The Most Useful Actinide Isotope: Americium-241 James D. Navratil The University of New South Wales, Kensington, NSW 2033, Australia Wallace W. Schulz Westinghouse Hanford, Richland. WA 99352 Glenn T. Seaborg Lawrence Berkeley Laboratory, Berkeley, CA 94720 Americium, a manmade element, was first isolated in weighable amounts in the fall of 1945. Now a useful hyproduct'of the nuclear industry, i t is produced in kilogram amounts by appropriate recovery, separation, and purification processes. This paper briefly reviews its discovery, nuclear and chemical properties, and the uses of 241Amwith emphasis on its production and separations chemistry. Americium was the fourth transuranium element to he discovered. The i s ~ t o p e ~ ~ was ~Am identified by Seaborg, James, Morgan and Ghiorsol late in 1944 at the wartime Metallurgical Laboratory (now Argonne National Laboratory) of the University of Chicago as a result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor. Bv analogy with the naming of its rare earth homologue, e. ur-~ &ium. after Eurone. element 95 was named americium after the Americas. ~ & i u m and the group of elements in a row at the bottom of the oeriodic table are referred to as the actinides. Actinium, thorium, protactinium, and uranium are naturallv occurring actinide elements, whereas the remaining areSmanmade: Neptunium was the first synthetic transuranium element to be discovered, followed by plutonium, both in 1940. The discovery and identification of americium and curium depended upon the fact that they are memhers of this family whose chemiral traits correspond to those of the known, naturally orcurring lanthanide family. Am-241 was first offered for sale in March 1962 at $1300/g by the U S . Ammic Energy Commission.The fir31 allotment of 200 e. tendered through the USAEC Isotupe Pool at Oak Ridge Rational ~ a b o r G o r came ~, from the Rocky Flats Plant in Golden, Colorado, as a byproduct of plutonium processing. Rocky Flats as a sole source of Am-241 has been periodically supplemented in recent years by material from nuclear fuel reprocessing facilities at Hanford, Washington, Savannah River, South Carolina, and Los Alamos National Laboratory in New Mexico. Americium has isotopes with mass numbers 232 through 247. Only three isotopes have half-lives greater than a few hours: Am-243 (7400 years), Am-241 (432 years) and Am-242 (152 years). Am-241 decays to Np-237, emitting alpha and gamma radiatiom3 Am(OHh was the first americium compound to be isolated (Table 1). As with other actinide elements, americium forms a variety of oxides with varying Am to 0 ratios, of which AmOz is the most common. Various halides, carbonates, and sulfates also have been prepared. Compounds of Am2+have been prepared on a tracer level and appear analogous t o those of Eu(I1). In aqueous solution, Am3+ has a light salmon color, has the most stable oxidation state, and is difficult to oxidize. Am4+ exists in solid compounds and concentrated fluoride or phosphate aqueous solutions. Light tan AmOz+ is unstable with respect to disproportionation to Am3+ and AmO2Zf. AmOz3+exists only in alkaline solutions and has a deep green color.
. ~ ~ ~ ~ ~ .
~
~
Table 1. Some Compounds ol Americium Oxides: HBlides:
Am02. Am&., AmO,,s-ls AmF3. AmFc Arn02F2, AmCh AffiI~.AmOCI. AmBr.. AmBrr, Ami,. Arnl., Am01
cabonAmdC0.h. MAmOP03 Sulfates: Am.(SO&, MAm(S04)2
ales:
Americium metal, which has a whiter luster and is more silvery than plutonium or neptunium, can be prepared by reducing AmF3 with barium metal. Americium is more malleable than uranium or neptunium and tarnishes slowly in dry air a t room temperature. In the processing of americium, the oxide (Amon),fluoride (AmF3)and oxalate [Amz(C204)3]arethe most common com~ o u n d encountered.' s Figure 1 shows a flow sheet for the process used to separateplutonium from 241Am, resulting from the beta decay of 2"Pu. In the first stage of the process, molten plutonium is contacted with a once-used KCI-NaC1MgC12 salt mixture for 1h in a resistance-type furnace under an argon atmosphere a t 750 OC. Americium is oxidized by the MgC12 to AmC13, which goes into the salt phase. The snent salt is seoarated from the ~ u r i f i e dplutonium metal and processed dyaqueous methods to recover and purify the americium. The plutonium then goes through a second stage of the process where it is contacted with fresh salt under the same ooeratinn conditions as stage . one to produce pure pluton&n meid. Various processes, using ion exchange and precipitation techniques, have been employed to separate americium from plutonium and other elemental impurities in molten salt extraction (MSE) residues or spent salts (Fig. 2). The present process includes the following unit operations: (1) The residues are first dissolved in dilute hydrochloric acid.
(2) Cation exchange or carbonateprecipitation is used to convert from a chloride to a nitrate system and to remove gross
amounts of monovalent impurities. Americium and plutonium form carbonate precipitates and can thus be separated from the soluble impurities. Likewise, the two elements can also be Loaded on a cation exchange resin and eluted with 7 M nitric acid.
'
Schulz, W. W. The Chemistry of Americium; ERDA Technical Information Center: Oak Ridge, TN, 1976. 2Navratll. J. D.; Schuiz, W. W.. Eds. Transplutonium Eismenfs Prcduction and Recovery; American Chemical Society: Washington.
-nr:-, is81 .- - ..
Katz, J. J.; Seaborg, G. T.: Morss, L. R. Eds. The Chemlstryofthe Actinide Elements; Methuen: London, 1987. Edelstein. N. M.; Navratil, J. D.; Schulz, W. W.. Eds. Americium and Curium Chemistry and Technology; Reidel: Dordrecht, Holland. 1985.
Volume 67 Number 1 January 1990
15
Table 2.
Appllcatlons of 24"Am
Uses 01 "'Am Alpha Ionizing Radiation Oap density determination
Ionization detenor in gas hromatography Smoke detector LUminOuS oaint Oreoaration Purified P" Metal
stape I PUO,
stage 2
Vsriable
Fieldllndu~ Utilizing Gamma Transmission Pmperty 01 24'Am Micine IndusniaIGauging Mineralqly Soil Science Hydrolcgy Fieldllndushy Using "'Am as Alpha Neuhon Source PebOleum Soil Science Coke and Concrete Analvtical Chemistw
Flgwe 1. Flow shaet of mlten sab extraction prnceJs used to separate americium241tram olutonium.
(3) The 7 M nitric acid solution is then passed through an anion exchange column where only plutonium is loaded. (4) Americium in the 7 M nitric acid effluent is purified further by oxalate precipitation. (5) The americium oxalate precipitate is calcined at 600 'C to
yield AmOz. Americium is highly radioactive and a hot cell and remote controls are used to handle gram quantities. The window and box have a two-foot-thick water wall to shield operator
Figure 2. Flow sheet 01 pmcess wed to pwify americium
from the gamma radiation of americium. This special equipment is necessary not only to protect the worker but to nrevent ineestion of radioactive narticles into the lunes and the s p r e a i of radiation througiout the laboratory. l ~ u c h work on the chemical properties of americium has been carried out in this way because of the high specific activity of 2"Am. However, its radioactive properties are the basis for the element's broad usefulness, including household-type smoke detectors. Most smoke detectors contain a very small amount of radioactive material to ionize the air in the sensing chamber so that it can conduct electricity. The flow of electricity is
lS
Journal of Chemical Education
interrupted and the alarm circuit triggered when tiny smoke narticles enter the detector's chamber. Since more than half bf all American households have a t least one such detector, the majority of us unknowingly harbor in our homes a minuscule, but completely harmless, amount of radioactive material. However. vou mizht be interested to know that if you do have a smoke detector operating in your home, your chances of surviving a fire are doubled. Americium bas other important uses. Because of its essentially monoenergetic alpha (5.4 and 5.5 MeV) and gamma (0.6 MeV) radiations, 24'Am is particularly suited for use as an x-ray excitation source, including a multitude of industria l and scientific gauging, thickness, density, and radiographic measurements. The list of applications is the largest for anv actinide i s o t o ~ e (Table 2). For instance, in medicine, the gamma emission of "!Am is used t o determine the mineral content of bones, lipid content of soft tissue, and body composition. In industrial gauging, it is used to determine thickness of plate glass, metals, wire, etc. (For exam~le.the decrease in eamma &a~smission through a material is related to the increased material thickness). In soil science, Am-241 is used t o determine the moisture and density of soil, while in hydrology, it is used for logging of groundwater and sediment concentration, and to determine ore concentrations in mineralom. Neutron sources containing ~ m - 2 4to i furnish alpha particles for the reaction 9Be(a,v)'2C find uses in oil well logging, determining soil density and moisture content, measuring moisture content of coke and concrete, and in activation analysis of many materials as well as for testing of neutron counters. Since there is no substitute for the useful properties of americium, much of the high technology we take for granted today would not be possible without it.
