Gamma Absorptiometer for Solutions of Heavy Metal Salts DONALD H. THURNAU' Savannoh River Laboraiov, E. I . du Pont de Nemours & Co., Inc., Aiken, S.
C.
t A gamma absorptiometer was developed for the analysis of solutions of heavy metal salts. A scintillation detector wos used to measure the transmittance of the solutions for gamma rays from americium-241. Apparent standard deviations of 0.05y0or better were obtained at concentrations in the range from 50 to 100 grams per liter.
centrations of other solutes is small. Corrections, when needed, can be made accurately. For example, a difference of 1 unit in the molar concentration of nitric acid is approximately equivalent in absorptance to a difference of 0.9 gram of uranium per liter.
I
Americium-241, vhich emits 60-k.e.v. gamma rays with a 470-year half life, was chosen to supply the y-rays. This isotope has been demonstrated to be nearly ideal for this purpose (4,6). The scintillation detector was chosen chiefly for its high sensitivity, so that a source of minimum intensity could be used. As a result, the radiation hazard in this instrument is negligible. The exposure rate for operating personnel is approximately 1 mr. per day. A Brown recorder mas chosen as the readout device because it is a selfbalancing potentiometer that can be calibrated directly in terms of solution concentration. A simple manually balanced circuit of the potentiometer type would provide an acceptable readout in a laboratory instrument. However, the self-balancing, direct-reading recorder was more suited to an instrument for use by nontechnical personnel. Cylindrical sample vials were used as absorption cells to avoid a change in an existing sampling procedure. One disadvantage in this choice was that the
utilizing the absorption of x-rays or y-rays by solutions of heavy metals have been found satisfactory for measuring the concentrations of such solutions (1, 5-5, 7, 8, 10, 11). Recent papers on the use of y-ray sources indicate that, in certain applications, analyticdl instruments based on the absorptiometric principle could he constructed compactly and economically (4,6, 7). The present work mas undertaken t o provide a simple, rugged instrument for the analysis of samples by nontechnical operators. NSTRUMENTS
PRINCIPLE
OF OPERATION
The principle of the gamma ahsorptiometer is analogous to that of the spectrophotometer. I n both instruments a beam of nominally monoenergetic photons traverses a given distance mithin a liquid sample. The relative intensity of the transmitted beam is measured by an appropriate radiation detector. The transmittance of an unknown sample relative to that of a standard is converted to concentration of solute by means of a calibration curve or formula. The error analyses commonly applied to colorimetry (9, 9) are valid also for the gamma absorptiometer, as the transmittance is governed by the analog of Beer's law. These error analyses are subject to the qualification that the statistical fluctuations of the indicated intensity are negligible. The fundamentals of gamma ahsorptiometry have been adequately discussed ( I , 3, 7). For the y-rays uscd here, the absorptance of the solutions of heavy metal compounds in concentrations above 10 grams per liter is high enough that the effect of varying con-
' Present address, Ohio Oil Co.,
ton, Colo.
1772
ANALYTICAL CHEMISTRY
Little-
DESCRIPTION
DESIGN CONSIDERATIONS
Figure 1.
vials did not prrscnt the ideal absorbing path length to the entire cross section of the beam. Either the vials had to be sorted from the ordinary supply by means of a pair of ring gages, or intolerable errors resulted from variations in their diameters.
The instrument consists of two separate enclosures connected by a pair of electrical cables. One enclosure contains the electrical panels associated !i-ith the detector, including the recorder, the high voltage power supply, and the control circuit. The detector, the sample holder, and the source of yrays are contained in a stainless steel enclosure, an inside view of which is seen in Fienre 1. A cross-sectional sketch of the source asscmbly is shown in Figure 2. About 10 mg. of americium, as the trifluoride, is enclosed in a cylindrical aluminum capsule which is sealed by a poured plug of an alloy xith a low melting point and a necative coefficient of thermal pxnxnsion.- The alloy is 52% ~bismuth,~~ZOY, lead, and 8% cadmium, by weight. The y-rays pass from the capsule through a 0.016inch aluminum window vhich transmits 98% of the incident beam 11-hile providing a barrier to the spread of contamination. The aluminum capsule is mounted in a lead shield which helps to define the beam and minimize stray radiation.
Gamma absorptiometer
Cover removed, electrical cabinet not shown
The sample holder positions the -ample containers to minimize errors arising from irregularities in their bhapt3. A spring-loaded mechanism grasps thr container aq it is pushed into the holticr. The oilentation of the points of contact betncen the sample holdrr and the container is such that variations in the out-de diameter of
Am241-PLASTIC MIXTURE
m A STAINLESS STEEL LOW MELTING POINT ALLOY, 5 2 2 81, 40% w, 8X Cd ALUMINUM LEAD
Figure 2. Cross source assembly
?IN
9
section
,
of
y-ray
the sample container cause minimum displacements perpendicular to the beam. Such displacement of the containers would cause large errors because the effective absorbing path length in the sample solution is strongly dependent on the transverse position of the circular c r o s section of the vial in the beam. The sample via1 has an outside dianieter of 2 1 / q ? inch. an inside diameter of 9,'16 inch, and an over-all height of 2 inches. The plastic container is 13/18 inches in diameter and 3114 inches in height. The detector is a thallium-activated sodium iodide crystal attached to an RCA 6655 multiplier phototube. The crystal is a Harshaw Type 6H S39, 1' '? inches in diameter and 2 'mm. thick. Optical coupling is obtained with DOTVCorning silicone fluid. Black elect,rical tape holds the crystal to the phototube aiid furnishes a light-tight' enclosure. A niu metal tube, concentric with the phototube, furnishes niagnetic shielding for thp multiplier dynodes. This assembly is shown in Figure 1. The voltage divider circuit which furnishes the proper voltages to t,he dynodes is shon-n in Figure 3. The components of this circuit are contained ivithin the phototube socket. A Nodel 321 high voltage pupply (Atomic Instrument Co.) is uscd to pon-er the voltage divider. This supply is operated to give negative high voltage. The control circuit has two functions: to integrate the pulses from t,he multiplier; and t o supply to the recorder a voltage which is proportional to the integrated anode currcnt, the constant
11
of proportionality being continuously variable. The first function is attained by means of 50 pfd. of capacitance between the anode and ground. The second function is fulfilled by a 10,000-ohm Helipot through which the anode curs ground. rent f l o ~ to The recording poteiitiometer had to he modifid for use in this instrument. It is a Brolin strip chart recorder equipped n i t h a 1 2 X amplifier. The large capacitance that is normally across the input to the recorder had to be reniored because of the high impedance of the control circuit. The balance circuit was changed (Figure 4) to provide five different ranges in the input to the recorder: 0 to 2.5, 3 to 5.5>6 to 8.5. 9 to 11.5, and 15 t o 17.5 niv. The zero suppression afforded by this modification permits expansion of the useful pait of the transmittance scale to minimize reading errors. OPERATING TECHNIQUE
Several techniques may be utilized in operating the gamma absorptiometer ( 2 , 9). The one chosen is simple, accurate, and readily adapted to the direct-reading system.
d standard absorber whose effective concentration is near that of the trst solution is used as a reference. With the recorder range switch set appropriately, the standard absorber is placed in the >ample holder, and the Helipot is set to cause the recorder pen to register a t a coiivenient reference point such as midscale. The unkiioivn sample is substituted for the standard absorber in the sample holder, causing the recorder to balance a t a point on the scale corresponding to the unknown solution concentration. The recorder scale can be graduated directly in terms of concentration, or an arbitrary scale can be used in conjunc-
T 50pfd.
\
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tJEGATIVE H l G h VOLTAGE %P?LY
1
ABCUT -1OOCV
Figure 3.
Electrical circuit associated with detector
Dashed line represents periphery of phototube socket
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R;. 509.5n R2, R J ,Rj. Standard 2.5-mv. range resistors VOL. 29,
NO. 12, DECEMBER 1957
e
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tion with a calibration curve to yield the concentration of the sample. A typical calibration curve for uranyl nitrate solutions in 0.2N nitric acid is given in Figure 5 . The standard absorber was a n ordinary sample container filled with lead acetate solution and sealed with plastic cement to prevent evaporation. The concentration of the lead acetate solution was adjusted to be equivalent in absorptance to about 82.5 grams of uranium per liter. The uranyl nitrate solutions used in these calibrations were obtained by dilution from a stock solution analyzed by standard chemical methods. PRECISION
The ultimate precision of the instrument was evaluated using only one sample vial. An average deviation of 0.1% (apparent standard deviation, 0.05%) at 60 grams of uranium per liter was obtained on the 6- to 8.5-mv. recorder range. This precision was essentially the same for concentrations up to about 200 grams per liter. The error was doubled a t about 30 grams per liter, and doubled again a t about 15 grams per liter. The effect of variations in the diameter of the sample vials on the precision of analyses was also evaluated. One hundred selected vials were filled with a solution corresponding to 60 grams of uranium per liter. An average deviation of 0.6% was found in analyzing these samples with the instrument. The vials had been selected from the common supply by means of a pair of ring gages differing in diameter by 0.003 inch. Approximately 15y0 of the regular stock of vials met this specification. Sorting the vials in this way improved precision by a factor of 3 over that obtained with unsorted vials. ACKNOWLEDGMENT
The information contained in this
Figure 5.
Uranium nitrate calibration curve
paper was developed during the course of work under contract AT(07-2)-1 with the U. S. Atomic Energy Commission, whose permission to publish is gratefully acknowledged. Appreciation is also expressed to B. B. Murray for taking preliminary data which led to this development and to J. M. -1IcKibben for his cooperation in the field evaluation of this instrument. LITERATURE CITED
(1) Bartlett, T. W.,ASAL. CHEX. 23, 705-7 (1951). (2) Hiskey, C. F., Ibid., 21, 1440-6 (1949). (3) Lambert, M. C., U. S. Atomic Energy Comm. Rept. HW-24717 (June 20, 1952).
(4) Leboeuf, h.1. B., Miller, D. G., Connally, R. E., ll’ucleonics 12, No. 8, 18-21 (1964). ( 5 ) Liebhafsky, H. 4., AXAL.CHEW26, 26-31 (1954). (6) Miller, D. G., U. S. Atomic Energy Comm. Rept. HW-39971 (Nov. 17, 1955). (7) Miller, D. G., Connally, R. E., Zbid,, HW-36788 (June 1, 1955) (claw-
fied). Peed, IT. F., Dunn, H. W.,Ibzd., ORNL-1265 (April 8, 1952). Reilley, C. N., Crawford, C. RI., ANAL.CHEM.27, 716-25 (1955). Vollmar, R. C., Petterson, E. C., Petruzzelli, P. A.. ANAL.CHEX.21,
1491-4 (1949). ’ (11) Wright, W. B., Barringer, R. E.,
U. S. Atomic Energy Comm. Rept. Y-1095 (Aug. 26, 1955). RECEIVEDfor review January 17, 1957. Accepted ilugust 6, 1967.
Radioassay of Low Specific Activity Tritiated Water by Improved Liquid Scintillation Techniques C. A. ZIEGLER, D. J. CHLECK, and J. BRINKERHOFF Tracerlab, Inc., Waltham, Mass.
b The performance of liquid scintillators in the radioassay of tritiated water of low specific activity is improved by making the water samplescintillating solution ratio optimum, and deoxygenating the scintillating soh1774
ANALYTICAL CHEMISTRY
tion by an ultrasonic irradiation technique.
T
liquid scintillation technique has achieved considerable popularity as a radioassay method for tritiated HE
water because it combines ease of sample preparation with adequate counting efficiency. However, for samples of low specific activity-i.e., producing a count approaching that of backgroundcounting times become long and the
