64.4 LONG: THE ESTIMATION OF TANTALUM IN MIXTURES p o l . 76 The Estimation of Tantalum in Mixtures by Neutron Activation Analysis BY J. V. P. LONG A neutron activation method is described for the determination of tantalum in mixtures containing niobium, zirconium, iron, tin, titanium and silicon. The method depends on the pile irradiation of the sample followed by measurement of the filtered y-ray activity. An accuracy of approximately k 3 per cent. can be achieved for mixtures containing more than 1 per cent. of tantalum. IT is known that whereas tantalum has a large slow-neutron activation cross-section and that the decay radiation includes a hard y-ray, many elements commonly associated with tantalum, for example, titanium, niobium, iron, tin, zirconium and silicon, have low activation cross- sections or give rise to isotopes with a short half-life, or with a short half-life combined with low-energy radiations.These facts, summarised in Table I, provide the basis for a neutron irradiation method for the determination of tantalum in the presence of the elements named above; but caution TABLE I ACTIVATION DATA Atomic activation Type and energy Target cross- Isotope of radiation Element nucleus Abundance, section, produced Half-life (in MeV) % barns Tantalum . . 18lTa 100 20.6 18Ta 120 days 8- 0.5 y 1.13, 1.22 Niobium . . OSNb Titanium . . &OTi Tin . . . . l12Sn U6Sn 120Sn 122Sn lZ4Sn Zirconium . . O*Zr Q8Zr Iron . . . . 64Fe 5@Fe 100 5.34 0.96 14.2 24.0 33.0 4.7 6.0 17.8 2.8 5.9 0.33 0.0099 - 0.01 ? ? ? ? 0.15 0.07 0.2 0.04 0.0026 MNb lUSn 85 Zr &I?e 5QFc 6.6 min.I.T. 100 years 6 minutes 112 days 14 days 100 days 27 hours 130 days 10 days 66 days 17 hours 2.9 years 47 days ,!3- 1-35 X /I- 1.6 K I.T. e Y ,!3- 0.4 18- 1.4 y 0.4 ,!3- 2.4 /3- 0.4, 1-0 y 0.7 0.8 8- 2.2 K /3- 0.26, 0.46 y 1-1, 1.3 is necessary in applying the method to minerals and ores of unknown composition, or to tantalum concentrates derived from them, because several other elements could, if present, cause interference. Some interference might be expected from the 17-hour half-life zirconium- 97 if zirconium is present in large amounts, but this can be easily overcome by delaying counting until several half-periods of this isotope have elapsed. In the absence of elements other than niobium and tantalum, niobium can be determined by difference.This method for the estimation of niobium and tantalum has provided in some instances a useful alternative to the difficult chemical separation of the two elements.Nov., 19511 BY NEUTRON ACTIVATION ANALYSIS 64.5 EXPERIMENTAL In the experiments described in this paper, materials for irradiation were sealed in silica capsules of 5 mm outside diameter and approximately 1.5 cm long. Other experiments have been carried out with graphite capsulesf (see Fig. l), which have since been found more section of for silica tubes aluminium holder section of graph i te capsule 0 I 2 3 cm I 1 1 Fig. 1. Details of apparatus convenient. These holders were constructed by drilling longitudinal holes in aluminium rod and then turning the rod in a lathe so that the holes were partially exposed and the holder would fit in a standard 30-ml irradiation can.Graphite capsules were arranged round a central graphite rod and tied in position with thread before insertion in the aluminium can. The samples were irradiated for one week in the Harwell pile a t a neutron flux of approximately loll neutrons per sq. cm per second. Unless otherwise stated, all oxides used were Johnson Matthey “Specpure” materials. The silica capsules were loaded into holders of the type shown in Fig. 1. Weight of Tho,, mg Fig. 2. Relation between counting rate and weight of tantalum646 LONG [Vol. 76 Gamma-ray measurements were made after four days had elapsed from the time of removal from the pile; this allowed isotopes of short life to decay.The experimental arrangement consisted of a copper y-ray tube screened with 5 mm of lead to filter out soft radiations from the source, which was placed in a light wooden trough at 10 to 15cm from the Geiger tube. A preliminary irradiation was carried out to determine the effect of the presence of other elements on the activity of the tantalum, 5-mg quantities of tantalum oxide being used together with 5 mg of an added oxide. Two sets of tubes were irradiated, the first containing 10 mg of the mixed oxides and the second 10 mg of the mixed oxides together with 30 mg of high-purity graphite. Oxides of tin, silicon, iron, titanium, zirconium and niobium were investigated. The values obtained for the specific activity of the tantalum showed a mean deviation of & 1.8 per cent.without graphite and &1.6 per cent. with graphite. These results indicate that the oxides of the elements investigated will have only a small effect on the tantalum activity . Two further experiments have shown that the activity as measured is proportional to the quantity of tantalum present. In the first, the tantalum oxide was mixed with niobium oxide before irradiation. In the second, the base material consisted of a matrix composed of the oxides listed above in the proportions in which they might be expected to occur with tantalum in the first stage of its separation from naturally occurring materials. The counting rates from these two series have been normalised to correct for different pile factors and are shown in Fig. 2.These results give a linear relationship between counting rate and weight of tantalum. In one further irradiation the specific activity was found to decrease with increasing weight of tantalum. Subsequent experiments have failed to confirm this effect and it is thought to have been due to the position of the tubes during irradiation. The standard deviation of the results used in plotting the graph in Fig. 2 indicates that tantalum can be estimated by this method to a standard deviation of approximately &3 per cent. on single determinations for a standard deviation of &l per cent. in the counting. The lower limit is set more by the variation in constitution, and consequently in the activity, of the base material than by the cosmic and y-ray background. For this reason the range covered extends upwards from approximately 0.2 per cent. of T+O, in the matrix used, and the method is probably of most use above 1 per cent. of T+O,. The work described was carried out on behalf of the Ministry of Supply, by whose permission it is published. REFERENCE 1. Long, J. V. P., J . Sci. Instv., 1961, 28, 191. CHEMICAL RESEARCH LABORATORY TEDDINGTON, MIDDLESEX Apvil, 1961