Determination of Stoichiometry of Vanadium and Titanium Oxides by 14-MeV Neutron Activation Carmine Persiani and James F. Cosgrove The Bayside Laboratory, Research Center of General Telephone & Electronics Laboratories, Inc., Bayside, N . Y .
ELECTRONIC PROPERTIES of vanadium oxides can be greatly affected by the large lattice defects formed in certain monoxides. These metal oxides are good conductors, having resistivities similar to metals but in some cases with negative temperature coefficients. The resistivity and the temperature coefficient of resistivity are strongly dependent upon the oxygen-vanadium ratio. The correlation of resistivity measurements with vanadium and oxygen concentrations for VO, type of compounds is essential in investigating the conduction mechanism. A rapid method for determining the chemical composition of prepared vanadium oxides was developed using fast neutron activation analysis. During the past several years, the majority of analyses dealing with the determination of vanadium, have been carried out using spectrochemical and spectrophotometric techniques. The inert gas fusion method has been used widely for determining oxygen. Most of these methods for vanadium and oxygen determination require a series of chemical analytical procedures and are therefore time consuming in addition to being destructive. Neutron activation analysis has been mainly confined to the determination of trace impurities. Little work has been reported for the analysis of minor and major amounts of vanadium and oxygen using this technique. Hoste (1) has determined vanadium in high alloy steels in the 0.01-1 % range using thermal neutron activation. Saltys and Morrison ( 2 ) have also used reactor activation for determining the stoichiometry of sodium-vanadium bronze crystals. Oxygen values were estimated by difference. The recent availability of neutron generators has greatly extended the capability of activation analysis. This technique, using a portable neutron generator, was found t o give results for vanadium and oxygen with accuracy comparable t o conventional methods. Also, the speed of the method greatly reduced the time normally spent on analysis. Vanadium monoxides and some mixtures of VO and T i 0 were analyzed, using 14-MeV neutron activation, in a wide composition range without interferences. EXPERIMENTAL
Neutron Source. Neutrons of 14.5-MeV energy are produced by the d,t reaction in a Technical Measurement Corp. Neutron Generator, Model 211, in which deuterium ions, accelerated t o 200 kV, bombard a tritiated titanium target. Neutron monitoring is accomplished by using a neutron scintillator which is fixed at a position near the drift tube of the generator and connected t o a radiation recording scaler located near the generator console. Sample Transfer System. Samples and standards are transferred t o and from the neutron source by means of a pneumatic tube system using compressed nitrogen gas. Sample capsules are threaded polyethylene containers approximately 1 inch long and l/* inch in diameter. Irradiation and counting
(1) J. Hoste, J. Pure Appf. Chem., 1,99 (1960). (2) M. N. Saltys and G. H. Morrison, ANAL.CHEM., 36,293 (1964).
1350
ANALYTICAL CHEMISTRY
times are automatically controlled with preset timers. The transfer time of a capsule from the neutron field to the detectors is two seconds. Irradiation of Standards and Samples. In order to obtain data on the precision and accuracy of the method, a series of prepared samples of known composition were analyzed using V206 and TiO, as the standards. Standards and samples ranging from 200-400 mg were weighed directly into the polyethylene capsules. The loaded capsules were transferred through the pneumatic tube system between the neutron field and the detectors. The scintillation spectrometer used two 3” X 3” NaI(T1) crystals coupled to a T M C Model 404C pulse height analyzer. Irradiation times were normally 1 minute for vanadium and 30 seconds for both titanium and oxygen. Five-minute counting times were used for vanadium and 1 minute for titanium and oxygen. Shorter irradiation and counting times were initially used because of the higher neutron fluxes produced by a new tritium target. No additional delay time was used for titanium and oxygen determinations because of their short half lives. Delays up t o 1 minute were used for vanadium analysis with no titanium present. As target life decreased, longer irradiation and counting times were used for vanadium analysis. Neutron fluxes of 1010 neutrons/cm*-sec were obtained with a new target. This output was calculated based on the measurement of radioactivity produced by the 63Cu(n,2n)62Cureaction using a copper activation monitor foil. Vanadium and titanium analyses were conducted first, followed by the oxygen analysis. This procedure permitted a rapid analysis without waiting for the longer lived 51Tiactivity formed from vanadium to decay. Radioactivity Measurement. After irradiation the polyethylene capsules containing the samples were sent automatically t o the counting equipment rjia the pneumatic tube transfer system and the activity measured. The activity caused by vanadium and by titanium were counted simultaneously. Their gamma ray energies are sufficiently separated to permit their determination without separations (3). The oxygen analysis was performed separately, and the 6-7 MeV activity due to 16N was measured. Photopeak areas were obtained by summing the information stored in the Analyzer Memory within the selected band of channels. Each channel count is recorded and totaled automatically using T M C Model 522 Spectrum Resolver-Integrator. Background activity on both sides of the photopeak is determined and subtracted from the total peak area. Photopeak areas were also normalized for dead time and neutron flux variation. Quantitative results were obtained from the ratio of the standard and sample activities. RESULTS AND DISCUSSION
The procedure described was used to analyze some vanadium and titanium oxides of known composition, the results of which are shown in Table I. Oxide sample designated GT-1 was synthetically prepared by combining known
(3) M. Cuypers and J. Cuypers, Air Force Weapons Lab. Rept.
DDC-AD-816 628 (1966).
~~
Oxides V?OJ
VO?
Ti0
GT- 1
Oxide
Table I. Analysis of Vanadium and Theoretical, % Found, % 0 V Ti 0 V Ti 32.0 68.0 ... 32.1 67.6 ... 31.6 66.1 32.5 68.5 32.5 67.9 32.6 67.0 38.6 61.4 ... 41.6 57.9 41.2 58.3 40.5 58.2 40.9 58.7 40.9 59.5 25.0 ... 75.0 24.8 ... 75.4 24.4 75.3 25.2 73.8 24.9 74.7 24.7 75.9 42.0 28.0 30.0 42.1 27.7 30.8 42.5 27.6 30.4 41.9 27.3 28.9
0
31.8 40.1 34.5 21.6 26.2 28.4 21.1 25.0 28.6
~
Titanium Oxides Average and standard deviation, L y 0 V Ti 32.3 h 0 . 4 6 7 . 4 h 0.9 ...
Total, 99.7
41.0 + 0 . 4
58.5 i 0.6
...
99.5
24.8 =I=0 . 3
...
75.0 rt 0.7
99.8
42.2h0.3
27.5h0.2
30.01!~0.6
99.7
Table 11. Comparison of Neutron Activation with Chemical Method, V Ti Total ... 99.4 67.6 ... 99.3 59.2 65.5 100.0 ... ... 98.2 76.6 ... 98.6 12.4 I.. 99.7 71.3 78.9 100.0 ... 75.0 100.0 ... 71.4 100.0 ...
Chemical Methods Neutron activation, 0 V Ti 67.4 32.3 ... 58.5 41.0 ... ... 34.0 65.4 77.8 22.0 ... 73.9 26.0 ... 72.0 28.0 ... ... 80.3 19.5 26.0 74.0 ... 70.9 ... 29.2
Total 99.7 99.5 99.4 99.8 99.9 100.0 99.8 100.0 100. 1
32.3
6.9
61.4
100.6
32.5
6.6
61.1
100.2
32.7
33.9
33.3
99.9
33.4
33.4
34.1
100.9
amounts of V 2 0 6and TiOz. The resulting sums for several compounds give totals less than 100%. This was assumed t o be due to the presence of nitrogen impurity and was confirmed by chemical analysis. Micro Kjeldahl analysis shows the presence of nitrogen, and in some cases amounts up to 0.5 were found. The results for vanadium and oxygen as shown in Table I are in good agreement with the theoretical amounts present in the metal oxides. Table I1 shows a comparison of results obtained by neutron activation and chemical methods ( 4 ) . A n inert gas fusion method was used for oxygen analysis and a spectrophotometric technique was employed for the determination of vanadium. The samples were prepared within the laboratories, and the nominal values are shown as the oxide samples in the table. I n addition t o the analytical methods mentioned, a n electrical measurement method has been developed for correlating electrical properties with the stoichiometry of vanadium and titanium oxides (5). Some samples were analyzed by this method. The results, shown as the oxygen-metal ratio, are (4) E. W. Lanning and R. P. Weberling, ANAL.CHEM., 40, 626-9 (1968). (5) R. Steinitz, General Telephone & Electronics Laboratories, Inc., Bayside, N. Y . , unpublished data, August 1967.
Table 111. Comparison of Stoichiometry by Electrical and Neutron Activation Measurements Electrical Neutron measurement, activation, Sample OIV OIV vo0.s 0.89 0.91 VOl.0 1.05 1.13 vo1.2 1.26 1.24 TiOl. o 1 .OO 1.02
compared t o those obtained by neutron activation in Table 111. Although the electrical measurement method is rapid and accurate, it is restricted to single phase compounds and cannot be used for multi-element compounds which are present in more than one phase. I n addition, unknown interstitial impurities can lead to serious errors in measurement. Activation analysis with a neutron generator is capable of analyzing suitable samples at the minor and major level of concentration. The method compares favorably with chemical methods in precision. The method is non-destructive, rapid, and suitable for laboratory control work. For the elements analyzed and under the conditions used, the precision was found t o be 3-5 relative per cent a t 95% confidence level. The precision and accuracy will be affected VOL. 40, NO. 8, JULY 1968
0
1351
t o a great extent by the target life. As the target life decreases, the neutron output decreases also. A greater variation can be expected due to the decreased activity measured. Errors due to neutron flux inhomogeneity were kept to a minimum by using standards having the identical geometry as the samples. In addition, the matrix material and form of the sample were the same as that of the standard. Rotation of sample and standard during irradiation and flux monitoring using a counter close to the irradiation station further reduced the errors caused by flux variation. No interferences were encountered during the analysis. However, chromium, if present in large concentrations, can cause some interference in the vanadium analysis through a
W r (n, cr)"Ti reaction. However, 54Crhas a low abundance and the cross section for the reaction is small, so that this interference is unlikely to be serious. ACKNOWLEDGMENT
The authors thank E. W. Lanning and R. P. Weberling for performing the comparative chemical analyses.
RECEIVED for review March 7, 1968. Accepted April 16,1968. Presented a t the 19th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1968.
Determination of Parts-per-Million Quantities of Plutonium-236 in Plutonium-238 by Alpha Pulse Height Analysis Mary Lou Curtis Monsanto Research Corporation, Mound Laboratory, Miamisburg, Ohio P L U T O N I U Mproduced -~~~ by irradiation of neptunium-237 contains small amounts of plutonium-236, the decay product of neptunium-236, produced by (n,2n) or (7,n) reactions in the neptunium-237. The plutonium-236 (2.86 yr) decays to uranium-232 (72 yr) and eventually to thallium-208 (3 min) which emits a 2.6-MeV gamma ray. The possible build-up of this high energy gamma emitter over a period of time is of concern where plutonium-238 is used in a power source, as in an artificial heart. A method of determining ppm (partsper-million) quantities of plutonium-236 in plutonium-238 is therefore required. This can be accomplished directly, quickly and simply by alpha pulse height analysis as described in the subsequent sections. The general principles involved in the method are well known; however, the specific techniques described resulted in the high sensitivity of the measurements. These techniques are applicable for the determination of parts-per-million concentrations of impurities in any alpha emitter, provided the trace impurity is higher in energy than the major component ( I ) .
Channel Number L
Figure 1. Alpha spectrum of plutonium-238 containing plutonium-236
EXPERIMENTAL
Apparatus. An Ortec Model SCCJ surface barrier detector, Industrial Development Products vacuum chamber, power supply, and amplifier system, and a Packard Instrument Co. 400-channel analyzer were used for the work. Resolution was 25 keV at full width, half maximum. Method. The system was calibrated to about 5 keV per channel, covering the range from 4.5 to 6.5 MeV. The alpha spectrum of approximately one part per million plutonium236 in plutonium-238 is shown in Figure 1 . The favorable half-life ratio of the two plutonium isotopes (87.4 yrj2.86 yr) ensures more counts by a factor of 30.82 per unit weight of plutonium-236 than from plutonium-238. The less energetic component of plutonium-236 is 221 keV more energetic than the highest energy alpha from plutonium-238. This represents a 44-channel separation. (1) An indirect and another direct technique are discussed by W. H. Smith, F. K. Tomlinson, D. W. Eppink, and G. R. Hagee, "Determination of Parts-per-Million Quantities of Plutonium-
236 in Plutonium-238," AEC Report MLM-1486 (to be published). 1352
ANALYTICAL CHEMISTRY
Samples were prepared by electroplating the solutions onto polished slides to get a uniformly thin sample deposit ( 2 ) . Activity levels were kept low to reduce the probability of pulse pile-up (the combination of two pulses occurring nearly simultaneously to form one larger pulse simulating that from a higher energy alpha), an effect which increases with counting rate. With the counting systems used for this work, no pulse pile-up was observed when a maximum of 5 x 103 alpha particles per minute was allowed to impinge on the sensitive area of the detector. To get the desired statistical accuracy for plutonium-236 at this counting rate, a minimum of five 200-min measurements were made with each sample. The analyzer memory was not cleared between measurements. If the sample counting rate exceeded 5 x lo3 cpm, part of the sample deposit was covered with cellophane tape. This reduced the counting rate without distorting the spectrum. Alpha particles emitted by the portion of the sample which was covered were totally absorbed in the tape. (2) M. Y . Donnan and E. K. Dukes, ANAL.CHEM., 36, 392 (1964).
