Multielement Neutron Activation Analysis of Rock Using Chemical Group Separations and High Resolution Gamma Spectrometry G. H. Morrison, J. T. Gerard, A. Travesi,’ R. L. Currie,*S. F. Peterson,2 and N. M. Potter Chemistry Department, Cornell University, Ithaca, N . Y. 14850 Instrumental neutron activation analysis and neutron activationanalysiswitha high-resolution lithium-drifted germanium detector, chemical group separations, and a coincidence-anticoincidence system were used to determine 45 elements in U. S. Geological Survey standard rock samples BCR-1, AGV-1, and 6-2. U. S. Geological Survey standard rock W-1 was used as the standard. The principal nuclides, their ?-rays used in the determinations, and the best time after irradiation for counting are listed. Nuclides are separated chemically into groups for counting by volatilization, extraction with hydrated antimony pentoxide, ion exchan e, and solvent extraction with TBP. Agreement o!our results with published data is good.
OF THE VARIOUS analytical methods available for the determination of the average abundance of elements in geological materials, neutron activation analysis has been one of the most sensitive and reliable. The recent advent of high resolution lithium-drifted germanium detectors for gamma spectrometry has further increased the value of neutron activation by providing the capability of multielement analyses. The technique has been applied to the nondestructive analysis of standard rocks by Gordon et al. ( I ) permitting the determination of 23 elements. However, the complexities of the gamma spectra from multielement samples such as rocks often prevent the determination of many elements by strictly instrumental methods. The high activities produced from abundant, high cross section isotopes such as Mn, La, and Na often mask lower level activities from other short and medium half-life elements, while the activities from such elements as Sc and Co interfere with long half-life measurements. For this reason chemical group separations were developed which permitted the full utilization of the desirable features of the high resolution semiconductor detector without the extra difficulty of separating individual elements. In addition, a coincidence-anticoincidence system developed by Currie, McPherson, and Morrison (2) was used for Compton suppression in conjunction with the semiconductor detector. This led to an improvement in the peak-to-background ratio of a factor of four or more permitting the more precise determination of many low intensity gamma transitions. Furthermore, the system permitted the determination of a number of elements not previously possible. This combination of chemical group separations, a high 1 Present address, Junto de Energia Nuclear, Nuclear Chemistry Division, Madrid, Spain. 3 Present address, Savannah River Laboratory, E. I. du Pont de Nemours and Company, Aiken, S. C. 29801.
(1) G. E. Gordon, K. Randle, G. G. Goles, J. B. Corliss, M. H. Beeson, and S. S . Oxley, Geochim. Cosmochim. Acta, 32, 369 (1968). (2) R. L. Currie, R. McPherson, and G. H. Morrison, “Modern Trends in Activation Analysis,” National Bureau of Standards, Gaithersburg, Md., 1968, p 712.
resolution Ge(Li) detector, and the coincidence-anticoincidence system has resulted in information on 45 elements. The method is applicable to a wide variety of rock samples. EXPERIMENTAL
Samples and Irradiations.
USGS diabase W-1 was used
as the multielement standard in the activation analysis. Both samples and standard were irradiated simultaneously in the Cornell University Triga Mark I1 reactor according to the schedule in Table I. A series of short irradiations permitted the nondestructive determination of eight elements not determined in the chemically processed groups. Following the long irradiation, the samples were allowed to cool 15 hours and then chemically processed. Measurement of Activity and Data Processing. Gamma counting was done using a nominal 30-cc coaxial lithiumdrifted germanium detector (Nuclear Diodes Inc.) and an RIDL 34-12 400 channel analyzer. System resolution was less than 2.8 keV (FWHM) and the peak-to-Compton ratio was better than 15:l for the 1330 keV gamma of W o . To obtain adequate dispersion with the limited memory size available, a biased amplifier was employed to permit counting in the energy ranges: 0 to 450 keV, 0 to 900 keV, and 800 to 1700 keV. The coincidence-anticoincidence system [Currie, McPherson, and Morrison (2)] consisted of the same 30-cc Ge(Li) detector surrounded by an 8-inch by 10-inch long NaI(T1) annulus. A Nuclear Data 181 1024 channel analyzer was made available for use with this system. The analyzer’s memory was divided into two halves, and each Ge(Li) detector energy pulse was routed to one half or the other depending on whether or not a coincidence was observed in the annulus. One memory half thus contained the standard Compton-suppressed spectrum and the other half the “discarded” events. Summing the two spectra reproduced the unsuppressed spectrum so that for those cases in which suppression produced poorer quantitative statistics, the original, unsuppressed spectrum was available. Punch papertape output from the multichannel analyzers was fed to a PDP-9 digital computer. The programs used provided graphical plots of the data and digital tables of the input data and peak locations. The peaks were assigned to isotopes being analyzed and were corrected for background. The decay-corrected peak areas were then used to determine elemental abundances in the standard rocks as compared to the W-1 standard. Chemical Reagents. Hydrated antimony pentoxide (HAP), 40-80 mesh, was used to selectively remove Na and Ta [Girardi and Sabbioni (3)]. Dowex 1-X8 anion exchange resin, 200-400 mesh, was washed with distilled water to remove fines and equilibrated with 8N HC1 before using. Tri-n-butyl phosphate (TBP) was preequilibrated with 8N HC1 before use in the liquid-liquid extraction separation step. Chemical Separation. The elements in each group are summarized in Figure 1. One gram of W-1 and the various (3) F. Girardi and E. Sabbioni, J . Radioanal. Chem., 1, 169 (1968). VOL. 41, NO. 12, OCTOBER 1969
1633
the HAP column. The HAP from the beaker and column was transferred with 30 ml of 8N HC1 into a 125-ml glass stoppered Erlenmeyer flask (group 2). The effluent from the HAP column was centrifuged to ensure complete removal of colloidally suspended HAP, and passed through a column of Dowex 1-X 8, 1.4-cm diameter and 10 cm high. The flow rate was 2 ml per minute. The column was washed with six 10-ml portions of 8N HC1. The effluent and wash solutions were evaporated to dryness. In the meantime the resin was eluted with fifteen 2-ml portions of 0.5N HC1 at a flow rate of 0.5 ml per minute. The resin was transferred to a 125-ml Erlenmeyer flask with 30 ml of 8N HC1 for counting (group 3). The eluate was collected into a 125-ml flask (group 4). The residue from the previously evaporated effluent was dissolved and transferred into a 125-ml separatory funnel with 20-25 ml 8N HC1. This solution was then extracted with 20-ml and 10-ml portions of TBP. The aqueous phase was carefully drawn into a 125-ml flask and diluted to 30 ml. The organic phase was transferred to a 125-ml flask (group 5 ) . The aqueous phase comprised group 6. To ensure complete recovery of each element, each group was examined for every element, and in the few cases where an element was found to be distributed in two groups (reported later) the combined abundances were used. Identiflcation of Species. In addition to the usual methods of identifying the nuclide producing observed gamma photopeaks [peak energy, relative intensity of associated peaks, and half-life, see Gordon et ai. (I)], two more pieces of information were available to assist us in our identifications. The first of these is the selectivity achieved through the chemical group separations, where the presence of an element in a given group is further defined by the chemistry of the element. This chemical scheme greatly reduces the number of nuclides that can be present in a given group. The second assistance comes from the coincidence-anticoincidence system. Because of the intimate source-detector geometry used in this work, the sample is inside the annulus tunnel. Therefore the "discarded" spectrum contains, in addition to unwanted Comptons, some photopeak events from cascade gamma emitters. The relative intensity of such peaks in the two memory halves permits estimation of the number and intensity of cascade gammas relative to those of the peak of interest. Such information can be used as a check on the decay schemes of presumed source nuclides. Abundance Determination. The radionuclides used for abundance measurements are given in Table I1 along with half-lives, photopeaks used, and the best time for counting.
Sample Carrlws ISe,Zn,La,As,0r) and HF+H+O, I
I
Residue Dissolved In 8N HCL
76
I
e2
As, Br
Solution HAP botch ertroetlon and column extroctlon
I Porred thfouqh 0 lorn column of Dowax I~8,200-400mrah
I
"Na'yTa
Collecte'd solution
1
Evoporol@d
Reiidue gI$d
55
69rn
Zn,
In
Zn,"%b,12'Sb
59Fe,wCo,%,"Ga "'Np
le7H9 ,lo7"H9
,"W '
,@'MO
Extracte with IO 8 20 rnl TBP I
,"Zr ,4'Sc
"'Hf
'5c
,'&I
"'kr,
'2
='pa
K , "R~,"'CCI,'~SC,~'~B~
a6Sr,"Ce,"7Nd,'6~mm''"Eu 154
Eu,I6%d ,"oTb:40Lo ,I%o
'mTm:"9vb,177LU
,%r , '%a
Figure 1. Radiochemical group separation scheme samples were processed bj this scheme. The irradiated samples, after cooling for 15 hours, were transferred to platinum dishes. One or two mg of carrier of Sc, Zn, La, As, and Br were added and the samples dissolved with 2-3 ml H2S04 and 10-15 ml 4 8 z H F with gentle heating. The volatiles in group 1 were trapped by drawing the fumes and gases through concentrated NaOH using an aspirator system. Heating was continued to the appearance of SOs fumes and more H F added. This was repeated several times until the samples were completely dissolved. The samples were heated to dryness and the residue was dissolved and transferred with 30-40 ml 8NHC1 to a 250-ml beaker containing 1-1.5 grams of HAP. The Pt dishes were counted to ensure no loss of activity. The contents of the beakers were stirred gently for 20-30 minutes. The HAP was allowed to settle and the supernatant was transferred to a centrifuge tube to separate colloidally suspended HAP. The supernatant was transferred onto a column of HAP, 1 cm in diameter and 11.5 cm high, with a flow rate of 2 ml per minute. The HAP in the beaker was washed 5 times with 10-ml aliquots of 8N HC1 and passed through the centrifuge procedure and onto
Sample size Nondestructive 100 mg 500 mg
100 mg 100 mg
Irradiation position Pneumatic transfer tube Pneumatic transfer tube Pneumatic transfer tube Pneumatic transfer tube
Pneumatic transfer tube
500 mg
Destructive 1.000gm
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Rotary specimen rack
ANALYTICAL CHEMISTRY
Table I. Irradiation Conditions Thermal neutron flux, n/cm*-sec Irradiation time
x 2 x 2x 2 x 2 x 2
Elements determined
10'0
30 sec
A1
10'0
30 sec
V, AI
10"
30 sec
Mg
10'1
45 sec
In, Na, Mn
10'2
45 sec
Ni, Na, K, Mn, Dy, C1
8 hours
As, Br, Na, Ta, Ba, Fe, Co, Cu, Ga, U, W, Mo, Th, Zn, Sb, Hg, Zr, Hf,Ti, Sc, K , Rb, Cs, Ca, Sr, Ce, Nd, Sm, Eu, Gd, Tb, La, Ho, Tm, Yb, Lu, Cr
7 x 10"
Table 11. Measurement of Nuclides Observed Best photopeak(s) Best time after Element used in determination (keV) irradiation for counting Product nuclide(s) measuredC cs 2 . 0 5 ~ 134Cs 605, (796)" 15-30 days Rb 1078 18.66d 86Rb 11-30 days Ba 12d I3lBa (373),. 496 14-30 days K 1524 12.4h 42K 2-6 days, 15 hours Sr 514 64d 8'Sr 15-30 days Ca 160 15-30 days 3.4d r7Scd 9.5m nMgd 1013c 9-30 minutes Mg Na (511),5 1369 15.0h 24Na 4-6 days, 15 hours La 40.22h l4oLa (328, 487, 816)," 1596 4-6 days Ce 145 20-30 days 33d I4lCe Nd 91 (531)' 14-30 days 1l.ld I4?Nd Sm (69),=103 47h 16aSm 1-6 days (122, 245),= 344 (779, 20-30 days Eu 12Y Ia2Eu 1408)o (724, 1277p 20-30 days 1 6 ~ Gd 103 after 25 days 242d Tb 299 (1177p 15-30 days 72. Id 2.3h 6-12 hours 95, (716)" DY Ho 3-6 days 81 27h 84 Tm 15-30 days 130d 177 Yb 10-30 days 32d Lu 8-14 days 208 6.7d 15-30 days Th (98),0 312 27. Od 106, (228, 278)" 4-10 days 2.35d U 1-2 days 17.0h 747c Zr 10-30 days (133)," 482 Hf 42.5d 14-30 days 100, 1222, 1231 Ta 115.ld 2-7 days 740 Mo 66h 479 (686)" W 1-3 days 24h (986),a 1040, 1314 1-4 days 1.82d Ti 1-5 hours Mn 2.58h 847 1-2 days 511 12.8h cu 5-30 days 1173, 1332 5.26~ co (1114),* 1481 2-6 hours 2.56h Ni 5-30 days 1100, 1291 45d Fe 1-2 days 13.8h 439 Zn 10-30 days (1114)" 245d 1-30 days 889, 1120 84d sc 1-10 minutes 1434 3,75111 V 20-30 days Cr 320 27.8d 2-5 days 77 65h Hg 2-10 minutes 1780 AI 2.31m 1-3 days 630, 835 Ga 14.l h 1-4 hours (1085)," 1293 In 54m 2-3 days 564, (686p Sb 2.8d 1C-30 days (603, 1691)" 60d 1-3 days 559 (657)a 26.4h As 1-3 hours CI 37.3m 1600 1-4 days Br 35.5h (619)," 777 a Photopeak(s) used for confirmation. b Principal group. c Coincidence-anticoincidence measurement. d See section in text. e Half-lives are from Lederer et a/. (4). Each chemical group was counted at several different times after irradiation in order to optimize determinations for isotopes of different half-lives, The decay-corrected photopeak areas were compared to those of the W-1 standard to obtain the abundances of the respective elements. Below is a discussion of special problems associated with determinations of the abundances of several of the elements. BARIUM.The 496-keV peak of 131Ba was used t o determine barium. Although it is found predominantly in group 6, some barium may also be retained o n the HAP so that the sum of both fractions must be used for the measurement. (4) C. M. Lederer, J. M. Hollander, and I. Perlman, "Table of Isotopes," 6th ed., John Wiley and Sons, Inc., New York, 1968.
Radiochemical group 6 6 2, 6b 6, INAA 6 6 INAA 2, INAA 6 6 6 6 6 6 6 6 INAA 6 6 6 6 3, 4, 5b 3, 4b 5 5
2 3, 46 4 5
INAA 4 4 INAA 4 3 3 5
INAA 6 3 INAA 4
INAA 3 3 1 INAA 1
STRONTIUM. The 514-keV peak of *%r was used. The absence of any contribution from 51 1-keV positron annihilation was confirmed by its absence in the "discard" spectrum of the coincidence-anticoincidence measurement. CALCIUM. The 160-keV line of 47Sc(3.4d) was used to B determine calcium ~ i a ~ B C a ( n , y ) ~ ----f Ca 47Sc. "Sc is 4.6d also produced via "Ti(n,p) 47Sc during irradiation. Upon chemical separation the 47Sc produced from 47Ti and that produced before separation by '?Ca decay went into the organic phase (group 5) leaving 47Cain group 6. Shortly after the "Sc resulting from the purified "Ca had passed its maximum activity, the half-life was approximately 7 days; after 30 days, it was 5 days. The 1300-keV line of 47Cawas also observed but was too small t o measure precisely. VOL. 41, NO. 12, OCTOBER 1969
1635
MAGNESIUM. A short irradiation at 10 kilowatts for 30 seconds permitted the 9.5-minute nMg to be determined. The coincidence-anticoincidence system greatly improved the peak-to-background ratio for the precise measurement of the 1013-keV line. The production of 27Mgin both the W-1 standard and BCR-1 via 27Al(n,p)nMgled to results for Mg which were 12 high; however this effect was corrected for from a knowledge of the fast-to-slow neutron ratio during irradiation. SAMARIUM.During the first few days after irradiation the 103-keV line of ls3Sm can be reliably used. After 1 week, the contribution of lsaGd (103 keV) becomes appreciable and must be subtracted. GADOLINIUM. According to Gordon et al. ( I ) , the 70, 97, and 103-keV peaks from 1baGd(247d)have interferences due to the 69 and 103-keV lines of ls3Sm, and the 68 and 100-keV lines of la2Taand the 94-98 keV lines of zaaPa. After group separation, the Gd could be determined after 30 days, the 153mhaving decayed away. HOLMIUM.The 81-keV peak of lBeHo(27h)was measured between 2-4 days. The contribution of the 84-keV line of 170Tm during this period was found to be negligible as confirmed by measurement of 170Tmafter 15-30 days. THULIUM. The 84-keV peak of 170Tm(l30d) was measured 15-30 days after irradiation. Correction was made for the contribution of the 87-keV peak of leoTb. The coincidenceanticoincidence technique was also used to partially remove this interference. THORIUM.The thorium content was determined via the B zazTh(n,y)2aaTh 22.3~~ 233Pa. The 312-keV peak of 2aaPa (27.0d) was used and confirmed by using the 98-keV peak. Thorium was found in groups 3,4, and 5 and it was necessary to combine the values for a material balance. B 2a9Np reaction was URANIUM.The 23*U(n,y)2agU used and the 106-keV peak of 239Np(2.35d)measured. The 228 and 278-keV peaks were used for confirmation. The best time for measurement was 4-10 days after irradiation. The activity was distributed 80% in group 4 and 2 0 z in group 3 so that both groups were considered to maintain a material balance. The contribution of the 106-keV peak from 23aPa(27d)was checked after 20 days and found to be
