Rapid extraction of short-lived isotopes from a buffer gas cell for use in gas-phase chemistry experiments. Part I: Off-line studies with  ²¹⁹Rn and  ²²¹Fr

Abstract

To study the chemical properties of the heaviest elements, a fast and efficient stopping and extraction of the highly energetic residues from heavy ion fusion reactions into the chemistry setup is essential. Currently used techniques like Recoil Transfer Chambers (RTC) relying on gas flow extraction provide high efficiencies for chemically non-reactive volatile species, but operate at extraction times $t_{\text{extr}}$ of about 0.5 s or more. Buffer Gas Cells (BGC) with electric and Radio-Frequency (RF) fields offer much faster extraction times. Here, we demonstrate the successful coupling of a BGC to a gas chromatography setup as is used for studies of chemical properties of superheavy elements. Using  ²²³Ra and  ²²⁵Ac recoil ion sources providing  ²¹⁹Rn and  ²²¹Fr ions for off-line tests, an extraction time $t_{\text{extr}}$ = 55(4) ms and an extraction efficiency of 35(3)% were achieved for the non-reactive  ²¹⁹Rn, while  ²²¹Fr was retained. The results show a BGC-based setup to be suitable for gas-phase experiments with short-lived volatile transactinide elements like Cn and Fl with half-lives substantially below 1 s.

Introduction

The seventh period of the Periodic Table of the Elements (PTE), introduced by Mendeleev 150 years ago, was officially completed by the acceptance of elements nihonium (Nh, $Z=$ 113), moscovium (Mc, $Z=$ 115), tennessine (Ts, $Z=$ 117), and oganesson (Og, $Z=$ 118) by IUPAC [1], [2]. The PTE is structured such as to reflect the periodic trends of the elements, with the groups containing chemically similar elements. The main interest in chemical experiments with Superheavy Elements (SHE, atomic number $Z\ge 104$, which are the transactinides in the periodic table) is on the study of their chemical behavior relative to that of their lighter homologs [3], [4]. Deviations from trends established by lighter group members are expected due to the influence of relativistic effects, which increase $\propto Z^{\text{2}}$ [5]. Experiments with SHE are challenging due to low cross sections and short half-lives. Only single atoms are available requiring one-atom-at-a-time experiments [6], [7]. A well established technique to study chemical properties of transactinides is to measure their interaction strength with heterosurfaces. Current experimental setups involve the synthesis of the element under study in nuclear fusion reactions, followed by thermalization in a gas-filled volume. This is either mounted directly behind the target [3], [8] or behind a physical preseparator [9], [10], [11]. Extraction from this chamber is the most time-consuming step in many contemporary setups of this type, and requires about 0.5 s or more. Furthermore, chemistry setups are typically coupled to these gas-filled chambers via plastic tubing. In case of strong interactions of the elements with the surface of these tubes, they adsorb on the surface and decay there before reaching the subsequent detection system. This approach allowed the study of volatile species of the transactinide elements including Sg(CO)${}_6$ [12], HsO${}_4$ [13] or elemental Cn [14] and Fl [15], which are the heaviest elements, whose chemical properties have been studied. Less volatile species were extracted from the gas-filled volume and carried over reactive surfaces by the aerosol-jet technique, including (oxy)halides of Rf, Db, Sg, and Bh [16]. For chemical studies of elements beyond Fl, which are short-lived and are expected to be

reactive [17], [18], [19], these limitations need to be overcome. Similar challenges are present in neighboring fields, including nuclear and atomic physics studies of short-lived exotic nuclei. A successful technique to forward these to a variety of experimental setups is by coupling using Buffer Gas Cells (BGC) with superimposed electrical fields providing very short extraction times down to milliseconds [20]. To explore the applicability of the BGC technique in the field of chemical studies of the heaviest elements, we present here exploratory studies on the combination of a BGC with a state-of-the-art chemistry setup suitable for studies of transactinide elements.