Anal. Chem. 1980, 52,2452-2454
2452
Determination of Lithium and Lithium-6 in Solutions by the Nuclear Track Technique B. Stephen Carpenter* Center for Analytical Chemistry, National Measurement Laboratory, National Bureau of Standards, Washington, DC 20234
Lawrence J. Pilione Physics Depadment, The Pennsylvania State University, Altoona, Pennsylvania 1680 1
The concentration and spatial distribution of elements that undergo thermal neutron induced reactions such as (n,p), (n,a), and (n,fission) as measured by the nuclear track technique have been extensively reported in the literature ( I ) . The energetic charged particles produced in such reactions strike and damage dielectric detectors positioned to optimize the registration of such events. The detectors are then chemically etched, thereby producing an enlarged track easily discernible with an optical microscope. Comparison of etched track density produced by a standard of known elemental composition t o the sample’s track density yields the concentration of the element of interest. The uniformity of tracks or lack of it is used to distinguish between homogeneous and heterogenous elemental distributions in the sample. When several elements that have nuclides undergoing thermal neutron induced charged particle emission (e.g., log, 6Li, 170)are present in the sample, it then becomes difficult to use the track technique to identify the specific reaction, as well as to make chemical analyses. The a particles from these different nuclear reactions produce tracks which for the most part are indistinguishable. However, the element lithium which undergoes the reaction 6Li(n,a)3His unique, since it is one of the few thermal neutron induced reactions of a light element with a relatively large nuclear cross section t h a t produces an exothermic triton. The triton’s energy, 2.73 MeV, i t s electric charge, +e, and its mass, -3 amu, make it a very penetrating particle as compared to a particles produced with the same energy. By insertion of an absorbing material between the sample and the detector, the a particles of equal energy can be screened out while the tritons penetrate and produce etchable tracks within the detector ( 2 ) . This insertion of an absorber then provides for determining lithium without interference from other a particles since the triton track density produced is proportional to the lithium concentration (parts per million) and lithium-6 isotopic abundance (percent). A measurement of the triton track density and knowledge of one of the variables allow determination of the other.
METHODS AND RESULTS The sensitivity of dielectric detectors to the formation of a n etchable track depends upon the type of charged particle and its entrance energy. For most plastic detectors there exist lower and upper energy limits (energy window) within which an etchable track is formed by a particular species of charged particle, and the specific range of this energy window is detector dependent ( I ) . Cellulose nitrate is considered to be one of the most sensitive plastics for recording tracks of light particles. The energy window for tritons in cellulose nitrate is approximately 400 keV with an upper energy of 600 keV (3). Tritons produced within the volume of a sample undergo inelastic collision with the constituent atoms as they move toward the surface, thereby altering their well-defined energy I (2.73 MeV) to a broad spectrum of energies (0 5 ETRITON 2.73 MeV). The maximum depth below the surface at which tritons are formed and produce etchable tracks in the detector
is set by the stopping power of the sample. Interposing an absorber between the sample and the detector reduces the maximum possible energy of the triton exiting the absorber and entering the detector. Therefore, the thickness of the absorber is the controlling factor associated with the maximum energy of tritons striking the detector. T o observe the effect of absorber thickness on the triton track density, we performed the following experiment. Cellulose nitrate detectors (Kodak Pathe CA 80-15) were cut into strips -0.5 X 1.0 cm and heat sealed inside various thicknesses of linear polyethylene. A standard lithium solution (NBS Reference Standard No. 9 ( 4 ) , 1358 ppm Li, 7.43% 6Li) was placed in 1/2-dramvials ( - 1mL maximum volume) with the absorber-detector package positioned in the middle of the vial. The samples were exposed to thermal neutrons in the D 2 0 facility a t the Breazeale Nuclear Reactor, Pennsylvania State University. The detectors were retrieved and chemically etched in 2.5 N NaOH at 50 f 0.5 O C for 25 min. The triton tracks in 50 random fields of view (both sides of the detector were used) were counted by using a Leitz optical microscope (1125X). The triton track density is plotted in Figure 1 as a function of absorber thickness. The data indicate t h a t absorbers between 12.5 and 28 ym can be used without a significant change in the triton track density. It can also be seen that a variation in track density due to nonuniformity of the absorber is minimal in the plateau region. (The thickness variation of the linear polyethylene used in this experiment is quoted a t &lo%.) The constant track density as a function of absorber thickness (12.5-28 ym) is due to several factors: the lithium-containing solution, the effective range of the triton through the solution + absorber, and the “energy window” response of the cellulose nitrate. This plateau response can be explained schematically, as in Figure 2. The “effective volume” is defined as that region of the sample where nuclear reactions take place such that tritons produce etchable tracks in the detector and its extent is determined by the factors listed above. In Figure 2a the effective volume is separated from the detector surface by the absorber and a layer of the sample, in Figure 2b the effective volume is not altered in size but is now closer to the detector surface (the stopping power of the linear polyethylene is greater than that of the solution) ( 5 ) ,and in Figure 2c the absorber thickness (>28 pm) is large enough so as to reduce the effective volume and thereby decrease the number of tritons reaching the detector surface. In this experiment no large variations (>lo%) in track density were observed between fields of view which would be indicative of complete dissolution of the lithium compound in the solvent. As a check of verification that optimum etching conditions were being used to reveal the triton tracks, detectors were sandwiched between 25-pm absorbers and placed in a standard solution and irradiated with thermal neutrons. The detectors were etched in 2.5 N NaOH a t 50 f 0.5 “C for varying times from 5 to 40 min a t 5-min intervals. The results from Figure 3 indicate that a 25-min etch lies within the plateau t h a t produces an optimum track density. The fact that the track
This article not subject to U.S Copyright. Published 1980 by the American Chemical Society
ANIALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
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2453
Trlton Track Denslty From a 1358 ppm Llthlurn Solullon vs Absorber mlckness Elchlng Condlllons: 2.5N NaOH a 50 OS0C 25 mlnutes
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Figure 1. The triton tract density as a function of absorber thickness.
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Figure 2. The "effective volume" is (a) separated from the detector
surface by the absorber and a layer of the sample, (b) not altered in size but is now closer to the detector surface due to thicker absorber, and (c) reduced in size and moved to the surface of the thicker absorber. T m h M s l t y vs Etch Tlme Absorber. 25 V r n Llnear Polyethylene Etchlng Condltlons: 2.5
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Flgure 4. Etched tracks in cellulose nitrate from the "Li(n,~x)~H reaction: (a:i triton tracks using 25 p m thick absorber; (b) a tracks.
cular, indicating that their entrance angle is approximately 0
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90" to the detector surface.
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Figure 3. The triton track density
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as a function of etching time.
density reaches a plateau demonstrates that the tritrons are entering the detector in a very narrow energy band, and most all of the particles are depositing their total energy a t or near the surface of the detector. If tritons entered with energies greater than 600 keV, the detector would slow them down to this critical energy and prolonged etching would produce a varying track density, which means the particles are stopping below the surface of the detector (i.e., under these conditions, new tracks would appear while at the same time others would be completely removed, so that the track density would vary with etch time). This plateau also indicates that reproducible results are not critically dependent upon a chemical etch time between 15 and 40 min. Figure 4 illustrates the marked difference between etched a particle and triton tracks in cellulose nitrate (25-pm absorber). I t can be seen that the triton tracks are nearly cir-
T o check the screening effect of the absorber against a particles, we sandwiched cellulose nitrate detectors between absorbers (12.5-25 pm) and placed them in boron solutions (1000 ppm, a certified Atomic Absorption Standard). The szmples were irradiated with thermal neutrons, retrieved, and chemically etched in 2.5 N NaOH at 50 f 0.i; "C for 25 min. No tracks above background were observed in the cellulose n.trate detectors. I t was found in the course of this investigation that the lower limit of detection of lithium (100 ppm Li, 7.4% 6Li) was set by the production of proton tracks (600 klsV) within the cellulose nitrate from t h e reaction: 14N(n,p)14C.The proton tracks were essentially identical with the triton tracks. In all cases blanks were exposed along with the samples, and the proton track density was substracted out from the total to yield the net triton track density of the sample. The triton track density is proportional to
CI 4t M
p a --
0
(1)
Anal. Chem. 1980, 52, 2454-2455
2454
Table I. Triton Track Density vs. Lithium Concentration triton track Li density, concn, 105t/cmZ sample PP m Atomic Absorption Standard 9.25 + 0.38 500 6.43 t 0.28 333 4.98 i 0.22 250 3.55 i- 0.16 167 Standard Reference Material No. 407 4.66 i 0.17 3.27 t 0.12 924 (NBS, 4.60% 6Li) 339 20 4 2.17 i- 0.08 1.53 i 0.06 136 Reference Standard (RS) No. 9 37 8 7.12 2 0.25 (NBS, 7.43% 6Li) 5.76 t 0.25 315 189 3.84 * 0.18 2.71 f 0.13 126 Oak Ridge National Lab Mate38.4 8.47 i 0.35 rial (ORNLM) (95.9% 6Li) 32.0 6.49 * 0.28 26.0 5.41 t 0.24 12.8 2.97 i- 0.13 Oak Ridge National Lab and 126 6.06 t 0.26 NBS RS No. 9 (20% 6Li) 95 4.72 t 0.21 3.12 + 0.15 63 1.37 t 0.07 32 Oak Ridge National Lab and 126 13.4 I0.44 NBS RS No. 9 (45% 6Li) 95 9.37 t 0.39 63 6.08 i- 0.27 where p 3 triton track density (tracks/unit area), C 3 concentration of lithium (pprn), I = isotopic abundance of lithium-6 (%), $t = neutron fluence (neutrons/cm2), u =thermal neutron cross section (cm2),and M atomic weight of lithium. Lithium solutions of known concentrations and 6Li isotopic abundance were selected to verify the above expression. These solutions were as follows: (i) Certified Atomic Absorption Standard, lo00 ppm; (ii) NBS Reference Standard No. 9, 378 ppm, 7.43%; (iii) NBS SRM No. 924, 407 ppm, 4.60%; (iv) Oak Ridge National Laboratories Li,C03 384 ppm, 95.9%; (v) Oak Ridge National Laboratories Li2C03+ NBS Reference Standard No. 9, 380 ppm, 45%; (vi) Oak Ridge National Laboratories Li2C03 NBS Reference Standard No. 9, 380 ppm, 20%. (The Atomic Absorption Standard's concentration was certified by the manufacturer, but its isotopic abundance was not.) Concentrations ranging from the maximum noted above down to -100 ppm were used for the first three standards. I t was found that the isotopically enriched (>7.43?&)samples with concentrations greater than -300 ppm were suppressing the neutron flux, thereby fewer triton tracks than anticipated were found in the detectors. T o avoid this
+
Table 11. Triton Track Density /Lithium Concentration vs. Lithium-6 Isotopic Abundance sample identification
triton track Li density/Li concn, abun103(t/cm2)/ppm dance, %
NBS SRM 924 NBS RS 9 ORNLM + RS 9 ORNLM + RS 9 ORNLM
1.08 t 0.08 1.97 i 0.11 4.75 i- 0.32 10.1 i- 0.6 21.6 f 1.3
4.60 7.43 20.0 45.0 95.9
difficulty, we diluted enriched samples to concentrations ranging from 10 to 100 ppm. These samples were placed in 1/2-dram vials and the cellulose nitrate detectors (25 km absorbers) positioned in the middle of the vial. In order to determine the background contributions from the detector and solvent, we filled additional vials with dilute nitric acid which was used to dilute the samples. All samples were exposed to thermal neutrons for 100 s a t a power level of 1 MW. The detectors were processed and counted as previously described (the total "hands on" time/detector was 15 min). Table I illustrates the data obtained from the study of the triton track density as a function of the lithium concentration (frepresents one standard deviation = l/(total number of triton tracks)'I2). By performance of a least-squares analysis of the data, it can be seen that a linear relationship exists between the track density and the lithium concentration. The effect of the isotopic abundance is found by calculating the ratio of the triton track density to the lithium concentration for each sample. This average of t h e ratio triton track density/lithium concentration at known isotopic abundance can be seen in Table 11. The deviation in this ratio, p / C , was -*6% (precision) for each standard. The isotopic abundance of lithium-6 for the Atomic Absorption Standard as determined from Table I1 is 7.44 f 0.44%.
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LITERATURE CITED (1) Fleischer, R. L.; Price, P. 6.;Walker, R. M. "Nuclear Tracks in Solids, Principles and Applications"; University of California Press: Los Angeles.
1975. (2) Carpenter, 6.S.; Pilione, L. J. NBS Tech. N o t e ( U . S . ) 1978, Nu. 969, 89-90. (3) Somogyl, G.; Varnagy, M.; Peto, G. Nucl. Instrum. Methods 1968, 59.
299. (4) NBS Tech. Note(U.S.)1960, Nu. 57. (5) Northcliffe, L. C.; Schilling, R. F. Nucl. Data Table 1970, 7 , 223.
RECEIVED for review May 1, 1980. Accepted August 14,1980.
A Device for Extraction and Elution at Elevated Temperature with Solvent Recycle G. J. Sloan E. I. du Pont de Nemours & Company, Central Research and Development Deparfment, Experimental Station, Wilmington, Delaware 79898
Kauffman and Bjorkman ( 1 )have described an apparatus for chromatographic extraction at elevated temperature. In their device, the column is suspended in a jacket, and solvent enters it through a few holes near the top. The perforations make the column fragile, and filling is awkward. A simpler, sturdier device can be made in a variety of sizes by inserting a column in a jacket having a standard-taper joint a t the bottom and an O-ring joint a t the top (Figure 1). The inner column is supported on a perforated disk of Teflon poly(tetrafluoroethy1ene) resin or of stainless steel. I t is centered in the jacket by three protuberances; these also serve 0003-2700/80/0352-2454$01 .OO/O
to retain a bail of stainless-steel wire which is useful for raising and lowering the packed column, especially in the largest sizes. The jacket may be creased vertically a t the level of the protuberances, to keep the column from rotating when a stirrer is used (see below). In one version of this device, the inner Pyrex column is 6.4 cm 0.d. x 80 cm long and the jacket is 8.0 cm 0.d. x 95 cm long. The jacket is conveniently insulated with commercial pipe covering of fiberglass or rigid foam. Viewing slots may be cut in the insulation. The condenser is mounted on the jacket with an O-ring of a solvent-compatible elastomer or of Teflon poly(tetrafluoroethy1ene) resin. 0 1980 American Chemical Society
