Anal. Chem. 1995, 67,4466-4470
Prompt ylRay Analysis of Boron with Cold and Thermal Neutron Guided Beams Chushiro Yonezawa* and Abdul Khalik Haji Woodt Depattment of Chemistly and Fuel Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan
Nondestructive determination of trace amounts of boron in various materials was wried out using neutron induced prompt y-ray analysis. A 100-250 mg sample was irradiated with cold or thermal guided neutrons of JRR3M, and the Doppler-broadened478 keV prompt y-rays were measured. To determine the B content accurately, analytical conditions such as the sample size, the hydrogen content of the sample, and any interferences due to other elements were examined. Nanogram to microgram per gram levels of B were determined in reference materials, including rocks, botanical samples, coal and coal fly ash, various types of sediments, animal tissue, environmental samples, and reactor materials, with accuracy and precision of better than 20%.The detection limits varied depending on the type of sample: 12-22 ng/g for graphite and Be; 120-260 ng/g for marine environmental materials, coal fly ash, and pepperbush; and 57-98 ng/g for the other materials. Accurate determination of traces of boron is important due to various specific requirements in such fields as geochemical, cosmochemical, agricultural, and material science and, more importantly, in the study of reactor materials. Popular nonnuclear methods for the determination of traces of B are inductively coupled plasma atomic emission spectrometry (ICP-AES) and spectrophotometryS2 A nuclear analytical technique based on neutron capture reaction is an alternative method with excellent high sensitivity. The large cross section of the nuclear reaction 'OB(H,~)~*L~, 3837 barn, is the main reason for this high sensitivity. There are two types of methods for detecting this reaction. The most common is the neutron induced prompt y-ray analysis OGA) ,3-6 in which the Doppler-broadened 478 keV prompt y-rays emitted from 7'Li are measured. Others involve detection of a-particles either by a-counting7 or a-track countin9 or by + Present address: Analytical Techniques Development and Services, Malaysian Institute for Nuclear Technology Research, Bangi, 43000 Kajang, Malaysia. (1) Winge, R IC: Fassel, V. A; Peterson, V. J.; Floyd, M. A Inductively Coupled Plasma-Atomic Emission Spectroscopy: An Atlas of Spectral Information; Elsevier: Amsterdam, 1985. (2) Marczenko, Z. Separation and Spectrophotometric Determination of Elements, 2nd ed.; Ellis Honvood: Chichester, 1986 pp 178-188. (3) Gladney, E. S.; Jurney, E. T.; Curtis, D. B. Anal. Chem. 1976, 48, 21392142. (4) Yonezawa. C.: Tojo, T.; Komori, T. Bunseki Kagaku 1986.35.782-785 (in Japanese). (5) Anderson, D. L.; Cunningham, W. C.; Mackey, E. A FreseniusJ. Anal. Chem. 1990,338, 554-558. (6) Sakai, Y.; Yonezawa, C.; Magara. M.; Sawahata, H.; Ito, Y. Nucl. Instrum. Methods 1994,A353, 699-701. (7) Chabot, G. E. /. Radioanal. Nucl. Chem. 1988, 123, 491-515.
4466 Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
measuring the product (He gas) using mass spectromehy (neutron activation mass ~pectrometry).~J~ There is also an application of the a-counting, called neutron depth profiling," in which the energies of the emitted a-particles are analyzed and the depth of the reaction site is determined. Due to the contamination and evaporation problems with B, destructive analytical methods such as ICP-AES and spectrophotometry are less accurate compared to the nondestructive methods, especially at the trace levels. However, traces of B can be determined nondestructively with good accuracy using PGA, Traces levels B in various samples have been determined with internal type PGA systems, in which the sample is irradiated in a reactor and the y-rays are measured outside the r e a ~ t o r and ,~ thermal neutron beam type PGA, which utilizes a thermal neutron beam from a horizontal or vertical experimental hole in the The PGA systems at the University of Maryland (UMd) and National Institute of Standards and Technology which produced most of the earlier important PGA data, belong to the conventional type. However, the conventional PGA systems suffered from low analytical sensitivity due to poor beam quality and low detection efficiency. This can be overcome by using cold and/or thermal neutron guided beams.12 The PGA systems at the Research Center (KFA) Jiilichl3 (Germany) and NIST14belong to this type and are being used for multielement ana1y~is.l~ A highly sensitive and low y-ray background PGA system using cold and thermal guided neutron beams has been constructed at the Japan Atomic Energy Research Institute UAERI),16and basic elemental and isotopic analyses have been performed using it.17J8 In this paper, we describe the determination of nanogram to microgram per gram levels of B in such reference materials as rocks, botanical samples, coal and coal fly ash, various types of (8) Fleischer, R L.: Price, P. B.; Walker, R M. Nuclear Tracks in Solids: Principles
and Applications; University of California Press: Berkeley, 1975: pp 489526. (9) Clarke, W. B.; Koekebakker, M.; Barr, R D.; Downing, R G.; Fleming, R. F. Appl. Radiat. Isot. 1987, 38, 735-743. (10) Iyengar, G. V.; Clarke, W. B.; Downing, R G. Fresenius]. Anal. Chem. 1990, 338, 562-566. (11) Downing, R G.; Lamaze, G. P. Neutron News 1993, 4, 15-20. (12) Lindstrom, R. M.; Yonezawa, C. In Prompt Gamma Neutron Activation Analysis: Alfassi, Z. B., Chung, C., Eds.; CRC Press: Boca Raton, FL, 1995: pp 93-100. (13) Rossbach, M. Anal. Chem. 1991, 63, 2156-2162. (14) Lindstrom, R M.: Zeisler, R.; Vincent, D. H.; Greenberg, R. R: Stone, C. A,: Mackey, E. A: Anderson, D. L.; Clark, D.D.J Radioanal. Nucl. Chem. 1993, 167, 121-126. (15) Rossbach, M.: Hiep, N. T. Fresenius J. Anal. Chem. 1992, 344, 59-62. (16) Yonezawa, C.; Haji Wood, A IC; Hoshi, M.:Ito, Y.; Tachikawa, E. Nucl. Instrum. Methods 1993,A329. 207-216. (17) Yonezawa, C. Anal. Sci. 1993, 9, 185-193. (18) Yonezawa, C.: Magara, M.: Sawahata, H.; Hoshi, M.: Ito, Y.: Tachikawa, E. J. Radioanal. Nucl. Chem. 1 9 9 5 , 193, 171-178. 0003-270019510367-4466$9.00/0 0 1995 American Chemical Society
sediments, animal tissue, environmental samples, and reactor materials, and we discuss the accuracy, precision, and detection limits of the PGA. EXPERIMENTAL SECTION Apparatus. The PGA system can be set at either the cold or the thermal neutron beam guides of the 20 MW reactor JRR-3M (upgraded Japan Research Reactor No. 3, JAERI). The system consists of a neutron beam shutter, neutron beam collimator, sample box, neutron beam stopper, shielding for neutrons and y-rays, and a multimode y-ray spectrometer. To obtain the lowest y-ray background neutron shielding material, lithium fluoride tiles (LiF, %F) and poly(tetrduoroethy1ene) (F'TFE) are used near the samples, and the sample box is made of F'TFE and filled with He gas. The neutron beam is collimated to 20 x 20 mm2 using the LiF collimator. The sample is mounted on a sample holder made of PTFE and is set with its face tilted 45" to the neutron beam. The average neutron flux at the sample position in the 20 x 20 mm2 area was 1.4 x 108 (2.4 x 107) neutrons cm-2 ssl, with a relative standard deviation of 18 (5)%for the cold (thermal) neutrons. The neutron beam intensity was monitored continuously by measuring scattered neutrons with a 3Hecounter placed beside the sample box and also by measuring twice a day the prompt y-rays from the 0.05 (0.5) mm thick Ti plates for the cold (thermal) neutrons. The flux fluctuation during one reactor operation cycle (26 days) was 0.8 (LO)%for the cold (thermal) neutrons. The multimode y-ray spectrometer consists of a closed-end coaxial type high-purity Ge detector, BGO (bismuth germanate, BiqGe3012) shielding detectors, and a multichannel analyzer controlled by a personal computer. Three modes of prompt y-ray measurements, single mode, Compton suppression mode, and pair mode, can be performed simultaneously in an energy range from 0 to 12 MeV. A detailed description of the PGA system can be found elsewhere.16-'* Standard Boron Samples. A standard boron solution (1.019 mg of B/g) was prepared by dissolving reagent grade boric acid in water. Standard B samples were prepared by evaporating portions of the B solution onto filter paper or 0.1 mm thick Sn foil of 13 x 13 mm2,which were then heat-sealed'in 25 pm thick fluorinated ethylene propylene resin PEP) film into an area smaller than 15 x 15 mm2. Analytical Procedure. Samples weighing 100-250 mg of various reference materials, prepared by Geological Survey of Japan (GSJ), NIST, National Institute for Environmental Studies (NIES) of Japan, and JAERI, were analyzed. The samples, except for the rocks, coal fly ash, graphite, and Be, were cold-pressed into a disk of 13.0 mm diameter and were then heat-sealed in FEP film into an area smaller than 15 x 15 mm2. For the rocks, coal fly ash, and graphite, which are dficult to press into a disk, the powder was heat-sealed in FEP film in an area smaller than 15 x 15 mm2. Reactor materials of Be were cut into an area smaller than 13 x 13 mm2 and were then heat-sealed in FEP film. The sealed samples were mounted onto the sample holder using 0.3 mm diameter PTFE string and were then placed into an airtight PTFE sample box. Air was purged from the box prior to and during the measurements by flowing He gas at a rate of about 1000 mL/min. A Compton suppression PGA spectrum was measured for 500-50000 s. The count rate of the Dopplerbroadened 478 keV y-ray was determined by Covell's method,Ig setting the region of interest at 466-490 keV. Other y-ray peaks
were analyzed using a conventional y-ray spectrum analysis program (SEIKO EG&G DSP201/MS). The B concentrationwas calculated by comparing the count rate of the 478 keV y-ray line with those of the standard samples. Absorbed moisture in the samples was measured separately by drying the same portion of the samples in an air oven at 85 "C for 4 h and making the necessary corrections. Safety Consideration. The PGA system is designed to achieve the lowest possible space radiation dose of neutrons and y-rays.16 Direct exposure to the neutron beam is strictly avoided by using the beam shutter. After measurements, the samples are activated. Although the induced activities are usually low due to the low neutron flux, the surface radiation levels must be monitored during sample handling. RESULTS AND DISCUSSION Analytical Sensitivity. The analytical sensitivitywas affected by the flux fluctuation as well as the type of matrix of the standards. Although the fluctuation of the count rate for the Ti flux monitor in an operation cycle was smaller than about LO%, the fluctuation for different operation cycles was larger than 1.096, and in a few cases it was as large as 30%. This is because the count rate of the Ti flux monitor reflects not only the neutron flux but also the conditions of the y-ray spectrometer. To compensate for these conditions, the B 478 keV peak count rate was divided by the average count rate of the 342 and 1381 keV peaks of the Ti plate monitor for normalization. The calibration curve for the normalized B 478 keV peak count rate versus the B amount in the standard samples was plotted at different operation cycles. Despite the different conditions for different operation cycles, the B amounts and the normalized peak count rates were in good linear agreement up to 0.1 (1) mg of B at the cold (thermal) neutron beams. The analytical sensitivity of 12.1 f 0.5 mg-1 of B was obtained at an average (1.23 f 0.06) counts count rate of the Ti monitor from 181 to 203 (121 to 133) counts ssl for the cold (thermal) neutron beams. The analytical sensitivity of B agreed within 2.7% for the standard samples in filter paper and in Sn foil. This indicates that although the filter paper is a hydrogenous material, it can be used as the base material for standard preparation without significant effect on the accuracy. Effect of Sample Size. The beam sizes of the JRR-3M cold and thermal neutron guided beams are as small and narrow as 20 x 50 and 20 x 91 mm2,respectively,and are further collimated to 20 x 20 mm2 in our PGA system. This size is substantially smaller than that of the cold neutron beam of KFA Jiilich,1350 x 100 mm2. However, the flux is not completely homogeneous in the area of the beam (20 x 20 "2). Thus, the effect of sample size was examined by measuring various sizes of square-shaped Ti (0.05 mm thick) and Ni (0.1 mm thick) plates. The count rate (counts s-l g-l) of the prompt y-rays of Ti (342 keV) and Ni (465 keV) remained constant up to 15 x 15 mm2,with relative standard deviations of 5.9 and 1.7%for the cold and thermal neutrons, respectively. Therefore, it was decided to make the sample size smaller than 15 x 15 mm2. For sizable samples, neutron scattering and self-absorptionof neutrons and prompt y-rays within samples may also affect the accuracy. This effect was investigated by analyzing various amounts of orchard leaves (NIST 1571) and basaltic rock (GSJ (19) Covell, D.F. Anal. Chem. 1959, 31, 1785-1790
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Orchard
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leaves (NIST 1571)
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. m m
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(GSJ J B - I ) 30
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D
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'0
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0 0,
0 1
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Figure 1. Effect of varying sample amount for analysis of orchard leaves (NIST 1571)and basaltic rock (GSJ JB-I). (0)B with the cold neutron, (0)B with the thermal neutron, (0) H with the cold neutron, and ( A ) Si with the cold neutron.
JBl),disk-shaped to a k e d diameter of 13.0 mm. For the rock samples, extreme care had to be taken in preparing the shaped disks. The results of B and H determination for the orchard leaves and of B and Si determination for the rock samples are shown in Figure 1 as a function of the amount of the samples. The determined contents are constant and are close to the certified and consensus values up to about 250 mg for the orchard leaves and rocks, but the contents become lower for larger sample amounts. Mackey et al.20-22 reported that analytical sensitivity is vaned depending on the thickness of disk-shaped hydrogenous samples, including the orchard leaves. They concluded that the phenomenon was due to neutron scattering with matrix H and recommended the use of spherical-shaped samples so as to minimize any detrimental effect. As can be seen in Figure 1,the sensitivity variation could not be observed in the present work. The neutron beam size of the present PGA system (20 x 20 mm2) was smaller than those of UMd and NIST (25 or 45 mm in diameter),zz2.'3 and the flux was lower near the outer fringe of the beam. Upon increasing the amount, the thickness of the sample became larger. For example, for orchard leaves the sample thickness was 7 mm when its amount was 1 g. In such a case, a part of the sample had to be situated in the outer fringe of the beam because the sample holder was tilted 45" in the beam direction. This may lead to a decrease in the signal intensity, as (20) Mackey. E. A; Gordon. G. E.; Lindstrom. R M.; Andenon. D. L A n d Ckem. 1991.63.288-292. . . (21) Mackey. E. A; Gordon. G. E.: Lindsbom. R M.: Anderson, D. L Anal. Chem. 1992. 64,2366-2371.
(22) Anderson, D. L.; Mackey, E.AJ Rodioonol. Nucl. Chem. 1993.167.145151.
(23) Failey. M. P.; Anderson. D. L.: Zoller, W. H.; Gordon. G. E.: Lindstrom. R. M. Anal. Chem. 1979.51. 2209-2221,
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observed. For the rock samples, an even greater decrease in the signal intensity may be observed, due to the difficulty of pressing the sample into well-shaped disks. Sample amounts from 100 to 250 mg were used in the present work. Effect of Hydrogen. As described above, Mackey et a1?n-22 reported that the analytical sensitivity and background count of B in hydrogenous materials are influenced by the H content due to scattering and moderation of neutrons with H in the usual thermal neutron beam type PGA The same effect has been examined in the NlSTs cold neutron beam recently, but it has not been confirmed?' We also examined this problem by measuring the prompt y-rays of B in samples with various B/H ratios, prepared by cold-pressingoxalic acid and the standard B solution. The H amounts were changed from 0 to 32 mg with 25 and 50 pg of B in the samples. Although the counting statistics (0) of the B peak count were normally less than 1.5%.the relative standard deviations of the counts of the B y-rays were 5%for H-free samples and 22%for H-containing samples. For the latter cases, no significant tendency of the B counts versus the H content was observed within the standard error of 22%. Although the H effect presents a serious problem for the usual thermal neutron beam type PGA, the effect seems rather small for the guided beam type PGA, because of the quality of the pure beam. The H effect in the cold and thermal guided neutron beams needs to be further investigated. Interference Correction. Due to the Doppler effect, the line width of the 478 keV prompt y-rays of B is broadened to 11.515.2 keV W M ) , which is 8-10 times those of normal y-rays.6 Thus, the B content in the samples was determined from the peak area in the region of interest, 466-490 keV. This implies increased possibility of spectral interference by other y-rays. Possible interferencewas examined by analyzing the y-ray spectra of standard samples of elements. The pray lines of 11 elements, Na, Si, P,CI, Mn, Co, Ni, Sr, Cd, Sm, and Hg, were found in the above-mentioned region of interest, for which the B equivalent of the interference peak was calculated. No interference was detected for H, Be, C, N. Mg, Al. S, IC Ca, Ti, Cr, Fe, and Gd, for which the detection limits of the B equivalent amounts were calculated from the 30 background count in the region of interest. The interference y-rays, the amounts of the B equivalent, the number of measured standard samples (n),and the reference y-ray lines for correcting the interferences of 24 elements are shown in Table 1. y-Ray lines for CI, Mn, Sm, and Hg were found at either the lower or the higher edge of the region of interest, for which the B equivalent amounts are shown as the upper limit value. The upper limit spectral interferences of CI were in the range 1.5-3.9% for the mussel @TIESno. 6). bovine liver @TIST 1577a), and fish tissue (NIES no. 11) and
