Беркелий: Синтез, қасиеттері және қолданылуы

Berkelium is a synthetic chemical element; it has symbol Bk and atomic number 97. It is a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory) where it was discovered in December 1949. Berkelium was the fifth transuranium element discovered after neptunium, plutonium, curium and americium. The major isotope of berkelium, 249Bk, is synthesized in minute quantities in dedicated high flux nuclear reactors, mainly at the Oak Ridge National Laboratory in Tennessee, United States, and at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. The longest lived and second most important isotope, 247Bk, can be synthesized via irradiation of 244Cm with high energy alpha particles. Just over one gram of berkelium has been produced in the United States since 1967. There is no practical application of berkelium outside scientific research which is mostly directed at the synthesis of heavier transuranium elements and superheavy elements. A 22 milligram batch of berkelium 249 was prepared during a 250 day irradiation period and then purified for a further 90 days at Oak Ridge in 2009. This sample was used to synthesize the new element tennessine for the first time in 2009 at the Joint Institute for Nuclear Research, Russia, after it was bombarded with calcium 48 ions for 150 days. This was the culmination of the Russia–US collaboration on the synthesis of the heaviest elements on the periodic table. Berkelium is a soft, silvery white, radioactive metal. The berkelium 249 isotope emits low energy electrons and thus is relatively safe to handle. It decays with a half life of 330 days to californium 249, which is a strong emitter of ionizing alpha particles. This gradual transformation is an important consideration when studying the properties of elemental berkelium and its chemical compounds, since the formation of californium brings not only chemical contamination, but also free radical effects and self heating from the emitted alpha particles.

Berkelium is a soft, silvery white, radioactive actinide metal. In the periodic table, it is located to the right of the actinide curium, to the left of the actinide californium and below the lanthanide terbium with which it shares many similarities in physical and chemical properties. Its density of 14.78 g/cm3 lies between those of curium (13.52 g/cm3) and californium (15.1 g/cm3), as does its melting point of 986 °C, below that of curium (1340 °C) but higher than that of californium (900 °C). Between 70 K and room temperature, berkelium behaves as a Curie–Weiss paramagnetic material with an effective magnetic moment of 9.69 Bohr magnetons (μB) and a Curie temperature of 101 K. This magnetic moment is almost equal to the theoretical value of 9.72 μB calculated within the simple atomic L S coupling model. Upon cooling to about 34 K, berkelium undergoes a transition to an antiferromagnetic state. Enthalpy of dissolution in hydrochloric acid at standard conditions is −600 kJ/mol, from which the standard enthalpy of formation (ΔfH°) of aqueous |Bk(3+) ions is obtained as −601 kJ/mol. The standard electrode potential |Bk(3+)/Bk is −2.01 V. The ionization potential of a neutral berkelium atom is 6.23 eV.

At ambient conditions, berkelium assumes its most stable α form which has a hexagonal symmetry, space group P63/mmc, lattice parameters of 341 pm and 1107 pm. The crystal has a double hexagonal close packing structure with the layer sequence ABAC and so is isotypic (having a similar structure) with α lanthanum and α forms of actinides beyond curium. Upon heating, α berkelium transforms into another phase with an fcc lattice (but slightly different from β berkelium), space group Fmm and the lattice constant of 500 pm; this fcc structure is equivalent to the closest packing with the sequence ABC. This phase is metastable and will gradually revert to the original α berkelium phase at room temperature.

Like all actinides, berkelium dissolves in various aqueous inorganic acids, liberating gaseous hydrogen and converting into the state. This trivalent oxidation state (+3) is the most stable, especially in aqueous solutions, but tetravalent (+4), and possibly divalent (+2) berkelium compounds are also known. The existence of divalent berkelium salts is uncertain and has only been reported in mixed lanthanum(III) chloride strontium chloride melts. A similar behavior is observed for the lanthanide analogue of berkelium, terbium. All of them are radioactive. The longest half lives are observed for 247Bk (1,380 years), 248Bk (over 300 years), and 249Bk (330 days); the half lives of the other isotopes range from microseconds to several days. The isotope which is the easiest to synthesize is berkelium 249. This emits mostly soft β particles which are inconvenient for detection. Its alpha radiation is rather weak (1.45%) with respect to the β radiation, but is sometimes used to detect this isotope. The second important berkelium isotope, berkelium 247, is an alpha emitter, as are most actinide isotopes.

All berkelium isotopes have a half life far too short to be primordial. Therefore, any primordial berkelium − that is, berkelium present on the Earth during its formation − has decayed by now. On Earth, berkelium is mostly concentrated in certain areas, which were used for the atmospheric nuclear weapons tests between 1945 and 1980, as well as at the sites of nuclear incidents, such as the Chernobyl disaster, Three Mile Island accident and 1968 Thule Air Base B 52 crash. Analysis of the debris at the testing site of the first United States' first thermonuclear weapon, Ivy Mike, (1 November 1952, Enewetak Atoll), revealed high concentrations of various actinides, including berkelium. For reasons of military secrecy, this result was not published until 1956. Nuclear reactors produce mostly, among the berkelium isotopes, berkelium 249. During the storage and before the fuel disposal, most of it beta decays to californium 249. The latter has a half life of 351 years, which is relatively long compared to the half lives of other isotopes produced in the reactor, and is therefore undesirable in the disposal products. The transuranium elements from americium to fermium, including berkelium, occurred naturally in the natural nuclear fission reactor at Oklo, but no longer do so. Berkelium is also one of the elements that have theoretically been detected in Przybylski's Star.

Although very small amounts of berkelium were possibly produced in previous nuclear experiments, it was first intentionally synthesized, isolated and identified in December 1949 by Glenn T. Seaborg, Albert Ghiorso, Stanley Gerald Thompson, and Kenneth Street Jr. They used the 60 inch cyclotron at the University of California, Berkeley. Similar to the nearly simultaneous discovery of americium (element 95) and curium (element 96) in 1944, the new elements berkelium and californium (element 98) were both produced in 1949–1950. The name choice for element 97 followed the previous tradition of the Californian group to draw an analogy between the newly discovered actinide and the lanthanide element positioned above it in the periodic table. Previously, americium was named after a continent as its analogue europium, and curium honored scientists Marie and Pierre Curie as the lanthanide above it, gadolinium, was named after the explorer of the rare earth elements Johan Gadolin. Thus the discovery report by the Berkeley group reads: "It is suggested that element 97 be given the name berkelium (symbol Bk) after the city of Berkeley in a manner similar to that used in naming its chemical homologue terbium (atomic number 65) whose name was derived from the town of Ytterby, Sweden, where the rare earth minerals were first found." The most difficult steps in the synthesis of berkelium were its separation from the final products and the production of sufficient quantities of americium for the target material. First, americium (241Am) nitrate solution was coated on a platinum foil, the solution was evaporated and the residue converted by annealing to americium dioxide (|AmO2). This target was irradiated with 35 MeV alpha particles for 6 hours in the 60 inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley. The (α,2n) reaction induced by the irradiation yielded the 243Bk isotope and two free neutrons:

^{238} {92}U >[\ce{(n,\gamma)}] ^{239} {92}U >[\beta^ ][23.5 \ \ce{min}] ^{239} {93}Np >[\beta^ ][2.3565 \ \ce{d}] ^{239} {94}Pu (the times are half lives)

Plutonium 239 is further irradiated by a source that has a high neutron flux, several times higher than a conventional nuclear reactor, such as the 85 megawatt High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Tennessee, US. The higher flux promotes fusion reactions involving not one but several neutrons, converting 239Pu to 244Cm and then to 249Cm:

Curium 249 has a short half life of 64 minutes, and thus its further conversion to 250Cm has a low probability. Instead, it transforms by beta decay into 249Bk:
^{249} {97}Bk >[\ce{(n,\gamma)}] ^{250} {97}Bk >[\beta^ ][3.212 \ \ce{h}] ^{250} {98}Cf

Although 247Bk is the most stable isotope of berkelium, its production in nuclear reactors is very difficult because its potential progenitor 247Cm has never been observed to undergo beta decay. Thus, 249Bk is the most accessible isotope of berkelium, which still is available only in small quantities (only 0.66 grams have been produced in the US over the period 1967–1983) at a high price of the order 185 USD per microgram. It is the only berkelium isotope available in bulk quantities, and thus the only berkelium isotope whose properties can be extensively studied. The isotope 248Bk was first obtained in 1956 by bombarding a mixture of curium isotopes with 25 MeV α particles. Although its direct detection was hindered by strong signal interference with 245Bk, the existence of a new isotope was proven by the growth of the decay product 248Cf which had been previously characterized. The half life of 248Bk was estimated as 23 hours, though later 1965 work gave a half life in excess of 300 years (which may be due to an isomeric state). Berkelium 247 was produced during the same year by irradiating 244Cm with alpha particles:

Berkelium 242 was synthesized in 1979 by bombarding 235U with 11B, 238U with 10B, 232Th with 14N or 232Th with 15N. It converts by electron capture to 242Cm with a half life of 7.0 minutes. A search for an initially suspected isotope 241Bk was then unsuccessful; 241Bk has since been synthesized.

The fact that berkelium readily assumes oxidation state +4 in solids, and is relatively stable in this state in liquids greatly assists separation of berkelium away from many other actinides. These are inevitably produced in relatively large amounts during the nuclear synthesis and often favor the +3 state. This fact was not yet known in the initial experiments, which used a more complex separation procedure. Various inorganic oxidation agents can be applied to the solutions to convert it to the +4 state, such as bromates (|BrO3( )), bismuthates (|BiO3( )), chromates (|CrO4(2 ) and |Cr2O7(2−)), silver(I) thiolate (|Ag2S2O8), lead(IV) oxide (|PbO2), ozone (|O3), or photochemical oxidation procedures. More recently, it has been discovered that some organic and bio inspired molecules, such as the chelator called 3,4,3 LI(1,2 HOPO), can also oxidize Bk(III) and stabilize Bk(IV) under mild conditions. is then extracted with ion exchange, extraction chromatography or liquid liquid extraction using HDEHP (bis (2 ethylhexyl) phosphoric acid), amines, tributyl phosphate or various other reagents. These procedures separate berkelium from most trivalent actinides and lanthanides, except for the lanthanide cerium (lanthanides are absent in the irradiation target but are created in various nuclear fission decay chains). A more detailed procedure adopted at the Oak Ridge National Laboratory was as follows: the initial mixture of actinides is processed with ion exchange using lithium chloride reagent, then precipitated as hydroxides, filtered and dissolved in nitric acid. It is then treated with high pressure elution from cation exchange resins, and the berkelium phase is oxidized and extracted using one of the procedures described above. Reduction of the thus obtained to the +3 oxidation state yields a solution, which is nearly free from other actinides (but contains cerium). Berkelium and cerium are then separated with another round of ion exchange treatment.

There is currently no use for any isotope of berkelium outside basic scientific research. such as lawrencium, rutherfordium and bohrium. A 22 milligram batch of berkelium 249 was prepared in a 250 day irradiation and then purified for 90 days at Oak Ridge in 2009. This target yielded the first 6 atoms of tennessine at the Joint Institute for Nuclear Research (JINR), Dubna, Russia, after bombarding it with calcium ions in the U400 cyclotron for 150 days. This synthesis was a culmination of the Russia US collaboration between JINR and Lawrence Livermore National Laboratory on the synthesis of elements 113 to 118 which was initiated in 1989.

The nuclear fission properties of berkelium are different from those of the neighboring actinides curium and californium, and they suggest berkelium to perform poorly as a fuel in a nuclear reactor. Specifically, berkelium 249 has a moderately large neutron capture cross section of 710 barns for thermal neutrons, 1200 barns resonance integral, but very low fission cross section for thermal neutrons. In a thermal reactor, much of it will therefore be converted to berkelium 250 which quickly decays to californium 250. In principle, berkelium 249 can sustain a nuclear chain reaction in a fast breeder reactor. Its critical mass is relatively high at 192 kg; it can be reduced with a water or steel reflector but would still exceed the world production of this isotope. Berkelium 247 can maintain chain reaction both in a thermal neutron and in a fast neutron reactor, however, its production is rather complex and thus the availability is much lower than its critical mass, which is about 75.7 kg for a bare sphere, 41.2 kg with a water reflector and 35.2 kg with a steel reflector (30 cm thickness). Most available berkelium toxicity data originate from research on animals. Upon ingestion by rats, only about 0.01% of berkelium ends in the blood stream. From there, about 65% goes to the bones, where it remains for about 50 years, 25% to the lungs (biological half life about 20 years), 0.035% to the testicles or 0.01% to the ovaries where berkelium stays indefinitely. The balance of about 10% is excreted. In all these organs berkelium might promote cancer, and in the skeleton, its radiation can damage red blood cells. The maximum permissible amount of berkelium 249 in the human skeleton is 0.4 nanograms.