638 Analyst, October, 1966, Vol. 91, p p . 638-646 Activation Analysis for Titanium and Niobium with Fast Neutrons BY V. T. ATHAVALE, H. B. DESAI, S. GANGADHARAN, M. S. PENDHARKAR AND M. SANKAR DAS (Analytical Division, Atomic Energy Establishment Trombay, Bombay, India) Fast-neutron activation methods have been developed for determining titanium and niobium. Scandium-47 formed by the (n,p) reaction on titan- ium-47 and yttrium-90 formed by the (n,cc) reaction on niobium-93 have been used for determining these elements. Irradiations were carried out in the swimming-pool reactor “Apsara” a t the Atomic Energy Establishment Trombay, a t a fission-flux of 3 x 10l1 n per cm2 per second. Radiochemical procedures were developed for the isolation of scandium and yttrium from the irradiated samples.The methods have been applied t o the determination of titanium in stabilised steels and the standard rocks G-1 and W-1 and of niobium in stabilised steels. The advantages and limitations of the methods are discussed. A FEW studies have been reported in the literature on the determination of titanium and niobium by the activation-analysis technique. Brooksbank, Leddicotte and Dean1 determined titanium in aluminium-based alloys by using the reactions 46Ti (n,p) 46Sc and 48Ti (n,p) 48Sc. Gruverman and Henninger2 applied the reaction 48Ti (n,p) 48Sc to determine high concen- trations of titanium in corrosion residues. Kim and Meinke3 reported on the analysis of titanium in different materials by using the (n,y) reaction on titanium-50. Activation analysis of niobium by the (n,y) reaction was investigated by Brownlee,4 Kim5 and Kim and Meinke,‘j who used rapid radiochemical separation for the isolation of the niobium-94m (half-life 6.6 minutes) activity.Leddicotte ef a1.’ used the (n,y) reaction for the non-destructive determination of niobium in steels. So, except for the studies of Brooksbank et al. and Gruverman and Henninger, these investigators have used the short-lived radionuclides formed by the (n,y) reactions on these elements. A major difficulty in the use of short-lived nuclides for activation analysis is that the induced activity decays rapidly with time so that pneumatic facilities are required for irradi- ation and delivery of samples. Even with these facilities, direct y-spectrometric analyses with 51Ti and S4nzXb are difficult, as the energy of the y-rays associated with the decay of these nuclides is low, and therefore subject to serious interference from other nuclides in the sample.This necessitates chemical separations, which have to be rapid in view of the time factor. The difficulties in developing satisfactory procedures for this purpose have been discussed by Brownlee and arise from the complex solution chemistry of these element^.^ 99 In some recent studies, Sankar DasloY1l and Yule, Lukens and Guinn12 have shown the possibilities of using fast neutrons for activation analysis, a field of study that has not yet been fully explored. The application of the (n,p) reaction on titanium and the (n,cc) reaction on niobium for the activation analysis of these elements is described.FEASIBILITY OF THE FAST-NEUTRON ACTIVATION METHOD- The different (n,p) and (n,ct) reactions on titanium and niobium, together with their threshold energies,13 the cross sections for a spectrum of fission neutronslO and the half-lives and the characteristics of the radiations emitted by the product nuclides14 are given in Table I. Reactions giving product nuclides of short half-lives are not listed as they suffer basically from the same disadvantages as the short-lived (n,y) products. The very long-lived products are also not included on practical considerations of sensitivity. The yields from these reactions for the irradiation of 1 mg of the element for 40 hours at a fission-flux of 3 x 10l1 n per cm2 per second are given in the sixth column of Table I.The flux value is as monitored by 32S (n,p) 32P, with an assumed cfs of 60 mb,15 near the coreTABLE I THRESHOLD REACTIONS IN FAST-NEUTRON ACTIVATION ANALYSIS Fission flux = 3 x 10l1 n per cm2 per second: time of irradiation = 40 hours Cross section for a spectrum of Yield, ET. fission neutrons, d per minute Radiation and energy, Element Reaction MeV * 6 8 (mb) Half-life Per mg MeV 8- 0.367 (100%) y 0.885; 1.119 /3- 0.439 (66%), 6- 0.622 (34%) ; y 0.155 y 0.99; 1.04; 1.314 y 0-48; 0-83; 1.31 1.61 8.6 84 days 2.3 x 103 3.4 days 1 x 106 Titanium . . . * . . 46Ti (n,p) 4'3Sc 47Ti (n,p) 47Sc - 0.17 20.4 4sTi (n,p) 48Sc 3.28 0.28 44 hours 2.2 x 101 18- 0.64 (100%) 3.37 0.047 4.7 days 1.1 x 102 /?- 1.94 (17%); 0.66 (830/,) 50Ti (n,cc) 47Ca Niobium .. . . . . 93Nb (n,a) -5.12 0.077 64 hours 3.1 x 103 8- 2-27 (100%) No gamma S%e)( (EjtiE * " , where ET is the threshold energy, o(E) the excitation function and $(E) is given by the relationship13 +(E) = -Ee sinh 1 / 2 E Of8 = -- --- j %El a E 43 W CD640 ATHAVALE et al.: ACTIVATION ANALYSIS FOR [Analyst, Vol. 91 of the “Apsara” Reactor, Trombay. It may be seen that the yields are high enough to be of interest in activation analysis. The application of scandium-47 and yttrium-90 for the analysis of titanium and niobium are described here. EXPERIMENTAL TRRADIATION- The samples and standards (spectrographically pure oxides of titanium and niobium) were irradiated in 8-mm diameter polythene tubes for 20 to 40 hours in the A, position of the “Apsara” reactor at Trombay.In this position, the samples are irradiated a t a distance of 9 cm from the nearest “13 plate 46 per cent. enriched uranium fuel element.” The samples and standards were wrapped in 30-mil cadmium foils in order to cut down the induced (n,y) activities. After irradiation, the samples were cooled for 10 to 12 hours before they were processed for isolating scandium and yttrium activities. C r ~ ~ ~ f ~ ~ ~ ~ SEPARATION- Stevenson and Xervikl6 have reviewed the different procedures reported for the radio- chemical separation of scandium and yttrium. In all of these methods, preliminary separation from common impurities is achieved by precipitating scandium and yttrium as fluorides. Brooksbank, Leddicotte and Dean1 used lanthanum as a non-isotopic carrier for scandium.Because of the known differences in the solubilities of the fluorides and oxalates of scandium and rare earths1’ it was decided to carry out the radiochemical separation of scandium in the presence of 5 mg of scandium and 10 mg of lanthanum carriers, the lanthanum being used as a collector for the scandium fluoride. The separation of yttrium-90 was carried out with about 20 mg of yttrium carrier. The final purification of scandium from rare earths was achieved by selective extraction into 100 per cent. tributyl phosphate from 8 M hydrochloric acid. This procedurels was preferred to the thiocyanate extraction procedure described by Kemp and S m a l e ~ , ~ ~ because of the difficulties experienced in destroying large amounts of ammonium thiocyanate.The only difficulty in following the tributyl phosphate extraction method is the need to spin the solution in a centrifuge to obtain clear phase separation. Scandium was back-extracted into 2 M hydrochloric acid, precipitated as hydroxy-quinolinate,20 and dried at 105” C to determine the chemical yield. The purification of yttrium from other rare earths is generally achieved by solvent extraction or ion-exchange separation methods.16 In the present studies, the final purification of yttrium was achieved by the anion-exchange separation with a methanol - nitric acid mixture which has been reported by Desai, Krishnamoorthy Iyer and Sankar Das.21 Yttrium is finally precipitated as oxalate, dried and mounted for P--counting. After the counting the chemical yield is determined by igniting the oxalate to yttrium oxide.MEASUREMENT OF RADIOACTIVITY- The photo-peak activity of scandium-47 was measured with a 14 x 1-inch NaI(T1) crystal coupled to an E.M.I. 9536 photomultiplier and a single channel analyser. After locating the 0.155-MeV photo-peak, the window width was opened so that the entire photo- peak was counted. The /3--activity of yttrium-90 was measured with a conventional mica end-window G.M. set-up. The activity measurements were made through a 184-mg per cm2aluminium absorber to cut off any heavy rare-earth activity that accompanies the yttrium fraction in the ion-exchange procedure.21 METHOD REAGENTS- Scandium carrier solution-Dissolve 0-7661 g of ignited scandium oxide (purity 99-9 per cent.) in 5 ml of concentrated nitric acid and dilute the solution to 100 ml (1 ml = 5.0 mg of scandium).Yttrium carrier solution-Dissolve 1.27 g of Specpure yttrium oxide in dilute nitric acid and make the solution up to 100 ml (1 ml = 10 mg of yttrium). 8-Hydroxyquinoline solution, 5 per cent. w l v 8-hydroxyquinoline in 2 M acetic acid. Ammonium acetate, 2 RE.October, 19661 TITANIUM AND NIOBIUM WITH FAST NEUTRONS 64 1 TributyZ Phosphate-Wash Eastman Kodak analytical-reagent grade tributyl phosphate with M sodium carbonate solution and then twice with distilled water and pre-equilibrate it with 8 M hydrochloric acid before use. Methanol - nitric acid, (i) 2.5 per cent. : 7.0 M-Dilute 2.5 ml of 7.0 M nitric acid to 100 ml with distilled methanol; (ii) 10 per cent. : 1.0 M-Dilute 10 ml of 1.0 M nitric acid to 100 ml with distilled methanol. All other reagents are of recognised analytical purity.PROCEDURE FOR THE ISOLATION OF SCANDIUM FROM IRRADIATED SAMPLES- (a) TITANIUM STANDARDS- (1) Evaporate 1 ml of the scandium carrier in a 25-ml silica crucible and ignite gently. (2) Weigh the polythene tube containing the irradiated standard. Cut open the irradiated polythene tube and transfer the oxide into the silica crucible. Collect the cut pieces of the tube in a tared paper and weigh; the difference in the weights gives the weight of the standard (5 to 10mg of titanium dioxide) used for analysis. (3) Fuse the standard with about 2 g of potassium bisulphate. (4) Dissolve the melt in dilute acid and transfer the solution into a 40-ml lusteroid centrifuge tube, add 10 mg of lanthanum carrier, precipitate the hydroxide with ammonia solution, spin the solution in a centrifuge and discard the supernatant liquid.(5) Dissolve the hydroxides in 1 ml of concentrated nitric acid, dilute the solution to about 10 ml and add 1 ml of 40 per cent. hydrofluoric acid. Warm the tube in a water-bath swirling it occasionally to facilitate the precipitation of scandium fluoride. Cool, spin the solution in a centrifuge and discard the supernatant liquid. (6) Dissolve the precipitate by warming it with 1 ml of saturated boric acid and 1 ml of 8 M nitric acid, precipitate the hydroxides with ammonia solution, spin the solution in a centrifuge and discard the supernatant liquid. (7) Repeat steps (5) and (6) twice.(8) Dissolve the hydroxide in 10 ml of 8 M hydrochloric acid and transfer the solution to a stoppered centrifuge tube. Add 10ml of 100 per cent. tributyl phosphate and extract scandium into the organic phase. Let the mixture stand for a few minutes, then spin it in a centrifuge to separate the phases. (9) With a transfer pipette, remove most of the organic phase into a second centri- fuge tube. (10) Repeat the tributyl phosphate extraction of the aqueous phase and combine the organic layers. (11) Scrub the organic phase once with 10 ml of 8 M hydrochloric acid, spin the solution in a centrifuge and transfer the organic phase into a separating funnel. (12) Back-extract scandium with 2 M hydrochloric acid, collecting the aqueous phase in a second separating funnel.Repeat the back-extraction twice and combine the aqueous phases. (13) Introduce 10 ml of chloroform into the second separating funnel containing the scandium, and extract to remove any dissolved tributyl phosphate. Discard the chloroform. Repeat the chloroform extraction twice. (14) Transfer the aqueous phase into a 150-ml beaker and evaporate the solution nearly to dryness. Destroy any organic matter by evaporating twice with 1 ml of concentrated nitric acid. When the nitric acid is almost completely removed, dilute with distilled water to about 100ml. (15) Add 2 ml of 8-hydroxyquinoline solution followed by 10 ml of ammonium acetate solution. Add ammonium hydroxide drop-wise until the pH is about 8 (test with indicator paper). Bring the solution to the boil.Cool and filter through a de-mountable glass filter unit containing a 2-8-cm Whatman No. 40 filter-paper that has already been washed with water, dried and weighed. Wash the precipitate with warm distilled water, and dry it in air for about 5 minutes.642 ATHAVALE et al.: ACTIVATION ANALYSIS FOR [Anabst, Vol. 91 (16) Transfer the filter disc containing the scandium 8-hydroxyquinolinate to a weighed cover-glass kept in a Petri dish and dry it at 105" C for about 45 minutes. Cool the residue and weigh it. (17) Mount the precipitate on a 1/32-inch aluminium card, cover with Scotch Tape and count the y-activity of scandium-47. (18) Correct the count-rate for chemical yield. (b) GRANITE AND DIABASE SAMPLES- crucible containing the scandium carrier. sulphuric acid.(1) Transfer a known weight (150 to 200 mg) of the irradiated sample into a platinum (2) Evaporate twice with 5ml of 40 per cent. hydrofluoric acid and a few drops of Ignite the residue and fuse with 2 g of potassium bisulphate. (3) Dissolve the melt in dilute acid and transfer it into a 40-ml lusteroid centrifuge tube. (4) Proceed through steps (4) to (13) as for the titanium standard, repeating the fluoride precipitation cycle 3 times. (5) Transfer the aqueous phase into a 100-ml beaker, destroy any organic matter by repeated evaporation with nitric acid and evaporate to dryness. (6) Dissolve the residue in 10 ml of methanol - nitric acid (2.5 per cent. : 7.0 M). (7) Transfer the solution to an anion-exchange column. (30 cm x 8 mm of 50 to 100- mesh Dowex-1 x 8 resin in nitrate form and pre-equilibrated with the same solvent.) (8) Elute with 30ml of the same solvent, collecting the effluent in a 100-ml beaker, evaporate the solution to dryness and precipitate scandium as hydroxy-quinolinate as des- cribed for the titanium standard. (c) STEEL SAMPLES- (1) Dissolve a known weight (200 to 300mg) of the steel sample by warming it in a 50-ml beaker with concentrated hydrochloric acid and adding a few drops of 30 per cent.hydrogen peroxide to hasten the dissolution. Add the scandium carrier to the beaker before sample dissolution. (2) When the metallic pieces have dissolved completely, dilute the solution to about 20 ml and filter the solution, collecting the filtrate in the lusteroid tube. (3) Incinerate the filter-paper containing any black residue in a silica crucible, fuse it with potassium bisulphate, dissolve the fused cake in dilute acid and combine with the filtrate obtained in step (2).(4) Precipitate the hydroxides with ammonia solution and proceed in the manner described for the standard. . PROCEDURE FOR THE ISOLATION OF YTTRIUM FROM IRRADIATED SAMPLES- KIOBIUM STANDARD- (1) With 2 ml of the yttrium carrier (and no lanthanum) isolate the yttrium-90 together with any rare-earth impurities from the standard, following steps (1) to (7) as described for the isolation of scandium from irradiated titanium standard. (2) Dissolve the hydroxide in dilute nitric acid and evaporate the solution to dryness in a 100-ml beaker. (3) Dissolve the residue in 10 ml of methanol - nitric acid (2.5 per cent.: 7.0 M). (4) Transfer the solution to an anion-exchange column. (5) Elute with 30ml of the same solvent and discard the eluate. (6) Continue the elution with 50 ml of methanol - nitric acid (10 per cent. : M) and collect the eluate in a 100-ml beaker. (7) Evaporate the solution to dryness on a water-bath, dissolve the residue in water, dilute to about 20ml and precipitate yttrium as oxalate with 2 ml of saturated oxalic acid solution. (30 cm x 8 mm of 50 to 100- mesh Dowex-1 x 8 resin in nitrate form and pre-equilibrated with the same solvent.)October, 19661 TITANIUM AND NIOBIUM WITH FAST NEUTRONS 643 Filter the solution through a de-mountable glass filtration unit containing a 2.8-cm disc of Whatman No. 42 filter-paper, wash the residue with water and finally with acetone.(9) Remove the paper containing the yttrium oxalate, dry it under an infrared lamp, mount it on a 1/32-inch thick aluminium card with Scotch Tape and count the P-activity. (10) When counting is over, remove the precipitate (with the minimum of the Scotch Tape) into a tared crucible, ignite it to the oxide and weigh it as yttrium oxide. Correct the observed count-rate for chemical yield. STEEL SAMPLES- fluoride by the procedure described for isolating scandium from irradiated steels. yttrium isolated from the irradiated niobium standard. (8) Bring the solution to the boil to granulate the oxalate. (1) With 2 ml of the yttrium carrier instead of the scandium carrier, isolate the yttrium (2) Purify yttrium-90 by following the steps (2) to (10) described for purifying the RESULTS Two types of samples were analysed by this method.One type consisted of analysed samples of titanium and niobium stabilised steels, which are in wide use as structural com- ponents in reactor technology. Although chemical colorimetric methods have been reported for determining these elements, it was desirable to have an independent check on the analysis procedure. The “standard” rocks G-1 and W-1 are also included in this work to evaluate the general applicability of the method. The results obtained are given in Tables I1 and 111. TABLE I1 ANALYSIS OF SAMPLES FOR TITANIUM BY THE 47Ti (n,p) 47Sc REACTION Certified value, Present results, Sample per cent. per cent. Stablised steel-1 . . 0*50* 0.56; 0.49; 0-50; 0.48 0.53 t 0.51; 0.48; 0.53 Mean 0.51 f 0.03 (lo) Mean 0.37 & 0.01 (lo) Mean 0-170 & 0.012 (lo) Mean 0.62 BCS/235/1 .. * . 0.36 0.36; 0-35; 0.38; 0.37 Granite G-1 .. .. 0.156: 0.158; 0,177; 0.164; 0.183 Diabase W-l , . .. 0*64$ 0.62; 0.62 * Chemical colorimetric.2a $ Calculated from the percentage of titanium oxide values, Fleischer and Stevens (Ref. 23, see p. 626). BCS = British Chemical Standards. By 48Ti (n,p) 46Sc.10 TABLE rrr ANALYSIS OF SAMPLES FOR NIOBIUM BY THE 93Nb (n,or) 9OY REACTION Certified value, Present results, Sample per cent. per cent. Stabilised steel-2 . . 0-76* 0.85; 0-70; 0 . S O ; 0.82; 0.86; 0.76; 0.67; 0.84 Mean 0.79 & 0.07 ( l u ) Mean 0-84 f 0.04 (la) BCS/246 .. .. 0.82 0.88; 0.85; 0.79; 0-S4; * Chemical colorirnetri~.~~ BCS = British Chemical Standards.The results given in Tables I1 and I11 indicate satisfactory precision and accuracy for the methods. The values obtained for titanium in G-1 and W-1 compared well with the preferred values reported by Fleischer and Stevens.23 The only other results reported for titanium by the activation-analysis technique are 0.133 per cent. for G-1, and 0-54 per cent. for W-1 (as obtained by Kim and Meinke6), which are lower in comparison with our results.TABLE IV INTERFERENCE FROM NEUTRON-INDUCED REACTIONS FissirJn flux = 3 x 10l1 n per cm2 per second; thermal flux = 5 x loll n per cm2 per second; thickness of cadmium foil = 30 mils Reactions studied (n,r) interference Conflicting threshold reaction A t-- mP - 7 - 1 7 ------ L--- Abun- Abun- Abun- F,* dance, 0 dance, Cadmium ii,, dance, mb percent.Reaction b per cent. ratio Reaction R, mb percent. Rl - - 46Ti (n,p) 46Sc 8.6 7.99 46SC-46SC 24 100 34 47Ti (n,p) 47% 20.4 7.32 46Ca-47Ca--%7S~ 4-7d 0.25 0.0033 ND 60Ti(n,o()47Ca ;$47Sc 0,047 5.25 48Ti (n,p) 48Sc 0.28 73.99 - - - - soV (n,ol) 47Sc 2*1* 0.25 "Nb @,a) 0.077 100 8 S y - s O y 1-2 100 26 51V (n,cc) 48% 0.027 99-75 ?Zr (n,p) QoY 0*9P 51-46 ND: 47Sc activity not detected. * Estimated cross ~ecti0.rr.l~ Ratio of activities R, to R, (for equal weight of parent element) (1 : 300) (6 : 1) (1 : 8) b ? .. r-- bOctober, 19661 TITANIUM AND NIOBIUM WITH FAST NEUTRONS 645 RADIOCHEMICAL PURITY- The radiochemical purity of the scandium-47 separated from different samples and standard was checked by following the decay of the 0-155-MeV photo-peak for about 7 days.The observed half-life was between 80 and 82 hours. (Half-life reported14 for scandium-47 = 81.6 hours.) Scandium-46 and scandium-48 are also produced with scandium-47 during the irradiation. For an irradiation time of 40 hours their relative intensities are in the ratio 1 to 10 to 44. Scandium-46 and scandium-48 do not emit any low energy y-rays and therefore do not interfere directly in the measurement of the scandium-47 activity. The interactions of the high energy y-rays from these nuclides, however, enhance the measured photo-peak activity of scandium-47. The extent of this interference was not more than 2.5 per cent. of the photo-peak activity of scandium-47 in the first few days of measurement.The purity of yttrium-90 was checked by following the p--decay for about 1 week. The half-life obtained was between 65 and 66 hours. (Half-life of yttrium-90 reported14 = 64.8 hours.) NUCLEAR INTERFERENCE- One of the reasons for not using the threshold reactions in activation analysis is because often the same active nuclide is produced at a higher rate from an (n,y) reaction. The neutron-induced reactions that interfere are given in Table IV. It may be seen that scandium, if present in the samples, interferes in the analysis of titanium with the 46Ti (n,p) 46Sc reaction. By shielding the samples with cadmium foil, the activity from the (n,y) reaction could be reduced by a factor of 34. However, it should be pointed out that this factor depends on the relative intensities of the thermal and epithermal fluxes in the neutron spectrum and, therefore, will vary with the irradiation position even in the same reactor.Neither 47Ti (n,p) 47Sc nor 48Ti (n,p) 48Sc has any interference from a primary (n,y) reaction. However, during the irradiation of samples, scandium-47 may be produced from a number of other reactions that are listed in Table IV. Of these, the (n,a) reaction on titanium-50 causes no interference in comparative activation analysis. Formation of scandium-47 from calcium-46 is small because of the low abundance of this isotope, and the low cross section for the (n,y) reaction. It is further reduced in cadmium-shielded irradiations. The calculated yield from the (n,a) reaction on vanadium-50 is 300 times lower than that resulting from an equal weight of titanium, as indicated in the last column of Table IV.The yield is calculated on the basis of an estimated cross section of 2-1 mb for this particular reaction.l3 The reaction 48Ti (n,p) 48Sc has a positive interference from 51V (n,a) 48Sc, the yield from the latter being one-eighth of that obtained from the former, as indicated in Table IV. Therefore, both from the point of view of sensitivity and freedom from nuclear inter- ference, 47Ti (n,p) 47Sc is to be preferred to the other (n,p) reactions on this element. Determination of niobium in samples containing yttrium or zirconium will be in error. The induced activity from SgY (n,y) 99Y could be reduced by a factor of 26 by shielding the samples with cadmium foil. From equal weights of zirconium and niobium in the sample, the yield of yttrium-90 from the (n,p) reaction on zirconium-90 is estimated to be 6 times greater than that resulting from the (n,a) reaction on niobium.It should be possible to correct for the zirconium interference as follows. A specimen of zirconium is irradiated together with the sample, and the ratio of the activities of yttrium-90 to yttrium-92 (formed by the reaction g2Zr (n,p) g2Y ; &, estimated = 0-16 mbI3) determined, The activity of yttrium-92 from the sample is also measured, and by using the ratio of activities from the zirconium specimen, the activity of yttrium-90 corresponding to the zirconium content of the sample is calculated. The total yttrium-90 activity from the sample can therefore be corrected for the contribution from zirconium. ’- 3-4 hour CONCLUSION The method described shows the advantages in using the reaction 47Ti (n,p) 47Sc over the other (n,p) reactions on titanium for the activation analysis of this element.The limit of sensitivities for these reactions, defined as that weight of the element which, on irradiation646 ATHAVALE, DESAI, GANGADHARA4N, PENDHARKAR AND SANKAR DAS for 40 hours at a flux of 3 x loll n per cm2 per second, would give a measured activity equal to the background (the measurements having been made 24 hours after irradiation) can be calculated to be 1.7 pg of titanium and 54 pg of niobium. In addition, our studies indicate the possibilities of using fast-neutron activation where the thermal neutron-activation technique is associated with definite practical limitations.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 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E., Geochim. Cosmochim. Acta, 1962, 26, 525. Madhava Menon, V. P., Mahadevan, N., Srinivasulu, K., and Venkateswarlu, Ch., Scient. Ind. Res., Received May 24th, 1965 1962, 21B, 20.