this nay the at)sorIxint~~ thcx solution incrcwwd, t h t ~ n doTable 111. Interference by HN, in twasetl to the original reading \\ ith c w h Determination of Fe+2 incroint~ntof azide. .it the vnd point a (Ceric titration in 5M H&O, at 5’ C.) pcrmanent increase in absorl)anccI \+as Fe+2,d l o l w n ed and succeeding incrcnients of “3, M Added Found n zidc increased t hc absorbance linearly. 0.0025 0,0495 0.0495 l‘hc true end point was obtaincd by 0.0062 0,0495 0.0495 plottiiig the absorbancy against thc 0,0125 0,0495 0.0499 azide concentration and c\tral)olating 0.0250 0.0495 0,0503 thr straight-line portion of the curve to ~01’0 absorbancy. Attempts to determine the end point visually gavc poorer procedure is e\;trcrndy sensitive and precision. large dilutions are necessary. The sulfamate concentration is obtaincd by RESULTS AND DISCUSSION difference from the hydrazine and The satisfactory results of the dcltcrsulfamate-hydrazine determinations; mination of ferrous ion in the prcscnce therefore, the precidion of the sulfamate 01 hydrazine are given in Table I. Data determination depends greatly on the for analyses of solutions containing precision of the hydrazine determination known concentrations of ferrous ion, and the ratio of hydrazine to sulfamate. sulfsniatc ion, and hydrazine are preI n nitric acid solution the hydrazinescntrd iii Table 11. Larger errors occur nitrous acid reaction produces low conin thtl hydrazine determinat,ion a t high centrations of hydrazoic acid that react conccntrations of hydrazine because the with ceric ion anti interfere in the I ~ Y I ~ I ~ I I111 .
of
ferrous tlt%miiii:ition. The rwults in Table 111 show that thc interference is not significant a t ~ ( J W concentrations of hydrazoic8 acid. LITERATURE CITED
( 1 ) .4udrieth, L. F., Ogg, B. A,, “The Chemistry of Hydrazine,” Wiley, New York, 1951. ( 2 ) Dukes, E. K., Wallace, R. M., L4NAL. CHEM.33,242 (1961). ( 3 ) Goddu, R. F., Hurne, D. N., Ibid., 26, 1740 (1954). ( 4 ) Pesez, hl., Petit, A,, Bull. SOC.Chim. France 1947, 122. (5) Pollard, F. H., Nickless, G., J . Chromatog. 4, 196 (1960). (6) Ryan, J. L., U. 8. At. Energy Comm., Rept. HW-59193 (February 1959). ( 7 ) Wallace, R. M., Dukes, E. K., J . Phys. C‘hem. 65, 2094 (1961). (8) Watt, G. W., Chrisp, J. D., ANAL. CHEM.24,2006 (1952).
RECEIVEDfor review March 20, 1962. Accepted June 14, 1962. Information developed during work under Contract AT(0iT2),-lwith the U. S. Atomic Energy Commission.
Determination of Deuterium Concentration in Heavy Water by the Reaction Oxygen-16 (d, n ) Fluorine-17 Induced by Reactor Neutrons SAADIA AMIEL and MAX PEISACH Israel Atomic Energy Commissionllaboratories, P. 0.Box 527, Rehovoth, Israel
b Recoiling deuterium nuclei from n-d collisions in heavy water react with oxygen. The 66-second fluorine-1 7 resulting from the reaction is measured, and its activity is used as a monitor for the deuterium concentration. In a fission spectrum neutron flux of 1 O I 2 n per cm.Z-sec., 1 gram of 100% D 2 0 produces about 14 pc. of F17 at saturation. The amount of F I 7 produced is proportional to the deuterium content over a wide range.
T
HE APPLICATION of reactions induced by knock-on particles in reactor irradiations has recently drawn considerable attention (7, 10, 11). Proton recoils i n hydrogenous media were used in reactions such as 0 1 8 (p, n)FlS for determining oxygen-18 abundance in hydrogen-normalized water samples (4). Other reactions of the types (P, n), (P, 71, b, 4, etc., with isotopes of light elements have been observed in aqueous solutions irradiated h-ith fast neutrons (7, 10, 11). I n this work the occurrence and applicability of the nuclear reaction 0 I 6 ( d , n)FI7 ini-
tiated by neutron-deuterium recoils were investigated. Fluorine-17 is a pure positron emitter decaying with a half life of 66.0 1 0 . 3 seconds to stable oxygen-17. It can be produced in water irradiated with fast neutrons by the secondary reactions 0 1 6 ( p , 7)F17, and 016(d, n)F17 following n-p or n-d collisions, and has already been identified as a radioactive ?onstituent of the heavy water moderator of some reactors (6). I n the neutron energy range of 1 to 8 m.e.v., the cross sections of n-p and n-d interactions are reported to be about equal, varying from 4.3 to 1.2 barns and from 3.0 to 1.3 barns respectively (9). The production of fluorine-17 as a function of deuterium content was studied as a means of determining the latter. The yields from the competing 0 1 6 ( p , y ) and 016(d, n) reactions, both of which give the same product, were determined for reactor neutrons. The advantage of using fluorine-17 as a monitor for deuterium content lies in the fact that, in the absence of other positron emitters, i t can readily be detected and measured by its positron
.
decay. Using this property, the irradiated water sample does not have to be processed further. among the usual methods for determining deuterium in water are mass spectrometry, interferometry, optical and infrared spectrometry, and density measurements ( 5 ) . The only method utilizing nuclear properties is that based on photoneutron emission (3, 8 ) . EXPERIMENTAL
Samples of 1 gram were prepared for irradiation by mixing known weights of heavy nater containing 94.5% D20 with natural water. It is essential that all samples be normalized with respect to natural oxygen. The DzO content of the prepared samples ranged from about 0.5 to 94.5%. The samples were contained in polyethylene vials which \\ere heat-sealed before use. Natural B ater samples, similarly prepared, were used as blanks. The samples were irradiated in the pneumatic tube system of IRR-1 (The 5 M w Israel Research Reactor No. 1 a t Nahal Soreq) for 66 seconds which produced half-saturation yields of fluorine-1i. Irradiations were carried out VOL. 34, NO. 10, SEPTEMBER 1962
1305
k,
10000
lot
I 2 3 4 5 6 7 8 9 1011 121314 (minutes)
Figure 1. Decay of positron annihilation y-radiation showing relative amounts of 66-second F17 and 10minute N13 produced in a 48% DzO sample irradiated for 66 seconds at an epicadmium flux of 6.7 X 10" n per cm.2-sec.
a t power levels of 100 to 500 kw., corresponding to fission spectrum neutron fluxes of about 1.4 to 6.7 X 1011 n per cm.z-sec. This flux was monitored by counting NaZ4 produced in aluminum foils. Effective deuteron and proton recoils occur with neutrons of energies of several million electron volts. Preferably slow and epithermal neutrons should be eliminated because they do not contribute to fluorine-17 production, and they contaminate the sample by radioactivation of impurities by (n, y) reactions. Cadmium covers, shaped from 0.75-mm. sheeting, proved efficient as no foreign activities were detected. Cross contamination from the cadmium cover was prevented b y wrapping the vials in polyethylene foil. In the absence of such covers, radioactive impurities could be discriminated against by coincidence counting of positron annihilation y-rays. After irradiation the vials were removed from the carrier rabbit, cover, and wrappings, and dropped into a well counter; counting was started about 10 seconds after the end of irradiation. The counting arrangement consisted of a 11/2 X 11/2 inch N a I (Tl) well-type scintillation counter connected to a single channel pulse height analyzer set to allow pulses from positron annihilation y-rays to pass t o an automatic recording scaler.
well. All counting was calibrated against a standard sodium-23 source. The relationship between the observed fluorine-17 activity and the deuterium concentration is shown in Figure 2. The curve was apparently linear over the entire range of concentrations from 0.5 to 94.501, DzO. I n n fission spectrum neutron flux of lo'* n per cm.2-sec., 1 gram of 100% DzO produced about 14 pc. of F17. This corresponds to an effective cross section for 016(d, n)Fl7 of 17 microbarns per DzO molecule for fission spectrum neutrons. When the specific activity, A, of fluorine-17 per mole water. whether in hydrogen or deuterium form, was plotted against the number of milligram-atoms deuterium, D, a straight line was obtained in the range of higher deuterium concentration (Figure 3, I). The extrapolated line passed through the origin, but the points representing samples low in deuterium all lay above the line. The extent of the deviation from proportionality was obA against D tained from the plot of 11) which showed that the (Figure 3, deviation started near 15 mg.-atoms of deuterium and increased rapidly as the deuterium content decreased towards zero. Above 15 mg.-atoms D per gram of water, the yield of fluorine-17 per mole deuterium was independent of deuterium content. DISCUSSION
The proportionality between the fluorine-17 yield and the deuterium content over a wide range (Figure 3), shows that the reaction 0l6(d, n)F17 is the predominating one in the production of fluorine-17. Any deviation from proportionality has to be attributed to 0 ' 6 ( p , y)F1' caused by proton recoils. While the relatively high contribution of the ( p , y) reaction to the fluorine-17 yield in the low deuterium range destroys the proportionality between the fluorine17 activity and the deuterium content, the linearity of the F17 yield with deuterium content is preserved over the entire range of concentrations (Figure 2). The over-all fluorine-17 yield is obtained from the two reactions whose
401 IO3
Figure 2. Calibration curve of deute rium concentration against observed F" activity of 1 gram of water at half saturation. Samples irradiated a t neutron Aux of 10" n per cm.2-sec.
6.7 X
individual yields are proportional to the content of the corresponding hydrogen isotope. Other reactions induced in pure water samples under neutron irradiation are unlikely to Iead to fluorine- 17. ilnalysis of the yields from the two competing reactions should take into consideration the relative yield-determining factors which are: The threshold for the ( p , y) reaction is much lower than that for the (d, n)Le., neutrons of lower energies could lead to F17 production in the former reaction but not in the latter one. (Qm values are given in Table I). From momentum considerations, recoiling protons will attain relatively higher energies than corresponding deuterons. Under the conditions of the experiment the enerav sDectrum of fission neutrons may x e distorted since the irradiation takes place a , certain distance from the fuel element in the reactor pool. This results in a shift of energies of neutrons to the lower energy range due to some moderation by the pool water. Accordingly, an effective cross section, if, can be obtained which incorporates the threshold energies and excitation functions for the corresponding reartions and the energy of recoils
RESULTS
Table 1.
The gamma spectra of heavy water samples, irradiated in an epicadmium neutron flux under the above conditions, consisted of a single photopeak at 0.51 m.e.v. which exactly matched the positron annihilation photopeak of sodium22. The decay of the photopeak is shown in Figure 1 from which the relative contributions of 66-second fluorine17 and 10-minute nitrogen-13 can be seen; prolonged counting reveals the presence of 112-minute fluorine-18 as 1306
ANALYTICAL CHEMISTRY
Reactions .Leading to Positron Emitters in Neutron irradiated Water Samples (2)
Source Sample
Reaction OIB(d, n)F17 ~ ' Y P ,r)F17 OIB(p,a)N13
OIB(d,y)F'8
0l8(p, n)F'* Container
Qm (m .e.v.) -1.631 0.596 -5.208 7.538 -2.450 1.941 -0.286 -3.005
Half Life of Product (Minutes) 1.1
10 112 10
at the prevailing conditions of the energy spectrum of the neutron flux. The ratio of the effective cross sections for the (d, n) and ( p , y) reactions wit,h Ole, R, should be given by the ratio of the fluorine-17 activities obtained from D 2 0 (100%) and H20(100%). It was found that
deuterons a t different isotopic concentrations of hydrogen-Le.,
D I milligram- atoms)
which showed that for equal numbers of atoms of the corresponding recoiling hydrogen isotope, the contribution of the ( p , y) reaction was about 11/2% of that of the (d, n) reaction. Because the container was made of I)olyethylene, a hydrogen-containing material, part of the ( p , y) contribution could have been due to protons rcroiling from the walls. Because the ratio, R. of yields of the (d, n) and ( p , y ) reartions is large, it enables analvsis of samples in the region where the fluorinp-li activity remains directly proportional to the deuterium content to be carried out relative to a single standard. Thi. procedure is suitable for samples rontaining above 15 mg.-atoms deuteriuin per gram of water. However, this limitation depends on the value of the ratio, R, which in turn is dependent on the energy distribution of the fast neutron flux due to the differenre in the threshold energies of the two competing reactions. Accordingly, the w l u c of R with an undistorted fission specatrum neutron flux might be evpected to exceed 60, and hence would extend the range of applicability of this proredure. When the deuterium content is less than about 1.5 mg.-atom< per gram of water, analyses can no longer he referred to a single standard. but a linear calibration curve such as given in Figure 2 has to be used. The mathod is then applicable to the entire range of conrentrations. Neutron activation of nater and container material componeiits which could yield positron emitters either directly or by proton or deuteron recoils' are given in Table I (d), which shows that N13and F1*are the nuclides most likely to accompany F17. These nudides were both observed but because of their widely different half lives, their presence can easily be allonvd for. However.
Figure 3. Variation of terium content
F17
with deu-
Specific activity of F17 per mole water, A Specific activity of F17 per mole water per mole deuterium, A / D , plotted against deuterium content, D
(1) Ill)
their formation introduces a source of error which becomes appreciable in samples of very low deuterium content, below about 1% DzO by weight, and may be considered as a limiting factor of the method. The relative accuracy of this method, using fluorine-17 as a monitor for the deuterium content, depends on the stability of the neutron flux, reproducibility of sample position and on counting statistics. The first two cease to be important if a standard is irradiated simultaneously with the sample. The relative standard deviation of the result due to the statistical errors of counting samples of high deuterium content is about O.l%, but in practice a value of 0.5% is generally obtained, considering the stability of the irradiation parameters and the counting equipment. The activity of the sample maj7 be obtained either by extrapolation of the decay curve or by measuring the integrated count over a fixed period. As the activity is proportional to the sample weight, the sample may be reduced at the expense of accuracy. The minimum sample size is determined by the accuracy of sample preparation and the activity obtained rather than by the loss of recoiling deuterons from the sample, because the range of deuterons of several million electron volts energy is of the order of 0.1 mm. (1). The observed constancy of the fluorine-li yield over a wide range of deuterium concentration suggests that there is no observable difference in the corresponding energy losses of the recoiling
The present method provides a simple and rapid nondestructive method for determining heavy water concentrations useful for heavy water reactors or heavy water process control. It could meet the required accuracies in both fields, 1% for DzO production and down to 0.1% for heavy water reactors. With a conventional counting arrangement near a reactor, several dozens of samples per day could be analyzed. The method is readily applicable to high-energy neutron distribution measurements in water systems, as in reactors. It is superior to foil techniques because the homogeneity of the medium is preserved. ACKNOWLEDGMENT
We t,hank the operating crew of
IRR-1 for the reactor irradiations and D. H. Samuel for supplying analyzed heavy water. Thanks are due t o I. Dostrovsky for his continuous interest and criticism. LITERATURE CITED
(l).Aron, W. A,, Hoffman, B. G., Wil. hams, F. C., U. S. At. Energy Comm AECU-663, (1951). (2) Ajzenberg-Selove, F., Lauritsen, T., Nucl. Phys. 11, 150 (1959). (3) Baranov, V. I., Kristianov, V. K., Karasev. B. V.. Dokl. Akad. -Vauk S.S.S.R. 129, 1035 (1959). (4) Dostrovsky, I., Samuel, D. H., Israel Atomic Energy Commission Laboratories, P.O. Box 527, Rehovoth, Israel, private communication, 1960. (5) Duke,, D. W., Babcock and Wilcox Co. Rept. BAW-1209, (1960). (6) Ernst, M. L., E. I. du Pont de Nemours and Co. Rept. DP-202, (1957). (7) Glickstein, S. S., Winter, R. G., Nucl. Znstr. Methods 9, 226 (1960). (8) Gokhale, V. A , , Navalkar, M. P., Srinivasan, M., Subbaramu, X. R., India Bt. Energy Establishment, U. S. At. Energy Comm. NP-9166, (1960). (9) Hughes, D. J., Schwartz, R. B., "BNL-325 Neutron Cross Sections." U. S.Govt. Printing Office, 1958. (10) Roy, J. C., Bresesti, M., Hawton, J. J., Can. J. Phys. 38, 1428 (1960). (11) Stehn, J. R., Trans. Am. lVucl. SOC. 3, 467 Paper 25-10, Dee. 13, 1960. for review December 18, I961 RECEIVED Accepted March 30, 1962.
VOC. 34, NO. 10, SEPTEMBER 1962
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