A Rapid,. Nondestructive Method of Precision Oxygen Analysis by Neutron Activation 0.U.
ANDERS and D.
W. BRIDEN
Radiochemistry Research Laboratory, The Dow Chemical Co., Midland, Mich.
b A new, highly ciutomated method of elemental analysis for oxygen is described, The method represents an improvement over previous techniques and is capable of determining as little as 100 p.p.m. of oxygen in all types of analyticcil samples. Only fluorine and fissionable materials interfere. Typical analyses of 7-ml. samples containing on the order of 1 to 100~o oxygen can be measured with relative errors of 2770/ requiring about 2 minutes of experimental time per sample. The techniques of spinning samples with a tangential air-jet during measurement is described. The techniques of fast-neutron monitoring are compared and CI method of measuring the effect of a variable flux on the production of a short-lived activation product is described. The dependence of the sample activation on the total macroscopic cross seciion and the vial diameter is demonstrated and a method is reported to minimize errors due to self-shielding.
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few years, several authors have reported methods of oxygen analysis by neutron activation. Osmund and Smales (7') , as well as Born and Wilkniss (3), employed secondary tritons obtained from admixed lithium in their samples to induce H.l-, F 1 d n and to the reaction 0 ' 6 obtain extreme sensitivities with reactor neutrons. Other authors (1, 4, 5 , 8-10) used the 0 ' 6 n + I;' N16 reaction to induce the N16 isotope with strong 14-m.e.v. neutrons which are available from small accelerators employing the H3 d + He4 n reaction for their production. This latter approach lends itself to rapid routine industrial analysis. The relatively low cost (9: of the equipment (about $25,000) , the shoi t time required for the activai ion to saturation, and the ready detxtability of the 6.1-m.e.v. gamma rays of N16, as well as the great demand for a reliable, fast, and cheap oxygen analysis method, make it attractive for industrial use. Three of the papers quoted report relative errors as low as 2.7% (9), 2.4y0 (10) and 1% (81 for special interference-free cases, while employing well crystal-type scintillation detectors URING THE PAST
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and sample-independent neutron flux monitoring. The reported accuracies were obtained for samples having approximately the same macroscopic cross sections as the standards with which they were compared. The well crystals employed yielded high counting efficiencies for obtaining the good sensitivity levels of down to 27 p.p.m. (8). When developing this technique for routine industrial precision analysis for a great variety of samples, the use of a well crystal would have disadvantages. Automation of sample handling by means of a one-way pneumatic system is difficult, if not impossible, to design for a well crystal counter and an expensive, rapid, reciprocating sample positioning and removing mechanism would be required. I n preliminary experiments with a 2-inch well crystal, interferences were encountered from high-energy beta emitters such as Bel', P34JF 2 0 , and Nez3; the short-lived fastneutron activation products of B; C1, S; Na, and Mg, respectively; and from short-lived fission products of uranium and thorium (8, 10). Even the high-energy beta particles of "6 might
have caused errors due to varying degrees of self-absorption in samples of different densities (6). We thus decided to employ for counting a n ordinary, cylindrical, %inch high and $inch diameter NaI (Tl) scintillation detector with a I-cm. thick plastic shield as beta absorber. Discrimination against interferences from all elements excepting fluorine and fissionable materials was effected by counting only gamma rays in the 4.5-m.e.v. to 7.5m.e.v. energy range, as was done previously for fluorine analysis by a similar technique ( 2 ) . The elimination of interferences of the boron activation product Bel1 was proved explicitly by obtaining single component decay curves from activated boric acid samples decaying with a half life of 7.3 seconds when measured 1 minute after the end of the irradiation, (cf. Table 11, sample 7). EXPERIMENTAL
The automated sample transfer system is depicted schematically in Figure 1. Its operation is as follows. The samples are weighed and packaged to completely fill 7-ml. polyethylene snap-
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Figure 1.
Pneumatic rabbit system for fast processing of oxygen samples VOL. 36, NO. 2, FEBRUARY 1964
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Figure 3. induced specific NI6activity per flux as a function of the self-shielding parameter D X 2 ,
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E C
I U M AIR P R E S S U R E PIN VIAL.LB.IIN2
ZT,macroscopic
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P O S I T I O N OF N O Z Z L E A 0 O V E ARRESTOR
Figure 2.
diameter.
total cross section for 14-m.e.v. neutrons.
D, sample
Flux measured with detector independent of sample
PIN,CY.
Performance of pneumatic vial spinner
Jet air pressure VI. jet position for vial 5.3 cm. long, 1.5 cm. in diameter. 1.d. of pipe, 2 cm.; nozzle diameter, 0.1 cm.
in a neutron flux of magnitude f (in neutrons per sq. em. per second) and decaying with the half life tllz is given by A = uiVf(1
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At)
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uNf (1 - e cap vials (2-dram polyvials from Olympic Plastics Co.). The vials are hermetically sealed by pressing their capped and trimmed end into the beveled hole of a 3/8-inch thick brass pan which has been heated on a hot plate. They are then introduced into the pneumatic system a t the sending station, S. Compressed air (about 18 p.s.i.) is applied from a pressure-regulated ballast tank, B , by the aircontrol solenoid to transfer the sample to the irradiation position, I . The sample is stopped there by a retractable arrestor pin, PI, inside the cadmiumcovered aluminum transfer pipe in front of the tritium target of the 150k.e.v. Cockcroft Walton deuteron accelerator. The sample is exposed to the nonhomogeneous, nonuniform, fast-neutron flux for 30 seconds and is spun by a cyclone set up below the sample by a laterally applied air-jet. The arrestor pin, P I , is then withdrawn by a solenoid, the sample falls free and is transferred by a second air blast to the counting position, C. While sample 1 is in transit, the arrestor pin, PI., is replaced and a second sample arrives to be irradiated. Sample 1 arrives a t a second arrestor pin, PpJa t the counting station approximately one second after leaving Z. Scaler, D , is turned on 2 seconds later and a 20-second count is collected. Only, pulses equivalent to gamma energies exceeding 4.5 m.e.v. are counted. After the count, the collected number of pulses is printed out via a Friden Add-Punch, F , and the scaler is reset for the next sample. After again 30 seconds, the arrestor pins, PI and Pz, are withdrawn, sample 1 is ejected
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ANALYTICAL CHEMISTRY
from the counter, sample 2 is transferred to the counter, and sample 3 arrives a t the irradiation position, etc. It is possible to process about 30 samples per hour and carry out triplicate determinations for each sample in this manner. It was necessary to measure the neutron flux to which each sample was being exposed to normalize conditions for the samples and the comparator standards. This became apparent when the neutron output for a steady deuteron beam was observed to decrease rapidly during such a 1-hour sample run because of aging of the tritium target. For this purpose then, a neutron counter was placed outside the shielding cube to measure the neutron flux a t a place where the reading could be assumed proportional to the neutron production rate. To facilitate the flux measurement for the samples following each other a t a rate of 2 per minute, and also to .compensate for changes in the flux during the irradiation of each individual sample, we employed an integrating count rate meter with the same 10.6-second RC integrating time constant as the mean life of the NIB activation product. The potential across a capacitor of size C (in farads) which is shunted by a resistor of size R (in ohms), when connected to a high-impedance charging circuit is given for time, t , after the initiation of the charging process, by
where Z is the charge rate (in amperes). The instantaneous disintegration rate of the radioactive activation product, A , induced in a sample during activation
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constant of the material, T its mean life, and u the activation cross section. By judiciously selecting the R and C of the ratemeter, we made R x C = 7.4/0.693 seconds, so that the ratemeter reading E a t any moment would be proportional to the instantaneous amount of "6 activity, A , in the sample, if Z and f are related linearly. The ratemeter is reset to zero and the integration is started at the beginning of the irradiation of each sample. The instantaneous ratemeter reading is thus dependent on the flux in the same way as the instantaneous "6 content of the sample. The final ratemeter reading, as displayed on a strip chart recorder a t the time the sample leaves the irradiation position, can thus be used for normalization of the data. It is a measure of the integrated effect of the neutron flux on the sample. This normalization factor can be read with less than 1% relative standard deviation and is linear with the neutron production rate of the source. RESULTS AND DISCUSSION
Early experiments carried out to test our system resulted in reproducibility data with relative standard deviations of =t3% for liquid standards made from varying concentrations of methanol in benzene. For solids and samples oi different composition, reproducibility was of the order of +60/, relative standard deviation. For averages of multiple runs of the same known sample as compared to the methanol-benzene
standards, the relativ? errors were of the order of 4 to 7%. To improve the reproducibility we installed pneumatic sample spinners both a t the irradiation and counting positions. These spinners each consist of an air-jet nozzle (latwally attached to the transfer pipe) which blows a jet of compressed air tangent [ally into the pipe just below the arrestor pins to set up an undisturbed cyclone wliich can spin the vials above it. Unreliable spinning, with occasional vials refusing to spin a t all, was esperienced initially with the air-jets positioned above the arrestor pins. Experiments proved that jets applied above the pin to the Fide of the vial will not affect spinning if they are applied more than one lengt? of the inside diameter of the pipe ahove the arrestor pin and belov the lop of the vial (Figure 2 ) . The present spinners are able to spin vials up to 50 grams in weight inside the 1.9-em. i.d. transfer pipe with about 15 p s.i. air pressure applied to the jets. The spinning reduced the relative standard deviation of individual activations of the solid samples to h2y0. It primarily eliminated errors caused by turning of the samples in transit to the counter. These errors were due to the nonhomogeneity of the flus and the action of the inverse :,quare law both during activation and counting. The turning of the vials in transit had resulted in higher or lcwer counts depending on whether the sample had faced the counter y;ith the more activated side of the vial which had been facing the tritium target during irradiation, or had exposed primarily the less activated side to thcb counter. By using spinners a t both irradiation and counting positions, errors due to the loose fit of the samples in the transfer pipe and nonhomogeneous packaging in the horizontal d i r t d o n of the vial (lumpy samples) were a so averaged out and the reproducibility improved. While spinning the vials markedly bettered the precision, it did nothing to the accuracy which is dependent, on three other parameters. Uniform density of the samples in the vertical direction and complete filling are needed to provide identical flux conditions for unknown5 and standards. The effect of the density was noticed when the second and t bird determinations of many samples \{ere consistently higher than the first, independent of the time elapsed between the determinations. Closer inspection revealed that, from the sudden impact'; of the vials a t the arrestor pins, the powdered materials would pack iensely a t the bottom of the vial while 1 he top portions were more loosely packed. Kneading of the vials so as to redistribute the sample in the vial just before introduction into
the transfer system eliminated this trend toward higher results and indicated the packing-down during the runs as the source of error. Loosely packed and only 3/4-full samples also gave high results with relative errors up to &20y0when compared to completely full standards. The geometrical relationships in the nonhomogeneous fast-neutron flux can readily account for theFe errors, a fact which indicates the great importance of constant geometry in fast-neutron activation work. We are now avoiding both these errors by filling the vials completely and compacting powdered samples tightly with a levered pestle. Even after these precautions were taken, relative errors for samples of different composition still were as high as 7% for certain substances. Plotting the induced specific NI6 activity per flux (counts per milligram per fluxmonitor reading) us. the total macroscopic cross section E T times the diameter of the vials, D , we obtained a graph seen in Figure 3. The values of the abscissa of this graph were calculated from the expression
the deeper layers of the sample and the geometric shadow behind the sample. The average flux inside a sample is thus a function of the macroscopic total cross section multiplied by the thickness of the sample as measured in the direction of the neutron flux. If a n exponential absorption law is assumed, the dependence of the relative flux on the depth x is given by
For a monodirectional flux, the average flux is the same in samples of parallel-piped configuration with one side facing the incoming particles, if the products of thickness time5 macroscopic cross section are the same; this average flux will vary with the sample thickness for the same type sample material. For cylindrical samples the same relationship holds. The average relative flux inside a cylindrical sample of macroscopic cross section Z T , arbitrary height, and diameter D , placed perpendicularly into a homogeneous monodirectional flux is given by:
D X Z r = D X
V is the volume of the sample, which in our case was a constant, 7 cubic em.;
i reprecents the element in a given compound, W %is the weight of element i in the sample, Mt its atomic weight, and c T ( 1 4 m.e.v.), is the microscopic total cross section of the element i for 14m.e.v. neutrons. The influence of the parameter D X Z r is readily recognized as the selfshielding effect for samples in a directional flux. ET is the microscopic total cross section times the number of nuclei per cubic centimeter and has the dimension cm.- l , that of an absorption coefficient. Self-shielding is the protection of deeper layers of a sample by nuclei lying closer t o the surface, and it results in a reduction of the flux to which the deeper layers are exposed. The total croqs sertion is used in our case, since even a single collision will, in general, eliminake from the effective flux a neutron which is capable of reacting with the 0'6 nuclei in the deeper layers of the sample. If the collision results in a nuclear reaction, the neutron is absorbed. I n the case of an inelastic scattering interaction, or elastic scattering on a nucleus of mass
