ADC10 Secondary ion mass spectrometry and alpha-spectrometry

J Radioanal Nucl Chem (2012) 292:973–981 DOI 10.1007/s10967-012-1670-9 Secondary ion mass spectrometry and alpha-spectrometry of electrodeposited thorium films Jozef Kuruc • Jana Strišovská • Dušan Galanda • Silvia Dulanská • Ľubomı́r Mátel • Monika Jerigová Dušan Velič • Received: 6 October 2011 / Published online: 11 February 2012 Ó Akadémiai Kiadó, Budapest, Hungary 2012 Abstract The main aim of this work was the preparation of samples with thorium content on the steel discs by electrodeposition for determination of natural thorium isotope by alpha spectrometry and secondary ion mass spectrometry and finding out their possible linear correlation between these methods. The analysis of the composition of surface was other aim of study. Discs were measured by alpha spectrometer. After that, alpha spectrometry discs were analyzed by TOF-SIMS IV, which is installed in the International Laser Centre in Bratislava. The integral and normalized intensities of isotope of 232Th and intensities of ions of ThO?, ThOH?, ThO2H?, Th2O4H?, ThO2-, ThO3H-, ThH3O3- a ThN2O5H- were measured. The linear correlation is between surface’s weights of Th and intensities of ions of Th? from identified in SIMS spectra. We found out the chemical binding between thorium and oxygen and hydrogen on the surface of samples by SIMS method. Obtained intensities of ions 232 ThO?, 232ThOH?, 232ThO2H? prove the presence of J. Kuruc (&)  J. Strišovská  D. Galanda  S. Dulanská  Ľ. Mátel  M. Jerigová  D. Velič Department of Nuclear Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynska dolina CH-1, 842 15 Bratislava, Slovak Republic e-mail: kuruc@fns.uniba.sk M. Jerigová  D. Velič Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynska dolina CH-1, 842 15 Bratislava, Slovak Republic M. Jerigová  D. Velič Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, International Laser Center, Ilkovičova 3, 812 19 Bratislava, Slovak Republic oxidized forms of thorium in the upper layers of surface. The oxidized ions predominate in univalent form of thorium up to deep about 3,000 nm. Keywords Thorium 232  Thorium compounds  Alpha spectroscopy  Ion microprobe analysis  Mass spectroscopy  Qualitative chemical analysis  Correlation functions  Thin films Introduction Thorium is a primordial actinide element, which naturally occurs as a common trace element in igneous rocks. It has several different isotopes, both natural and man-made, all of which are radioactive. The most common isotope of thorium is thorium-232, found naturally [1]. Thorium and its compounds have found a number of industrial and technical applications due to its chemical, physical and nuclear properties. Thorium is used for producing of thorium compounds in industry, for thorium-fuelled reactors [2, 3] in nuclear industry and isotope geologists who use 230Th/234U, in particular, for age dating [4–6]. The standard radiometric method is alpha spectrometry [7], which determines the isotopic concentration in samples. Thorium-isotopes usually have been electrodeposited on stainless steel discs [8, 9] and measured using semiconductor detectors for charged particles. Spectral interference is the problem by determination of thorium because of the overlap of alpha peaks of similar energy if they are present in the sample. Uranium always occurs together with thorium in samples of natural origin and the 234U peak (4.77 MeV) would interfere with the estimation of 230Th and 229Th if present together on the final counting source [10]. 123 974 Since the naturally occurring isotope 232Th is a longlived alpha emitter, analytical techniques need to be available for the accurate assay at concentration levels ranging over many orders of magnitude. The development of surface analytical techniques such as SIMS [11, 12], GDMS [13] and LA-ICP-MS [14, 15] focuses on improvements to sensitivity and detection limits in order to obtain precise and accurate analytical data. SIMS is the most important mass spectrometric surface analytical technique and can be applied for analysis of long-lived radionuclides. In SIMS the solid sample surface is sputtered by bombardment with a focused primary (keV) ion beam (clustered ions of Aun?, Bi3?, SF5?, C60?) and the sputtered ions are analysed by mass spectrometry. Fig. 1 SIMS spectrum of sample No. 1 with electrodeposited thorium in the positive polarity 123 J. Kuruc et al. Technique is able to perform a micro-local analysis in the sub-micrometer range (e.g. for analysis of local inclusion or impurities) and can be used for the determination of lateral element distribution and isotope analysis, e.g. for the characterization of small particles, aerosols, and liquid or solid inclusions. SIMS can be applied for the characterization of bulk material with detection limits down to the low nanogram per gram range [16]. In this study the natural thorium isotope was determined and we found out the characteristic of the surface of thorium samples. Thorium was determined by alpha spectrometry and secondary ion mass spectrometry. The results from both methods were used for finding of the correlation between them. Secondary ion mass spectrometry and alpha-spectrometry of electrodeposited thorium films Methods and experiment Preparation of samples with thorium The samples with the thorium content on the steel discs were prepared by electrodeposition [17, 18] according to the procedure developed by Strišovská et al. The procedure of preparation and results are published in the article Surface’s weights of electrodeposited thorium samples determined by alpha spectrometry [9]. 975 made OrtecÒ, USA). The counting efficiency was about 20% and time of measuring was 60,000 s. Discs were measured by TOF-SIMS IV instrument (IONTOF, Muenster, Germany). In the measurements, the samples were bombarded with a primary Bi? beam of 25 keV with a current intensity 1 pA and static limit was used 5.1012 ions cm-2. The samples were measured in positive and negative polarity. The area of 150 9 150 lm2 on the sample surface was scanned. Results and discussion Measurements Stainless steel discs were counted by low-level alpha spectrometer (OrtecÒ Dual Alpha Spectrometer 576 A–919 by The discs were analysed by the instrument TOF-SIMS IV. The samples were measured in the positive and negative polarity. The measuring of samples in the positive polarity Fig. 2 SIMS spectrum of sample No. 1 with electrodeposited thorium in the negative polarity 123 976 J. Kuruc et al. Table 1 Values of integral intensities of 232 Thþ ;232 ThHþ ;232 ThOþ ;232 ThOHþ ;232 ThO2 Hþ and their proportion in the samples of electroplated thorium measured out in the positive polarity No. of disc mp ½lgcm2  Th 7 9 1.69 1.86 10 2 ? ThH ? 8,294 8,888 10,981 9,580 2.12 8,663 2.63 10,069 6 2.70 8 2.80 3 4.23 a I Pa Average integral intensity (counts) ? ThO ThOH ? ThO2H Proportion ? Th? ThH? ThO? ThOH? ThO2H? 58,428 44,499 51,366 46,626 138,524 105,779 267,593 215,372 0.03 0.04 0.04 0.04 0.22 0.21 0.19 0.22 0.52 0.49 10,476 94,363 65,743 201,938 381,183 0.02 0.03 0.25 0.17 0.53 13,878 115,763 78,976 184,316 403,002 0.02 0.03 0.29 0.20 0.46 7,143 9,580 88,412 59,845 224,691 389,671 0.02 0.02 0.23 0.15 0.58 7,511 8,538 5,749 66,716 237,010 325,524 0.02 0.03 0.02 0.20 0.73 6,579 8,350 84,654 52,602 144,357 296,542 0.02 0.03 0.29 0.18 0.49 Sum of intensities Table 2 Values of normalized intensities of 232 Thþ ;232 ThHþ ;232 ThOþ ;232 ThOHþ ;232 ThO2 Hþ and their proportion in the samples of electroplated thorium measured out in the positive polarity No. of disc mp ½lgcm2  IPa Average normalized intensity (counts) ? Th ThH ? ThO ? ThOH ? ThO2H Proportion ? ThH? ThO? ThOH? ThO2H? 0.03 0.04 0.04 0.04 0.22 0.21 0.19 0.22 0.52 0.49 7 9 1.69 1.86 15.93 15.77 21.06 17.00 112.30 78.95 98.74 82.73 266.30 187.74 10 2.12 22.30 26.96 242.98 169.26 519.93 981.44 0.02 0.03 0.25 0.17 0.53 2 2.63 35.70 49.20 411.51 280.33 655.12 1431.88 0.03 0.03 0.29 0.20 0.46 6 2.70 19.81 26.59 245.15 165.97 622.93 1080.46 0.02 0.03 0.23 0.15 0.58 8 2.80 18.62 21.17 259.59 165.40 587.59 1052.37 0.02 0.02 0.25 0.16 0.56 3 4.23 29.95 38.02 385.75 239.74 658.10 1351.56 0.02 0.03 0.28 0.18 0.49 a 514.34 382.20 Th? Sum of intensities Table 3 Values of integral intensities of sured out in the negative polarity No. of disc ms ½lgcm2  232 232 ThO ThO3 H a232 ThH3 O 2; 3 and their proportion in the samples of electroplated thorium mea- IPa Average integral intensity (counts) ThO2- ThO3H - ThH3O3- Proportion ThO2- ThO3H- ThH3O3- 7 1.69 6,331 13,498 9,892 29,721 0.21 0.45 0.33 9 1.86 2,488 6,073 5,338 13,899 0.18 0.44 0.38 10 2.12 10,277 23,948 30,531 64,756 0.16 0.37 0.47 2 2.63 19,060 28,113 36,262 83,435 0.23 0.34 0.43 6 2.70 14,143 32,901 29,147 76,191 0.19 0.43 0.38 8 2.80 11,692 31,886 36,329 79,907 0.15 0.40 0.45 3 4.23 26,729 38,188 47,091 112,008 0.24 0.34 0.42 a Sum of intensities means to a negative extraction voltage sets. The positive ions are getting through the extractor. In the negative polarity a positive extraction voltage sets, the negative ions are getting through the extractor. The mass spectrum is the result of measuring. Both the positive and negative SIMS spectra are necessary for the identification of ions. The material sputtered from the sample surface consists not 123 only of mono-atomic ions but molecular species, which can interfere with analysis of other elements in mass spectrum. The examples of SIMS spectra of analysed samples are shown on the next figures (Figs. 1, 2). The significant ions important for the analysis were chosen from spectra. The spectrum of positive ions can be seen in the Fig. 1. The main signals refer to this ions: 232Th?, 232ThO?, 232ThOH?, Secondary ion mass spectrometry and alpha-spectrometry of electrodeposited thorium films Table 4 Values of normalized intensities of measured out in the negative polarity No. of disc 232 232 ThO ThO3 H a232 ThH3 O 2; 3 and their proportion in the samples of electroplated thorium I Pa ms ½lgcm2  ThO2 977 - - ThO3H ThH3O3- Proportion ThO2- ThO3H- ThH3O3- 7 1.69 16.86 31.94 26.35 75.15 0.22 0.42 0.35 9 1.86 14.81 36.99 32.65 84.46 0.17 0.44 0.39 10 2.12 21.55 50.44 64.30 136.29 0.16 0.37 0.47 2 2.63 55.63 82.02 105.53 243.19 0.23 0.34 0.43 6 8 2.70 2.80 29.79 26.01 69.39 70.91 61.39 80.81 160.58 177.73 0.19 0.15 0.43 0.40 0.38 0.45 3 4.23 101.36 144.52 178.29 424.18 0.24 0.34 0.42 a Sum of intensities Fig. 3 The dependency of intensity of integral ions of surface’s weight mp of electrodeposited thorium 232 ThO2- on Fig. 4 The dependency of intensity of integral ions of 232ThO3H- on surface’s weight mp of electrodeposited thorium Fig. 5 The dependency of intensity of integral ions of on surface’s weight mp of electrodeposited thorium 232 232 ThH3O3- ThO2H?, 232Th2O4H?. It can be seen in the Fig. 2, the spectrum acquired by measuring in the negative polarity is characterized by the presence of subsequent ions, for example: Fig. 6 The dependency of intensity of normalized ions of on surface’s weight mp of electrodeposited thorium 232 ThO2- Fig. 7 The dependency of intensity of normalized ions of 232ThO3Hon surface’s weight mp of electrodeposited thorium H-, CH-, O-, OH-, Cl-, NO2-, NO3-, SO3-, SO4-, 232 ThO2-, 232ThO2H-, 232ThO3-, 232ThO3H-, 232ThH3O3and 232ThN2O5H-, which are not found in the positive mass spectrum. The spectra provided the analysis by method of SIMS in which intensities were assigned to real ions and fragments. The values of integral and normalized intensities (calculated by software IonSpecÒ), which were registered in the positive and negative polarity, are presented in the Tables 1, 2, 3 and 4. The intensities of 232ThO?, 232 ThOH? and 232ThO2H? have higher values than the intensities of 232Th?. We were interested in those ions with thorium which values of intensity was higher than 6,000 counts for integral intensities and 15 counts for normalized intensities. 123 978 Fig. 8 The dependency of intensity of normalized ions of 232 ThH3O3- on surface’s weight mp of electrodeposited thorium We performed the dependence of intensities of ions 232Th acquired by method of SIMS from surface’s weight mp. Values of surface’s weights of isotope 232Th mp are presented in paper (surface’s weights of electrodeposited thorium samples determined by alpha spectrometry) [9]. The analysis by SIMS method was performed in the positive and the negative polarity therefore we plotted the dependences for every mode of polarity. We plotted also the dependences of normalized intensities on surface’s weight mp. The thorium occurred not only as a metal but it was bounded with oxygen and hydrogen J. Kuruc et al. and it created subsequent ions: 232Th?, 232ThO?, 232ThOH?, 232 ThO2H?; 232ThO2-, 232ThO3-, 232ThO2H-, 232ThO3Hand 232ThH3O3-. In the Figs. 3, 4, 5, 6, 7 and 8 are illustrated dependency of intensity of some integral and normalized ions on surface’s weight mp of electrodeposited thorium. The Figs. 3, 4, 5, 6, 7 and 8 show that better correlations provide normalized intensity then integral intensity. Because of that we use a normalized intensity that is defined here as the count of the secondary ions of each element divided by the total count of ions recorded. The best regression between intensity of thorium oxide ions and thorium hydroxide ions on surface’s weight of thorium was obtained for normalized ions of 232ThO3H(r2 = 0.975) and 232ThH3O3- (r2 = 0.909). The FisherSnedecor F-test of significance of the regression model was used [19]. It is based on the testing criterion to confirm the significance of the proposed models for the significance level of a = 0.05. If we get an accidental variable that has F-distribution with m-1 degrees of freedom, we compare it with a table value. If it is true that the F [ F (1-a, m-1), the proposed model is accepted as significant. However, if F \ F (1-a, m-1) stands true, proposed Fig. 9 2D distribution of elements and compounds on the electroplated disc No. 10 (originally scan were obtained in colour) 123 Secondary ion mass spectrometry and alpha-spectrometry of electrodeposited thorium films model isn’t accepted as significant. The proposed models are received for almost all the dependences of integral and normalized intensities of thorium ions from a surface’s weights mp except for the dependence of integral intensities of ions 232ThO2H?. The two-dimensional distribution of elements and molecules obtains the screening of the bundle of primary ion. This two-dimensional distribution provides the information not only about chemical composition of surface, where the organic and inorganic areas are rotated, but also about the concrete location of selected chemical particles. The more intensive is the light colour in the pictures, the higher was the emission of ions of that type from that place, and vice versa. Black colour means low or none emission, thus their local representation [20]. The 2D-distribution of elements and molecules was studied for the disc 10. The area on the disc sized 9 9 9 mm was analysed in the positive polarity. The image of covering is the result of the analysis in the Fig. 9. In the concrete pictures the distribution of elements of 23Na, 28Si, 39K, 40Ca, 56Fe, 63Cu, 64Zn, 208Pb, 232Th and the distribution of organic molecules are represented. Thorium was localized sufficiently homogeneously. The aim of depth profiling is to obtain information on the variation of composition with depth below the initial surface—such information is obviously particularly useful for the analysis of layered structures. Since the SIMS technique itself has relied upon the removal of atoms from Fig. 10 The deep profile of electrodeposited of thorium in the sample No. 11 979 Fig. 11 The deep profile of electrodeposited of thorium in the sample No. 12 Fig. 12 The deep profile of electrodeposited of thorium in the sample No. 13 123 980 the surface, it is due to its very nature a destructive technique, but also, ideally suited for depth profiling applications. Thus a depth profile of a sample may be obtained simply by recording sequential SIMS spectra as the surface is gradually eroded away by the incident ion beam probe. A plot of the intensity of a given mass signal as a function of time is a direct reflection of the variation of its abundance/ concentration with depth below the surface. One of the main advantages that SIMS offers over other depth profiling techniques is its sensitivity to very low concentrations (*ppm) of elements [21]. The samples No. 11, 12 and 13 were measured in the positive polarity by oxygen with energy 2 kV, ions of Bi were used as the primary ion and the current was 1 pA. The dependencies of intensity form depth are the result of measuring of depth profiling (Figs. 10, 11 and 12). The individual elements and molecules give us information about sample composition. Thorium and metals, that should characterize plate, are being found out. In the case of thorium it always gives out at break when its concentration is constant. This could be a place where the sample ended and the plate begun. The depth ablation is approximate: sample No. 11*3,150 nm, sample No. 12*3,000 nm and sample No. 13*3,800 nm. If we assume that we used the same sputtering speed and the same matrix, we can say that the layer was the most rough at sample No. 13. It is caused by the biggest concentration that is present in the sample. Conclusion The alpha spectrometry was used for the measuring of activity in the prepared thorium sample. The weights and surface’s weights were calculated from the activity of 232 Th. We used these values for finding the linear correlation between SIMS method and alpha spectrometry. We assumed that thorium excluded in metal form by electrodeposition but SIMS analysis revealed that it is chemically bound in another forms—ions. This fact decreased the electrodeposition efficiency. Obtained intensities of ions 232 ThO?, 232ThOH?, 232ThO2H? prove the presence of oxidized forms of thorium in the upper layers of surface. The oxidized ions predominate in univalent form of thorium up to deep about 3,000 nm. The sensitivity of method SIMS is in area 10-9 (ppb) so the different interferences could be found by sample analysis. The different thickness of layer of electroplated thorium and unstability current during electrodeposition belong to the matrix effects. We used the surface’s weights acquired from alpha spectrometry and the intensities of ions from SIMS method for acquisition of the linear correlations. The deviation of linearity was caused thereby that thorium was bound in another chemical forms. We 123 J. Kuruc et al. made the Fisher-Snedecor test of significance of regression to confirm that the suggested dependences are received as significant. For all dependences the suggested models are accepted as significant except for one model (for the dependency of intensities of ions 232ThO2H?). Acknowledgments Supports by Slovak Research and Development Agency under APVV-20-007105, APVT-20-029804 and by Scientific Grant Agency of Ministry of Education of the Slovak Republic under VEGA project 1/2447/05 and 1/3577/06 are gratefully acknowledged. References 1. 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