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Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19

214 Actinide Elements

Alfred Nobel was immediately accepted by the IUPAC. is reduced by 2, the mass number by 4. With emission
However, experiments at Berkeley and the Kurchatov of a β − particle, the mass number remains unchanged,
Institute in Moscow showed that the original Swedish whereas the atomic number increases by 1. As a result,
claim to have prepared element 102 was in error. Attempts in these decay series the mass number can differ only by
to synthesize and identify isotopes of element 102 in multiples of 4 and there are four such families, desig-
heavy ion bombardments of actinide targets dragged on nated 4n + 0 (thorium series), 4n + 1 (neptunium series),
for many years at the laboratories in Berkeley and Dubna, 4n + 2 (uranium or uranium-radium series), and 4n + 3
Russia. Thus, scientists from Berkeley suggested that the (actinium series). The neptunium series is missing in na-
credit for the discovery should be shared. But, in 1993 the ture. It was probably present in nature for some million
IUPAC-IUPAP Transfermium Working Group concluded years after the genesis of the elements, but decayed due to
that the Dubna laboratory finally achieved an undisputed the relatively short half-life of 237 Np, compared with the
9
synthesis. age of the Earth (about 5 · 10 years). Each series contains
Also, the discovery of element 103, the last actinide el- a number of short-lived nuclides, and the final members
ement, was contested by Berkeley and Dubna for a long of each series are stable nuclides. α Decay is the domi-
time. At Berkeley mixtures of californium isotopes were nant decay mode of long-lived heavy nuclei with atomic
bombarded with boron ions, whereas at Dubna the bom- numbers Z > 83. With increasing atomic numbers spon-
238
bardment of americium targets with oxygen ions was ap- taneous fission begins to compete with α decay. For U
−4
plied. Finally, both groups accepted the conclusion of the the probability of spontaneous fission is about 10 %of
256
Transfermium Working Group, that full confidence was that of α decay and is already about 90% for Fm.
built up over a decade with credit for discovery of ele- The radioactive decay is the simplest form of a nuclear
ment 103 attaching to work in both Berkeley and Dubna. reaction according to equation [Eq. (6)]:
The name lawrencium after E. O. Lawrence, the inventor A → B + x +
E. (6)
of the cyclotron, suggested by A. Ghiorso and co-workers
from Berkeley and accepted by IUPAC, was finally rec- This is a mononuclear reaction. In nuclear science, how-
ommended by IUPAC in 1997 together with the names for ever, binuclear reactions are generally understood by the
the transactinide elements up to element 109. term “nuclear reaction.” They are described by the general
Table I summarizes the discovery or synthesis of all of equation [Eq. (7)]:
the actinide elements. A + x → B + y +
E, (7)

where A is the target nuclide, x is the projectile, B is the
product nuclide, and y is the particle or photon emitted.
II. RADIOACTIVITY AND NUCLEAR Equations (3)–(5) are examples for neutron- and deuteron-
REACTIONS OF ACTINIDES induced nuclear reactions. With heavy ions (heavier than α
particles) as projectiles, the heaviest actinides have been
All isotopes of the actinides and actinium are radioac- synthesized. Targets made from heavy actinide nuclides
tive. Table II presents data on several of the most avail- such as 248 Cm and 249 Bk have been used to synthesize
able and important of these. The unstable, radioactive ac- several transactinide elements in heavy-ion reactions.
tinide nuclei decay by emission of α particles, electrons, Nuclear fission of actinides is, without doubt, the most
−
+
or positrons (β or β decay, respectively). Alternatively important nuclear reaction. Nuclear fission by thermal
to the emission of a positron, the unstable nucleus may neutrons may be described by the general equation
capture an electron of the electron shell of the atom (sym- [Eq. (8)]:
bol ε). In most cases the radioactive decay leads to an
A + n → B + D + νn +
E. (8)
excited state of the new nucleus, which gives off its excita-
tion energy in the form of one or several photons (γ rays). The fission products B and D have mass numbers in the
In some cases a metastable state results that decays in- range between about 70 and 160, the number of neutrons
dependently of the way it was formed. Spontaneous fis- emitted is ν ≈ 2–3, and the energy set free by fission is
sion (symbol sf) is another mode of radioactive decay,
E ≈ 200 MeV. This energy is relatively high, because
which was discovered in 1940 by G. N. Flerov and K. A. the binding energy per nucleon is higher for the fission
Petrzhak. products than for the actinide nuclei. In the case of nu-
Thenumerousradionuclidespresentinthoriumandura- clei with even proton and odd neutron numbers, such as
nium ores are members of genetic correlated radioactive 233 U, 235 U, and 239 Pu, the binding energy of an additional
decay series, which are represented in Fig. 1. In all of neutron is particularly high, and the barrier against fission
these decay series, only α and β − decay are observed. is easily surmounted. Therefore, these nuclides have high
4
With emission of an α particle ( He), the atomic number fission yields for fission by thermal neutrons.
2