Determination of Micro Amounts of Tantalum in Niobium by Using Neutron Activation and G a m m a Spectrometry FRANCOIS DUGAIN Pechiney Cie., L.R.M., 8 Rue Ampere, Grenoble, France JEAN LAVERLOCHERE Commissariat a I’Energie Atomique, C.E.N.G., S.A.R., Boite Posfale 269, Grenoble, France The purpose of this work was to develop a simple and accurate method for the routine determination of tantalum in niobium and niobium oxide. The neutron activation method has the advantage of being extremely sensitive and selective, owing to the very different characteristics of the two radioisotopes produced: Nbg4and The main effort was centered on the interferences which arise in the case of TalS2, the radiochemical separation processes which would avoid these interferences, the limits of destructive and nondestructive methods, and the accuracy obtained. The sensitivity is excellent, even for an irradiation of only 1 hour in a flux of 10l2n . cm.7 saT1
T
are difficult to determine quantitatively by standard chemical methods ( 4 ) , either because of the similarity of their chemical properties or because of the lack of sensitivity of certain of these methods. These difficulties are increased further when one of the above elements is to be measured in the form of an impurity in the other, which is the problem investigated here- Le., the quantitative determination of tantalum in niobium or niobium oxide. These obstacles can be overcome by the use of neutron activation analysis, which possesses certain advantages in addition to those (1-3, 8 ) described elsewhere. These advantages are as follows : (1) The radioisotopes formed from ?;b and T a have very different characteristics (Kbg2,N b g 4 m , Nbg4, Talezm, Tals2) (6,9). (2) Tal**has a much longer half life than the other radioisotopes formed and can therefore be distinguished from them. (3) Very high sensitivity can be expected for the tantalum determination ( g e = 19 barns), even if the irradiation time is very short compared with the half life of TalS2. Moreover, this isotope ANTALUM AND NIOBIUM
998
ANALYTICAL CHEMISTRY
can be measured very efficiently by gamma sDectrometrv since the dis-
EXPERIMENTAL
Equipment. T h e atomic reactor “Melusine” ( C E N Grenoble, enriched uranium, light water swimming-pool pile) was used; it provides irradiation facilities in thermal neutron fluxes up to 2.1013 n. cm.+ s-l A gamma spectrometry unit was used which included a 400-channels “Intertechnique” analyzer with a 7.5X 7.5-cm. KaI(T1) probe placed in a lead castle of 80 X 80 X 80 cm. with 5-cm. thick walls. For chemical separations Dowex 1 ion exchange resin (X8, 100-200 mesh) and transparent polyvinyl chloride columns of 0.6-cm. diameter and 6 cm. in length were used. Procedure. Standards were prepared from a tantalum oxalate solution containing 10 mg. of Ta per ml. Twenty microliters were placed on small pieces of chromatographic paper. After evaporation each standard was enclosed in a polyethylene tube or bag. Samples were prepared from about 0.5 to 1 gram of niobium (oxide or metal) placed in a polyethylene tube or bag. Samples and standards were irras.-l for 1 diated in a flux of 10l2n. hour. hfter irradiation, a cooling period of 5 days was allowed if no chemical separations were performed. Standards were dissolved first in concentrated HNOa to destroy the paper, and were evaporated to dryness in concentrated HF-HC1 (20 ml. of HF, 2 ml. of HCI). Samples were dissolved in hot concentrated HF-HCl (20 ml. of HF, 2 ml. of HC1). The solutions (standards and samples) were evaporated almost to dryness over a water bath, redissolved in 5 ml. of 9N HC1-1,V HF, and the volume was brought to 10 ml. after rinsing. When chemical separations were performed, the sample solutions were passed over 0.6-cm.-diameter columns of transparent polyvinyl chloride, filled to a height of 6 cm. with Dowex 1
resin (X8, 100-200 mesh) and prepared in 9N HCI-1N HF (flow rate
of 0.005N HCl-lAr HF for Zn. Tantalum was eluted with 10 ml. of 4N NHa Cl-1N NHAF. Two countings were performed on each sample, with the counter settings a t 0-200 k.e.v. and 1-2 m.e.v. for a sufficient length of time to obt.ain about 10,000 counts in each of the energy ranges 0.055 to 0.080 and 1 to 1.3 m.e.v. A comparison of the results corresponding to these two measurements gives an indication of the radioactive purity of the spectrum obtained. RESULTS
The initial tests were carried out without chemical separation to determine the limits of the technique and the interferences which may arise. Figure 1 shows the importance of the cooling time of the sample after irradiation. Several peaks are observed on the spectrum recorded after 30 hours of cooling, in particular that corresponding to NaZ4(1.38 m.e.v.); its Compton contribution would lead to an overestimate of the Ta value; the relative error might reach 2f1’3~ for a series of samples measured in this way. ri decay time of 8 to 10 half lives of K a 2 e i . e . , about 150 hours-eliminates this error. The other peaks observed have been attributed to WlE7 (0.480 and 0.680 m.e.v.) and NbgZ(0.93 m.e.v.) but they do not interfere between 1 and 1.3 m.e.v. Other interferences are possible from long-lived radioisotopes which emit gamma radixtions in this energy range. These are: Fe59, Coco, S C ~ and ~, Cd115. The importance of these interferences is as follows: 1 pg, of tantalum gives the same counting rate (in the energy range 1.0 to 1.3 m.e.v.) as 1800 p g . of Fe, 5 p g . of Co, 400 p g . of Zn, 0.6 p g . of Sc, and 18,000 pg. of Cd.
~~
Finally, interferences may arise due to the nuclear reactions W182(n,p)Ta182 and Re185(n,cr)Ta18Z,but the cross sections of these reactions are so small that neither tungsten nor rhenium can cause errors in the tantalum determination when present as impurities. From these considerations i t is clear that chemical separation of tantalum from the elements cited is necessary when the tantalum concentration is very low. Although this problem has been partially investigated in other cases (3, 6) using ion exchange resins, the chemical characteristics of tantalum and niobium are such that both chloride and fluoride ions must be present. Some difficulties are involved with the pretreatments of the samples. According to their types, the samples were dissolved as follows : (1) Solid metal sample : dissolved by contentrated HF to which concentrated HXO3is added drop by drop. (2) Niobium oxide (calcinated for 15 hours at 900' C.) or fresh hydroxide precipitates: dissolved with 25 ml. of concentrated HF 3 ml. of concentrated HC1 (for about 500 mg. of oxide). The purpose of the chemical separations is the fixation of tantalum and elution of the impurities. The distribution coefficients were measured in HCl-HF for two different HC1 concentrations; the results are given in Table I. [ K Dand K D Vare the mass and volume distribution coefficients, E the elution constant, d the displacement of each element for an elution volume V. (d in cm., V in nil.)]. The selected medium was 9 M HC11 ~ HF, 1 because it allows good redis-
+
1.
Table I.
HCl9M H F 1M HCl HF
3M 1M
Measured Distribution Coefficients (Dowex 1 )
Ta Nb
co sc
Ta
Nb
co
sc
KD
KDV
E
150 11.1 70 2.7
60 4.4 28 0.11
0.017 0.21 0.03 2.5
0.6 7.5 1.5 90
0.0018 0.26 0.9 0.9
( V = 20 ml.) 0.2 18 64 64
1400 8.9 2.1 2.0
solution after evaporation of the attack solution previously described, and because it is possible t o use standard glass columns. Eluting in 9M HCl-1M HF through a Dowex 1 ion exchange column, prepared in the same medium and containing resin to a height of 6 cm., we thus find that tantalum and cobalt are very strongly fixed, scandium is not fixed, and niobium is partially eluted. Beside these elements, Fe and Zn are very strongly fixed in this medium ( 7 ) . Cobalt, iron, and zinc can be eluted with the following solutions: 313: HClI N HF (6 volumes) for Go and for total elution of the N b a t the same time; 0.4N HC1-1N H F (6 volumes) for Fe; and finally 0.005N HC1-IN HF (6 volumes) for Zn ('7). Tantalum is eluted with a yield of 98% by 5 column volumes-Le., 9 m1.of 4N NH&l-lN NH4F. The elution constant of tantalum in this medium is close to 1. All these elutions are carried out at a flow rate of 0.2 to 0.3 ml./minute. With the idea of using this process industrially, the irradiation time was voluntarily restricted to 1 hour. Under
560 3.6 0.8 0.8
Table II.
(V
d = 10
ml.)
Reproducibility of Technique Used
Activity
counts/min./rg. of Ta
Mean value
Dev. from mean value
5 8 . 5 -58.5 6.6& 6.80 8 . 6 0 - 8.80 5 4 . 5 -55.2 8.72- 9 . 6 4 13.2 -13.8 7.57- 7 . 7 5 9.17- 9 . 2 8 6.88- 7 . 0 6 6.85- 7 . 1 7 13.40-13.43
58.5 6.73 8.70 54,85 9.18 13.50 7.66 9.22 6.97 7.01 13.41
0 1.0 1.5 1.5 5.0 2.2 1.2 0.7 1.5 2.3 0.1
of standards,
Table 111. Comparison of Results between the Three Methods
Ta content in p.p.m.
Nondestructive (a) (b) 78 42 600 5100
Solution
After chemical separation
(b) (a) (b) 110 100 94 91 155 157 155 154 140 146 135 140 770 660 695 690 655 7400 5610 6220 6000 6000 (a)
125 90
0.480 m.e.v.
i
-
p
---..___
Figure 1 . Gamma spectrum of a sample of niobium irradiated at lo1*n. cm.-2s.-1 for 1 hour
- - - after -after
(1 ) (2)
30 hours' cooling 150 hours' cooling
these conditions the sensitivity remains excellent and is more than adequate for our tantalum determinations in the niobium oxides analyzed. In fact, with a 1-hour irradiation in a flux of 1012 n. s.-l, we measured a counting rate of about 50 counts/minute and 200 counts/minute, respectively, in the energy bands centered on 1.2 m.e.v. and 0.067 m.e.v., for 1 pg. of tantalum (10-ml. sample, 1 cm. from the counter). This is not a sensitivity limit, because with a longer irradiation time (1 to 2 weeks for example) in the high fluxes available (up to 1014n. cm.-2 s.-l) it is possible to reach a tantalum determination better than gram. In order to estimate the precision of this measurement, 11 different irradiations have been listed in Table 11, giving the values obtained on the two standards placed in each irradiation tube. VOL. 37, NO. 8, JULY 1965
999
The maximum relative deviation observed with respect to the mean value is f 5%. (The comparison of values obtained from one irradiation to another has no significance.) The three methods thus developed were compared by treating five samples according to the technique described (Table 111). The first value, columns (a), corresponds to the 0.067- m.e.v. peak measurement ; the second, columns (b), to that of the 1.2-m.e.v. peak. The results show that chemical separations are not indispensable when a very accurate result is not required. Under these conditions the analysis has the obvious advantage of saving time. Aicomparison of the results obtained a t 0.067 and 1.2 m.e.v. gives an estimate of the value which can be attached to the analysis. In fact, these examples, confirmed by routine use of the technique, show that the difference between these two figures decreases when the
material is taken into solution, and decreases again when the tantalum is separated chemically. CONCLUSION
The industrial analysis of tantalum in niobium by neutron activation is possible because of the suitable nuclear characteristics of these two elements. h wide range of concentrations can be measured as a result of the development of anefficient chemical separation procesb which is simple enough to be performed on a routine basis and effective for concentrations of from a few p.1i.m. to 1%. Moreover, from the viewpoint of reproducibility and accuracy, satisfactory reiults can be obtained from this method by working carefully according to a clearly defined, but straightforward, procedure. This is largely a result of the possibility of checking, a t each analysis, the radiochemical purity of the tantalum.
LITERATURE CITED
(1) Albert, P., “L’Analyse par Radioactivation,” Dewister and Gauthier Villars, eds., Paris, 1963. ( 2 ) Bowen, H. J. AI., Gibbons, D, “Radioactivation Activation Analysis,” Clarendon Press, Oxford, 1963. (3) Compt. Rend. Journees ‘Etude sur I’Analyse par Activatzon, Grenoble, May 4-5 (1961). Presses Universitaires de France. Paris. 1963. (4) Elwell, W. T:, Wood, D. F., Anal. C h h . Acta 26, 1-31 (1962). ( 5 ) Hughes, 1). J., Harvey, J. A , , Brookhaven National Lab. Rept. B.VL-3251955. (6) Laverlochere, J., Chim.Anal. (Paris) 44(9), 388-91 (1962). (7) Selson, F., Kraus, K. A., J . Am. Chem. SOC.77, 4508 (1955). 18) Proc. Intern. Conf. on Xodern Trends in Activation Anblyszs, 1961, Texas A . and M. College, College Station,
Texas, 1962.
( 9 ) Strominger, I)., Table of Isotopes, Rev. Mod. Phys. 30, Part I1 (1958).
RECEIVED for review November 10, 1964. Accepted AIarch 17, 1965.
Computer Program for Multicomponent Spectrum Analysis Using Least-Squares Method JAMES A. BLACKBURN’ Attorney General’s laboratory, Toronto, Ont., Canada
b A least-squares method of resolving the components of a complex spectrum is presented. The program is written in the Fortran IV language and may be used on any computer similar to the IBM 7044. The method allows the resolution of component spectra which have coincident peak regions. A mathematical discussion of the method is given as well as a general description of the program. General capabilities of the program are reviewed with special regard to the maximum number of components which may be resolved.
E
data are very often expressed in the form of spectra. These spectra are not normally due to single sources but rather are the superposition of various component spectra. Thus a gamma ray spectrum may be the sum of many different spectra, each of which is due to a pure radioactive element. The computer program discussed in this paper evaluates the contributions to the resultant spectrum made by each of the components, provided a pure spectrum of each of the components is available. The mathematical discussion and subsequent discussion of results are given Lvith particular reference to the decomposition of complex gamma spectra. h XPERINENTAL
1000
0
ANALYTICAL CHEMISTRY
section is included in which other applications of the computer program are discussed. MATHEMATICAL DISCUSSION
Suppose we have a composite (gamma) spectrum consisting of S(Z) counts in the Zth channel, where S(Z) is determined for N channels. Further, let R(Z,J) be the number of counts in the Zth channel due to the J t h component (or standard) spectrum. Each standard spect’rum is taken as being defined over Y channels also and there are .If of these standard spectra available. Finally, let X ( J ) be a number (at present undetermined) which multiplies each channel count for the J t h standard spectrum. The values of the X ( J ) ’ s must, be determined so that the sum of all the standard spectra, when multiplied by their appropriat,e X ( J ) ’ s , gives a best, fit, in the sense of least squares, to the composite spectrum. This can be accomplished by minimizing the following expression : I=1
5 TV(I)S(Z)R(Z,K)
I=1
This is satisfied i f :
c
x!J)R(zIJ:12}
.U
=
J= 1
J-1
Since K runs through the yalues 1 to M, we have in fact N equations with the X ( J ) ’ s as *\I unknoi\ns. The solution of this set of equations produces the values for X ( J ) such that a least squares fit to the composite spectrum is obtained. For the case of gamma spectrum analysis, the channel counts S(Z) and R(1,J) are not known exactly. A statistical uncertainty of the order of [S(1)]1’2 or [R(Z,J)]1’2 is present for each particular channel count. Let us assume that the standard spectra are known with no uncertainty. If this is not the case, mathematical filters may be applied to the standards to remove statistical variations in the channel counting (1). A weighting factor, W ( I ) ,is put into Equation 3 to yield
=
0 (2)
x
J-1
1 Present address, Department of Physics, I-niversity of Waterloo, Waterloo, Ont., Canada.
