One major problem in many transactinides (TAN) chemistry experiments is the substantial amount of nuclear transfer products produced in incomplete fusion reactions as well as products of the interaction of the beam with the target assembly and impurities in the target. The resulting background arises from isotopes of elements from many different groups of the periodic table. Some of these isotopes may exhibit decay properties similar to the TAN element of interest and complicate or even render impossible its unambiguous identification. In past experiments, choice of a chemical system was dominated by the need for a good separation from these elements, which necessitated favoring selectivity between different groups of the periodic table over selectivity between the different members of a given group. The differences in chemical behavior among the different elements in the same group could thus only be studied in limited detail.
All of these limitations can be overcome by employing physical preseparation of the desired nuclear reaction products formed as evaporation residues (EVRs) in heavy-ion-induced fusion reactions. In this technique, a physical device such as a gas-filled magnetic separator or a vacuum separator comprising electrostatic and/or magnetic deflection elements is used to remove the intense heavy-ion beam and a substantial fraction of the nuclear transfer products from the desired radio- nuclide. The separation is based on differences in magnetic rigidity, electric rigidity, or velocity. The desired atoms are guided to a so-called Recoil Transfer Chamber (RTC), which transfers the separated EVRs to the appropriate chemistry setup. The RTC consists of a gas-filled (usually 1–2 bar) chamber mounted at the focal plane of the separator and is isolated from its vacuum chamber or low-pressure (0.5–1 mbar) filling gas by a thin Mylar foil referred to as the RTC window. The transport from the RTC to the chemistry apparatus can be performed, e.g., by using a gas-jet (either with or without aerosol particles).
In contrast to experiments without preseparation where thermally unstable com- pounds were added to the carrier gas outside the recoil chamber, these can now be fed directly into the RTC which is expected to lead to increased yields. Removal of most unwanted elements is performed in the separator. In contrast to conventional chemistry experiments without preseparation, where targets with thicknesses up to 1.5 mg/cm2 have been used, only rather thin targets with thicknesses up to about 500 mg/cm2 can be used for preseparation due to the acceptance limitations of the physical separators. Together with losses due to separator efficiency, this can lead to production rates that are smaller than in experiments without preseparation. However, this possible limitation is usually outweighed by the advantages of our method.
So far, only one TAN element has been chemically investigated using preseparated isotopes and the Berkeley Gas-filled Separator (BGS) installed at the Lawrence Berkeley National Laboratory (LBNL) was used as a physical separator. In these liquid–liquid extraction experiments, the behavior of Rf was studied and compared to its lighter homologs Hf and Zr, which were investigated in separate studies. Use of preseparated isotopes resulted in nearly back- ground-free experiments, which enabled unambiguous identification of 257Rf (T1/2 = 4.3 s).
Chemical investigations of the TAN elements usually compare their behavior to that of their lighter homologs in the periodic table. The most accurate comparisons can be obtained when all studied elements are produced and investigated simultaneously. The influence of differing experimental conditions, which can lead to irreproducible results and erroneous conclusions, is an important factor to consider when assessing the results of studies where the compared elements have not been investigated simultaneously. The transport yield of aerosol-gas-jets, for example, is often not reproducible, which is disadvantageous for studies that depend on a constant production and transport yield. Other examples of differences in gas-phase experiments are unequal surface conditions or different trace amounts of reactive impurities (e.g., oxygen or water), which are less important in studies of macroamounts. In liquid-phase experiments, differences in the size and composition of the aerosol particles may affect their solubility. It is also preferable to use the same solutions for all chemistry experiments since aging effects (e.g., uptake of oxygen or carbondioxide) can alter the results. All such problems can be avoided by direct comparison of all the elements under study in the same experiment. This is commonly done by irradiating targets containing a mixture of different elements that lead to the production of all homologs using the same projectile.
However, when physical preseparation, which is based on differences in magnetic (as in case of the BGS) or electric rigidity or in velocity, is employed, these isotopes cannot be guided to the RTC simultaneously. Additionally, the BGS is not suited for separating Zr isotopes produced in almost symmetric (e.g., Ti-induced) nuclear reactions. This is due to the very similar magnetic rigidities of the beam and evaporation residues in this reaction type, which make a separation of the two impossible. Choosing more asymmetric reactions, e.g., based on 18O as a common projectile, is generally undesirable, since the slow Hf and Rf EVRs produced in such reactions cannot penetrate the Mylar foil used as the RTC window. The minimum thickness of this foil is defined by the maximum acceptable leakage from the high- pressure chemistry side into the separator and is an even bigger concern should a vacuum separator be used instead of a gas filled one. In case of the BGS and the current RTC window design, a limit of about 2.5 mm was determined for Mylar. It follows that it is not possible to investigate Zr, Hf, and Rf simultaneously when isotopes preseparated in the BGS are used.
The next best approach is switching between short-lived isotopes of these elements without long delay times and without having to open or change any part of the experimental setup, minimizing variations in experimental conditions in studies of the different homologous elements. This can be achieved when isotopes of all these elements are produced in heavy-ion-induced fusion reactions employing projectiles of similar E/m and m/q, where E is the beam energy in the lab frame, m the mass, and q the charge state. Such projectiles can be simultaneously injected into a cyclotron and switching from one beam to the other can be achieved solely by adjusting the frequency of the cyclotron, which is fast if the m/q difference is small. This near-simultaneous acceleration of different ions is referred to as a ‘‘heavy-ion cocktail’’ and similar cocktails are already routinely employed at the LBNL 88-Inch Cyclotron for applied research. The kinematical differences of the nuclear reactions lead to different recoil ranges of isotopes of different elements. In order to compensate for this effect, fast EVRs are degraded so their recoil range matches the one of the slowest EVRs. In the work presented here the technique was used for the near-simultaneous production and preseparation of short-lived Zr and Hf isotopes. This allows the study of the chemistry of these elements on an atom-at-a-time level and development of a chemical system that can be used in a future experiment where the three group 4 homologs, Zr, Hf, and Rf, are investigated in one single experiment.
Switching from the production of Zr to Hf involves
- changing the beam,
- changing the target (since no mixed targets were used),
- changing the BGS magnet settings, and
- introducing or removing a Mylar degrader foil.
The time required for these changes is usually about 15 min. A slight increase in beam energy over the value used here, which does not affect the conclusions of this work, allows for the production of all three elements including Rf. The method can be applied to homologous elements within other groups of the periodic table without major conceptual changes.
