transuranium elements

chemical elements beyond uranium in D. I. Mendeleev’s periodic system of the elements, that is, elements with atomic number (Z) greater than or equal to 93. Fourteen transuranium elements are known.

Owing to the relatively rapid rate of their radioactive decay, these elements have not been retained in the earth’s crust in significant amounts. The age of the earth is about 5 × 10⁹ years (yr), while the half-life (T_1/2) of the longest-lived isotopes of the transuranium elements is less than 10⁷ yr. In the period since the formation of the earth, transuranium elements, which were synthesized by nuclear reactions, have either completely decayed or have greatly reduced in quantity (by a factor up to 10¹²). Microscopic amounts of ²⁴⁴Pu, the longest-lived transuranium element (T_1/2 ~ 8 × 10⁶ yr), have been found in natural ores. This isotope possibly has remained in the earth since the time of the earth’s formation. Uranium ores have yielded traces of ²³⁷Np (T_1/2 ~ 2.14 × 10⁶ yr) and ²³⁹Pu (T_1/2 ~ 2.4 × 10⁴ yr), which were formed as a result of atomic reactions with the participation of uranium nuclei.

The first transuranium elements were synthesized in the early 1940’s in Berkeley (USA) by a group of scientists under the direction of E. McMillan and G. Seaborg, both of whom received a Nobel Prize for the discovery and study of these elements. There are several methods of synthesizing transuranium elements, all of which essentially reduce to the bombardment of targets with beams of neutrons or charged particles. When uranium is the target, powerful neutron beams produced in nuclear reactors or by the explosion of nuclear devices may be used to obtain the transuranium elements through fermium (Z = 100) inclusively. The synthesis process consists either in (1) the consecutive capture of neutrons, with each capture step accompanied by an increase in the mass number A, leading to beta decay and an increase in nuclear charge Z, or (2) the instantaneous capture of a large number of neutrons (by way of an explosion), with a long chain of beta-decay steps. The possibilities of this method are limited because of the insufficient density of neutron beams, the low probability of capturing a large number of neutrons, and, most important of all, the very rapid radioactive decay of nuclei with Z > 100, which precludes the obtainment of nuclei with Z > 100.

The element with Z = 101, Mendelevium, was discovered in 1955 by the bombardment of ²⁵³₉₉Es with accelerated alpha particles. Five elements with Z > 101 were obtained in charged particle accelerators (the cyclotron of the Joint Institute for Nuclear Research at Dubna, USSR, and the Heavy Ion Linear Accelerator at Berkeley, USA) through nuclear reactions with accelerated heavy ions. The scientific groups under the direction of G. N. Flerov at Dubna and of Seaborg and A. Ghiorso at the Lawrence Radiation Laboratory in Berkeley have made the major contributions in this work. Significant results have also been obtained at the Oak Ridge National Laboratory in the USA.

Two types of nuclear reactions, namely, fusion and fission, are used for the synthesis of the heavier transuranium elements. In the first case, the nuclei of the target and the accelerated ion fuse completely and the excess energy of the excited compound nucleus is eliminated by neutron “evaporation.” Upon using ions of carbon, oxygen, and neon and targets of plutonium, curium, and californium, a highly excited compound nucleus is formed with excitation of about 40–60 megaelectron volts (MeV). Each evaporated neutron is capable of removing, on the average, energy of the order of 10–12 MeV from the nucleus, and consequently up to five neutrons must be ejected to “cool” the compound nucleus. For elements with Z = 104–105, the probability of the evaporation of one neutron is 500–100 times less than the probability of fission. This accounts for the low yield of new elements: the fraction of nuclei that “survive” as a result of the loss of excitation is only 10^–8–10^–10 of the total number of target nuclei that fuse with the particles. This accounts for the synthesis of only five new elements (Z = 102–106) in the past 20 years.

A new method, based on nuclear fusion, has been developed at the Joint Institute for Nuclear Research for the synthesis of transuranium elements. In this method, densely packed stable nuclei of lead isotopes are used as the target, and the relatively heavy ions of argon, titanium, and chromium as the bombarding particles. The excess energy of the ions is used for the “unpacking” of the compound nucleus; the excitation energy is low, only 10–15 MeV. The evaporation of one or two neutrons is sufficient to eliminate the excitation of such a nuclear system. As a result, a very noticeable gain in the yield of new transuranium elements has been obtained. This method was used to synthesize the transuranium elements with Z = 100, Z = 104, and Z = 106.

In 1965, Flerov proposed the method of induced fission using heavy ions for the synthesis of transuranium elements. Fragments resulting from such nuclear fission have a symmetrical distribution relative to mass and charge with a large dispersion; thus, elements with Z significantly greater than half the sum of Z of the target and Z of the bombarding ion may be observed in the fission fragments. The distribution of fission fragments was found experimentally to become broader with the use of heavier particles. The use of accelerated xenon or uranium ions would permit the production of new transuranium elements as heavy fission fragments upon the bombardment of uranium targets. In 1971 xenon ions were accelerated using two cyclotrons at the Joint Institute for Nuclear Research for the bombardment of a uranium target. The results indicated that this new method is suitable for the synthesis of heavy transuranium elements.

Transuranium elements undergo all types of radioactive decay. However, electron capture and beta decay are relatively slow processes, and their role becomes minor in the decay of nuclei with Z > 100, which have short lifetimes with respect to alpha decay and spontaneous fission. With increasing atomic number, the competition between spontaneous fission and beta decay becomes more significant. Instability with respect to spontaneous fission apparently determines the limit of the periodic system of the elements. While the half-life of ₉₂U in spontaneous fission is about 10¹⁶ and of ₉₄Pu about 10¹⁰ yr, the half-life for ₁₀₀Fm is measured in hours, the half-life of element 104 (seeKURCHA-TOVIUM) is measured in seconds, and the half-life of element number 106 is only several milliseconds. (SeeACTINIDES for the chemical properties of transuranium elements to Z = 104 and the structure of their electron shells.)

Theory suggests that the existence of very heavy nuclei with enhanced stability with respect to spontaneous fission and alpha decay is possible. This “island of stability” should be located near the magic number nucleus, which has 114 protons and 184 neutrons. If this hypothetical region of stability is found to be real, then the limits of the periodic system of the elements will be expanded significantly, to which end work is under way in search of new experimental methods that would make possible the discovery of new elements. It is relatively simple to obtain 114 protons in a new nucleus but difficult to obtain 184 neutrons. Failure to reach the magic number 184 even by a few units sharply decreases the stability of a nucleus with respect to spontaneous fission.

Analysis of the fission barriers and lifetimes of the superheavy elements have indicated that some such elements, may have a half-life of about 10¹⁸ yr, and microscopic amounts of these elements may have been retained on the earth to the present time. In 1968 work was begun under the direction of Flerov to search for superheavy elements in nature. Terrestrial minerals, products of volcanic eruption, and geothermal waters are being investigated, as well as substances capable of accumulating the heavy component of cosmic rays (iron-manganese concretions from the ocean bottom, bottom silts of lakes and seas, meteorites, and lunar regolith). Samples in which theoretically there may be chemical elements with Z = 108 are being studied. At the same time research is being conducted using multicharged ion accelerators.