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| How Heavy Can Atomic Nuclei Be? |
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How Heavy Can Atomic Nuclei Be?
With the synthesis and detection of five new chemical elements—the
elements with atomic numbers 107, 108, and 109 in the years 1981-1984,
and those with atomic numbers 110 and 111 at the end of 1994—a GSI
research group stands uncontested at the forefront of worldwide efforts to
expand the table of the elements upwards. And the possibilities have not yet
been exhausted. Even the "magic" element 114 now appears to be within
reach.
The question concerning the upper limits of stability of atomic nuclei - practically speaking, the
question of the maximum size and weight they may attain - is not only of fundamental
importance for the physics of nuclear structure. It also has immediate significance for our
understanding of the structure of matter in the universe. Answering this question has thus been
one of GSI's major research goals since its inception, and was indeed a major reason for its
founding.
Many properties of atomic nuclei can be described by analogy with a drop of liquid. The "drop
model" of the nucleus, based on this idea, gives reliable predictions of nuclear masses and mean
binding energies. However, it says nothing about the internal arrangement of protons and
neutrons in a nuclear "drop." This inner arrangement essentially determines the properties of a
nuclear system, for example, its exact binding energy. Just like the electron cloud of the atom,
atomic nuclei also exhibit a shell structure, whose arrangement for certain numbers of protons
and neutrons, the so-called "magic numbers," leads to particularly stable configurations.
Significant examples of this phenomenon include the double magic nuclei helium 4, oxygen 16,
calcium 40 and calcium 48, as well as lead 208. In these nuclei, both the protons and neutrons
form filled shells, so that these nuclei all have particularly high binding energies.
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. A doubly magic configuration, similar to lead
208, was anticipated for the isotope 298114 (the superscript indicates the total number of
nucleons, i.e. protons and neutrons), with 114 protons and 184 neutrons in the nucleus. Early
calculations from 1966 predicted an "island of stability" in this region, with the isotope 298114
at its center. This was the birth of the idea of the superheavy elements (SHE), and marked the
start of experimental efforts to synthesize them.
That in short, is how things stood when Christoph Schmelzer recommended the construction
of a universal heavy ion accelerator, the UNILAC, in Germany. Such an accelerator would
allow the systematic investigation of all nuclear reactions that could conceivably produce
superheavy elements. With the founding of the GSI in 1969, the stage was set for German
nuclear physicists to begin research into heavy ions.
After a euphoric beginning, an initial setback
At first it all seemed so simple: the predicted superheavy elements were expected to have a life
span comparable to that of uranium or thorium, and to be producible in macroscopic quantities.
For chemistry, it seemed the door was opening to new compounds; for materials research, the
door to new materials. Atomic physicists hoped for new atoms; nuclear engineers hoped for
new fuels. But by the beginning of the Eighties, after just a few years of worldwide research, it
was clear that the superheavy elements were short-lived and difficult to produce. All attempts
to synthesize them or to find trace amounts of them in geological specimens had met with
failure. As a research area, the superheavy elements seemed to be finished. The finding of a
way to synthesize superheavy elements in the face of these difficulties was and remains one of
GSI's great successes.
UNILAC was the first key to achieving heavy element synthesis. With its chain of individual
resonators - the first time such a concept had been realized in a linear accelerator - it was
possible to change the energy of the ions by small increments and set the ion energies in a
reproducible fashion. The second key was the velocity filter SHIP (Separator for Heavy Ion Reaction Products),
which was built in collaboration with the Second Physical Institute of the University of Giessen
and went into operation at about the same time as UNILAC. SHIP had the task of filtering out
the sought-after, extremely rare fusion products. This was no mean feat given that it meant
detecting about 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.
The experiment
set-up is sketched in the next figure.
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| Using a chain of individual resonators, UNILAC makes it possible to change the energy of the ions by small increments and set ion energies in a reproducible fashion. The photo shows construction work in the Alvarez tank. |
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| The target wheel, which rotates in the projectile beam at high speed, has an outside diameter of 42 centimeters. It is covered with thin layers of lead or bismuth, the target nuclei, mounted on a very thin film of carbon. The superheavy nuclei are created here. Knocked out of the thin film by the impact of the projectile nuclei, they are then selected and analyzed while passing through the SHIP filter. . |
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| The superheavy nuclei created through fusion and then selected from the beam are implanted in a silicon detector and then identified by their decay characteristics. The entire detector system in a temporary plexiglass covering can be seen in the picture above; ordinarily it is enclosed in a steel tank. Below the actual detector is shown in close-up. It is made up of 16 vertical silicon strips, each 35 mm high and 5 mm wide.. |
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| The alpha decay chain that was decisive for the proof of the
discovery of the third nucleus of the new isotope 272111. The event took
place on December 17, 1994 at 6:03 a.m., 25 years to the day after the
founding of GSI. The numbers on the arrows show the measured alpha
energies and the time intervals between the decays. CN stands for the
compound nucleus first produced, which subsequently changed into the
isotope 272111 through the emission of a neutron. The intermediate
products 268109 and 264107 are the heaviest isotopes of those elements. |
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| The successful SHIP group led by Dr. Sigurd Hofmann (center)
and Prof. Peter Armbruster (behind him on the right), pictured here with the
confirmation of the 269110 decay chain. The event itself occurred on
November 9, 1994 at 4:39 p.m.. |
The particle beam coming from UNILAC is incident 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 separation process which subsequently takes place is realized
by a two-stage velocity filter, the principle of which dates back to the German physicist
Wilhelm Wien (1864-1928). The filter employs a combination of electrical and magnetic
deflecting fields which have been arranged so that the electrical and magnetic deflections
cancel each other out for a specified particle velocity - that of the synthesized superheavy
element. In other words, the strengths of the two fields are adjusted so that only the
superheavy nuclei can pass - the thousands of billions of other projectile particles and reaction
products cannot follow this path. In this manner it is possible to find the proverbial needle in
the haystack; only the few candidates representing potential superheavy elements are
winnowed out.
The third key to success was provided by the methods of detection. The velocities of all heavy
nuclei passing through the SHIP are remeasured by means of a time-of-flight array as they
leave the filter. This represents a third velocity measurement, following the repeated speed
selection in the spectrometer. Position-sensitive silicon surface barrier counters, in
which the particles are then implanted, determine each particle's point of impact and energy.
Thanks to this highly sensitive process, even isotopes with extremely low rates of production
can be detected at GSI. At present, only a few nuclear physics spectrometers are capable of
approaching the sensitivity of the SHIP.
For the precise identification of the implanted nuclei, their decay characteristics are determined
with the help of the silicon detectors: the entire decay chain is traced from the implanted parent
nucleus through the daughter isotope down to the granddaughters and great-granddaughters.
The process is referred to as a correlation method, because the decay products of an implanted
parent nucleus have the same spatial coordinates in the counters of the detectors and can thus
be correlated with one another. The decay of a new, previously unknown isotope, for example,
of a superheavy element, must thus be unambiguously correlated with the already known
properties of the following generations of daughter isotopes. Decay chains are observed down
to the fifth generation.
The probability of such an event occurring purely by chance
is smaller than 1 in 1016, or about one billion times less likely than the probability of getting six
numbers right in the lottery. Every event detected in this way is thus fully significant,
amounting to a demonstration of the presence of the sought-after superheavy element.
Gently does it
The triad of UNILAC, SHIP, and decay correlation alone still would not have sufficed to reach
the goal, however. There also had to be a discovery in physics. Even before UNILAC had been
put into operation, a Russian research group led by Yuri Oganessian in Dubna had shown that
in the fusion of double shell-stabilized lead 208 and argon 40, compound nuclei are created
whose excitation energy is very low. This conclusion can be drawn from the small number of
emitted ("evaporated," in physicists' jargon) neutrons. "Soft fusion" is also spoken of in this
connection: since in this kind of reaction the compound nucleus is only slightly heated, such a
reaction approaches ideal nuclear fusion. As we know today, successful synthesis of
superheavy elements depends crucially on a low excitation energy in the compound nucleus
system. In any other circumstances the intermediate nucleus formed divides at once into two
lighter fragments.
The application of this discovery broke down the last barrier on the road to producing the
superheavy elements. First successes in this respect were notched up by GSI researchers in
1980. Through the fusion of lead 208 and titanium 50, they were able to produce the isotope
257104 with the emission of just one neutron. Here, soft fusion technology offered still another
experimental advantage: compared to the radioactive actinide targets used in other synthesis
reaction methods, lead and bismuth are more easily accessible and easier to handle.
Since then, GSI has been a worldwide leader in research into superheavy elements. In the years
from 1981 to 1984, the elements 107, 108, and 109 were discovered. These elements were
later given the names Nielsbohrium (after the Danish physicist Niels Bohr), Hassium (from
GSI's location in the German region of Hesse) and Meitnerium (in honor of the Austrian
physicist Lise Meitner). After several confirmations of the experimental results through
replication of the experiments, improvements to the SHIP detection device were carried out
after 1988. These modifications increased sensitivity by more than an order of magnitude, i.e.
more than a factor of ten.
By 1994 all preparations had been finalized and the hunt for the next heaviest elements could
begin. The first success came on November 9, 1994, when - following soft fusion of lead 208
and nickel 62 with the emission of just one neutron - a nucleus of the isotope 269110 was
identified. Element 110 had finally been discovered, and GSI had won the race against
competing laboratories in Dubna and Berkeley.
On November 23, 1994, the use of
the same recipe with the substitution of nickel 64 projectiles yielded the isotope 271110, two
neutrons heavier. On December 8, 1994, the discovery of element 111 represented the icing on
the cake. The isotope 272111 was created through soft fusion of bismuth 209 with nickel 64
and the emission of one neutron.
All the isotopes of elements 107 through 111 discovered at GSI are, with one exception, alpha
emitters. This means that they decay through the emission of helium nuclei. The observed
chains of alpha decay are a characteristic of the superheavy elements. They are based on a
drastically reduced probability of fission - the consequence of shell stabilization - so that
practically speaking the nuclei can only make the transition to more stable configurations
through alpha decay. In the meantime, thanks to theoreticians we now know substantially more
about the mechanism of shell stabilization. According to theory, it is expected that the nuclei
synthesized in Darmstadt will turn out to be deformed shell-stabilized isotopes. Experimental
proof of the deformation, however, has not yet been forthcoming.
The deformation changes around the neutron number of 170. The nuclei should at this point
take on a ball-shaped structure. For every superheavy element, there are thus lighter isotopes
that are deformed and heavier isotopes that are spherical. The latter should have longer half-
lives. However, because they possess a high number of neutrons they are difficult to obtain
through the fusion of stable isotopes. To date they have not been successfully synthesized.
On the path to element 114
The experiments carried out at the end of 1994 demonstrated the existence of five new
isotopes of elements 107 through 111. As a result, the number of known superheavy elements
has been almost doubled. Moreover, these experiments also pointed the way to even heavier
elements, such as the "magic" element 114. In the production of the isotopes 269110 and
271110 - achieved by fusing lead 208 with nickel 62 and nickel 64 respectively - the addition
of two neutrons for the isotope 271110 increased the measured rate of production by a factor
of four. However, when it comes to producing the next higher element, the probability of
fusion is reduced by approximately the same factor. As a consequence, the production rate of
the isotope 272111 of element 111 - produced by the fusion of bismuth 209 with nickel 64 - remained about the same as that of the isotope 269110.
If nickel 64 is replaced by even heavier projectiles, the reaction mechanism should not change
essentially, and it should even be possible to synthesize elements 113 and 114 with the current
set-up. With the help of the projectiles zinc 70 and germanium 76, the isotopes 278113 and
283114 would then be produced. All these isotopes should decay by alpha decay chains, with
half-lives of less than one millisecond. In the wake of the discovery of elements 110 and 111,
the successful synthesis of elements 112 through 114 would now seem to be well within reach.
A feasible path to the predicted center of the superheavy elements has thus appeared on the
horizon for the first time, some thirty years after this center was first postulated. The
production of element 114 would represent a great triumph for the physics of nuclear
structure, and consequently the coming years will witness substantial research efforts aimed at
achieving this goal.
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