Instrumental Multielement Proton Activation Analysis of HighPurity Niobium Using Both y-Ray and X-Ray Spectrometry Viliam Krivan' Center for Trace Characterization, Department of Chemistry, Texas A&M University, College Station, Texas 77843
Trace concentrations of nine metallic impurities have been determined simultaneously in high purity niobium by a nondestructive technique based on irradiation of the sample with 12-MeV protons and observation of both the y-and Xrays from the reaction products with a Ge(Li) and a germanium low energy detector, respectively. The activation of the trace elements is based on (p,n) reactions. The elements determined are: Ti, V, Cry Fe, Zr, Mo, Hf, Ta, and W. Three other elements, i.e., Pd, Sn and Sb, could have been determined with high sensitivities if at all present. For Ti, V, Fe, Zr, and Mo, this method is more sensitive than other techniques which have been applied to the determination of these elements in niobium excluding solid mass spectrography. Detailed data on possible interferences, precision, and limits of detection are given.
Niobium has several special properties enabling its extensive application in various scientific and industrial fields. In the center of interest is the superconductivity exhibited by niobium and its compounds ( I ) . Among the transition elements, the maximum transition temperature occurs for niobium (9.5 OK). Niobium compounds and/or solid solutions, e.g., NbsGe, Nbs(Ge,Al), NbsSn, Nb-Zr, and Nb-Ti, have become technologically the most important superconducting materials. The highest known superconducting transition temperature, T , = 22.3 OK for Nb:jGe, was announced recently (2). Because of its suitable nuclear properties, niobium metal finds application as an end-plug in fuel elements for nuclear reactors ( 3 ) .Niobium is widely used to impart special properties to alloys with regard to their corrosion and fracture resistance, hardness, fabricability, ductility, etc. Many other uses of niobium as the element, as its alloys and compounds are of great practical significance. For superconductivity, trace impurities are of basic importance. They also play a very important role in other applications of niobium. The most common metallic impurities in niobium are tantalum, tungsten, molybdenum, zirconium, and iron. Important trace metallic impurities include titanium, vanadium, and chromium. There are several sensitive methods for the determination of each of the mentioned elements, but unfortunately most of them including atomic absorption ( 4 ) ,spectrophotometry (5-7) and electrometric methods (8-10) require the trace element to be separated from the matrix and in some cases also enriched before the determination. The main obstacle to the use of such destructive techniques for analyzing high purity materials is the blank often causing the limits of detection to be insufficient. But, in some cases, the determination methods themself are not sensitive enough. On leave of absence from Max-Planck-Institut fur Metallforschung, Laboratorium fur Reinststoffe, D-7070 Schwabisch Gmund, KatharinenstraPe 17, W. Germany, t o which address any correspondence should be sent.
Three methods enable direct instrumental multielement analysis of niobium: optical emission spectrography (111 3 ) ,solid mass spectrography (14, 1 5 ) and neutron activation analysis (16-19). Optical emission spectrography, however, can determine trace impurities only at or above the ppm range and therefore is sufficiently sensitive only for the analysis of low and medium purity niobium. In addition, this method can be considered as a direct one only with a certair? limitation as the sample has to be powdered, converted into niobium oxide, or fused with a suitable salt before the analysis. Solid mass spectrography can detect trace impurities in the ppb range. However, its accuracy is dependent on the reference standards used-a very difficult problem in some circumstances. Niobium is a convenient matrix for instrumental neutron activation analysis using y-ray spectrometry. However, only a few elements can be determined in niobium by this ,method because the impurities normally present in the highest concentrations, i.e., tantalum and tungsten, have high thermal neutron activation cross-sections and relatively complicated spectra. Our experience of routine niobium analysis shows that mostly only tantalum, tungsten, and sodium can be determined by instrumental neutron activation analysis in spite of using up-to-date equipment. A number of elements, e.g., Cr, Mn, Co, and Zn can be determined only after using radiochemical separations (20, 21 ). Other elements, e.g., Ti, V, Fe, and Zr, cannot be practically determined in high purity niobium even after using radiochemical separations because of the short half-life of the radionuclides produced and/or the low cross-sections and isotopic abundances involved. Recently, proton activation analysis was applied to the nondestructive multielement analysis of tantalum by y-ray spectrometry (22).The introduction of X-ray spectrometry increases the potentiality of instrumental activation analysis (23-25). McGinley and Schweikert ( 2 6 )were the first to point out the analytical use of X-ray emitting radioisotopes produced by charged particle activation. In the present work, proton activation ana!ysis was applied to the instrumental multielement analysis of niobium by making use of both y-ray and X-ray spectrometry.
EXPERIMENTAL Targets. Analyses were performed on niobium samples of good purity, ES type obtained from Heraeus, Hanau, W. Germany; high purity, WCT type obtained from Teledyne Wah Chang Albany, Ore.; and highest purity prepared in the Laboratory for High P u rity Materials, Max-Planck-Institute for Metal Research, Stuttgart, W. Germany, by molten salt electrolysis in LiF-NaF-KF eutectic containing 2.5 mol % KZNbF7. From the niobium samples, targets with a thickness -1 mm and a diameter of a t least 1 cm were cut. To remove possible surface contamination, the samples were etched for -1 min with a 2:2:1 mixture of concentrated ultra pure HNO3 and H F and water. The targets were covered with thin aluminium foils carefully precleaned to prevent possible surface contamination during the preirradiation manipulation. Particularly, there was a danger of iron and tantalum contamination during the mounting of the targets on ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
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Table I. Survey of Proton-Induced Reactions on Niobium Producing Radionuclides Reaction
93Nb(p,n)93mMo
93Nb(p,p)""'Nb "Nb(p, ~ n ) " ~ N b
Q - v a l u e , MeV
-1.2
0.0 -8.8
I3Nb(p, ~ 2 n ) ~ l ~ N b-16. 7
Half-life
?-Rays, MeV
Intensity, 96
6.95 h r
0.1139 0.2632 0.4552 0.6846 0.9479 0.9662 1.3630 1.4772 2.1618
0. 71 61.20 p a i r peak 91.90 0.20 p a i r peak 0.62 99.40 0.08
10.16d
0.9128 0.9345 1.8475
1.80 99.00 0.80
64.0 d
0.1040 1.2050
0.50 3.40
X-Rays, keV
Intensity, 94
17.45
22.0
19.70
4.0
13.6 y 16.60 18.60 14.90 16.60 16.80 18.70
1.5 40.0 0.3 7.0
Table 11. Data on Production and Properties of Radionuclides Observable with Ge(Li)- and Low Energy Detectors in Proton-Irradiated Niobium Element to be d e termined
Ti
Isotopic abundance, Principal reactions
"TTi(p, n)lKV
@
