5. Element 117 And Beyond
(K. P. Rykaczewski)
The synthesis and identification of new elements is a long standing challenge, with major experimental contributions made over the last 50 years by the US, Russia, Germany and Japan, see, e.g., [Due10, Hof00, Mor04, Oga04, Oga05,Oga07,Oga10, Sta09]. The questions like "What is the heaviest atomic nucleus which can be bound against immediate disintegration?" or "Where does the Periodic Table of Elements end?" are of large interest not only to physicists but to the whole scientific community and general public. Most theoretical calculations point to the existence of elements 119 and up, to at least 126, and this has motivated many scientists to continue the effort of searching for the next superheavy element.
This year a team including scientists from Joint Institute for Nuclear Research (JINR Dubna, Russian Federation), Oak Ridge National Laboratory, Lawrence Livermore National Laboratory, Vanderbilt University, Las Vegas University, and the Research Institute of Atomic Reactors (RIAR, Dimitrovgrad, RF) reported the discovery of new chemical element Z=117 [Oga10]. The synthesis of 293(117) and 294(117) isotopes and other 9 new isotopes of Superheavy Elements (SHE) was achieved through a close collaboration of Russian and US laboratories. Unique ORNL contributions to this discovery included 22.2 mg of very high purity 249Bk (T1/2=320 days) material with only 1.7 ng of 252Cf, and no other detectable impurities. The 249Bk material was produced in a two-year campaign at the High Flux Isotope Reactor and was purified during the three-month long chemical separations at the Radiochemical Engineering Development Center at ORNL.
Six arc-shaped targets, of total area 36.0 cm2 area and about 0.3 mg/cm2 thick, were made at RIAR. Target disk was rotated at 1700 rpm perpendicular to the beam direction during the irradiation with over 1 particle-microAmp of 48Ca beam. This high intensity beam was accelerated at the heavy-ion cyclotron U-400 at JINR. The experiments were performed by means of the Dubna Gas-Filled Recoil Separator (DGFRS) [Oganessian1998]. A total target exposure to 4.4 x 1019 48Ca ions was achieved during a 150-day bombardment.
Evaporation residues (ER) passing through the DGFRS were transmitted and registered by two Multi-Wire Proporional Chambers with a total detection efficiency of over 99%, and were implanted into a Si-detector array with 12 vertical position-sensitive strips (10 mm wide, and 40 mm long) surrounded by eight 4cm x 4cm side detectors. The preamaplifier outputs with two amplifications allowed to measure alpha-energies in the range of up to 12 MeV with a reasonable resolution as well as to detect high energy fission events. The energy resolution for implanted alpha-particles was 60 to 140 keV. The total kinetic energy released in the spontaneous fission (SF) of nuclei with Z>101 was determined from the measured energy release Etot + 23 MeV. The 23-MeV correction corresponds to the mean energy loss of fission fragments in the dead layer detector as determined from a 252No measurement. The position resolution of the strip detector in registering correlated decay chains of the ER followed by alpha chain and SF event, was below 1.2 mm. The 48Ca beam was switched off for several minutes after a recoil signal was detected with parameters of implantation energy expected for Z=117 ERs, followed by an alpha-like signal with an energy between 10.7 MeV and 11.4 MeV, in the same strip, within a 2.2-mm-wide position window.
Two 70-day long irradiations were performed, at 252-MeV and 247-MeV energies of 48Ca projectiles, corresponding to about 39-MeV and 35-MeV excitation of the compound nucleus 297(117). Five decay chains observed at higher beam energy, with three subsequent alpha emission and ending in fission with Total Kinetic Energy (TKE) of 218(5) MeV, were assigned as starting at 293(117) nucleus, see Fig. 5-1. A single event consisting of six alpha decays and again ending with fission of TKE=219(5) MeV was observed at lower beam energy. The observation of this single long decay chain has been very recently confirmed at JINR Dubna, by two events observed during the chemistry-focused experiment using stationary 249Bk targets of 0.5 mg/cm2 [Dmi10].
Figure 5-1: The observed decay chains were interpreted as originating from the 294(117) isotopes and from 293(117). Average time and energy values obtained from five identified events are displayed for the 293(117) of the new element Z=117. The measured and predicted [Sob10] lifetimes and alpha-particle energies are shown in black and blue, respectively (figure adapted from [Oga10]).
The properties of 11 new isotopes identified among the products of 249Bk+48Ca reaction [Oga10] indicate a strong rise of stability for heavier isotopes with Z=111 to 117, validating the concept of the long sought island of enhanced stability for superheavy nuclei, see Fig. 5-2.
Figure 5-2: Alpha decay energy (plot a) and half-lives (plot b) of Superheavy nuclei with an odd atomic number are displayed as a function of neutron number for the isotopes of elements with Z=111 to 117. All the nuclides with N>165 have been produced in 48Ca induced reactions. The isotopes identified in [Oga10] are shown as solid symbols. In the plot (b) the values of Tαexp are given only for the decay chain originating from the 293(117) isotope.
Presently, most of nuclear models point to the N=184 spherical shell closure. However, the superheavy nuclei identified to date are still several neutrons away from N=184. The observed decay properties do not point clearly to the one of predicted proton shell closures at Z=114, Z=120 or Z=126. Therefore, more experimental data and more theoretical work on the production and properties of superheavy elements are needed before the coordinates of the Island of Stability can be determined with high confidence.
Figure 5-3: Potential future chart of superheavy nuclei. The green squares indicate previously produced isotopes. The blue background shows predicted stability of isotopes (the more intense color indicates the more stable nuclide). The colored squares with colored frames indicate nuclei which may be potentially created in the fusion reactions between actinide targets and beams of 48Ca and 50Ti followed by evaporation of 3 and 4 neutrons. The solid colors indicate targets of 249Cf (pink), 249Bk (yellow), 251Cf (orange) and 254Es (grey). The frames correspond to the use of 48Ca (black) and 50Ti (red) beams. There are other new isotopes of Z = 118 elements to be potentially produced with 50Ti beam not indicated in this figure, e.g., 295(118) produced in the 3n evaporation channel of the 248Cm + 50Ti reaction. Note that reactions between 254Es target nuclei and 50Ti projectiles would theoretically produce the new element Z=121, predicted to start a new row of elements, after lanthanides and actinides.
One of the methods to push towards new isotopes of the heaviest nuclei is to employ intense 50Ti beams as well as new targets like 251Cf and even 254Es, see Fig. 5-3. One should note that attempts to use heavier beams like 54Cr, 58Fe and 64Ni on 238U and heavier radioactive targets have not been successful, probably due to the drop of the cross section, see, e.g., [GSI120, Oga08,Oga09]. 50Ti projectiles represent the stable beam closest to 48Ca, with the same magic neutron number of N=28 and Z larger by 2 protons. While larger Z helps to create heavier elements, the 2-proton excess with respect to the doubly-magic 48Ca will likely make the cross section for production of new isotopes smaller. It implies that the reaction between 50Ti and actinide targets should be well mapped and understood before attempting a long experiment aimed at producing new element (e.g., 249Bk + 50Ti to create new element Z=119). The development of high intensity50Ti beams and the excitation function mapping can be pursued using the longer-lived targets between 238U and 248Cm. However, since the production rates are expected to be low, an upgrade of the detection system to higher sensitivity is necessary. We believe that it is possible to reach these nuclei using ORNL-made target materials and a set of improved detectors.
The first step to meet this challenge is to build a new detector system using novel techniques. ORNL main contributions to the research on superheavy nuclei are currently related to the world-unique production capabilities of actinide target materials and to the theoretical work on the structure of the heaviest nuclei, see, e.g., [Cwi05]. These ORNL contributions to the SHE studies are getting extended, by constructing new detectors and their signal processing system at the Holifield Radioactive Ion Beam Facility. This new detection scheme should be capable of identifying the radioactivities having sub-microsecond lifetimes and offer a better discrimination between the high energy alpha signals and partial recoil energy deposition. This will be achieved with a large area and high granularity Double-sided Silicon Strip Detector with matching alpha-escape detectors around, all connected to the digital signal processing units like 100 MHz Pixie-16 modules of Xray Instrumentation Associates (XIA) [Grz2007]. The new recoil detection chamber should be able to distinguish the recoils having atomic number around Z=118 from the Z=84 contaminants, by tracking their energy loss. The new detection scheme should allow us to largely purify the alpha decay spectra and would increase confidence in the obtained results, even the cases where a single event is observed. New detectors and data acquisition system proposed for the SHE research will profit from the experience gained at the at HRIBF [Lid06, Maz07, Dar10] as well as from the joint experiments performed with Optical Time Projection Chamber [Mie07] in collaboration with the Warsaw University team at the National Superconducting Cyclotron Laboratory (Lansing, MI).
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