Determination of Oxygen by Activation Analysis with Fast Neutrons Using a Low-Cost Portable Neutron Generator EDGAR L. STEELE' and W. WAYNE MEINKE Deparfment of Chemistry, University of Michigan, Ann Arbor, Mich.
b Fast neutron activation analysis, using a low-cost Cockcroft-Walton design accelerator as a source o f 14m.e.v. (deuterium-tritium) neutrons, has been found satisfactory for trace oxygen determination. This method i s rapid, sensitive, and selective, and i s free from most matrix interferences, Yet it uses equipment costing no more than good infrared or spectrographic 10 instruments. Fast neutrons m.e.v.1 convert oxygen-1 6 b y an (n,p) reaction to 7.4-second nitrogen-1 6. This in turn emits 6 to 7 m.e.v. y-rays which are measured b y scintillation spectrometry. Samples containing 10 mg. or more o f oxygen have been analyzed to within & 10% with a fast flux of -108 n cm.-2 sec.-1 Larger samples give smaller errors. By using all the sample area available with an average flux for irradiation of 108 n cm.-2 set.-' and using a proper transfer system, it should be possible b y this nondestructive method to analyze to f 10 to 15% for as low as 10 within p.p.m. of oxygen. The average time for an analysis, including weighing, is approximately 7 minutes. The only interference encountered i s from fluorine and this can be compensated for a t F/O ratios below 10.
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-
T
important effects of oxygen content on physical properties of materials and the wide distribution of this element in nature necessitated a rapid and reasonably accurate method for trace oxygen determination, Ivhich would be free of matrix interferences, get remain in the price range approachable by the average analytical laboratory. d number of specialized niethods have been reported for the determination of small amounts of oxygen in materials (4, 7-9), but only activation analysis (16) offers a general approach to the problem. There is a wide variety of nuclear reactions available to the analyst for oxygen determination-e.g., 018(p,n)F'8 (6) ; O''(d,n)F" ( 1 7 ) ; O'6(t,n)F'8 ( 1 6 ) ; 0 ' 6 HE
1 Present address, Esso Research and Engineering Co., Linden, N. J.
(io); o ~ ~ , ~ )( 5o) ; ' 0 Q 17( ~ , c u ) C(I); ' ~ and 0 1 6 ( n , p ) P(2, 3 , I S ) . From a consideration of time, equipment, expense, and convenience, the 016(n,p)S I 6 reaction appears to be the best suited for the average analytical laboratory. This paper dcscribes the application of 14-m.e.v. ncutron irradiation for ouygen determination, using a l o r voltage Cockcroft-n'alton accclcrator as a neutron sourcr and y-rap scintillation spectromctry to mrasure thc i.4-second radioactive nitrogen-16 produced. The neutron gencmtor uscd by Coleman and Pcrkin is not deicribed in their paper (a),but from the fact that it uscd
[email protected].~-. deuterons one can surmise that it TI a i an clectrostatic machine of some sort. They rcport 47r total yirlds of fast ncutronr a t the zirconium-tritium tarpct of about 1010 neutrons per vcond, while T'cal and Cook made their runs a t neutron qource strengths up to 10'0 neutrons pclr second and normalizrd to lo9 (18). Runs on the low-cost neutron generator used by Steele and Alrkdw were made at yields betmern 2 x 10'0 and 2 x 109 neutrons per sccond under roughly the same circumstanccs. The limiting factor in the work of all three groups has been the decrease in strength of the tritium target with use. S e w concepts of target design available no!?- promise to improve this situation considcrahly. (y,n)~lj
APPARATUS, REAGENTS, AND PROCEDURE
Apparatus. Texas Kuclear Corp. Model 150 neutron generator. This is a machine of Cockcroft-Walton design which accelerates deuterium ions t o 150 k.e.v. It uses a target of tritium absorbed onto a thin layer of titanium, which in t u r n is backed by 2-mil stainless steel. Total cost for t h e facility was a b o u t 622,000 for generator and about $3000 for concrete shielding. No major building modification was required-only ground floor space to accommodate heavy shielding (12, 1 3 ) . Radiation Instrument Development Laboratory special 100-channel pulseheight analyzer with duplicate memories for use with short half-lived radio-
nuclides and two 3 X 3 inch XaI(T1) scintillation crystals (11, 12, 1 6 ) . Scintillation well counter using a 1 1 / 2 x 2 inch NaI(T1) crystal with 4 inches of lead shielding and scaler. Two-T proportional counter, fabricated locally according to Los Alamos design. Special transfer system to take samples from the generator to the detector in 3 to 4 seconds (12, I S ) . I'rrliminary information on our neutron goncrator facilities and twhiiiqurs is arailahlc (12-14), but it is also planncd t o publish a separate detailctl r i i x u s k m in thc near future. Reagents. ;inalytical reagent grade oxalic acid, sodium carbonate, and aninionium nitrate. Eastman Kodak p-aminophenol. 99.9970 copper foil. Procedure. Individual samples were irradiated for 30 seconds and transferred t o t h e scintillation detectors through t h e transfer tube x i t h a n average travel time of 4 seconds. The y-ray spectrum was measured and rworded with a multichannel analyzer. The counts in the nitrogen-16 photopeaks were taken as a measure of the amount of oxygen present. Known amounts of pure oxalic acid were employed as standards for quantitative measurements. IRRADIATION METHODS
Neutron Generator. Fourteen million electron-volt neutrons were produced by the H3(d,n)He4reaction in the 150-kv. Cockcroft-Walton neutron generator. At the renter of the irradiation position during operation, neutron fluxes varied from 5 X 10' t o 5 x 108 n set.-', depending upon the condition of the tritium target. These fluxes were measured continuously, however, by monitoring (with a Geiger counter) the target cooling water for i.4-second 5 ' 6 content produced by the 016(n,p)N'6 reaction on the water. The monitoring system in turn was standardized with copper foils using the Cu63(n,2n)Cu62 reaction (see Figure 1). The positron from the Cue2 was measured in a calibrated 27r proportional counter and this sample in turn used to standardize a well-type scintillation counter for copper activity measureVOL. 34, NO. 2, FEBRUARY 1962
* 185
5x10'
FAST
NEUTRON FLUX
S A M P L E : 0.0795 g H 2 C2 0 4 ' 2 H 2 O IRRADIATION T I M E ' 30 SECONDS DELAY: 4 SECONDS REACTION: o f 6 ( n , p ) N ' ~ NEUTRON ENERGY: 14 MEV NEUTRON FLUX: 1.79 x IO' n CM-* S E C - ' COUNTING T I M E : I M I N U T E COUNTS IN PEAKS' 3869 A" : 5779 COUNTS /Mlh
i
\
i
\
I
-4
LI
lo" 0
IO
20
I
I 40
30 50 TARGET L I F E ( M ! N )
60
73
Figure 1. Standardization of fast neutron beam monitor with copper foils IC
ment. A cross section of 0.51 barn was assumed for the copper reaction. Irradiation Facilities and TechOxygen samples, varying niques. from 0.005 to 0.1 gram, were sealed in medical grade polyethylene tubing and placed in a screw-cap, machined polyethylene capsule 1 inch long and 0.5 inch in diameter. With the accelerator beam switch in the off position, the samples were pulled into the irradiating position by a vacuum transfer system. The system consists of '/,-inch i.d. aluminum tubing and a vacuum cleaner along with the necessary solenoids, switches, and photoelectric circuits to start and stop the timers, cleaner motor, andmultichannel analyzer ( 1 2 ) . Irradiations were started by turning on the accelerator beam switch and were timed with a stop watch. At the end of 30 seconds, the transfer system was turned on and the sample sent to the detectors. Two feet in front of the detectors, the sample capsule passed through a photoelectric beam, which turns on the multichannel analyzer. The response time of the system is approximately the same as the
Figure 2.
travel time for the remaining 2 feet; therefore, the analyzer was started a t the same time as metal stops positioned the sample capsule between the two NaI detectors. The radiations from the sample were detected, analyzed, and stored for 1 minute. This information was simultaneously recorded both graphically on an X - Y recorder and typed out in digital form by a Hewlett-Packard printer (11). A record of the multichannel pulse analyzer dead time was taken continuously during the counting operation. The number of scintillation counts in the 6- to 7-m.e.v. photopeaks of nitrogen16 is proportional to the weight of oxygen present. All counts were normalized for decay (to the end of irradiation), for analyzer dead time (to O%),
Calibration Data for Oxygen Determination Normalized to a Fast Neutron Flux of 108 N Cm.-2 Set.-' (Sample, H&04.2Hz0. Irradiation time, 30 seconds. Transfer time, 4 seconds)
Gram 0.0052 0.0135 0.0370 0.0606
Flux,
N Cm. --2 Sec. -l 1.4 X 1.8 X 2.5 X 1.8 X
lo8 lo8 lo* lo8
A . Photopeaks Counts, Min.-I at End of h a d . 726!' 2,254 9,303 10,896
Specific Activity, Counts Min.-1 G.-1 96,919 95,402 100,984 100,435 98;435 f 2.4%.
Av. Corrected for analyzer dead time and neutron flux variation. Values for sample determination. 0 Error is "standard deviation" of four values. Statistically, higher counting values of Aq ?re more significant and hence some weighting factor should probably be used in determining error. Such a procedure would tend to reduce the value of this error.
186
ANALYTICAL CHEMISTRY
30 40 50 RELATIVE ENERGY
High energy
y-ray
60
70
spectrum of Nle
Showing extrapolation of base line under 6- to 7-m.e.v. photopeaks to elirninote contribution of other activities
Table I.
Oxygen,
20
and for fast neutron flux (to 108 n cm.-2 see. -I). Background radiations from other activities in the sample were eliminated by an extrapolation of the base line under the photopeaks as illustrated in Figure 2. The small oxygen contamination from the packaging materials was determined by irradiations of identical polyethylene tubing and capsules. Samples containing less than 15 mg. of oxygen were sealed and packaged in an atmosphere of nitrogen for the best results. DISCUSSION A N D RESULTS
Bombardment of 0 1 5 with fast neutrons (above 10 m.e.v.) produces We by a neutron-proton reaction. Radioactive nitrogen-16 decays with a half life of 7.4 seconds by beta emission to an excited state of oxygen hiid the oxygen returns to its ground state by emission of 6.13-, 6.91-, and 7.12-m.e.v. ?-rays in approximately second. Figure 2 shows a spectrum of these y-rays taken with a 100-channel analyzer on a neutron-activated sample of oxalic acid. The activity in the photopeaks was measured, normalized, and compared with a calibration curve to determine the weight of oxygen. The calibration curve was prepared by irradiating known weights of oxalic acid for 30 seconds, transferring these samples to the NaI(T1) detectors, and measuring the radioactivity for 1minute with a 100-channel analyzer. Table I lists the results of this calibration. A
number of standard samples of ssalio acid, sodium carbonate, p-aminophenol. and ammonium nitrate were then analyzed routinely to check this method, with the results given in Table 11. The relatively large errors for certain runs are probably due to the instability of the neutron beam monitoring system. It is expected t h a t larger counting rates, which could be made available with modification of this experimental arrangement, would reduce the relative error in the 5- to 100-mg. range to *5%. The calibration curve was established at the neutron flux level of lo8 n cmSW2 see.-' from a consideration of target life and sample area. Figure 1 shows that fluxes of this order of magnitude are available for more than 1 hour at the irradiation position. Since the fast neutron fluu varies approximately with l / r 2 in air (where r is the distance from target t o sample), a total sample area of 100 cc. is available for activation analvsis for periods up to 1 hour per targrt with an average flux of lo8 n sec.-' Neutron scattering in large samples (- 100 cc.) varies with the matrix; however, if the sample is homogeneous, this scattering can be determined readily by using thin copper foils. Self-absorption of the 6- to 7m.e.v. 0l6y-ray radiation in the sample is neglected. Kork is progressing on t h e design of an irradiation facility t o utilize such a volume. It is probable t h a t concentrations as low as 1 mg. of oxygen per 100 grams of oxygen-free matrix can be analyzed with this ac-
celerator, providing the neutron flux is monitored accurately. The only other isotope that produces N16 under 14-m.e.v. neutron bombardment is F19. This is accomplished by the neutron-alpha reaction F19(n,a)N16. Fortunately, F19 also undergoes a neutron-proton reaction F19(n,p)01Q t o produce 29-second Olg. By establishing calibration curves for both reactions, the interference of fluorine with oxygen analysis can be eliminated. Fluorine-oxygen ratios as large as 10 can be tolerated before the signal-noise ratio becomes too small for accurate analytical determination. ACKNOWLEDGMENT
The authors thank H. W. Kass for helpful discussions of the problem and R. W. Shideler for the design and construction of the transfer and neutron monitoring systems. LITERATURE CITED
(1) Ajzenberg, F., Lauritsen, T., Rev. Modern Phys. 27, 77 (1955). (2) . , Coleman. R. F.. Perkin. J. L.. Analust
84, 233 (1959). ' (3) Ibid., 8 5 , 154 (1960). (4) Elbling, P., Goward, G. IF7., ANAL. CHEM.32, 1610 (1960). (5) Faull, N., Brit. At. Energy Research Establ., Rept. AERE-RJR-1919(1956). (6) Fogelstrom-Fineman, I., HolmHanson, O., Tolbert, B. M.,Calvin, M., Intern. J. Appl. Radiation Isotopes 2,280 (19571. (71 Griffiths, V. S., Jackman, M. I., ANAL.CHEY.31, 161 (1959). (8) Harris, W. F., Hickam, W. M., Ibid., 31, 1115 (1959).
Table 11. Analysis of Known Oxygen Samples by Fast Neutron Activation Analysis Oxygen, RIg. %
Sample Oxalic acid
Added 5.2 5.2 5.2 5.2 5.2 37.0 31.2
Sodium carbonate p-Aminophenol 23.8 Ammonium 10.4 nitrate 40.3 54.0 58.2 58.2
Found 5.0 6.0 5.5 4.9 5.4 37.7 33.0 22.1 11.3 40.6 55.1 58.0 59.1
Error
- 3.8
t-15.4 5.8 - 5.8 3.8 1.9 5.8
+ ++ +
- 7.1 +10.4 0.7 2.0 - 0.3 1.5
++ +
(9) Iiallamann, S., Collier, F., Ibid., 32, 1617 (1960). (10) Leveque, P., Proc. Intern. Conf. Peaceful Uses of Atomic Energy, U.N., Kew York, 1956, Vol. XV, Paper 342. (11) Maddock, R. S., hIeinke, W. W., U. S. At. Energy Comm., Progress Rept. 8, Rept. AECU-4438 (1959). (12) Ibid.,Rept.9,Rept.TID-11009(1960). (13) Ibid., Rept. 10 (Kovember 1961). (14) hIeinke, W.,W., IAEA Conf. Use of Radioisotopes in Physical Science and Industry, Paper RICC/283, Copenhagen, Denmark, 1960. (15) Rleinke, W. W., I17ucZeonics17, S o . 9, 86 (1959). (16) Osmund, R. G., Smales, A . A., Anal. Chim. Acta 10, 117 (1954). (17) S,ue, P., Compt. rend. 242,770 (1956). 118) Teal. D. J.. Cook. C. F..ANAL.CHEM. 34, 178'(1962). '
RECEIVED for review September 27, 1961. Accepted November 17, 1961.
Nitrogen-13 in Hydrocarbons Irradiated with Fast Neutrons J. T. GILMORE and D. E. HULL California Research Corp., Richmond, Calif.
b The concentration of nitrogen in hydrocarbons can b e measured b y means of the 10-minute nitrogen-1 3 activity produced by irradiation with 14-m.e.v. neutrons. The analysis is simple and gives valid results in the per cent concentration range. Attempts to extend it to the part-permillion range have failed because even the purest hydrocarbons irradiated with fast neutrons give rise to a NI3 activity corresponding to several hundred parts per million of nitrogen. This activity does not arise from nitrogen or any other impurity, but it is produced
inherently in hydrogen-carbon systems at this energy by the reaction
H'
+ ClS = N13 + n1.
The protons receive the necessary energy b y recoil from collision with fast neutrons.
A
of hydrocarbons for nitrogen in concentrations greater than 0.1% is possible by fast neutron irradiation. Nitrogen-13 is formed by a n n,2n reaction on the nitrogen-14 in the sample (4) and can be measured by NALYSIS
counting the positrons or the annihilation photons. The 10-minute halflifeis convenient for irradiation and counting. The positrons are detected with a liquid scintillation counter and photons with a sodium iodide crystal. The neutrons are produced in the H2-H3 reaction in a 150-k.e.v. Texas Nuclear accelerator with energies ranging from 13.3 to 14.9 m.e.v. This energy is well over the 10.5-m.e.v. threshold for the n,2n reaction in N1* but is too low to excite the corresponding reaction in C12 and form 20-minute (211. At a deuteronbeam current of 0.1 ma., the average flux VOL. 34, NO. 2, FEBRUARY 1962
187
