Nondestructive Determination of Some Trace Elements in Tantalum by Proton Activation Analysis Viliam Krivan Max-Planck-lnstitut fur Metallforschung, Laboratorium fur Reinststoffe, 0-7070 Schwabisch Gmund, West Germany
Dale L. Swindle and Emile A. Schweikert Center for Trace Characterization, Deparfment of Chemistry, Texas A&M University, College Station, Texas 77843
Trace concentrations of titanium, iron, zirconium, niobium, molybdenum, and tungsten have been determined nondestructively and simultaneously in high purity tantalum. The determination is based on the analysis of the y-ray spectra of the radionuclides produced by bombarding thick samples with a 15-MeV proton beam via the following principal reactions: 48Ti(p,n)48V, 56Fe(p,n)56Co, goZr(p,n)goNb, 93Nb(p,n)93mM~, and la2W(p,2n)181Re. The trace elements have been determined at levels comprised between 0.1 to 35 ppm, with a relative precision of 8.5 to 25%. The detection limits for a 5 PA-hr irradiation range from 0.005 ppm for zirconium to 0.5 ppm for tungsten.
The trace characterization of medium and high purity tantalum for metallic impurities is a difficult analytical task. Among the impurities of particular interest in connection with metallurgical and physical studies are titanium, iron, zirconium, niobium, molybdenum, and tungsten. The two most significant trace elements are niobium and tungsten. Niobium, for example, is present a t a concentration level of several tens of parts-per-million even in the highest purity tantalum commercially available, prepared by electron beam zone refining. The second main impurity, tungsten, is usually present in the part-per-million range in the same type of material. Although several sensitive determination methods are available for the elements mentioned, most of them are destructive. Compounding this drawback is the fact that separation procedures applicable in the analysis of tantalum are relatively complicated. Clearly, an instrumental method of analysis would in this case be of significant advantage. Among the instrumental techniques, only emission spectrography (1-3) and mass spectrometry ( 4 ) are applicable, both carrying, however, some inherent limitations (sensitivity and standardization, respectively). Another powerful method, neutron activation analysis, can be used only in connection with radiochemical separations (5, 6 ) , mainly because of the high matrix activity induced. Several studies reported in recent reviews ( 7 , 8 )and orig"Methods for Emission Spectrochemical Analysis," Fifth ed., American Society for Testing and Materials, Philadelphia, Pa., 1968, p 580. (2) L. C. Chandola, V. S. Dixit, and M. D. Sakena, India, At. Energy Comm., (1)
Bhabha At. Res. Cent., BARC-465, 1970, 5 pp. (3) N . F. Zakhariya, A. I. Staikov, and I. S. Aubinder, Spektrosk. At. Mol., 88-91 (1969). (4) K. Sato, N. Yamaguchi, R. Suzuki, and R. Matsumoto, J. Jap. Inst. Metals, 34, 610 (1970). (5) T. Kawashima, "Proceedings 1965 International Conference: Modern Trends in Activation Analysis,'' Texas A 8 M University, College Station, Texas, 1965, p 61. (6) E. N. Gilbert, V. A. Pronin, and V . G. Torgov, Radiokhimiya, 10, 500 (1968). (7) E. Ricci, "Charged Particle Activation Analysis," in "Advances in Activation Analysis," Vol. 2, J.M.A. Lenihan, S. J. Thomson, and V. P. Guinn, Ed., Academic Press, New York, N . Y . , 1972, p 221.
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inal communications, e.g. (9-12), have shown the feasibility of highly selective and sensitive determination of medium and high 2 elements using charged particle activation analysis. These dealt, however, mostly with destructive or nondestructive single element determinations. In the present study, the application of this technique to the nondestructive multielement analysis in high purity tantalum is presented.
EXPERIMENTAL Irradiation. Samples and standards were irradiated a t the Texas A&M University 88-inch cyclotron. Typical bombarding conditions for thick target yield measurements on Mo, Nb, Zr, W, Ti, and Fe and for standardization purposes were: one-minute irradiations with 15 MeV proton beams of 0.05 t o 0.2 PA. Activation curves were established using proton energies up t o 30 MeV. High purity tantalum samples for analysis were irradiated in a watercooled sample holder with beam intensities of 2-4 PA for 1-3 hours. The details of the experimental arrangement have been described previously (13). Samples. Trace characterization was performed on samples of high purity tantalum obtained from Materials Research Corporation, Orangeburg, N.Y. The VP grade tantalum supplied in rod form was cut into sample wafers of approximately 1-mm thickness. A pre-irradiation chemical etch was accomplished with a 3 to 1 mixture of concentrated "03 and H F to remove possible surface contamination. Counting Equipment. Gamma-ray measurement was performed using an Ortec Ge(Li) y-ray spectrometer. The detector characteristics included energy resolution of 2.3 keV FWHM for the 1.332 MeV y-ray of 6oCo,photopeak efficiency of 8.0% relative to a 3 X 3 inch NaI(T1) detector and a peak to Compton ratio of 28:1. The absolute counting efficiency for this detector was determined using calibrated radioisotope sources of 5 7 c ~*03Hg, , **Na, 54Mn, and 6oCo supplied by NEN Chemicals. Data were collected on a 4096-channel Canberra multichannel analyzer. Quantitative Calculations. Quantitation of results was accomplished using thick metal targets of Mo, Nb, Zr, W, Ti, and Fe. These were irradiated and used as reference standards after making the appropriate corrections for both the length of irradiation and the beam intensity.
RESULTS AND DISCUSSION Trace Element and Matrix Activation by Proton Bombardment. The selection of the type of the bombarding particle and energy is paramount for optimizing the performance of nondestructive multielement charged particle activation analysis. For the trace element-matrix combination considered, proton activation appeared best suited both from the standpoint of sensitivity and selectivity. A bombarding energy of 15 MeV was chosen, based on considerations outlined below. The trace elements that can be detected in tantalum by (8) W. S. Lyon, E. Ricci, and H. H. Ross, Anal. Chem. 44, 438R (1972). D. L. Swindle and E. A. Schweikert. Anal. Chem., 45, 21 11 (1973). J. L. Debrun and J. W. Barrandon, J. Radioanal. Chem., 17, 291 (1973). D. C. Riddle and E. A. Schweikert, Anal. Chem., 46, 395 (1974). B. Parsa and S. S. Markowitz. Anal. Chem. 46, 186 (1974). H. L. Rock and E. A . Schweikert, Anal. Chem., 41, 958 (1969)
(9) (10) (1 1) (12) (13)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
Table I. Activation D a t a on the Trace E l e m e n t s of Interest
Principal reaction
48Ti(p,n)4%' 56F
e (p,n)56C o
@value, MeV
Half-life
-4.8
16.0 d
-5.4
78.5 d
%Zr(p,n)gONb
-6.9
14.6 h
g3Nb(p,n)g3mMo
-1.2
6.9 h
94Mo(p,n)94Tc
-5.0
4.8 h
96Mo(p,n)9 6 T ~ 182W(p,2n)181Re '' For
-3.7
4.3 d
-11.2
19 h
Major y-rays, MeV
Intensity,
0.983 1,312 0.847 1.238 1.771 2.598 1.129 2.319 0.141 0,263 0.685 1.477 0.702 0.850 0.871 0.778 0.850 0.812 0.366
100 99 99 69
70
Thick target yield, dps/&A"
3.9
x 105
Q-value, MeV
Half-life
-1.0
121.5d
181Ta(p,3n)ligrn.gW
-16.7
181Ta(p,4n)li8W 18LTa(p,pn)l B o n l T a lB1Ta(p,p2n)li9Ta 18LTa(p,p3n)1791nsgTa
-23.0 -7.6 -14.2 -22.1
l*lTa(p,p4n)"7Ta 181Ta(p,2p)la0mHf 18lTa(p,3p)li9Lu 'Ta (p,3pn) liBrn.gLu
-29.0 -5.9 -13.9 -20.9
6m 38 m 21.5d 8 . 1h 600 d 9.3m 2.2 h 56.6h 5.5h 4.6 d 21m 28 m 161 d
Reaction
181Ta(p,n)l*'W
3p2n) 177Lu
181Ta (p,
-26.8
Major y-ws, MeV
0.136 0.152 0.222 0.031 no 0.093 no 0.093 0.093 0.113 0.093 0.217 0.089 1.341 0.115
nondestructive proton activation and pertinent nuclear data (14-26)are listed in Table I. Also included are thick target yields for 15-MeV protons. These data indicate the possibility of determining simultaneously the elements listed with a single irradiation. Additional more detailed information was obtained by measuring the activation curves (relative excitation functions) for all of the reactions of interest. The experimental threshold energies observed under used experimental conditions are given in Table I. The activation curves further comfirmed that a proton energy of approximately 15 MeV would be best suited for the combination of trace elements of interest. In Table 11, a listing is given of the proton-induced nuclear reactions on tantalum and their relevant nuclear data. The examination of the Q-values shows that there are only a few reactions energetically possible below -15 MeV proton energy. For most of them, the y-ray energies and inten(14) C. M. Lederer, J. M. Hollander, and I . Perlman, "Table of Isotopes," 6th ed., John Wiley & Sons,New York, N.Y., 1967. (15) K. A. Keller, H. Munzel, and H. Lange, "Q-Values for Nuclear Reactions," Landolt-Bornstein. Neue Serie, Gruppe I, Band 56, Springer, Berlin, 1973. (16) Ch. Meixner. "Gammaenergien:" Jul-811-RX. Julich, W. Germany, Dec. 1971.
6.5
+ 0.5
5 . 1 x 104
8 . 0 31 0 . 5
4 . 4 x 106
7.5 i 0.5
4 . 6 x 105
4.0
5 . 4 x 105
8.0 i0.5
x 105
6.5 i 0.5
15
17 89 82 67 53 92 100 100 100 100
100 97 88 100
2.8
2 . 2 x 105h
1-minute irradiations of pure metal targets (natural isotopic composition) at the end of irradiation. 'I Assuming I.,
Table 11. Proton-Induced Reactions on Tantalum
Experimental threshold energy, MeV
=
+ 0.5
11.5 i 0 . 5 100%.
sities are low or the half-lives of the product nuclides are long. An important advantageous factor further minimizing the activation of the tantalum matrix is the relatively high Coulomb barrier, which for protons has a value of 15.7 MeV. Experimental evidence confirmed that proton energies 5 1 5 MeV can indeed be readily used on tantalum. Nuclear Interferences. At a proton energy of 15 MeV, the following interfering reactions are energetically possible: a) 52Cr(p,an)48V (Q = -14.2 MeV), affecting the titanium determination; b) 6oNi(p, ( Q = -11.6 MeV), affecting the determination of iron; c) 94Mo(p,~xn)~ONb (Q = -9.0 MeV), affecting the zirconium determination; d) 94Mo(p,pn)93mMo(Q = -9.7 MeV), affecting the niobium determination; e) 99Ru(p,a)96gTc (Q = +3.1 MeV), 100Ru(p,an)96gTc,(Q = -8.4 MeV), and 101Ru(p,a2n)g6gTc, (Q = -13.4 MeV), affecting the molybdenum determination; f) 1840s(p,a)1s1Re, (Q = +6.5 MeV) and 18sOs(p,a2n)181Re,(Q = -8.4 MeV), affecting the determination of tungsten. These possible interferences were checked experimentally by irradiations of thick targets made from the corresponding elements. In all cases, no measurable interference was observed when the element to be determined and the interfering element were at equal concentrations. y R a y Interferences. Possible interferences due to overlapping y-rays originating from different nuclides are listed in Table 111. This type of interference is most serious in the tungsten determination for which the 18zW(p,n)182Rereaction combined with measuring the 1.122-MeV and 1.222-MeV y-rays would be most suitable. Unfortunately, this reaction cannot be used because of a strong interference for both of these lines caused by the presence of ls2Ta emitting also y-rays of exactly the same energies. The formation of 182Ta can only be explained by reaction lslTa(n, y)lS2Ta. The following experimental evidence supports this interpretation: a) this interference occurs only in the presence of a tantalum matrix (Figure 1); b) the y-ray energies, their intensities and the half-lives indicate the presence of 182Ta;c) the activation curve for the formation of the interfering radionuclide shows that this is not a threshold reaction (Figure 2). The production of lS2Ta is not surprising when one considers that by bombardment with protons of medium energy, neutrons can be produced by (p,xn) reactions and that the activation cross
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
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Table 111. G a m m a - R a y Interferences Encountered in Nondestructive T a n t a l u m Analysis Interference Principal reaction
Ti,?
182W(p,n)182Re
13 h
E,, MeV
1.122 1.222 0.264 0.848 0.850 1.127 1.129
6.9 h 77.3 d 4.3 d
3Nb (p,n)93n1Mo 56Fe(p,n)56Co 96Mo(p,n)g6Tc 90Zr(p,n)90Nb
14.6h
17, %
E,, MeV
strong strong 58 100 100
1.122 1.222 0.265 0.850
1S2Ta g6Tc
0.848
56Co
97
1.129 1.127
96T~
TIME ( h r s )
Figure 1. y-Ray interferences in the nondestructive determination of Ta via 18'Re: experimental decay curves for E, = 1.122 and 1.222 MeV; the decay of '*'Re from pure W is given for comparison
W in
/
I
1122 MeV Y-Ray 0
,0/
I1 -5
o
o
1222 MeV Y-Ray
I
I
0
5 10 15 PROTON ENERGY (MeV)
I
I
I
20
I
25
Figure 2. Production of 18'Ta (4 = 1.122 and 1.222 MeV) by secondary neutrons via I8'Ta(n, y) reaction at different primary proton bombarding energies section for the reaction IslTa(n, y)lS2Ta has a relatively high value for neutrons in the MeV range (17). A part of the neutrons produced may be thermalized, giving a very high thermal activation cross section for tantalum. These conclusions make it evident that the use of unnecessarily thick samples should be avoided. Consequently the determination of tungsten must in this case be based on the reaction ls2W(p,2n)lS1Reand the measurement of a 0.366MeV y-ray. Because of the higher threshold energy and the lower thick target yield for 15-MeV protons and also because of high Compton continuum at 0.366 MeV, this determination is less sensitive than that based on the reaction 1s2W(p,n)1s2Re.The lslRe is, however, adequate in the case of a tantalum matrix where the tungsten concentrations are (17) M. D. Goldberg, S. M. Mughabghab, S. N. Purohit. B. A. Magurno, and V. M. May, "Neutron Cross Sections," Vol. iIC. BNL 327, 2nd ed., Suppl. 2, Brookhaven National Laboratory, Aug. 1966.
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Nuclide
1s2Ta
Ti/.
115 d
Mode of production
'81Ta(n,y) 1eZTa
lszTa
@Nb
115 d 4.3 d 77.3 d 14.6h 4.3 d
18 'Ta (n, y) l8ZTa 96Mo(p,n)96Tc jfiFe(p,n)5 T h 90Zr(p,n)g0Nb Q6Mo(p,n)g6Tc
a t the ppm level. The detection of lslRe is further aided by the high intensity of its 0.366-MeV y-ray. It should be emphasized that owing to the complexity of the y-ray spectra in this energy region, an unequivocal identification of lslRe must be based on both y-ray and half-life data. For similar reasons, as discussed above, in the determination of niobium via g3Nb(p,n)Y3mMo, the 0.264-MeV y ray cannot be used. In this case, however, there are two other interference-free y-rays available which, by virtue of their higher energies, have as an added advantage a much lower Compton background. In the determination of molybdenum, the 0.850-MeV yray of 96Tc can be affected by the presence of the 0.848MeV y-ray of 56C0 due to iron. These two y-rays could not be separated by the detector at our disposal. Other interference-free y-rays of 9 6 Twere ~ therefore used for the molybdenum assay. Similarly in the case of the determination of iron, specific y-rays are available, although even the 0.848-MeV y-ray could be used, provided sufficient waiting ~ decay. time is allowed for 9 6 Tto Finally, in the determination of zirconium via the 1.129MeV y-ray of 90Nb,a weak interference can occur with the ~ from molybdenum. The 1.127-MeV y-ray of 9 6 T arising error introduced in this case into the zirconium determination is less than 2% when both elements are at the same concentration level. Further, if necessary, the interferencefree y-ray of 90Nb can be used for determining zirconium. Analysis of Samples. To illustrate the practical usefulness of proton activation, a series of high purity tantalum samples were examined. The nuclides and pertinent y-ray energies used for the determination of the trace elements sought are indicated in the y-ray spectrum shown in Figure 3. The concentrations determined for the six elements of interest are given in Table IV. The results for titanium, iron, molybdenum, and tungsten represent the average of three determinations on replicate samples. Included in these data are the average deviations based on the individual determinations. These results reflect the capabilities of the method on a multielement basis using a single irradiation of 2 to 4 hours and counting periods in excess of 10 hours. The performance of the method in terms of accuracy could not be evaluated because of the lack of tantalum material characterized by another independent method. In this context, the constant concern in this work for assessing possible sources of systematic errors might be emphasized again. Also listed in Table IV are the estimated limits of detection based on experimental data obtained in a typical tantalum irradiation (2.5-hour bombarding time with a beam intensity of 2 /.LAfollowed by a cooling period of 10 to 50 hours). The measured y-ray peak intensities corresponding to known concentrations were compared with the minimum detectable peaks calculated using the 3a criterion. In the case of zirconium and niobium, six additional determinations involving shorter irradiation and counting
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 1 2 , OCTOBER 1974
Figure 3. y-Ray spectrum of a tantalum sample measured under tne rollowing cononions: counr rime or o nours, sIan or counr u nours aner end of bombardment
Table V. Reproducibility of Niobium and Zirconium Determination in Tantalum IVP Grade)
Table IV. Results of Tantalum Analysis and Experimental L i m i t s of Detection Impurity element
Concn detd, ppm
Av. devu
Exptl detection limits, ppm
Ti Fe Zr Nb Mo W
0.09 0.77 0.12 32.80 0.67 6.60
*0.02 +O .15 3=0.02* *2. 80b 10.12 h1.40
0.02 0.10 0.005 0.30 0.20 0.50
a Based on 3 determinations. terminations.
* Standard deviation calculated from 9 de-
Detn
1 2 3 4 5 6 7 8 9 Average and
o
N b content, ppm
Zr content, ppm
33.3 35.9 31 . 0 35 . 0 37 . O 29.7 30.6 37 .O 32.4 32.8 i 2 . 8
0.10 0.10 0.11 0.10 0.12 0.16 0.17 0.12 0.11 0.12 + 0.02
times were carried out to establish the standard deviations. The reproducibility on a total of nine determinations for these two trace elements is illustrated in Table V. The standard deviations were found to be 8.5% for niobium at 30 ppm and 16.6% for zirconium at 0.1 ppm. It should be noted that these repetitive determinations were carried out on nine adjacent samples from the original tantalum rod. Possible inhomogeneities in the trace element distribution would be included in the results obtained and the standard deviations computed.
tive assay with high sensitivity and selectivity for most trace elements of major interest. The combination of these important features is particularly attractive when considering currently available methods for the analysis of tantalum.
CONCLUSIONS
RECEIVEDfor review February 5 , 1974. Accepted May 1,
Proton activation is well suited for the trace characterization of tantalum. This technique provides a nondestruc-
1974. This work was supported by the National Science Foundation Grant GP-34877X.
ACKNOWLEDGMENT The assistance of the cyclotron operations personnel is gratefully acknowledged.
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