International Conference on Modern Trends in Activation Analysis

NOTICE

' INTERNATIONAL CONFERENCE ON MODERN TRENDS IN ACTIVATION ANALYSIS

Determination of Rare Earths by High Resolution X-ray Spectromotry Following Neutron Activation

M. Mantel and S. Amiel Nuclear Chemistry Department Soreq Nuclear Research Centre Yavne, Israel

ABSTRACT

Reactor neutron activation followed by high resolution- X-ray spectrometry has been applied to the nondestructive quantitative determination of the rare earth elements in naturally occurring materials. The various sources of error as well as the possibilities of minimizing them arc discussed.

I. INTRODUCTION Activation analysis of the rare earths has been the subject of intensive studies. References to the large number of papers written.on the subject may be found in the NBS Activation Analysis Bibliography *• ' as well as in a number of other recent reviews ^ , In previous work ^ '' we studied the application of high resolution X-ray spectromctry to activation analysis using reactor neutrons. It was shown that S9 elements lend themselves to such analysis, since the decay o£ their corresponding neutron-induced activities is accompanied by emission of characteristic X-rays, This method has several advantages over gamma-spectrometry: the direct correlation between an element and the energies of its characteristic К and L X-rays permits its identification by energy f7

measurements alono, without requiring lengthy half-life determination and gamma ray peak assignments; the small number of lines in the characteristic X-ray spectrum of each element allows tho determination of many elements in the same sample; and finally, the low sensitivity of the Si(Li) detectors to electromagnetic radiation above 70 koV eliminates interference of high energy gamma rays. In the present work we have applied this method to the quantitative, nondestructive determination of the rare earth elements present in naturally occurring materials. In the case of the rare earths, the gamma-ray spectra obtained upon neutron activation are generally very crowded due to the mul- titude of radioisotopes formed during the activation, and X-ray spectrometry therefore becomes especially attractive.' Of the 14 lanthanide elements, 12 (excepting La and Pr) were found to produce, upon neutron capture, radioactive isotopes which decay with emission of X-rays.

II. EXPERIMENTAL High purity oxides ("Spec-pure", Fluka or Johnson Matthey) of the rare earth elements were used. Standards Since no certified' standards were available, mixtures of the lanthanide elements had to be prepared. Dilute solutions (a few ug/ml) of the rare earths were prepared.by dissolving accurately weighed quantities of the oxides in nitric acid. Aliquots of these solutions were introduced into small polyethylene irradiation capsules (i.d. • 10 mm), weighed, evaporated to dryness and sealed. Procedure A few milligrams (10-20) of the sample were weighed into small irradiation capsules. To prevent dispersion of the powder on the walls of the vessel during irradiation, a few drops of hot paraffin were added f^ К The capsules were sealed and irradiated together with the standard in the pneuma- tic tube of the IRR-1 reactor. Counting 2 A Si(Li) detector*, 100 mm in area and 4mm in depth was used. The signals from the detector were passed through a preamplifier, amplifier, biased amplifier, and pulse stretcher**. The resulting pulses were analyzed by a 400 channel analyzer***.- The resolution (F»!) of the systera for the 6.4 keV Fe К X-rays (from 'Co) and 31.7 keV Ba К X-rays (from w'Cs7 ) was 450 and 550 еУ, respectively.

•Seforad, Israel **0rtec 118A, 410, 408 and 411, respectively ***T.M.C. - 3 -

III. RESULTS AND DISCUSSION The yields of the X-rays obtained from 12 of the rare earths are summarized in Table 1. (La and Pr do not produce X-rays in neutron activation.) The sensitivities (dps/yg) were calculated for irradiation times of one half- lifo or five min, whichever is shorter. Higher sensitivities could of course be obtained for the longer-lived isotopes by longer irradiations. The advantage of the X-ray method is demonstrated in Fig. 1, which shows a comparison between the у and X-ray spectrum of neutron activated ytterbium. Of the seven naturally occurring stable isotopes of ytterbium, three, 31.8 d 169Yb, 101 h 1/sYb and 1.9 h 177Yb produce, by the (n,y) reaction, radioactive isotopes which decay by emission of gamma-rays. The same three •radioisotopes emit X-rays during their decay: 169yb (Tra-X-rays) and *77Yb and * Yb (Lu X-rays) (see Table 1). Nevertheless, as clearly illustrated in Fig. 1, the Y-ray spectrum is much more complicated than the X-ray spectrum. As previously mentioned, the X-ray method permits simultaneous determination of many elements. If there is no overlap of X-ray peaks, the possibility of determining two neighboring elements will depend only on the resolution of the detector. For К X-rays the difference between the energy of the Kg, У-roys of one element and that of the K yttrium or cerium minez'als. One such mineral is xenotime CY-O-'P.O ), which contains up to 10 percent rare earths. The absorption coefficient of xenotime was calculated and the results obtained are summarized in Table II. Based on those values (u=5.2u Cf)' /8 an^ 2.9 cm /g for 40 and SO keV respectively) and a sample of 1 cm area and 10-15 mg weight (0.01-0.015 g/cra") the absorption will be small. As seen from the table of sensitivities, the above sample sizo will be sufficient for the assay of the short-lived isotopes while for the long-lived ones longer irradiation and counting times will meet the requirements of sensitivity. The case of a rare car.th having its К absorption edge just below or at the energy of the К X-ray peak to be measured should be taken into account. Table III shows pairs of such elements. Prom the absorption coefficients listed, it is concluded that even for minerals containing 1-7Л rare earth* this effect is still negligible for the small sizo samples used. The effect becomes of con- cern if rare-earth impurities are sought in high purity rare earths. In this case,..since the absorption will dopend essentially on the major rare earth con- stituent and not on the small variations in concentrations of the impurities, it is poss?.b?e to prepare a standard similar to the sample to be analyzed *• ' and then to determine the extent of absorption. Another possible source of error is the self-absorption of neutrons during activation due to the high thermal neutron cross section of most of the rare earths. This effect has been previously studied "• ' -1 and will therefore not be discussed here in detail. The use of small samples, which as shown above is essential to X-ray spectrometry, overcomes this source of error too; the quantity of every individual element in the sample to be analyzed will always be smaller than that permissible according to the self-absorptin curves obtained by Vöight (8). In special cases where absorption of X-rays by the mat- rix or self-absorption are feared, the determination of one element by another method, for instance atomic absorption, and its use as an internal standard for the evaluation of tho others ^/'ö-i' may be applied.

IV. APPLICATIONS A sample was proparcd containing a mixture of the 12 i*are earths studied, in proportions corresponding to their relative sensitivities of detection (Table I). This sample was analyzed repeatedly, using samples of pure lanthanidcs as standards. Table 1У shows the results obtained in comparison to tho weighed quan- - 5 titles. As may bo soon, an overall accuracy of - 4% and a precision of *'6.5-i wer e obtained. Based on those satisfactory results, the mixed sample was usod as a standard for the determination of the lanthanides in two minerals: xenotime and monazito. Figures 4 and S show the spectra obtained from these two minerals. Dy and Nd were determined immediately after irradiation, whereas the remaining lanthanides were detccinined 24 h later, after the short lived isotopes had decayed. Table V shows the results obtained. Yttrium, which always accompanies the rare earths in na- Ne ture, could also easily be detected and determined (Fig. 4). The results for xenotime could be compared with those obtained before by gamma-spectrometry ^ '\ good agroemont was found. Sa:

INFERENCES 6a 1. N.B.S.Technical Note 467-Fart 1: Activation Analysis, Л Bibliography, Lutz, G.Y., Boreni, R.Y., Maddock, R.G., Meinlce, W.W. Te_ 2. Bereanai, Т., J. Radioan. Chem. ID, 81 (1971). 3. Lutz, G.I., Anal. Chem. 43., 93 (1971). 4. Amiel, S., Mantel, M., An. Chem. 44_, Б48 (1972). 5. Fine, S., Hcndee, C.F., Nucleonics, data sheet No. 1, Physical Constants (1969). 6. Strom, E., Israel, H. I., Nuclear data tables A7, 565 (1970). 7. Gilat, Y., Shenberg, C, Mantel, M., IA-1168, 79 (1967). 8. Becknell, D.E., yoigt, A.F,, J. Radioa», Chem. IB, 89 (1971). 9. Mantel, M., Amiel, S., to be pubUshed. ТАВЬИ I Sensitivities obtained for the rare earths by X-ray spectroinctry following the (n,Y) reaction.

X-rays Sensitivity ^ H lenient Radionuelide Half-life Decay measured dps/Wp

6 100m 2 75 Yttrium ^ y 3.1 h I.T. ) Y -K« 1.3 Cerium 137 Co • 9 h B.C V La-Ka 8 x 10"2 143 Co 3S h IT4) Pr-Ke 12 x 10"2 Ncoilynium 149 Nd (2СЛУ 1.8 h 6" Pm-Ka 6.5 x 10 5' 1S0 Nd (74%) 12 m 6T Pm-Ka Samarium 155 Sm (55%) 23.5 m S" Eu-Ka 8 x 102 152 Sm (45%) 46.5 h r • Пи-Ka Europium 151 Bu 9.6 h E.C. Sm-K« 3 x 104 Gadolinium 159 Gd (3.8%) 18 h 67 ТЬ-Кй 1.4 x 102 100 Gd (96.2%) 3.6 m R~ Tb-Ко 16Ü Terbium Tb 72 d 0" Dv "а 2.7 Dysprosium 165 Dv 2.3 h B" Ив К« 5.9 x 103 163n 5 Dy 3.26m I.T. Dy Кв 4.2 x 10 Holmium 166 Ho 26.9 h (Г Er Ka 3.2 x 102 Erbium 171 Er 7.5 h $~ Tm-Ka 5 x 10 163 Er (61%) 75 m E.C. Ho-Ka 2.6 x 10 164 Er (39%) 10.3 h E.C. Ho-Ka Thulium 170 Tm 134 d tT Yb-Ko 8 x 10"Г Ytterbium 177 Yb (74%) 1.9 h er Lu-Ka 2.4 x 10 175 Yb (26%) 4.2 d tr Lu-Ka 169 Yb 31.8 d EX. Tm-K-ï 5.1 177 Lutecium y" Lu (4.4%) 6.7 d (Г Kx-Ka 8.4 x 102 176mLu (95.6%) 3.7 h 8" Hf-Ka

Vj Calculated from integrated peal;, (background„subtracted) for 5 min. irradiation.at a thermal-neutron flux of 10 n/cm , sec, extropolated to zero'time after end of irradiation with an overall geometry of 2%, •) counted through 1.5 mm plastic absorber.

2) Internal conversion-yields X-rays of clement Z.

3) Electron capture-yields X-rays of element (Z-l).

4) B-" decay followed by internal conversion of gamma rays - yields X-rays of . element (Z+l). . • .••'.- ' ' 'Cont'd on next page...///..... Footnote to Table 1 (eont'd): 5. Evaluated for the sum of both isotopes at 0 time after irradiation 6. Studied since present usually in rare earth minerals.

1', * A ТАШ.П II Calculation of absorption coefficient of xenctirae

Element Absorption coefficient ftpcm /g) *• ^ (as oxido) % 4Ü koV 50 keV Y2°3 44.6 3.71 2.04 P2°S 28.4 0.14. 0.08 Fe2°S 15.0 0.39 0.30 SnP 2.0 0.35 0.18 ttare fc'artli oxide* ' ioip' 0.65 0.37 xenotiine •5.24 2.97 •Calculated as s^ncc D/ predominates,

TADLB Щ Mutual interference of rare earths

Pairs of elements Element Sought Interfering Element X-ray KctT K. absorptiorj Mass. abs. coef? Element Produced keV Element edge keV cm /я Sm Ea 40.87 Ce ' 40.45 26.25 Dy Ho 47.5 Sm 46.80 20. ГА Er Ho 47.5 Sm 46.80 20.84 Ho . Пг 49.1 Eu . 48.50 19.57 Пг Tin 50.7 Gd 50.20 18.39 Yb Tm • 50.7 Gd 50.20 18.39 Yb \JA 54.1 Dy 53.76 . 16.13 *At К absorption odgv. И -8

TABLE IV Composition of mixed rare-earths standard

Ce Ы Sin Ku Gd Tb Dy llo Tm Yb Lu ' Er taken 1300 200 SO 2 120 800 3 100 1100 Ш 110 32C found* 1342 209 48 2.13 125 765 3.10 £>6 10S2 S82 116 307 - 8.3 ^2.3 -0.11 f 4.7 Ï22 to. is 1зз tS7 t22 is.( Ï12.S *mean of 8 determinations

TABLE V Results of analysis of minerals.

xenot-ime monazite ' X-ray X-ray Element spectromotry (Ti spectrometry vspectrornetry4 ' Ce 0.-04 - 0.002 8.4 - 0.26 Nd 0.12 t O.OOS 2.2 * 0.05 Sm 0.42 1 0.015 0.5 t 0.01 p.9 - 0.03 Пи n.d. n.d. Gd 0.31 * 0.012 0.51 * 0.02 Tb 1.1 * 0.033 1.5 - 0.06 n.d. Dy 5.4 - 0.135 5.0 - 0.20 0.015- 0.0007 Ho 2.2 1 0.077 2.0 - 0.08 n.d. Er 0.82 - 0.032 ' n.d. Tm 0.07 * 0.005 n.d. Yb 0.42 * 0.018 n.d. Lu 0,22 * 0.011 n.d. Y 32.3 * 0.95 0.28 - 0.012

The results are the mean of S determinations. * n.d. в not detected M ü.

Figure Captions Figure 1 Comparison between X-ray and y-ray spectra of neutron irradiated Yb. 800 щ Yb 0,, irradiated for S inin. A « y-ray ? spectrum measured with a - 40 cc._Gc(Li) detector. В a x-ray spectrum measured with a - 100 nun Si(Li) detector.

Figure 2 .X-ray spectra of neutron irradiated Sm and Eu,Eu/Jm 1/100, irradiated for 1 inin., counted for 4 min. A e 20 min. after irradiation. В я 24 h. after irradiation.

Figure 3 X-ray spectra of neutron irradiated Dy, Ио, Er and Yb. 2}ig Dy,lSjig По, 220j4g Er, 450)ig Yb, irradiated for S min. Л = 10 win. after irradiation, counted for 10 min. at 10 cm. from crystal. В в 24 h. after irradiation counted for 10 min.

Figure 4 X-ray spectra of neutron irradiated xenotime. 21 nig. c^snotime, irradiated for 5 min. A = 10 min. after irradiation, counted for 10 rain, at 10 cm. from crystal. В = 24 h. after irradiation, counted for 400 min. at 0.5 сш. from crystal.

Figure S X-ray spectrum of neutron irradiated monazite. 18 mg. monazito irradiated for 5 min. counted for 10 roin., 15 min. after irradiation at 5 cm. from crystal. "f О.

l7öYb

300 . 500 700 S00 1(00 IS00 Energy koV В 14

. 2

О 10 20 30 40 SO 60 70 Energy koV

•H'o.d. ш .o I .3 <аа. E üк o (Л в. (О 5Г I^MK^s o LU s o to Ю JU es o Ф Ene i

o Ю

о О Е

(ЙОР<) ш о»

Ю о Oi Energ y

Ю êfio

V (0 со (0 cu со ГО Ю СМ см I l$m> (Tm) EuK YbK , «I e В 41,5 52.8 (Er+YblfYb) М5Се-У-гау 57.5

а 9 о О 10Э

10 30 SO 70 Energy keV -.4 Л

30 50 70 Energy kcV

ЧНЪ