alcohol, nitrobenzene, and not extracted into carbon tetrachloride, ethyl acetate, ethyl ether, or bromobenzene. Beer’s Law. A straight line was obtained over the range 1-10 ppm when absorbances were plotted against concentrations. Precision and Accuracy. The precision and accuracy of the method was studied by analyzing solutions containing known amounts of gold using the outlined procedure. The results are summarized in Table 11. Effectof Diverse Ions. Five milligrams of a foreign ion was added to a separatory funnel containing 90 pg of gold and the extraction was performed according t o the procedure outlined above. The following ions did not interfere: Na+, K+, Mg+2, Ca+2, VO+3, Cr+3, Mn+2,Zn+2, Ba+*, Sn+4, Cd+2, Pb+2, Al+3, Zr+4, UOz+2,T1+2, Pt+4, R u + ~Re+’, , Rhf3, NH4+,S04-2,
F-, NO3-, Clod-, Br-, acetate ion, citrate ion and EDTA. When 5 mg of a foreign ion interfered, masking agents were employed. The following ions did not interfere in conjunction with the corresponding masking agents: 5 mg of Be+* masked with 25 mg of F-; 5 mg of N P 2 masked with 2 ml of a 1 EDTA solution; 5 mg of C u f 2 masked with EDTA solution; 0.5 mg of Fe+3masked with 5 mg of F-; 5 mg of Bi+3 masked with 200 mg of citrate. The system could also tolerate 0.5 mg of W04-2; 1 mg of Hg+2; 3 mg of Cr207-2; 1 mg of Ag+ or 5 mg of Ag+ if the major portion of the AgCl formed was filtered off. Interferences due to Pd+2 and Co+2 could not be eliminated. RECEIVED for review September 23, 1966. Accepted November 21, 1966. Work supported by the National Research Council of Canada.
Uniform Neutron Irradiation of Inhomogeneous Samples G . L. Priest, Forrest C. Burns, and Homer F. Priest U. S.Army Materials Research Agency, Watertown, Mass. 02172 THE USE OF NEUTRON generators utilizing the 3H(d,n)4He reaction to produce 16Mev neutrons for activation analysis has presented some special problems because such a source is neither isotropic nor exactly reproducible in flux configuration from irradiation to irradiation. Studies reported elsewhere (1-2) have shown that a n isotropic flux configuration is not realized near the target but only a t a distance of several inches away. Uniform irradiation of samples a t a distance of several inches could be achieved, but unfortunately the neutron flux a t this point is decreased by several orders of magnitude, a n intolerable reduction in most cases. If the samples are placed as near to the target as possible to expose them to the maximum neutron flux, the asymmetry of the flux distribution is severe. There are two generally accepted procedures for neutron activation analysis. I n the first, the unknown and standard samples are irradiated separately with a flux monitor such as a neutron scintillator or a standard foil being used to determine the magnitude of the neutron flux for each. The flux monitor reading is used to normalize the data obtained for the sample and the unknown. This procedure has been quite successfully used by Anders (3) and his associates. In the second procedure the unknown and standard are irradiated simultaneously employing some kind of motion to ensure that they are both exposed to the same neutron flux. The authors prefer this second procedure. Planetary rotators designed t o simultaneously expose the unknown and the standard to the same neutron flux are available commercially ( 4 - 3 , but such devices give accurate results only for samples having a high degree of uniformity of distribution of the constituent being determined (6). A (1) H. F. Priest, F. C. Burns, and G. L. Priest, Nuclear Instruments and Methods, in press. (2) B. T. Kenna and F. J. Conrad, Health Physics, 12, 564 (1966). (3) 0. U. Anders and D. W. Briden, ANAL.CHEM., 37, 530 (1965). (4) Kaman Nuclear Corp., Colorado Springs, Colo., “Activation
Analysis Transfer System,” company bulletin.
( 5 ) Technical Measurements Corp., Ellison Division, Chamblee,
Ga., company bulletin. (6) D. E. Wood, P. L. Jessen, and R. E. Jones, Kaman Nuclear,
Colorado Springs, Colo., 1966 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 1966. 110
ANALYTICAL CHEMISTRY
simple calculation shows that for the case where the level of irradiation at the ends of 1.5-inch long samples is one half that in the middle, inhomogeneity of the magnitude experienced in many metals and semiconductors can give rise to either negative o r positive errors as large as 25 %. Various simple motions have been studied singly and in combination to determine what motion or combination of motions will ensure not only uniform irradiation of both unknown and standard but also that both receive an identical neutron dose. Two types of samples were considered: long, thin cylinders where radial effects are negligible, and disks where radial effects become important and longitudinal effects are minimized. EXPERIMENTAL
To simulate samples having a known distribution of impurities, a cylindrical configuration 0.31 3 inch in diameter and 1-2 inch long was chosen. The radioactive isotope chosen for the studies was 18F, 109.5 minute half life (7). The cross section for the reaction 19F(n,2n)18F(109.5min) is about 60 millibarns (8) so that for the size of samples considered adequate activity for good counting statistics is obtained by short irradiations. The half life is such that accurate corrections are easily made but is short enough that the radioactivity decays overnight permitting re-use of the samples. Allowing the samples to decay for 2 min eliminates interference due to z°F. Teflon (Du Pont) was used as the fluorine-containing material. Disks 0.313 inch in diameter and 0.125 inch thick were machined from Teflon rod. Each disk was numbered and weighed. Polyethylene disks of identical dimensions were prepared for use as inert fillers to enable preparation of cylinders having a variety of distributions of fluorine. By restricting sample diameter to 0.313 inch the radial neutron absorption was negligible obviating the need to rotate the cylinders on their own axes although such an addi(7) F. C. Burns, G. L. Priest, and H. F. Priest, U. S. Army Materials Research Agency, Watertown, Mass., unpublished data, 1965. (8) Texas Nuclear Corp., Austin, Texas, Table of Cross Sections for Fast Neutron Reactions, 2nd. ed., January 1964.
Figure 1. Parallel cylindrical rotator with symmetrical sample and holder read) for assernhly
spacers ready for assembly
tional motion would be important for dense materials such as metals. Disks of Teflon 0.625 inch in diameter and 0.125 in. thick were punched from Teflon sheet to provide samples for the inclined disk technique. Aqueous H F samples were contained in sealed polyethylene vials 0.5 inch in diameter X 1 inch long. Cylindrical irradiation containers 0.5-inch 0.d. by 0.313 i.d. were machined from polyethylene. Each container had a closure which held either a spring or a plastic rod against the sample to ensure that the sample did not move during a n irradiation. A hole in the bottom facilitated removal of the samples. All of the studies reported here except for the HF analyses and the inclined disk were made with the polyethylene and Teflon disks in various arrangements to simulate both homogeneous and inhomogeneous samples. Carriers to hold the sample containers were fabricated from acrylic plastic and mounted on a variable speed motor for rotation of the samples. While no problem existed in the present work from partial thermalization of the 14-Mev
by using cadmium sleeves over the sample containers (9). A ball slide was used to traverse the carrier motor assemblies across the target by means of a variable speed drive. The traverse was always symmetrical about the center of the target using the geometrical center of the sample as the reference point. The configuration of the simulated planetary rotator was identical to that of the commercial units, but the samples did not rotate on their own axes. This carrier was mounted with the plane of the two containers parallel to and 0.125 inch from the target cap and with the center of rotation as close as possible to the beam center. Figure 1 shows the parallel cylindrical rotator with one sample container nearly all the way out, a simulated uniform sample, and the closure as it would appear before assembly. In this carrier the axis of rotation is centered between the two containers, parallel to the plane of the target cap and centered on the beam with the outside of the carrier 0.125 in. from the cap.
Table I. Simulated Planetary Rotator Ratio Individual samples Unknown, counts/g Max Z dev Av Z dev Re1 std from mean Standard, counts/g from mean de+ Irradiation configuration. ~
Sample Unknown T-P-T-P-T-P-T-P-T-P-T-P-T Standard T-P-T-P-T-P-T-P-T-P-T-P-T T-P-T-P-T-P-T-P-T-P-T-P-T Unknown T-P-T-P-T-P-T-P-T-P-T-P-T Standard T-P-P-T-P-P-T Unknown T-P-P-T-P-P-T Standard T-P-P-P-T-P-P-P-T Unknown T-P-P-P-T-P-P-P-T Standard Unknown T-T-P-P-P-P-P-P-T-T P-P-P-P-T-T-P-P-P-P Standard T-T-P-P-P-P-P-P-T-T Unknown P-P-P-P-T-T-P-P-P-P Standard T-P-P-T-P-P-T Unknown T-P-P-T-P-P-T Standard T-P-P-T-P-P-T Unknown T-P-P-T-P-P-T Standard Unknown T-P-P-T-P-P-T Standard T-P-P-T-P-P-T Unknown P-P-T-T-T-P-P Standard T-P-P-T-P-P-T Unknown T-P-P-T-P-P-T Standard T-P-P-T-P-P-T Unknown T-P-P-T-P-P-T Standard T-P-P-T-P-P-T 0 T = Teflon. P = polyethylene. b Only for those cases with 5 or more samples,
0.973 1.075 1.017 1.075 1.142 0.910 0.959 1.054 1.038 0.978 1.113 0.917
13.7 12.0 36.8 31.4 22.7 24.2 18.0 13.8 8.0 1.6 19.4 2.7 3.8 4.8 3.4 2.7 2.5 4.5 1.4 10.6
6.4 5.8 18.4 16.4 15.1 16.1 12.0 9.2 6.6 1.6 13.4 2.7 2.5 3.2 2.3 1.8 1.7 3.0 0.9 7.0
10.6 10.4 10.1 17.4
7.0 7.0 6.7 11.6
7.8 7.2 22.3 20.0
Remarks Traversed and rotated Rotated only Traversed only Traversed only Rotated and traversed Rotated only Rotated and traversed Rotated and traversed Rotated and traversed Rotated only Rotated only Rotated only
VOL. 39. NO. 1, JANUARY 1967
11 1
Table II. Planetary Rotator Individual samples ,-
radiation counts/g dev conStandard, from Sample figuration counts/g mean Unknownn T-T-T 0.9899 10.4 Standarda T-T-T 10.6 Unknownb T-T-T 0.9893 21.6 Standardb T-T-T 22.6 0.
3 Teflon samples, '/z inch diameter X 3 Teflon samples 6/16 inch diameter X
inch long. inch long.
OEV. FROM MEAN
-20
10% OUNTS/QRAM
'c
6/~e
I/z
RESULTS
I n Table I the results are shown for a number of runs using the simulated planetary rotator. The sample configuration, the ratio between the unknown and the standard, and the maximum % deviation from the mean as well as the average deviation from the mean for individual disks of both standard and unknown are given for each run. Traversing the planetary rotator past the target improves the uniformity of irradiation, but has little effect on the agreement between unknown and standard. The lack of agreement between standard and unknown samples with the same configuration reflects the fact that this simulation device is not as highly accurate a device in terms of repetitive positioning as the commercial planetary rotation irradiation units. In Table I1 the results are shown for some ANALYTICAL CHEMISTRY
I
< "
dev from mean Remarks 6.9 (T.M.C. unit) 7.1 14.4 (Kaman nuclear 13.3 unit)
I n Figure 2 the carrier, samples, and sample spacers for the inclined disk method are shown prior to assembly for a n irradiation. This carrier is interchangeable with the parallel cylindrical carrier and is rotated and traversed in the same way. In this carrier the 0.625-inch diameter disk samples are centered on and at 45 O inclination to the axis of rotation of the carrier which also places them a t a 45" angle to the plane of the target cap. Thus, alternate sides of the disk are exposed to the neutron flux as the carrier rotates, ensuring uniform radial irradiation of the whole disk, In general, the carriers were rotated at 1000-1500 rpm, although this is not a critical parameter. Traverse rates from 4-12 inch per min were used and were not found to be a critical parameter. The samples were counted using a Harshaw No. 128-12-13 integral line 3- X 3-inch thallium-activated sodium iodide well crystal with a T M C DS13 preamplifier feeding into a T M C 404 pulse height analyzer. A T M C 522 resolver integrator was used to integrate the I S F peaks. The data was printed out o n a n IBM typewriter. Background counts were taken as frequently as necessary to ensure an accurate background correction. In the cases where the counting geometry was different between the standard and unknown samples, the samples were rearranged before counting to eliminate this effect, which, although a real problem in inhomogeneous samples, w o d d serve only to confuse the results in the present study. Each individual Teflon disk was counted as well as the complete samples. In this way a comparison of the uniformity of irradiation of each sample was obtained as well as the comparison of the reproducibility of the irradiation of the standard and unknown samples. All measurements were converted to counts per gram for comparison purposes.
112
20
P 0 SI1ION
1
I
1
2
1 3 14
1
S
6
1
7
~8 l 9 1 l 0 1 11
l
~
I
l
IZ1314ISlS
I
Figure 3. Comparison of planetary rotator and paralle cylindrical rotator results 0 Planetary unknown @ Planetary standard 0 Parallel cylindrical unknown 0 Parallel cylindrical standard irradiations using commercial planetary rotation units (10-11). Here the agreement between standard and unknown is excellent, although the large nonuniformity from end to end is still present. I n Table 111the results are shown for the parallel cylindrical rotator. Not only is the agreement between the unknown and standard excellent but the agreement between individual disks is also excellent indicating uniform neutron irradiation of the samples. B. & A. Electronic Grade 48% aqueous HF was analyzed using the parallel cylindrical rotator and a Teflon standard. Despite the fact that the sealed, polyethylene vials of HF were not full and had a markedly different density and fluorine content from the Teflon standard, excellent results were obtained. Three samples gave 48.7 %, 48.5 %, and 47.9 %. For the inclined disk method, the agreement between unknown and standard is equivalent to that obtained with the parallel cylindrical rotator, an average ratio of 1.01 being obtained for five runs. To confirm that adequate radial uniformity of activation was obtained, one disk was cut into a central disk and two nesting annular rings. The results of several runs showed a n average deviation from the mean of 1.93 % for the three samples. The outer ring was consistently higher as would be expected, indicating the desirability of keeping the diameter of the disk as small as possible. The level of activation was similar for all three methods, the planetary rotator being 1.4 times higher than the other two methods. DISCUSSION
In Figure 3 the results for symmetrical samples run o n the simulated planetary rotator and the parallel cylindrical rotator are compared graphically. The agreement between samples and the uniformity of irradiation of each sample is superior for the parallel cylindrical rotator. Off-center displacement of the center of rotation relative to the samples in the planetary rotator system can give rise t o marked differences between sample and unknown (12), while (10) Kaman Nuclear Corp., Colorado Springs, Colo., private
communication, December 1965. (1 1) Technical Measurements Corp., Ellison Division, Chamblee,
Ga., data taken on samples, November 1965. (12) Wm. E. Mott and J. M. Orange, ANAL. CHEM.,37, 1338, (1965).
I
I
Table 111. Parallel Cylindrical Rotator: Rotated and Traversed
Sample
Irradiation configuration“
Individual samples Ratio Unknown, counts/g Max dev Av dev Re1 std devb Standard, counts/g from mean from mean
z
z
Rotated and traversed across in. below ctr. of target
Unknown Standard
T-P-P-1-P-P-T T-P-P-’I-P-P-T
0.994
1.40 1 .oo
0.94 0.67
Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard Unknown Standard
T-T-T-1’-T-T-T-T-T-T-T-T-T-T-T T-T-T-7’-T-T-T-T-T-T-T-T-T-T-T
0.993
1 .oo 1.70
0.49 0.34
0.59 0.55
1.004
1.14 1.80
0.68 0.54
0.77 0.75
0.994
1.52 0.67
1.01 0.45
1.007
1.50 1.98
1.12 0.98
0.996
0.83 1.10
0.55 0.74
1.004
0.12 0.01
0.12 0.01
1.005
0.41 1.40
0.41 1.40
1.006
0.41 1.00
0.22 0.51
0.993
0.44 0.16
0.29 0.16
0.997
1.80 1 .oo
1.80 1 .OO
1 .ooo
1.90 0.98
1.17 0.98
1.013
0.60 0.86
0.40 0.57
1.006
0.55 1.05
0.37 0.69
0.976
0.90 1.50
0.60 0.98
1.003
0.50 0.45
0.34 0.30
1 .ooo
0.25 0.33
0.17 0.22
1.009
1 .00 1.10
0.68 0.74
T-T-T-1’-T-T-T-T-T-T-T-T-T-T-T
P-T-T-’I-T-T-T-T-T-T-T-T-T-T-T T-T-T T-T-T T-P-T-P-T-P-T-P-T-P-T T-P-T-P-T-P-T-P-T-P-T T-P-P-P-T-P-P-P-T T-P-P-P-T-P-P-P-T T-P-P-P-P-P-T T-P-P-P-P-P-T P-P-P-P-T-P-P-P-T T-P-P-P-T-P-P-P-P T-P-P-T-P-P-T-P-P-T T-P-P-T-P-P-T-P-P-T T-P-P-T-P-T-P-P-T P-P-T-P.P-P-T-P-P P- P-P-T-P-P-P-T T-P-P-P.T-P-P-P T-P-P-T.P-T-P-P-T P-P-T-P. P-P-T-P-P P-T-P-P.P-T-P-P-P-T P-P-P-P-T-T-T-P-P-P P-P-P-P-P-T-T-T T-T-T-P .P-P-P-P P-P-P-P-P-T-T-T T-T-T-P-P-P-P-P T-P-P-P-T-P-P-P-T P-P-P-T-T-T-P-P-P P-P-P-P-P-T-T-T T-T-T-P.P-P-P-P T-P-P-P-T-P-P-P-T P-P-P-T-T-T-P-P-P
Remarks
1.30 1.20
Traversed only
a T = Teflon. P = Polyethylene Only for those cases with 5 or more samples.
b
a 0.5-inch off-center displacement produces no degradation of results in the parallel cylindrical rotator method. A planetary rotator gives adequate results only for samples of homogeneous composition, where sample and standard are similar in properties, identical in size, and where the center of rotation is at the precise geometric center of the plane array of the two samples which in turn must be precisely positioned parallel and with the ends a t identical distances from the center of rotation. Any sever12 sample inhomogeneity will cause serious errors in the results even if the above conditions are met. A system in which a s1.andard and a n unknown cylindrical sample rotating o n their own axes and mounted parallel to each other in a mount which revolves the two samples about
a n axis centered between them and traverses across the neutron generator target gives high reproducibility and uniformity of irradiation of inhomogeneous samples. The positioning of the samples in this method is not critical in any respect, For the special case of disk-shaped samples, the inclined disk rotator has a great deal of merit in terms of simplicity and reproducibility. Also, if the disk size is matched to the well size of the scintillation crystal, the counting geometry is nearly perfect from unknown to standard.
RECEIVED for review July 18, 1966. Accepted October 28, 1966. Presented in part at the Fifth National Meeting Society for Applied Spectroscopy, June 1966. VOL. 39, NO. 1, JANUARY 1967
e
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