Applied Radiation and Isotopes 87 (2014) 238–241

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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso

Preparation of thick uranium layers on aluminium and stainless steel backings L. Benedik a,n, G. Sibbens b, A. Moens b, R. Eykens b, M. Nečemer a, S.D. Škapin a, P. Kump a a b

Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium

H I G H L I G H T S

 Target preparation procedures included electrodeposition and molecular plating.  Organic and inorganic solutions used as electrolytes.  Characterisation of layers for activity, thickness, homogeneity and morphology.

art ic l e i nf o

a b s t r a c t

Available online 14 December 2013

The methods of electrodeposition and “molecular plating” were studied for the production of uranium targets with an areal density up to 0.6 mg cm  2 on aluminium and up to 1.5 mg cm  2 on stainless steel backings from aqueous and organic electrolytes. For characterisation of the deposited material, gammaray spectrometry, alpha-particle spectrometry, X-ray fluorescence, X-ray powder diffraction, scanning electron microscopy and autoradiography were applied. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Uranium Target preparation Characterisation of target Electrodeposition Molecular plating

1. Introduction Uniform layers of actinides are required in experimental nuclear physics to measure accurate nuclear data. The production of these layers on different metallic backings is very difficult due to the fact that their stability depends on the chemical composition, as well as on the thickness of the deposit, the mode of preparation and the surface of the metallic backing. If the deposited material after drying does not adhere to the backing material, the layer can contain cracks and damage which is problematic for experiments in nuclear laboratories, research reactors and accelerators. Numerous methods for radioactive source preparation have been published (Sibbens and Altzitzoglou, 2007): electrolytic deposition, spontaneous deposition, micro-precipitation, direct evaporation, vacuum sublimation, drying of liquid drops directly deposited on the substrate, electrospray, electrostatic deposition etc. The techniques available for preparation of layers of actinides on metallic backings involving electroplating from organic and aqueous media and the characterisation of such targets have been described in numerous papers (Mirashi et al., 1986; Ingelbrecht et al., 1997; Lobanov et al., 1997; Eberhardt et al., 2004; Liebe et al.,

n

Corresponding author. Tel.: þ 386 1 588 5347; fax: þ 386 1 588 5346. E-mail address: [email protected] (L. Benedik).

0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.052

2008; Jobbágy et al., 2013; Sadi et al., 2011; Vascon et al., 2012, 2013; He et al., 2013). The objective of this work was to prepare uranium layers on aluminium and stainless steel backings with an areal density up to 1.5 mg cm  2 and to characterise the deposited material by gammaray spectrometry, alpha-particle spectrometry, X-ray fluorescence (XRF), X-ray powder diffraction (XRDF), scanning electron microscopy and autoradiography.

2. Experimental 2.1. Target preparation Targets of uranium were prepared by electrodeposition in the Department of Environmental Sciences at the Jožef Stefan Institute (JSI) in Ljubljana, Slovenia and in the Target Preparation Laboratory at the Institute for Reference Materials and Measurements of the Joint Research Centre (JRC-IRMM) in Geel, Belgium. Two different types of backing material, mirror polished stainless steel (SS) and commercially available aluminium (Al), both with a diameter of 25 mm and a thickness of 0.4 mm, were used. As anode a flat spiral platinum wire was used at the JSI (Klemenčič and Benedik, 2010) and a vertical rotating rectangular platinum grid at the JRC-IRMM (Ingelbrecht et al., 1997). The active area of

L. Benedik et al. / Applied Radiation and Isotopes 87 (2014) 238–241

the deposited material was about 3.4 cm2. As material stock solutions of uranium with natural isotopic abundances were prepared at the JRC-IRMM and at the JSI in the form of UO2(NO3)2. Targets were prepared by electrodeposition in isopropanol (IP), isobutanol and ammonium oxalate (Ox) (Ingelbrecht et al., 1997; Puphal and Olsen, 1972; Mirashi et al., 1986). Approximately 10 cm3 of the solution was transferred to the cell and an aliquot of the uranium solution was added. The cathode–anode distance was set between 8 mm and 10 mm. When the targets were prepared by electrodeposition using an aqueous electrolyte, the targets were rinsed with water and ethanol containing a few drops of an ammonia solution. The discs with deposited material were dried in air and then heated at 100 1C to fix the uranium.

2.2. Target characterisation The uranium sources were characterised for their activity, mass and areal density. A first approximate assay of the deposited uranium was done by gamma-ray spectrometry. Therefore the γ line of 235U at 185.72 keV was measured. At the JRC-IRMM the activity was determined by low solid angle α-particle counting (Heyse et al., in press). The solid angle subtended by the 20 mm diameter diaphragm in front of the silicon surface barrier detector was 1.4% of 4π sr. Table 1 U activity in Bq, mass in mg and areal density in mg cm  2 of four typical uranium targets. The uncertainties, unc, are combined standard uncertainties. Target no.

Backing Electrolytea Activity (Bq)

17 22 23 27

SS Al SS Al

a

Ox Ox IP IP

133.1 38.0 62.3 51.4

unc (Bq)

Mass U unc (mg) (mg)

Areal density (mg cm  2)

1.4 0.4 0.7 0.6

5453 1558 2554 2105

1574 450 737 608

Ox ¼ 5.7% (NH4)2C2O4 in 0.3 M HCl; IP¼ (CH3)2CHOH.

57 18 28 24

239

The distribution and homogeneity of the deposited uranium layer were checked by phosphor imaging autoradiography using Fuji Photo Film BAS-IP TR 8 and BAS-IP MS 2025 imaging plates at the JSI, while at the JRC-IRMM the CycloneTM Storage Phosphor Imaging System (Packard) was employed (Sibbens et al., 2003). The uranium samples were analysed non-destructively for their elemental composition by energy dispersive X-ray fluorescence spectrometry (EDXRF). A 109Cd disc radioisotope excitation source was used to irradiate the samples. Analysis of the complex X-ray spectra was performed by the AXIL spectral analysis program (Van Espen and Janssens, 1993). Quantification of the metal content after the necessary calibration of the XRF system was then performed by the QAES (Quantitative Analysis of Environmental Samples) software developed by P. Kump (Nečemer et al., 2011). Surface characterizations of the deposited material to visualise the layer structure were performed with a field emission scanning electron microscope (FE-SEM, Zeiss ULTRA plus).

3. Results and discussion At the JRC-IRMM the experimental work was focused on the preparation of the targets containing thick layers of deposited uranium from aqueous and organic electrolytes onto SS and Al backings and the characterisation by low solid angle alpha-particle counting. The measured activity and calculated mass and areal density are presented in Table 1 for four typical targets out of thirty prepared. Fig. 1 shows pictures of the prepared targets and the corresponding alpha-particle spectra obtained by low solid angle alphaparticle counting. Samples no. 17 and no. 22 were prepared from 5.7% ammonium oxalate electrolyte on SS and on Al, respectively, while samples no. 27 and no. 23 were prepared from isopropanol on SS and on Al, respectively. The spectrum of sample no. 22 demonstrates its low resolution and long tailing in the low-energy region because of self-absorption. And from Fig. 1, it is evident that the aluminium disc (no. 22) during electrodeposition of uranium

Fig. 1. Pictures of uranium layers on SS and Al backings as listed in Table 1 prepared by electrodeposition and molecular plating using various electrolytes and corresponding alpha-particle spectra obtained by low-solid angle counting.

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L. Benedik et al. / Applied Radiation and Isotopes 87 (2014) 238–241

Fig. 2. SEM pictures taken at 5k  (a) and 100k  (b) magnifications of 500 mg cm  2 uranium targets prepared from ammonium oxalate on a SS (1) and Al (2) backing and from isopropanol on a SS (3) and Al (4) backing.

L. Benedik et al. / Applied Radiation and Isotopes 87 (2014) 238–241

from 5.7% ammonium oxalate in 0.3 M HCl was partially dissolved, and then the dissolved aluminium was re-deposited on the backing. The autoradiographs of the targets showed that the uranium material was distributed over the whole area and not concentrated on one side or in the centre or border of the backing. The thickness of the deposited material was checked by energy dispersive XRF (Nečemer et al., 2011; Van Espen and Janssens, 1993). It was observed that all electrolytes gave approximately the same thickness of deposited uranium on the same backing material except when electrodeposition from 0.2 M oxalate on an Al backing was performed. The X-ray fluorescence spectrum of U electroplated on a SS disc from 5.7% ammonium oxalate in 0.3 M HCL showed that during the electrodeposition the Pt anode was also partly dissolved and slightly electroplated on the surface of the deposit. The deposited material was also examined by scanning electron microscopy; this technique allowed insight into the depth of the material and thus a good view of the prepared layers. Fig. 2 shows the deposited material obtained from aqueous and non-aqueous electrolytes on SS and Al backings pictured at low (5k  ) and high (100k  ) magnifications. The areal densities of uranium on the investigated targets were approximately 500 mg cm  2. All targets showed visible cracks. 4. Conclusion Two different backing materials, stainless steel and aluminium, were used for the investigation of the preparation of uranium layers with an areal density up to 1.5 mg cm  2 by electrodeposition and molecular plating from an ammonium oxalate solution and isopropanol, respectively. The targets were characterised for their activity, thickness, homogeneity and morphology. Uranium targets could be prepared with an areal density up to 0.6 mg cm  2 on aluminium and up to 1.5 mg cm  2 on stainless steel backings. The uranium material was distributed over the whole area. Detailed analysis of the surface by scanning electron microscopy showed damage in the form of cracks over the whole deposited layer. The best layers on a stainless steel backing were produced from an ammonium oxalate solution and on an aluminium backing from isopropanol. Acknowledgement This work was supported by the Ministry of Education, Science and Sport of the Republic of Slovenia within the Research Programme P1-0143.

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Dr. Benedik acknowledges support received for completing this work in the framework of the ERINDA Project (European Research Infrastructures for Nuclear Data Applications), PAC SV2/3.

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Preparation of thick uranium layers on aluminium and stainless steel backings.

The methods of electrodeposition and "molecular plating" were studied for the production of uranium targets with an areal density up to 0.6 mg cm(-2) ...
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