Application of High Resolution X-Ray Spectrometry to Activation Ana lysis Marianna Mantel and Saadia Amiel Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yaune, Israel High resolution X-ray spectrometry was applied to neutron activation analysis, The feasibility of applying this method to all naturally occurring elements has been investigated. Thirty elements were studied experimentally (27 5 z 5 92). The detection sensitivity for these elements ranged from lo5 to le2dps/pg, using 5-minute neutron irradiations at a flux of 1018 neutrons/cm2 sec and employing a 1% or 4% counting geometry. Resolutions of 320 eV obtainable with the Si(Li) detectors make possible the simultaneous determination of neighboring elements in most cases. Several practical examples are given to demonstrate some of the advantages of X-ray over gamma-ray spectrometry in activation analysis.
RADIOACTIVATION of many elements results in the production of nuclides whose decay is accompanied by emission of characteristic X-rays. These X-rays result from orbital (K or L) electron capture, or from internal conversion of isomeric transitions or of gamma rays accompanying beta emission. By measuring the energy of such characteristic X-rays, the parent element may be identified by using the simple relationship between X-ray energy and atomic number given by Moseley’s law :
-
EK or (2- const)2 The use of X-ray spectrometry for unambiguous identification of elements is further facilitated by the small number of lines in the characteristic X-ray spectrum of each element. It is thus possible, if high-resolution spectrometers such as solidstate Si(Li) diodes are employed, to detect simultaneously any number of elements in the same sample, since overlapping of characteristic X-ray peaks is minimal. Furthermore, because of the low sensitivity of the small Si(Li) spectrometers to electromagnetic radiation above 60 keV, no interference by high-energy gamma rays is expected. However, two possible sources of interference exist: one is induced fluorescent X-rays from the detector and sample environment; the other is Bremsstrahlung of the beta particles emitted from the radioactive sample. This may raise the background and substantially increase the dead time of the detector, upsetting the resolution. This problem may be partly overcome by using low detection geometries or magnetic deflection of the beta particles-both procedures operate at the expense of good geometry (and hence of sensitivity), because of the need for collimation. Since the X-rays emitted by lighter elements are too soft to be detected without special care to minimize absorption, X-ray spectrometry of radioactive samples is applicable in practice mainly to elements heavier than sodium or magnesium. Furthermore, due to their low range, the X-rays emitted are subject to self-attenuation in the sample, which may lead to erroneous quantitative results if the volumes and matrices of samples and standards are not sufficiently well matched. One of the main features of the present method is the unambiguous correlation between the elements and the energy of their characteristic K or L X-rays. In contrast, the commonly used gamma-ray spectrometry requires assignment 548
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
of gamma rays by determination of half-lives and very precise energy measurements, and reference to detailed gamma-ray catalogues ; the assignments may sometimes be ambiguous in cases where a multitude of gamma rays are emitted form a complex matrix. Shenberg et al. ( I ) of our laboratory were the first to demonstrate the use of X-ray spectrometry in activation analysis. They used a low-resolution proportional counter, which was sufficient for the determination of traces of bromine in the presence of a large excess of sodium. Recently, Pillay (2) extended this work and surveyed the use of a proportional and scintillation counter as X-ray spectrometers in activation analysis with (n,r) reactions. In the present work we investigated the use of high-resolution Si(Li) X-ray spectrometers in activation analysis, and attempted to assess the potentialities and limitations of the method for general use as a complementary technique to activation analysis with gamma-ray spectrometry. EXPERIMENTAL
Materials. High-purity (Fluka or Johnson Matthey) compounds of the elements to be tested. High purity paraffin (BDH)-congealing point, 120 O F . Preparation of Samples. Dilute solutions (a few pg/ml) of the different elements to be studied were prepared by dissolving an appropriate compound of the element in water, nitric acid, or any other solvent which would not interfere with the irradiations. Aliquots of these solutions were introduced with a micropipet into small polyethylene cups (i.d. = 12 mm), weighed and evaporated to dryness under an infrared lamp. In this way, very thin and uniformly distributed samples were obtained, because of the small diameter of the cups. For elements whose compounds are insoluble in any suitable solvent, the smallest possible quantity (a few milligrams) of metal powder or oxide was weighed into the polyethylene cup. To prevent dispersion of the powder on the walls of the vessel during irradiation, a few drops of hot paraffin were added. Upon cooling, the paraffin encloses the metal powder in a solid block. Because of its higher density, the metal powder remains on the bottom of the vessel and possible losses by attenuation of X-rays, due to sample thickness, are avoided. The polyethylene containers were sealed with suitable polyethylene stoppers, and irradiated as described below. Irradiation. The irradiations were carried out in the pneumatic tube of the IRR-1 reactor. Counting. Two Si(Li)detectors were used: one of 25 mm2area and 3 mm depth (from Nuclear Equipment Corporation, San Carlos, Calif.), and another of 100mm2 area and 4 mm depth (manufactured by Seforad, Israel). The output signals from the detectors were passed through a preamplifier (N.E.C.-N.C.70 and Ortec 118A, respectively), and a linear amplifier (N.E.C. and Ortec 410). The resulting pulses were (1) C . Shenberg. 39, -. J. Gilat, and H. L. Finston, ANAL.CHEM., 780 (1967). (2) K. K. S. Pillay and N. W. Miller, J. Radioanal. Chern., 2, 97 .
I
(1969).
analyzed by a T.M.C. 400-channel analyzer. The resolutions of the systems for the 6.4 keV Fe K X-rays (obtained from V o ) and 31.7 keV Ba K X-rays (obtained from I W s ) were 315 and 750 eV (FWHM), respectively, for the N.E.C. detector; and 450 and 550 eV (FWHM) for the Seforad detector, Accordingly, the former was used for the measurement of X-rays with energies up t o 10 keV, and the latter for those with higher energies. For comparison with gamma spectrometry, a 29 cm3, drifted to 15 mm, Ge(Li) detector (manufactured by Elscint, Israel) was used. The detector was connected to an Elron CA-N-1 preamplifier, an Ortec 410 amplifier, Ortec 408 biased amplifier, and Ortec 411 pulse stretcher. The resolutbn of this system was 2.6 keV (FWHM) for the 57C0 122 keV gamma ray, and 3.5 keV (FWHM) for the W o 1,330 MeV gamma ray.
I 1
Cd Ka 23.1 keV
I
ttt++tts Pd
RESULTS AND DISCUSSION
Ka
The X-rays accompanying the decay of radioactive isotopes may be classified into three groups: those due to internal conversion, which yield X-rays of the parent element (2); those due to orbital electron capture, which yield X-rays of the daughter element (2-1); and those due to beta decay followed by internal conversion, which yield X-rays of the daughter element (2 1). In all three cases, the half-lives of the X-rays are those of the parent radionuclide (Z). An element may give a mixture of X-rays because of various modes of decay of one or more of its radioactive isotopes. For example, neutron capture in cadmium (2= 48) results in X-rays from Cd (Z), Ag (2 - l), and In (2 I), with energies of 23.2, 22.1, and 24.2 keV, respectively (each peak decays with a different half-life). Because of the high resolving power of the Si(Li) detector used (450 eV in this energy range), all these X-rays could be well resolved (Figure 1). In the present work, the yield of X-rays after neutron activation has been determined experimentally and expressed as sensitivity of detection for the element concerned. In addition to the common parameters which influence the sensitivity of neutron activation (natural abundance of the stable nuclide, thermal-neutron cross section, half-life of the radioactive isotope obtained, intensity of the neutron flux, etc.), some other factors specific to X-rays will influence the overall sensitivity of the method. These factors are the atomic number 2, the internal-conversion coefficient, the energy of the gamma ray undergoing conversion, the per cent of electron capture as compared with positron emission, the fluorescence yield of the element in question, and finally, especially for heavy elements, the preference for emitting K or L X-rays, which is determined by the electron-shell structure of the element. Of the elements of the periodic table, 59 can, in principle, be determined by X-ray spectrometry following neutron activation, Le., these elements produce, after neutron activation, radioactive isotopes which decay with X-ray emission through one of the above-mentioned processes.(3) Not all of these nuclides could be investigated under the working conditions of the present study, because of either the too low intensity of the X-rays emitted or the too short half-life of the parent radioisotope (less than 30 sec). The study also excluded the noble gases, Ar, Kr, and Xe, whicharepotentially assayable by this technique but require special handling suitable for gaseous samples. Thirty of the remaining elements were studied by irradiating and counting as described in the previous section.
+
21.17 krV 2.42~1
Tile
P 0.4 c
0.3 0
+
(3) C . M. Lederer, M. Y. Hollander, and I. Perlman, “Table of
Isotopes,” 6th ed., John Wiley and Sons, New York, 1967.
0.4 -
0.5
-
--
0.3
0.2 0.1
-
Figure 1. X-ray spectra of the neighboring elements, Cd, Ag, and Pd The results are summarized in Table I. Elements are listed according to increasing atomic number. The sensitivities obtained are expressed as dps/pg, for overall geometries of 1 and 4 x for the N.E.C. and Seforad detectors, respectively. The sensitivities are calculated for irradiations of one half-life or 5 min, whichever is shorter, and a neutron flux of IOl3 neutrons/cm* sec. It is obvious that for the longer-lived isotopes, better sensitivities could be obtained by longer irradiations, but for purposes of easy comparison the sensitivities are given for a maximum irradiation of 5 min in all cases. If two or more radioisotopes which decay with different X-ray emission are formed from one element, the different sensitivities obtained in each case are indicated. If the same type of X-rays are formed from different radioisotopes, the percentage due to each isotope at zero time after irradiation has been calculated, and the sensitivity indicated is calculated for the sum of these isotopes. If the irradiation results in intense beta activity, it is necessary to reduce this by the use of an appropriate absorber. A 1.5 mm plastic plate was found to be satisfactory.
ANALYTICAL CHEMISTRY, VOL. 44,NO. 3, MARCH 1972
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Element Cobalt Copper Zinc Germanium Selenium Bromine Rubidium
Strontium Niobium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium
Iodine Cesium Barium Cerium Tantalum Tungsten Rhenium Osmium Iridium Platinum
Table I. Sensitivities Obtained by X-Ray Spectrometry Following the (n, y) Reaction Sensitivityn X-rays Half-life Decay measured dpslllg 1 . 7 x 104 10.5 min Co-Ka 1.T 12.8 h Ni-Ka 1 . 8 X 10 E.C 13.8 h 7 x 10-1 Zn-Ka 1.T 46 sec Ge-Ka 8 . 5 x 103 1.T 1 . 2 x 103 3.9 min Se-Ka 1.T 57 min Se-Ka 1.T 1.2 x 104 6.2 rnin Br-Ka 1.T 4.4 h Br-Ka 1.T 1.1 x 103 1.06 rnin Rb-Ka 1.T 20 min Rb-Ka 1.T 2.8 h 6 . 3 min 4.4 h 4 . 4 min 4.7 mit 13.6 h 2 . 4 rnin 6.5 h 48.6 rnin 54 min 20 min 40 rnin 4 . 2 min 2.8 d 25 rnin
69 min 25 rnin 2.9 h 82.9 rnin 14.5 min 2 . 5 rnin 9h 33 h 16.5 rnin 115 d 24 h 18.7 rnin 91 h 14 h 1.45 rnin 80 rnin
1.T 1.T
P1.T 1.T PE.C E.C 1.T 1.T 1.T E.C P1.T E.C P-
1.T E.C 1.T
P-
1.T 1.T E.C
P-
1.T
PP1.T E.C 1.T 1.T 1.T
Sr-Ka Nb-Ka Rh-Ka Rh-Ka Pd-Ka Ag-Ka Pd-Ka Ag-Ka Cd-Ka In-Ka Sn-Ka In-Ka Sb-Ka Sb-Ka Sn-Ka I-Ka
4.0 3.9 x 104 5.1 4 . 1 x 104 2 . 2 x 101 3 x 102 1 . 3 x 104 9.0 2 . 5 X 10 1 x 102 3.4 5 . 7 x 10-1 7 x 10-1 1 . 4 x 103 2.9 6.5
Te-Ka Te-Ka CS-K~ La-Ka Ba-Ka Ba-Ka La-Kff Pr-Ka Ta-Kaz W-Kai Re-Kal Re-Kap W-KaI Os-La1 Ir-LP, Pt-KPi
2.0 1 . 2 x 102 2 . 5 x 103 2 . 7 X 10 2.0
LYl
8X 1 . 2 x 10-1 1.25 x 102 2 2.5 X 10 8 . 5 X lo2 2.7 5.5 6 . 5 x 104 1.6 2.5 1 . 7 x 10-l 5 x 10-1 7.0 1 x 10 4 . 4 x 102 1.1 x lo*
Remarks Ab A A A
from 86Rb (n, 2n) reaction
A A
Calculated according to peak height A
A A A A
A A
A Au-Lal A Au-Kai Mercury Hg-Kai 1.T Au-Kai E.C A Thorium Pa-Lal PA Uranium Np-Lai PCalculated from integrated peak, for 5-min irradiation at a thermal-neutron flux of 1013 n/cm2 sec, extrapolated to zero time after irradiation, with an overall geometry of 1% and 4% (0.8z for X-rays with energies higher than 50 keV). Background tests using an irradiated paraffin sample yielded an activity smaller than 1 dps. A = counted through 1.5 mm plastic absorber. 18 h 31 min 43 min 65 h 22.4 rnin 23.5 min
PP-
In order to assess the possibility of determining an element in the presence of its nearest neighbors in the periodic table, several factors have to be considered: the energies of the X-rays involved, the relative sensitivities as listed in Table I, the respective half-lives of the X-rays, and the ratio of the concentrations of the two elements. Clearly, if there is no overlap in energies, the possibility of determining two neighboring elements in the presence of each other will depend only on the resolution of the detector. For the working conditions med in this study, this possibility will start from element 32-germanium; from this element on, the difference between the energy of the Kal X-rays of one element and that of the KP1 X-rays of its immediate 550
*
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
predecessor becomes more than 315 keV-the resolution of the N.E.C. Si(Li) detector, and this difference increases with atomic number. If overlapping occurs, the chances for determining one element in the presence of the other will increase with increasing differences in sensitivity and half-lives. Last, when a trace concentration of an element is to be determined in the presence of large quantities of its neighbor, the shape of the tail of the peak is the decisive factor, rather than the resolution a t the FWHM. This point needs further experimental investigation. As an example, let us consider the element silver (47), and its two neighboring elements, cadmium (48), and palladium (46). Ag produces by neutron capture only one radio-
I-
4
10.7 kev X ray
s 5
TI,,
- spectrometry
(si(Li)-IOOm 2 1
= 1.45m
0 2
4
Channels
Figure 3. Comparison between X-ray and p r a y spectra obtained from a sample containing cesium and rubidium in the ratio l / l O G
I
1 pg Cs
+ 100 mg Rb, irradiated for 1 min, counted for 40 min after
a cooling time of 2 hours
0.1 Channels
Figure 2. X-ray spectra of the neighboring elements, Ir, Os, and Pt active isotope which decays with X-ray emission--'08Ag; the latter decays by electron capture and emits Pd X-rays of 2.42 min half-life. On the other hand, Pd produces two radioisotopes which decay with X-ray emission : logmPd,which by internal conversion emits Pd X-rays of 4.7-min half-life, and lo9Pd,which after beta decay emits Ag X-rays with 13.6-h half-life. (The Rh X-rays which could be obtained from l03Pd are too weak to be detected.) Figure 1 shows the spectra obtained. As may be seen, direct determination of Ag in the presence of Pd is difficult, since both emit Pd X-rays (Ag emits only Pd X-rays) and the half-lives are rather close. On the other hand, Pd can be determined quite conveniently in the presence of Ag by measuring the 13.6-h Ag X-rays from Pd after a longer cooling time. Having established the Pd content in the sample, it is now possible to calculate its contribution to the Pd X-rays measured shortly after irradiation, and thus to deduce the contribution arising from the Ag content. The radioisotopes and X-rays obtained from Cd have been discussed above, and are shown in Figure 1. It will generally be possible to determine Cd and Ag,in the presence of each other, since, as seen from the Figure, there is no overlap between the energies of the X-rays emitted by the two
elements. Pd and Cd may be determined together from the Pd and Cd X-rays, respectively, at short times after irradiation. A second interesting example is the group of elements, Pt (78), Ir (77), and Os (76). Because of their high atomic number, both K and L X-rays can be detected, and the different ratios (K/L and a//3/7)corresponding to the three elements may be used in order to simplify their simultaneous determination. Figure 2 shows the spectra obtained. In short irradiations, as used throughout this study, both Os and Ir produce only L-X-rays. As seen from Figure 2, it will be difficult to resolve Ir and Os La1 and LOl X-rays. But, because of the great difference in sensitivities and half-lives (see Table I), and of the different La/LP/Ly ratios, it will be possible to determine Ir in the presence of Os, and vice versa, by counting the sample twice: immediately after irradiation, for a short time (0.5-1 rnin), and after one hour, for a longer time (40 min or more). From the first measurement the amount of Ir may be calculated, since the contribution of Os will be negligible up to an Ir/Os ratio of 1/10; from the second measurement the amount of Os may be calculated, since Ir will have decayed completely in the meantime. Of the five stable Pt isotopes which, after neutron capture, all produce radioactive isotopes that decay with emission of X-rays, only two, 1 9 W and 1 9 8 P t contribute to the X-rays obtained by short irradiations. The latter yields lg9Pt,which emits only Au K X-rays, whereas the former yields 1g7mPt, which emits Pt L and K X-rays, and 197Pt, which emits Au L and K X-rays. Because of the relatively low natural abundance of lgeR(25.2%), and its low thermal-neutron cross section for producing 197mPt (0.05 b), as well as the K/L ratio
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
0
551
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-1 Channels
Figure 5. Comparison between X-ray and y-ray spectra obtained after neutron activation of a phosphate-rock sample containing 50 ppm iodine 100 mg phosphate rock, irradiated for 5 min, counted for 10 min through a plastic absorber at 2.5 cm from the crystal after a cooling time of 30 min Channels
Figure 4. Comparison between X-ray and y-ray spectra obtained from a sample containing niobium and zirconium in the ratio 1/104
+
10 mg Zr, irradiated for 1 min, counted for 4 min after a cooling time of 2 min
1 pg Nb
of about 2 (determined experimentally) for the F't X-rays obtained, the sensitivity for the determination of Pt according to its L X-ray will be low (see Table I). Therefore, the determination of Ir in the presence of Pt will be easily carried out at a short time after irradiation, despite the fact that the energies of the Lal and Lol X-rays of the two elements overlap, as seen from Figure 2. The L X-rays of Os and F't are resolved by the detector; thus Os may be determined in the presence of Pt, although the difference in sensitivities is small. The determination of Pt in the presence of Os and Ir will be more accurate if carried out by measuring the Pt or Au K X-rays emitted. Practical Examples. The above considerations summarize the general capabilities of analysis by X-ray spectrometry following neutron activation. Four cases of practical interest were studied in order to compare the merits of this method with those of gamma spectrometry of the neutron-activated samples. The determination of traces of cesium in the presence of large quantities of rubidium, and that of niobium in the presence of large quantities of zirconium, are both analytical 552
problems known to be difficult and tedious(4). Thermalneutron irradiation of the only stable isotope of cesium, 33Cs, gives rise to two isomeric activities: short-lived la4mCs (2.9 h) and long-lived 134Cs (2.05 y). Only 1 3 4 % ~ can be used for the determination of cesium, since the use of la4Cs would necessitate too long irradiation times. Figure 3 shows the gamma and X-ray spectra obtained from a sample containing a Cs/Rb ratio of l / l O 5 . In both cases, the sample was irradiated for 1 min and counted for 40 rnin after a cooling period of 2 hours. The sensitivity of cesium determination by gamma spectrometry is greatly impaired by interference of the Compton continuum from the 1080 keV Rb peak with the 128 keV 134mCsgamma-ray peak. Therefore, radiochemical separation is necessary. , In contrast, X-ray spectrometry can be applied directly, due to the large separation of the atomic number of Cs (55) and Rb (37), and the difference in the half-lives of the radioisotopes involved: 2.9 h for 1 3 4 m C1.02 ~ , min for BsmRb, and 20 min for s4mRb. As may be seen from the Figure, the Kal and KO1 X-rays peaks of Cs are well resolved, and the background due to the beta activity of Rb is reduced to a low continuum which has no influence on the integration of the Cs peak. Like cesium, niobium has only one stable isotope, g3Nb (100 which upon thermal-neutron activation produces two radioactive isomers, 94mNb(6.6 min) and 94Nb(2 X 104y). Again only 94mNbcan be used for the determination of N b
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
z),
(4) I. M. Kolthoff, J. P. Elving, and B. L. Sandell, "Treatise on
Analytical Chemistry, Part 11," Vol. 1 (1963) and Vol. 6 (1964), John Wiley and Sons, New York, N.Y.
-
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~
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-
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X ray
-
spectrometry
1 I
Channels
Figure 6. Comparison between y-ray and X-ray spectra obtained after neutron activation of a sample of platinum metal containing 0.2 Ag, 0.1 Ir, 0.1 Rh, and 1 Os
x
x
10 mg of platinum metal, irradiated for 1 min, counted for 1 min through a plastic absorber after a cooling time of 10 min
by neutron activation. Figure 4 shows the gamma and X-ray spectra of a sample containing a Nb/Zr ratio of 1/104, irradiated for 1 min. As in the previous example, the weak (0.273 871 keV gamma ray of 94mNbis resolved only with difficulty from the background activity and gamma-ray peaks due to Zr. On the other hand, Nb may be determined in the presence of large quantities of Zr by X-ray spectrometry, since neutron activation of Zr produces no radioactive isotope
which decays with emission of X-rays. Thus, the 16.6 keV K a X-ray peak of Nb is obtained on a low flat background. The analysis can easily be used to determine a concentration of 1 ppm Nb in Zr, or less, by changing the irradiation and counting time to one half-life (6.6 min) or more. Figure 5 shows the gamma and X-ray spectra obtained by irradiation of a phosphate-rock sample containing 50 ppm of iodine. As seen from the Figure, both gamma and X-ray spectrometry may be used for the determination of iodine. Nevertheless, in this case too, the background is much lower in X-ray spectrometry, making the integration of the peak easier and more accurate. This example demonstrates the advantage of using X-ray spectrometry, without chemical processing, for matrices which contain Na (about 1.5% in this case) and C1 (about OS%), elements which are known to interfere with thermal-neutron activation analysis by gamma spectrometry. The last example shows the direct determination of a number of trace elements in a metal matrix. Rh, Ag, Ir, and Os are generally found in platinum metal, and accurate quantitative determination of these elements is difficult (5). If wet-chemical methods are used, a complete analysis could take several days, and would have to be carried out by a trained analyst. If instrumental activation analysis followed by gamma spectrometry is used, a very complex spectrum is obtained (see Figure 6). By X-ray spectrometry, however, well-resolved peaks of Ir, Rh, and Ag on a low background are obtained by counting for 1 min shortly after irradiation; for the determination of Os, the sample is counted a second time, after 24 hours, for 40 min. The cases discussed above clearly illustrate the advantage of using X-ray, rather than gamma spectrometry, for the analysis of the activated sample. Even when it was possible to resolve the gamma peaks of interest from the gamma spectrum of the matrix, the analysis would require computation of half-lives and subtraction of Compton backgrounds, treatments which are time-consuming and may introduce errors. All these complications are avoided in X-ray spectrometry. ACKNOWLEDGMENT
The authors wish to thank Raia Nutman for technical help throughout the experiments. RECEIVED for review July 26, 1971. Accepted October 5, 1971. This work is part of an investigation performed by Mrs. M. Mantel in partial fulfillment of the requirements for a Ph.D. degree to be presented to the Hebrew University, Jerusalem. This work was supported by the U.S. National Bureau of Standards. ( 5 ) F. E. Beamish et al., Tulunta, 14 (4), 1 (1967).
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