Optimizing the synthesis of SHE metal carbonyl complexes andpreparing nuclear structure studies of 265Sg: preparatory experiments
at Tandem Accelerator, JAEA Tokai, and at TRIGA Mainz

Ch.E. Düllmann1,2,3, M. Götz1,2,3, M. Asai4,, J. Ballof1,5, A. Di Nitto1,2, K. Eberhardt1,3, S.
Götz1,2,3, J. Khuyagbaatar2,3, J.V. Kratz1, L. Lens1,2, C. Mokry1,3, Y. Nagame4, J. Runke1,2, T.K.
Sato3,4, A. Toyoshima4, P. Thörle-Pospiech1,3, K. Tsukada4, V. Wolter1, A. Yakushev2,31Johnannes Gutenberg University Mainz, Germany
2GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany
3Helmholtz Institute Mainz, Germany
4Japan Atomic Energy Agency, Japan
5CERN, Geneva, Switzerland

Abstract
In a series of complementary experiments at the TRIGA Mainz and the JAEA tandem accelerator,
the in-situ synthesis of volatile transition metal carbonyl complexes with short-lived
radioisotopes has been investigated to develop a new approach, which promises to allow more
efficient carbonyl-complex formation than the previously used combination of gas phasechromatography with a physical recoil separator. Using a setup with decoupled recoil ion
thermalization and chemical complex formation in two separate volumes, the feasibility of themethod was demonstrated using fusion-evaporation-produced radionuclides at JAEA Tokai andshort-lived fission products generated at the research reactor TRIGA Mainz. According to thecurrently available preliminary data, superior efficiencies were obtained, validating this novel
approach. In parallel, construction of the ALpha-BEta-GAmma decay spectroscopy detection
setup ALBEGA continued.

1. Research Objectives
In case of superheavy elements, the investigation of carbonyl complexes is of broad interest, asrelativistic effects, which scale with Z2, are predicted to strongly influence the chemical behavior

of these elements [1]. The bonding in carbonyl complexes emerges from electron pair donationfrom the CO molecules surrounding the central atom into its empty d orbitals and subsequentπ-back bonding due to overlapping of the filled d orbitals with antibonding π∗–orbitals of CO [2].

Comparative studies of the transactinides with their lighter homologs help to understand theinfluence of relativity on the electron shell and test the predictive power of the periodic table.
Currently, the method of choice is the isolation of the respective ions behind a recoil separator,
followed by the chemical carbonyl complex formation [3], which allowed synthesis of Sg(CO)6 [4].
However, the evaporation residues of asymmetric fusion reactions, needed for the productions ofsuitably long-lived isotopes of the transactinides Sg, Bh, and Hs have a wide angular and energy
distribution, thus the transmission efficiency through a forward-angle recoil separator isrelatively low. In case of RIKEN GARIS, which was used in the Sg work, this partial efficiency
is in the order of 13% for Sg [5]. Thus, the overall efficiency of the synthesis of carbonyl complexes
in combination with physical preseparation is rather low. For future experiments a more efficientconcept is highly desirable due to decreasing production rates for elements heavier than Sg.

2. Research Contents
2.1. A novel synthesis approach for volatile carbonyl complexes of transition metal radioisotopes
The possibilities for the chemical investigation of these systems without a physical preseparation
stage is currently explored within a so called two-chamber approach, whereby the non-volatilefusion-evaporation products will be flushed out to a second, directly connected reaction chamber.
Scientific goal is to avoid the high losses of about 80-90% currently inherent to the techniqueusing a physical preseparator. First experiments performed at the JAEA tandem accelerator at
Tokai, Japan, suggested that the successful synthesis of Os and W carbonyl complexes is feasible
if the thermalization of the evaporation residues is spatially decoupled from the chemicalsynthesis.
forming
Removable
glass fiber
filterNeutrons
fromTRIGA

Removable
Teflon discFissionproducts
101Mo,
101Tc

This

allowsthecarbonyl

complexes in the absenceofbeam.

Furtherexperiments

wereconductedusingfission

products

obtainedat

TRIGA Mainz. Figure 1
illustrates the schematic
structureof

thetwo-chamber approach.
Thefission

products,

Fig 1. Schematic illustration of the experimental setup usedeitherfor carbonyl chemistry studies at TRIGA Mainz (with a 235U

producedbyneutron-inducedtarget) and off-line (with a 248Cm) fission fragment source).

(red, with a 235U target) orfission

by spontaneous fission (blue, with a 248Cm source), were thermalized in a gas-filled chamber
behind the target in the thermalization chamber. To obtain absolute rates of fission fragments
entering the thermalization chamber, a Teflon disk acting as a catcher for fission fragments
emerging from the target could be mounted in close proximity covering the 248Cm source (see Fig.
1). In case of the on-line studies at TRIGA, previously obtained absolute rates were used [6]. The
thermalization chamber was continuously flushed with inert gas in both cases. Directly at theexit of this thermalization chamber a reaction chamber was added, into which the fission
products were flushed with the inert gas. In case of the flush-out efficiency studies, performed
with the 248Cm-source, a removable charcoal filter was installed between the exit of thethermalization- and the entrance of the chemical reaction chamber. This filter was used to collectthe non-volatile fission products that were flushed out of the thermalization chamber into thereaction chamber and was, after being removed, counted via γ spectrometry. For the chemical
experiments, using the 235U target, carbon monoxide gas was injected into the second chamber,bypassing the thermalization chamber. The inert gas and the carbon monoxide were pre-cleaned
by passing gas purification cartridges and hot titanium getters. Fission fragments volatile intheir elemental form or forming volatile complexes with CO were transported with the gas
stream through a Teflon capillary to a chemistry laboratory. Here the products were trapped ona charcoal filter and isotopically identified by examining the characteristic γ rays. All samples(Teflon disk; charcoal filter at the entrance of the chemical reaction chamber, and charcoal filter
collecting volatile species after transport through the capillary) were counted in the same
geometry to ensure identical efficiency.

2.2. On the way to the ALpha-BEta-Gamma multidetector system ALBEGA.
A next generation setup for measurements of ALpha-BEta-GAmma decays (ALBEGA) after
chemical isolation is currently under development. Building up on a first prototype [7], anadvanced version, overcoming some limitations encountered during commissioning of the firstversion, is being built. It will allow nuclear spectroscopy studies of chemically isolated samples,including volatile carbonyl complexes. Therefore, this activity nicely complements the chemicalstudies described in Sect. 2.1.

3. Research results
3.1. Carbonyl compound synthesis
After first indications about the success of the approach were obtained at JAEA Tokai, moredetailed and quantitative studies were carried out at TRIGA Mainz. Preliminary results from
TRIGA for the total yield (comprising flush-out yield from thermalization chamber to reaction
chamber and chemical yield for complex formation and transport to the external charcoal filter)as a function of gas composition (starting from a concentration ratio of N2 : CO of 2:1 over

Relative Yield

decreasing N2 content to a limiting ratio of

1.0 bar
1.2 bar
1.5 bar

501:2) at constant total flow rate, and atdifferent pressures, is shown in Figure 2. The
efficiency

40for

thetransferfrom

thethermalization chamber into the reaction was30

found to reach values up to around 45%,
which is substantially higher than the20transmission of the GARIS separator. The10

positive influence of a high CO concentrationon the yield is likely explicable by an increasein statistical collision probability with CO,
N :CO

N :CO

N :CO

2:1

1:1

1:2

2

2

2Fig 2. Transport efficiency for non-volatile
reaction products like Tc and Mo, collected
at the entrance of the reaction chamber.

which is necessary for the formation of ametal-CO bond and eventually a volatilecarbonyl complex. This observation agrees
with previous experiments performed by Evenet al. [3, 8] behind a physical preseparator.
The yield increases with increasing pressure
inside the chamber and can be explained byshorter mean free path between collisions with CO molecules and slower diffusion to the wall,
where non-volatile species are lost. Further studies aimed at investigating the influence offurther parameters are currently under final evaluation.

3.2. ALBEGA development
Advanced prototypes of the two ALBEGA core detectors, whose inner surfaces are covered witha thin Al and SiO2 layer, respectively, have been tested. They were developed to provide i)
improved energy resolution and sensitivity to low energy signals produced by electrons, ii) higher
photon detection efficiency, and iii) better mechanical stability against pressure differences. The
tests performed with several radioactive sources including 241Am (α particles) and 133Ba(conversion electrons) confirmed performance according to specifications, including theuniformity and desired very thin thickness of the dead layers, as well as the sensitivity to thelow energy signals. In a next step, the detectors will be mounted in a sandwich configuration toproduce the final version of the ALBEGA core detector.

4. Conclusion
After initial promising indications from the JAEA Tokai experiments, the main scientific goal of
the TRIGA studies was to distinguish and optimize the transport efficiency for volatile carbonyl
complexes of non-volatile radioisotopes between the thermalization- and the reaction chamber

as well as to increase overall efficiency for the in-situ synthesis of carbonyl complexes. The
transport efficiency was investigated at the TRIGA Mainz. Experiments with an 235U-target andoffline measurements, performed with a 248Cm spontaneous fission source, revealed transport
efficiencies up to 40% and thus, the new setup looks to be a promising alternative to the current
method of choice.

5. Acknowledged
The experiments were conducted at the JAEA tandem accelerator and at the research reactor
TRIGA Mainz. We acknowledge support from the mechanical workshop and the actinide target
lab staff at Institute of Nuclear Chemistry at Johannes Gutenberg University Mainz and fromJörg Krier and Egon Jäger. Financial support from BMBF for the construction of ALBEGA(project 05P15UMFNA) and from the JAEA ASRC’s Reimei program is gratefully acknowledged.

6. References[1] P. Pyykkö and J.P. Desclaux Acc. Chem. Res. 12, 276 (1979).
[2] G. Frenking and N. Fröhlich, Chem. Rev. 100, 717 (2000).
[3] J. Even et al., Inorg. Chem. 51, 6431 (2012).
[4] J. Even et al., Science 345, 1491 (2014).
[5] H. Haba et al. Phys. Rev. C 85, 24611 (2012).
[6] J. Grund, PhD Thesis, Johannes Gutenberg University Mainz (2018).
[7] A. Di Nitto et al., GSI Sci. Rep. 2015, 184 (2015); contribution MU-NUSTAR-SHE-C06.
[8] J. Even et al., Radiochim. Acta 102, 1093 (2014).