipped with reusable imaging plates and a 650 nmlaser, was used to record images that were later overlaid by a gridof square spots with an area of 0.25 mm2 each. The spatialresolution of the technique waso200 μm (Liebe et al., 2008).Fig. 2a and b show, as examples, RI results of targets 4 and 8,respectively, whose deposition details are presented in Table 1.

Despite the large areas (42 cm2) that had to be plated, Fig. 2shows that the produced layers are characterized by a very goodhomogeneity.

Table 1

Mass (mg), areal density (μg/cm2), and deposition yield (%) of the produced 242Putargets. Error propagation was used to determine the uncertainties of the data.

TARGET 242Pu mass(mg)

242Pu areal density (μg/cm2)

Deposition yield(%)

1 6.4770.21 15575 96742 5.7070.18 13674 99743 5.0270.16 12074 100744 4.8970.15 11774 99745 4.9570.15 11874 99746 5.0870.16 12274 100747 4.5370.14 10873 100748 5.7270.18 13774 9974

73 mm73 mm

0

73 mm

0

73 mm

Fig. 2. RI normalized 3D graphs of 242Pu targets number 4 (a) and 8 (b) producedfor the neutron-induced fission experiments at HZDR.

A. Vascon et al. / Applied Radiation and Isotopes 95 (2015) 36–4338

3. Scanning electron microscopy–energy dispersive X-rayspectroscopy (SEM–EDX) studies

3.1. Experiments

Two 242Pu targets were prepared in addition to the eightsamples required for the fission experiments at HZDR. The MPprocedure described in Section 2.4 was adopted. One of the layerswas deposited on the very smooth Ti-coated Si wafer substrate, theother on the rougher Ti foil (see Section 2.2). The 242Pu areal densitiesof the two targets were close to the range of 100–150 μg/cm2 and thedeposition yields were also quantitative (�95%).

The morphology of these layers was studied by using scanningelectron microscopy coupled with energy dispersive X-ray spectro-scopy (SEM–EDX) (Philips XL40 with EDX detector). The microscopewas operated at 25 kV. The pressure inside the vacuum chamberwas 10�6 mbar. Secondary electrons were detected. Magnificationfactors of 200� , 500� , 1000� , 2000� , 5000� , and 10,000�were used.

3.2. Results

Fig. 3a and b show 500� SEM images of the layers grown onthe Ti-coated Si wafer (hereafter referred to as layer A) and on theTi foil (hereafter referred to as layer B), respectively.

Both deposits show the presence of cracks, which destroy thesurface homogeneity creating smaller island-like areas in thelayers. Despite this, layer A has a less defective surface than layerb, where both flaking of material (see white arrows) and severecracking can be noticed. These cracks create islands with largerareas but less sharp edges than those of layer A. In particular, alarger magnification (2000� ) clearly shows that the width of thecracks (i.e., the distance between the islands) is much larger inlayer B (Fig. 4b) than in layer A (Fig. 4a). This width can beapproximately quantified to be r1 μm in the case of layer Aand Z1 μm in the case of layer B (see Fig. 5a and b, respectively, atmagnification factors of 10,000� ).

EDX spectra recorded from measurements performed at thepoints indicated by the masks in Fig. 5a and b mainly showed MαX-ray lines of Pu, Kα X-ray lines of Ti, and Kα X-ray lines of Si.When the measurements were performed in the islands Pu, Ti, andSi X-ray lines could be clearly seen. Conversely, the Pu X-ray linealmost disappeared when the spectrum was recorded from thecracks. Fig. 6a and b show, as examples, EDX spectra measuredfrom the highlighted island and crack, respectively, of layer B (seeFig. 5b). The cracks between the islands clearly expose the surfaceof the deposition backing. In the case of layer B, the amount ofexposed Ti foil is significant.

In order to get more detailed information on the quality of layersA and B, EDX spectra were recorded during scanning of the targetsacross their diameters. Eleven 1000� images were taken persample at distances of ca. 1, 7, 14, 21, 28, 35, 42, 49, 56, 63, and70 mm from the border between the deposit and an outer ring ofbacking material. This outer ring where no layer was deposited wasthe portion of target area used to fix the substrate to the Ti block bymeans of a Teflon ring (see Section 2.1). For each of the eleven1000� images a (50�50 μm2) mask was used to select an area inthe image and record an EDX spectrum of it. A 1000� image of theouter ring of backing material was also taken (see Fig. 1) and a(50�50 μm2) mask was used to record an EDX spectrum. Fromthese measurements two different ratios were evaluated for eachlayer. For layer A the denominator of both ratios was the mostintense signal obtained from the EDX measurement of the outerring of the backing material, i.e., the Kα X-ray line of Si. For layer Bthe most intense signal recorded from the outer ring of the backingwas instead the Kα X-ray line of Ti. Both these line intensities were

considered as standard values for the calculations and are hereafterreferred to as Sistd and Tistd, respectively. The intensities of the PuMα, Si Kα, and Ti Kα X-ray lines recorded during the scanning of thetargets were instead used as numerators of the ratios. Each of thefour ratios was normalized to the value of the largest fractioncalculated. Fig. 7a shows the normalized ratios Pu/Sistd and Pu/Tistdfor layers A and B, respectively, whereas for the same layers Fig. 7bshows the normalized ratios Si/Sistd and Ti/Tistd. Average values ofthe ratios with corresponding error bars are reported in the panelson the right-hand side of each figure.

It is clear that in the case of layer A both ratios have almostconstant trends with very little scattering of the data points. This isnot the case for layer B. The areal density of Pu thus seems to bemuch more constant in layer A than in layer B. Moreover, layer Ahas a more homogeneous distribution of exposed substrate surfacethan layer B.

4. Discussion

Several publications describing the preparation of Pu targets bymeans of MP have reported deposition yields that are usually quitesmall (ca. 60%) (Evans et al., 1972; Sibbens et al., 2014; Ingelbrechtet al., 1997). Only on few occasions and for rather small depositionareas Pu yields Z90% have been obtained (Trautmann and Folger,

Fig. 3. 500� SEM images of the 242Pu layers grown on the following depositionsubstrates: (a) Ti-coated Si wafer, (b) Ti foil.

A. Vascon et al. / Applied Radiation and Isotopes 95 (2015) 36–43 39

1989; Getoff and Bildstein, 1965). In all cases, isopropanol has beenused as the organic medium to perform Pu electrodepositions.

In one of our recent publications (Vascon et al., 2013a) we haveexplained the influence of the solvent on the properties of the MPproduced layers. Solvent electrolysis occurring during the platingprocess causes the deposited layers to be composed of bothphysisorbed and chemisorbed solvent molecules (Vascon et al.,2012, 2014), whose volatility affects the stress experienced by thedeposit during drying (Vascon et al., 2013a). In general, the largerthe volatility of the solvent (i.e., the lower the boiling point, Bp), thelarger the stress and thus the probability of cracking and delamina-tion of the layer (Malzbender and De With, 2000). In the case ofisopropanol, the boiling point is rather low, i.e., 82.2 1C (Izutsu,2002) and for this reason during experiments with lanthanides theuse of N,N-dimethylformamide—DMF (Bp¼153.0 1C) was suggested(Vascon et al., 2013a). MPs with this solvent showed that layerswithout cracks can be produced. However, for the experiments withPu the same choice was not made because the chemistry of thisactinide is so peculiar that the results obtained from the combina-tions DMF/lanthanides might not be reproduced. This together withthe reduced availability of the costly Pu material and its highradiotoxicity led us not to perform tests with DMF. Therefore, byadopting isopropanol as organic medium for the electrodepositions,the production of defective 242Pu layers (i.e., with surface cracks)was expected. The SEM images of Section 3 show that the volatilityof this solvent is indeed too high to produce crack-free layers.Despite this, the application of other parameters found previously

effective for the quantitative preparation of quality lanthanidelayers (Vascon et al., 2012, 2013a, 2014) proved useful also for theMP of Pu. Variables like current density, electrolyte concentration,and surface roughness of the deposition substrate were indeedcarefully selected in order to reproduce the results obtained withlanthanide elements. In particular, the application of a constantcurrent density of 0.6 mA/cm2 for a plating time of 3 h allowed thequantitative deposition of Pu. To the best of our knowledge,reproducible deposition yields 495% have never been reportedfor this actinide, especially for such large deposition areas (42 cm2).In addition, the use of substrates with the smallest possible surfaceroughness favored the production of high quality layers. Despite infact the seeming good quality of the 242Pu layer grown on therougher Ti foil substrate—layer B—the performed SEM–EDX mea-surements clearly show that the layer deposited on the smootherTi-coated Si wafer—layer A—has superior properties. When a sub-strate is defective the surface mobility of the deposited atoms isreduced by trapping in the defects (see Fig. 1b). This causes thegrowth of rough and inhomogeneous layers (Vascon et al., 2013a,2013b). Besides the stress normally experienced by the depositduring drying, the surface defects grown from the deposition foilcause additional stress by acting as fragility points in the layer(Vascon et al., 2013a). This is probably the reason why layer B ismore defective than layer A. The very heavy cracking of the surface

Fig. 4. 2000� SEM images of the 242Pu layers grown on the following depositionsubstrates: (a) Ti-coated Si wafer, (b) Ti foil.

Fig. 5. 10,000� SEM images of the 242Pu layers grown on the following depositionsubstrates: (a) Ti-coated Si wafer, (b) Ti foil. The masks indicate points where EDXmeasurements were performed.

A. Vascon et al. / Applied Radiation and Isotopes 95 (2015) 36–4340

of layer B is such that the amount of exposed deposition substrateper each (50�50 μm2) area investigated by SEM–EDX is variableand generally large. The Ti/Tistd trend shown in Fig. 7b is a clearindication of this. In the case of layer A, the almost constant Si/Sistdtrend determined across the diameter of the target indicates insteadthat the amount of exposed substrate surface is almost alwaysidentical. Moreover, from the analysis of the recorded SEM images itis obvious that the area of uncovered Ti-coated Si wafer is muchsmaller than that of the corresponding Ti foil. The small and nearlyconstant amount of exposed Ti-coated Si wafer is also the reasonwhy the flat Pu/Sistd trend of layer A can be considered as anindication of almost constant Pu areal density. The intensity of thePu Mα X-line depends in fact on the total number of Pu atomspresent in the investigated area, i.e., it depends on both the degreeof Pu coverage of each (50�50 μm2) mask and the thickness of thedeposited Pu layer. In the case of layer A, where the amount ofsubstrate covered by Pu is rather constant, each Pu/Sistd ratio can beexpected to depend mainly on the thickness of the deposited layer.As the Pu/Sistd trend is nearly constant, the Pu areal density (i.e., thethickness) of the deposited layer can then be assumed to be alsoalmost constant. Therefore, despite the surface cracks caused by the

large volatility of the isopropanol used to perform the MP, the Pulayer grown on the Ti-coated Si wafer has a very good homogeneity,as also confirmed by the RI results of Fig. 2. The same of course doesnot hold true for layer B, where the Pu/Tistd trend is clearly variable.In this case the intensity of the Pu Mα X-ray line depends on boththe degree of coverage and the thickness of the deposited Pu layer.Consequently, no detailed information about the Pu areal density oflayer B can be inferred. However, it is reasonable to assume that thelarge defectivity of this layer causes the areal density values to beaffected by considerable degrees of uncertainty. This is of course theopposite of what is observed in the case of layer A, where thesmooth deposition substrate helps reduce the scattering of arealdensity values across the entire deposition area. In the perspectiveof performing experiments to reduce the uncertainty on theneutron-induced fission cross section of 242Pu this is rather impor-tant. Therefore, optimum targets for nuclear science experimentsshould be always characterized by both quantitative depositionyields and improved layer properties.

The outcomes of this work clearly prove that the recent studiesaiming at producing better quality layers of lanthanide elements(Vascon et al., 2011, 2012, 2013a, 2013b, 2014) have led to an

TiPu

Energy (keV)

Ti

Pu

Energy (keV)

Fig. 6. EDX spectra recorded from the highlighted island (a) and crack (b) of the 242Pu layer grown on the Ti foil—layer B (see Fig. 5b).

A. Vascon et al. / Applied Radiation and Isotopes 95 (2015) 36–43 41

understanding of the MP process that enables now also thepreparation of actinide layers with optimum properties.

5. Conclusion

Large-area (42 cm2) 242Pu targets were produced from isopro-panol by means of constant current density molecular plating.Radiochemical separations were performed prior to the platingsin order to remove the ingrown 241Am from the Pu base material.The deposition yields of the targets were evaluated by α-particlespectroscopy and turned out to be all quantitative (495%). Theobtained 242Pu areal densities are in the range of 100–150 μg/cm2.Eight 242Pu targets were prepared on very smooth Ti-coated Siwafers, which proved to be superior to normal Ti foils in theproduction of layers with improved properties. Indeed, after testexperiments followed by detailed SEM–EDX characterizations, 242Pulayers grown on the smooth Ti-coated Si wafers were found to beless defective and more homogeneous than layers grown on therougher Ti foils. The uncertainties on the areal density values of the242Pu targets produced for the fission experiments are thus smallerthan would be the case for layers obtained by using normal Ti foils.

The neutron-induced fission cross section measurement of242Pu will benefit from the high quality of the produced targets.

Acknowledgments

The authors would like to thank Dr. H. Keller from the Instituteof Nuclear Chemistry of the University of Mainz as well as Prof.Dr. K. Lützenkirchen and Dr. T. Wiss from the Institute for

Transuranium Elements of the Joint Research Centre of the Eur-opean Commission for their assistance during the carrying out ofthis scientific work. The authors thank also Dr. T. Lauer forperforming the coating of the Si wafers. A. Vascon acknowledgesfinancial support from the German Federal Ministry of Science andEducation (BMBF) under contract numbers 02NUK13A and02NUK13E.

References

Ali, M.N., Garcia, M.A., Parsons-Moss, T., Nitsche, H., 2010. Polymer-assisteddeposition of homogeneous metal oxide films to produce nuclear targets.Nat. Protoc. 5, 1440–1446.

Klug, J., Altstadt, E., Beckert, C., Beyer, R., Freiesleben, H., Galindo, V., Grosse, E.,Junghans, A.R., Légrády, D., Naumann, B., Noack, K., Rusev, G., Schilling, K.D.,Schlenk, R., Schneider, S., Wagner, A., Weiss, F.P., 2007. Development of aneutron time-of-flight source at the ELBE accelerator. Nucl. Instrum. Methods A577, 641–653.

Becerril-Vilchis, A., Cortès, A., Dayras, F., De Sanoit, J., 1996. A method for thepreparation of very thin and uniform α-radioactive sources. Nucl. Instrum.Methods A 369, 613–616.

Evans, J.E., Lougheed, R.W., Coops, M.S., Hoff, R.W., Hulet, E.K., 1972. The use ofelectrodeposition methods to prepare actinide targets for cross-section mea-surements and accelerator bombardments. Nucl. Instrum. Methods 102,389–401.

Filippone, B.W., Wahlgren, M., 1986. Preparation of a 7Be target via the molecularplating method. Nucl. Instrum. Methods A 243, 41–44.

Getoff, N., Bildstein, H., 1965. Molecular plating IV, a rapid and quantitative methodfor the electrodeposition of Plutonium. Nucl. Instrum. Methods 36, 173–175.

Ingelbrecht, C., Moens, A., Eykens, R., Dean, A., 1997. Improved electrodepositedactinide layers. Nucl. Instrum. Methods A 397, 34–38.

Izutsu, K., 2002. Electrochemistry in Nonacqueous Solutions, first ed. Wiley-VCH,Darmstadt, Germany.

Liebe, D., Eberhardt, K., Hartmann, W., Häger, T., Hübner, A., Kratz, J.V., Kindler, B.,Lommel, B., Thörle, P., Schädel, M., Steiner, J., 2008. The application of neutronactivation analysis, scanning electron microscope, and radiographic imaging for

Fig. 7. Trends of the normalized ratios Pu/Sistd, Pu/Tistd (a) and Si/Sistd, Ti/Tistd (b) measured at distinct points across the diameters of layers A and B. The panels on the right-hand side show the arithmetic means and corresponding standard deviations of the eleven data points calculated for each type of ratio.

A. Vascon et al. / Applied Radiation and Isotopes 95 (2015) 36–4342

the characterization of electrochemically deposited layers of lanthanide andactinide elements. Nucl. Instrum. Methods A 590, 145–150.

Malzbender, J., De With, G., 2000. The effects of drying, aging and curing on anorganic–inorganic coating. J. Mater. Sci. 35, 4809–4814.

Runke, J., Düllmann, Ch.E., Eberhardt, K., Ellison, P.A., Gregorich, K.E., Hofmann, S.,Jäger, E., Kindler, B., Kratz, J.V., Krier, J., Lommel, B., Mokry, C., Nitsche, H.,Roberto, J.B., Rykaczewski, K.P., Schädel, M., Thörle-Pospiech, P., Trautmann, N.,Yakushev, A., 2014. Preparation of actinide targets for the synthesis of theheaviest elements. J. Radioanal. Nucl. Chem. 299, 1081–1084.

Salvatores, M., Aliberti, G., Dunn, M., Hogenbirk, A., Ignatyuk, A., Ishikawa, M.,Kodeli, I., Koning, A.J., McKnight, R., Mills, R.W., Oblozinsky, P., Palmiotti, G.,Plompen, A., Rimpault, G., Rugama, Y., Talou, P., Yang, W.S., 2008. NuclearEnergy Agency status report 6410.

Sibbens, G., Moens, A., Eykens, R., Vanleeuw, D., Kehoe, F., Kühn, H., Wynants, R.,Heyse, J., Plompen, A., Jakopič, R., Richter, S., Aregbe, Y., 2014. Preparation of240Pu and 242Pu targets to improve cross-section measurements for advancedreactors and fuel cycles. J. Radioanal. Nucl. Chem. 299, 1093–1098.

Trautmann, N., Folger, H., 1989. Preparation of actinide targets by electrodeposition.Nucl. Instrum. Methods A 282, 102–106.

Vascon, A., Düllmann, Ch.E., Eberhardt, K., Kindler, B., Lommel, B., Runke, J., 2011.Toward large-area targets for “TRAKULA”. Nucl. Instrum. Methods A 655,72–79.

Vascon, A., Santi, S., Isse, A.A., Reich, T., Drebert, J., Christ, H., Düllmann, Ch.E.,Eberhardt, K., 2012. Elucidation of constant current density molecular plating.Nucl. Instrum. Methods A 696, 180–191.

Vascon, A., Santi, S., Isse, A.A., Kühnle, A., Reich, T., Drebert, J., Eberhardt, K.,Düllmann, Ch.E., 2013a. Smooth crack-free targets for nuclear applicationsproduced by molecular plating. Nucl. Instrum. Methods A 714, 163–175.

Vascon, A., Wiehl, N., Reich, T., Drebert, J., Eberhardt, K., Düllmann, Ch.E., 2013b. Theperformance of thin layers produced by molecular plating as α-particle sources.Nucl. Instrum. Methods A 721, 35–44.

Vascon, A., Santi, S., Isse, A.A., Reich, T., Drebert, J., Christ, H., Eberhardt, K.,Düllmann, Ch.E., 2014. Fundamental aspects of molecular plating and produc-tion of smooth crack-free Nd targets. J. Radioanal. Nucl. Chem. 299, 1085–1091.

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