An Irradiation, Transfer, and Counting System for Neutron Activation Analysis of Shod-Lived Components in lnhomorreneous Samples Homer F. Priest, Forrest C. Burns, and Grace L. Priest Army Materials and Mechanics Research Center, Watertown, Mass.
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ible irradiation of samples to be analyzed by neutron activation using a 14-MeV neutron generator. The svstem simultaneouslv rotates. revolves. and traverses -< both an unknown~andistandaid sample'past the target of the neutron generator, and then transports the samples at completion of irradiation to a remote counting station where the samples are removed from the irradiation capsules and counted. Data are given to show uniformity of irradiation and precision obtained in the analyses for oxygen in a series of NBS standard reference metal samples. ~~~~~~
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Tm AUTHORS have previously described (1-3) some of the problems associated with neutron activation analyses using 14 MeV neutrons from a neutron generator utilizing the aH(d,n)4He reaction. To eliminate the imprecision in 14 MeV neutron activation analysis caused by nonhomogeneity and anisotropy in the usable neutron flux,dual sample-hiaxialrotation and pneumatic tube transfer have been used (4). The authors solved the problem 3f nonuniform irradiation of an unknown and a standard sample by means of a complex motion ( I , 2) of the two samples in the neutron flux to give uniform neutron irradiation regardless of the distribution of the component being analyzed in the samples. The present paper describes an automatic irradiation, transfer, and counting system'which was designed and constructed to our performance specifications by Cambridge Engineering Inc., Waltham, Mass. This system gives uniform irradiation of inhomogeneous samples, transports them to a counting station, removes the samples from the irradiation containers, and counts the activity. SYSTEM
The complete system is made up of three subsystems: irradiation and transfer, uncapping, and counting. The system is operated from a central control console which also indicates the status of each subsystem at all times. Irradiation and Transfer Subsystem. Most of the irradiation head is constructed of nylon and delrin to reduce attenuation and backscatter. It is designed so that either two cylindrical samples, 0.313 inch diameter by 1.5 inches long, or two disks, 0.625 inch diameter by 0.25 inch thick may be reproducibly irradiated simultaneously and subjected to the same integrated neutron flux. The two cylindrical samples (the unknown and the standard) are held firmly in high-density polyethylene irradiation capsules while they undergo three motions; rotation, revolution, (1) F. C. Burns, G. L. priest, and H. F. Priest, 5th Natl. Meeting, Soc. for Applied Spectroscopy, Chicago, Ill., June, 1966. (2) G. L. Priest, F. C. Burns, and H. F. Priest, ANAL.CHEM., 39, 110 (1967). (3) H. F. Priest, F. C. Burns, and G . L. Priest, Nucl. Insfrum. Mefhods, 50, 141 (1967). (4) F. A. Lundgren and S. S. Nargolwalla, ANAL.CHEM., 40, 672 (1968).
Figure 1. Irradiation and transfer system: two cylindrical samole mode ..
-
I . .
1
..
..
and traverse. LTY means or tne planetary gear assemmy, tne standard and unknown samples are rotated on their own axis by the same synchro which causes them to revolve about an axis equidistant from the two sample axes and in the same plane. The rotation is nonsynchronous, ensuring that a differentaspect of each sample is closest to the target with each revolution. The speed of revolution can be varied from 0-600 rpm with tberotation speed varying from 0-180 rpm. The disk samples which are subjected to two motions, rotation and =averse, are held firmly by polyethylene inserts in their high-density polyethylene irradiation capsule so that the disks are inclined at an angle of 45" to the axis and are 0.75 inch apart with their faces parallel. The speed of rotation can be varied from 0-600 rpm. Changing between the cylindrical sample mode and the inclined disk mode involves only one simple gear release or engagement. Theirradiation head isattacbed to atraversetable which has a rack and pinion drive to traverse the samples across the target with the capsules 0.030 inch from the target cap (Figure 1). The distance of traverse can be varied from 0-8 inches, and the time for traverse can be varied from 0-10 minutes. In order to ensure that the traverse is always symmetrical about the target, the complete traverse assembly can be translated relative to the target. The traverse table, which is accurately located by scales, has overtravel safety, stops and override switches for manual positioning. The control console indicates the various functions occurring with panel meters for speed of rotation, traverse speed, and table position. Safety interlocks are provided to prevent improper sequence of operations. The sequence of operations is as follows: The samples in their capsules are placed on the irradiation head, and the ANALYTICAL CHEMISTRY, VOL 42, NO. 4, APRIL 1970
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Figure 3. Both capsules uncapped, sample No. 1 ejected, sample No. 2 in counting position samplesis exactly '/%thedesired traverse distance to the right of the target center. After sample rotation is started from the console. a start-irradiation button closes a..latching ......... relay -.-which starts the generator and the traverse simultaneously. At .. the ...completion of the traverse, the accelerator and roteitor are ~, . . rransrer . " system PICKS . . up the sroppeu, ana me pneumaric samples. Uncapping Subsystem. The capsules travel the 40 feet of plastic transfer tubing in approximately 1.3 seconds. Paper diaphragms at the uncap end of the transfer tube, 16" from the uncapping mechanism, maintain the vacuum of 40" H,O in the transfer lines until the capsules penetrate the diaphragms and coast the rest ofthe way. The sequential uncapping mechanism for the two capsules is shown in Figure 2. The capsules, with the cap leading, slam into a pocket where the cap is held by the cap pawls. Bounce springs keep the capsules from bouncing hack. When the cap pawls on station I spring out to let the cap through, they activate a microswitch which activates its uncap solenoid uncapping capsule 1 and drawing hack the sleeve allowing the capsuleheld by the capsule pawls to swing down, thereby dropping the sample down the drop tube to the counting position, where either a time-delay relay or a photocell relay opens the gates on the counting system. In station 2, the cap holds a microswitch in a closed position. When the preset time on the counting system for sample I is reached, a signal activates the uncap sequencefor station 2 and simultaneously opens the dump valve to dump sample I and blow it away from the shield. The dump valve closes before sample 2 reaches position, and counting of sample 2 starts as soon as it is in position between the crystals. After counting sample 2, its dump is activated manually. Figure 3 shows both stations with the capsules uncapped and the samples dropped. Total time from completion of irradiation to start of count 1 is 3.5-5 seconds and elapsed time between end of count 1 and start of count 2 is 1.5-2.5 seconds. In Figure 4, the dump valve and blowout tube are shown with the scintillation crystals. Samples are blown into a box two feet from the cave where they do not affect background. In order to allow for simultaneous counting of two samples, uncap station 2 is removable from its position adjacent to station I and fits on top of another similar shield and counting unit. The disk samples are manually removed from the capsule and dropped into position for counting. ~
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500
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
Counting Subsystem. The counting systems are in 4-inch
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Lead covers on rollers allow access to the counting systems. Shield I , used for sequential and for simultaneous counting, has two 5-inch X 5-inch sodium iodide thallium-activated The mounting of the crvstals with sinele - obotomultioliers. . two crystals is shown in Figure 4. A vertical adjustment system ensures that the sample is I:entered between the crystals to maximize the approximate 48 geometry. The drop ...:-L.-.L L , . P_._. LUOCS are~nrc~nanycirvlc L I W U ,ne rop of the shield to allow for different sample configurationssuch as disk:S. The counting equipment, as shown inI block diagram in iinrl Figure 5, is based on the AEC modular systLllru.ly hy Ortec. The preamplifier outputs go to individual Model 410 linear amplifiers followed by Model 420 single channel analyzers. The outputs of the single channel analyzers are combined in a Model 19 Ortec Fanning Network, the output of which goes to the Model 431 scaler. Five Model 431 timerscalers are used for sequential mode counting. The first is a timer for setting preset time for scaler 2 which counts the activity in sample 1. When timer I reaches preset time, it 0 signal which dumps sample I and uncaps supplies a +5V -, capsule 2 dropping sample 2 into counting position. Timer 5 is controlled externally by the relay which opens the gates on timer I and scaler 2 and is stopped by the completion of preset time on timer 3, thus giving total elapsed time and thereby time between the completion of count I and count 2. When timer 3 reaches its preset time, all data for an analysis in sequential mode has been taken and is then automatically . . . . printed out on a teleprinter.
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EXPERIMENTAL
The system was designed for analysis of short half-life elements, principally oxygen in solids using a 14-Mev neutron generator. Samples were prepared by machining to size and degreasing. Repeat runs could be made on many samples, hut in some steels there was a build up of 56Mnwhich gave excessively high backgrounds after several analyses, so repeat runs were usually made on consecutive days. The selection of standards is a serious problem in such analyses. For the best results, the composition and dimensions of the unknown sample and the standard sample should be as nearly the same as possible. In our work, we used various synthetic standards based on lucite as the oxygen containing material. All lucite standards were not generally used. Instead, metal cylinders having compositions
10 T l L r T I P E
Figure 5. Counting system: block diagram m =
. (interval 1) - background X Val 2) - background] (corr. factor)
Figure 4. Counting assembly sbowir out tube.
[grams 01 std. and dimensions similar to those of the unknowns were prepared with four symmetrically-spaced0.060 inch holes parallel to the axis. The oxygen content of the specific holder was determined. Lucite rods were inserted into the holes in sufficient number to give approximately the same number of counts as the unknown. Asymmetry of the lucite rods did not produce any erroneous results. Nargolwalla et al. (5) have reported an excellent treatment of the problem of neutron .and gamma attenuation within the sample and the standard. This problem is reduced in OUT case by using samples of 0,313-inch diameter or less. Two methods were used to correct for the decay of “N to calculate the amount of oxygen present. Usual methods are not applicable because several half lives are involved in each counting period. The first method experimentally determines a correction factor by counting one lBN-containing standard sample as both sample 1 (interval 1) and as sample 2 (interval 2). The ratio of these counts corrected for background is used to correct any count on sample 2 (interval 2) to an equivalent count for the counting interval of sample 1. The oxygen in the unknown is computed as follows: Correction Factor
dN dt
gives N
=
count std. sample 1 (interval 1) count std. sample 2 (interval 2)
(9 S.
This method presumes complete reproducibility of all times from completion of irradiation to end of counting. In systems using pneumatic transfer where it is desirable to start counting as soon as the sample is in position, as is the case for any short-lived isotope, T, (the time from completion of irradiation to start of the first count) is variable depending on the weight of the sample. In our system, accurate measurement shows that the variation is of the order of 0.1-0.5 second in going from a lucite to a steel sample which is significant, a 0.1 second change in T,causing a 3 % change in the ratio. The second method eliminates the effect of variation of TI by utilizing the property of radioactive decay where the rate of radioactive nuclei disintegration depends on the number present at any time and integration of the relationship
S. Nargolwalla, M. R. Crambes, and
=
Nee+
where N = number of radioactive atoms at any time t No = number of radioactive atoms present at time
- background - background
t = O
X = radioactive disintegration constant (characteristic of radioactive nuclei under consideration)
-0.693 _ -
J. R. Devoe, ANAL.
CHeM., 40,666 (1968).
Unknown Standard Unknown Standard Unknown Standard Unknown Standard i
Teflon. P
?,I2
Table I. Uniformity of Activation Ratio Unknown countslgram M ~ ~dev.. Irradiation configurationa Standard, wunts/gram from mean
Sample
TITITITIT
0.998
TITTm?T TPTPTPTPTT
0.996
PTFTPTPTPT 0.994
TTTMTnT TITTm?T =
-AN
0.987
0.996 1.35 1.88 1.16 0.71 0.68 0.62 0.82
Individual samples Av. % dev. from mean
Rel. std. dev., %
0.36 0.68 0.74 0.48
0.46 0.86 1.16 0.68
0.29
0.44 0.46
0.37
~. 0.53 ~
Polyethylene
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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Table 11. Determination of Oxygen in Inhomogeneous Samples
Number of deterSample minaNo. Irradiation configuration of unknowna Standard tions 1 NN"N"NNNNNNNN"NLLLc Lucite 2 1 LLLNNNNNNNNNNNNNNNNNNc Lucite 2 2 2
LNLNLNLNLNLNLNLNLLNNLC LNNLLNLNLNLNLNLNLNLNLO
Lucite Lucite
6 5
Averageb
0 found,
0 found,
0 theory
grams 0.0419 0.0637
grams
grams 0.0561 0.0561 0.0561 0.2025 0.2025 0.2025
0.2028 0.2010
0.0528 0.2019
Experimental rel. dev., Z
0.265
0.297
zfrom Dev. theory 25.37 13.00 5.94 0.12 0.78 0.33
N = Nickel. L = Lucite Average of two orientations. c Traversed target first. a
b
Table In. Analysis of Oxygen in NBS Standard Reference Materials
SRM 355 titanium SRM 355 titanium
Sample diameter, inches 0.307 0.307
SRM 355 titanium
0.307
SRM 355 titanium SRM 356 titanium
0.307
Sample
...
Standard Lucite Lucite inserts in steel Lucite inserts in titanium
...
Lucite
14 MeV neutron activation analysis Number determinaOxygen Experimental tions content, rel. std. per sample PPm dev., 72 11 2749 0.267 8 2930 0.589 6
...
2862
...
Oxygen content, ppm Activation Certifieda analysish
0.712
10
1319
0: 373
6
1323
0.502
6
1362
0.739
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5
485
1.659
5
477
1.676
3
462
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6
121
1.48
5
129
1.44
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10
44
2.28
4
33
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6
34
2.20
6
38
1.96
12
33
2.21
...
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3031 f 57
2896 f 44
1332 f 77
1279 zk 28
484 f 14
492 f 29
131 f 8
132 =k 7.6
28 f 2
27.8 f 5.4
alloy
Lucite inserts 0.307 in titanium alloy Lucite inserts 0.307 SRM 356 titanium in steel alloy SRM 356 titanium ... alloy SRM 1090 (#I) 0.250 SRM 355 titanium Ingot iron SRM 355 0.250 SRM 1090 (#2) titanium Ingot iron Lucite inserts 0.250 SRM 1090 (81) in steel Ingot iron SRM 1090 Ingot ... iron Lucite inserts SRM 1091 (#I) 0.250 in steel Stainless steel (AISI 431) Lucite inserts SRM 1091 (#l) 0.250 in nickel Stainless steel (AISI 431) SRM 1091 Stainless ... *.. steel (AISI 431) 0.250 SRM 355 SRM 1092 (#I) titanium Vacuum melted steel SRM 356 SRM 1092 (#l)" 0.250 titanium Vacuum melted steel alloy Lucite inserts SRM 1092 (#1)" 0.250 in steel Vacuum melted steel 0.250 Lucite inserts SRM 1092 (#l)c Vacuum melted in nickel steel 0.250 SRM 356 SRM 1092 (#2) titanium Vacuum melted alloy steel ... SRM 1092 Vacuum melted steel a NBS certificate of analysis. b NBS Technical Note 458, March, 1969. Filed and cleaned with acetone. SRM 356 titanium
. . I
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502
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4,APRIL 1970
e . .
No is calculated for both the unknown and the standard. The counts in each counting interval is a definite integral. No
No (unk)
=
N =
q
e
t
(unk)
(unk)
The ratio of eN is then the ratio of the oxygen in the stanNo std. dard and the unknown, and
N o unk grams 0 in unk = -X [grams 01 std. No std. This method of calculation is more flexible and does not depend on the exact reproducibility of times, only their measurement.
RESULTS The uniform irradiation of samples was checked using the Teflon (DuPont) polyethylene sample configurations similar to those previously reported ( I , 2) with '*F isotope in Teflon as the radioactive isotope. Table I shows the results obtained in these studies. Although the data shown in Table I prove a high degree of uniformity of activation of I8F(109.5min) there is still the possibility that for a short lived isotope like 'eN(7.2 sec.), where the traverse time. is several half lives, inhomogeneity of the sample may present a problem. In order to check this point, synthetic samples were made up using 0.25 inch X 0.0625 inch lucite disks and nickel spacers in a nickel holder. The nickel used has 70-80 ppm oxygen. Table I1 shows the results obtained with two synthetic samples. The first sample had 98.4% of the oxygen concentrated 0.188 inch from one end, a highly unlikely distribution. The results indicate two things; gross inhomogeneity will give rise to different results depending on which end of the sample is leading in the traverse, giving a useful way to check for inhomogeneity; even this type of inhomogeneit) gives results within 6% of theory if the average of the results for the two orientationsis taken.
The second sample configuration is a more realistic type of inhomogeneity showing a higher local oxygen concentration at one point followed by a depleted region to give an overall average uniform composition. The results show that there is nonuniformity, but either value is excellent, and the average value of the two agrees within 0.33x of the theory. In Table I11 the results are shown for the analyses on samples of a variety of NBS Standard Reference Materials. The results in Tables I, 11, and I11 show the high degree of precision obtained with this system for either homogeneous or inhomogeneous samples. In no case is the precision below that expected from counting statistics. One added advantage of this system is the ability to detect inhomogeneity by reversing the samples in the irradiation capsules so that the leading end in the traverse is reversed. If different results are obtained, there is gross inhomogeneity and the true result is closest to the average of the two positions. The most accurate results were obtained with a standard utilizing lucite rods in a cylinder whose composition was close to that of the unknown. The sensitivity using this system is of the order of 1200-1 300 counts per milligram of oxygen under idealized conditions. While sensitivities of approximately 2200 counts per milligram of oxygen at equivalent neutron output are reported for a system using a planetary rotator (6),the system reported here has a distinct advantage in that the sample is removed from the irradiation capsule which eliminates the blank due to the capsule, a serious problem in the analysis of small amounts of oxygen. We believe that this system can analyze 1 ppm of oxygen in steel with a reliability of f50 %. While the system was designed for samples 0.313 inch in diameter, special capsules designed for 0.25-inch diameter samples have been successfully used, and samples having variable diameters as small as 0.188-inch diameter were analyzed without difficulty and with excellent precision. To analyze samples other than cylindrical rods, holders of low oxygen nickel or plastic have been used, but of course such holders reintroduced a blank correction, and corrections must be applied for counting geometry effects.
RECEIVED for review November 19,1969. Accepted February 9,1970. Presented in part at 1st Northeast Regional Meeting, American Chemical Society, Boston, Mass., October 1968.
(6) L. C . Pasztor and D. E. Wood, Tulanta, 13, 389 (1966).
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