Instrumental Neutron Activation Analysis Using TRIGA Reactor High-Resolution Gamma Spectrometer and Computer Oswald U. Anders Radiochemistry Research Laboratory, The Dow Chemical Co., Midland, Mich.
A system of elemental analysis by neutron activation is described which is capable of analyzing and assaying samples for eight or more elements simultaneously by entirely instrumental means. The gamma-ray spectra of the reactor-activated samples are collected at various times after the irradiation with a Ge(Li) detector of 13-cc sensitive volume. The computer program processes the time-dependent spectra and identifies the elemental constituents by the energy and half-life of the corresponding peaks in the spectra. The peak counting rates extrapolated to the time of the end of the activation are compared with the equivalent counting rates per milligram for the individual elements as obtained fromexperimentswith standard samples. Typical samples were analyzed and six to eight constituents identified and assayed with relative errors of 1 to 10%.
INSTRUMENTAL neutron activation analysis was initiated at this laboratory in 1958 with a project to develop a method utilizing the lOgn/cm*sec flux of an available 2-MV Van de Graaff accelerator. The method based on the abilities of the accelerator used a 7.6-cm-thick, 7.6-cm-diameter Na(T1)I detector connected to a 200-channel analyzer with punched paper tape output and involved a computerized scheme for resolving gamma-ray spectra into elemental constituents. A library of time-dependent gamma-ray spectra for 70 elements was used to peel off spectra components identified by the experimenter in the smoothed and normalized sample spectra which were plotted by the computer ( I , 2 ) . This first method has been used for several years, but had some severe drawbacks. The resolution of the Na(T1)I detector, on the order of 50 keV FWHM, was relatively poor and caused considerable peak-overlapping in the spectra. Small shifts in gain caused spectra distortions which were exaggerated after one or more components had been peeled off. This made it difficult t o utilize standard spectra taken at earlier times and in turn restricted the number of elements which could be identified simultaneously t o about four. The associated computer program written in machine language for the slow and limited memory of a LGP 30 computer required considerable time for processing the spectra (1 hour per set of five spectra). Active assistance from the operator was needed for the peak identification and the peeling procedure. Other authors have treated several aspects of instrumental activation analysis with varied success. Guinn and Wagner (3) reported on the use of the Na(T1)I detector for activation analysis in 1959. Wainerdi and coworkers discussed their efforts to develop a computerized activation analysis system in several reports since 1960 (4-7). Heath and coworkers (1) 0. U. Anders and W. H. Beamer, ANALCHEM., 33, 226 (1961). (2) 0. U. Anders, “Gamma-Ray Spectra of Neutron-Activated Elements,” Dow Chemical Co., May 1960. (3) V. P. Guinn and C. D. Wagner, ANAL.CHEM.,32, 317 (1960). (4) W. E. Kuykendall, and R. E. Wainerdi, Trans. Am. Nucl. Soc., 3, 95 (1960). ( 5 ) D. Gibbons, L. E. Fite, and R. E. Wainerdi, “Computercoupled, Automatic Activation Analysis,” TID-7629, 221, Oct. 1961. (6) R. E. Wainerdi, L. E. Fite, D. Gibbons, W. W. Wilkins, P. Jimenez, and D. Drew, Radiochemical Methods of Analysis, IAEA Vienna, Vol. 11, 149 (1965). 428
ANALYTICAL CHEMISTRY
4
*
12 cm
f 1.7 cm
1 TRIGA
Rabbit
Figure 1. Positioning of sample, flux monitor, and spacer inside the rabbit of the TRIGA reactor Sample and monitor are sealed in l/Z-drampolyethylene vials which in turn are packaged together in a 2-dram polyethylene vial before insertion into the rabbit. The rabbit is positioned inside the reactor core with its cap-end pointing up
approached the problem of computer analysis of gamma spectra from Na(T1)I detectors by a least-squares curve fitting procedure involving gain shifting of multichannel analyzer spectra (8, 9). Schonfeld wrote a n extensively used least-squares program t o resolve spectra into pre-identified components ( 1 0 , I I ) . Youle (12), Mundschenk (I.?), and Hoste (14) devoted themselves to the problem of overlapping peaks in the gamma spectra. The increased capabilities of computers, now operating from magnetic tape inputs and conversing in understandable languages, seemed to warrant another attack on the overall problem. During this time the semiconductor detector was introduced and has become commercially available. With resolutions of 3 t o 5 keV FWHM, it promised to overcome the problems of the Na(T1)I detector (15). The groups of Girardi (16), Hollander (17), and Schroeder (18) were among the first to try the lithium-drifted germanium detector for activation analysis in 1965. The present-day lithium-drifted germanium diode is rela(7) M. P. Menon, and D. W. Berry, Anal. Chim. Acta, 38, (3), 349 (1967). (8) R. G. Helmer, R. L. Heath, D. D. Metcalf, and G. A. Cazier, “A Linear Least-Squares Fitting Program for the Analysis of Gamma-Ray Spectra Including Gain-Shift Routine,” IDO-17015, Oct. 1964. (9) R. L. Heath, Nucl. Instr. and Methods, 43, 209 (1966). (10) E. Schonfeld, Proc. 1965 Internat. Conf. Modern Trends in Activation Analysis, College Station, Texas 279-283 (1965). (11) E. Schonfeld, Nircl. Instr. and Methods, 42, 213 (1966). (12) H. P. Youle, ANALCHEM.,40, 1480 (1968). (13) H. Mundschenk, Internat. J. Appl. Radiation Isotopes, 18, 365 (1967). (14) D. DeSoete, and J. Hoste, Proc. 1961 Internat. Conf. Modern Trends in Activation Analysis, College Station, Texas, pp 59-63 (1961). (15) F. S. Goulding, Nucl. Instr. and Methods, 43, 1 (1966). (16) F. Girardi, G. Guzzi, and L. Pauly, Radiochim. Acta, 4 (2), 109 (1965). (17) J. F. Lamb, S. G . Prussin, J. A. Harris, and J. M. Hollander, ANAL.CHEM.,38, 813 (1966). (18) G. L. Schroeder, H. W. Kraner, R. D. Evans, and T. Brydges, Science, 151, 815 (1966).
S-
Figure 2. Germanium (lithium) detector and shielding Aluminum cup containing 13 cc Ge(Li) detector in high vacuum on the end of a dip-stick cryostat Housing of cryostat Collar for charging cryostat dewar with liquid nitrogen Liquid nitrogen loading tube 25-liter dewar Preamplifier A low capacitance lead inside the cryostat connects the Ge(Li) diode to the preamplifier input Counting thimble Sled suspension of counting thimble Slot in roof of lead cave Crank handle and threaded block for moving sled Air operated sample positioner Threaded rod ( N ) Compressed air inlet for sample positioner to move sample from the end of pipe Q to end of pipe P and drop it into the counting thimble H (0) Inlet for compressed air to return sample positioner (P) Transfer pipe to magazine (Q)Transfer pipe from magazine (R) Sample positioner drive piston ( S ) Lead shielding cave 5-cm wall thickness (7') Opening in front side of cave (U)Lead door (VI Tracks for lead door ( X ) Penlight bulb I Y ) Photodiode monitoring the lightbeam of the penlight - bulb to verify arrival of sample in thimble H ( Z ) Inlet of compressed a& to transfer sample from counting position-to storage magazine tively inefficient in converting incoming gamma-ray energy to photopeak pulses. Instrumental activation analysis methods using the detector thus require considerably stronger sources than are available from small accelerators. Nuclear reactor fluxes, being about 10,000 times stronger than those of typical accelerators could, however, compensate for this deficiency in many instances. In 1967 our laboratory acquired the DOW TRIGA Research Reactor which achieved its full power of 100 kW on July 11, 1967. It was thus possible to undertake the development of a new integrated system of instrumental activation analysis. This system was to be capable of qualitative identification and
quantitative assay of up to 10 and more elements. A single activation of a sample in the reactor was to be followed by routine gamma-spectra collection at different times after the activation. All calculation, identification, and measurements were to be made by the computer. The experimenter was to be permitted a high amount of flexibility in selecting the number of spectra to be taken and the times for the counts. He should thus be able t o favor detection of minor constituents suspected to be present in the sample and obtain optimum counting statistics for the elements of interest. Except for routine flux monitors, no standards of suspected elements were to be activated with the sample. VOL. 41, NO. 3, MARCH 1969
429
c X
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I
1
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1
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E N E R G Y (MeV.) Figure 3. Detection efficiency of high-resolution gamma spectrometer as a function of energy o Photopeak efficiency (peak counting rate/photon emission rate of sample) x Total spectrum effciency INSTRUMENTATION AND EQUIPMENT
The DOW TRIGA Research Reactor is a below-ground installation of General Atomic’s Mark I swimming pool reactor using stainless-steel clad fuel elements. It consists of a core suspended in a well of approximately 6.4-m depth and 2-m diameter filled with deionized water. The details of the Mark I TRIGA reactor are available elsewhere (19). The thermal neutron flux level achievable at the terminal of the pneumatic rabbit system, in the outermost core position, is on the order of 1.6 X 1012n/cmz sec. The pneumatic system of the reactor extends to the loading and receiving terminal in the hot-laboratory hood. The rabbits for it, made of Nylon, are 12 cm long and have inside diameters of 1.7 cm (Figure 1). The semiconductor detector which has been used for this work is made of a cylindrical piece of germanium 2.95 cm long and 3.25 cm in diameter coaxially drifted with lithium to a depletion depth of 1.06 cm. It has a nominal sensitive volume of 13 cc. The detector is mounted in a standard dip-stick cryostat with right-angle mounting and aluminum window (Figure 2). The cryostat is mounted in a reinforced table with the detector positioned inside a lead cave of 5-cm wall thickness with a sliding lead door on its front end. The inside dimensions of the cave are: length 84 cm, height and width both 50 cm. The detector is located coaxially with respect to the length of the cave. Its distance from the front end is 28 cm. A plastic beta absorber 1.27 cm thick and 7.3 cm in diameter is positioned 0.4 cm in front of the detector window, or 1.3 cm from the face of the detector. Samples are positioned inside a 1.9-cm 0.d. aluminum thimble which can be moved reproducibly with a crank. The normal position of the axis of this thimble was kept at 6.2 cm from the Ge(Li) detector face for the work reported below. The counting efficiency of this counting geometry was measured as a function of energy and is given in Figure 3. A timer-controlled pneumatic sample changer system loads and unloads the thimble by remote control in a given sequence from a lead-shielded magazine placed at some distance from (19) J. D. Buchanan, Proc. 1961 Internat. Conf. Modern Trends in Activation Analysis, College Station, Texas, pp 7-77 (1961). 430
ANALYTICAL CHEMISTRY
the detector cave. A series of up to 10 samples can be loaded into this magazine. A small photodiode detects the interruption of the light beam from a penlight bulb through two holes in the side of the thimble near its bottom, and indicates proper arrival and seating of the counting sample. For automatic operation, this “arrival” signal is used to start spectrum collection. The specialized electronics of the high-resolution gamma spectrometer were all commercially available. They included a n ORTEC 118A preamplifier mounted on the cryostat of the germanium detector, a John Fluke high voltage supply, an ORTEC precision pulse generator, an ORTEC 440 amplifier operated with 2-psec time constants, and a Nuclear Data 2200 analyzer system with 50-MHz digitizing rate, 4096-channel A D C and 2048-channel memory. The analyzer was equipped with a gain stabilizer operated by pulses from the precision pulser which were admixed t o the spectrum at the preamplifier. The read-out modes available were: a Tally paper-tape punch operating at a read-out rate of about 425 channels per minute, a digital printer with an output rate of 1310 channels per minute, and a Moseley x, y-recorder for plotting the spectra as raw data. The counting of the potassium tartrate flux monitors was accomplished with a Nuclear Chicago sample changer utilizing a 7.6-cm-diameter, 7.6-cm-thick Na(T1)I well-type scintillation crystal. The major load of the data handling was performed by Dow’s Burroughs 5500 computer using the program described below. This computer has a 28,000-word core memory and random access disk storage. Our present procedure involves a translation of the punched-paper-tape data onto magnetic tape by means of a Mohawk reader. The tape is then used together with the data-parameter punched cards as the input to the computer. One of the outputs of the computer program is a plotter program tape to generate plots with a CalComp plotter. The flux calibration data is currently handled externally from the general analysis program. The simple but tedious operation is readily performed by the computer from a remote station Teletype console available in the laboratory. SAMPLE PREPARATION
It was decided early, for convenience, to utilize medical grade %-dram polyethylene snap-cap vials from Olympics Plastics Co. of Los Angeles, Calif., as sample containers for the project. Initial experience with the reactor indicated that considerable amounts of 41Ar are encountered in the activated samples when the vials contain air and are exposed t o air before and during the activations. The samples were thus loaded in a nitrogen-purged glove box and stored under nitrogen until activation. Even when using small samples to be activated in a reactor, self-shielding poses a problem in samples containing highcross-section elements. One can reduce the resulting errors by dilution of the samples. The dilution of the solid materials used for this work as standards was carried out, more or less effectively, in the form of solid dispersions. Several materials were investigated as diluants for the samples. Reagent grade sucrose was finally chosen as the most satisfactory for the purpose. The samples were weighed out on a microbalance and ground together with a weighed amount of sucrose in an agate mortar. Weighed aliquots of the mixtures were then packaged into the polyethylene vials. To adjust the volumes to approximately 1 ml, additional pulverized sucrose was added and the vial contents were mixed with a shaker. The contents of the vials were then compacted tightly while inside the nitrogen glove box and the remainder of the vial was filled with pulverized sucrose to fix spatially the sample in the vial. This assured that the samples would not pack nonuniformly or segregate by particle size when handled by the pneumatic systems (20). The vial was then (20) 0. U. Anders and D. W. Briden, ANAL.CHEM., 36,287 1964)).
heat-sealed and stored under nitrogen inside a glass bottle. A typical analysis of a sucrose-filled vial prepared by the above process gave: Ca, 300 ppm; C1, 11 ppm; Na, 3 ppm; Br, 0.1 ppm; and Ar, from air. FLUX MEASUREMENTS
A measurement of the neutron flux to which a sample is being exposed during the activation is essential for comparison with a flux-normalized library of standard data taken at an earlier date than the sample. Instead of gold-foil monitors as used in the earlier work ( I ) , or pieces of A1-Co alloys, as are employed by other laboratories ( 2 4 , it was decided to use flux monitors prepared from a standard solution. Potassium tartrate was chosen, as it can be obtained in a highly purified form. Because of the low activation cross section of potassium, it can be used in sufficient concentration to avoid wallabsorption losses during preparation of the monitors. The induced activity consists of a single long-lived species 42K, whose well known half-life is convenient for counting many monitors overnight in a sample changer after the experiments are completed. The monitors are reuseable after a week. A large 18-liter batch of standard solution was prepared by dissolving 541.6 grams of potassium tartrate in distilled water. The solution was empirically calibrated to serve as flux monitor in the following manner: Four solution samples were weighed out and packaged into the 1/2-drampolyethylene vials. They were then irradiated for 30 minutes at different positions in the rotary sample rack of the reactor. After a cooling period of several hours they were counted several times. Comparison of the decay-corrected data indicated equal counting rates per gram of solution for the four samples with an error range of only 0 . 4 7 z . This proved the equivalency of the activation positions in the rotary sample rack. To avoid self-shielding errors, a carefully weighed 1-cm-long piece of 0.0025-cm-diameter gold wire was dissolved inside a drop HC1. I/2-dram polyethylene vial with nitric acid and This was evaporated to near dryness and diluted to fill the vial. The gold solution was then irradiated together with several aliquots of the potassium tartrate standard solution in the rotary sample rack for 30 minutes. The samples were then counted repeatedly after a 1-day delay in the Na(T1)I well crystal and the disintegration rates were extrapolated to the end of the activation. The entire 19*Au spectrum was used for the counting energy range and a 9 5 z counting efficiency of the well crystal was assumed for the 0.41 MeV gamma rays. Using this information and the cross section value for gold as 95 barns, the flux during the activation at 10-kW reactor power was calculated as 0.6634 X lOlln/cm%ec at the rotary sample rack. The sum of channels 107 through 123, with the maximum of the 1.52-MeV gamma-ray photopeak of 42K in channel 116 of the 400-channel analyzer, was used as the peak area of the spectra of the activated potassium tartrate solution. For these conditions the counting rate per gram solution extrapolated t o the end of the activation was found to be 59,993 c/m per lO%/cm2sec for the 30-minute activations. For the work reported below this number was rounded off to 60,000 c/m/g per 1 X lO%/cm*sec for 30-minute exposures, and 10,000 c/m/g per 1 x lO%/cmzsec for 5-minute activations. For practical purposes it appeared convenient to calibrate a stable single-channel analyzer with respect to the above data and then use the count rates from the single-channel analyzer as the flux monitor data. It was found experimentally that the neutron flux at the sample position in the rabbits exceeded that measured at the monitor position by 8 . 1 8 z (Figure 1). This correction factor has been applied to the flux measurements for the samples in the subsequent study. (21) M. A. Wahlgren, Argonne National Laboratory, private communication, 1968.
ACTIVATION OF STANDARDS
For convenience in testing out the system under development, it was decided to start with the collection of standard spectra using 5-minute activations in the pneumatic rabbit system of the reactor. The standard samples were carefully prepared as described above using quantities of activatable elements, their oxides, carbonates-or in the case of the halogens-their ammonium salts or organic compounds, chosen so as to yield counting samples with reasonable counting rates. This avoided overloading the multi-channel analyzer and yet permitted acquisition of statistically significant data during the short counting intervals. For activation, the rabbit (Figure 1) was loaded into the pneumatic transfer system and inserted into the reactor for the predetermined time. The rabbit was then retrieved, opened, and the radiation dose rate of the recovered sample vial was measured with an ionization chamber survey meter. The sample was then placed into a new, uncontaminated, 2-dram vial and transferred to the counting room. These operations could be completed within 85 sec after the end of activation. TIMING AND COUNTING
The counting interval-Le., the time interval during which the collection of a gamma spectrum is being carried out-is supplied to the computer in three different ways : (1) The clock-time interval is read by the experimenter and supplied on punched cards. It permits the computer to calculate corrections for decay occurring during the counting interval (22). (2) The live-time of the analyzer is reported on punched tape as channel zero of the spectrum. (3) The “pulser time” which the computer uses to calculate the actual counting rates of the peaks identified in the spectra is obtained by summing eight channels under the 60 c/sec pulser peak at the high-energy end of the spectra. A detailed discussion of the “pulser time” concept is given in reference (23). For the present purpose it suffices to note that peak distortions in the spectra due to pulse pileup at high counting rates are similar for all peaks of the spectrum. The fraction of the pulses which are lost from the energy interval occupied by the peak at low counting rates is of the same magnitude for all peaks of the spectrum, including the peak of the admixed pulses from the precision pulse generator. Using then this pulser peak for timing of the spectra collection, one can obtain a simultaneous correction for the peak distortions and the analyzer dead time. The admixed pulse train from the precision pulser is also used for gain stabilization of the spectrum by the spectrum stabilizer of the analyzer. This device compensates for any gain shifts occurring during a day or more of spectra collection and maintains the peaks of the spectra reproducibly within one channel width of their theoretical positions. The gain and zero position of the analyzer are checked and adjusted before each set of experiments with samples of ZOSHg, ISiCs, and W o so that the maxima of the peaks corresponding to the gamma rays emitted by these isotopes fall into the channels calculated from the chosen energy interval per channel and the energies of the respective gamma rays. The peak due to the admixed pulse-generator pulses falling between channels 2025 and 2041 is then locked into its proper channel by setting the switches on the spectrum stabilizer panel. The calibration stays fairly constant during periods of weeks (23). This is verified every day before the first use of the system. Experience with the system indicated that the energy equivalent is a rather well-defined parameter. With a FWHM resolution of less than 3.5 keV even at count rates of 60,000 (22) B. W. Hoffman and S. B. Van Carnerik, ANAL.CHEM., 39,1198
(1967). (23) 0. U. Anders, Nucl. Instr. and Methods, in press. VOL. 41, NO. 3, MARCH 1969
431
Table I. Priritout of the Library Information for Some Typical Elements Legend : First line for each element contains the chemical symbol, the number of peaks used for the recognition of the element, the weight of the sample used to obtain the data, and the self-shielding parameter for the element. The other lines contain in column 1 the energy of the gamma rays in keV. Columns 2 and 3 are the half-life used for the peak. The last column is the count rate at the end of the 5-minute activation in units of counts/sec/mg per 1 X 1011n/cm-2/secflux.
K 312.3 1524.0 Ca 177.5 1145.0 2064.0 2576.0 c1 621.O 1132.0 1145.0 1642.0 1656.0 2169.0
4 12.440 12.440 4
10.430 37.480 8.800 8.800
481.200 Hour Hour 519.100 Min Min Min Min
6
23.330
37.700 37.700 37.700 37.700 37.100 37.700
Min Min Min Min Min Min
3.187 x 0.003 0.019 6.600 X 0.007 0.003 0.067 0.015 5.738 x 10-l 0.097 0.023 0.301 0.710 0.058 0.661
c/sec, the spectrometer is capable of reproducing the positions of the peak maxima within i 1.5 channels or i 2.2 keV, if sufficient counts can be collected to eliminate the influence of the counting statistics. In order to assure proper correlation of peaks by the computer, even if somewhat distorted by the statistics of low counting rates or slight errors in setting up the analyzer, an arbitrary error range of i 4 keV was assigned to the peak energies. The computer program leaves the scheduling of the counting intervals to the discretion of the experimeter. From the point of view of economics, one would like to schedule in such a way that the counting of several samples activated about I-hour apart can be interleafed. Several spectra collected in short order after the end of the activation to measure and identify the short-lived activities are followed by additional spectra taken at later times, after much of the short-lived contributors have decayed, to permit better identification of the longer-lived gamma peaks. Longer counting periods at the later time can serve to obtain good statistics for the then less active samples. Some such schedule permitting the handling of six samples in a 9-hour workday was used for the processing of the samples for the standards library. It is possible to forego the early counts when unknowns are to be processed and one is not interested in the short-lived activation products. This time can be spent to purify the samples chemically or handle several samples simultaneously. As long as compared with standards which are activated for the same length of time as the unknown, the results obtained by the comparison remain correct. COMPUTER PROGRAMS
The flux calculations program uses as input data the weights
of the monitors, the time of the end of the activation, the length of the activation, the counting data, the times of the counts, and the counting intervals. It requires the count of one sample in the well crystal counter to normalize the counting efficiency. The computer calculates the counting efficiency relative to standard conditions and extrapolates the counts to the time at the end of the activation. From the extrapolated count rate the flux is calculated. Repeated counting and good counting statistics permit flux determinations with relative errors of less than 1 %. The general data analysis program is based on the following 432
ANALYTICAL CHEMISTRY
premises, the validity of which will have to be proven by experience with the method: The high resolution of the germanium detector makes it unnecessary to fit the peaks to a gaussian curve for analysis. The relatively high gain stability of the spectrometer makes it unnecessary to gainshift the data. Composite peaks with unresolved maxima and two different half-lives do not have to be considered specially, because they do not occur with sufficient frequency. It is sufficient to consider Compton continua in the region of a photopeak as a base upon which the photopeak is superimposed and to approximate the continuum by a straight line over the channels of the peak (24). The background, as well as pulse-pileup distortions, can be treated simultaneously with the Compton continuum except in regions where there are peaks in the background spectrum. The flux is known to about 1 standard deviation. The half-lives of the isotopes giving gamma-ray peaks are assumed to be known exactly from the literature or other sources. MAJOR FEATURES OF GENERAL ANALYSIS PROGRAM
The beginning and end of the activation and counting intervals are given to the computer as input. This frees the experimenter from adhering to a fixed counting schedule and also permits him to choose the lengths of the counting intervals. The counting times to calculate the counting rates are calculated from the data and are independent of operator error. The computer will apply decay corrections to the counting times for each indiuiduul peak to obtain the correct time of the count as applicable to this peak. This compensates for appreciable decay of the short-lived isotopes during the time of the counts (22). The program requires no peeling or stripping of standard spectra, nor does it require spectra of standards taken at the same time as the data. The standards library obtained from experiments carried out with known samples consists of an array with the identification of the element, a self-shielding parameter, and the energies and half-lives of the more significant peaks found in the gamma spectra of the element after activation. It furthermore contains the count rate equivalents of the peak areas extrapolated to the end of the bombardment and normalized to 1 mg of the element and IO%/cm2 sec flux. This library is small and requires little memory space. The program recognizes the peaks, measures the peak areas, and applies corrections. It correlates peaks in consecutive spectra belonging to the same-energy gamma rays. It calculates the half-life for the peak, then identifies the element to which the peak belongs from the library, substitutes the correct half-life for the peak as obtained from the library and extrapolates the counting data to the time of the end of the irradiation. The statistical certainty of the counting data is maintained by weighting the counting data to permit proper error evaluation of the extrapolated counting rate. The program compares the extrapolated peak area with the equivalent information in the library and calculates the amount of the element present in the sample as well as its concentration. If there are several peaks recognized belonging to the same element, the computer calculates a weighed average of the results and prints out the averaged result for the element. It furthermore reports all gamma-ray peaks it recognized, but which it was unable to correlate with the library. These include minor peaks from the identified elements which were considered unimportant for the analysis of the element and thus eliminated from the library in the editing process. Gamma peaks which the computer could have recognized, if the identified elements had been present in sufficient amounts, are also listed in the print-out. The program estimates the amount of self-shielding experienced by the sample and prints an appropriate warning. (24) D. F. Covell, ANAL.CHEM., 31, 1785 (1959).
0
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VOL. 41,NO. 3, MARCH 1969
433
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ANALYTICAL CHEMISTRY
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VOL. 41, NO. 3, MARCH 1969
435
Table 11. Results of the Analysis of Typical “Unknown” Samples Processed by the Computerized Instrumental Activation Analysis Method Wt. of element in mg Percent of element y Energy keV cl=l Present Found Known Found Present Found Isotope Half-life mg flux Element (Sample I)
Sr I
3.15 0.418
3.58 0.418
0.197 0.0262
0.224 0.0262
87mSr
AI La
0.273 0.542
0.0171 0.0339
1.516 2.31
Na
1.045
0.0160 0.0377 0.0390 0.183 0.164 0.156 0.0653
28A1 140La
As c1
0.256 0.603 0.624 2.93 2.62 2.49 1.042
0.095 0.144 0.0654
1281
16As 3 8 c 1
24Na
388 442 526 1779 328 815 2099 1642 2169 2157
388 442 526 1781 328 815 2097 1644 2171 2760
2.87 hr 25 min
388 442 526 1779 486 1598 655 2169 1642 2757 1368 1434
388 442 526 1781 487 1598 657 2171 1644 2760 1370 1434
1.019 84.68 6.429 2.3 rnin 47.1 40.2 hr 1.43 0.652 26.6 hr 0.331 37.7 rnin 0.661 0.709 15.0 hr 0.315 0.784 3.77 rnin 525.
1779 331 1524 1145 843 2757 1733
1781 331 1526 1146 843 2760 1734
2.3 9.7 12.4 37.7 9.5 15.0
rnin min hr rnin rnin hr
0.150 20.12 0.0194 0.301 0.702 0.315 0.327
1779 161 331 1524 1642 2169 843 2757 1368
1781 161 331 1526 1644 2171 843 2760 1369
2.3 41 9.7 12.4 37.7
rnin rnin min hr rnin
0.150 1.139 20.12 0.0194 0.709 0.661 0.702 0.315 0.784
2.3 rnin 40.2 hr 26.6 hr 37.7 rnin 15.0 hr
1.019 84.68 6.429 47.1 1.142 0.370 0.00655 0.709 0.661 0.315
(Sample 11)
Sr I
2.69 0.219
A1 La
0.297 0.659
As
c1
0.930 1.97
Na V
0.0876
0.185 0.0151 0.0134 0.0199 0.0484 0.0489 0.0591 0.140 0.143 0.0889
24Na
0.00278
0.00270
52V
58.8 3.71 52.4 1.97 6.23 1.45 1.46
3.72 0.243 3.38 0.101 0.400 0.0956
3.93 0.248 3.50 0.131 0.416 0.0969 0.0976
55.9 2.78 2.80 44.38 2.37 2.39 6.93 1.626 1.69
3.57 0.187
3.74 0.186 0.187 2.97 0.158 0.160 0.464 0.1090 0.113
0.178 0.0145
1.32
2.79 0.229 0.203 0.300 0.730 0.737 0.891 2.11 2.17 1.34
0.0412
0.0408
0.0197 0.0437 0.0617 0.130
87mSr 1281
28A1 140La 6As 3 8 c 1
2.87 hr 25 rnin
(Sample In)
P Sn
55.7 3.63
K
50.5
c1 Mg Na
1.51 5.95 1.43
P Sn
53.3 2.80
K
44.1 2.35
28A1
i2smsn
42K 3 8 c 1
27Mg 24Na
(Sample IV)
c1 Mg Na
6.53 1.627
2.95 0.157 0.437 0.1090
Finally the program generates a 10-cycle semilog plot of the normalized spectra for the CalComp plotter t o graph the count rate in cjm per channel normalized to l g sample weight for a flux of 1 X 1011n/cm%ec. It plots the recognized peaks point for point and averages 10 channels for the intermediate regions t o reduce statistical noise. RESULTS
For the initial testing of the method a set of standard samples was prepared for some 40 elements. They were each activated in the pneumatic tube of the reactor for 5 minutes and then counted with the germanium counter. Spectra were obtained a t five different times during the 2-hour periods following the activations (Figure 4). The numerical information from the standards was carefully edited t o prepare the library. Data for peaks identified but belonging t o impurities in the samples were suppressed. Peaks found in the standards, but small relative t o others from more intense gamma rays from the same element, were eliminated so as not t o confuse the computer during the analysis of unknowns. Peaks nonspecific for a n element were omitted as far as possible. Such peaks included the noise-peaks due t o Bremsstrahlung in the low energy region of many of the spectra, 436
ANALYTICAL CHEMISTRY
18A1
imsn izsmsn
42K 38c1
27Mg 14Na
9.5 rnin 15.0 hr
backscatter peaks, the positron annihilation peak occurring in the spectra of many elements having high-energy gamma rays, peaks for elements in energy regions in which commonly encountered impurities would contribute to the spectra-e.g., 41Ar-as well as the pulser peak used for timing and spectrumstabilization during data collection. The editing also involved the selection of the “true” half-life for each identified peak to be used for the standards. A sample listing of the resulting library cards is given in Table I. The drastic reduction of information-from about 10,OOO data points per element t o some 20 numbers-to be retained in the computer memory constituted one of the major achievements of the method. After the reference library had been accumulated to include the majority of the elements, the method could be tested. This was done with the use of four artificial unknowns. Mixtures of the chemicals used for the preparation of the standards were prepared by weighing out small quantities and then mixing them together after addition of sucrose providing the bulk for the sample. Two samples containing six, and one each containing seven and eight identifiable elements were prepared. The elements were chosen so that the activated samples would contain both short- and long-lived isotopes, some narrowly spaced peaks as well as peaks in every region of the spectrum
range: 0 to 3000 keV nominally. The amounts of the chemicals were adjusted so that the expected counting rates of the samples would not exceed 45 dead time at the beginning of the first count, two minutes after the end of the activation. Seven spectra were collected for the unknowns spaced over a period of 25 hours to permit a good lever for the half-life estimates of the longer-lived activation products (Figures 5 and 6). Flux monitoring and other experimental detail followed closely the procedures used to collect the spectra for the standard library prepared several months earlier. The results of the analyses of the four samples are given in Table 11. DISCUSSION
The computer was able to identify positively each of the added elements and assay them with good accuracy. Some of the error indicated by the data may be accounted for by the difficulties encountered in weighing the small amounts of sample constituents. The large error for the As assay in Sample I is due to the fact that the computer used a relatively minor peak of ‘6As for the analysis. The computer reported that it did recognize a peak at 657 keV for this sample, but estimating a half-life of 38 hours could not make the assignment to arsenic. Using the 38-hour half-life, however, it calculated the resulting count rate at the end of the activation for this peak as 0.485 c/sec.
It was left to the experimenter to calculate an arsenic assay of 0,485 c/sec/0.331 c/sec/mg = 1.465 mg, which is in much closer agreement with the actual arsenic content of Sample I. The choice as to how large a span of error in the half-life the program should tolerate for making an assignment will have t o be improved from time to time as experience is gained with the use of the method. CONCLUSION
It could be shown by the present work that computerized instrumental activation analysis, as conceived several years ago, is indeed feasible, and that it is able to give both qualitative and quantitative acceptable results for multi-element determinations. The method described is free of the previous requirement of processing standards with the sample, leaves the experimenter a large amount of choice in the timing of the counts, andcan be readily carried out by non-technical personnel. ACKNOWLEDGMENT
The author Computation program and the operation
is indebted to Thomas G. Eichinger of the Dow Research Laboratory for writing the computer to Robert P. Madison for his assistance with of the reactor.
RECEIVED for review October 11, 1968. Accepted January 13, 1969.
Gas Enthalpimetry Pier Giorgio Zambonin‘ and Joseph Jordan Department of Chemistry, The Pennsylvania State Unioersity, 212 Whitmore Laboratory, University Park, Pa. 16802
Gas Enthalpimetry (GE) is an “equilibrium heat pulse” method applicable to reactions between gases and liquids. Gaseous samples were injected, under judiciously controlled experimental conditions, into thin walled adiabatic calorimeters-e.g., platinum lined Styrofoam. A temperature change was concomitantly monitored with the aid of automatic thermistor instrumentation which had the sensitivity of a thousandjunction thermocouple. Aquation and Bronsted acidbase reactions of the gases COz, SO,, NO,, and N204 were investigated systematically. Heats of reaction were determined by GE in a few minutes with a precision and accuracy of 1%, which is comparable to the capabilities of classical calorimetry with such systems. GE is ideally suited for quantitative gas analysis: CO, and SOs, were determined accurately in amounts between 0.5 and 1000 pmoles, in gas mixtures ranging from 0.1 to 100 ml in volume and containing from 1 to 50% of the “unknown.” Based on the comprehensive study of aqueous carbon dioxide, sulfur dioxide, and nitrogen dioxide systems, A H assignments and associated confidence intervals are assessed critically.
AN AUTHORITATIVE REVIEW on the status of gas analysis published 16 years ago ( 1 ) includes the interesting idea of utilizing the heat evolved in a hetereogeneous reaction as a means for monitoring concentrations in a gas phase. Specifically, the response of a hundred-junction thermopile was used as a measure of the oxygen content of a gas which was bubbled On leave from Istituto di Chimica Analitica, University of
Bari, Italy (1) H. Guerin, Bull. SOC.Chim. Fr., 19, 24 (1952).
through an aqueous solution of chromous chloride. An obvious development suggests itself, ciz to substitute a single thermistor sensor (which, if appropriately wired, has the sensitivity of a thousand-junction thermocouple) in lieu of the thermopile. Indeed, gaseous reagents have been successfully-albeit sporadically-employed in conventional thermometric enthalpy titrations (2, 3) as well as in the newer heat pulse method, “direct enthalpimetry” (4, 5). In this paper we report the first systematic assessment of a methodological approach, which relies on the measurement of temperature pulses engendered by the injection of gaseous samples into suitable reagent solutions. “Gas enthalpograms” were recorded automatically as the unbalance potential of a thermistor Wheatstone bridge. Results are presented and discussed which indicate that gas enthalpimetry is a generally applicable instrumental method for gas analysis on a macro-or-micro scale. Gas enthalpograms also provide rapid and convenient means for the determination of heats of reaction and the elucidation of relevant thermodynamics. The “model reactions’’ selected for this study included aquation and Bronsted acid-base equilibria of the type: COZ(g)
+ H~0(1)
=
CO?.aq
(1)
(2) B. J. Duffield, Thesis, MIT (1960). (3) D. N. Hume and B. J. Duffield, Abstracts, 147th National Meeting, American Chemical Society, Philadelphia, Pa., 1964, p 13B. (4) J. C. Wasilewski, P. T-S Pei, and J. Jordan, ANAL.CHEM., 36, 2131 (1964). (5) J. Jordan, P. T-S Pei, and R. C. Buchta, Jr., Abstracts, 149th National Meeting, American Chemical Society, Detroit, Mich., 1965, p 30B. VOL. 41, NO. 3, MARCH 1969
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