Preparation and Use of lead-21 2 Tracer Claude W. Sill and Conrad P. Willis, Health and Safety Division, U.
AN INVESTIGATION on the separation of lead-210 from ores and mill products by dithizone, a convenient radioactive tracer was sought to facilitate the work. The radioactive isotopes of lead most suitable for general use as tracers are those of masses 203, 210, and 212. All other isotopes have half lives too short for other than special purposes. Lead-203 is produced by nuclear bombardment reactions and is quite expensive and not readily available. Its half life of 52 hours would not permit shipment to locations very far from its point of preparation economically, and the tracer would require frequent replacement to permit continuous experimentation over any significant period of time. Lead-210 is very difficult to measure directly because of the low energy of both the beta particles and gamma radiation resulting from its decay. Its bismuth-210 daughter has an energetic beta emission but its half life of 5 days would require an 18-hour delay in counting to obtain even 10% of the lead-210 activity taken. Particularly, bismuth-210 is a pure beta emitter and has to be obtained essentially free of absorbing solids and liquids before counting can be accomplished. I n contrast, lead-212 has a 0.239-Mev. gamma ray of 45% abundance which permits direct counting of aqueous solutions in a scintillation well counter. I t s half life of 10.6 hours is long enough to present no problems due to decay and short enough that the tracer will be regenerated to about 75% of its maximum equilibrium value on standing overnight. Since this nuclide is present in appreciable activities in natural thorium compounds, it is readily available a t very low cost. Both the lead212 and its bismuth daughter can be removed quantitatively from dilute acid solution by the well-known dithiocarbamate extraction ( I ) . Thus, a continuing and virtually inexhaustible supply of inexpensive gamma-emitting tracer is easily available at all times. The disadvantage of having to prepare the tracer for each days work is essentially nullified by the simplicity and speed of the procedure described in the present work, only about 45 minutes being required after the initial preparation. Other methods of preparing lead212 tracer have been summarized in a recent excellent review of the radiochemistry of lead (1) but are generally not as rapid, efficient, or easily adaptable to large-scale preparation as
URING
1 176
ANALYTICAL CHEMISTRY
S. Atomic
Energy Commission, Idaho Falls, Idaho
the present procedure. The only significant disadvantage associated with use of lead-212 as a tracer is the necessity of waiting for 8 hours to obtain more than 99.597, ingrowth of the 60.6minute bismuth-212 and 3.1-minute thallium-208 daughters in the lead fraction and decay from the bismuth fraction before results can be obtained by gross gamma counting when tracing reactions in which lead and bismuth are separated from each other. EXPERIMENTAL
Preparation of Tracer. Place 200 grams of reagent grade Th(NO&.4 H z 0 and 110 ml. of 72y0 perchloric acid into a 600-ml. beaker. Heat the mixture on a high-temperature hot plate until all solid salts have dissolved and the evolution of nitric acid changes to heavy white fumes of perchloric acid. Continue the evaporation of excess perchloric acid until the froth at the surface of the solution tends to crust over while still on the hot plate. Cool the solution and add 20 ml. of 72y0 perchloric acid and 400 ml. of water. Boil the solution gently for a few minutes to dissolve the cake and to volatilize decomposition products of perchloric acid and cool. The solution will be about 0.7Jf in perchloric acid and 450 ml. in volume. Transfer the thorium perchlorate solution into a 500-ml. separatory funnel and shake vigorously for 1 minute with 20 ml. of a 1% solution of diethylammonium diethyldithiocarbamate (DDTC) in chloroform. Draw off the extract into a 250-ml. separatory funnel containing 100 ml. of water and 10 ml. of 72% perchloric acid. Repeat the extraction with another IO-ml. aliquot of the 1% D D T C solution and add the extract to the separatory funnel containing the first one. Shake the 250-ml. funnel vigorously for 1 minute to wash the combined extracts and draw off the organic layer into a 250-ml. Erlenmeyer flask. Add 2 ml. of 5Oy0 sodium hydrogen sulfate (NaHS04) and 2 ml. of concentrated nitric acid to the flask containing the combined extracts and heat carefully on the hot plate until the chloroform has evaporated. Place the flask on a high-temperature hot plate covered with a thin piece of asbestos cloth and heat strongly until nitric acid has been expelled and a black cake of charred organic matter is obtained. As soon as the charred mass is completely dry and immobile and no further charring is evident, remove the flask from the hot plate to prevent small losses of lead by volatilization. Cool the flask and add 3 ml. of a 1 to 1 mixture of concentrated nitric and 72% perchloric acids and boil until the
black suspension becomes colorless and excess perchloric acid has evaporated. Do not attempt to evaporate the D D T C extract to dryness with perchloric acid before the thermal charring or the organic matter will ignite in a sudden burst of white-hot fire and over 75% of the tracer will be volatilized. Add 10 ml. of water and 1 ml. of concentrated hydrochloric acid and boil the solution for 1 minute, cool and dilute to 10 ml. Each 1-ml. aliquot will contain approximately 8.5 X 105 c.p.m. under the conditions used. Extract the thorium perchlorate solution in the 500-ml. separatory funnel with 20 ml. of straight chloroform for I minute to clean up traces of diethyldithiocarbamate and decompssition products. Discard the chloroform extract and store the aqueous solution until more tracer is needed. I t is advisable to discard the first batch of tracer obtained from a new sample of natural thorium to remove all extractable impurities including natural lead that might be present. After standing overnight, the freshly-ingrown tracer will be carrier free. Counting Procedure. The lead-212 is determined by gross gamma counting in a 3-inch thallium-activated sodium iodide well crystal in aqueous solutions of up to 75-ml. volume contained in 100-ml. polystyrene bottles. Each sample should be compared to a standard within a few minutes under the same counting conditions to ensure constant counting efficiency and to eIiminate decay corrections. The standard should contain approximately the same salt content as those samples containing a significant fraction of the total activity. Particular care should be taken to adjust the level of each sample solution by dropwise addition of water until the height is within about 0.1 mm. of that of the standard for maximum precision, especially on high-counting samples. This height adjustment should be made immediately before counting to eliminate any effect of differences in temperature between sample and standard on their volumes. Since lead-212 decays initially a t a rate of 0.1% per minute, sufficient activity should be used so that the desired statistical precision can be obtained on 1-minute counts to maximize the number of samples that can be counted before the standard must be recounted. If all low-counting samples are counted first and the standard is then counted midway through the remaining group of high-counting samples, more samples can be counted with less error due to decay. At least 2 x 105 c.11.m. should he present at time of counting to keep the statistical error on 1-minute counts less than 0.5% at the %yoconfidence level but more than
TO
1.0
0.5
1.5
2.0
ENERGY, Mev Figure 1.
Separation of lead-2 12 from thorium nitrate Curve 1. Curve 2.
about 5 X 105 c.p.m. should not be present because of increasing coincidence losses. Since the samples will generally have to stand overnight to allow reestablishment of equilibrium with the bismuth-21 2 and thallium-208 daughters, approximately four times as much activity should be taken initially for both standards and samples as is desired to have present at time of counting to compensate for the decay occurring on standing. DlSCUSSiON
Two hundred grams of 13-year-old thorium nitrate gives approximately 8.5 x io6c.1i.m. of lead-212 tracer under the conditions described, 75YGof which will be regenerated in 21 hours. Even starting with as much as 106 c.1i.m. per run, a t least 5 experiments and 1 standard can be made each day and maintained indefinitely on this quantity
Aqueous solution after extraction Diethyldithiocarbamate extract
of thorium. Many more samples can be maintained if slightly lower precision is acceptable or if the samples can be counted after 8 hours instead of having to wait until the following morning. Larger quantities of activity can be obtained by direct scaleup of the conditions used in the present work. Even freshly-manufactured natural thorium compounds will give virtually maximum activity due to the retention of the 1.9-year thorium-228 during manufacture a t the equilibrium concentration present in the original ore. The efficiency of the present separation is demonstrated graphically by the gamma ray spectra shown in Figure 1. The upper curve was obtained from the aqueous thorium solution immediately after extraction and consists entirely of actinium-228 with no trace of lead-212, bismuth-212 or thallium-208 being
detectable. The lower curve is a spectrum of the DDTC extract and shows the same equilibrium distribution of all three nuclides that existed in the original sample of thorium nitrate. Other tests using tracer under similar conditions showed that recovery of both lead-212 and bismuth-212 was greater than 99%. Thallium-208 is not removed quantitatively under the conditions employed b u t grows back to equilibrium very quickly because of its short half life. KOtrace of actinium-228 could be detected in the extract. Furthermore, the activity of the entire extract was followed by gross gamma counting through several half lives of lead-212 without finding any evidence of contamination from thorium or radium precursors which would prevent complete decay of the tracer with the half life of lead-212. VOL. 37, NO. 9, AUGUST 1965
1177
Table 1.
Distribution of Lead-212 Tracer
Recoverv. “ , 7% ,” Run Run Run A B C
Fraction 1 2 3
1.0 0.7
5
0.0 0.0
0.3
4
97.8 0.1 0.3
6
7
8 9 10
0.1 0.3 0.0 1.0 1.1 0 . 2 0.0 0.0 0 . 0 97.8 0.0 0.4 0.0 0.0 0.2 0.0 1.5 0.1 0.2 0.0
92.8 4.1 0.1
11
12 13
Material balance
100.2 100.1 99.8
Some of the data obtained in connection with other work are shown in Table I to demonstrate the effectiveness of lead-212 as a tracer. Dithizone was being used to separate lead and bismuth both from the samples and from each other. Every fraction obtained through the entire analytical procedure was counted separately so that the distribution of lead could be studied. About 1.5 X lo6 c.p.m. of tracer was taken initially for both the experiment and standard. All solutions were counted the following day at which time the standard counted approximately 4 X 105 c.p.m. The data shown are not specially selected but represent the first three experiments performed using the lead-212 tracer. The material balances obtained show the excellent precision with which the tracer could be accounted for even over the large
number of fractions obtained. hll subsequent work has further confirmed the effectiveness and convenience of this tracer. I3y using gamma spectrometry, information on the distribution of bismuth can be obtained simultaneously. Use of a gamma-emitting tracer has proved particularly useful in demonstrating the losses of lead produced by volatilization, ion exchange to glass surfaces and silica, and reduction and consequent alloying during fusions in platinum dishes. The relatively short-lived lead-212 can also be used to determine the yield in the radiochemical determination of lead-210. LITERATURE CITED
(1) Gibson, W. hI., “The Radiochemistry of Lead,” National Academy of Sciences, Xuclear Science Series, NAS-SS 3040 (1961).
Zone Refiner with Temperature Control Fred Ordway,’ National Bureau of Standards, Washington 25,
the heat is usually by radiation (1, 3). The temperature of the sample is not directly controlled. For many samples, however, elevated temperature endangers purity because of rearrangement, decomposition, or reaction with atmosphere. Therefore, it is often desirable to control the maximum temperature to which any part of the sample is subjected, or at least to know it accurately. When the zone refining process is in smooth operation it may be possible to estimate the temperatures to which the sample is being exposed. When the process is starting up, on the other hand, the temperature in any given region of the sample may be highly unpredictable. If the container is initially filled with granular material, or even melted material with voids, the isolated portions of the sample may often reach significantly higher temperatures than does any portion during the ideal steady-state process. After all voids have been eliminated the subsequent passes may or may not remove impurities resulting from the overheating. With some samples the complete elimination of voids may be difficult or impossible. The fundamental problem is that the temperature is determined by the accidental value of thermal resistance, together with a fixed hcat flow from a source of very high temperature. T o
I applied ’
N ZONE REFININQ,
Present address, Melpar, Inc. Falls Church, Ya. 1178
ANALYTICAL CHEMISTRY
D. C.
control the temperature properly the heat should flow to the sample, from a source whose controlled temperature is close to t h a t of the sample, through a comparatively low thermal resistance. The maximum sample temperature is then limited to the source temperature required for the steady state of the process. This objective has been attained by arranging to circulate a fluid of controlled temperature over a narrow band around the sample tube. The constant-temperature band is made capable of motion along the tube by the use of shaft seals. DESCRIPTION OF APPARATUS
The construction of the fluid carrier is shown in Figure 1. The assembly consists of alternating partitions, P , (bearing shaft seals that closely fit the sample tube) and collars, C, (each with two tubes for fluid circulation) , with a window section, W7, for each chamber. These parts are separated by gaskets, G . The assembly is aligned by the internal shoulders on the collars and by three symmetrically disposed bolts, B, and is clamped together by nuts, N , bearing on two end plates, E . Outside diameter of the end plates is 2.5 inches (63 mm.). ’4 series of three annular compartments is formed by four of the partitions. Each of the compartments has two connecting tubes for fluid circulation and a glass wall for viewing the sample inside. The shaft seals were automotive-type oil seals (Garlock “Klozure” or equivalent), but O-ring seals or other types
might possibly have been used. Interchangeability of units designed for different-sized shafts is desirable. A suitable tolerance to the sample tube’s deviations from perfect centering and roundness is necessary. The fluid carrier was made by assembling a set of parts, all clamped together with gaskets, to permit variations in the number and lengths of the compartments. Any future units having just three compartments can be constructed of fewer pieces and require no more than three gaskets. The compartments may be as short as 1-2 cm. each. The present unit was slipped over the sample tube while loosely assembled, and then given the final tightening, to ensure correct alignment of the seals. The fluid carrier was attached to the translation mechanism of a Fisher zone refiner. The upper end of the sample tube, protected by a short length of elastomer tubing, %as clamped in a three-jaw drill chuck of 0.5-inch capacity, which projected downward from the 10: 1 worm gear reducer of a 1/15-h.p. variable-speed motor mounted on top of the zone refiner cabinet. The sample t’ube was in contact only with the driving chuck and the elastic seals in the fluid carrier. Temperature-controlled heating fluid was circulated through the center compartment and coolant through the top and bottom compartments. The fluids were supplied, through silicone tubing, from two small baths by immersiontype centrifugal circulating pumps. The fluid pressure was slight, flow rates being less than 0.1 liter/minute. The hot bath was filled with DowCorning 510 silicone fluid. The cooling
