that do not exist in naturethat have been created in a laboratoryare
unstable. After hours or days of one element bombarding another
with enough energy for both to fuse, the resulting new element typically
is born and begins to decay instantly.
Neptunium and plutonium (elements
93 and 94) were the first elements created in a laboratory, at the
University of California at Berkeley in 1940. Scientists have since
fabricated many more elements, each one heavier and with a shorter
half-life than the one before it.
In the 1960s, a few physicists
predicted that some elements around element 114 would survive longer
than any of their synthesized predecessors. Early estimates for
the half-lives of these more stable elements were as high as billions
of years. Later computer modeling reduced the anticipated half-lives
to seconds or minutes before the element began to decay.
Half-lives of seconds or
minutes may seem brief. But consider that various atoms of element
110 created in the laboratory have had half-lives ranging from 100
microseconds to 1.1 milliseconds. The only atom of element 112 that
had been created before 1998 had a lifetime of 480 microseconds.
As described further in the box below, the long-lived nuclei of
elements around element 114 would comprise an island of stability
in a sea of highly unstable elements.
When a collaboration of Russian
and Livermore scientists at the Joint Institute for Nuclear Research
in Dubna, Russia, created element 114 in 1998, the first atom survived
for 30 seconds before it began to decay, a spontaneous process that
leads to the creation of another element with a lower number on
the periodic table. (See the box below for more information on stability
and instability.) A total of 34 minutes elapsed before the final
decay product fissioned, splitting in two the surviving nucleus.
These lifetimes may seem brief, but they are millions of times longer
than those of other recently synthesized heavy elements.
of the voyage to the island of stability.
Since that groundbreaking
effort in 1998, the team has created another atom of element 114.
This one has a different number of neutrons and thus a different
mass, thereby making it a different isotope of element 114. The
team has also created several previously undiscovered isotopes of
elements 112, 110, and 108 to which element 114 decayed. More recently,
the team added element 116 to the periodic table with the creation
of three atoms of the element in a series of experiments.
Nuclear chemist Ken Moody
leads the Livermore portion of the international collaboration.
In 1998, we proved that there really was an island of stability,
he said. We proved that years of nuclear theory actually worked.
The collaboration began in
1989. with heavy element chemist Ken Hulet representing Livermore
and Yuri Oganessian, scientific director of the Flerov Laboratory
of Nuclear Reactions at the Joint Institute, leading the Russians.
In the early 1990s, the U.S.Russian team discovered two isotopes
of element 106, one isotope of 108, and one of 110 at the Dubna
In 1990, when Ron Lougheed,
who has since retired, and I went to Dubna, we were the first U.S.
scientists to perform experiments at that institute, adds
Moody. Remember what was happening then. The Berlin Wall had
just fallen, and Eastern Europe was in turmoil. The early days of
the collaboration were definitely interesting.
Primer on Stability and Instability
Why should element 114 be so much more stable and long-lived
than so many of its synthesized predecessors? The answer
lies in basic chemistry.
The nucleus of
an atom is surrounded by one or more orbital shells of
electrons. The electron configurations of atoms of the
many elements vary periodically with their atomic number,
hence the periodic table of the elements.
unfilled shells seek out electrons in other elements to
fill them. These include carbon, oxygen, and all of the
reactive elements that want to react with
other elements. This is the basis of covalent bonding.
The noble gases (on the far right column of the periodic
table) have a completely filled outer electron shell and
hence are highly stable. They are termed noble because
they are aloof, with no desire to react with
Protons and neutrons
are in analogous shells within the nucleus. The proton
shells of helium, oxygen, calcium, nickel, tin, and lead
are completely filled and arranged such that the nucleus
has achieved extra stability. The atomic numbers of these
elements2, 8, 20, 28, 50, and 82are known
as magic numbers. These same numbers plus
126 are magic numbers for neutrons. Notice that the magic
numbers are all even. No truly stable element heavier
than nitrogen has an odd number of both protons and neutrons.
Elements with even numbers of protons and neutrons make
up about 90 percent of Earths crust.
A nucleus is doubly
magic when the shells of both the protons and neutrons
are filled. Lead-208 has 82 protons and 126 neutrons,
both of which are magic numbers. Lead-208 is thus doubly
magic and seems to be virtually eternal.
stable element such as lead does not decay. However, all
elements with an atomic number greater than 83 (bismuth)
exhibit radioactive decay. Decay may happen in several
ways. For heavy elements, an unstable or radioactive isotope
usually decays by emitting helium nuclei (alpha particles)
or electrons (beta particles), leaving a daughter nucleus
of an element with a different number of protons. This
process typically continues until a stable nucleus is
reached. Plutonium, for example, decays ultimately to
The heavy elements
that have been created in the laboratory are so unstable
that they decay almost immediately and have extremely
short half-lives and thus lifetimes. How quickly a particular
isotope decays is measured by its half-life. Plutonium-239,
which decays very slowly, has a half-life of about 24,000
years, while plutonium-238s half-life is just 88
years. Half-lives are a result of a statistical process.
If an experiment produces only one atom, then a half-life
cannot be determined. Thus, with one or a few atoms, scientists
talk instead about lifetimes.
In the mid-1960s, a physicist in the U.S. predicted
that the next magic proton number above 82 would be
114, not 126, and that an atom with a doubly magic nucleus
of 114 protons and 184 neutrons should be the peak of
an island of stability. Russian scientists had come
to the same conclusion at about the same time.
In the years
since, increasingly sophisticated computer models have
indicated that element 114 would exhibit significant
nuclear stability even with neutron numbers as low as
175. Note that element 114 is expected to lie in the
same column (or group) of the periodic table as lead.
The two elements are expected to share many properties.
of deuteriumtritium fusion fuel would produce
the same energy as 30 million kilograms of coal. Other
major advantages include no chemical combustion products
and therefore no contribution to acid rain or global
warming, radiological hazards that are thousands of
times less than those from fission, and an estimated
cost of electricity comparable to that of other long-term
theory (top) in the U.S. and (bottom) in Russia, in
Days and Nights
flood could have come and gone in the time it took the collaboration
to create the first atom of element 114. For 40 days of virtually
continuous operation, calcium ions bombarded a spinning target of
plutonium in Dubnas U400 cyclotron. While the first atom of
element 114 was actually created on November 22, 1998, Russian researchers
discovered it in data analysis and communicated the news to Livermore
on December 25, 1998quite the Christmas present.
The box below shows the recipe
for the early Dubna experiments that created isotopes of element
114. Plutonium, with an atomic number, or Z, of 94, and calcium,
Z = 20, add up to the necessary Z = 114. By fusing plutonium-244,
an isotope of plutonium with 150 neutrons, and calcium-48, a neutron-rich
isotope with 28 neutrons, a compound nucleus with 114 protons and
178 neutrons (150 + 28) would in theory be possible. In fact, however,
when the plutonium-244 and calcium-48 nuclei collide with enough
energy to overcome their mutual electrostatic repulsion, the compound
nucleus has excess energy. A few neutrons evaporate to de-excite
the nucleus and produce an isotope with 175 neutrons.
To discover whether new
elements were created by the bombardment of plutonium, the team
was interested in finding events comprising a series
of alpha decays ending with spontaneous fission. In alpha decay,
an isotope loses an alpha particle, which is two protons and two
neutrons (or a helium nucleus). For example, an atom of element
114 with 175 neutrons (described as isotope 114-289) would emit
an alpha particle, thereby becoming isotope 112-285, having lost
2 protons and 2 neutrons. The atom of 112-285 would become 110-281,
which would become 108-277. At some point, fission would occur,
ending the process. At the same time, however, unwanted nuclei generated
by the experiment also undergo alpha decay and fission, mimicking
the decay sequence of element 114. Trillions of these unwanted nuclei
are produced every day, whereas the expected production rate for
an element 114 isotope was much less than one atom per day. To deal
with the problem of unwanted nuclei in earlier experiments, Dubna
scientists had developed a gas-filled mass separator to separate
unwanted nuclei from the desired ones. It worked marvelously,
Heavy-element reaction products
recoil from the spinning plutonium target wheel and enter the mass
separator, a chamber filled with low-pressure hydrogen gas confined
between the pole faces of a dipole magnet. The magnetic field is
adjusted so that, for the most part, only the nuclei of interest
pass through to the detector array.
The desired nuclei are focused
with a set of magnetic quadrupoles, pass through a time-of-flight
counter, and are captured by a position-sensitive detector. A signal
from the time-of-flight counter allows the team to distinguish between
the effect of products passing through the separator and the radioactive
decay of products that are already implanted in the detector. The
flight time through the counter is also used to discriminate between
low- and high-Z products, because heavier elements travel more slowly.
The position-sensitive detector lowers the rate of background interference,
allowing scientists to identify and ignore unwanted products.
During 40 days in November
and December 1998, with ten-thousand trillion ions per hour of calcium-48
bombarding the plutonium target, the team observed the signals of
just three spontaneous fission decays. Three synthesized compound
nuclei had been created and passed through the separator before
fissioning. Two of them lasted about 1 millisecond each and proved
to be products from the decay of the nuclear isomer of americium-244.
Only one of the events involved
an implant in the detector followed by three alpha decays in the
detector array. This isotope of element 114 (114-289) had a lifetime
of 30.4 seconds. It decayed to element 112, which, with a lifetime
of 15.4 minutes, decayed to element 110. Element 110, with a lifetime
of 1.6 minutes, then decayed to element 108, which decayed by spontaneous
In 2000 and 2001, the collaboration
performed three experiments in which a curium-284 target was bombarded
with calcium-48 ions to create element 116. The team chose this
combination of isotopes because they would produce isotopes of element
116 that should decay to the previously observed isotopes of element
Researchers produced the
super-heavy isotope 116-292 once in each of these experiments. They
also created some other isotopes repeatedly. Isotopes 114-288, 112-284,
and 110-280 have been found five times, lending credibility to several
experimental results. However, the first atom of 114-289 with the
30.4-second lifetime has yet to be replicated.
The Dubna gas-filled separator uses a combination of chemistry
and physics to suppress the products of unwanted reactions.
(b) The high-efficiency detector.
the Final Analysis
The recipe for element 114
in the box below refers to the analysis of 7 gigabytes of data from
the first experiments. The team has since accumulated another 23
gigabytes of data, all requiring extensive analysis to verify the
times and energies of the alpha decays. Valid decay sequences must
fall within the alpha decay time and energy parameters of what is
known as the GeigerNuttall relationship.
Scientists at Livermore and
Dubna analyzed the data in parallel. Livermore gave the Dubna institute
a computer workstation for the Russian scientists to use on that
mountain of information. Nuclear chemists John Wild and Nancy Stoyer
analyzed the data at Livermore. These duel analyses were independent
but were calibrated. In the end, our results agreed, says
The team must also confirm
that the sequences they saw were not composed of random events.
The problem of randomness is real, especially for long-lived
elements, adds Wild. The longer the lifetime of a member
of a decay sequence, the greater the probability that the decay
could be random.
A novel Monte Carlo method
to estimate the probability of whether a decay chain was random
or the real thing was the brainchild of nuclear chemist Mark Stoyer.
It is a pseudo-random number generator that places random fission
events into the real data throughout the duration of the experiment.
Nancy Stoyer developed the search code that sifted the data, including
Monte Carlogenerated random fissions, for decay sequences
similar to the 114-289 decay sequence that had been observed experimentally.
Because the actual decay
chains end with a spontaneous-fission event, Nancy Stoyers
search algorithm looks backward from the planted fission event for
candidate alpha-decay chains that match actual decay chains and
end with a fission event. The number of returned accidental
decay chains defines the probability that a decay sequence is random.
For the first atom of element 114, the random probability was 0.6
percent. If we eliminate decay chains in which all alpha events
do not meet the GeigerNuttall relationship, says Moody,
the random probability falls to 0.06 percent. Thats
Stormy Voyage to the Island of Stability
of November 2001, scientists throughout the world had
synthesized 20 elements that do not exist in nature. The
ones up to meitnerium (109) have been given official names.
Elements 110, 111, 112, 114, and 116 will not be named
until their existence has been corroborated with several
experiments or by several different groups. Recall that
one of the fundamental tenets of science is reproducibility.
In 1940, Ed McMillan
and his team at Berkeley bombarded uranium with neutrons
to create neptunium (element 93). Then Glenn Seaborg and
his colleagues created plutonium-238, the first isotope
of plutonium (element 94), through the decay of neptunium-238,
which they produced by bombarding uranium with deuterium
(heavy hydrogen). Elements 99 and 100 were discovered
in the debris of the first hydrogen bomb test in 1952
from the simultaneous capture of many neutrons by uranium.
The heavy, highly radioactive uranium isotopes decayed
quickly by beta emission down to more stable isotopes
of elements 99 (einsteinium) and 100 (fermium). Elements
95, 96, 97, 98, and 101 were created by irradiating heavy
nuclei with beams of alpha particles to boost the atomic
numbers two steps at a time.
Beginning in the
late 1950s, the new particle accelerators were capable
of accelerating ions heavier than helium. First, ions
of the lightest elements were directed at the heaviest
elements. But it took excess energy to cause them to fuse,
producing a very hot nucleus that tended to fission almost
immediately. Known as hot fusion, this method
yielded elements 102 through 106 by 1974. Many of these
experiments included Livermore scientists.
In 1974, Yuri
Oganessian at the Joint Institute at Dubna found that
if heavier ions are directed at lead and bismuth, less
energy was needed to create new elements. These two elements
are extra-stable, and thus the resulting compound nucleus
has less energy and is more likely to remain intact. This
process is known as cold fusion, not to be
confused with the discredited fusion energy process of
the same name. Even with cold fusion, so few nuclei of
the new element are produced during an experiment that
existing detection techniques were not sensitive enough
to find them.
The field of synthesizing ever heavier elements went
on hiatus for several years until sophisticated new
separation and detection methods were developed in the
early 1980s in Germany. German researchers were then
able to create and detect elements 107, 108, and 109
in experiments that have since been corroborated such
that these synthetic elements now have names. They also
created isotopes of 110, 111, and 112, but these results
have not yet been fully corroborated.
The German group,
the Consortium for Heavy Ion Research at Darmstadt,
Germany, has produced an isotope of element 112 that
decayed into the same isotope of 110 that the DubnaLivermore
team found in 1994. This isotope had the same energy
and lifetime, which is encouraging validation.
The voyage to
the island of stability has been a stormy one. It took
until 1998 to even reach the beach. As shown in the
figure below, the islands peak is still tantalizingly
just out of reach.
Element 114 in context—on the beach of the island of stability.
for a New Element
Livermore chemist with a sense of humor developed this
recipe to describe the creation of element 114.
2 grams calcium-48, a rare neutron-rich isotope
of calcium. Out of every 100,000 atoms of calcium, only
187 atoms are calcium-48.
30 milligrams plutonium-244, the most neutron-rich,
long-lived isotope of plutonium. The worlds supply
of this isotope is only 3 grams.
The U400 cyclotron at Dubna, Russia, to accelerate
calcium ions to 10 percent the speed of light (236 megaelectronvolts).
A gas-filled recoil separator for removing unwanted
A position-sensitive detector for capturing decays
of reaction products.
2 computers, one for data acquisition and another
for data analysis.
Numerous Russian technicians and accelerator operators.
19 Russian scientists.
5 American scientists.
the first seven ingredients, using 0.3 milligrams per
hour of calcium-48. Add lots of patience, a dash of luck,
and a dollop of inspiration. Simmer for about 6 months,
24 hours per day, 7 days a week. Use the last two ingredients
to analyze 7 gigabytes of data for signature decay sequences
of element 114. Garnish with several papers describing
Serves: Very few.
In two experiments, makes one atom of 114-289, the lifetime
of which is 30 seconds, and two atoms of 114-288, each
with a lifetime of 2 seconds.
Elements Still to Come
The Livermore researchers
are continuing its work to explore the southwest shores of the island
of stability. With funding from the Laboratory Directed Research
and Development program, they have begun efforts to add elements
115 and 113 to the periodic table. They are in the process of sending
22 milligrams of pure americium-243 to Dubna for the work on element
Current exploration of the
island of stability, or its beaches, is limited to stable targets
and projectile beams. There exists no suitable combination of projectile
and target to produce 114-298, the long-predicted highly stable
isotope. The isotopes 114-289 and 114-288 require the most neutron-rich
isotopes of plutonium and calcium. In the future, when radioactive
beam accelerators are capable of producing intense beams of even
more neutron-rich isotopes, researchers may venture farther toward
the center of the island. For example, calcium-50 has a half-life
of 14 seconds, far too short to gather material together to put
into a conventional ion source. However, plans are for a radioactive
beam facility to produce calcium-50 and accelerate it to energies
required for the experiments well before it can decay. Thus, an
isotope of element 114 with a mass of 290 or 291, two neutrons closer
to the center of the island, may well be possible.
As scientists continue to
explore for new elements, they expect that more spherical and longer-lived
isotopes will be produced, which will most certainly require more
sensitive detection schemes. Challenges abound.
schematic representation of the decay sequence of the first
atom of element 114, isotope 114-289.
Livermore researchers also
want to study the chemical properties of elements 112 and 114. The
combination of chemical and nuclear properties defines the usefulness
of any nuclide. Most heavy elements exist in such small amounts,
or for such short times, that no one has pursued practical applications
for them. However, several heavy elements do have usesamericium
is used in smoke detectors, curium and californium are used for
neutron radiography and neutron interrogation, and plutonium is
elemental in nuclear weapons. Although elements 114 and 116 have
no immediate use, they do exist, and more of them can be manufactured
when uses for them are found. Adds Moody, Showing that the
isotopes of element 114 produced by the collaboration have unique
chemical properties will also provide proof that they are indeed
a new element.
Key Words: element
114, element 116, heavy elements, island of stability, Joint Institute
for Nuclear Research in Dubna, Russia.
information contact Ken Moody (925) 423-4585 (firstname.lastname@example.org).