Actinide targets used in nuclear physics as well as some samples studied in solid state physics must, generally, meet very precise specifications which determine the quality of the results. This is particularly true for thin film deposits for which the usual characteristics are following: Elements: Th, Pa, U, Np, Pu, Am, Cm, Cf... Chemical state: metal, oxide or salt Deposit area: 1–10 cm 2 Thickness: 10–100 μ cm −2 Homogeneity: better than 10% Amount of material used: a few milligrams or less One has also to take into account the radioactivity of the actinides (especially the short half life ones) and the fact that some isotopes are scarce. These restrictions impose to set up special equipment in glove boxes for the deposition and the control. The examples of electrospraying and focused ion beam sputtering are presented. Although the electrodeposition in aqueous or organic media and the classical vacuum deposition are widely used, these techniques will not be discussed in this paper. Electrospraying. The principles of electrospraying were first stated by Zeleny in 1917 [1]. However the technique only became efficient after the work of Verdingh and Lauer at the CBNM (GEE1, Belgium) [2]. A solution containing the element to be deposited flows through a capillary. A positive potential with respect to the substrate is applied to a wire lodged into the capillary. At its exit, due to the electrical forces, the drop becomes unstable and explodes forming a spray which sprinkles the substrate. Usually the solvent evaporates before the microdroplets reach the backing. Moreover the electrical field accelerates the formed particles improving their adhesion on the foil. For examples, in the case of actinide acetates dissolved in methanol and with usual experimental conditions, the number of such droplets is about 4× 10 11 per cubic centimeter. A special mechanism allows the substrates to move in x and y directions in order to improve the film uniformity. The deposits can be heated at 450 °C to get the oxides. The whole system can easily be mounted in a glove box with exterior monitoring. We have realized in our laboratory targets or samples of the following isotopes: 230, 232Th, 231Pa, 237Np, 233, 235, 238U, 239, 243Am, 244Cm, 252Cf as acetates or oxides. Focused Ion Beam Sputtering One drawback of electrospraying is that only a limited number of chemical species can be deposited i.e. mainly acetaes and eventually oxides. Thus focused ion beam sputtering was developed in our laboratory for actinide target preparations as this technique allows the use of only milligrams of starting material. Moreover practically all the chemical forms of actinide compounds, including the metals, can be sputtered. The principles is the following: ions (generally from rare gas) produced by an ion source (duoplasmatron-type or cold cathode-type) are accelerated and then focused by mean of einzel lens on the source material. The sputtered atoms are then collected on the substrates suitably disposed around the beam-stop. The typical running parameters are the following: -ion current: 1–2 mA -extraction voltage: 10 kV -focus voltage: 1–2 kV Since actinide compounds and particularly the oxides have low sputtering yields, deposits in the 100- 100 μ cm −2 thickness range require deposition times ranging from several minutes to a few hours. The glow box adaptation is made by disconnecting the control unit from the deposition chamber and the pumping unit. The glove box is divided in two compartments: one containing the deposition chamber and the other the pumping group. When metallic samples are handled, the glove box is flushed with pure argon or nitrogen. Deposits of the following isotopes have been prepared: 230,232Th, 231Pa, 233, 2355, 238U, 237Np, 239, 241Pu, 241, 243Am, 244Cm as oxides and 232Th, 235U, 238U, 239Pu, 241Am as metals. Generally actinide thin film deposition starting from metals leads to oxide deposits. The realization of metallic layers requires care towards the glove box atmosphere purity, particularly in the case of Pu and trans Pu isotopes. The characterization of the oxidation state can be achieved using ESCA and the deposition rate is monitored either by integrating the ionic current or by a quartz oscillating monitor. Electrospraying and focused ion beam appear as complementary techniques for thin film depositions of actinide compounds and particularly the scarce isotopes when classical methods such as vacuum evaporation or electrodeposition are ineffective.
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Jan 1, 1988
R Aihara + 6
