RESEARCH
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observation that signal sequences of secretory proteins are not always exchangeable,
suggesting that at least for some proteins the
signal may carry information necessary for
its correct folding. Only time will tell if this
new dimension to the role of signal sequence
in protein biogenesis can be established. But
for now we must agree that Blobel's findings
are undisputedly deserving of the highest
recognition that the Nobel committee has
bestowed.
Bohrium - A New Element
in the Periodic Table
ber of trans-uranium elements by similar
experiments.
Srinivasan Natarajan
Introduction
The periodic table of elements, the basic and
most important component of research in
chemistry and physics is growing continuously. It is interesting to note that until the
16th century, only a handful of elements
have been known to mankind (10, to be precise) and during the 18th century, 10 more
elements have been identified. It is only in
the 19th century that most of the elements
have been discovered and a form for the
periodic arrangement of the elements has
been proposed (Mendeleyev's periodic table).
The modern periodic table as presented in
Figure 1 was arranged by Moseley in 1914. In
1934, Enrico Fermi proposed that newer elements could be made by bombarding the
atomic nucleus of an element by particles
such as neutrons. Thus, the first man-made
element, technetium (Tc), was discovered in
1936 by bombarding Mo by deuterons. This
was followed by the discovery of a large num-
Utpal Tatu, Department of Biochemistry, Indian
Institute of Science, Bangalore 560 012, India, Tel:
91.80.3092823, Email: [email protected]
The chemistry of the heavy elements (transuranium) requires separations that come to
equilibrium very rapidly, and these must be
valid on an atom-by-atom basis. Such atoms
are created in the laboratory by bombarding
heavy target nuclei with an accelerated beam
of projectile ions. The nuclei of interest,
which are created by the evaporation of few
nucleons are only a very small fraction of the
large number of reaction products produced.
The above process is illustrated in Figure 2.
In such a fusion-evaporation experiment, two
heavy nuclei collide at energies just above
the Coulomb barrier (the energy required to
overcome the electrostatic repulsion between
the two nuclei) forming a fused nucleus. In
Figure 2, a typical example of a reaction of
40Ca incident on 92Mo target nucleus is presented. When such a reaction occurs, the
first step is the nuclei fuse together to form a
compound nucleus with mass 132 which includes 62 protons i.e., an isotope of Ce (Z =
62). Such a system is liable to fission into two
parts (Figure 2a) very rapidly (10-22 s) but, if
it survives, it will now exist for quite a long
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RES lARCH
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WebElements: the periodic table on the world-wide web
http://www.webelements.com/
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time (10-19s). What has been created is effectively a hot, charged, rapidly rotating liquid
drop with nucleons (neutrons and protons)
instead of molecules. Like any hot liquid
drop, it cools down by evaporating particles
(in this case protons, neutrons and a particles). Sufficient particles are evaporated to
reduce the temperature (internal excitation
energy) to the point where no further particles can be emitted. This has reduced the
temperature, but the nucleus is still rotating
very rapidly (Figure 2b). The only way to get
rid of the remaining rotational energy is by
emitting a long cascade of about 40-50 y-rays,
which pass through a series of excited states
until we reach the ground state (Figure 2c).
Since the rates of these transitions are governed by electromagnetic forces the slowest
of them takes approximately 10-9 s. Finally,
because the compound nucleus formed is
inevitably short of neutrons, the nuclei produced following particle evaporation in such
reactions are neutron-deficient and unstable.
The ground state will decay back towards
stability by positron emission or if it is a
heavy nucleus by a-decay or spontaneous
fission (Figure 2d).
The present periodic table ends with the
element 106 named seaborgium (after Glenn
Seaborg). In 1981, the fusion products resulting from collisions between' a heavy-ion
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(a) Fa t fIssion
(eg 40Ca + 92 Mo
116Ce
(b) Compound
+ u + 2p)
nucleus formed
E,oln - 075 MeV
- 2 x 1020 Hz
(1.
p
n
n
(e) Evaporation
of nucleons
and 1996, elements 110-112 have
been discovered, but names for
these elements have not yet been
given. In Figure 3 the scenario
that existed during 1996 is presented. The red dots indicate the
various isotopic species that had
been made, culminating in 277112.
Formation of heavier elements by
this approach (fusion-evaporation)
will require orders of magnitude
increases in accelerator beam currents and greatly improved target
technologies so that they can withstand the higher currents.
)I- Ray
The stability of these artificial elements depends on the number of
neutrons and protons in the
nucleus. Thus, certain isotopes of
(ei) Ground tr
Years
state
Days ve
decay
elements are more stable than othSees
ers. Calcium (40Ca) and lead
(l08Pb) are very stable. The isotope of element 114 with 114 proFigure 2. The fusion-evaporation reaction between
tons and 184 neutrons is expected
40Ca and 92Mo is shown.
to be stable. Thus, the elements
113 onwards have a hidden valley
of stability, so that they could be isolated and
beam from the Universal Linear Accelerator
(UNILAC) and a target of lead or bismuth studied in detail. This prediction is essenlead to the discovery of the element 107. The tially based on the nuclear shell structure of
element was named bohrium (symbol = Bh) these super heavy elements, which would
make them stable for thousands of years rather
after the Danish physicist Niels Bohr. In
addition to Bh, during 1983, other elements than mere seconds or milliseconds, as is the
heavier than Bh were identified including case in many of the trans-uranium elements
Hassium (108, named after the state of (half-lives). In Figure 4, the current status
with respect to the island of stability is preHessen) and Meitnerium (HI9, after Austrian Physicist Lise Meitner). Between 1994 sented. It is clear from Figure 4, that ele-
I
11
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R'ESEARCH I NEWS
110
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GSI
110 1
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190
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Figure 3. The status of experiments towards
heavy atom synthesis (1996).
Figure 4. The current status towards the island
of stability.
ments with extra neutrons are needed in
order to approach closer to the island and
create the much longer-lived isotopes.
wards the formation of heavy elements. These
experiments facilitated the successful isolation of the element Bh.
The Bohrium Story
Thus, recently at the Phillips cyclotron at
PSI, Switzerland the first chemical study of
Bh was carried out. It is to be noted that
elements beyond 100 are .made one atom at a
time with very low production rates and very
short half-lives. In the first attempt at isolating Bh in 1999, a long lived isotope of Bh
with a mass number of 267 was produced by
the reaction between 22Ne ions and a 249Bk
(Berkelium) target. This isotope was found
to have a half-life of about 20 s. This is long
enough to study the chemical properties, especially considering that many elements
above the atomic number 100 have half-lives
in the region of few milliseconds.
There has been some speculation that there
is yet another region of reasonable stability
due to the 'deformed shells' at lower neutron
and proton numbers and the element
Bohrium fits into this region. This, according to the researchers is the beginning of a
long march up the periodic table towards the
island of stability described above. A large
international collaboration of radio chemists
from various research establishments consisting of Paul Scherrer Institute (PSI), Switzerland, the Lawrence Berkeley Laboratory
(LBL), USA, the Flerov Laboratory, Dubna,
Russia, University of Bern, Switzerland, the
Forschungzentrum Rossendorg, Gesellschaft fur Schwerionenforschung (GSI), Germany, Technical University, Dresden, Germany and the Japan Atomic Energy Research
Institute, Japan conducted experiments to-
In a typical experiment a 600 g/Cm 2 target of
249Bk was bombarded with 2 x 10 12 22Ne ions
per sec. The experimental set-up used for
such a study in shown in Figure 5. Immediately after the bombardment, the reaction
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He
f
as oxychlondes:
very volatile: boIIttuM
--=------volatile:
transaCtlOldtls
e. . rutherfOfdium, dubntum, seaoor um
not volatile: actinides
(e.g. fermium , mendelevium ,lawrencium)
<;::::)
Because the positive charge of
a heavy nucleus is so great, the
electronic structure of the atom
is distorted. These so-called
'relativistic effects' can produce
unexpected deviations from
chemical properties extrapolated from the element's lighter
homologues in the periodic
table. Bohrium is found to
behave in similar fashion as a
member of group 7.
Figure 5. A schematic view of the apparatus for bohrium
chemistry, illustrating the various steps that are involved
in producing and isolating the element's oxychloride.
Because of its volatility Bh can be distinguished from its
neighboring elements in the on-line gas chromatogra-
Thus, assuming Bh to be a
member of group 7 in the periodic table, a suitable chemical
isolation procedure can be dephy apparatus (OLGA) and its presence can be deterveloped based on several model
mined from its decay chain in the rotating wheel
experimen ts using the various
multidetector analysis (ROMA).
nuclides of Tc and Re. The
experimental set-up used for
such
studic
is
duplicated for the study of Bh
products were swept into an automated isothermal system called the on-line automated (Figure 5). fhe recoiling reaction products
were thermalized and transported with
gas analyzer (OLGA, see Figure 5), which was
1 litre/min He/C aerosol gas-jet to the reaccapable of measuring the volatility of the pre
tion oven OLGA. A reactive gas mixture of
formed oxy-chlorides. Confirmation of the
50 ml/min HCI and 50 ml/min O 2 was added
presence ofbohrium with single-atom sensiin order to oxidize the C-aerosols and to form
tivity was achieved using a rotating wheel
oxychloride compounds. The various prodmulti detector analyzer (ROMA, Figure 5).
The analyzer was equipped with solid-state ucts of the reaction were separated using
detectors to register both the alpha-particle ROMA. The aerosol particles were impacted
emission and spontaneous fission events,
in vacuum (-5 mbar) on thin (30-40 mg/
cm2 ) polyethylene foils, which were mounted
which are characteristic of the decay of such
heavy nuclei. 267Bh was unambiguously idenon the circumference of the wheel with a stetified by the pattern of its a decay, first to
pping time of lOs. The oxychloride thus
263Db (dubnium), then to 259Lr (lawrencium) formed is volatile at 180°C similar to its lighter
and subsequently to 255Md (raendelevium).
homologues in group 7 of the periodic table
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RESEARCH I NEWS
such as Re (rhenium) and Tc (technetium).
It is to be noted that, the OLGA technique
involves a 'reclustering' step. In reclustering,
the separated volatile compounds pass the
isothermal part of the set-up and are then
reattached to new aerosol particles in order
to transport them with a gas-jet to the detection system. In experiments with Re, Tc and
Bh CsCl/Ar gas-jet (Figure 5) was used.' It is
observed that Bh0 3CI reclusters with CsCI,
similar to Re0 3CI, indicating that Bh is closer
to Re. Tc0 3CI, however, did not form
reclusters with CsCI and could only be
reclustered with FeCl 2 aerosol particles.
Conclusions
As can be seen from the above, due to the
marginally higher half-life of Bh, some experiments could be carried out to find out
about the chemical nature of the element.
Since the elements are produced in extremely
small quantities (a few atoms in many cases),
it is difficult to confirm the stability of most
of these elements. If the half-lives are as long
as predicted (region of stability), and there
are sufficient yields, there are exciting possibilities for chemical studies of the superheavy elements. It islikely that the heavy
elements may also have multiple oxidation
states. In this context, 10-100 atoms of the
super-heavy species are needed depending
on the half-life. If, on the other hand, we are
limited to a few atoms of these super-heavy
species, it is likely that any further use of
them would be very restricted. However, the
Holy Grail of super-heavy elements has been
one in which one began by creating a handful
of atoms and perhaps, one day we may learn
to produce them in sufficient quantity for
real chemical investigations.
For further information
1. PSI Annual report, Jan 2000.
2. http://www.psi.chl
3. http://www.lbl.gov
4. B Gelletly, Chemistry in Britain, p. 40, March 2000.
Srinivasan Natarajan, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced
Scientific Research, Jakkur P.O., Bangalore 560 064,
India, E-mail: [email protected]
Please Note
Resonance, Vol.S, No.3, March 2000, page 20.
Tide: Pollen grains, random walks and Einstein
Author: Sriram Ramaswamy
Third sentence from the end of the last complete para of the published
article: 'less dense' should be read as 'denser'.
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