Speaker
Description
Recently we have determined the first ionization potential (IP$_1$) of lawrencium (Lr, $Z = 103$), which is the last member of the actinide elements, using an ISOL (Isotope Separator On-Line) equipped with a surface ion-source coupled to a He/CdI$_2$ gas-jet transport system at the JAEA tandem accelerator facility [1]. A good agreement between the measured IP$_1$-value and the theoretical one, predicted by state-of-the-art relativistic calculations, strongly indicates that Lr would have a valence electron in the 7p$_{1/2}$ orbital. This results from strong relativistic effects in Lr, even though the 6d orbital is anticipated from the analogy to lutetium (Lu), the lanthanide homolog of Lr, which has the [Xe]4f$^{14}$5d6s$^2$ configuration.
It has been discussed that the Lr atom could possibly be much more volatile than other actinides and lanthanides since the valence electronic orbital of Lr is similar to that of group 13 elements [2]. Surface ionization used in Ref. [1] involves the adsorption-desorption process of atoms on a surface, which characterizes the chemical stability of the atomic state and its reactivity. In this work, the adsorption behavior of elemental Lr on tantalum has been studied by using the surface ion-source as a vacuum chromatographic column.
Tantalum metal was selected as a working surface for surface ionization process, superimposed by adsorption-desorption processes. The short-lived Lr isotope $^{256}$Lr ($T_{1/2} = 27$ s), produced in the reaction of $^{249}$Cf with $^{11}$B [3], was deliverd to the ion-source by the gas-jet transport system and was surface ionized. Ionized Lr was extracted by an extraction electrode and accelerated for mass-separation. The mass separated ions were collected and measured to determine apparent ionization efficiency ($I_{\rm eff}$). In case of a sufficiently high temperature in the ion-source, the adsorption retention and subsequent decay loss of atoms on the surface can be neglected. However, at lower temperature, atoms may be retained on the surface according to its adsorption interaction and decay. In order to deduce how many atoms are lost in the absorbed state inside the ion-source, we compared the measured apparent $I_{\rm eff}$ value with estimated one obtained from the Saha-Langmuir equation [4]. The $I_{\rm eff}$ measurements of Lr and lanthanide isotopes were conducted in the temperature range from 2100 K to 2600 K. From the observed temperature dependence of $I_{\rm eff}$ of Lr and various elements, we found that Lr behaves as a reactive element with low volatility similarly to Lu and contrary to possible expectation based on its ground-state electronic configuration.
In the presentation, details of the experiments and the results will be presented.
References
[1] T. K. Sato et al. Nature 520 (2015) 209-211.
[2] B. Eichler and S. Hübener, Inorg. Chem. Acta 146 (1988) 261-265.
[3] N. Sato et al. Radiochim. Acta 102 (2014) 211–219.
[4] R. Kirchner, Nucl. Instrum. Methods A 292 (1990) 203-208.
