comparison of routine inaa procedures based on k0 and kzn standardizations

Journal o f Radioanalytical and Nuclear Chemistry, Articles, Vol. 152, No. 2 (1991) 507-518

C O M P A R I S O N O F R O U T I N E I N A A P R O C E D U R E S B A S E D

O N ko A N D k z n S T A N D A R D I Z A T I O N S

I. OBRUSNIK,* M. BLAAUW, P. BODE

lnterfaculty Reactor Institute, Delft University o f Technology, Delft 'The Netherlands)

(Received February 19, 1991)

Performance characteristics (especially accuracy) of a routine INAA with k0 standardization were verified and eompaxed with those of INAA with a "e!assieal" single ~ompaxator. For this purpose, samples of three certified reference materials of environmental origin (Fly Ash, Orchard Leaves and Buffalo River Sediment - all supplied by NIST) were irradiated with both kinds of compaxatorS (Au-Zr for k 0 and Zn for classical k method) in one irradiation rabbit. Also the following steps of INAA procedure were practically the same for both standardization methods used (counting, spectral processing, etc.). The results have shown that the k0 method gives sufficiently accurate results comparable with those of the well established and routinely used single comparator (Zn) method, provided proper neutron flux monitoring, efficiency calibration and also Coincidence summing corrections axe applied. This work shows that modem k 0 standardization method in INAA can be sucess- fully used in routihe practice and applied with an advantage in INAA laboratories subject to changes Of neutron spectra or counting conditions,

Introduction

The ko standardization method introduced by SIMONITS and DECORTE 1 has

become available, after many recent refinements, for use in routine INAA. Results

of analyses o f various materials obtained by this method have been reported and

discussed by several l abora to r ies . : - s It is remarkable that the k0 method has not yet

spread in INAA laboratories to the extent which could be expected from the advantages

o f this method. One of the reasons m~ght be that the k0 method suffers from a com-

plicated image because of , the necessary calibrations and computat ions prior to routine

implementat ion o f the method. However, most INAA laboratories have nowadays

access to computer systems, and the necessary data - the ko factors, have been re-

cently published in an upda ted form. 6,~

In the INAA laboratory at IRI Delft, INAA is routinely performed on basis o f the

single comparator method, using Zn as a comparator element (the system has been

*On leave from Nuclear Physics Institute, Czechoslovak Academy of Sciences, l~e~., Czecho- slovakia.

Elsevier Sequoia S. A., Lausanne Akad~miai Kiad6, Budapest

I. OBRUSNIK et al.: COMPARISON OF ROUTINE INAA PROCEDURES

extensively described elsewhere s,9). The single comparator method h.as produced suf-

ficiently accurate results in many applications of INAA. It is well known that this method of standardization is only well applicable as long as reactor parameters do not change between the time of calibration and the time of analysis.

In the forthcoming 5 years the neutron spectrum in the irradiation facilities is expected to change regularly because of the gradual conversion of highly enriched to low enriched fuel. As a result, some of the Zn calibration factors have to be regularly updated and checked, and a time-consuming re-calibration period cannot be excluded. It is obvious that under such circumstances, the use of the ko method may have distinct advantages, as the neutron spectrum parameters can rather simply be determined.

The g0al of this work has been to check the accuracy of the "new" standardization of a routine INAA with k 0 factors and to compare it with that of the "older" kzn method. For this purpose, samples of three typical certified reference materials (CRMs) have been analyzed with the use of both kinds of comparators (Zn and Zr-Au, respectively) in such a way that the samples have been irradiated together in one irradiation rabbit. Moreover, the results for both standardization methods have been calculated from the same gamma-ray spectra and evaluated by the same program ICPEAX (peak processing) 1~ to avoid any bias caused by different irradiation and counting conditions. Our aim has not been an achievement of the lowest detection limits or the highest accuracy but to compare both methods under day-to-day routine experimental conditions.

Practical aspects of the k 0 method

In the case of the ko method a procedure employing Au-Zr comparators 1~ ,12 has been followed. The samples are coirradiated with Au and Zr comparators and the induced 198Au ' 95Zr and 97Zr activities are counted on an efficiency calibrated

detector. This so-called bare triple-monitor method for ko standardization results in an in-situ determination of parameters f (thermal to epithermal neutron flux ratio) and a (a measure of the non-ideal epithermal neutron flux distribution).

Preliminary experiments (neutron flux mapping of the irradiation rabbit) and detector calibrations (full.energy peak detection efficiency, peak-to-total ratio, etc.) have been carried out before the comparison of both standardization methods. The neutron flux mapping has been carried out with a set of Zr-Au comparators-

monitors. This experiment has shown the distribution of neutron flux parameters f and cr over the length and diameter of an irradiation rabbit - the results of the mapping experiment will be published separately) 3

508

I. OBRUSN][K et al.: COMPARISON OI~ ROUTINE INAA PROCEDURES

The efficiency curves for the detector to sample distances used (4 and 10 cm) have been obtained by means of a set of calibration standards. The geometry of 10 cm (point source) has been used as a "reference" one 12 while a 4 cm distance has been used for measurements of CRM samples. The a and f values have been calculated by means of an iterative procedure using the results of Au-Zr comparator-monitor

measurements) 4 Corrections for true coincidence losses, self-shielding for Zr foil

and different sample shapes have been applied. The k0 factor data were taken from the literature. 6,7

The concentration Cx of an element x in the sample is determined by counting the photons at the 3,-line (xi) emitted by the isotope formed, and can be calculated by using k0 standardization as follows:

Cx = [Asp(Xi)/Asp(Au)ko,Au(Xi)][f+Qo,Au(Ot)/f+Qo,x(Ot)][ep,Au/ep(Xi)] (1)

where Asp is the specific count rate at the end of irradiation, calculated from the peak area, irradiation, decay and measuring times, half-life of the isotope formed and weights of comparator and sample. The ep(Xi) and ep,Au are the full-energy peak detection efficiencies for the measured 7-lines of the relevant nuclide and the related Au comparator (411.8 keV), respectively. The ko,Au(Xi) value is an experimentally measured composite nuclear constant defined as follows:

MAu Ox trO,x ')'x k0,A u(Xi) = (2)

Mx 'OAu O'O,Au~Au

where M - atomic mass,

O - isotopic abundance, Oo - 2200 m �9 s -1 (n, 7) cross section,

- absolute 7-intensity of the line measured. Qo(a) denotes the experimentally measured ratio of the resonance integral to the

thermal cross-section corrected by means of the a parameter (for the method of cal- culation see References 15, 16).

Experimental Samples

The certified reference materials NIST SRM-1633a - Coal Fly Ash, SRM-2704 - Buffalo River Sediment and SRM-1571 - Orchard Leaves have been used for the comparison of both INAA procedures.

509

I. OBRUSN][K et al.: COMPARISON OF ROUTINE INAA PROCEDURES

Preparation of samples

The samples of CRMs, each containing approximately 100 mg (SRM-1633a) or 150 mg (SRM-2704 and SRM-1571) of CRM have been prepared for the analysis (drying, handling, etc.) with the recommended precautions 17-a 9 and then heat-sealed in poly- ethylene irradiation capsules (i.d. 9 mm, length 9 mm).

Compamtors

Zn comparators containing 2.0 mg of Zn have been prepared by pipetting of 20/11 aliquots of a standard Zn solution on small filter paper discs. Then, the discs have

been dried and heat-sealed in small 5 mm long irradiation capules. Z r -Au comparators have been prepared by careful pipetting of a standard Au solution on filter paper discs (2.012/~g Au), similarly as in the case of Zn comparators. A weighed small Zr disc containing 25 mg of Zr prepared from an ultrapure 125 ~ thin Zr foil (Goodfellow Metals) has been added to each Au monitor and fixed on the bottom of the capsule

by a piece of a clean filter paper before sealing.

Irradiation

8 samples of CRIb, eight Au-Zr and eight Zn comparators-monitors together with

3 blank PE capsules have been placed into each irradiation rabbit. This configuration ensured that for each sample a defined pair of Au-Zr and Zn comparators existed. The irradiation has been carried out in file BP3 facility of HOR reactor for 4 hours. The characteristics of this facility are (average values): thermal neutron flux 3.9 �9 10 a 2

n �9 em -2 �9 s ~1 , thermal to epithermal neutron flux ratio f = 46.4+4 and the deviation from the ideal 1/E law a = 0.04+0.004. Though the standard deviations in f and

are relatively high, their contributions to the error in the elemental concentrations found are usually smaller than 2% relative.

Counting

All measurements have been made with a calibrated Ortec Ge(Li) coaxial detector (relative efficiency 16%, FWHM 1.75 keV and peak to Compton ratio 47:1 - all measured for 1332.4 keV photons of 6~ A standard distance 4 cm from the detector A1 end cap has been used for both CRM samples and Zn and Au-Zr comparators. Standard Ortec linear electronics (preamplifier, shaping amplifier) have

been connected through a TN1242 100 MHz ADC (Tracor Northern) to the 4096 channel memory buffer of a CAMAC interface. The gamma-ray spectral data obtained have been transferred to a PDP 11/44 computer (DEC) and evaluated by the program

510

I. OBRUSNfK et al.: COMPARISON OF ROUTINE INAA PROCEDURES

ICPEAX.1 o The dead time and pile-up losses have been corrected by means of the pulser method. 2 o An automatic sample changer employing a pneumatic tube transport into detector lead shield has been used for all measurements.

1NAA procedure

The samples and comparators in eacb INAA run have been counted twice after 3 - 5 days decay [t(meas) = 60 min, first count] and after 20--25 days [t(meas) = 120 min, second count]. The longer counting time, 180 minutes, has been employed in the second measurement of biological samples of SRM-1571 Orchard Leaves. The comparators have been measured usually for 20 minutes in the same geometry as the samples (Au-Zr comparators have been measured only once after 3 days decay).

Results and discussion

A simple variant of the k 0 method has been used in this study (before a routine use) - usually only a single 3,-line of each nuclide has been chosen for a quantitative determination of elemental concentration (Table 1). A few corrections for spectral interference contributions have also been applied in the case of Cu (contribution of 24Na and 6SZn to 511 keV annihilation peak) and Se (contribution of 182Ta to the

264.6 keV line). The content of Mo has been corrected for the contribution of U(n, 0 reaction using an experimentally determined factor characteristic for the irradiatioff facility in use.

Table 1 also shows the type of measurement (first or second) and whether the corrections for coincidence summing and spectral interferences have been applied. The coincidence summing correction factors (COI) for the counting geometry of 4 cm have been calculated in a similar way as in Reference 12. The corrections for sample shape ('Tilling height" of the capsules) were determined experimentally.

The results obtained by the Zn comparator method have been calculated by the program ICPEAX as weighted averages from several 3,-lines of particular nuclides corrected for interferences 1 o and sample shape (filling height).

Tables 2, 3, 4 and 5 summarize the results obtained by both standardization methods for three CRMs investigated. The last table contains the results classified into three groups with bias intervals of 0-5%, 5-10% and over 10% relative for all three reference materials; bias has been calculated as the difference between certified (or consensus) and experimentally found values.

It can be seen from the results that there is practically no difference between the accuracy (bias) of two methods of the INAA standardization (differences can be found

511

I. OBRUSN]~K et al.: COMPARISON OF ROUTINE INAA PROCEDURES

Table 1

Radionuclides and gamma-ray energies used in INAA of CRMs with k o standardization

Element. Nuclide E-/, keV Measurement Remarks

As ~ ~ As 559.1 a Preferred line, COI 6 As 657.1 a COI

Ba 131Ba 123.8 a , b COI

Bt 82 Br 776.5 a COI

Ce 141Ce 145.4 b

Co 60 Co 1173.2 b Both lines used, COI 60Co 1332.4 b Both lines used, COI

Cr s I Cr 320.1 b Correction for blank

Cs i s 4 Cs 795.8 b COI

Cu 64 Cu 511.0 a Correction for interf.

Eu 1 s ~ Eu 1408 b COi

Fe s 9 Fe 1099.2 b Both lines used s ~ Fo 1291.6 b Both lines used

Ga ~ 2 Ga 629.9 a Preferred Line

Hg 203 Hg 279.2 b Corrected for interf.

Hf a 81 Hf 482.2 b COI

K 42K 1524.7 a

La a 40La 487.0 a COI

14 o La 1596.5 a Preferred line, COI

Lu 1 ~ 7Lu 208.4 a, b

Me 99mTc 140.5 a Corrected for interf.

Na 24Na 1368.6 a COl

Rb 86 Rb 1076.6 b

Sb 122 Sb 564.1 a

:24Sb 1691.0 b CO1

Sc 46 Sc 889.3 b COI

Se 7 SSe 264.7 b Correct. for inter., COl

Sm 153 Sm 103.2 a

Ta 182 Ta 1221.4 b COl

Tb a 6 o Tb 298.6 b CO1

Th 23apa 312.0 b

U 239Np 228.1 a Preferred (BRS) 239Np 277.6 a Preferred (CFA)

512

I. OBRUSNIK et al.: COMPARISON OF 'ROUTINE INAA PROCEDURES

Table 1 (cont 'd)

Element Nuclide E% keV Measurement Remarks

W l ~ *W 685.7 a

Yb 1 7 s y b 396.3 a, b

Zn 65Zn 1115.5 b 6 9 m Z n 438.6 a

Preferred nuclide

a, b - first and second measurement , respectively. BRS - Buffalo River Sediment , C F A . . . Coal Fly Ash. CO1 - corrected for coincidence summing.

in about 5% of results investigated in accordance with statistics). Also the precision of the ko results is comparable with that of the kzn method.

However, a small number of results show a non-negligible bias (usually by both methods) as can be seen from Table 5. It could by mostly explained by quite com- mon difficulties, such as interference corrections (Cu in fly ash), possible losses during irradiation (Br in Buf. Riv. Sed.), difficult resolution of spectral shapes (Zn in fly ash- strong influence of the very intense 1120.5 keV line of 46 Sc on the rather weak 1115.5 keV line of 65Zn) or by a very low counting statistics (Se in Bur. R.iv. Sed. etc.). Some other biases found, like for W and Hf in.SRM-1571, can be explained by the use of a consensus values with a rather low level of confidence (relatively rare elements detectable in environmental matrices with difficulties and also calculated from a limited amount of data (see Ref. 21) for the bias estimation. Small biases found like for U in SRM-1633a (ko-method) and Na in SRM.2704 (both methods) will need a further study for an explanation.

Eu and Lu give biased results only by the ko-method (concentrations found for Lu were so biased that they were not included in the tables). These biases can be explained by the low accuracy of available k0 factors (see Refs 6, 7, 12); the values of

k0 factors for these elemer~ts are not recommended. The results show that even a simple variant of the ko-method tested allows an

accurate (with rather low bias) determination of most of the elements in all three materials: As, Fe, K, Th and also Co, Cr, Cs, La, Sb, Sc, Sm, Ba, Na and U (U is below the detection limit in Orchard Leaves). These elements occur in environmental matrices investigated at concentrations higher than a determination limit and all sources of bias like interferences, etc., can be easily corrected. Small biases found for several elements have been explained and could be diminished to a negligible level by means of refinement of the k0 procedure used (more sophisticated corrections for interferences, coincidence summing and shapes of samples, etc.), by an opti-

513


Table 2 CovaP~SOn of Xesults obtained by INAA with k o and kZn standardization fox SRM 1633a Coal Fly Ash

Concentration, ppm Difference, % Element

k0 kZn Cert. (cons) Cert-ko Cert-kzn

As 144.2 + 1.9 144,3 -+ 2.6 145 -+ 15 +0.55 +0.48 Ba 1345 • 97 1323 ,+ 85 1420 +_ 100" +5.3 +6.8 Br 1.99• 0.17 2.00• 0.25 [2,31 Ce 170 • 3 174 ,+ 3.3 1.75 • 7* +2.9 +0.6 Co 43.9 • 0.6 47.2 • 0.9 43 ,+ 3* -2.1 -9 .8 Cr 1 9 8 • 4.8 198 • i .0 196 • 6 -1 .0 -1 .0 Cs 10.0 • 0.24 10.0 + 1.0 10.5 ,+ 0.7* +4.8 +4.8 Cu 102 ,+ 6.7 66 ,+ 7.3 118 • 3 +13,6 +44 Eu 1.9 • 0.05 3.6 • 0.08 3.7 ,+ 0.2* +48.6 +2.7 Fe 92420 ,+1400 92660 • 94000 • +1.7 +1.4 Ga 53.9 • 5.6 60 • 12.7 56 • 3* +3.8 -7.1 Hf 6.83+_ 0.13 7.66-+ 0.26 7.4 ,+ 0.3* +7.7 -3.5 K 19360 ,+ 220 17980 • 360 18800 • 600 -3 .0 +4.4 La 83.2 ,+ 1.1 75.4 • 1.3 84 • 8* +1.0 +10.2 Lu - 1,.19,+ 0.06 1.12• 0.18 -6.3 Me 35.0 • 3.5 31.6 • 4 30 • 3* -16~7 -5 .3 Na 1800 • 21 1740 ,+ 30 1700 +- 100 -5.9, -2 .4 Rb 132 • 13 122 • 11 131 ,+ 2* -0 .8 +6.9 Sb 6.87-+ 0.19 6.02-+ I}.6 6.8 • 0.4 -1 .0 +3.2 Se 38.5-+ 0.6 39.3 • 0.7 39 ,+ 3* +1.3 -0.8 Se 10.3 +- 1.1 8.03• 1.0 I0.3 ,+ 0.6 +0.0 +19.1 Sm 17.2 ,+ 0.21 15.8 • 0.22 17.0 • 1.5" -1 .2 +7.1 Ta 1.59• 0.14 1.70_+ 0.13 2 . 0 • 0.2* +20 -I-15 Tb 2.35-+ 0.06 2.31,+ 0.17 2.5 ,+ (~.3" +6.0 +7.6 Th 24.3-+ 0.7 24.6 ,+ 0.5 2.4.7 ,+ 0.3 +1.2 +0.4 U 9.47• 0.4 9.87• 0.28 10.2 ,+ 0.1 +7.2 +3.2 W 6~19,+ 0.4 5.09• 0.3 5.7 • 0.7* -8 .6 +10.7 Yb 8.0 • 0.4 6.9 -+ 0.25 7.4 • 0.7* -8.1 +6.8 Zn 262 • 7.1 256 • 6.3 220 • 10" - 1 9 -16

Remarks: Experimental results are arithmetic moans + one standard deviation (8 samples analyzed). Certified and consensus values ( ,) show uncertainties (NIST) and standard deviations, respectively (see GLADNEY 2 a ).

m iza t ion o f exper imenta l cond i t i ons for the e lements o f in teres t 22 and also by an

improved qual i ty assurance sys tem ta i lored for tile rout ine use o f the k o - m e t h o d

u n d e r the exper imenta l cond i t i ons at IRI,

514


Table 3 Comparison of results obtained by INAA with k 0 and kZn standardization

for SRM 2704 Buffalo River Sediment

Concentration, ppm Difference, % Element

ko kZn Cert. (cons) Cert-ko Cert-kZn

As 22.3 5 1.8 22.3 • 0.9 23.4 • 0.8 +4.7 +4.7 Ba 413 5 29 384 5 99 414.0 5 12 +0.2 +7.2 Br 5.735 0.37 5.56• 0.35 [7] +18.1 +20.6 Ce 64.9 5 3 63.3 • 6.3 [72] +9.9 +12.1 Co 13.8 • 0.64 14.8 5 1.1 14.0 5 0.6 +1.4 -5 .7 Cr 143 • 8.1 136 5 i i 135 5 5 -5 .6 - 0 . 7 Cs 5.535 0.25 5,685 0.43 [6] +7.8 +5.3 Cu 98.5 5 13.5 80.8 5 7.5 98.6 5 5.0 +0.1 + t 8 Eu 0.75• 0.t 1.34• 0.11 [1.3] +44 -3 .1 Fe 40650 • 40380 52430. 41100 51000 +1.1 +1.8 Ga - 12.7 5 1.2 [15] +15.3 Hf 7.68• 0.54 8.41• 0.89 181 +4 -5 .1 K 20160 • 420 19500 5 800 20000 • 400 -0 .8 +2.5 La 30.3 5 0.6 29.3 5 1 . 2 [29] -4 .5 -1 .0 Lu - 0,51• 0.04 [0.6] +15 Mo 4.0 + 1.5 4.6 • 1.9 - Na 5960 • 140 6000 5 290 5470 5 140 -9 .0 -9 .7 Rb 104 5 6.8 95.2 5 9.2 [1001 -4 .0 +4.8 Sb 3.53• 0.27 3.595 0.34 3.79• 0.15 +6.9 +5.3 Sc 11.7 5 0.33 11.8 • 0:69 [12] +2.5 +1.7 Se 1.7 • 0.6 1.245 0.48 [1,!1 -54.5 -12 .7 Sm 6.06• 0.15 5.445 0.91 [6.7] +9.6 +18.8 Ta 0.945 0.08 0.93• 0.08 - Tb 0.815 0.04 0.815 0.05 - Th 9.105 0.32 9.02• 0.54 [9.21 +1.1 +'2.0 U 3.205 0.25 3.055 0.28 3.135 0.13 -~ .2 +2.6 W 1.785 0.14 1.79• 0.27 - Yb 3.13• 0.09 2.535 0.19 [2.81 -11 .8 +9.6 Zn 457 5 8.5 447 • 20 438 5 12 -4 .3 -2 .1

Remarks: Experimental results are arithmetic means + one standard deviation (8 samples analyzed), Certified values show uncertainties, nonc~rtified values (for information only) are without uncertainties.

Conclusions

The resul ts o f th i s w o r k show t h a t t he ko s t a n d a r d i z a t i o n m e t h o d gives very

p romis ing resul ts for a r ou t i ne app l i ca t ion in ou r and p r o b a b l y in m a n y o t h e r N A A

labora to r ies . The m e t h o d is r a t h e r s imple and can be easily a d a p t e d to possible changes

o f n e u t r o n s p e c t r u m pa rame te r s . The m o s t i m p o r t a n t s teps are a p r o p e r ca l ib ra t ion

515


Table 4 Comparison of results obtained by INAA with k 0 and kZn standardization

for SRM 1571 Orchard Leaves

Concentration, ppm Difference, %

Element k o kZn Cert (cons) Cert-k o Cert-kZn

As 11.0 +- 0.3 11.8 -* 0.4 10 +- 2 ~10 -18 Ba 39.3 +- 12.8 50.0 +_ i0;4 43 -* 4* +8.6 -16.3 Br 8.73 +- 0.19 9 . t l +- 0.25 9.5 +- i .1" +8.1 +4.1 Ce 0.80 +- 0.12 0.85 ~ 0.t4 0.99 • 0.12" +19.2 +14.1 Co 0,15 -* 0.03 0.17 -+ 0.03 O.16 -* 0.037* -6 .3 +6.3 Cr 2.82 *- 0,32 2,86 + 0.3.4 2.6 +- 0.3 -8.5 ~10 Cu 14.0 -+ 0.32 14.3 +- 0.43 12 +- 1 -16.7 -19.2 Eu 0.021+- 0.005 0.029 + _ 0.012 0.024+- 0.003* +12.5 -20.8 Fe 286 -+ 50 309 ~ 48 300 + 20 +4.7 -:3.0 Hf 0.055~: 0.03 0.064-* 0.03 0,030+- 0.005* -83 -113 K 1.4390 +-350 14320 -*340 14700 +-300 +2.1 +2.6 La 1.09 ~ - 0 . 0 3 1.05-~ 0.03 1.17 • 0.11" +6.8 +10.3 Na 85.4 -* 5,6 88,0 +- 6,6 82 +- 6 -4.1 -7.3 Rb 10.3 +- 1.5 10.0 -+ 1,5 12 -* ! +14.2 -16.7 Sb 2.70 +- 0d7 2.80 -* 0.3 2.9 -* 0.3 +6.9 +3.4 Sc 0.068-* 0.007 0.075-* 0.007 0.063-* 0.014" -7.9 -19 Sm 0.108_+ 0.004 0.111+_ 0.004 0.i14-* 0.02 * +5.3 +3.0 Th 0.063-* 0.0I 0.067_+ 0.012 0.064-* 0.006 +1.6 -4.7 W 0.059+- 0.015 0.056 + _ 0.012 0.030+- 0.020 ~ -97 -87 Zn 22;9 + - 1.5 23.8 -+ 1.6 25 + 3 +8.4 +4.8

Remarks: Experimental results-are arithmetic means • one standard deviation (8 samples analyzed). Certified and consensus values (*) show uncertainties (NIST) and standard deviations, respectively (see

GLADNEY21 ).

o f the detectors (eff ic iency, peak- to- tota l ratio), careful prepara t ion o f A u - Z r com-

parators and also their reasonable space dis t r ibut ion in a rabbit to cover the range o f

possible changes o f neu t ron flux parameters . Then, the f and ~ values can be found

af ter some i terative calculat ion using the results o f the A u - Z r compara to r measure-

ments . In principle, only minor changes are needed for subst i tu t ion o f "c lass ical" k -method

for the ko -me thod in the final de te rmina t ion o f e lementa l concent ra t ions . Of course,

all wel l -known precaut ions and care c o m m o n in m o d e r n N A A (good qual i ty assurance

sys tem) are essential for obta in ing accurate results. Sensit ivi ty can be main ta ined or

drnproved by using closer sample to detector-dis tances , provided that p roper correct ions

for coincidence summing are employed . Even at the distance o f 4 cm and the no t t o o

large Ge de tec tor used in this work such correct ions are essential.

516


Table 5 Comparison of bias levels of results obtained by INAA with ko and kZn standardization

Bias range CRM Elements

k 0 method kZn method

0-5 % Fly Ash As__ 2, Ce, Co, Cr._._, Cs, Fe.__, Ga, K, As__ Ce, Cr, Cs, Eu, Fe.., Hf, K, Na, La, Rb, Sb, Sc, Se, Sm, Th Sb_... z, So, Th__..~, U2

5-10% Fly Ash Ba, Hf, Na....~, Tb, U__.~, W, Yb Ba, Co, Ga, Lu, Mo, ~ Sm, Tb, Yb

> 10% Fly Ash Cu___~, Eu, Ta, Zn Cu_._z La, Se_.. 2, Ta, W, Z__nn

0-5% B.R. Sed. As, Ba, CO, Cu, Fe, Hf, K__ 2, La,. As._2, Cr_~, Eu, Fe, K, La, Rb, Se, Th, Rb, Sc, Th, U U__ z Zn

5-10% B.R. Sed. Cr._.~, Cs, N_aa, Sb__.~, Sin, Co, Yb Ba, Co, Cs, Hf, Na, Sb

> 10% B.R. Sed. Br, Se, Yb Br, Ce, Ca__ z, Ga, Lu, Se, Sm

0-5% O. Leaves Fe, K, Na, Th Br, Fe, K, Sb, Sm, Th, Zn

5-10% O. Leaves As__ z, Ba, Br, Co, Cr__. 2, La, S_bb, Sc, Co, Cr, Na Sm, Z__nn

> 10% O. Leaves Ce, Cu__z Eu, Hf (?),.Rb, W (?) As__, Ba, Ce, C'U___z, Eu, HI" (?), La, R_~bW (?)

Remarks: Xx Means elements with certified concentration in. CRM, (?) means elements having large uncertainties of certified or consensus values.

This work has shown that the change from a "classical" single compara tor me thod

wi th Zn moni tors to a more m o d e r n and versatile me thod wi th k0 factors - no t so

sensitive to changes o f exper imental parameters (neu t ron spectrum, detectors) - is

feasible wi thou t any decrease of qual i ty (especially accuracy) of results.

One of the authors (I. O.) is greatly indebted to Delft University of Technology and to Inter- faculty Reactor Institute for the providing of a fellowship.

References

1. A. SIMONITS, F. DE CORTE, J. HOSTE, J. Radioanal. Chem., 60 (1980) 461. 2. LIN XILEI, F. DE CORTE, L. MOENS, A. SIMONITS, J. HOS'I'E, J. Radioanal. Nuel. Chem.,

81 (1984) 333.

517

I. OBRUSNfK et al.: COMPARISON OF ROUTINE INAA PROCEDURES

3. F. DE CORTE, A. DEMETER, LIN XILEI, L. MOENS, A. SIMONITS, A. DE WISPELAERE, J: HOSTE, Isotopenpmxis, 20 (1984)223.

4.. A. DEMETER, Anal. Chim. Acta, 186 (1986) 195. 5. M. C. FREITAS, E. MARTINHO, Anal. China. Aeta, 219 (1989) 317. 6. F. DE CORTE, A. SIMONITS, A. DE WISPELAERE, A. ELEK, J. Radioanal. Null. Chem.,

133 (1989) 3. 7. F. DE CORTE, A. SIMONITS, J. Radioanai. Nucl. Chem., 133 (1989) 43. 8. M. DE BRUIN, P. J. M. KORTHOVEN, Anal. Chem., 44 (1972) 2382. 9. M. DE BRUIN, P. J. M. KORTHOVEN, P. BODE, J. RadioanaL Chem., 70 (1982) 497.

10. P. J. M. KORTHOVEN, Report IRI-133-70-04, IRI Delft, 1970. 11. F. DE CORTE, L. MOENS, A. SIMONITS, A. DE WISPELAERE, J. HOSTE, J. Radloanal.

Chem., 52 (1979) 295. 12. F. DE CORTE, The k 0 Standardization Method - a Move to the Optimization of NAA, Thesis,

Rijksuniversiteit Ghent, 1987. 13. P. BODE, M. BLAAUW, I. OBRUSNIK, Variation of neutron flux and related parameters in an

irradiation container, in use with k o-based neutron activation analysis; to be published in J. Radioanal. Chem.

14. M. BLAAUW, P. BODE, M. DE BRUIN, An alternative convention describing the (n, ~/) reaction rate, suited for use in the k o-method; to be published in ft. Radioanal. Chem.

15. J. OP DE BEECK, J, Radioanal. Chem., 89 (1985) 169. 16. F. DE CORTE, A. SIMONITS, A. DE WISPELAERE, J. HOSTE, J. Radioanal. Nucl. Chem.,

113 (1987) 145. 17. Certificate of Analysis, Standard Reference Material 1633a, Trace Elements in Coal Fly Ash,

Office of Standard Reference Materials, NBS, Washington, D. C., 1979. 18. Certificate of Analysis, Standard Reference Material 1571, Orchard Leaves, Office of Standard

Reference Materials, NBS, Washington, D. C., 1976. 19. Certificate of Analysis, Standard Reference Material 2704, Buffalo River Sediment, Office of

Standard Reference Materials, NIST, Washington, D. C., 1988. 20. O. U. ANDERS, Nucl. Instr. Methods, 68 (1969) 205. 21. E. S. GLADNEY, B. T. O'MALLEY, I. ROELANDTS, T. E. GILLS, NBS Spec. Publ. 260-111,

Washington, 1989. 22. I. OBRUSN~K, K. ECKSCHLAGER, I. Radioanal. Nucl. Chem., 112 (1987) 233.

518

comparison of routine inaa procedures based on k0 and kzn standardizations

Documents