Tracer americium

Many systems using either HDEHP or Aliquat 336 as the stationary phase have been studied for extraction chromatographic separation of americium tracer. An Aliquat 336 (nitrate-form)-kieselguhr system has been used recently both in the USA and in Europe to separate milligram to gram amounts of americium from curium [70, 71]. 

Of the radioassays that are commonly used to quantify americium, a-spectroscopy is used when isotopic analyses of americium must be conducted (e.g., 241Am and 243Am). 243Am is often added as a tracer to estimate the efficiency of the sample preparation method when quantifying 241Am in biological matrices. 

Adsorption experiments were performed by removing rock-equilibrated water from the fissures and injecting stock solution which was made by dissolving tracer amounts of americium-241 in rock equilibrated water. The stock solution was allowed to equilibrate within the fissures for different periods of time and was then removed from each fissure. The stock solution was assayed before injection and after removal from the fissures so that the change in americium concentration was determined for a different time in each fissure. Ten adsorption experiments were performed in this manner and the results are presented in table I. Figure 3 is a graphical representation of the initial part of the adsorption curve. 

Figure 3 illustrates the distributions found for Pu and Am when a mixed sample of these tracers was infiltrated into a large (30 cm x 30 cm) block of Bandolier tuff (6). The nuclide activities were determined simultaneously by coring sections of the tuff and represent the activity distributions in the rock. It is obvious that although the activities are both normalized at 100% at the surface the increased dispersion of the plutonium concentration during elution leads to an increase of almost an order of magnitude in its activity relative to Am at the 5-6 cm depth. It is highly unlikely that abnormal flow paths or movement of colloidal clay particles would discriminate between americium and plutonium therefore this experimental result tends to discount these possible types of mechanisms. However, a pure Pu polymer could carry the Pu more rapidly downstream. 

There is tracer scale evidence455 for the formation of the Cf02 ion during ozonization of 249Bk and subsequent jS decay to 249Cf. However, the only compounds isolated in this oxidation state are all americium(V) species. 

It can be seen that the (111) state is highly stable with respect to disproportionation in aqueous solution and is extremely difficult to oxidize or reduce. There is evidence for the existence of the (II) state since tracer amounts of amencium have been reduced by sodium amalgam and precipitated with barium chloride or europium sulfate as earner. The (IV) state is very unstable in solution the potential for americium(III)-ameridum(IV) was determined by thermal measurements involving solid Am02. Amencium can be oxidized to the (V) or (VI) state with strong oxidizing agents, and the potential for the americium(V)-americium(Vl) couple was determined potentiometrically. 

The isotope 242Cm was first isolated among the products of a-bombardment of 239Pu, and its discovery actually preceded that of americium. Isotopes of other elements were first identified in products from the first hydrogen bomb explosion (1952) or in cyclotron bombardments. Although Cm, Bk, and Cf have been obtained in macro amounts (Table 20-2), much of the chemical information has been obtained on the tracer scale. For the later elements, i.e., those with Z > 100, identification of a few atoms of short lifetime has required the use of very rapid separation techniques and detection based on their nuclear properties. 

Plutonium-242 and Americium-243 Pu-242 and Am-243 are produced in fuel by multiple nuclear reactions. They therefore appear in items contaminated by fuel. They were found to exceed the GQ limit in HNA and HPA MCI and HPA SPF waste (high uncertainty). In addition in IX resins at HPA and HNA, Am-243 was above the GQ. Neither of these radionuclides are currently analysed in Magnox wastes because they are used as yield tracers in other analyses. To measure these two radionuclides, it is possible to simply repeat the current analyses for Pu and Am with and without tracers. No development work should be required. It has been possible to use these isotopes as tracers because the amount present (in terms of activity) is very low. FISPIN predicts the following radioisotope ratios in fresh waste Am-241 to Am-234 of 111 to 1 and Pu-239/-240 to Pu-242 of 2,500 to 1. At these activity levels, it may be more accurate to estimate the activities rather than measure them. 

First, the trivalent actinide and lanthanide elements are separated from the other elements in the waste. In the second step, americium and curium are then separated from the lanthanide elements. Experimental studies have largely been laboratory-scale in which synthetic waste solutions and tracer levels of radioactivity were utilized. A few laboratory-scale experiments were made in hot cells on the coextraction of trivalent actinides and lanthanides. The two most promising methods investigated for co-removal of trivalent actinides and lanthanides are ... 

The main radionuclides that are measured in this system are isotopes of thorium, uranium, and plutonium. Others are the longer-lived isotopes of heavy elements such as radium, protactinium, neptunium, americium, and curium. Conventionally, an isotopic tracer of known activity that emits alpha particles is added at the beginning of the radiochemical procedure (see Section 6.3.2). The solution from which the sample for counting will be deposited is thoroughly purified by the radiochemical procedure to remove interfering radionuclides and solids. 

As mentioned above, a very important point is the presence of seven 5f electrons in stable tripositive curium (element 96), making this element very actinium-like. A series of thoride elements, e.g., would imply stable IV oxidation states in elements 95 and 96 and the presence of seven 5f electrons and the IV state almost exclusively in element 97. A series of this type seems to be ruled out by the now-known instability of americium in solution in the IV state and by the apparent non-existence in aqueous solution of any oxidation state other than III in curium. Moreover, the III state of uranium would be surprising on this basis, because this element would be the second member of a thoride, or IV oxidation state , series. The fact that nearly a year was spent in an unsuccessful effort to separate tracer amounts of americium and curium from the rare earths, immediately following the discovery of these two elements, illustrates how unnatural it would be to regard them as members of a thoride, or IV oxidation state, group. 

The silver(i)-catalysed oxidation of americium(iii) by SgOg " has been studied using Am tracer techniques, coprecipitation with lanthanum trifluoride representing a good method of monitoring the micromolar... 

Some work has also been reported on a californium dipivaloylmethanato complex, where the volatility of the complex was compared to complexes of other lanthanides and actinides [151]. These tracer studies suggested that the californium complex deposited at lower temperatures than the americium or plutonium complexes. 

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