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DdayEd -Neutron Activation DEtErmination Of Uranium in Thirteen French Rock REfmmcc Samples

E.B. LEDGER,* T.T. T I E H " AND M.W. ROWE§

Departments of Geology* and Chemistry5 Texas A&M University, College Station, Texas 77843, USA

The uraniwn contents of t h i r t een rock refe- rence samples from the Centre de Recherches P&trographiyues e t G&ochimiques, Nancy, France, were determined by delayed-neutron act ivat ion analysis. F o w others from the same laboratory were found t o hme uraniwn concentrations beZm o w l i m i t of detection. Meaningfui! comparison of o w data with other workers i s presently not possible (i) because of the paucity o f uranium data and ( i i l because of the d i s p m i t y of uraniwn data on the samples fo r uhich uraniwn data e x i s t .

Seventeen different geochemical reference samples prepared and distributed by the Centre de Recherches PBtrographiques et G6ochimiques (CRPG) were analysed for their uranium content. The major, minor and trace element contents of most of the samples studied have been collected and reported in detail by Roubault et al. (l), de la Roche and Govindaraju (2-4) and Govindaraju and de la Roche ( 5 ) . There have been relatively few determinations of the uranium contents in these samples. Furthermore, even for those samples where several uranium results are available, there is not adequate agreement for an acceptable value to be assigned for any of them. We, therefore, determined the uranium contents of these rock standards to partially alleviate this serious lack of data.

DELAYED-NEUTRON COUNTING

Delayed-neutron counting was introduced in 1957 as an analytical method by Echo and Turk (6) and by Dyer and Leddicotte (7) and later refined mathematically and experimentally by Dyer et al. (81, Amiel (9) and by Cavallari et al. (10). Rock and mineral samples were first determined by Hamilton (11) and Gale et al. (12, 13). Uranium contents of meteorites are generally much lower (10 ppb) than most rocks but were nevertheless successfully measured by Amiel et al. (14). Uranium has also been studied in marine sediments (15) and in Portuguese basalts (16) to list a few

selected examples. We have not attempted an exhaustive review - or even a complete listing of delayed-neutron activation analysis studies applied to geological samples.

Several advantages are realized by using delayed-neutron counting as an analytical method for determination of uranium: (i) It is sensitive enough for most purposes; our limit of detection was 50.2 ppm uranium. (ii) It is rapid. The method requires only 2.5 minutes for irradiation and counting of each sample analysed. (iii) No sample treatment is necessary. No preparation is required except for placing the sample in a clean polyethylene irradiation vial. (iv) Except for irradiation effects, the method is non-destruc- tive. Multiple analyses are therefore possible on a single sample. (v) There are usually no serious interferences. Although thorium interference is possible, the efficiency for thorium is only 51.37% of that for uranium, as determined experimentally for the same irradiation position used here, and this is generally not a problem (16). (vi) The counting equipment is relatively inexpensive, simple and easy to maintain.

ANALYTICAL TECHNIQUE

Analyses were conducted on multiple samples of each of the seventeen standards by measurement of the delayed neutrons emitted by fission products (primarily for 235U) after the samples had been irradiated at the face of the core with a flux of neutrons cm-2 sec-' in the research reactor located at the Nuclear Science Center at Texas A&M University.

The samples studied here were sent to us as powder (in one case small grains). Aliquots of this material were then transferred to small (-lcc) polyethylene vials for irradiation. Sample sizes ranged from -0.8 to -1.3 g. No special treatment nor handling was used for these standard rock samples; they were analysed as routine samples.

Gw%tmthrdr Nsmkttcr, Vol. 4, N o I, A v r i Z 1980, p . 5 a' 8

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The neutron counting system used eight " B-enriched, BF3 -filled counters which surround the sample. This system was built and described in detail by Riley (17). Samples are irradiated for sixty seconds, held for twenty seconds and counted for sixty seconds. Count rates naturally varied markedly for the different samples depen- ding upon the uranium contents and ranged from back-ground levels for four of the rock standards to -17,500 counts for the Biotite-Mica-Fe. The uranium concentrations were determined by compa- rison with primary standards (New Brunswick Laboratory) and secondary standards (Columbia River Basalt, BCR-1, and Lujavrite, NIM-L of the National Institute of Metallurgy, Johannesburg. South Africa) which were previously calibrated by comparison with primary National Bureau of Standards glass standards and Corning Glass glass standards.

Our system was tested previously by measure- ment of various rock standards and it was thus demonstrated that serious systematic errors are unlikely (Figure 1). The average relative devia- tion of our results from the 'accepted' or 'preferred' values was only *6.9% (18-19).

ppm Uranium ('accepted')

F igure 1. Canparison o f the uranium contents i n s i x standard rocks; values from t h i s system (19) versus "accepted" values (22-24). The cross-hat- ched area represents our estimated unce r ta in t y of one standard dev ia t i on . The e r r o r bars on the ou t l y ing JG-1 p o i n t a re the standard dev ia t ions on the average o f our data (19) and t h a t canpi led by Amdo e t a l . ( 22 )

RESULTS

Table 1 shows our measurements of the uranium contents of the seventeen French rock reference samples; Of those, four a m below o w limit of detection which we estimate to bes0.2 ppm uranium. The uncertainties shown on our analyses represent our estimate of the standard deviation involved in the measurement. The standard deviation of 20.09 ppm uranium was estimated by pooling the data between 0.5 and 3 ppm. The staadard deviation thus estimated agrees with the average deviation of +-6.9% noted between results from our system and the accepted values (18,19). The standard deviations listed on the other determinations in Table 1 are calculated in the usual way. Calculation of the standard deviation for the data for uranium between 0.5 and 3 ppm referred to earlier were always less than or equal to the standard deviation calcula- ted with the pooled data.

Table 1. Uranium contents o f CRPG standard rock samples by delayed-neutron a c t i v a t i o n ana lys is

SAHPLE No. of measurements M P P )

Basalt BR

Glauconite GL-0 p d e r

Glauconite GL-0 grains

Dior i te DR-N

feldspath FK-N

Mica-Fe

Serpentine US-N

Anorthosite AN-G

Granite MA-N

Granite GA

Bauxite BX-N

Mica I%

Etalon-traces VS-N

Granite GH

Basalt BE-N

Granite GS-N

Disthene DT-N

3

6

6

3

3

3

3

3

3

5

3

2

5

3

3

3

5

2.15t0.09

0.78t0.09

0.74t0.09

1.4020.09

50.2

50.2

20.2

75.921.1

11.7t0.6

4.9t0.3

7.6tO. 3

20.2

0.50t0.09

17.3tO. 4

2.27t0.09

7.520.3

I . 72t0.09

Table 2 shows a comparison of our analyses with previous determinations on the CRPG standard rock samples. In no case are there data with sufficient agreement so that an acceptable recommended value could be assigned. In general, our data appear to differ least from thwe of N.H. Gale. This is perhaps not surprising since both Gale and we used delayed-neutron activation analysis. Nevertheless, the differences found between our values and Gale's are fairly large, being 19%, 13% and 12% for basalt BR. granite GA

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and granite GH, respectively. Obviously many more determinations are needed on the uranium in these standards before meaningful conclusions can be drawn with respect to the choice of a recommended value.

Table 2 . Uranium i n CRPG rock standards: canpari- son o f our r e s u l t s v ia delayed-neutron ac t iva t ion analysis with other researchers using various methods

- ~ _ _ SAMPLE ppm U( th is work) ppm U analyst method* reference

Basalt BR 2.15+0.09 0.9 G. Branche CFM ( 5 ) 1 Z. Votara DFX ( 5 ) 7 . 6 5 N.H. Gale EMN ( 5 ) 3.15 G. Dei Fiore , CMN (21)

Granite GS-N

Granite GA

7.5fO.j

4.gt0.3

J.M. Peters, I . Roelandts

4.1 H. Z a f f r e r i e , CMN (4) J . L. Joron

10 G . Kaplan and N . Bouchet ESM

1 . 7 G. Branche CFM (5) ( 5 ) 2 H. Agr in ie r CCP

4.26 N.H. Gale EMN 4.86 J . Morel CFF!

Granite G! 17.3~0.4 10 H. Agr in ie r CCP 12 G. Branche CFM 14.2 J. Morel CFM 19.71 N.H. Gale EMN

B i o t i t e Mica-Fe 75.911.1 35 G. Branche CFM 40 H. Agr in ie r CCP 57 G . Kaplan ESM 94 A . cornu, ESM (21 )

J . Brun

+We use here the method symbolism of Roubault et aZ. (1) in which the first letters refer to the method of preparation while the second and third letters refer to the method of determi- nation. Thus, the first letters: C=Dissolution and separation; D=Simple mixing with buffers; E=Simple physical conditioning of samples and the second and third letters: FM=Fluorimetry; FX=X-ray fluorescence; MN=Nuclear methods; SM=Mass spectrometry; CP=Paper chromatography; under this system the symbol for delayed neutron activation analysis would be EMN as would y-ray spectrometry and instrumental neu- tron activation analysis.

ACKNOWLEDGEMENTS

We are grateful to the staff at the Nuclear Science Center of Texas A&M University for their cooperation in providing the necessary irradia- tions and for their assistance in maintaining the counting equipment. We acknowledge support for the Office of University Research and the Center for Energy and Mineral Resources, both of Texas A&M University. The assistance of M. Hyman is gratefully acknowledged.

RESUME

La teneur en uranium dans t r e i z e gchantil- ions gfiochimiques de reference provenant d u Centre de Recherches Petrographiques e t Geochimi- ques, Nancy, France a ete determinee par l ' ana- lyse par ac t iva t ion au "neutrons r e t a rdes" .

Quatre a u t r e s echant i l lons de la mhe sowce ont ete Qalement analysBs mais leur teneur se trou- vent en dessous de la limite du dosage ( 0.2 ppn) . I1 s lavere d i f f i c i l e d e cmparer nos rCsul- t a t s avec ceux publihs car ce s derniers sont peu abondants e t sont fortement disperses.

REFERENCES

(1) M. Roubault, H. de la Roche and L. Oovindaraju (1970) Etat actuel (1970) des Ltudea coopdratives sur les standards g6ochimiques du Centre de Recherches PdtrOgra- phiques et G€ochimiques, Sciences de la Tern. XV (4): 351-393.

12) H. de la Roche and K. Govindaraju (1973) Etude coopdrative our un verre synwdtique V6-N prOpOS6 comme €talon analytique par le dosage des Cldments en traces dans les silicates, Analusis. 2: 59-70.

( 3 ) H. de la Roche and K. Govindaraju (1973) Rapport (1972) sur quatre standards g6ochimigues de 1'Association Nationale de la Recherche Technique: diorite DR-N. serpentine UB-N. bauxite' BX-N and disthhe DT-N. Extrait du Bulletin de la Socihtb Franqaise CCralque. 100: 49-75.

( 4 ) H . de la Roche and K . Govindaraju (1976) Nouveaux 6talons gCochimiques: granite GS-N and feldspath FK-N, Analusis, 4: 347-372.

(5) K. Govindaraju and H. de la Roche (1977) Rapport (1966-1976) sur les blCments en traces 4ans trois roches standards ghchimiques du C.R.P.G.: basalt BR and granites GA et GH. Geostandards Newsletter, 1: 67-100.

(6) N.W. Echo and E.H. Turk (1957) Quantitative determination of "'U by delayed neutron counting, TID-7531.

(7) F.F. Dyer and G.W. Leddicotte (1960) Analytical applications of delayed neutron counting, 4th Conference on Analytical Chemistry of Nuclear Reactor Technology, Cattlinburg. TID-7606.

( 8 ) F.F. Dyer, J.F. Emery and C.Y. Leddicotte (1962) A comprehensive study of the neutron activation analysis of uranium by delayed neutron counting, ORNL-3342: 71.

(9) S. Amiel (1962) Analytical applications of delayed neutron emission in fissionable elements, Analytical Chemistry: 1683-1692.

(10) F. Cavallari, M. Terrani and 5. Terrani (1970) The analysis of fissile nuclide mixturss by delayed neutron emission, Nuclear Instrumentation Methods. 79: 69-76.

(11) E.I. Hamilton (1966) The determination of uranium in rocks and minelrals by the delayed neutron method. Earth and Planetary Science Letters, 1: 77-81.

(12) N.H. Gale (1967) Development of the delayed neutron technique as s rapid and precise method for the determinat€on of uranium and thorium at trace levels in rocks and minerals with applications to isotope geochronology, in Radioactive Dating and Methods of Low Level Counting, International Atomic Energy Agency. Vienm: 431-452.

(13) P. Henderson, A. Mackinnon and N.H. Gale (1971) The distribution of uranium in some basic igneous cumulates and its petrological significance. Gemhimica et Cosmochimica Acta. 35: 917-925.

(14) S. Amiel, J . Gilat and D. Heymann (1967) Uranium content of chondrites by thermal neutron activa- tion and delayed neutron counting, Ceochimica et Cosmochi- mica Acta, 31: 1499-1503.

(15) T. Mo. A.D. Suttle and W.M. Sackett (1973) Uranium concentrations in marine sediments, Geochimica et Cosmochimica Acta. 37: 35-51.

(16) M?P. Ferretca. R . Macedo, V.'.Coat6.: 3.H: Reynoldtl'. J.E. R I M Y . J F . and, M . L : R o w ;(1975) Rare-gas dating. I I.., Attqmptep uranium-helium dating of young volcanic pocks from the Madeira hhhibelago. Ea'kh and Pbnetciry Stiedce. Ldtter%:'25: 142-150:

(17) J.E. Riley Jr. (1971) A system tor the determination of uranium in meteorites by delayed-neutron counting, 49 pp.. Masters Thesis. Depart- ment of Chemistry, Texas A W University.

! l H ) Y.W. Rowe and J.W. Herndon (1976) Uranium in rock standards JG-1 and JE-1, Geochemical Journsl, 10: 163-164.

(19) M.W. Roue and J.M. Herndon (1976) Uraclum in NTMRCC standard igneous rock samples, Geochemi- cal Journal. .10:"219-221.

(20) G. Del Fiore. J.M. Peters and 1. Roelandts (1974) Precision determination of the U in rocks and minerals by neutron activation analysis us4ng 1331 fission products, Chemical Geology, 13: 309-319.

(21) A . Cornu and J.C.5 B n m (1968) Application da 11 spoctrors6trie de mause B btincelles P certainti problbes g6ologiquas. Colloques Nationaux du C.N.R.S., No. 923. Nancy, 4-6 d k m b r e 1968: 215-223.

(22) A. Ando, H. Kuraaaua. I. Dhmori and E . Takeda (1974) 1974 compilation of data on the GSJ geochemical reference samples J G 1 granodloslte and JB-1 basalt. Ceochmnlcel JOUI'M~, 8 : 175-192.

(23) F.J. F l a ~ ~ a @ n (1973) 1972 values for international geochemical reference samples, Gemhimica et Coalnochimica Acta. 38: 1731-1744.

(24) S. Abbey (1978) U.S.G.S. I1 revislted, Geostandards Newsletter, 2: 141-146.