Rapid determination of matrix components by neutron activation

ACKNOWLEDGMENT

multiples and submultiples thereof] form a geometric sequence with ratio loll6. Similarly, ladders with g values of l o l l 3 , 101i4, erc. are readily constructed. Frustratingly, however, the preferred values do deviate significantly from being a perfect geometric sequence. Thus, to two significant figures, 1.0, 1.5, 2.2, 3.2, 4.6, 6.8, and 10 are the numbers which form a sequence with ratio

It is a pleasure to acknowledge the experimental assistance

of Marten G~~~~~~~ and the financial assistance of the N ~ tional Research council of Canada. RECEIVED for review July 12, 1972. Accepted August 22, 1972.

Rapid Determination of Matrix Components by Neutron Activation Analysis: The Analysis of Gold Alloys Michel Heurtebise, Francois Montoloy, and J. A. Lubkowitz Centro de Incestigaciones Tecnoldgicas, Instituto Venezolano de Inoestigaciones Cientificas, Apartado 1827, Caracas, Venezuela The possibility of analyzing the macroconstituents of a matrix by using neutron activation without utilizing any standards has been studied, provided all major constituents can be activated and detected. Equations have been derived that directly relate the concentration of each of the constituents to the ratio variations of the accumulated counts due to the activation of these elements present. The general theory has been successfully applied to the determination of gold alloy constituents. A relative standard deviation of 0.92, 3.3, and 0.91% has been obtained for gold, silver, and copper determinations, respectively. Average deviation of the results obtained by this method and atomic absorption was 0.58%.

ACTIVATION ANALYSIS is most commonly used in trace analysis. In this case, standards are generally prepared and irradiated simultaneously with the samples being determined. Alternatively, it is common practice to incorporate a reference material in the sample. This technique presents the advantage over the former method in that errors due to flux inhomogeneity are avoided. Differences due to counting geometry are also minimized. Nevertheless, this method is dependent on standard preparation. In an earlier paper, it has been shown how to eliminate errors due to counting geometry, flux inhomogeneity, and standard preparation, by utilizing a matrix component as a standard ( I ) . This technique can be used only in case of trace analysis when the quantitative composition of the macroconstituents is constant. Thus, this method is inadequate for the determination of macroconstituents which vary from sample to sample. Other techniques, for the analysis of macroconstituents in alloys, based upon isotopic dilution after activation have been described (2). Although this technique does not require standards, it has the disadvantage of requiring radiochemical separations which may not be necessary using conventional methods. This paper describes a novel technique for the rapid determination of the macroconstituents of a matrix. If after an appropriate irradiation and decay period of a qualitatively known matrix, the y-activities due to activation of each and every constituent can be observed, then the ratio (1) M. Heurtebise and J. A. Lubkowitz, ANAL.CHEM.,43, 1218 ( 197 1). (2) C. Capadona, “Modern Trends in Activation Analysis,” J. R.

De Voe, Ed., Nat. Bur. Stand. (US.)Spec. Publ., 312, Volume I, 574 (1 968).

of the accumulated counts of the different isotopes will be a function of the matrix composition. A study of the variations of the ratios of accumulated counts due to the activation of the elements present in the sample has permitted the development of a mathematical model that relates this ratio variation with composition. The principles developed in this work have been successfully applied to the analysis of gold alloys employed in the manufacture of coins and ornamental jewelry. Das and Zonderhuis (3)have considered the problems that occur in analyzing samples containing gold, silver, and copper, such as high cross sections and high sample density which cause flux depressions in and around the sample. In such cases, the comparison of the induced activity with standard foils of the pure metals yields incorrect results and corrections have to be made. The work described here shows that this method is independent of flux depression and self neutron shielding effects at least for a homogeneous matrix. Mixtures of gold and copper have also been analyzed previously by activation analysis ( 4 ) although no quantitative data have been given. THEORY

The following reactions are considered during the irradiation of a matrix constituted by two elements:

‘E (n,y) ‘+lE where the first element has a cross section ul,decay constant XI, isotopic abundance el, number of atoms N I . The induced activity immediately after irradiation is Aol. For the second element present in the matrix, the following reaction is considered :

yF (n,y) y+lF where this second element has a cross section u2, decay constant X2, isotopic abundance &, number of atoms N 2 . The induced activity immediately after irradiation is A,>. The ratio of the activities of ‘+lE and Y f l F immediately after irradiation is given by: A,, _ = u1 _ . _a . -Nl. [1 - e-”’’] (11 1 A,, u2 0 N2 where ti is the irradiation time.

___

(3) H. A. Das and J. Zonderhuis, Rec. Trav. Chim., 85, 837 (1966). (4) M. Okada and Y.Kamemoto, Nature, 197, 278 (1963).

ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973

47

o\o

3C

Figure 1. Efficiency curve of the 30 cm3 Ge(Li) detector

P, .’

0 = 11emIn 0 = mBr A = 78% A = calibrated sources

2

1x10-

*,

G

500

IO00

I500

2000

Energy, Kev

For small irradiation times (see note b in Table I) Equation l can be approximated to :

to the decay during the counting time t are given b y f i and The decay factorfis given by:

fi for both elements.

f =

t.X.(l

- e-’t)-l

(6)

Combining Equations 5 and 4 yields : The ratio of the activities can be expressed in terms of the respective weights of the elements in the matrix by the following equation: Each isotope can be characterized by a constant k, which is defined as : k

uxeI -

where MI and M2 are the atomic masses of the two elements and WI and W Zare the respective weights of the elements in the matrix. The activity ratio after a period of decay t d , which is defined as the elapsed time between the end of the irradiation and the beginning of the counting time, is represented by the following equation:

This constant k will be called “activation factor” since it represents the ease with which an element will be measured by gamma counting after activation with a neutron flux. Incorporating Equation 8 into Equation 7 and solving for the weight ratio of the two elements yields :

The activities A1 and Az are measured by counting the gamma transitions y1 and y2 which have absolute intensities I1 and 12. If the counting time is long relative to the halflife, a decay correction must be made. The ratio of the accumulated counts can now be expressed as:

In the case of similar samoles analyzed in series, td and f can kz f; e X l t d E? be kept constant and thus, K l v z = - . - . - . -- is conk , f i e’2td €1 stant. Consequently Equation 9 is written as:

-Ci_ -- .Ai- . - €1 G

fi

M

(5)

(10)

where C1and C2are the accumulated counts due to the gamma transitions y1 and yz,respectively. These gamma transitions are measured at efficiencies E , and e2. The corrections due

Suppose a matrix is composed of several elements 1,2,3,. . . i , I . . .j,J and whose composition is not known. For the sake of simplicity, let’s assume that two separate irradiations

Cz

48

Aa

€2

,

12 f;


Table I. Comparison of Experimentally and Calculated k Values Radionucleide formed by (n,y) reaction

y-photopeak Experimental k valueso Maximum irrad. timeb used (MeV) Calculated k values “V 2.95 x 10-4 2.99 i 0.09 x 10-4 31 s 1.433 “A1 4.37 x 10-6 4.33 zt 0.10 x 10-6 19 s 1.78 6eMn 1.79 x 10-5 Reference 21 rnin 0.835 1osAg 1.52 x 10-5 1.51 + 0.05 x 10-6 20 s 0.633 66CU 2.27 x 10-6 2.04 i 0.08 x 10-6 42 s 1.04 198Au 1.47 X 1.48 i 0.02 X 9 hr 0.412 ‘Ti 2.98 x 10-7 3.01 i 0.14 X lo-’ 48 s 0.322 24Na 2.95 x 10-7 2.99 i 0.12 x 10-7 2 . 1 hr 1.368 76As 1.97 x 10-7 2.03 -I: 0.09 x 10-7 3.7 hr 0.559 z7Mg 1.37 x 10-7 1.39 rt 0.08 X 10-7 79 s 0.842 126Sn 8.26 X 8.28 i 0.76 X 81 s 0.331 lz2Sb 6.04 x 10-8 6.52 rt 0.21 x 10-8 9 hr 0.561 lolMo 3.33 x 10-8 3.22 i 0.07 X 10-8 2 min 0.592 66Ni 4.20 x 10-9 4.17 i 0.23 x 10-9 21 min 1.49 69mZn 3.74 x 10-9 2.69 zt 0.12 x 10-9 2 hr 0.440 116Cdc 2.85 X lo-$ 2.80 zt 0.11 x 10-9 7.4 hr 0.335 60Co 2.62 x 10-9 2.60 f.0.07 X 10-9 ... 1.173 lCr 3.69 x 10-1O 3.84 =t0.31 X 10-lo 3.8 days 0.332 69Fe 7.08 X 10-l2 7.22 rt 0.36 x 10-12 6.3 days 1.097 All k values expressed relative to kasnin = 1.79 X 10-6. Cross sections are expressed in barns and half-lives in seconds, Irradiation selected so that [ A t , - (1 - e-Xti)][l - e-Xtt]-l -57& Measured after t d > 33 hr when equilibrium establishes that A I I S ~=~ 1.09 I ~ A116Cd.

will be sufficient to activate and count preselected gamma transitions of all constituents in the sample. In a short irradiation time, elements 1,2. . .i,Z are activated and after a decay period tal and a counting time 11, the ratio of counts due to this set of elements can be accurately determined. During a longer irradiation time of the same sample, elements 1 , 3 , ~. .j,J are activated and after a decay period f d P and a counting time t2 the ratio of counts due to this set of elements can also be accurately determined. It will also be assumed that all elements present in the sample give a measurable activity a t either of the two irradiations. Thus the sum of the masses of the constituents expressed in any units is given by

1

=

w1+

w2+

....

-w3 = - . c3 K3,l w 1

1

= w1+

C1’

w,

Solving for W 1yields the generalized expression:

wi+ w, + ws +

,

...

wj

+ WJ

(11)

In the first irradiation the reactions mA (n,y)m+lA yield a y1 whose C1 counts are a function of the weight W1. OB (n,y) g+lB which yields y ~ C2, , and Wz. Y (n,y) ‘ + l Y which yields yi,Ci, and Wi. Applying Equation 10 for the above reactions gives:

In the second irradiation, the reactions nA (n,y) ‘ + l A yields a yl’ whose C1’ counts are a function of the weight Wl. hC (n,y) h+lCwhich yields 7 3 , C,, and W 3 . *Z ( n , y ) * + l Zwhich yields y3’,C j ,and W j . Since a n element may have more than one stable isotope, irradiation by a neutron flux will produce different radioisotopes. However, the different gamma transitions could be referred, in both cases of short and long irradiations, t o the same weight Wl. Similarly one can write the following equations for the second irradiation:

Once W I has been determined, W2,.. . W a W , 3 .. . W , can be directly determined by the use of Equations 12-15. The same mathematical treatment can be applied to the case where elements C,. . . Z are long lived relative to elements A , B . . . Y . Thus after an appropriate decay and counting time elements A , B , . . . Y can be counted simultaneously. After a certain time has elapsed and without the need of a second irradiation, element A , B , . . . Y can be counted. However the Cl and Cl’ for element A will be different in the two counting experiments since a longer decay time has elapsed since the end of irradiation. In the case of gold alloys, the silver and copper can be measured by means of lo*Ag (t1/2: 2.4 min) and 66Cu (t1iz: 5.8 rnin). The favorable neutron cross sections of l97Au (f1/2: 2.7 days) would permit measuring these three radioisotopes after a td of 150 sec. In this case C1 = C1’ and thus Equation 17 is simplified as follows:

+

WAIJ = CAII[CALIC A . & A ~ , A-f~ C C ~ K C ~ , A (18) ~~-~ EXPERIMENTAL

A 30-cm3 Ge(Li) detector coupled to a 1024 channel pulse height analyzer was used in measuring the activities; since the ratio of counts of the two y-transitions due to different elements is proportional to the weight ratios, it is necessary to determine the proportionality constant K1.* which includes


49

Figure 2. Typical spectrum of a 16-karat gold alloy sample Irradiation time, 5 sec; decay time, 150 sec; counting time, 120 sec

the relative counting efficiencies of these two y-transitions. A rapid and precise method to establish the relative detector efficiency can be realized by utilizing point sources of *2Br, llsmIn, and 75Se obtained by neutron activation. The k values defined by Equation 8 were measured experimentally by using the k value of manganese as a reference. Mixed solutions of manganese and the element whose k value was to be determined were prepared. Aliquots of the solutions were placed on small disks of filter paper which were subsequently irradiated. The concentrations of these solutions were chosen so that after a preselected irradiation time, the activities could be determined conveniently. Then the k value of the chosen element was calculated by the use of Equation 9. In certain cases, due to differences in the halflife and cross-section it was not possible t o irradiate simultaneously hln and the element in question. Thus another element was chosen as standard and one whose k value was previously established. These k values, obtained by interpretation of experimental data, were compared with those obtained by simple and direct calculation using Equation 8. The gold alloy samples were laminated so that they would have a thickness of 0.14 mm. Disk-type samples were used for irradiation which had a weight of about 30 mg. After a n irradiation of 5-9 seconds at a thermal neutron flux of -10’2 n cm-2 sec-l, and after a n exact decay time of 1.50 sec, the sample was counted for a real time period of 120 sec which does not include a dead time correction. The reaction logAg(n,y)lloAghas a cross section of 89 barns and the formed isotope of 24.4 sec half-life emits a p- of 2.87 Mev and a gamma that cannot be used since its negligible intensity is not well known. A decay time of 150 sec is necessary to eliminate the greatest portion of the bremsstrahlung interference of lloAg so as t o reduce the dead time of the instrument. The laminated samples were placed 5 cm from the detector and a similar geometry was maintained as when counting the 82Br, llsrnIn, and 75Sesources. After counting, peak areas were determined by Covell’s method (5). Gold was measured by the 412 keV y-transition of 1g8Au, silver was measured by the 633 keV y-transition of lmAg, while copper was measured by the 1040 keV y-transition of Wu. -~ (5) D. F. Covell, ANAL.CHEM.,31, 1785 (1959). 50

ANALYTICAL CHEMISTRY, VOL. 45,

The weight of gold in the sample was calculated by using Equation 18 while the copper and silver content was calculated by the use of Equations 12 and 13. A classical NAA technique was also used to corroborate the results obtained by the mathematical development. On a plastic planchet, a gold standard was placed in the center and two identical samples were placed on each side of the gold standard. After an irradiation of 30 sec at a flux of -10’2 n cm-2 sec-1 and after a decay period of 2 or 3 days, the samples and the standard were counted with the Ge(Li) detector system. Then, the activities obtained for the two samples were summed and divided by the sum of the sample weight in order to obtain an average specific activity. This value was compared with the specific activity of the standard; thus the flux deviation error was compensated. Since the thickness of the standard was slightly different from that of the samples and since the gold content varied from sample to sample, it was necessary to use a neutron self shielding correction (6). A correction for y-absorption was also necessary (3). The copper and silver content was also determined by atomic absorption (7-9). About 20 mg of sample was dissolved in 3 ml of aqua regia. The mixture was heated to a slightly moist residue. The mixture was solution of KCN was added. then cooled and 3 ml of a The solution was then diluted to the required concentration for direct reading by atomic absorption.

.5z

RESULTS AND DISCUSSION Efficiency Curve of the Ge(Li) Detector. As shown in Equation 9, it is necessary to measure accurately the relative efficiency values and E ? . The calibration curve of relative efficiency us. energy determined by the use of 82Br, ‘lsrnIn, and 75Se sources, was further checked with additional ab(6) F. H. Helm, Nucl. Sci. Eizg., 16,235 (1963). (7) A. Strasheim, L. R. D. Butler, and E. C. Maskew, J . S. Afr. Inst. Mining Met., 62, 796 (1962). (8) V. C. D. Schuler, A. V. Jansen, and G. S. James, ibid., p 807. (9) “Analytical Methods for Atomic Absorption Spectrophotometry,” Perkin-Elmer Corporation, Norwalk, Conn., 1968, p Ag-6.

NO. 1, JANUARY 1973

solutely calibrated standard sources of T o , W r , I37Cs, 54Mn, 22Na, and 88Ywhich were measured in the same geometry used in counting the 82Br, 116mIn,and 75Se sources. Figure 1 shows that the relative points obtained with the prepared activated sources are in agreement with the points obtained with the absolutely calibrated sources; thus the rapid and accurate construction of relative efficiency us. energy curve can be obtained by the use of activated sources using recent reported values of intensity (10-12). With this calibration curve, it is easy to obtain the ratio of €1 and €2 in the energy range of interest for the elements in alloys. Determination of the Activation Factor, k . I n order to establish the weight composition of an element in a matrix, it is necessary to obtain the activation factors k , as shown in Equations 9 and 10. The k value as defined by Equation 8 can be calculated from literature data. However, due to numerous discrepancies in the literature, particularly in the gamma intensity values, it is more convenient to determine the “activation factor” k experimentally relative to the k value of a n element whose intensity, cross section, half-life, and isotopic abundance are well known. Manganese presents these characteristics. The calculated activation factor k for j j M n ( r ~ , y ) ~ ~ M is n1.79 X 10-j. The activation factor is not a dimensionless constant but for the sake of simplicity it is calculated by using the half-life in seconds and the cross section in barns. The experimentally determined k values are shown in Table I for nearly all elements found in metallic alloys for which this method is useful. The elements considered are formed by (n,y) reactions. The experimental values obtained are within the experimental error of the calculated values. The uncertainties of the k values given in Table I take into account the accumulative precision of the experimental measurements such as weighing errors, statistical errors of the counting, and the uncertainties of the points that define the detectors calibration curve. The good agreement between the experimental and calculated values implies reliability upon the calibration curve and the development of the mathematical model. In the case of T u , 1*2Sb, and 69rnZn,there are discrepancies between the k values which are probably due to errors in the reported values of the yintensities or cross sections. For these three elements, the k values were checked by utilizing different salts and varying the experimental conditions. Nevertheless, similar results were obtained. The y-transitions used in the measurement and calculation of the k values are also given in Table I. The mathematical treatment is based upon the approximation of Equation 2 which is valid for small irradiation times. Table I also shows the maximum permissible irradiation time allowed so that the error in the approximation of Equation 2 is less than 5 %. Analysis of Gold Alloys. A typical spectrum of a n irradiated 16-karat gold alloy sample is shown in Figure 2. The spectrum shows gamma transitions a t 412 keV, 633 keV, and 1040 keV due to I9*Au, IOSAg, and T u , respectively. The peaks are well defined. A strong bremsstrahlung due principally to IlOAg of 24-sec half-life is present along with its peak at 656 keV. The selected time of decay of 150 sec is a compromise between a high dead time due to IlOAg, that is near 50% a t the beginning of the counting, and the relationship between the peaks of lg8Au, loSAg,and 66Cu. Ef(10) C. R. Meredith and R. A. Meyer, Nucl. Phys., 142, 513 (1 970). (11) D. Rabenstein, Z . Phys., 240, 244 (1970). (12) T. S. Nagpol and R. E. Gaucher, Nucl. Instrum. Methods, 89, 311 (1970).

Table 11. Typical Analysis of a 10-Karat Gold Alloy Count Count ratio, Trial ratio, Cu/Au AglAu Au, Sample No. Ag, % cut 41.81 13.38 44.80 0.662 10K 1 0,259 41.44 12.78 45.77 0.638 10K 2 0.267 42.25 13.52 44.22 0.662 10 K 3 0.253 41.92 14.03 44.05 0.692 10 K 4 0.254 42.44 13.32 44.24 0.649 10K 5 0.252 41.97 13.41 44.61 Average 0.45 0.39 Standard deviation 0.41

z

z

fectively T u and Io8Ag have half-lives of 5.1 min and 2.3 min, respectively; consequently the measurement has to be done quickly to obtain well defined peaks. An example of a typical analysis is shown in Table 11. The value of K A ~ , has A ~ been found to be 0.4835 and for K c ~ , A4.137. ~, These k values take into account the decay prior to counting, the decay during counting, and the relative counting efficiency of these elements. Utilizing the counting ratios Cu/Au and Ag/Au shown in Table 11, in conjunction with Equations 12, 13, and 18 yields the concentration of Au, Ag, and Cu. Table I1 shows the results of analyzing the same sample five times. Relative standard deviations of 0.92 %, 3.3 %, and 0.91 are obtained for Au, Ag, and Cu, respectively. These values indicate good reproducibility of the method. It is noted that in classical activation analysis, it is difficult to obtain a standard deviation below 4 %. The standard deviation values reported here could not be obtained in cases where the method is influenced by flux depressions and flux inhomogeneities, which is the usual case of classical techniques in NAA . As the concentration of silver increases, the irradiation time must be lowered to avoid large formations of lioAgwhich yields a large bremsstrahlung effect. Thus, the samples present, after a decay period of 150 sec, varying activities which are reflected in a varying instrumental dead time. It must be remembered that the mathematical model implies a constant counting time. Since the method is based upon obtaining the relative counts of the sample components, the ratio is independent of the dead time variation during counting and it is also independent of dead time differences from sample to sample. The results in Table I1 obtained for replicate samples but with different dead time corroborate this fact. Table I11 shows a comparison of the analytical results obtained by analyzing gold alloys by three different approaches. The gold content in the alloys obtained by the mathematical model is compared to the results obtained by using neutron activation analysis based upon standard comparison. The average deviation between these two methods is 0.58 %. This is a good indication of the accuracy of the procedure. Furthermore, there is a n excellent agreement between the per cent Ag and Cu obtained by atomic absorption. Since these are two different techniques, the good agreement is also a n indication of the accuracy and reliability of the method developed in this paper. It is also to be pointed out that the sum of the results obtained by using neutron activation analysis and atomic absorption is close to 100%. The uncertainties given in Table I11 are the standard deviations. The standard deviation of the neutron activation analysis method based upon standard comparison is somewhat lower than in conventional neutron activation analysis since the method is based upon irradiating a standard which is placed


51

Table 111. Comparison of the Analysis of Gold Alloys Samole nominal gold content in karats

Mathematical treatment and NAA

10 12 14 16 18 20 22

41.97 f 0.39 46.56 + 0.55 61.13 f 1.40 62.62 f 0 . 9 4 73.46 f 0.72 79.18 f 0.86 87.63 f 0 . 3 8

~

-

_

Au, _ Z _

Standard comparison, NAA 40.79 46.64 60.53 62.64 72.80 79.02 88.14

Ag,

-

f 0.34 f 0.34 f 0.68 f 0.81 f 0.90 =t 0.49 f 0.36

Mathematical treatment and NAA 13.41 f 0.45 10.39 =t 0.61 16.00 f 0.27 12.79 f 0.27 16.33 f 0.34 9.34 f 0.94 0.16 =t0.04

between two identical samples. The comparison of a n average specific activity of the samples with the specific activity of the standard diminishes some of the effects due to flux depression and flux inhomogeneity. CONCLUSIONS The method developed in this work can be applied to the quantitative analysis of macroconstituents when their identities are known and provided they can be activated by n,y reactions. Moreover, the method can be generally applied to determine the concentration ratio between two elements present in any matrix with the aid of Equations 9 and 10. Once the concentration of one of these constituents is known, it is possible to render quantitative data about their concentrations. Thus, it is not necessary, as is often done in conven-

Z

cu, %

Atomic absorption

Mathematical treatment and NAA

Atomic absorption

14.20 f 0.34 11.06 i 0.26 17.62 f 0.30 14.52 f 0 . 3 5 17.28 f 0.51 10.03 i 0.78 0 . 0 8 + 0.03

44.61 f 0.41 43.05 f 0.99 22.86 f 1.55 24.59 i 0.96 10.20 f 0.49 11.47 =t 0.81 12.20 f 0.36

45.36 f 0.41 44.52 f 1.02 21.73 f 0.30 24.10 =I= 0.86 9.83 f 0 . 3 3 11.62 f 0.61 11.91 =t0.20

tional NAA, to prepare standards for each of the constituents to be analyzed. The method is amenable for the treatment of the spectral data with computers. For this reason, a large number of k values of the majority of elements normally present in alloys are given in this work. The method is rapid and precise since errors due to neutron irradiation are minimized. ACKNOWLEDGMENT

Thanks are due to E. GonAlez for the atomic absorption analysis. The authors are also indebted to T. Monsalve for technical assistance. RECEIVEDfor review April 18, 1972. Accepted August 22, 1972.

Determination of Fluorine and of Oxygen in the Presence of Fluorine by Selective Neutron Activation Using Californium-252 and a 14-MeV Generator J. J. Lauff, E. R. Champlin, a n d E. P. Przybylowicz' Research Laboratories, Eastman Kodak Company, Rochester, N . Y. 14650 A method has been developed for neutron activation analysis of fluorine using a californium-252 neutron source. In addition, a differential method has been developed for the analysis of oxygen in the presence of fluorine by the complementary use of a californium-252 neutron source and a 14-MeV neutron generator. The sensitivity for the fluorine determination is 0.4 mg fluorine in a 10-gram sample; that for oxygen i s 0.04 mg. Both methods are nondestructive, interferencefree, and applicable to either organic or inorganic matrices. A precision of 1% is obtained for the macrolevel fluorine determination and 2% or more for the differential oxygen measurement. Fluorine analyses of the same materials by a spectrophotometric method and by neutron activation with californium-252 provide a basis for comparison of the two methods. The californium-252 procedure yields results with better precision and accuracy based on fewer determinations, although a much larger sample (1 gram) is required.

SEVERAL UNIQUE physical properties make californium-252 attractive as a maintenance-free, continuous neutron source for activation analysis. Its high specific activity (2.34 X lo'? To whom correspondence should be addressed. 52

neutrons per second per gram) coupled with a relatively low alpha yield allows fabrication of neutron sources with significantly smaller physical dimensions than was previously possible. An equivalent americium-beryllium neutron source must be over l o 4larger in volume to accommodate the helium produced by alpha decay. A californium-252 source with a yield of 5 x 10'0 neutrons/sec dissipates only 0.8 W of thermal energy to its environment (as. 750 W for Am-Be), thus essentially eliminating the need for heat-dissipating surfaces (e.g., fins, heat exchangers). These characteristics greatly lower source fabrication and shielding costs and permit analytical samples to be placed very close to an intense, encapsulated isotopic source of neutrons. Two of the strongest advantages of neutron activation analysis are that the technique is nondestructive, and sample solubility is not required for analysis. Several wet chemical methods for the determination of macro-level fluorine exist, but, in addition to being prone to interference from a number of common elements, they require complete dissolution of the sample for an accurate determination. Activation analysis of fluorine with a californium-252 source has been investigated by the National Bureau of Standards and a preliminary report


Rapid determination of matrix components by neutron activation

Recommend Documents