First successful ionization of lawrencium by surface ionization process

T. K. Sato1), M. Asai1), Y. Kaneya1,2), K. Tsukada1), A. Toyoshima1), S. Miyashita1), N. Sato3),
K. Ooe4), A. Osa5), S. Ichikawa1,6), T. Stora7), Y. Nagame8), M. Schädel1), J. V. Kratz1,9)1) R.G. for Superheavy Elements, JAEA 2) Ibaraki Univ.
3) Nuclear Engineering Research Collaboration Center, JAEA 4) Niigata Univ.
5) Department of Research Reactor and Tandem Accelerator, JAEA
6) RIKEN, 7) ISOLDE, CERN 8) ASRC, JAEA 9) Univ. MainzThe first ionization potential (IP) is one of the fundamental
physical and chemical properties of an element. It directlyreflects the electronic configuration of the element. The
relativistic effects affect the electronic configuration and would
be noticeable for the heaviest elements. With an experimentally
determined IP value of heavy elements, therefore, we cancontribute to a better understanding of shell effects and how
relativistic effects play a role in the electronic structure of heavy
atoms. The ground-state electronic configuration of the heaviest
actinide, lawrencium (Lr, the atomic number Z = 103), ispredicted to be [Rn]5f147s27p1/2, which is different from that of
the lanthanide homolog lutetium (Lu) [Xe]4f146s25d. The reason
for this change in the ground-state configuration is that the 7p
orbital of Lr is stabilized below the 6d orbital by strong
relativistic effects [1]. The weakly-bound outermost electron
results in a significantly lower IP of Lr as compared with its
neighboring heavy actinides [2]. However, the elements heavier
than fermium (Fm, Z = 100) are produced at accelerators using
reactions of heavy ions with heavy target materials. Moreover,both half-lives and production cross sections of the isotopes ofthe still heavier elements are rapidly decreasing. Thus, they areusually available in quantities of a few atoms at a time.
Consequently, beginning with about the end of the actinides,
properties of the heaviest elements must be studied on an atomat-a-time scale. The IP values of the heavier actinides with Z >100, therefore, have not been measured using well-establishedmethods.
The surface ionization is a process in which an atom is ionized
via the interaction with a solid (metal) surface at hightemperature. The ionization yield depends on the surface
temperature of the solid material, a work function and theionization potential of the element of interest. In order todetermine the IP value based on the surface ionization process at
the atom-at-a-time scale, we have developed a surface ionization
type ion-source as part of the JAEA-ISOL (Isotope Separator
On-Line) setup, which was coupled to a He/CdI2 gas-jet
transport system [3]. As the first step to the determination of theIP value of Lr, we conducted ionization and mass-separation ofa short-lived Lr isotope with the ion-source and the ISOLsystem.
The isotope 256Lr (T1/2 = 27 s) was produced in the reaction of249,250,251
Cf(11B, xn)256Lr [4]. A 249,250,251Cf target was irradiatedwith a 67.9-MeV 11B beam from the JAEA tandem accelerator.
For comparison, the lutetium isotopes 168mLu and 168gLu, withhalf-lives of 6.7 min and 5.5 min, respectively, were alsosynthesized in the 162Dy(11B, 5n) reaction. The nuclear reaction
products recoiling from the targets were transported to the ionsource by a He/CdI2 gas-jet transport system. The products wereionized in the ion-source, accelerated with 30 kV, and massseparated with a mass-separator in ISOL. The amount of ionscollected at the end of the ISOL after mass-separation wasdetermined by -particle or -ray measurements. To calculate

ionization efficiencies, nuclear reaction products transported
from a target recoil chamber were collected directly andmeasured in advance of the ionization experiments. To compare
the ionization efficiencies on different surface materials of theion-source, a rhenium (Re) surface and a tantalum (Ta) surface
were employed in this study.
An -particle spectrum of ions ionized on the Re surface andmass-separated with mass number A = 256 is shown in Fig. 1.
Alpha-particles originating from 256Lr and its decay products,
256
No and 252Fm, are clearly observed. This result
unambiguously demonstrates that we have successfully ionizedand mass-separated 256Lr for the first time, verifying the surface
ionization ion-source is a promising tool for the measurement ofIP values of single atoms. The ionization efficiencies of Lr andLu on the Re surface were 42+20-19% and 19.9 ± 7.0%,respectively. In the case of the Ta surface, the efficiency of Lr
was 19+9-8% while that of Lu was 4.0 ± 1.4%. The ionization
efficiencies of Lr on both the surfaces were higher than those ofLu. The results suggest that the IP of Lr should be lower thanthat of Lu. This is consistent with the theoretical prediction froma coupled cluster calculation that takes into account relativisticeffects [2]. Using the present system, the ionization efficiency ofLr is being examined in more details for the determination of its
IP value.

Fig.1

Measured -particle spectrum of mass-separated ions withmass number A = 256 with the Re surface.

References[1] Y. Zou et al., Phys. Rev. Lett. 88, 183001 (2002).
[2] A. Borschevsky et al., Eur. Phys. J. D 45, 115 (2007).
[3] T. K. Sato et al., Rev. Sci. Instr. 84, 023304 (2013).
[4] N. Sato et al., JAEA Rev. 2010-056, 52 (2010).