substitute other instrumental methods for this analysis. Recent investigations indicate that the gas chromatograph is suitable for the determination of carbon dioxide in the presence of silicon tetrafluoride and fluorine oxide. This instrument also offers increased sensitivity at a reduced equipment cost. However, sufficient data are not yet available for a valid evaluation using this instrument for the gas analysis.
LITERATURE CITED
(1) Bernhardt, H. A,, Brickell, W., Harris, W. W., Rickard, R. R., U. 8. At. Energy Comm. Rept. K-130(1958). (2) Freier, H. E., Nippoldt, B. W., Olson,
P. B., Weiblen, D. G., ANAL. Cnm. 27, 146 (1955). (3) McCoy, R. N., Baatin, E. L., Zbid., 28, 1776 (1956). (4) Milner, 0. I., Zbid., 22, 315 (1950). (5) Teston, R., McKenna, F. E., Zbid., 19, 193 (1947).
(6) Throckmorton, W. H., Hutton, G. H., Zbid., 24, 2003 (1952). (7) Van Kooten, E. H., Gardner, R. D., U.S. At. Energy Comm. Rept. LA4947 (1955). ( 8 ) Warf, J. C., U.S. At. Energy Comm. Rept. CC433 (1943). RECEIVEDfor review February 5, 1962. Accepted May 7, 1962. Work carried out under Contract No. W-7405-Eng-26 at the Paducah Plant, operated by Union Carbide Nuclear Go., Division of Union Carbide Corp., for the U. S. Atomic Energy Commission.
Liquid Scintillation Counting of Tritium Improvements in Sensitivity by Efficient Light Collection ROYAL H. BENSON and ROBERT L. MAUTE Monsanto Chemical Co., Texas City, Texas
b The use of light guides to improve routine tritium counting efficiency has been investigated. By optimizing the tritium spectrum with respect to the discriminators and using an effective light guide, a differential tritium counting efficiency of 44% can be obtained for routine use. Counting errors arising from varying sample volumes and condensation on the vial are eliminated by use of the light guide.
L
IQUID SCINTILLATION COUNTING has
become the method of choice for the routine determination of tritium and other weak beta emitters in liquid and solid samples. Comparatively high sensitivity and ease of sample preparation have been the major contributing factors to the wide acceptance of this technique. Because of the low energy of the tritium beta particle, efficiencies of about 15% for nonquenching systems are obtained routinely with commercially available equipment. Specialized techniques have been described (1, 8) which can give the extrapolated theoretical maximum tritium efficiencies for single and dual multiplier phototube systems. Such techniques, however, have not met the requirements for routine use. The work done in this laboratory to obtain a %fold increase in tritium efficiency for routine counting is herein described.
The polished aluminum sample chamber was removed to permit optical coupling of light guides with the multiplier phototube faces. Scintillator Solution. The solution used in these studies consisted of reagent grade toluene containing 0.5% diphenyl oxazole and 100 mg. per liter of 1,4-di-2-(5-phenyl oxazolyl benzene) (POPOP). The toluene was or otherwise not distilled,. degassed, pretreated. Linht Guides. Several lieht guide designs were investigated. The iiitial elliptical designs used vertical aluminum reflecting planes in the center of the guide, parallel to the phototube faces, to reduce the possibility of backward light reflection. I n the final design this was eliminated, as the thin side walls acted as an effective barrier. The early designs were vacuum aluminized on the outside to obtain total internal reflection of scintillation light. The final design used aluminum foil, frosted side in, optically coupled to the cell with epoxy resin.
EXPERIMENTAL
Apparatus. A commercially available liquid scintillation spectrometer, Tri-Carb Model 314, Packard Instrument Co., was used in this work. 1 122
ANALYTICAL. CHEMISTRY
All light guides uere machined from Lucite and were highly polished. The guides were coupled t o the multiplier phototube faces with Dow Corning silicone fluid. Standards. The tritium standards used in our work were prepared from an NBS tritiated water standard. An accurately weighed sample (300 mg.) of tritiated water was diluted with dry n-butanol t o give about 3 x 104 d.p.s. per milliliter. Small aliquots (0.02 ml.) of this were used for counting. This technique reduced quenching to a minimum, eliminated the possibility of adsorption of significant amounts of tritiated water on the vial walls, and permitted accurate sample measurement. Efficiencies were calculated with the following equation. Percent absolute efficiency = c.p.8. obtained from sample x 100 d.p.8. in sample (calculated from NBS standardization) Procedure. DISCRIMINATOR SETAll differential counting was done with 5- to 100-volt discriminator settings. A major increase in efficiency is obtained by lowering of the A and A’ discriminators to the 5-volt level. The reasons for this increase are obvious from an examination of the tritium beta spectrum shown in Figure 1. The normal settings of the Tri-Carb discriminator for tritium counting are usually 10 to 50 volts. These values are dictated by the maximum (efficiency)2 background ratio which is obtained a t these values. With the light guide, however. this is no longer the case. (efficiency)2is now obThe background tained with a 5- to 100-volt window. To use the &volt setting, it is frequently necessary to select, by trial and error, TINGS.
I
Figure 1 .
0
A
Tritium beta spectrum
1 1 90 v. with AI coated rectangular light PiPo 1200 v. with conventional AI chamber
background counts should be taken a t different monitor tube voltages. The best compromise value based on the (efficiency)a relation may then be background selected. Fignre 2 shows the variations in count rate us. voltage normally eucountered during optimization. RESULTS
Figure 2.
Optimization of tritium peak
Rxed at 1300 V. Analyzer tube Yoltogc fired (It 1 1 60 Y.
0 Monitor tube voltage
A
the 6AH6 tubes in the ;I and A' discriminators. There are two such tubes in each discriminator. The input (front) tube is the most important. This selection is specific for the vacuum tube Tri-Carb and may not be possible or of value with other types of liquid scintillation spectrometers. Increased high voltage, with decreased amplifier gain, will also permit 1ower.A and A' discriminator settings to include the low energy part of the spectrum. In OUT case, the elevated background due to the higher coincident noise rate nullified the increased efficiency so obtained. This factor will vary according to the particular multiplier phototubes and should be investigated accordingly. The object of selection is to obtain tubes which will cause preamplifier noise to be cut off' a t the 3- to 4-volt level (high voltage off, selector in single channels position). These tubes will usually last several months. OPTIMIZED OPERATING VOLTAGE. The optimum high voltage settings for each multiplier phototube were determined as follows: The monitor tube high voltage was increased to the maximum using the fine adjustment. Short counts (ea. 10 seconds! are taken a t different high voltage tap settings to locate approximately the tritium pe;tk. The analyzer tube voltage is then increased in IO-volt increments with the fine adjustment, taking counts a t each new value. The results are plotted and the optimum analyzer tube voltage is determined from the results-i.e., the peak value. The sample is then recounted with the analyzer tube voltage fixed at the optimum value wbile r e ducing the monitor tube voltage at 10volt increments and the results are plotted. A compromise may have to be reached in this latter case, as it is conceivable that a small increase in efficiency due to increased monitor voltage may result in an inordinate increase in background. Thus, severa.1
Maximum Tritium E5ciency. The maximum efficiency obtainable for a 12-ml. sample volume was measured with a I-inch diameter, I-inch long flat-ended cylindrical quartz cell tightly wrapped with aluminum foil. The cell was coupled with silicone fluid directly to the phototube faces. The differential efficiency ( 5 to 100,volts) was 50.5% with a background of 0.90 C.P.S. Normal Trifium E5ciency. The maximum efficiency of the instrument for the standard aluminum chamber was determined using 15 ml. of scintillator solution in a crystallite vial. Under optimiaed conditions an extrapolated integral efficiency of 27% was obtained using a technique previously described ( I ) . The measured differential efficiency (5 to 100 volts) was 23.1% with a background of 0.75 c.p.s. Light Guides. Several designs of light guides (Figure 3) were investigated to determine the factors affecting light collection. The first design, a truncated elliptical shape, with a central reflecting plane, was coated with aluminum by a metallizing process. A differential efficiency of 33% was obtained (5 to 100 volts) with this guide. The two subsequent designs were similar in shape but different in length and were vacuuni nluminized to obtain better internal light reflection. The longer guide gave a differential efficiency of 30.3% (5 to 100 volts). The shorter guide gave a differential efficiency of 37.5y0 (5 to 100 volts). These results indicated that regardless of the efficiencyof the reflecting wall enough light is lost in longer guides, by multiple reflections, to cause major decreases in efficiency. The shortest possihle guide was then designed in the form of a rectangular cell with extremely thin walls at all sides and without a central reflecting plane. This design is shown in detail in Figure 4. The rectangular guide was first evaluated without reflectors other than
the polished cell walls. .4 differential efficiency of 40.4% (5 to 100 volts) was obtained. When aluminum foil was coupled to the walls the efficiency increased to 43.9y0 (5 to 100 volts), and an extrapolated integral efficiency of 51.3qb was obtained. Also, greater efficiencyresulted from using the frosted side of the aluminum foil as the re0ector rather than the shiny side. The use of metallic oxide coatings previously described (4)were investigated and sho\ved no advantape over the frosted aluminum foil reflector. Scintillator Solution Volume. Figure 5 shows the effeets of scintillator solution volume on efficiency, using a polyethylene vial in the well of the A1 chamber compared to the rectangular light guide. The two curves s h o r greater independence of counting efficient.? for different sample volumes in the case of the light guide. Further, the decrease in counting efficiency for larger sample volumes is minimized by the light guide. This feature alone justifies the use of the light guide in minimizing errors due to varying sample volumes and vial configurations. DISCUSSION
Tritium counting is the most severe test of the liquid scintillation spectrometer. Because of the low E, (18 k.e.vJ and much lower E., (5.69 k.e.v.), all of the factors affecting efficiency and stability are magnified. Electronic noise, sample quenching, fluorescence, phosphorescence, and light collection by the multiplier phototubes become problems of maior importance in obtaining reliable and accurate results, much more so than in the case of more energetic beta emitters. Light guides have been widely used with success in gamma scintillation
I I'*'
BORE I 2 3 1 ~ ;
Figure 4.
Optimum light guide design
AI foil reflectors used on dl outer rurfacor
Figure 3.
Light guide designs for tritium counting
except rider 1 ond 2
VOL 34, NO.
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5
10
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SAMPLE VOLUME (ML.1
Figure 5.
Variation of count rate with sample volume
0 Uchamber
A
Rectangular light guide
counting where considerable light is produced. The problems in designing e5cient guides have been investigated (S,4) with the conclusion that it is nearly impoasible to calculate light transmission for guides of any but the most simple configuration. In the case of tritium where the amount of scintillation light produced is extremely small, our results indicate that the best guide is the shortest possible and that to increase e5ciency, the probability of multiple reflections must be minimized regardless of the e5ciency of the surface reflector. Figure 1 shows the tritium beta spectrum obtained on the same sample with the conventional aluminum chamber and with the most e5cient light guide. Better resolution is apparent in the latter case aa well as much higher efficiency due to reduced losses of light resulting from low energy particles. The tailing of the spectrum is presumably due to nonlinear amplification in the multiplier phototubes. The e5ciency of 43.9% obtained with the rectangular light guide represents 7’9% of the theoretical maximum e5ciency of a two-multiplier phototube system, and the extrapolated integral value of 51.3% represents 92% of theoretical. These are calculated based on a theoretical maximum of 56% (4). These values show that with the light guide lower discriminator setting, not light collection, is now the important factor in obtaining even greater e 5 ciency. That is, if the discriminator could be reduced to near zero, the dif-
1124
ANALYIlCAL CHEMISTRY
ferential efficiency should closely approach the integral value. Preamplifier noise at present obviates this possibility. A further increase-i.e., from 43.9% to 46.3% may be realized by silicone fluid coupling of the vial to the well of the light guide to reduce light losses at the vial-air-lucite interface. This expedient is not justified because of the difEculties in routine application. Examination of Figure 5 shows the sensitivity of tritium counting efficiency to varying sample volumes. The very sharp initial increase can be explained only by the fact that the vial itself must be acting as a light guide and conducting the light vertically for distribution to the multiplier phototubes. This sharp increase is certainly due in part to the large solid angle geometry of the system at small volumes. The slow decrease above 12 ml. is probably produced by the decreasing solid angle. Elevated counting rates from tritium samples are frequently observed when condensation occurs on the vial surface during insertion into the aluminum counting chamber. If the aluminum chamber is dull or dirty, the effect is especially pronounced. The count rate usually decreases to a stable value after 10 to 15 minutes in the chamber. This elevation has been traced to better light collection from the frosted vial surface by the multiplier phototubes rather than light induced phosphorescence of the sample or vial. This unfortunate effect is completely eliminated by the light guide. This is due to the high
light collection efficiency of the guidei.e., the frosted vial does not further improve the system. The light guides described here have been in use and under investigation for about three years. The fact that the aluminum reflecting surfaces are completely protected from dirt and oxidation has resulted in much more stable long-term operation than was possible with the conventional chamber. The sample wells of the light guides are occasionally wiped clean with lens tissue; however, no problems attributable to dirt have been observed. Although the light guides described here were developed for the manuallyoperated Tri-Carb, the small rectangular guide is easily adapted to automatic operation by extending the well completely through the guide to permit movement of the piston through the sample well. The steel vial elevator surface should be covered with a polished aluminum disk to obtain better collection of light emitted through the bottom of the vial. LITERATURE CITED
(1) Horroch, D. L., Studier, M. H., ANAL.CHEM.30. 1747 (1958). (2)Ibid., 331 615 (i96l). (3) Krivitskii, V. V., Leksin, G. A., Pribory i Tekhn. Ekspcrim. 1,79 (1960). (4) . . Swank, R. K., “Liquid Sciatillation Counting,” C. ‘G. gell, ed., p. 23, Pergamon Press London, 1958.
.
- I
RECEIVEDfor review March 12, 1962. Acceptad May 8,1962. National Meeting of the American Nuclear Society, June 1962.
Correction Magnesium Iodate Tetrahydrate as Primary Standard for (Ethylenedinitri1o)tetraacetate Solutions In this article by Frederick Lindstrom and B. G. Stephens [ANAL.CHEM.34, 993 (1962)], the title ww incorrectly shown as Magnesium Iodate Tetrahydrate as Primary Standard for (Ethylenedinitri1o)tetrahydrate Solutions.
