tellurium interference in 1311determinations. Coal samples from various US.mines were analyzed for uranium in our laboratory by the TIOA technique and counted on a Ge(Li) detector using a pulse height analyzer. These values were then compared to the results obtained by Weaver (9) using a low-energy photon detection system on similar coals (Table 111). As previously mentioned, liquid IC1 round-robin samples were separated by the TIOA technique prior to analysis. Table I shows the excellent agreement between the two methods used (NAA and FAA). Without compromising accuracy or precision, it was possible to separate the element of interest (Hg) from other elements having interfering y energies while eliminating the NBS combustion step. In addition to isolating the element to be analyzed, another advantage is that of time reduction in the separation process. Without the NBS combustion step, the liquid anion exchange process takes approximately 5 min for completion. Similar analyses have been performed on scrubber water samples from the phosphate fertilizer industry with comparable results. As a result of the change in sample geometry, some efficiency was lost by counting the y emitters in liquid solution rather than in a precipitate form. This is demonstrated in Figure 5 . A 1-ml sample containing mixed radionuclides was counted. After the addition of water and mixing, the sample was repeatedly counted a t different volumes. Note that there is a 20% drop in counts from 1 to 10 ml and a further drop of 21% from 10 to 25 ml. These data demonstrated the need for minimizing all volumes.
CONCLUSION This technique of liquid anion exchange is applicable in performing rapid and accurate radiochemical separations in a wide variety of samples prior to y-ray spectrometry. The versatility of the technique is demonstrated by its applicability to both solid and liquid samples. Future studies in this laboratory may include the analysis of trace elements in total suspended particulate matter from environmental high-volume filter samples. Also being considered is the rapid separation and analysis of irradiated petroleum samples without using the NBS combustion procedure.
* o o l 90
80
a U
60
50 *..
40
0
5
10
15
20
25
4 30
VOLUME 0FSOLUTION.ml
Figure 5. Effect of sample geometry (volume) on counting accuracy
ACKNOWLEDGMENT The authors express their gratitude for the assistance given them by William Mitchell of EPA, Tom Gills of NBS, and Jack Weaver, James McGaughey, and Mark Jensen of North Carolina State University.
LITERATURE CITED (1) U.S. Congress, "Clean Air Amendments of 1970." Public Law 91-604, 91st Congress, H.R. 17255,Dec. 31, 1970. (2)D. von Lehmden, R. Jungers. and R. E. Lee, Anal. Chem., 46, 239-245 (1974). (3)R. E. Lee and D. von Lehmden J. Air Pollut. Control Assoc.. 23, 853-857 (1973). (4)E. Orvini, Thomas E. Gills, and Philip D. LaFleur, Anal. Chem., 46, 12941297,(1974). (5) F. E. Butler, A. R. Boulogne, and E. A. Whitley, Health Phys., 12, 927933 (1966). (6)Fed. Reg., "National Emission Standards for Hazardous Air Pollutants: Asbestos, Beryllium, and Mercury", Volume 38, No. 66,Pari II, 88318840 (April 6,1973). (7)K. A. Kraus and F. Nelson, Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1955,Paper No. 837. (8) E. S.Gladney and H. L. Rook, Anal. Chem., 47,1554-1557 (1975). (9)J. Weaver, Anal. Chem., 46, 1292-1294 (1974).
RECEIVEDfor review May 19, 1975. Accepted September 15, 1975.
Determination of Lithium, Boron, and Carbon by Quasi-Prompt Charged Particle Activation Analysis John R. McGlnley and Emile A. Schwelkert Center for Trace Characterization, Department of Chemistry, Texas A&M University, College Station, Texas 77843
A novel approach for rapid nondestructive trace anaiysls is presented, based on the detection of short-lived high energy 0 emitters (10 msec I t j I 2 I 1 sec) produced by charged particle bombardment. Lithium, borQn, and carbon are determined via 'LI(d,p)'LI, "B(d,p)I2B and I2C(p,n)l2N, respectively. These elements have been measured at the l-to 350-ppm level with a relative precision of 5 to 30% In glasses, semiconductor materials, botanical specimens, and metals. Experimental detection limits are 0.50 ppm for lithium, boron, and 50 ppm for carbon.
Most of the work so far in charged particle activation analysis has been based on the detection of radioisotopes having half-lives greater than 1 min. Only a few recent studies have been concerned with the analytical exploitation of short-lived nuclides ( t l l z < 1 min) (1-4). An interesting aspect of this approach is that several light elements yield very short-lived species (tllz < 1 sec) under charged particle bombardment. The objective of the present study was to examine these possibilities from the analyst's standpoint, focusing on those cases where the product nuclides
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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I
6
I
Ob
‘2
4
6
8
IO
I2
ENERGY (MeV1
Figure 2.
h
6
‘8
Activation curve for ’Li(d,~)~Li 7
Figure 1. Block
--
diagram of the 6-detection system.
decay by emission of high energy p particles. It is apparent from the half-life range considered, that detection of the radioisotopes of interest must be carried out a t the irradiation site immediately following bombardment. For this reason, this approach is termed “quasi-prompt” activation. Charged particle activation analysis is particularly well suited for this purpose because of the inherent simplicity of producing short bursts of activating particles with accelerators. In this study, the following topics are examined: a) review of possible activation reactions yielding high energy p emitters; b) irradiation and counting requirements; c) application of quasi-prompt activation to the determination of traces of Li, B, and C.
Figure 3.
Activation curve for “B(d,p)l*B
I
“C(p,n) “N
i
,
EXPERIMENTAL
22
24
26
20
,
,
,
30
32
34
/
36
I
38
ENERGY (MeV)
Irradiation. The irradiations were performed a t the 88-inch variable energy cyclotron of Texas A&M University. A novel setup was used for pulsing the cyclotron beam, to achieve maximum build-up of the nuclides of interest while minimizing the production of longer lived species. Previously developed methods either produced too long a beam pulse ( 2 ) , were not applicable to high energy charged particles ( 5 ) , or were difficult to manipulate. The method used in this work consisted of pulsing the cyclotron Dee voltage ( 6 ) .The duration of a voltage pulse applied to the Dee determined the duration of the beam pulse on target. This technique produced a high intensity beam pulse with a duration which was variable from 1 msec to 10 sec. All irradiations except those for carbon were carried out in air, with the particles emerging from the beam line through a thin tantalum window (25 pm thick). For carbon analyses, the irradiations were performed in vacuum, using an experimental set-up described previously (2). In all cases, the samples were in a fixed position a t 45O with respect to the beam and detector axis, respectively. Nominal irradiation energies were 20 and 40 MeV for protons and 20 MeV for deuterons and 3He particles. Beam intensities of up t o 2 pA were used. Typically the beam was focused on the sample to a spot of -2 mm in diameter. The beam intensity was monitored immediately before each irradiation/data collection cycle with a movable Faraday cup. The beam varied less than 10% from one irradiation to the next and, over a large number of repetitive irradiation/counting cycles, these fluctuations tended to average out. Each sample was subjected to a t least 10 and sometimes up to 100 repetitive irradiation/counting sequences. Samples. Thin targets were prepared by depositing 1-5 mg/cm2 of LiF, H3B03, or graphite onto vanadium foils (14.9 mg/cm*). Stacks of metal foils or pelletized powders were used for thick target irradiations. Commercially available samples used for analyses included: Standard Reference Materials (No. 610, 612, 1143, and 1571) from National Bureau of Standards, Washington, D.C., “VP iron” from Materials Research Corp., Orangeburg, N.Y. The standards used consisted of thick pellets containing from 1 t o 50% of the element of interest mixed with a non-interfering binding material (e.g., LiF with graphite binder). Counting. A major experimental requirement was to detect high energy emitters selectively and efficiently in samples con2404
Figure 4.
Activation curve for I2C(p,n)l2N
taining high levels of y activity. This was accomplished with a coincidence counting technique utilizing two thin (2.5 mm) discs (7.6-cm diameter) of “NE 102” plastic scintillator (Nuclear Enterprises, Inc., San Carlos, Calif.). A block diagram of the counting system used is shown in Figure 1. The linear gate width was -200 nsec. The output from the linear gate was fed into a 400-channel analyzer operating in the multiscale mode. The entire sequence, bombardment, delay ( 2 to 10 msec to allow for decay of very short lived species), and counting was carried out automatically with preset electronic timers. This provided for the reproducibility needed to carry out on the same sample repetitive irradiations and accumulation of the decay data for better counting statistics. During irradiation, the photomultipliers were electronically disabled to prevent possible overload of the counting system and dynode damage. This procedure has been described by Black et al. (7). The use of thin detectors allowed high p detection efficiency with relative insensitivity to y irradiation. The coincidence counting requirement further reduced the level of y events recorded by more than two orders of magnitude without seriously affecting the p count rate. Additional background reduction was obtained by: a) using the single channel analyzers to discriminate against the smaller pulses produced by the interaction of y rays with the scintillator; b) placing aluminum absorbers between the sample and detectors to screen out low energy p particles emitted by a host of activation products. Several parameters can be varied to maximize the ratio of Ply events recorded: type of detector material, size and shape of the detectors, threshold height of the single channel analyzers, timing of the linear gate, thickness of the aluminum absorbers. A detailed study evaluating these parameters in terms of detector performance is being published elsewhere (8). Two points must be emphasized when assessing this p detection system: a) the large amount of low energy p activity usually produced by charged particle bombardment, which must be eliminated with absorbers, precludes in practice the detection of nuclides emitting p+ or 8- particles with Eomax< 5 MeV; b) the optimum counting conditions must be found experimetally for each matrix-trace element combination.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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Table I. Thick Target Yields and Interference-Free Detection Limits Using Quasi-Prompt Activation Reaction
Q Value, MeV
fx,msec
Epmax, MeV
count'slsec
C(p.n)' Z N Nip; 2n)l 0 24Mg(p , n ) 2 4 m A1 8Si(p,n)z8 P
18.1 19.5 14 14.6 13.9 7.9 8.4 9.0 0.2 -1.2
11
16.4 16 13.3 11.5 9.4 6.6 6.1
1.07 X lo8 8.23 x 104 8.08 x 105
I
Interference-free detection limits: ppm
Thick target yieldsa
8.7 129 270 297 486 288 194 850 20 600
8.33
0.25 2000 350 25
x 106
7.50 X l o 6 30 6Ti(p,n)4 V 5.40 x 105 300 450 'Cr(~ , nOMn ) ~ 6.06 x 105 5 4 F e ( p , n ) 5Co 4 6.47 X l o s 500 7.3 Li( d, P ) Li ~ 6.23 x 107 0.15 13.0 I I B(d,p)l Z B 13.4 1.32 X l o 8 0.25 'Ca(d, n)41Sc 1.1 1.27 x 104 1000 5.5 880 3.68 x 103 4000 40Ca(d,dn)39Ca 15.6 5.5 ... 6Li(3Hen)8B 774 4.2 x 1 0 3 ~ . 14.1 1.2 'B('He',n)' 2 N 11 -1.5 5.4 x 1 0 3 b 16.4 ... a Incident particle energy 4 0 MeV for 'ZC(p,n)'ZNand 14N(p,2n)130reactions. All other reactions studied at 20 MeV. Activity normalized t o a I-pA irradiation of an infinitely thick pellet for the half-life of the product nuclide. Determined using non-standard counting arrangement. Based on 1 0 repetitive irradiations at 2 pA, giving 1 0 0 counts at the end of irradiation with irradiation and count times equal to the half-life of the product nuclide. 32s(p,'t1)32~1
I
Quantitative Calculations. The results obtained are based on the activities measured in thick standards and samples with appropriate corrections for the number of irradiations/counting sequences and the beam intensities.
200
R E S U L T S A N D DISCUSSION Activation Data. A first assessment of the feasibility of trace determination for the elements surveyed can be made based on the thick target yields obtained (Table I). T h e highest specific activities were measured for proton and deuteron reaction products of Li, B, C, S, and Si. Proton activation of S and Si has been discussed in an earlier paper by Thomas e t al. (2). That study was particularly concerned with the analysis of sulfur via 32Cl and possible limitations of this procedure due to the closeness of the nuclear characteristics of 32Cl and 28Pproduced from silicon. In the present work, attention was thus focused on further assessing the quasi-prompt activation technique for lithium, boron, and carbon. T h e relative excitation functions for the reactions 7L~(d,p)*Li, 11B(d,p)12B,and 12C(p,n)12N are shown in Figures 2, 3, and 4, respectively. The low threshold and peak energies of the lithium and boron reactions indicate that these reactions could also be used with low energy accelerators ( E d = 5 MeV for ex.). The high threshold of the carbon reaction requires a proton beam of E , 2 25 MeV. Interferences. Also included in Table I are interference-free detection limits, calculated according t o the definition given by Currie (9) for a "zero blank" at a level of precision satisfactory for quantitative measurement. Detection limits given in subsequent Tables take into account the activities produced in the samples analyzed, and were calculated assuming the case of "paired observations" (9). No nuclear interferences were found for the reactions studied and the bombarding energies used ( E , I40 MeV, E d < 20 MeV). However, the possibility of producing different nuclides with similar decay characteristics had t o be considered (Table 11). Procedures to achieve selectivity are outlined in the last column of Table 11. In cases where the interfering radioisotope emits a medium energy @ particle, absorbers can be used to screen out the interfering activity. If the interfering nuclide is a y emitter, the thresholds of the single channel analyzers can be raised t o discriminate against the smaller pulses due to y radiation. It is clear that either of these steps affects the sensitivity of the method. The only interference that cannot be eliminated in this fashion deals with the contribution of I3O from nitrogen in determination of carbon via 12N.However, the yield of the
y. -> -
f\
b-
4"
10-
I
I
I
\
\
.-... .'.. . .. .* ...*. . . . .. .. .. .. . .. \
100
200
300
400
0.
500
Figure 5. Decay curve for "B in Si
reaction 14N(p,2n)130is sufficiently low a t E , = 40 MeV, that nitrogen does not interfere with carbon analyses provided the nitrogen concentration does not exceed 100 times the concentration of carbon. As noted earlier, carbon deter: minations were carried out in a nitrogen-free environment (vacuum). Sample Analysis. T o test the capabilities of the quasiprompt technique, different types of samples were analyzed for their lithium, boron, or carbon content. The results along with the detection limits for each element/matrix combination are given in Tables 111, IV, and V. The data obtained are in good agreement with results from other techniques. This confirms the inherent accuracy of the method which is free from nuclear interferences under the conditions used. The errors indicated for each determination were estimated by differences between results of repetitive analyses (for each sample, ten determinations for lithium and boron, three determinations for carbon). Comments pertaining to the different determinations are given below. These should be viewed by noting again the critical dependence of the quasi-prompt technique on how well the Ply activity ratio can be maximized. From this standpoint, the performance achieved could still be considerably improved by utilizing faster timing electronics. Indeed a de-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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Table 111. Results of Boron Determinations and Experimental Limits of Detection Amount found, ppm Sample
Other methods
Quasi-Prompt
Experimental detection limit, ppm
NBS Glass SRM 6 1 0 3 5 2 t 70 3516 2-3 NBS Glass SRM 6 1 2 38 t 5a 32b 2-3 Si IX 225 7 3 i 5a 77C 0.5 Si IX 252 11 t 20 llc 0.5 Si IX 253 2.3 t 0.7a 1-2C 0.5 NBS SRM 1571 3 6 t 5a 33 t 3b 0.75 (orchard leaves) a Average deviation based on 1 0 determinations. b Noncertified value, National Bureau of Standards. e Data supplied by F. A. Thrumbore ( I 0).
___
Table IV. Results of Lithium Determinations and Experimental Limits of Detection
Sample
X
Si(Li) A Si(Li) B Ge(Li) MBS Glass SRM
5
Experimental Amount found, ppm detection limits, Quasi-Prompt Other methods ppm
0.98 t 0.40a
