Measurement of the First Ionization Potential of Lawrencium (Lr, Z=103) by a
Surface Ionization Method
T. K. Sato1), M. Asai1), Y. Kaneya1,2), K.Tsukada1), A. Toyoshima1), A. Osa5),
S. Ichikawa1),Y. Nagame6), T. Stora7), A. Borschevsky8),
Ch. E. Düllmann8,9,10), M. Schädel6), J. V. Kratz1, 10), N. Trautmann10)
1)
R. G. for Heavy Element Nuclear Science, JAEA 2) Ibaraki Univ. 3) Hiroshima Univ.
4)
Niigata Univ. 5) Department of Research Reactor and Tandem Accelerator, JAEA
6)
ASRC, JAEA 7) ISOLDE, CERN, 8) Helmholtz-Institut Mainz, 9) GSI, 10) Univ. Mainz

Relativistic effects influence the electronic structure of heavy
elements. The ground-state electronic configuration of theheaviest actinide, lawrencium (Lr, Z = 103), is predicted to be[Rn]5f147s27p 1/2 , which is different from that of the lanthanide
homolog lutetium (Lu) [Xe]4f146s25d. The reason for thischange is that the 7p orbital of Lr is stabilized below the 6dorbital by strong relativistic effects [1].
The first ionization potential (IP 1 ), one of the mostfundamental physical and chemical properties of an element,gives direct information about the binding energy of an electronin the outermost electronic orbital of an atom. Accurate IP 1
values of heavy elements provide crucial tests for our
understanding of their electronic structure. IP 1 values of heavy
elements with Z ≧ 100, however, could not be determined
experimentally, because production rates drastically decrease forthese elements as their atomic number increases. The study ofthese elements therefore requires new techniques, on an atomat-a-time scale.
In order to determine the IP 1 value of Lr experimentally, weemploy the surface ionization process in which an atom isionized via interaction with a solid surface at a high temperature.
For this purpose, we have developed a surface ion-source
installed to the JAEA-ISOL system and an efficient detection
system for α-particle measurements [2]. In this work, we carriedout a measurement of the IP 1 value of Lr based on thedependence of the ionization efficiency (I eff ) on IP 1 in thesurface ionization process.
The isotope 256Lr (T 1/2 = 27 s), produced in thereaction 249Cf(11B, 4n), was used for studying the Lr ionization.
A 249Cf target (thickness 260 μg cm-2) was irradiated with a67.9-MeV 11B beam from the JAEA tandem accelerator. To
obtain a relationship between I eff and IP 1 in our present system,
shortlived 142,143Eu, 143Sm, 148Tb, 153,154Ho, 157Er, 162Tm, 165Yb, 168Lu
and 80Rb were also produced by irradiating mixed targets witha 11B beam. The reaction products recoiling from the targets
were transported to the surface ion-source by a He/CdI 2 gas-jettransport system [3]. The products ionized on a tantalum surfaceof the ion-source cavity were extracted, accelerated with 30 kV,and mass-separated. The ionization experiments were performedat the ion-source temperatures of 2800 and 2700 K. The amountof ions collected at the end of the ISOL was determined with an
α-particle detection system. The I eff was evaluated bycomparing detection rates of ionized 256Lr to the rate at
which 256Lr transported directly from the target to the detection
system were registered.
By using the present system, we successfully ionized andmass-separated 256Lr with efficiencies of 36 ± 7% and 33 ± 4%
at 2800 K and 2700 K, respectively. From these I eff values of Lr,
the Lr IP 1 value was determined to be 4.96+0.08 -0.07 eV using therelationship between I eff and IP 1 obtained by the I eff
measurements of the short-lived isotopes.

A theoretical calculation of the IP 1 of Lr was also performed,
using the relativistic coupled cluster approach with single,
double, and perturbative triple excitations (DC CCSD(T)), andcorrected for the Breit contribution and Lamb shift. The
calculated IP 1 for Lr is 4.963(15) eV. Our experimental result is
in excellent agreement with the theoretical value (Fig. 1) [4].
Thus, we have experimentally shown that the IP 1 of Lr is
distinctly lower than that of Lu (5.425871(12) eV). Lr, theheaviest actinide element, has the lowest IP 1 value of alllanthanides and actinides; this quantitatively reflects andconfirms the theoretically predicted situation of closed 5f14 and7s2 shells with an additional weakly-bound electron in thevalence orbital. We note that the surface ionization method,
successfully applied here to determine the IP 1 of Lr, can provide
experimental data that can benchmark quantum chemical
calculations of the heaviest elements.
References[1]A. Borschevsky et al., Eur. Phys. J. D 45, 115 (2007)..
[2]T. K. Sato et al., Rev. Sci. Instr. 84, 023304 (2013).
[3]T. K. Sato et al., J. Radioanal. Nucl. Chem. 303, 1253
(2015).
[4]T. K. Sato et al., Nature 520, 209-211 (2015).

Fig.1 Ionization potential of heavy lanthanides (blacksymbol) and actinides (red symbol) including ourpresent results for Lr. A closed and open symbol
indicates an experimental and estimated value,respectively. The blue star * depicts the present
theoretical value for Lr.