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Quantification of uranium, thorium and gadolinium spectral interferences in instrumental neutron activation analysis of samarium
Chemical Geology, 62 (1987) 223-226 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
223
[31
QUANTIFICATION OF URANIUM, THORIUM AND GADOLINIUM SPECTRAL INTERFERENCES IN INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS OF SAMARIUM S. L A N D S B E R G E R * a n d A. S I M S O N S Nuclear Reactor, McMaster University, Hamilton, Ont. LSS 4K1 (Canada)
(Received July 20, 1986; revised and accepted August 14, 1986)
Abstract Landsberger, S. and Simsons, A., 1987. Quantification of uranium, thorium and gadolinium spectral interferences in instrumental neutron activation analysis of samarium. Chem. Geol., 62: 223-226. The spectral interferences arising from U, Th and Gd were determined for the analysis of Sm by instrumental neutron activation analysis (INAA). Due to the different half-lives of the respective radio-isotopes used in INAA the degree of quantification is expressed in apparent ppm of Sm per ppm of U, Th or Gd. A compilation of twelve geological standards revealed that errors on the Sm concentrations determined by INAA may vary from 0.6% to 81% after a 7day decay period. These errors increase from 2% to 139% after a 2-week decay period. Although the dominant error is associated with the U/Sm ratio, high concentrations of Th and Gd may also increase the error of Sm analysis significantly. This is particularly true when Sm concentrations are at low levels.
1. I n t r o d u c t i o n S a m a r i u m is one of t h e m o s t sensitive t r a c e e l e m e n t s t h a t can be d e t e r m i n e d b y n e u t r o n a c t i v a t i o n analysis ( N A A ) w i t h d e t e c t i o n limits at the ng g-1 level. T h i s is p r i m a r i l y due to: (1) s a m a r i u m ' s relatively large n e u t r o n crosssection; (2) the fairly a b u n d a n t stable isotope 152Sm used in i n s t r u m e n t a l n e u t r o n a c t i v a t i o n analysis ( I N A A ) ; (3) a large b r a n c h i n g ratio for t h e 103.2-KeV y - r a y used; a n d (4) m o s t yray d e t e c t o r s have t h e i r highest efficiency a r o u n d this p a r t of t h e e n e r g y s p e c t r u m . H o w ever, t h e analysis o f S m m a y suffer f r o m signif*Present address and to whom all correspondence should be sent: Nuclear Engineering Laboratory, Department of Nuclear Engineering, University of Illinois, 103 South Goodwin Avenue, Urbana, IL 61801, U.S.A.
i c a n t spectral i n t e r f e r e n c e s arising f r o m t h e p r e s e n c e of U a n d to a lesser e x t e n t T h a n d Gd. Few a u t h o r s have p o i n t e d out t h e i n t e r f e r e n c e s while m a n y m o r e have p u b l i s h e d results without m e n t i o n i n g w h e t h e r t h e s e i n t e r f e r e n c e s were t a k e n into account. H e n d e r s o n a n d P a n k h u r s t (1984) have discussed t h e p o t e n t i a l inaccuracies in S m analysis arising f r o m U, G d a n d T h . B a r n e s a n d G o r t o n (1984) r e p o r t e d correct i o n factors for T h a n d U, a s s u m i n g t h e decay t i m e was 7 days while Ila et al. (1983) suggested t h a t the U a n d T h i n t e r f e r e n c e s could be evalu a t e d using a c o m b i n a t i o n o f i n t e n s i t i e s a n d efficiencies. Considering that the concentrations and r e s p e c t i v e half-lives of Sm, U, T h a n d G d v a r y f r o m e a c h o t h e r b y several o r d e r s of m a g n i t u d e t h e p o t e n t i a l for m i s c a l c u l a t i o n of S m c o n c e n -
224 trations in geological materials can be severe. This is even more important when one then uses the results to interpret chondrite-normalized abundances of rare-earth elements. Germani et al. (1980) found a 30% greater value for Sm when comparing the results from INAA to prompt-gamma NAA (a technique essentially interference free for Sm analysis). As part of a major program to determine uranium fission interferences in NAA (Landsberger, 1986), we have extended our research to examine closely the potential interferences to Sm. In this respect we have quantified the degree of interference in terms of apparent ppm concentration of Sm per ppm of U, T h or Gd over a decay period between 7 and 11 days after the end of irradiation. A compilation study of twelve geological reference materials reveals that errors associated with NAA of Sm ranged from 0.6% to 81% after a 7-day decay. 2. N u c l e a r c h a r a c t e r i s t i c s The usual method for NAA determination of Sm is the use of the 152Sm (n,y) 153Sm reaction employing the 103.2-KeV 7-ray from 153Eu observed in the fl decay of 15~Sm (tl/2 =46.7 hr). The first main interference arises from the characteristic 103.7-KeV K X-ray produced in the 2:'8U (n,7) 239U (tl/2 =23.5 min.) -~239Np (tl/2 = 2.35 days) sequence. A second interference comes from the 232Th (n,y) 233Th (tl/2=22.2 min.)--*233Pa (tl/2=27.0 days) sequence. The 233Pa isotope decays with a plethora of y-rays, twenty-five in all, including a 103.9-KeV y-ray which has an intensity of ~ 2% relative to the strongest decay line at 311.9 KeV. The last interfering reaction is 152Gd (n,y) 153Gd (tl/2=246.1 days). This radioactive isotope decays by//÷ emission to l~3Eu giving the same 103.2-KeV gamma-ray as in the f l - decay of 153Sm. It is therefore clear that the analysis of Sm by neutron activation can be fraught with interferences and reliable results may be severely hampered due to high concentrations of U, T h and Gd. However, the ratios of these
latter three elements to Sm are the deciding factors in the degree of interference. 3. E x p e r i m e n t a l section Natural purified uranium dioxide along with atomic absorption standard solutions of Sm, Th and Gd were prepared and heat-sealed in 1.5cm 3 polyethylene vials. All samples were prepared in triplicate and irradiated for 1 hr. at a nominal flux of 8" 1012 n cm '~ s 1. Spatial variations in neutron flux ( _+10% ) were very carefully monitored using Co wires. After a decay period of almost 7 days the samples were counted using an Ortec ® 12% efficient hyperpure Ge detector possessing a resolution of 1.75 KeV at the 1332-KeV 6°Co peak. Samples were subsequently counted at ~ 24-hr. intervals up to the eleventh day of decay. Counting periods were relatively short (10 min. or less per sample) but were sufficient to achieve good statistics. Errors associated with the counting regimes were between 2% and 5%. 4. R e s u l t s and discussion The results for the degree of interference are described as apparent ppm of Sm per ppm of U, Th, or Gd in two ways. A visual interpretation is shown in Fig. 1. With these numbers the reader can extrapolate for longer or shorter decay periods, depending on the experimental design. Error symbols have been enlarged for convenience. Clearly when there are equal concentrations of U, Th and Gd the dominating interference is from U. However, such is not usually the case for geological materials. In Table I, a matrix expressing apparent Sm concentration vs. U, Th or Gd concentration for a 7-, 10- and 14-day decay time is shown. These decay times have been arbitrarily chosen and are evaluated from the straight line fit from Fig. 1. To emphasize the gross errors that can be associated with INAA of Sm, we have compiled the elemental concentrations of Sm, U, T h and
225
APPARENT SAMARIUM CONCENTRATION O.10 PER PPM OF U, Th OR Gd
days and then to 14 days the error increases accordingly. The presence of high levels of U (e.g., SY-3) is not the deciding factor for the per cent error. It is the ratios of U/SIn, T h / S m and G d / S m that ultimately determine the error. Although Sm, U and Th can be routinely determined by INAA the analysis of Gd is some what more difficult since one must wait several months for the e33pa ( tl/2-= 27.0 days) isotope to decay sufficiently for reliable Gd analysis. Gd can also be determined by prompt-gamma neutron activation analysis ( Gladney et al., 1985 ) ; however, very few such facilities exist for routine analysis of Gd. Thus for most researchers the interference from Gd has to be estimated. Although the interference is small (0.0013 ppm of Sm per ppm of Gd at a 7-day decay time),
D,. Q.
Y
Z
_o I.-If: I-Z LLI
Z 0.05 O O rO OU OTh
I,,Z w
• Gd
a.
0.01 6
7
8
9
10
11
DAYS AFTER END OF IRRADIATION
Fig. 1. A p p a r e n t S m concentration per p p m of U, T h or Gd. TABLE I
Gd for twelve international geological reference materials (Govindaraju, 1984) along with the associated per cent error when the interferences are not taken into account. The results are shown in Table II. As can be seen after a 7day decay period the per cent error ranges from 0.6% for BHVO-1 to a very large 81% for SG1A. As the decay time increases from 7 to 10
A p p a r e n t p p m of S m per p p m of U, T h and Gd
Sm
U Th Gd
td = 7 days
ta = 10 days
td=
14 days
0,061 0.0018 0,0013
0.075 0.0048 0.0045
0.093 0.0088 0.0087
T A B L E II Per cent error on the analysis of S m due to presence of U, T h a n d Gd Reference material
U (ppm)
Th (ppm)
Gd (ppm)
Sm (ppm)
Type
% Error ( t ~ = 7 days)
FK-N STM-1 NIM-L BVHO-1 RGM-I BCR-1 G-2 GSP-1 Sy-3
SG-IA GSD-2 NIM-S
0.15 9.1 14 0.4 5.8 1.7 2.1 2.2 650 63 17 0.6
0.4 31 66 1.0 15 6.1 25 105 990 120 70 1
10 6.0 3.7 6.6 4.1 13 55 7 9.5 -
0.07 13 5 6.1 4.3 6.6 7.2 26.8 100 5 10.8 1
feldspar syenite lujavrite basalt rhyolite basalt granite granodiorite syenite albitized granite stream sediment syenite
14 5 19 0.6 9 2 2.5 1 41 81 11 4
( Q = 10 days) 19 7 27
( Q = 14 days) 25 9 37
1
2
12 3 4 3 53 106 15 5
16 4 6 5 69 139 21 6
226
unusually enriched Gd concentrations may yield falsely high Sm concentrations. Therefore, for some samples it may be imperative that a reliable Gd result be obtained by methods other than INAA. However, a recent study of rareearth elements in thirty-seven geochemical reference materials (Gladney et al., 1985) has shown that Gd is never more than twice the concentration of Sm. It therefore can be concluded that for most rocks the interference arising from Gd can be assumed to be negligible.
5. Conclusions We have demonstrated that the analysis of Sm by instrumental neutron activation analysis is sensitive to the presence of U, Th and Gd. A compilation of twelve international standard reference materials has shown that the per cent error of INAA for Sm depends on the actual ratios of U, Th and Gd to Sm and on delay time to counting. These errors can vary from 0.6% to 81% (or even higher for other geological materials) for a 7-day decay and subsequently increase for longer decay times. A quantification of apparent ppm level of Sm per ppm of U, Th and Gd has been evaluated. Gd may have to be determined by other chemical methods such as isotope dilution. The reasons being that a very long decay time or the use of a promptgamma system are not feasible. However, both U and Th should be determined simultaneously with Sm in each geological sample as to properly correct for the interferences.
Acknowledgements We would like to thank E.L. Hoffman, G. Kajrys, D. Shaw, and M. Truscott for critical reading of the manuscript.
References Barnes, S.J. and Gorton, M.P., 1984. Trace element analysis by neutron activation analysis with a low flux reactor (SLOWPOKE-II): results for international reference rocks. Geostand. News., 9:17-23. Germani, M.S., Gokman, I., Sigleo, A.C., Kowalcyzk, G.S., Olmez, I., Small, A.M., Anderson, D.L., Failey, M.P., Gulovali, M.C., Choquette, C.E., Lepel, E.A., Gordon, G.E. and Zoller, W.H., 1980. Concentrations of elements in the National Bureau of Standards bituminous and subbituminous coal standard and reference materials. Anal. Chem., 52: 240-244. Gladney, E.S., Curtis, D.B. and Perrin, D.R., 1985. Determination of selected rare earth elements in 37 international geochemical reference materials by instrumental thermal neutron capture gamma-ray spectroscopy. Geostand. News., 9: 25-30. Govindaraju, K., 1984. 1984 compilation of working values and sample description for 170 international reference samples of mainly silicate rocks and minerals. Geostand. News., Spec. Iss., 7: 3-16. Henderson, P. and Pankhurst, R.J., 1984. Analytical chemistry. In: P. Henderson (Editor), Rare Earth Element Geochemistry, Ch. 13. Elsevier, Amsterdam, pp. 467-499. Ila, P., Jagam, P. and Muecke, G.K., 1983. Multielemental analysis of uraniferous rocks by INAA: special reference to interferences due to uranium and fission of uranium. J. Radioanal. Chem.,. 79: 215-232. Landsberger, S., 1986. Spectral interferences from uranium fission in neutron activation analysis. Chem. Geol., 57: 415-421.
223
[31
QUANTIFICATION OF URANIUM, THORIUM AND GADOLINIUM SPECTRAL INTERFERENCES IN INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS OF SAMARIUM S. L A N D S B E R G E R * a n d A. S I M S O N S Nuclear Reactor, McMaster University, Hamilton, Ont. LSS 4K1 (Canada)
(Received July 20, 1986; revised and accepted August 14, 1986)
Abstract Landsberger, S. and Simsons, A., 1987. Quantification of uranium, thorium and gadolinium spectral interferences in instrumental neutron activation analysis of samarium. Chem. Geol., 62: 223-226. The spectral interferences arising from U, Th and Gd were determined for the analysis of Sm by instrumental neutron activation analysis (INAA). Due to the different half-lives of the respective radio-isotopes used in INAA the degree of quantification is expressed in apparent ppm of Sm per ppm of U, Th or Gd. A compilation of twelve geological standards revealed that errors on the Sm concentrations determined by INAA may vary from 0.6% to 81% after a 7day decay period. These errors increase from 2% to 139% after a 2-week decay period. Although the dominant error is associated with the U/Sm ratio, high concentrations of Th and Gd may also increase the error of Sm analysis significantly. This is particularly true when Sm concentrations are at low levels.
1. I n t r o d u c t i o n S a m a r i u m is one of t h e m o s t sensitive t r a c e e l e m e n t s t h a t can be d e t e r m i n e d b y n e u t r o n a c t i v a t i o n analysis ( N A A ) w i t h d e t e c t i o n limits at the ng g-1 level. T h i s is p r i m a r i l y due to: (1) s a m a r i u m ' s relatively large n e u t r o n crosssection; (2) the fairly a b u n d a n t stable isotope 152Sm used in i n s t r u m e n t a l n e u t r o n a c t i v a t i o n analysis ( I N A A ) ; (3) a large b r a n c h i n g ratio for t h e 103.2-KeV y - r a y used; a n d (4) m o s t yray d e t e c t o r s have t h e i r highest efficiency a r o u n d this p a r t of t h e e n e r g y s p e c t r u m . H o w ever, t h e analysis o f S m m a y suffer f r o m signif*Present address and to whom all correspondence should be sent: Nuclear Engineering Laboratory, Department of Nuclear Engineering, University of Illinois, 103 South Goodwin Avenue, Urbana, IL 61801, U.S.A.
i c a n t spectral i n t e r f e r e n c e s arising f r o m t h e p r e s e n c e of U a n d to a lesser e x t e n t T h a n d Gd. Few a u t h o r s have p o i n t e d out t h e i n t e r f e r e n c e s while m a n y m o r e have p u b l i s h e d results without m e n t i o n i n g w h e t h e r t h e s e i n t e r f e r e n c e s were t a k e n into account. H e n d e r s o n a n d P a n k h u r s t (1984) have discussed t h e p o t e n t i a l inaccuracies in S m analysis arising f r o m U, G d a n d T h . B a r n e s a n d G o r t o n (1984) r e p o r t e d correct i o n factors for T h a n d U, a s s u m i n g t h e decay t i m e was 7 days while Ila et al. (1983) suggested t h a t the U a n d T h i n t e r f e r e n c e s could be evalu a t e d using a c o m b i n a t i o n o f i n t e n s i t i e s a n d efficiencies. Considering that the concentrations and r e s p e c t i v e half-lives of Sm, U, T h a n d G d v a r y f r o m e a c h o t h e r b y several o r d e r s of m a g n i t u d e t h e p o t e n t i a l for m i s c a l c u l a t i o n of S m c o n c e n -
224 trations in geological materials can be severe. This is even more important when one then uses the results to interpret chondrite-normalized abundances of rare-earth elements. Germani et al. (1980) found a 30% greater value for Sm when comparing the results from INAA to prompt-gamma NAA (a technique essentially interference free for Sm analysis). As part of a major program to determine uranium fission interferences in NAA (Landsberger, 1986), we have extended our research to examine closely the potential interferences to Sm. In this respect we have quantified the degree of interference in terms of apparent ppm concentration of Sm per ppm of U, T h or Gd over a decay period between 7 and 11 days after the end of irradiation. A compilation study of twelve geological reference materials reveals that errors associated with NAA of Sm ranged from 0.6% to 81% after a 7-day decay. 2. N u c l e a r c h a r a c t e r i s t i c s The usual method for NAA determination of Sm is the use of the 152Sm (n,y) 153Sm reaction employing the 103.2-KeV 7-ray from 153Eu observed in the fl decay of 15~Sm (tl/2 =46.7 hr). The first main interference arises from the characteristic 103.7-KeV K X-ray produced in the 2:'8U (n,7) 239U (tl/2 =23.5 min.) -~239Np (tl/2 = 2.35 days) sequence. A second interference comes from the 232Th (n,y) 233Th (tl/2=22.2 min.)--*233Pa (tl/2=27.0 days) sequence. The 233Pa isotope decays with a plethora of y-rays, twenty-five in all, including a 103.9-KeV y-ray which has an intensity of ~ 2% relative to the strongest decay line at 311.9 KeV. The last interfering reaction is 152Gd (n,y) 153Gd (tl/2=246.1 days). This radioactive isotope decays by//÷ emission to l~3Eu giving the same 103.2-KeV gamma-ray as in the f l - decay of 153Sm. It is therefore clear that the analysis of Sm by neutron activation can be fraught with interferences and reliable results may be severely hampered due to high concentrations of U, T h and Gd. However, the ratios of these
latter three elements to Sm are the deciding factors in the degree of interference. 3. E x p e r i m e n t a l section Natural purified uranium dioxide along with atomic absorption standard solutions of Sm, Th and Gd were prepared and heat-sealed in 1.5cm 3 polyethylene vials. All samples were prepared in triplicate and irradiated for 1 hr. at a nominal flux of 8" 1012 n cm '~ s 1. Spatial variations in neutron flux ( _+10% ) were very carefully monitored using Co wires. After a decay period of almost 7 days the samples were counted using an Ortec ® 12% efficient hyperpure Ge detector possessing a resolution of 1.75 KeV at the 1332-KeV 6°Co peak. Samples were subsequently counted at ~ 24-hr. intervals up to the eleventh day of decay. Counting periods were relatively short (10 min. or less per sample) but were sufficient to achieve good statistics. Errors associated with the counting regimes were between 2% and 5%. 4. R e s u l t s and discussion The results for the degree of interference are described as apparent ppm of Sm per ppm of U, Th, or Gd in two ways. A visual interpretation is shown in Fig. 1. With these numbers the reader can extrapolate for longer or shorter decay periods, depending on the experimental design. Error symbols have been enlarged for convenience. Clearly when there are equal concentrations of U, Th and Gd the dominating interference is from U. However, such is not usually the case for geological materials. In Table I, a matrix expressing apparent Sm concentration vs. U, Th or Gd concentration for a 7-, 10- and 14-day decay time is shown. These decay times have been arbitrarily chosen and are evaluated from the straight line fit from Fig. 1. To emphasize the gross errors that can be associated with INAA of Sm, we have compiled the elemental concentrations of Sm, U, T h and
225
APPARENT SAMARIUM CONCENTRATION O.10 PER PPM OF U, Th OR Gd
days and then to 14 days the error increases accordingly. The presence of high levels of U (e.g., SY-3) is not the deciding factor for the per cent error. It is the ratios of U/SIn, T h / S m and G d / S m that ultimately determine the error. Although Sm, U and Th can be routinely determined by INAA the analysis of Gd is some what more difficult since one must wait several months for the e33pa ( tl/2-= 27.0 days) isotope to decay sufficiently for reliable Gd analysis. Gd can also be determined by prompt-gamma neutron activation analysis ( Gladney et al., 1985 ) ; however, very few such facilities exist for routine analysis of Gd. Thus for most researchers the interference from Gd has to be estimated. Although the interference is small (0.0013 ppm of Sm per ppm of Gd at a 7-day decay time),
D,. Q.
Y
Z
_o I.-If: I-Z LLI
Z 0.05 O O rO OU OTh
I,,Z w
• Gd
a.
0.01 6
7
8
9
10
11
DAYS AFTER END OF IRRADIATION
Fig. 1. A p p a r e n t S m concentration per p p m of U, T h or Gd. TABLE I
Gd for twelve international geological reference materials (Govindaraju, 1984) along with the associated per cent error when the interferences are not taken into account. The results are shown in Table II. As can be seen after a 7day decay period the per cent error ranges from 0.6% for BHVO-1 to a very large 81% for SG1A. As the decay time increases from 7 to 10
A p p a r e n t p p m of S m per p p m of U, T h and Gd
Sm
U Th Gd
td = 7 days
ta = 10 days
td=
14 days
0,061 0.0018 0,0013
0.075 0.0048 0.0045
0.093 0.0088 0.0087
T A B L E II Per cent error on the analysis of S m due to presence of U, T h a n d Gd Reference material
U (ppm)
Th (ppm)
Gd (ppm)
Sm (ppm)
Type
% Error ( t ~ = 7 days)
FK-N STM-1 NIM-L BVHO-1 RGM-I BCR-1 G-2 GSP-1 Sy-3
SG-IA GSD-2 NIM-S
0.15 9.1 14 0.4 5.8 1.7 2.1 2.2 650 63 17 0.6
0.4 31 66 1.0 15 6.1 25 105 990 120 70 1
10 6.0 3.7 6.6 4.1 13 55 7 9.5 -
0.07 13 5 6.1 4.3 6.6 7.2 26.8 100 5 10.8 1
feldspar syenite lujavrite basalt rhyolite basalt granite granodiorite syenite albitized granite stream sediment syenite
14 5 19 0.6 9 2 2.5 1 41 81 11 4
( Q = 10 days) 19 7 27
( Q = 14 days) 25 9 37
1
2
12 3 4 3 53 106 15 5
16 4 6 5 69 139 21 6
226
unusually enriched Gd concentrations may yield falsely high Sm concentrations. Therefore, for some samples it may be imperative that a reliable Gd result be obtained by methods other than INAA. However, a recent study of rareearth elements in thirty-seven geochemical reference materials (Gladney et al., 1985) has shown that Gd is never more than twice the concentration of Sm. It therefore can be concluded that for most rocks the interference arising from Gd can be assumed to be negligible.
5. Conclusions We have demonstrated that the analysis of Sm by instrumental neutron activation analysis is sensitive to the presence of U, Th and Gd. A compilation of twelve international standard reference materials has shown that the per cent error of INAA for Sm depends on the actual ratios of U, Th and Gd to Sm and on delay time to counting. These errors can vary from 0.6% to 81% (or even higher for other geological materials) for a 7-day decay and subsequently increase for longer decay times. A quantification of apparent ppm level of Sm per ppm of U, Th and Gd has been evaluated. Gd may have to be determined by other chemical methods such as isotope dilution. The reasons being that a very long decay time or the use of a promptgamma system are not feasible. However, both U and Th should be determined simultaneously with Sm in each geological sample as to properly correct for the interferences.
Acknowledgements We would like to thank E.L. Hoffman, G. Kajrys, D. Shaw, and M. Truscott for critical reading of the manuscript.
References Barnes, S.J. and Gorton, M.P., 1984. Trace element analysis by neutron activation analysis with a low flux reactor (SLOWPOKE-II): results for international reference rocks. Geostand. News., 9:17-23. Germani, M.S., Gokman, I., Sigleo, A.C., Kowalcyzk, G.S., Olmez, I., Small, A.M., Anderson, D.L., Failey, M.P., Gulovali, M.C., Choquette, C.E., Lepel, E.A., Gordon, G.E. and Zoller, W.H., 1980. Concentrations of elements in the National Bureau of Standards bituminous and subbituminous coal standard and reference materials. Anal. Chem., 52: 240-244. Gladney, E.S., Curtis, D.B. and Perrin, D.R., 1985. Determination of selected rare earth elements in 37 international geochemical reference materials by instrumental thermal neutron capture gamma-ray spectroscopy. Geostand. News., 9: 25-30. Govindaraju, K., 1984. 1984 compilation of working values and sample description for 170 international reference samples of mainly silicate rocks and minerals. Geostand. News., Spec. Iss., 7: 3-16. Henderson, P. and Pankhurst, R.J., 1984. Analytical chemistry. In: P. Henderson (Editor), Rare Earth Element Geochemistry, Ch. 13. Elsevier, Amsterdam, pp. 467-499. Ila, P., Jagam, P. and Muecke, G.K., 1983. Multielemental analysis of uraniferous rocks by INAA: special reference to interferences due to uranium and fission of uranium. J. Radioanal. Chem.,. 79: 215-232. Landsberger, S., 1986. Spectral interferences from uranium fission in neutron activation analysis. Chem. Geol., 57: 415-421.