better, because dense ionization tracks from the soft tritium P-particles were originating throughout the sensitive volume of the counter. Figure 3 shows the effect of varying the amount of n-ater vapor in the counter. Good plateaus were obtained with 5 and 10 pl. of tritiated water, but the plateau had practically disappeared with 20 pl, Figure 4 shows the effect of varying the temperature. K i t h increasing temperature the plateau shortened, until at 105” C. it had almost disappeared. A temperature of 90” C. was chosen for routine operation. The system was calibrated using a standard tritiated water sample (Table I). By using the known disintegration rate of the standard, it was possible to calculate an efficiency factor for the counter n-hich n a s used to correct the observed counting rates. The standard deviation of the determinations was less than =t2%,
DISCUSSION
The only “memory” effect, or cross contamination between samples, was found to be due to the accumulation of tritium in the vacuum pump. For low activity samples (counting rates up to 10 times background), it is sufficient to allow the pump to flush itself with free air for a minute between determinations. For high activity samples (lo2 to lo3 times background) it is necessary t o flush the counter with one or two samples of pure water between determinations. The use of a cold trap in the pumping line is recommended for use with high activity samples. The sensitivity of the method depends on the background count. Because extreme sensitivity was not one of the design considerations, the counter was unshielded. If the very rigorous criterion is used that to be valid a
count should be equal to the background, the sensitivity of the counter is 0.0003 pc. or 0.03 p c . per ml., as a 10-pl. sample is used. By shielding the counter and relaxing the statistical requirements, the sensitivity can be stated to be about 50 times the above. The small quantity of water Tapor that the counter can tolerate places a lower limit on the specific activity of the water sample which can be measured. Xevertheless, There extreme sensitivity is not a major consideration, the system forms a fast, accurate method of tritium determination. LITERATURE CITED
(1) Drever, R. W. P., Moljk. A., Rev. Sci. Instr. 27, 650 (1956). ( 2 ) Nucleonics 16, KO.3, 62 (1958,~. (3) Ibid., p. 67.
RECEIVEDfor revien- April 28, 1958. Accepted July 21, 1958.
Low Level Plutonium-241 Analysis by Liquid Scintillation Techniques DONALD L. HORROCKS and MARTIN H. STUDIER Argonne National laboratory, lemont, 111,
b The plutonium-241 content of plutonium samples can be determined with a high degree o f confidence with the liquid scintillation spectrometer. The plutonium-24 1 P-particles are counted with a relatively high and easily reproduced efficiency of 37y0. The very low limit of detection, gram of plutonium-241, and the ease of recovery of the plutonium for further investigations give this method added advantages over other methods of analysis.
T
principal isotope of plutonium formed in reactors is plutonium239; however, isotopes of higher mass are also formed in lesser amounts by successive capture of neutrons. Because the nuclear properties of the various isotopes of plutonium are different, it is frequently desirable to know the isotopic composition of plutonium. Usually this isotopic analysis is made with EE
The term “multiplier phototube,” rather than “photomultiplier,” has been adopted b y ANALYTICAL CHEMISTRYasrecommended b y the Institute o f Radio Engineers [IRE h o c . 45,
No. 7, 1000 ( 1 957)].
a mass spectrometer. Frequently, however, the sample and/or its plutonium241 content is so small that the plutonium-241 may be below the limit of detection of the mass spectrometer. It is very difficult to recover a sample from the mass spectrometer after the analysis has been made. For these reasons a nondestructive method of analysis for plutonium-241 with a sensitivity of 10-’6 gram has been developed. This sensitivity is not seriously affected by the presence of relatively large amounts of the other plutonium isotopes. The method consists of putting the plutonium into an organic scintillating solution and measuring the light pulses produced by the @-particles from plutonium-241 with a coincidence-type liquid scintillation spectrometer. Upon completion of determination of the plutonium-241 content, the plutonium is easily recovered for other measurements. The sensitivity of this method depends on the fact that plutonium-241 is a relatively short-lived (13-year) beta emitter. Unfortunately, the very low beta energy (18 k.e.v. maximum) makes the measurement of the plutonium-241 activity somewhat difficult. Because the range of the 0-rays is less than 1 mg. per sq. cm., the use of a n
end window counter is not practical. Although plutonium-241 beta activity can be measured with an internal proportional counter, it is difficult to attain good reproducibility by this method. The problems usually encountered with the internal proportional counter method hare been eliminated with the liquid scintillator method. By dissolving the plutonium sample in the liquid scintillator (internal sample technique) the losses of the plutonium241 p r a y s due to absorption are essentially eliminated, n hereas measurement of the 0-rays by internal proportional counters is greatly dependent upon the method of sample preparation. For internal proportional counters, the usual procedure of sample preparation has been to evaporate saniples on platinum disks and drive off volatile salts or organic matter with heat. Even very tiny amounts of extraneous material will seriously lower the amount of beta activity n hich can be measured. If the platinum is heated too hot, the sensitivity for the measurement of the 0-particles may be reduced by as much as 20%. even though no extraneous material is present. Only relatirely large amounts of extraneous materials n-ill interfere with the measurement of the light pulses produced in the liquid scinVOL. 30, NO. 1 1 , NOVEMBER 1958
1747
Table I.
Counting Efficiencies and Background
Relative Background, Counting C.P.M. Efficiency 1. Reduction of Sise.of Light Guide and Surface of Multiplier Phototube Clear Lucite 85 ... %inch diameter 60 ... I-inch diameter I-inch diameter with aluminum rings 25 ... 2.
rfaee Materials Interface
Absorber, %inch diameter Shiny reflector, Zinch diameter Eter Diffuse reflector Diffjse %inch diameter 1-inch diameter Outside unvainted unpainted Outside painted
tillator and hence the determination of plutonium-241. When plutonium-241 6-rays are counted in an internal proportional counter, a correction has t o be made for the alpha counting rate. Because of possible absorption of the n-particles by extraneous materials and uncertainties in the geometry, it is sometimes very difficult to determine this correction. Also in samples of high alpha and low beta counting rates the uncertain correction may lend to large errors in the determination of amounts of plutonium-241. I n the liquid scintillation spectrometer the alpha and beta counting rates are determined independently of each other and no correction is needed. INSTRUMENT
A commercially available coincidencetype liquid scintillation spectrometer, Tri-Carb, Model 314, Packard Instrument Co., LaGrange, Ill., was used for the plutonium-241 measurements. Two Dumont 6292 2-inch multiplier phototubes were selected for their high sensitivity and low thermal noise. The tubes have an S-11 response-Le., a spectral response range of 3400 to 5200 A. with their peak response at 4200 A. (6). Scintillator Solution. M a n y scintillator solutions have been reported (2, 3,6). The composition of the scintillator solutions used in this work was chosen mainly because of its commercial availability, spectral characteristics, and relative pulse height properties. Xylene was used as the solvent with the primary and secondary solutes of p-terphenyl at 4 grams per liter and 1, 4-di-[2-(5phenyloxasolyl) ]-benzene (POPOP) at 0.1 gram per liter, respectively. POPOP shifts the wave length of the primary scintillator, pterphenyl, to higher wave lengths peaking at about 4200 A. The emission spectrum of the scintilhtor solution is in this way matched to the peak spectral response of the multiplier phototubes, giving the maximum efficiency for electron multiplication. Counting E5ciency end Background. T o measure precisely t h e
1748
.
ANALYTICAL CHEMISTRY
18 20
1.00
27
1.38
15 15
1.22 1.41
1.24
very soft beta activity of plutonium-241 or tritium in samples of very low disintegration rates, it is important to maximize the efficiency for counting and t o minimize the background. The Packard instrument was modified t o improve its light collection efficiency. The air coupling was replaced by a system involving a very short Lucite light guide coupled directly t o the faces of the two multiplier phototubes with a silicone grease. A well was drilled in the middle of the light guide for insertion of sealed tubes containing the scintillating samples. The well was filled with silicone oil to coude the sample tube optically to the light guide. With this type of arrangement the counting efficiencies for tritium and plutonium-241 &rays were found to be 50% and 37%, respectively. The three principal sources of background of coincidence-type scintillation counters in general are:
1. Accidental coincidences of the thermal noise of the multiplier phototubes. 2 . External racliations from radioactive sources or from cosmic rays. 3. Coincident events produced by photons arising in one multiplier phototube and entering the other (“lightdark current”) (4). By selecting multiplier phototubes with low thermal noise and keeping them cold (about -9’ C.), the background counting rate due to accidental coincidences was less than one count per minute with the window levels set for counting plutonium-241. When the sample size was reduced t o solution volumes of about 0.3 ml. and the sample tube made of thin-walled quartz, there was no detectable background due to external radiations. The major background was produced by the “light-dark current,” which was reduced by three methods: 1. Using a small light guide. ’ 2. Masking off that part of the face of the multiplier phototube which was not coupled t o the light guide. 3. Introducing a reflective interfacr into the light guide. The background was reduced 41% by reducing the size of the light guide from 2 inchcs to 1 inch in diameter (Table I). Aluminum rings with a 1-inch conceutric hole were placed over the ends of the 1-inch light guide and against the faces of the multiplier phototubes t o cover that part not directly coupled t o the light guide. This reduced the background by a factor of 2.4 (Table I). The ability to reduce the background by reducing the area of direct vision between the two multiplier phototubes led t o the designing of a special light guide.
Two pieces of clear Lucite were sealed
Figure 1. light guide with special titanium dioxide interface
60 k e v
10 RELATIVE
Figure 3. - 0
20
40
60
RELA?lVE
Figure 2. sample
300
400
WINDOW
500
600
together with a material between them for making a n interface which will not transmit light. After sealing and turning on a lathe t o the desired size, a sample well was drilled a t the interface so that the multiplier phototubes were optically coupled through the sample only (Figure I).
or decreasing the solubility of the scintillators. Plutonium is prepared as the dibutyl phosphate (DBP) complex, which is very soluble in xylene and for the required amounts of dibutyl phosphate did not interfere n ith the scintillation properties of the solution.
Three types of interfaces were investigated: an absorber, a “shiny” reflector, and a diffuse reflector. Table I shows the results of the relative counting efficiencies and backgrounds for light guides 2 inches in diameter and 0.5 inch thick with the various interfaces.
After suitable chemical purification, the plutonium was extracted from about 1 ml. of a 1M hydrochloric acid solution with a minimum amount of dibutyl phosphate (usually 5 to 25 ~ 1 . )diluted with a n equal volume of xylene. The organic phase was scrubbed with water to remove any dissolved acid 1% hich will lead to quenching. Samples extracted from hydrochloric acid solutions showed less quenching than those extracted from nitric acid solutions. It appears that hydrochloric acid is less soluble in dibutyl phosphate, is easier to scrub out of dibutyl phosphate, and/or does not quench the light as much as nitric acid. After scrubbing, the dibutyl phosphate-plutonium solution is transferred to a thin-walled quartz tube, using the scintillator solution as a transferring agent. The total volume of the sample is usually about 0.3 ml. Sitrogen is bubbled through the sample to remove any dissolved oxygen. which also quenches light from the scintillator. Because the concentration of the components of the sample affects the scintillation properties of the solution, any solvent lost by evaporation during this nitrogen treatment was replaced with nitrogen-pretreated solvent to make all samples the same volume. The samples were frozen in liquid nitrogrn, evacuated. and sealed by collapsing the wall of the quartz tube with a hot flame. If the qainples are counted immediately after thawing, the mensurement of plutonium-241 content always high. However. the measurements were qtabilized hy allowing the samples t o remain in the dark in the deep freeze unit for 2 t o 3 hours before the measurements. A sample of plutonium of known isotopic composition was used to determine the operating conditions for the maximum counting efficiency of Plutonium @-rays. A standard solution of plutonium as the dibutyl phoqphate
EXPERIMENTAL
Procedure. It is necessary t o prepare the sample in a form which is soluble in the scintillator solution but will not interfere with the scintillation process by quenching the light produced
20
of Amz4’
25
30
4
LEVEL
Spectra of external standards
700
LEVEL
Beta and alpha spectra of C.R. 1 plutonium
The diffuse reflector, which gave the highest counting efficiency of the three interfaces investigated, was made by painting the surfaces of two Lucite pieces with a special paint made of titanium dioxide suspended in Lucite n hich had been dissolved in ethylene dichloride. The paint also served as a binder for the two pieces of Lucite. The loss of light at the outside surface of the light guide was reduced by painting the outside surface with this special titanium dioxide paint. The results of this painting were a marked increase in the counting efficiency of plutonium-241 W a y s and essentially no change in the bnckground (Table I). A light guide 1 inch in diameter and 0.5 inch thick, with the special titanium dioxide interface and a sample well 7 min. in diameter and 19 mm. deep, was made for use n i t h the small samples employed in this work. K i t h aluminum rings placed over the ends of the light guide and the out3ide surface painted with the Epecial titanium dioxide paint, the background was reduced to only 15 counts per minute (Table I). S o loss in the counting efficiency rmulted from the r~dnctionin size of the light guide.
I5
WINDOW
7
complex in xylene n-as prepared from plutoniuin \I hich had been irradiated a t Chalk River for 6 months (C.R.-1) and whose isotopic composition had been determined mass spectrometrically in April 1952 ( 1 ) . After the plutonium had been purified by a solvent extraction process and corrected for the decay of plutonium-241, the isotopic composition of the C.R.-l plutonium on the date of the purification was: Isotope
c
ITass,
This standard plutonium ha. a beta disintegration-alpha disintegration ratio of 30.2 to 1. The operating conditions were selected such that the maximum counting rate of the @-raysof pIutoniun~-241was obtained. The low energy “cutoff” was adjusted so that the noise from the preamplifier was just eliminated a t an energy level of about 3 or 4 k.e.v. A counting efficiency of 36% to 38% for plutonium-241 &rays was obtained with the selected operating conditions. A typical plutonium-241 beta spectrum is s h o r n in Figure 2. This spectrum was obtained at a constant voltage by setting the loiv energy cutoff, Helipot A in the notations of the Tri-Carb instrument, a t a fixed value and observing the number of events within a limited window width by varying helipots B and C over the spectrum with a fixed difference between them. The other plutonium isotopes normally present are the alpha emitters, plutonium-238, -239, and -240. The aparticles give rise to pulses a t a n energy level corresponding to that of a 0.5-m.e.v. beta ray. Because the a-particles are monoenergetic, they gire rise to pulses of a definite pulse height and do not interfere with the plutoniuin-241 beta counting. There are several soft y-rays and conrersion electrons ascociated with the alpha emitters, nhich fortunately are all coincident n-ith the a-particles and thus merely add to the energy of the a-particles. Only about 0.1% of the a-particles in a liquid scintillator sample of plutonium-239 produce counts in the plutonium-241 beta counting region. This permits the determination of plutonium-241 to be VOL. 30,
NO. 11, NOVEMBER
1958
1749
made, even though it is present in plutonium samples with very high plutonium-239 plus plutonium-240/ plutonium-241 ratios. The CY particles are counted with 100% efficiency and a background of 0.2 c.p.m. A sample of C.R.-l plutonium Tvas used to determine the operating conditions for counting the alpha particles. Figure 2 s h o m the alpha spectrum of a C.R.-l plutonium sample. Table II. Nonquenching Effects of Increased Dibutyl Phosphate Concentrations
Vol.
70
Phosphate Dibutyl
Pu241p-Ray Counting Efficiency, %
Calibration Peak. (Window Level)
a Kindon- level for eak of spectrum of external standard of $e109-rlg109m.
Table 111.
Sample
Counting Efficiencies of Two Counting Systems
P L : *B-Ray ~ ~ Counting Efficiency, % Internal Liquid proportional scintillation counter spectrometer
1 2 3 4 5
47.5 41.5 44.9 41.5 44 7
Table IV.
35.9 36.3 36.2 37.0 36 4
quenching plutonium sample is shown in Figure 3. A second external standard which can be used is the 60-k.e.v. gamma ray of americium-241 ; however, the efficiency for conversion of this y-ray in the scintillator solution is low. A typical y-ray spectrum for the 60k.e.v. y-ray of americium-241 is also shown in Figure 3. Bpplication of this method to tritium analysis is now being investigated. Employment of the operating conditions for counting the P-rays of plutonium-241 with a sample of tritiated xylene in the scintillation solution gave a 50% counting efficiency for the @-raysof tritium. RESULTS
The possibility of quenching of the light output of the scintillator solution by dibutyl phosphate was investigated by varying the concentration of dibutyl phosphate over a limited range, 0.8 to 8.1% by volume (Table 11). No quenching due to the increase in dibutyl phosphate concentration was detected, as there rvas essentially no change in the plutonium-241 beta counting efficiency and no shift in the peak of the x-ray spectrum of the external standard of cadmium-109-silver-lO9m. The counting efficiency was betm-een 36 and 38% and the calibration x-ray spectrum peak was essentially the same for all the samples. -4series of duplicate samples of the standard C.R.-1 plutonium was counted in a n internal proportional counter and in the liquid scintillation spectrometer
Determination of Plutonium-24 1 Content of Plutonium Sample
Liquid Scintillation Spectrometer 01
8
dis./niin. dis./min.
3 3 (100% eff.) 7 15 (2 64 c.p.m. at 37% eff.)
An external standard was used when samples of unknown plutonium-241 content were counted, to determine any possible quenching in the sample which would lower the counting efficiency. The 22.6-k.e.v. x-rays of silver from a source of cadmium-lO9-silver-109m were alloved to activate the sample and the position of the peak of the x-ray spectrum indicated the presence of any quenching in the sample. A typical spectrum of silver x-rays on a non-
1750
e
ANALYTICAL CHEMISTRY
Internal Proportional Counter 3 . 3 ( 1 . 7 c.p.m. at 51% eff.) 7.15 (2.29 c.p.m. at 32% eff.)
(Table 111). The counting efficiency for plutonium-241 P-rays was consistently between 36 and 37%. Slthough the counting efficiency may be somewhat higher, 41 to 477,, for samples plated on platinum disks and counted in a n internal proportional counter, it is also very erratic because of deposits of extraneous material on the disks and/or degree of heating of the disks during and after the plating of the sample. However, with extreme care in the
sample preparation and plating, this erratic counting efficiency in the internal proportional counters can be minimized (Table 111). Upon completion of the scintillation measurements, the entire solution can be evaporated on a platinum disk and the organic matter ignited. With extra precaution a sample is obtained which can be alpha-energy-analyzed with good resolution. However, the counting efficiency for plutonium-241 P-rays from such a sample in a n internal proportional counter is only of the order of 30%. A sample of unknon-n plutonium-241 content was analyzed in the liquid scintillation spectrometer and after the measurements the solution was plated on a platinum disk and the plutonium241 content was redetermined by counting in a n internal proportional counter. Using the counting efficiencies determined with C.R.-1 plutonium standards, the results obtained by the two methods showed very good agreement (Table IV). ACKNOWLEDGMENT
The authors wish to acknowledge very helpful discussions with F. T. Porter, L. E. Glendenin, and R . K. Swank. LITERATURE CITED
(1) Fields, P. R., et al., Suclear Sci. and Eng. 1, 62-7 (1956). (2) Hay:;, F. N., “Liquid Solution Scintillators, Los Alanios Scientific Laboratory Rept. LA-1639 (October 1953). (3) Hayes, F. N.,et al.. Sucleonics 13, 38-41 (1955). (4) Hayes, F. N., Hiebert, R. D., Schuck, R. L., Science 116, 110 (1952). (5) Hayes, F. IT.,Ott, D G., Herr, V. N., Sucleonics 14, 42-5 (1056). (6) Phototube Section, Commercial Engi-
neering, Tube Department, Radiot.20r of America, Harrison, N. J., R& Tube Handbook, HB-3, Vol. 3-4.” RECEIVED for review April 28, 1958. Accepted July 28, 1958. Based on work performed under the auspices of the U. 8. Atomic Energy Commission.
Separation of Glycerol from a Polyhydric Alcohol Mixture by Nonionic Exclusion-Correction On page 1676 the name of the author should have been printed as Ira T. Clark.
