Thomas K. Keenan
Americium and Curium
10s Alarnos Scientific Laboratory 10s Alamos, N e w Mexico
Americium aud curium are two completely man-made elements, having atomic numbers 95 and 96, respectively. Their existence was unknown 15 years ago and their existence was hardly imagined as little as 20 years ago. They were formed in the reactors which were built to produce plutonium during World War 11. Americium, the next element after plutonium, is formed by the series of nuclear reactions:
+
PuPa9 neutron
+ neutron
PuS4O
--
Pu2'0 Pu"~
produced by bombarding plutonium-239 with cyclotronaccelerated helium nuclei. After such a particle enters the plutonium nucleus, a neutron is "boiled" off so that one is left with the mass 242 isotope of curium. The second method of producing curium-242, involving the neutron bombardment of americium, is the more customary method employed today. Because its short half life prevents any significant buildup, prohably not over one ounce of curium-242 is in existence today. 1.
2.
+ .4mPu + neutron
PuaJQ a ( H e + + )
--
Cma"
+ neutron
.4mZ""'
P.4mP41"' A Cm"% 16.0 hours
As plutonium is created in a reactor, the most common isoto~eis that of mass 239. However. this isotove can also capture a neutron turning itself into plutonium240 which in turn captures a second neutron to form plutonium-241. While all of these plutonium isotopes are radioactive and disintegrate themselves, it so happens that plutonium-241 (which disintegrates with a 13.2 year half life) is the only one that disappears a t any noticeable rate. Both plutonium-239 and plutonium-240 have half lives of thousands of years so that they will not disappear within any reasonable time. Plutonium-241 emits a beta particle to transform itself into americium-241. Americium of mass 241, which is formed as a result of all of these processes, has a half life of 458 years (I$). As a result of such transformations, americium was first detected in 1944 by Seahorg, James, and Morgan (5). As with most other synthetic elements, the first discovery was made by using tracer techniques where perhaps only a few billion of the atoms concerned were available for study. It was almost a year following the discovery that a weighable quantity, a few micrograms, mas isolated by B. B. Cuuningham (4). The availability of americium has increased from this rather modest beginning to quantities of the order of grams. In fact, some papers recently appeared which described the isolation of 4.5 g of americium (6, 6). However, it could hardly be said that there are plentiful amounts of americium available. Probably less than half a pound exists today in the whole world, both inside and outside the Iron Curtain. Curium, element number 96, was discovered also in 1944 hv Seahore. James. and Ghiorso (7). It was f i s t Presented before the Division of Chemical Education at the 133rd Meeting of the American Chemical Society, San Francisco, April, 1958. This work was sponsored by the U. 6 . Atomic Energy Commission.
a
CmP41 PuZS8 162.7 days
Therefore, because large quantities of either element have not been available in any one laboratory, americium and curium chemistry has generally been carried out on the micro scale. It is quite common to do fairly elaborate experiments on a few hundred micrograms of americium or curium. Whole new techniques of rnicrochemistry were developed for experimentation nith such elements. Since it is possible, electronically, to detect aud record a single alpha disintegration event, the problem of analysis in this type of chemistry is greatly simplified. One can easily determine amounts of americium and curium of the order of 10-'O g by counting techniques. The Chemistry of Americium
Americium is known in the formal valence states of (0), (111), (IV), (V), and (VI), but only americium(III), (V), and (VI) have been isolated in solution. Americium metal has been prepared as shown in Table 1 by the reduction of americium trifluoride with barium metal vapor a t 1100-1200' in vacuum (8,9). Some thermal data are also shown such as the vapor pressure of the metal a t 1000" (10). The heat of reaction of americium metal in hydrochloric acid a t infinite dilution is -163.2 kcal/mole (9). Using this value, with some estimates for the entropy changes for such a reaction, the formal potential of the metal versus the trivalent ion is estimated as +2.32 volts (9). The metal reacts with excess oxygen to form Am02while with a minimum of oxygen, Am0 is formed (11). Americium also reacts with a minimum of hydrogen to form AmH2 (11). These latter compounds, Am0 and AmH2, have been shown to contain metallic and not ionic americium and, therefore, should not he construed as evidence for diVolume 36, Number I , January 1959
/
27
Table 1.
AmF.
Reactions of Americium M e t a l
+ Barium--------+
Arn(c) 1100-1200°C Am vapor pressure 1000°C, 1 . 7 X 10-a mm Hg Ami.,, +a; AH = -163.2 keal mol-I Amro = Am(,,,+* + 33e-, E," = +2.32 volts
0, (minimum)
Am
Am0 (presence of Am metal)
\
valency in americium (18). When the fact became established that the actinide elements were counter parts of the lanthanide elements, one of the apparent anomalies x a s the failure of americium to form a divalent state. Americium is the homologue of europium which does have a readily attained divalent state, and some of the early tracer work on americium did give certain indications of divalency (13). However, later work with macro amounts gave only negative results. Table 2 shows the results of such experiments in more detail. Under conditions where the Table 2.
Reactions of Analogous Elements Not Observed in American Chemistrv to Yield AmOl)
SmCIJ + H,
600-8W" SmCL; AmCll
A
Sm
52 P
3955
I
z -= Curium ($44only)
HOURS
I I I
I
0
I
1 1
CmFs
Fr, 400"
\
CmF,
1 atmos.
though curium is the homologue of gadolinium and has the 5f' or half-filled shell structure, it had often been considered that a higher valence state than (111) might be formed in aqueous solution. However, to date, there has been no good evidence for the production of any aqueous valence state of curium other than the trivalent.
I
2000 Figure 4.
2500
I
I
I
I
I
MOO 3500 4000 4500 5000 WAVELENGTH IN ANGSTROMS
I
5500
8
Absorption spectrum of aqueous Cm(lll1.
used curium-242 entirely, only gradually increasing absorption in the region
