devices, a rotating disc electrode assembly, or a Petrey table. Future accessories strictly for analytical use are now being fabricated. An instrument function that was used extensively in initial photographic focusing and alignment and is of analytical importance involves slowly rotating the spectrograph camera mirror using the worm gear wheel attached to the trigger mirror shown in Figure 5 . The synchronous motor for highspeed mirror drive is turned off, and a small clock or stepper motor is engaged to the worm wheel through a sliding platform and gear reduction train. The slow drive motor is programmed with relays and timers to rotate one sixth of a revolution at adjustable time intervals, which when coupled through a 720:l reduction ratio allows rapid and precise spectral displacements on the order of 2 mm at the camera. The slow drive motor may also be run continuously, sweeping out a continuous tracing. This function allows programmed split-burn and moving-camera arc-discharge studies, as well as pre-spark and hysteresis studies on time-integrated spark discharges. SUMMARY
It has been shown previously with an oscillatory spark discharge (13, 19) that it is possible to extend analytical sensitivity, substantially simplify qualitative analysis, reduce or even eliminate matrix effects, and successfully use rather atypical discharge regimes for analysis. The above ends were (19) R. M. Barnes, Ph.D. Thesis, University of Ilhois, Urbana,
1966.
achieved by combining a time-resolved approach to spectrometric analysis with some leading knowledge about the discharge mechanism. In many cases, simplicity and convenience were sacrificed, the analysis or study requiring long exposure times and sensitive manipulations of the apparatus. The instrument discussed here was designed to retain highquality time-resolved readouts, while at the same time eliminating sensitive adjustments, decreasing photographic exposure times, extending time ranges into the afterglow for many electrical discharges, and emphasizing the optical characteristics of modern analytically-oriented spectrographs and monochromators. ACKNOWLEDGMENT
The author thanks Ivel Reithmeyer, Russell Riley, and Robert Schmelzer for their many helpful contributions to the construction and design of the apparatus. James B. Peters, and Robert Darlington of the Whitnon Co., were instrumental in the design and construction of the sine-bar and rotatingmirror mechanisms, respectively. The donation of two highvoltage spark sources by the General Motors Research Laboratories is appreciated. RECEIVED for review January 16, 1967. Accepted March 17, 1967. Mid-American Symposium on Spectroscopy, Chicago, Ill., May 1967. The research was supported in part by the Wisconsin Alumini Research Foundation, and by the National Science Foundation under Grant GP-5073. Special financial assistance was given by the Research Corp. and the U. S. Rubber Co.
Use of X-Ray Spectrometry in Activation Analysis: Determination of Bromine Cesia Shenberg, Jacob Gilat, and Harmon L. Finston' Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yavne, Israel X-ray counting of *OBr and EomBris applied to the determination of submicrogram amounts of bromine in neutron-irradiated samples containing a large excess of common interfering elements. The precision of the method is about 2%, and an accuracy of 2 4 % can be attained for Na/Br or K/Br ratios up to 500:l. The method also provides a means of fast analysis of milligram amounts of bromine in organic matter. The advantages of x-ray counting and spectrometry over conventional scintillation gamma spectrometry in nondestructive neutron activation analysis are discussed.
THEPOSSIBILITY OF nondestructive analysis of selected elements in a complex matrix is one of the most attractive features of radioactivation analysis. The prerequisites for such analyses are that some of the characteristic radiations emitted by the activated species should be discernible in the presence of large amounts of other radiations. Countless examples of the uses of characteristic gamma rays, delayed neutron emission, Israel AEC Fellow; present address, Brooklyn College, City University of New York, New York, N. Y. 780
a
ANALYTICAL CHEMISTRY
positron decay, etc. can be found in the literature (1-5). However, little attention has been paid to the analytical applications of x-rays emitted in radioactive decay. The two main processes responsible for the production of x-rays in radioactive decay are electron capture and internal conversion of gamma rays. In both cases, vacancies are created in the low-lying electron shells (mainly the K shell) of the decaying atoms. The filling of these vacancies results in the emission of x-rays characteristic of the decaying element. Thermal neutron activation leads mainly to isotopes on the neutron excess side of the stability line. Therefore, relatively few isotopes which decay by electron capture (e.g., 37Ar, 61Cr, 55Fe, 7lGe, 7sSe,*OBr, g3M0) can be produced in the predominantly thermal neutron flux of a reactor. Large internal conversion coefficients are expected only for high multipolarity gamma transitions (isomeric transitions). Thus the probability of x-ray emission from nuclides produced in a (1) W. W. Meinke, ANAL.CHEM., 32 (3,104 R (1960). (2) G.W .Leddicotte, Ibid.,34 (9,143 R (1962). (3) G. W .Leddicotte, Ibid.,36 (5), 419 R (1964). (4) W. S.Lyon, E. Ricci, and H. i-I. Ross, Ibid.,38 (3,251 R(1966). ( 5 ) F. Adam and J. Hoste, AI. Energy Reu. 4 (2), 113 (1966).
nuclear reactor is usually quite Iow, except for some special cases. X-ray detectors, such as proportional counters, thin scintillation crystals, and solid state devices are quite insensitive to gamma rays of energ:y greater than a few tenths of an MeV. Soft x-rays can be effectively detected and measured over a large background of harder gamma rays. For optimum performance, the detector parameters (dimensions, nature, and pressure of the counting gas, etc.) should be chosen to suit the energy of the x-rays. The selectivity imparted by this inherent discrimination effect is enhanced by the rather small number of x-ray-emitting species and by the differences in their x-ray energy and half-life, so that a high degree of specificity is attained. In this work we investigated the possibility of using x-ray spectrometry for. the nondestructive determination of bromine in various media. Activation of 79Br (natural abundance 50.54z) leads either to the ground state of 80Br = 18 minutes, a = 8.5 barns) or to the 4.5-hour metastable state (a = 2.9 barns), which then decays to the 18-minute ground The ground state decay has an electron capture state branch of about 5z--i.e., in approximately llz0of all disintegrations an 11-keV Se K x-ray is emitted. The excited 4.5-hour state decays tiy a cascade of 48 keV (M3, fully converted, K / L = 3) and 36 keV (El ,-50 % K converted) gamma rays-Le., about 1.2 Br K x-rays of 11.7 keV are emitted per disintegration (7). In addition, the 6-minute isomer of 82Br produced from 8lBr (u = 3 barns) also decays through a fully converted 46keV garnma transition, and yields about 0.7 Br K x-rays per disintegration (8). Apart from convent onal chemical analysis (9), bromine is usually determined by neutron activation and assay of the 36-hour 82Br(10-13). Because the half-life of this isotope is relatively long, long pzriods of irradiation and counting are required for sensitivities comparable with those attained with shorter-lived isotopes such as 80Br or 8cmBr. Long irradiations generally in\rolve more elaborate facilities and high levels of activity from irradiated sample containers. In addition, the amounts of common interfering activitiese.g.. *4Na-prod~~edin the sample also increase when the time of bombardment is longer. The gamma spectrum of 82Br is complex, with ti peaks between 554 and 1475 keV (7). Its determination in tbe presence of even moderate amounts of interfering activities requires multichannel pulse height analysis and lengthy processing of the spectra. Alternatively, an equally time-consuming radiochemical separation prior to the p- or y-counting of the 82Brhas to be used. The x-ray spectrum on the other hand is simple, and 80Bror ‘OmBr can be assayed nondestructively using single channel analyzers only. Analysis of bromine based on the production of 5-second 79mBrby 14-MeV neutrons has also been proposed (14). However, the very short half-life of this isomer is rather inconvenient, and the sensitivity that can be reached
(a.
(6) D. T. Hughes and R . B. Schwartz, U.S.A.E.C. Repf. BNL 325 (1958). (7) Nuclear Data Sheets, National Academy of Sciences, National Research Council, Washington, D. C. 20418. (8) 0. U. Anders, Phys. Reu., 138 B, 1 (1965). (9) A. Turner, J . Sci. Food Agr., 15,265 (1964). (10) C. E. Castro and R . A. Schmitt, J . Agr. Food Chem., 10, 236 (1962). (11) V. P. Guinn andJ. 12.Potter, Zbid.,10,232(1962). (12) V. P. Guinn and R. A. Schmitt, “Residue Reviews,” 5, F. A. Gunther, Ed., Springer Verlag, Berlin, 1964, pp. 148-174. ( 1 3) P. Thoresen, Acta C’hem. Scand.,18,1054 (1964). (14) 0.u. Anders, ANAL. CHEM., 34, 1678 (1962).
with currently available 14-MeV neutron generators is much lower than that attainable with a reactor. The determination of bromine by means of the ,-ll-keV Br and Se x-rays emitted in the decay of SOBY,80”Br,and 82mBr offers many advantages over other methods. Proportional x-ray spectrometry was applied to the analysis of traces of bromine in the presence of a large excess of interfering elements. Possible practical applications include the determination of pesticide residues in crops or air pollution studies. EXPERIMENTAL
Irradiation. All irradiations were carried out in the pneumatic transfer facility of the IRR-1 reactor, at a thermal neutron flux of -4 X 1OI2neutrons/cm2second. The irradiation times varied between 1 and 10 minutes. Counting. A cylindrical gas flow proportional counter, 9 cm in diameter, 29 cm in length, with a 7-mg/cm2 aluminum window was used for the x-ray detection. The counting gas was a 90% argon-loz methane mixture. The detector was coupled through a low noise preamplifier to a 400-channel analyzer. The multichannel analyzer was used to permit evaluation of the performance of the system. For routine analyses, the multichannel analyzer can be replaced by a single-channel analyzer and scaler, Under optimum conditions, the resolution was 1.2-1.5 keV. Some of the results reported in the following section were obtained with an earlier model of the counter, with -2.5-keV resolution. Cobalt-57 (6.8-keV x-ray and 14.4-keV gamma ray) was used for the energy calibration. To reduce the beta-ray background in the counter, thin plastic absorbers or a 2000-gauss permanent magnet were placed, when necessary, between the source and detection window. A 2-3-mm thick layer of Lucite (,-1-MeV beta energy equivalent) was found sufficient to remove essentially all the beta particles emitted by the common interfering elements. The attenuation of the x-rays by a 2-mm absorber was about 40%. For highest sensitivity, this attenuation can be avoided by means of a more powerful magnet (8-10 kgauss). Sample Preparation. For x-rays of the order of 10 keV, the effects of source thickness can be quite pronounced. Samples and standards should be mounted so as to have similar self-absorption. This can be achieved by the use of very thin (effectively weightless) sources. The practical thickness of a weightless source depends on the source material. Reproducible self-absorption may also be achieved by the use of infinitely thick sources (>>1 gram/cm2, depending on the source material), or by having all sources equal in thickness and of a similar chemical composition. In this work we adopted the third method. Organic Material. A weighed 50- to 100-mg sample of powdered organic material was spread evenly over an area 2 x 2 cm, and sealed in thin polyethylene or Mylar. In this way samples of approximately uniform thickness not exceeding 25 mg/cm2 were obtained. Because activation of the Mylar or polyethylene was found to be negligible, these samples were left intact for the counting. For standards, 1-cm3 aliquots of an irradiated NH4Br solution of known bromine content were absorbed on a 2- X 2-cm piece of filter paper on a Mylar or polyethylene sheet, evaporated to dryness under an infrared lamp, covered with another piece of Mylar (or polyethylene), and sealed. The thickness of the filter paper was chosen to be approximately equal to the thickness of the solid organic samples. Aqueous Solutions. In order to evaluate the effects of various interfering elements, synthetic solutions of 5 0.25 mg/cm3 total solid content were irradiated and mounted on filter paper as described above. Under these conditions the attenuating effect of sample constituents other than the filter paper could be neglected. VOL. 39,
NO. 7, JUNE 1967
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5
Figure 1. Effect of source thickness W on x-ray counting efficiency The dependence of the counting rate on the thickness of the filter paper is shown in Figure 1. The self-absorption effect is negligible up to -70 mg/cm2. RESULTS AND DISCUSSION
Typical x-ray spectra of activated bromine, taken at various times after irradiation (-1 pg of bromine, irradiated for 10 minutes, and counted at close geometry through 2 mm of Lucite) are shown in Figure 2. Figure 3 shows the decay of the above spectrum, resolved into three components of 6 minutes, 18 minutes, and 4.5 hours, corresponding to SzmBr, S0Br,and 80mBr, respectively. The relative activities are consistent with these assignments. The initial counting rate in the x-ray peak is ~ 4 0 , 0 0 0cpm, over a background of -100 cpm. Under the conditions of this experiment, and in the absence of strong interfering activities, the sensitivity is of the order of 0.001 Mg. Precision and Accuracy. The standard deviation of 12 separate determinations of the bromine content of an NHrBr solution (nominal concentration 50 pg/cm3) was 1.7 %. This errors in transfer and includes reactor flux instability (-1 sample mounting (-1 self-absorption, and counting
z),
z),
Channel Number
Figure 2. Typical bromine x-ray spectra 1 pg Br, irradiated for 10 minutes and counted for 1 minute through 2 mm of Lucite at (a) &minutes A.E.B.; (b) 12-minutes; (c) 28minutes; (d) 255-minutes. Insert shows a T o spectrum taken under identical conditions
Channel Number
L
Figure 5. Scintillation gamma spectra (10-minute irradiation, 1minute count, bhour A.E.B.)
Figure 4. Proportional counter spectra of common interfering elements
statistics and geometry (-0.5 %). The precision is thus comparable with that normally found in activation analysis. When the bromine content of the samples is much lower (less than 0.1 pg), counting statistics become the error-determining factor, and the precision drops accordingly. The precision is also affected by self-absorption in the samples. With the sample-mounting procedure described above, this source of error is negligible for aqueous solutions and thin samples of organic matter (see Figure 1). The effect can become a p preciable, especially when the matrix contains a large proportion of high Z elements, and its influence should be carefully evaluated for each case. The self-absorption problem also affects the accuracy of the method. As in most analytical procedures, the accuracy depends on the quality of the standards used. It is therefore impossible to discuss the accuracy without referring to a specific system. For the case of dilute aqueous solutions described in this paper, the accuracy is about 2 %(see Table I).
L
looo
50 Channel Number
100
(6)“Ma, ( c ) **Ai,(4“Na, 4zK, (f)‘OFe
(a) Wl, (e)
Table I. Results of Bromine Determination in Mixed S - p h Bromine, pg Interfering element, p i : Known content Found 0.500 0.498 K: 100 0.500 0.499 K: 250 0.500 0.515 Na: 100 0.500 0.491 Na: 250
(Spectrum c displaced slightly for display purposes)
t
Channel Number
Figure 6. Proportional x-ray spectra (10minute irradiation, 1-minute count, -7-minute~ A.E.B.) (a) 0.5 pg Br, (15) 100 pg Na, (c) 100 pg
M Br
Na
+ 0.5
Time [min)
Figure 7. Decay of Br x-ray spectrum in the presence of excess Na (Na/Br
=
200)
VOL 39, NO. 7, JUNE 1967
a
783
0
50
100
Channel Number
Figure 9. Proportional x-ray spectra (10minute irradiation, 1-minute count, -8minute A.E.B.) (a) 0.5 0.5 pg Br
Channel Number
Figure 8. Scintillation gamma spectra (10minute irradiation, 1-minute count, 27-hours A.E.B.) ( a ) 0.5 pg
Br, (b) 250 rg K,
(c)
250 pg K
+ 0.5
M Br Interferences. Figure 4 shows the spectra of some radioactive isotopes commonly found in activated natural materials ( W 1 , 66Mn, 28A1,24Na,42K,and 69Fe), taken with the x-ray spectrometer. All have the same shape, with a broad peak at about 12 keV, corresponding to the mean energy loss of a minimum ionizing electron in the counter. Because the energy loss depends on the nature of the counting gas and on counter dimensions, the position of the interference peak with respect to the x-ray peak of interest can be shifted by varying these parameters. In a smaller counter, for example, the broad peak would appear at lower energies; on the other hand, the use of heavier counting gases or higher gas pressures would shift the interference peak to higher energies. In principle, the signal-to-interference ratio can be optimized by proper choice of the counter parameters, thus rendering the technique even more selective. The same factors also affect the efficiency of the counter for both the x-rays of interest and the interfering gamma-rays. A careful evaluation of all the effects is therefore required before these general principles can be applied to any specific case. The intensity of these spectra was not reduced by the insertion of additional Lucite absorbers, up to 12 mm. Thus the interference is due to gamma- rather than beta-rays, and its intensity is proportional to the specific gamma activity of the isotope in question. Of the isotopes we tested, 56Mn interferes most strongly with the determination of bromine. This is due to its high activation cross-section, relatively short half-life, and large gamma branching ratios. The limiting M n p r ratio is ff3O:l. 784
ANALYTICAL CHEMISTRY
Br, (b) 250 pg K, (c) 250 pg K
+
In order to check the feasibility of determining the bromine x-ray peak over the gamma-ray background arising from an excess of interfering elements, solutions containing about 0.5 pg of bromine and varying amounts of KNO,, NaNOs, and NH4Cl were irradiated and counted as described above. The same samples were also counted with a 3-inch NaI(T1) scintillation crystal. Figure 5 shows the scintillation spectra obtained in a I-minute measuring period, 3 hours after the end of a 10-minuteirradiation, for the case of: ( a ) 0.5 pg of Br, (b) 100 pg of Na, and (c) a mixture of (a) and (b). Spectra (6) and (c) are almost indistinguishable, and the 8zBr activity is almost completely masked by the excess of 24Na.
(a) 80-minutes A.E.R.; (b) 140-minutes; (c) 200-minutes; ((i,260minutes; (e) 320-minutes; (f) 380-minutes
Channel Number
Fk
T,!,=37
38cL
100
50
0
min
~
Time (hours)
Figure 11. Decay of x-ray spectrum of Figure 10 Spectra of the same samples, taken with the x-ray detector -7 minutes after irradiation, are shown in Figure 6. The bromine x-ray peak is clearly visible over the z4Na background. The decay of the peak activity is shown in Figure 7. The 18-minute 80Br component can be resolved easily. Figures 8 and 9 show a similar comparison between the scintillation and x-ray spectra of an irradiated K-Br mixture (500:l weight ratio). In this case the E2Br gamma-rays can be resolved from the 421C background, but the advantage of the x-ray method is quite obvious. Figure 10 shows a series of x-ray spectra of a C1-Br mixture (1O:l weight ratio) taken at various times after irradiation. In this case a magnet was used to remove the beta-ray background. At short times after irradiation the bromine x-rays are completely masked by the gamma-rays and high energy (up to 4.8 MeV) beta particles of T I . A few hours later, after the chlorine activity has decayed, the x-ray peak of the 4.5-hour 'OrnBr begins to emerge. A decay curve of the activity is shown in Figure 11. Calculation of Result!;. The following equation can be used to determine the bromine content of a complex matrix.
where P is the number of counts per unit time in a window centered on the Br x-ray peak, V the number of counts per unit time in a window of approximately the same width far from the x-ray peak, and the subscripts s, st, and i denote sample, standard. and interference, respectively. The standard count P,,and Vstshould be corrected for decay to the time the unknown sample was counted. The interference spectrum shape factor Pi'Yi needs to be determined only once for a given counter and sample geometry, because most interfering elements have similar spectra. Results obtained with a number of mixed solutions are given in Table I. Fast Quantitative Analysis of Milligram Amounts of Bromine in Organic Matter. The x-ray counting method was also applied to the problem of fast quantitative analysis of wood brominated to increase its fire resistance. The samples were powdered, and weighed portions of about 100 mg (3-5 mg of bromine) were sealed in plastic as described above and
-
-
I
I
I
I
I
l
l
1
I
l
l
1
1-
B
-
Sample
-
analyzed. In this case, interference from the trace elements normally present in wood was negligible due to the small amounts present relative to bromine. However, some of the samples also contained Na and C1 in amounts comparable with those of the bromine. In scintillation gamma-ray spectra %C1, Z4Na, and S0-s2Br were all prominent, and the bromine activity had to be resolved from the interfering 24Na. In the x-ray spectrometer the spectra of the irradiated samples were identical with those of the pure bromine standards, and showed a single-component decay corresponding to 4.5-hour 'OrnBr,over several half-lives. This is shown in Figure 12. Due to the high bromine content of these samples, they could only be counted several hours after irradiationi.e., only after the initially produced 18-minute 80Brhad decayed. In view of the simplicity of the x-ray spectra and the high counting rates, the method is easily adaptable to single channel analysis and routine fast assay of many samples. ACKNOWLEDGMENT
We thank Saadia Amiel for continued interest and useful discussions. Thanks are also due to Jerry Cuttler who built the x-ray counter and to Rachel Billig and Yair Gat for their assistance with the measurements. Menahem Levin and Jacob Zabicky of the Institute of Fiber and Forest Products Research, Jerusalem, supplied the brominated wood samples. RECEIVED for review November 17, 1966. Accepted January 23, 1967.
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