From Physics.com:
Viewpoint: Heavy into Stability
In 1940, the first synthetic element heavier than
uranium—neptunium-239—was produced by bombarding uranium with neutrons.
Since then, nuclear scientists have ventured into the search for new
heavy elements, expanding the frontiers of the physical world. The
creation of elements with atomic number beyond that of uranium is
challenging, as the half-life of elements decreases with increasing
atomic number. However, nuclear theories have predicted that a so-called
“island of stability” exists for certain superheavy elements of the
nuclide chart, which should have half-lives ranging from minutes to many
years.
The search for this island of stability has led to the
creation of elements with up to 118 protons. The last element to be
discovered was 117 [
1] (see
9 April, 2010 Viewpoint), filling in the final gap on the list of observed elements up to element 118. Now, writing in
Physical Review Letters,
Yuri Oganessian at the Joint Institute for Nuclear Research (JINR),
Russia, and colleagues report on a second production campaign for
element 117 [
2],
which verifies their initial findings and provides a new comprehensive
characterization of the decay chains of two isotopes of the 117 element.
Their results confirm that we are indeed approaching the shores of the
island of stability.
The stability of nuclides is a function of proton (
) and neutron (
)
number, as illustrated in Fig. 1. A connected region (“continent”) of
stable elements is found for lighter elements, ending at the
lead–bismuth “cape.” All elements with an atomic number exceeding 82
(lead) are unstable, with decreasing half-life for higher atomic
numbers. However, a first region of relative stability appears around
the isotopes of thorium and uranium (
equal to 90 and 92, respectively) whose lifetimes are comparable with
the age of the universe. Elements with atomic number greater than that
of uranium (transuranium elements) have only been produced in laboratory
experiments (see the historical review in Ref. [
3]).
The progress in this field is impressive: 26 new, manmade heavy
elements have been synthesized within 60 years. Some of these elements
(up to californium) can be produced in macroscopic quantities in nuclear
reactors, using neutron capture processes to form heavier elements from
actinides.
Elements beyond uranium should become more and more
unstable as they get heavier, as Coulomb repulsion starts to be stronger
than the strong force that holds the nucleus together. But in the late
sixties, Glenn T. Seaborg postulated the existence of a relatively
stable region of superheavy elements, an island of stability. This idea
is based on the nuclear shell model, which describes the atomic nucleus
as made of shells, similar to the well-known electronic shell model for
atoms. Nuclear theorists, including myself [
4,
5],
predicted that the stability of nuclei with so-called closed proton and
neutron shells should counteract the repelling Coulomb forces. In
isotopes with so-called “magic” proton and neutron numbers, neutrons and
protons completely fill the energy levels of a given shell in the
nucleus. Those particular isotopes will have a longer lifetime than
nearby ones. According to theory, this second island of stability should
be located around proton number
or
and neutron number
.
Reaching this island of stability would open new horizons in nuclear
physics and technology, enabling the production of superheavy nuclides
in macroscopic quantities and with sufficiently long half-life to carry
out actual experiments. This would allow us to test our understanding of
nuclear matter and to possibly exploit such long-lived elements for
applications in medicine or chemistry.
Substantial progress in the synthesis of superheavy
nuclei was achieved at the GSI Helmholtz Centre for Heavy Ion Research
in Germany, where the elements with
to
have been synthesized for the first time in fusion reactions of heavy
projectiles (from iron to zinc) with lead and bismuth targets [
6].
Unfortunately, in these projectile–target combinations only the
proton-rich isotopes of superheavy elements with very short half-lives
can be produced, as they lie outside the island of nuclear stability.
Within the last ten years, researchers at JINR have successfully
synthesized six new heavy elements with
–
by following a different approach: instead of a heavy projectile, a
high-intensity beam of lighter atoms (calcium-48) is aimed at heavy
actinide targets made of uranium or transuranium elements. The use of
neutron richer calcium-48 allows the synthesis of nuclides with neutron
number closer to that needed for stability.
Up until 2010, there was a gap between elements 116 and
118. The obstacle towards the production of element 117 was that the
appropriate target material, berkelium-249 (
) with 97 protons, has a short half-life of only 330 days. In 2009, several milligrams of
were produced at Oak Ridge National Laboratory in the US—enough to
prepare a target and to perform the first experiment for the synthesis
of element 117 at JINR [
1]. In early March of 2012, a new portion of
,
,
was shipped again from Oak Ridge to JINR, where physicists started the
second production campaign for the synthesis of element 117.
The results of this campaign, reported in the paper by Oganessian
et al. [
2],
confirm that a reliable method for the production of 117 exists. The
authors can now state with confidence that two isotopes of this element,
and
, have been synthesized and provide a comprehensive characterization of their decay properties. Two decay chains of
and five decay chains of
were detected. Oganessian
et al. also observe a concomitant decay chain of element 118. This occurs because, at the time of the experiment, part of the
target material had already decayed into californium-249, which can
generate element 118 in a fusion reaction with calcium-48. The measured
lifetimes of the 117 isotopes and other elements along its decay chain
are long, lying in the millisecond-to-second range. This is consistent
with shell-model predictions, confirming that these elements are indeed
located at the southwest shores of the island of stability. The
consistent results emerging from the two productions campaigns at JINR
may get the authors close to laying claim on naming the new element.
What are the prospects of reaching deeper into the center
of the island of stability? Although fairly long lived, the isotopes of
superheavy elements produced in the experiments with calcium-48 are
still neutron deficient: each isotope needs six to eight more neutrons
to lie within the island. This occurs because heavier stable atoms must
have a larger neutron/proton ratio that lighter atoms. Thus creating a
heavy atom by fusion of two lighter ones inevitably leads to an atom
that has too few neutrons and too many protons to be stable. One would
then deduce that there is no way to the island of stability. However,
pathways towards the center of the island of stability may exist. Recent
theoretical studies carried out in my research group suggest that
superheavy nuclei located at the top left side of the island of
stability, formed in ordinary fusion reactions, could get rid of excess
protons via
decay [
7].
Other alternatives to get to the right neutron number might exploit
neutron capture, rather than fusion: such techniques would require the
exposure of heavy elements, such as uranium, to very high neutron
fluxes. Theory shows that this could be achieved in hypothetical
small-scale underground nuclear explosions [
8]
or by using pulsed nuclear reactors of the next generation, if their
neutron fluence per pulse is increased by about three orders of
magnitude.
While the island of stability is now more firmly in
sight, the jury is still out on what navigation plan will turn out to be
successful.
References
- Yu.Ts. Oganessian et al., “Synthesis of a New Element with Atomic Number Z=117,” Phys. Rev. Lett. 104, 142502 (2010).
- Y. T. Oganessian et al., “Production and Decay of the Heaviest Nuclei 293,294 117 and 294 118,” Phys. Rev. Lett. 109, 162501 (2012).
- G. T. Seaborg and W. D. Loveland, The Elements Beyond Uranium (John Wiley and Sons, New York, 1990)[Amazon][WorldCat].
- S. G. Nilsson, S. G. Thompson, and C. F. Tsang, “Stability of Superheavy Nuclei and Their Possible Occurrence in Nature,” Phys. Lett. 28B, 458 (1969).
- U. Mosel and W. Greiner, “On the Stability of Superheavy Nuclei Against Fission,” Z. Phys. A 222, 261 (1969); Also in the Proposal for the Establishment of GSI: Frankfurt-Darmstadt-Marburg (1967).
- S. Hofmann and G. Munzenberg, “The Discovery of the Heaviest Elements,” Rev. Mod. Phys. 72, 733 (2000).
- V. I. Zagrebaev, A. V. Karpov, and W. Greiner,
“Possibilities for Synthesis of New Isotopes of Superheavy Elements in
Fusion Reactions,” Phys. Rev. C 85, 014608 (2012).
- V. I. Zagrebaev, A. V. Karpov, I. N.
Mishustin, and W. Greiner, “Production of Heavy and Superheavy
Neutron-Rich Nuclei in Neutron Capture Processes,” Phys. Rev. C 84, 044617 (2011).