Employing the new methods, water and silanol analysis of silicas which have becn trimethylsiloxy-treated have consistently shown two hydroxyl-group loss per trimethylsiloxy group added to the surface. (This loss, a possible result of steric effects, has not been completely explained.) The data from analysis of three silicas before and after this treatment are given in Table VIII. Carbon analysis for the trimethylsiloxy group was used to predict the decrease in surface hydroxyl groups. After correction for the two hydroxylgroup loss per trimethylsiloxy group added, the experimental values agreed with the predicted values. Observed and estimated precision for the various methods is summarized in Table 1. The lower detection limit of the MKFR system was abcut 0.005 % water with a practical lower limit of approximately 0.01%. The precision of the azeotropic distillation analyses should approach that of the catalytic condensation procedure where a similar separation
of water is made. The practical lower detection limit of the AZD procedure should be 0.01% water. The RSD for TGA adsorbed water analysis was estimated assuming a maximum weighing error of i 0.2 mg in evaluation of weight losses when employing a 300-mg sample. This RSD would apply to the silanol values by TGA also. The lower detection of water loss by TGA is limited to the maximum error in evaluation of weight loss. The practical lower limit of analysis using the catalytic condensation hydroxyl procedure was approximately 0.05 hydroxyl, but this limit could be extended to 0.005 or 0.01% hydroxyl or lower with care. Speed, sensitivity, and precision were identical for both untreated and treated silicas. RECEIVED for review August 17, 1966. Accepted January 9, 1967.
Determination of Boron by Thermal Neutron Capture Gamma-Ray Analysis B. W. Garbrah and J’. E. Whitley Scottish Research Reac,for Center, East Kilbride, Glasgow, Scotland
Thermal neutron capture y r a y analysis is applied to the determination of boron using a 2- by 2-inch NaI(TI) crystal. Analysis is based on the well known 477-keV -pray which i s emitted in 93% of all thermal neutron captures in boron. Correction factors provided for thermal neutron flux depression in boron also make the technique applicablle to the determination of large boron samples.
THEINHERENT ADVANTAGES of thermal neutron capture y-ray analysis as an analytical technique compared with conventional activation analjsis have been suggested by several authors (1-5). Isenhcur and Morrison have determined boron by thermal neutron capture y-ray analysis using a modulation technique ( 3 ) . Pierce, Peck, and Henry ( 5 ) and Sippel and Glover (6) hz.ve applied the measurement of prompt radiation emitted during charged particle bombardment to the determination of some light elements. The complexity of capture y-ray spectra often makes it difficult to identify uniquely the elements giving rise to the spectra, and also hindtxs satisfactory analysis. The determination of a single element in a matrix is complicated by the production of overlapping peaks due to other elements, in the matrix. In some situations the high background of ~
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(1) R. C . Greenwood and J. H. Reed, “Scintillation Spectrometry Measurements of Capture Gamma Rays from Natural Elements,” U.S. At. Energy Comm ARF 1193/6 (quarterly report) 1962. (2) L-V. Croshev, A-M. Demidov, V. N. Lutsenko, and V. F. Pelekov, “Thermal Neutron Capture ?-Ray Atlas,” Atornizdat (1958). (3) T. L. Isenhour and G. H. Morrison, ANAL. CHEM.,38, 167 (1966). (4) A. F. Para and M. M..Bettoni, Energia Nucleare, 11 (ll), 612 ( 1964). (5) T. B. Pierce, R. F:. Peck, and W. M. Henry, Analyst, 90, 339 (1965). (6) R. F. Sippel and E. D. Glover, Nucl. Znstr. Meihods, 9 , 3 (1960).
y-rays and neutrons around the reactor can also produce undesirable peaks in the spectrum. These difficulties have hindered widespread application of thermal neutron capture y-ray analysis. Because the y-rays arising from other elements in the sample which are not being determined cannot be avoided, a given spectrum can only be improved by a reduction of reactor background spectrum. To this end the methods of Hammermesh and Hummel (7) and of Greenwood and Reed ( I ) have been combined and modified to permit satisfactory quantitative analysis for boron. The method reduces the four-cycle measurements of Hammermesh and Hummel to two, and in some cases the total live counting time is reduced by a factor of four. The reduction in counting time has the additional advantage of reducing the effects of the activity of radioisotopes that build up during irradiation. EXPERIMENTAL
Apparatus. The apparatus is schematically shown in Figure 1 . Irradiations were performed using the UTR lOOKW reactor of the Scottish Research Reactor Centre. The beam tube chosen for the experiments passed through the thermal column of the reactor and so was shielded from fission product y-rays by a lead wall (Figure la). The fraction of epithermal and fast neutrons in the beam tube at this location was considerably less than in other beam tubes in the reactor. At the sample position the thermal neutron flux was 8 X 105 neutrons/cm*/sec; and the beam was 2 cm in diameter. Detector and Shielding. The detector and shielding assembly is shown in Figure 1b. The detector was a 2 in. x 2 in. NaI(T1) crystal detector with built-in photomultiplier (Ecko Type N 609). This was shielded by a lead castle of wall
(7) B. Hammermesh and V. Hummel, Phys. Reu., 88, 916
(1952). VOL. 39, NO. 3 , MARCH 1967
0
345
/H[P.MIL
COLUMN
Figure 1. A. Location of beam tube relative to reactor core 1. 2. 3. 4. 5.
thickness 4 inches. The top, however, was built with 2-in.thick lead bricks on 3/&-thi~k steel plate. A central hole of diameter 1.5 in. served as collimator for the y-rays. Twoinch rather than 4-in. lead bricks were used to build the top of the lead castle to keep the sample-crystal distance as small as possible. A 6LiF of thickness 450 mg/cm2 placed between the sample and detector absorbed neutrons scattered by the sample and sample holder in the direction of the crystal. By surrounding the lead castle with a mixture of boric acid in. on the top), the number and wax of 1.5-in. thickness ("is of neutrons scattered into the detector was reduced to the extent that iodine capture y-rays, which appeared to be the main constituent of the background spectrum, were removed completely. Increasing only the 6LiF neutron absorber between the sample and crystal did not have any significant effect on the background. Pulse-Height Analyzer. The pulses from the detector were fed, after amplification, to a 512-channel analyzer located in a counting room outside the reactor hall. Only 256 channels of the analyzer were used for the experiments. Samples and Sample Holder. Powder samples were sandwiched as evenly as possible between two aluminum disks of 2-cm diameter and 0.002-inch thickness and compressed together in a hand vice. Large powder samples were put into small aluminum cups of 2-cm diameter. Steel and some other metal samples were machined or punched into disks. The samples were held at the center of a square aluminum frame, between two thin steel wires which crossed each other at right angles at the center of the frame. Tension in the wires could be adjusted to accommodate samples of different sizes. The sample to detector distance was 13 cm. Method. Hammermesh and Hummel (7) have suggested that to eliminate the effects of background from reactor neutrons and y-rays, a cycle of four measurements should be made as follows: 1. Source in-no cadmium between source and neutron beam 2 . Source in-cadmium in the beam 3. Source out-cadmium in the beam 4. Source out-no cadmium in the beam The required capture y-ray spectrum was obtained from the relation 1 - 2 3 - 4. In Figure 2a is shown the relation 4 - 3 measured before the boric acid and wax shielding was placed around the lead castle. Figure 2b shows the same spectrum after the shielding
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B. Detector collimator and shielding assembly
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ANALYTICAL CHEMISTRY
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