Supernovae and the Superheavy Elements

Richard C. Kohler

  During the 15 billion years that have elapsed since the big bang, from which we trace the origin of the universe, a complex array of processes has shaped our present environment.  The emergence of galaxies, stars, planetary bodies, and eventually life itself arose from the primordial big bang.  This evolution represents the consequence of interactions between natures basic forces and the fundamental particles.  Underlying the great diversity of our present day universe has been the formation of the chemical elements.  Chemical element synthesis or nucleosynthesis is now understood  in terms of nuclear reaction that occur in a variety of cosmological settings.  Within this realm, the chemical elements are created.  While scientist have a good understanding of the processes that occur in the creation from hydrogen through uranium, it is the transuranium and more specifically the super heavy elements that remain the most illusive and least understood.  The transuranium traditionally refers to those elements that have an atomic number greater than that of uranium.
 Research over the last 30 years indicates there are three main sources responsible for the synthesis of the chemical elements.  They include (1) cosmological nucleosynthesis in the big bang, (2) nucleosynthesis in the interstellar medium induced by galactic-cosmic rays and (3) nucleosynthesis during stellar evolution (Cox 89).  A brief overview will cover each of these processes with greater detail being given to those processes necessary for the creation of the transuranium and the superheavy elements (SHE).
 

Nucleosynthesis during the Big Bang

  The earliest era to which we can trace the origin of our universe is that of the big bang, which is believed to have occurred 15 billion years ago.  Under the initial conditions of the big bang, all matter existed and energy existed in the form of a hot, dense fireball that contained only the elementary particles.  When the universe had cooled to 1010 K, it consisted of a sea of photons, electrons, and positrons, neutrinos and antineutrinos, in addition to the stalwart protons, and neutrons.  These particles existed with one another according to the following equation (Zin 79):
 

 
As the universe cooled to a bone chilling 109 K, a more complex nuclei was formed.  The universe continued to expand, the temperature decreased to a point where 2H nuclei could survive for a finite length of time.  The following network of equations applied during that period of the universe (Zin 79):
 
 
 Within 3 minutes, the universe had expanded and cooled to a point where nuclear reactions could no longer to be sustained.  The remaining neutrons then decayed to protons as follows (Zin 79):
 
 
The unreacted protons and neutrons from the big bang formed the large residual hydrogen abundance that is seen in the universe today.  The primary nuclear reaction product of the big bang was 4He.  The hydrogen and helium elements that today comprise 98% of nature’s elements.
 

Nucleosynthesis in the Interstellar Medium

   While the big bang and stellar evolution can account for most of the elements in the periodic table, there are a small number of elements that are produced in the interstellar medium. Cosmic rays are very energetic nuclei, primarily from H and He that permeate our galaxy. While their origin is poorly understood, their properties have been studied extensively in balloon and satellite flights above the Earth’s atmosphere. Li, Be, and B nuclei are believed to be formed when cosmic rays interact with the 4He, carbon, nitrogen, and oxygen nuclei present in the interstellar medium (Cox 89). These reactions occur at energies much higher than those characteristics of the big bang or stellar evolution but in an environment which has a very low density. The temperature is low and the Li, Be and B products do not burn up after their formation, as they would in the stellar interiors.
 

Nucleosynthesis during Stellar evolution

 We know move on from the big bang to the inner processes of stars.  When the core of a star reaches unusually high temperatures and densities, the protons in the core acquire sufficient kinetic energy to overcome their mutual electric charge repulsion and initiate nuclear reactions (Emp 91).  The process, known as hydrogen burning, characterizes main sequence stars.  Such stars burn protons into He by means of the following series of nuclear reactions (Zin 79):
 

 
The hydrogen burning reactions stabilize a condensing star and provide a vital source of energy by producing He from H.  In order to synthesize the more complex elements that provide the diversity of our solar system, the advanced stages of stellar evolution must be explained.  As a main sequence star becomes older, it begins to develop into two phases:
          1) a core composed largely of the He produced during hydrogen burning,
          2) and an outer envelope consisting largely of unburned hydrogen (Emp 91)
     If the mass of a star is sufficiently large, the force of gravity begins to contract the core once again, leading to still higher temperatures and densities.  This causes the envelope of the star to expand greatly and gives rise to a new stage in its evolution, call the red giant phase.  Stars that do not contain sufficient mass to sustain more advanced stages of nuclear burning simply exhaust their hydrogen fuel and undergo no further evolution.  They are destined to become white dwarf stars.  Our on sun will eventually meet this mundane end in 4 to 5 billion years.
 During the red giant stage of a star, gravitational pressure continues to compress and heat the core.  When the temperature reaches about 108 K, which corresponds to a density of 104 g/cm3, a new type of nuclear reaction becomes possible (Bau 89).  The exothermic reaction called He burning, is represented by the equation (Zin 79):
      Once He burning begins, the core of the star becomes stabilized against further gravitational contraction by the evolution of nuclear energy, producing a new equilibrium situation.  At the same time oxygen can be produced via the reaction (Zin 79):
   If the mass is sufficiently great, a much more dramatic sequence of processes ensues.  As a massive star passes through the red giant phase, new core condition eventually develop.  For the most part, the core contains 12C and 16O surrounded by envelopes composed of H and He.  The electrostatic repulsion arising from the large nuclear charges of 12C and 16O inhibit nuclear reactions at He burning temperatures, leading to further gravitational contraction and heating (Atk 91).  A star’s subsequent fate under these conditions is one of the least understood phases of stellar evolution.  The stellar core may continue to evolve via processes similar to the equilibrium situations that exist in main sequence and red giant stars, although on a much shorter time scale.  Explosive conditions may develop under which nucleosynthesis occurs very rapidly, leading to supernovae explosions.
     If the core temperature and density reach approximately 500 * 106 K and 5 * 105 g/cm3, new avenues of nuclear burning become available.  One such reaction involves the fusion of the 12C and 16O remnants from He burning to form still heavier nuclei.  These reactions are complicated, but can be summarized as follows (Zin 79):
 
 
 
 
     Because these reactions can occur relatively rapidly at high temperatures, the evolution of the star proceeds much faster at this stage and a more varied nucleus composition develops.  As the life cycle of a heavy star continues, a core composed largely of nuclei near 28Si evolves.  At temperatures near 109 K and densities approaching 106 g/cm3, a new process referred to silicon burning begins (Bau 89).  These reactions involve both the ejection of an alpha particle (4He) by high-energy photons present in the hot core and the inverse process in which 4He is captured by the surrounding nuclei. It can be summarized as (Zin 79):
 
 
 This chain of reaction stops near mass number A=56.  Iron is nature’s most stable nucleus.  When an Fe core develops in a star, the ability of energy-liberating nuclear reactions to provide support for resisting gravitational contraction becomes limited.  At this point, a rather complex unstable star has developed, containing most of the elements up to Fe in various layers. At each stage of evolution, the synthesis processes becomes less efficient and more diverse.  This accounts for the steady decrease in the abundance of the elements.  Due to the special ability of 56Fe nuclei, a sink is created that accumulates ironlike elements, producing the abundance peak at Fe.  It also accounts for the low abundance of the heavier elements.
     The accumulation of iron-group elements in the core of stars with masses greater than 10 times the mass of the sun lead to catastrophic conditions.  Without the stabilizing influence of nuclear energy evolution, the gravitational force causes the core to collapse.  The implosion occurs on a time scale as short as seconds, during which the density of nuclear matter may reach 10^8 g/cm^3 with a corresponding temperature well over 109 K in the center of the core (Bau 89).  This rapid heating is followed by a massive shock wave that leads to the explosion of the star, a process believed to be associated with supernovae such as SN1987A.
     There are two important consequences of gravitational collapse and the rapid heating that follow: First, the temperature increases triggers an extensive network of nuclear reactions throughout the outer envelopes of the star.  This leads to a diversity of nuclear species for the elements previously formed. Second, the condition in the very center of the core, where the temperature and density are highest, cause the iron nuclei to break up by means of photodisintegration reactions, leading to the following processes (Zin 79):
 
 
As far as nucleosynthesis is concerned, the important point is that large numbers of neutrons are produced in the central core region.  Because neutrons have no electric charge, they can interact with previously processed nuclear matter without the constraint of electromagnetic repulsion.  This further enriches the variety of nuclei and produces nature’s heaviest elements.
    This stage of nucleosynthesis is commonly referred to as the r process (r is for rapid) and proceeds according to the following series of abridged reactions (Zin 79):
 
These reactions produce highly neutron rich nuclei that are well submerged below the sea of instability.  As neutron addition continues, nuclear beta decay (conversion of a neutron into a proton within the nucleus) becomes increasingly probable.  This produces the next higher element (Zin 79):
 
 
     It is the r process that forms thorium and uranium (Z=90 and 92 and may account for the existence of any of the “superheavy” elements in nature. Unfortunately, it now becomes necessary to move our investigation from the cascades of supernovae to the inner workings of man-made accelerators.

 

Transuranium Research
 

 By using neutron and positive-ion bombardment, scientists have been able to extend the periodic table. Prior to 1940, the heaviest known element was uranium (Z=92), but in 1940 neptunium (Z=93) was produced by neutron bombardment of 238U. The process initially gives 239U, which decays to 239Np by b -particle production (Mas 88):

                                        239U + 1n Þ 239U Þ 239Np + -1e

    In the years since 1940, the elements with atomic numbers 93 through 112, have been synthesized. Many of these elements have very short half-lives, as illustrated in Table 1.

                                   Table from Chemistry 3rd Edition, A. Jackson

    During the late 1940’s, it was predicted that transuranium elements were expected to have a life span comparable to that of uranium or thorium, and to be produced in macroscopic quantities. For chemistry, it meant that the heavy elements would open up discoveries for new compounds, and for physics it meant a better understanding of matter. One question that arose as long ago as the early Sixties was whether such shell effects in nuclei much heavier than uranium would cause them to be stable enough that they might still occur in trace amounts in nature, or could be synthesized. Early calculations from 1966 predicted an "island of stability" in this region for the isotope 298114(Akt 91). This was the birth of the idea of the superheavy elements (SHE), and marked the start of experimental efforts to synthesize them. But much like life, their early optimism was dealt a setback when research indicated that the superheavy elements were short lived and difficult to produce.
    Many properties of transuranium elements can be described by analogy with a drop of liquid. The classical liquid drop model, which treats nuclear binding as an interplay of attractive nuclear forces (acting between both protons and neutrons) keeping the nucleus bound, and repulsive electrostatic forces ( acting only between protons) driving apart. The drop model should give reliable predictions of nuclear masses and binding energies (Sea 63). The binding energy is the energy required to decompose a nucleus into it component parts. However, it says nothing about the internal arrangement of protons and neutrons in a nuclear drop(Sea 63). This inner arrangement essentially determines the properties of a nuclear system, for example, its exact binding energy. Figure 1 indicates the binding energy per nucleon as a function of the mass number.
                                                    Graph from Chemistry 3rd Edition, A. Jackson
The most stable nuclei are at the top of the curve. The most stable nucleus is 56Fe, which has a the highest binding energy per nucleon of 8.79 MeV. Just like the electron cloud of the atom, atomic nuclei also exhibit a shell structure. While the atoms of noble gases (He, Ne, Ar, Kr, Xe, and Rn) owe their stability to closed shells of electrons, certain atomic nuclei are extremely stable because they have closed shells of neutrons and protons. This stability of nuclei with proton number Z or neutron number N equal to 2, 8, 20, 28, 50 and 82 becomes evident in two fundamental properties: the nuclear binding energies, as deduced from nuclear masses, are exceptionally high for such so-called "magic number" nuclei (Sea 90). For neutrons, N=126 is also identified as a "magic" number. Figure 2 and 3 refers to the stability of nuclei.  The pennisula illustrates known nuclei and an island of superheavy nuclei that is relatively stable in the sea of instability:

                                                 
Common examples of this occurrence include the double magic nuclei 4He, 16O, 40Ca, and 48Ca, as well as 208Pb. In these nuclei, both the protons and neutrons form filled shells, so that these nuclei all have particularly high binding energies.

    The magic numbers were successfully explained by the nuclear shell model developed in 1948. It deals with the local stabilization of certain proton and neutron configurations brought about by quantum mechanical effects. It took another twenty years to resolve the conflict that arose between the shell model and the classical liquid-drop model. One such issue, according to the liquid-drop model, involved the belief that repulsive electrostatic forces would prevail around element 110 and effectively terminate the periodic table of the elements because heavier nuclei, if formed, should disintegrate into two fragments (Sea 90). The shell model suggested that it should overcome the electrostatic forces and even stabilize heavier elements. Three theoretical papers were presented in the late l960’s : by V.M. Strutinsky, who combined shell- and liquid-drop models; by W.D. Myers and W.J. Swiatecki, who predicted strong shell effects beyond the existing elements; and by H. Meldner, who concluded that 114 should be the next "magic" proton number after 82 (Wal 92).
    Further theoretical studies predicted a whole superheavy "island of stability" around proton number Z=114 and neutron N=184. This was well separated from the "mainland" of known nuclei by the "sea of instability." Element 114 of atomic weight 298 (Z + N) became the Holy Grail for nuclear chemists and physicists.
    In the late sixties, it was recommended that a universal heavy ion accelerator be constructed. Such an accelerator would allow the study of all nuclear reaction that could be involved in the production of SHE. In principle, the method of producing the nuclei is always the same: a beam of accelerated particle from the linear accelerator, or the heavy ion synchrotron, is directed against a foil or a piece of matter, referred to as the target. Nuclear reaction in the target produce exotic nuclei which are then separated by separators according to nuclear charge and mass.
    The universal heavy ion accelerator was only the first step in achieving heavy element synthesis. With its chain of individual resonators , it was now possible to change the energy of the ions by small increments and set the ion energies in a reproducible fashion (Gru 97).
    The second key involved the velocity filter SHIP (Separator for Heavy Ion Products). SHIP had the task of filtering out the rare fusion products. This process was extremely difficult. It meant detecting one superheavy nucleus per day from the flood of more than three thousand billion beam particles and reaction products incident on the filter every second. Figure 4 displays a schematic diagram of the velocity filter SHIP (Mun 81).
    The particle beam coming from the universal heavy ion accelerator is on thin layers of lead or bismuth, which form the outer circumference of the target wheel. The target wheel itself is rapidly rotated to avoid overheating. The target wheel makes is possible to irradiate lead or bismuth metal foils with high currents of particle beams despite their relatively low melting points. Eight target now rotate with 1,125 rpm through the beam.
    The reaction products are identified by a sophisticated detector system which registers the time and the position, measured vertically and horizontally, at which time an ion passing through SHIP is implanted in the detector.
    The fate of the initially implanted new element can be followed step by step through several generations of known radioactive decay products. Each such decay chain has it own fingerprint characterized by the lifetimes of the isotopes and the energies of the particles emitted.
    On November 9, 1994, a German team at Darmstadt beat out the American team in Berkely and the Russian team in Dubna with the creation of element 110. The Germans bombarded a 208Pb target with a beam of 62Ni ions. Four atoms of element 110 with a mass number of 269 were observed. During the subsequent week, isotopes of element 110 were also created. Then on December 8, 1994, the first element of 111 was created using 64Ni projectiles and a 209Bi target. The mass number of the produced isotope was 272. In addition to the Germans discovery of element 111, they observed that the decay chains of 272111 produced new isotopes elements 109 and 107. Figure 5 shows three representative decay chains of the new elements with a -decay energies and lifetime of the chain links indicated (Gru 97).
 

 

On the Road to Element 114
 

 With researchers recent successes , the number of known SHE have nearly doubled. These experiments suggest there are even heavier elements waiting to be discovered, including the "magic" element 114. The whole landscape presented in (figure 6) from Pb to the superheavy elements represents a contour map of the stabilizing shell energy.

It is from this map that predictions can be made beyond element 111. In particular elements 112 and 113 should suffer from destabilization. Heavier isotopes of these two elements would already profit from the superheavy island, but they can not be made with stable isotope targets and projectiles (Gru 97). Element 114 and 115 are already in the shore region of the island of stability, but element 116 will be close to its center (Wal 97). This indicates a more stable chain path may be found with element 116.
    Similar decay properties are calculated for the three succeeding a -decays through elements 114Þ 112Þ 110 down to elements 108 and 106. The chain probably ends at element 104 by spontaneous fission of 266104 (Gru 97). All nuclei of this decay chain are currently unknown, but the half lives and decay properties are unique for the decay of superheavy elements.
    The road to SHE development began nearly 60 years ago with the first human made elements. This research has taken us on many paths and in many directions. Some discoveries have had grave consequences for humanity.  For example, the production and use of plutonium in nuclear weapons, while other discoveries have permitted us to theorize about the unknown, not for profit or weapons but just for a greater understanding.  This purity of thought is akin to why humanity attempts difficult pursuits, because they can.
 
 
 
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