Just over one gram of berkelium has been produced in theUnited States since 1967. There is no practical application of berkelium outside scientific research which is mostly directed at the synthesis of heaviertransuranium elements andsuperheavy 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 elementtennessine for the first time in 2009 at theJoint Institute for Nuclear Research,Russia, after it was bombarded withcalcium-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-energybeta particles and thus is relatively safe to handle. It decays with ahalf-life of 330 days tocalifornium-249, which is a strong emitter of ionizing alpha particles. This gradualtransmutation 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.
Double-hexagonal close packing with the layer sequence ABAC in the crystal structure of α-berkelium (A: green, B: blue, C: red)
Berkelium is a soft, silvery-white, radioactiveactinide metal. In theperiodic table, it is located to the right of the actinidecurium, to the left of the actinidecalifornium and below the lanthanideterbium 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).[6] Berkelium is relatively soft and has one of the lowestbulk moduli among the actinides, at about 20GPa (2×1010 Pa).[7]
Berkelium(III) ions shows two sharpfluorescence peaks at 652 nanometers (red light) and 742 nanometers (deep red –near-infrared) due to internal transitions at thef-electron shell. The relative intensity of these peaks depends on the excitation power and temperature of the sample. This emission can be observed, for example, after dispersing berkelium ions in a silicate glass, by melting the glass in presence of berkelium oxide or halide.[8][9]
At ambient conditions, berkelium assumes its most stable α form which has ahexagonal symmetry,space groupP63/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 isisotypic (having a similar structure) with α-lanthanum and α-forms of actinides beyond curium.[13] This crystal structure changes with pressure and temperature. When compressed at room temperature to 7 GPa, α-berkelium transforms to the β modification, which has aface-centered cubic (fcc) symmetry and space groupFm3m. This transition occurs without change in volume, but theenthalpy increases by 3.66 kJ/mol.[14] Upon further compression to 25 GPa, berkelium transforms to anorthorhombic γ-berkelium structure similar to that of α-uranium. This transition is accompanied by a 12% volume decrease and delocalization of the electrons at the5f electron shell.[15] No further phase transitions are observed up to 57 GPa.[7][16]
Upon heating, α-berkelium transforms into another phase with anfcc lattice (but slightly different from β-berkelium), space groupFm3m and thelattice constant of 500 pm; thisfcc 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 atroom temperature.[13] The temperature of thephase transition is believed to be quite close to the melting point.[17][18][19]
Like allactinides, berkelium dissolves in various aqueous inorganic acids, liberating gaseoushydrogen and converting into the berkelium(III) state. Thistrivalentoxidation state (+3) is the most stable, especially in aqueous solutions,[20][21] buttetravalent (+4),[22]pentavalent (+5),[23] and possiblydivalent (+2) berkelium compounds are also known. The existence of divalent berkelium salts is uncertain and has only been reported in mixedlanthanum(III) chloride-strontium chloride melts.[24][25] A similar behavior is observed for the lanthanide analogue of berkelium,terbium.[26] Aqueous solutions ofBk3+ ions are green in most acids. The color ofBk4+ ions is yellow inhydrochloric acid and orange-yellow insulfuric acid.[24][27][28] Berkelium does not react rapidly withoxygen at room temperature, possibly due to the formation of a protective oxide layer surface. However, it reacts with molten metals,hydrogen,halogens,chalcogens andpnictogens to form various binary compounds.[10]
In 2025 anorganometallic compound containing berkelium was synthesized from 0.3 mg of berkelium and named berkelocene.[29]
Nineteen isotopes and sixnuclear isomers (excited states of an isotope) of berkelium have been characterized, with mass numbers ranging from 233 to 253 (except 235 and 237).[30] All of them are radioactive. The longesthalf-lives are observed for247Bk (1,380 years),248Bk (almost surely over 300 years), and249Bk (327.2 days); other isotopes are less than a week. The isotope which is the easiest to synthesize (reactorneutron capture) is berkelium-249. This emits mostly softβ-particles which are inconvenient for detection. Itsalpha radiation is rather weak (1.45×10−3%) with respect to the β-radiation, but is sometimes used to detect this isotope. The second important berkelium isotope, berkelium-247, isbeta-stable and analpha emitter, as are most long-lived actinide isotopes.[30][31]
All berkelium isotopes have a half-life far too short to beprimordial. Therefore, any primordial berkelium − that is, berkelium present on the Earth during its formation − has decayed by now.
Nuclear reactors produce mostly, among the berkelium isotopes, berkelium-249. During the storage and before the fuel disposal, most of itbeta 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,[33] and is therefore undesirable in the disposal products.
The transuranic elements up tofermium, including berkelium, should have been present in thenatural nuclear fission reactor atOklo, but any quantities produced then would have long since decayed away.[34]
The name choice for element 97 followed the previous tradition of the Californian group to draw an analogy between the newly discoveredactinide and thelanthanide element positioned above it in theperiodic table. Previously, americium was named after a continent as its analogueeuropium, and curium honored scientistsMarie andPierre Curie as the lanthanide above it,gadolinium, was named after the explorer of therare-earth elementsJohan 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 homologueterbium (atomic number 65) whose name was derived from the town ofYtterby,Sweden, where the rare earth minerals were first found."[36] This tradition ended with berkelium, though, as the naming of the next discovered actinide,californium, was not related to its lanthanide analoguedysprosium, but after the discovery place.[39]
The most difficult steps in synthesising 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 aplatinum foil, the solution was evaporated and the residue converted by annealing toamericium dioxide (AmO2). This target was irradiated with 35 MeValpha 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 the243Bk isotope and two freeneutrons:[36]
241 95Am +4 2He →243 97Bk + 21 0n
After the irradiation, the coating was dissolved withnitric acid and then precipitated as thehydroxide using concentrated aqueousammonia solution. The product wascentrifugated and re-dissolved in nitric acid. To separate berkelium from the unreacted americium, this solution was added to a mixture of aqueousammonia andammonium sulfate and heated in the presence of atmospheric oxygen to convert all the dissolved americium into theoxidation state +6. Unoxidized residual americium was precipitated by the addition ofhydrofluoric acid as americium(III)fluoride (AmF3). This step yielded a mixture of the accompanying product curium and the expected element 97 in form of trifluorides. The mixture was converted to the corresponding hydroxides by treating it withpotassium hydroxide, and after centrifugation, was dissolved inperchloric acid.[36]
Further separation was carried out in the presence of acitric acid/ammoniumbuffer solution in a weakly acidic medium(pH ≈ 3.5), usingion exchange at elevated temperature. Thechromatographic separation behavior was unknown for element 97 at the time but was anticipated by analogy with terbium. The first results were disappointing because no alpha-particle emission signature could be detected from the elution product. With further analysis, searching forcharacteristic X-rays andconversion electron signals, a berkelium isotope was eventually detected. Itsmass number was uncertain between 243 and 244 in the initial report,[26] but was later established as 243.[36]
Berkelium is produced by bombarding lighter actinidesuranium (238U) orplutonium (239Pu) withneutrons in anuclear reactor. In a more common case of uranium fuel, plutonium is produced first byneutron capture (the so-called (n,γ) reaction or neutron fusion) followed by beta-decay:[40]
Plutonium-239 is further irradiated by a source that has a highneutron flux, several times higher than a conventional nuclear reactor, such as the 85-megawattHigh Flux Isotope Reactor (HFIR) at theOak Ridge National Laboratory in Tennessee, US. The higher flux promotes fusion reactions involving not one but several neutrons, converting239Pu to244Cm and then to249Cm:
Curium-249 has a short half-life of 64 minutes, and thus its further conversion to250Cm has a low probability. Instead, it transforms by beta-decay into249Bk:[30]
The thus-produced249Bk has a long half-life of 330 days and thus can capture another neutron. However, the product,250Bk, again has a relatively short half-life of 3.212 hours and thus does not yield any heavier berkelium isotopes. It instead decays to the californium isotope250Cf:[41][42]
Although247Bk is the most stable isotope of berkelium, its production in nuclear reactors is very difficult because its potential progenitor247Cm has never been observed to undergo beta decay.[43] 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[44]) at a high price of the order 185USD per microgram.[6] It is the only berkelium isotope available in bulk quantities, and thus the only berkelium isotope whose properties can be extensively studied.[45]
The isotope248Bk 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 with245Bk, the existence of a new isotope was proven by the growth of the decay product248Cf which had been previously characterized. The half-life of248Bk was estimated as23±5 hours,[46] though later 1965 work gave a half-life in excess of 300 years (which may be due to an isomeric state).[47] Berkelium-247 was produced during the same year by irradiating244Cm with alpha-particles:[48]
Berkelium-242 was synthesized in 1979 by bombarding235U with11B,238U with10B,232Th with14N or232Th with15N. It converts byelectron capture to242Cm with a half-life of7.0±1.3 minutes. A search for an initially suspected isotope241Bk was then unsuccessful;[49]241Bk has since been synthesized.[50]
The fact that berkelium readily assumesoxidation state +4 in solids, and is relatively stable in this state in liquids, greatly assists separation of berkelium from many other actinides. These are 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 berkelium(III) solution to convert it to the +4 state, such asbromates (BrO−3),bismuthates (BiO−3),chromates (CrO2−4 andCr2O2−7), 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[clarification needed] molecules, such as thechelator 3,4,3-LI(1,2-HOPO), can also oxidize Bk(III) and stabilize Bk(IV) under mild conditions.[22] Berkelium(IV) is then extracted withion exchange, extractionchromatography 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 andlanthanides, except for the lanthanidecerium (lanthanides are absent in the irradiation target but are created in variousnuclear fission decay chains).[51]
A more detailed procedure adopted at theOak Ridge National Laboratory was as follows: the initial mixture of actinides is processed with ion exchange usinglithium chloridereagent, then precipitated ashydroxides, filtered and dissolved in nitric acid. It is then treated with high-pressureelution fromcation exchange resins, and the berkelium phase is oxidized and extracted using one of the procedures described above.[51] Reduction of the thus-obtained berkelium(IV) 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.[52]
In order to characterize chemical and physical properties of solid berkelium and its compounds, a program was initiated in 1952 at theMaterial Testing Reactor,Arco, Idaho, US. It resulted in preparation of an eight-gram plutonium-239 target and in the first production of macroscopic quantities (0.6 micrograms) of berkelium by Burris B. Cunningham andStanley Gerald Thompson in 1958, after a continuous reactor irradiation of this target for six years.[44][53] This irradiation method was and still is the only way of producing weighable amounts of the element, and most solid-state studies of berkelium have been conducted on microgram or submicrogram-sized samples.[17][54]
The world's major irradiation sources are the 85-megawatt High Flux Isotope Reactor at theOak Ridge National Laboratory in Tennessee, USA,[55] and the SM-2 loop reactor at theResearch Institute of Atomic Reactors (NIIAR) inDimitrovgrad, Russia,[56] which are both dedicated to the production of transcurium elements (atomic number greater than 96). These facilities have similar power and flux levels, and are expected to have comparable production capacities for transcurium elements,[57] although the quantities produced at NIIAR are not publicly reported. In a "typical processing campaign" at Oak Ridge, tens of grams ofcurium are irradiated to producedecigram quantities ofcalifornium,milligram quantities of berkelium-249 andeinsteinium, andpicogram quantities offermium.[58][59] In total, just over one gram of berkelium-249 has been produced at Oak Ridge since 1967.[17]
The first berkelium metal sample weighing 1.7 micrograms was prepared in 1971 by the reduction of berkelium(III) fluoride withlithium vapor at 1000 °C; the fluoride was suspended on a tungsten wire above atantalum crucible containing molten lithium. Later, metal samples weighing up to 0.5 milligrams were obtained with this method.[13][60]
BkF3 + 3 Li → Bk + 3 LiF
Similar results are obtained with berkelium(IV) fluoride.[15] Berkelium metal can also be produced by the reduction of berkelium(IV) oxide withthorium orlanthanum.[60][61]
Two oxides of berkelium are known, with the berkeliumoxidation state of +3 (Bk2O3) and +4 (BkO2).[62] Berkelium(IV) oxide is a brown solid,[63] while berkelium(III) oxide is a yellow-green solid with a melting point of 1920 °C[64][63] and is formed from BkO2 byreduction with molecularhydrogen:
2 BkO2 + H2 → Bk2O3 + H2O
Upon heating to 1200 °C, the oxideBk2O3 undergoes a phase change; it undergoes another phase change at 1750 °C. Such three-phase behavior is typical for the actinidesesquioxides. Berkelium(II) oxide, BkO, has been reported as a brittle gray solid but its exact chemical composition remains uncertain.[65]
Inhalides, berkelium assumes the oxidation states +3 and +4.[66] The +3 state is the most stable, especially in solutions, while the tetravalent halidesBkF4 andCs2BkCl6 are only known in the solid phase.[67] The coordination of berkelium atom in its trivalent fluoride and chloride is tricappedtrigonal prismatic, with thecoordination number of 9. In trivalent bromide, it is bicapped trigonal prismatic (coordination 8) oroctahedral (coordination 6),[68] and in the iodide it is octahedral.[69]
Berkelium(IV) fluoride (BkF4) is a yellow-green ionic solid and is isotypic withuranium tetrafluoride orzirconium tetrafluoride.[70][72][73] Berkelium(III) fluoride (BkF3) is also a yellow-green solid, but it has two crystalline structures. The most stable phase at low temperatures is isotypic withyttrium(III) fluoride, while upon heating to between 350 and 600 °C, it transforms to the structure found inlanthanum trifluoride.[70][72][74]
Visible amounts ofberkelium(III) chloride (BkCl3) were first isolated and characterized in 1962, and weighed only 3 billionths of agram. It can be prepared by introducinghydrogen chloride vapors into an evacuated quartz tube containing berkelium oxide at a temperature about 500 °C.[75] This green solid has a melting point of 600 °C,[66] and is isotypic withuranium(III) chloride.[76][77] Upon heating to nearly melting point,BkCl3 converts into an orthorhombic phase.[78]
Two forms of berkelium(III) bromide are known: one with berkelium having coordination 6, and one with coordination 8.[54] The latter is less stable and transforms to the former phase upon heating to about 350 °C. An important property of radioactive solids has been studied on these two crystal forms: the structure of fresh and aged249BkBr3 samples was probed byX-ray diffraction over a period longer than 3 years, so that various fractions of berkelium-249 hadbeta decayed to californium-249. No change in structure was observed upon the249BkBr3—249CfBr3 transformation. However, other differences were noted for249BkBr3 and249CfBr3. For example, the latter could be reduced with hydrogen to249CfBr2, but the former could not – this result was reproduced on individual249BkBr3 and249CfBr3 samples, as well on the samples containing both bromides.[68] The intergrowth of californium in berkelium occurs at a rate of 0.22% per day and is an obstacle to studying berkelium properties. Beside a chemical contamination,249Cf, being an alpha emitter, brings undesirable self-damage of the crystal lattice and the resulting self-heating. The chemical effect however can be avoided by performing measurements as a function of time and extrapolating the obtained results.[67]
Thepnictides of berkelium-249 of the type BkX are known for the elementsnitrogen,[79]phosphorus,arsenic andantimony. They crystallize in therock-salt structure and are prepared by the reaction of either berkelium(III) hydride (BkH3) or metallic berkelium with these elements at elevated temperature (about 600 °C) under high vacuum.[80]
Berkelium(III) sulfide,Bk2S3, is prepared by either treating berkelium oxide with a mixture ofhydrogen sulfide andcarbon disulfide vapors at 1130 °C, or by directly reacting metallic berkelium with elemental sulfur. These procedures yield brownish-black crystals.[81]
Berkelium(III) and berkelium(IV) hydroxides are both stable in 1molar solutions ofsodium hydroxide. Berkelium(III)phosphate (BkPO4) has been prepared as a solid, which shows strongfluorescence under excitation with a green light.[82] Berkelium hydrides are produced by reacting metal with hydrogen gas at temperatures about 250 °C.[79] They are non-stoichiometric with the nominal formulaBkH 2+x (0 <x < 1).[81] Several other salts of berkelium are known, including an oxysulfide (Bk2O2S), and hydratednitrate (Bk(NO 3) 3·4H 2O), chloride (BkCl 3·6H 2O),sulfate (Bk 2(SO 4) 3·12H 2O) andoxalate (Bk 2(C 2O 4) 3·4H 2O).[67] Thermal decomposition at about 600 °C in anargon atmosphere (to avoid oxidation toBkO2) ofBk 2(SO 4) 3·12H 2O yields the crystals of berkelium(III) oxysulfate (Bk2O2SO4). This compound is thermally stable to at least 1000 °C in inert atmosphere.[83]
Berkelium forms a trigonal (η5–C5H5)3Bkmetallocene complex with threecyclopentadienyl rings, which can be synthesized by reacting berkelium(III) chloride with the moltenberyllocene (Be(C5H5)2) at about 70 °C. It has an amber color and a density of 2.47 g/cm3. The complex is stable to heating to at least 250 °C, and sublimates without melting at about 350 °C. The high radioactivity of berkelium gradually destroys the compound (within a period of weeks).[75][84] One cyclopentadienyl ring in (η5–C5H5)3Bk can be substituted by chlorine to yield[Bk(C5H5)2Cl]2. The optical absorption spectra of this compound are very similar to those of (η5–C5H5)3Bk.[85]
Berkelium also forms berkelocene, anactinocene complex, with substituted cyclooctatetraenides.[86]
22milligrams of berkelium (asnitrate) prepared atHFIR in 2009 at a cost of approximately one million dollars, used for the synthesis oftennessine inJINR[87]
There is currently no use for any isotope of berkelium outside basic scientific research.[17] Berkelium-249 is a common target nuclide to prepare still heaviertransuranium elements andsuperheavy elements,[88] such aslawrencium,rutherfordium andbohrium.[17] It is also useful as a source of the isotope californium-249, which is used for studies on the chemistry ofcalifornium in preference to the more radioactive californium-252 that is produced in neutron bombardment facilities such as the HFIR.[17][89]
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 oftennessine at theJoint 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 andLawrence Livermore National Laboratory on the synthesis of elements 113 to 118 which was initiated in 1989.[90][91]
Thenuclear 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 capturecross section of 710barns forthermal neutrons, 1200 barnsresonance 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.[92][93][94] In principle, berkelium-249 can sustain anuclear chain reaction in afast breeder reactor. Itscritical mass is relatively high at 192 kg, which can be reduced with a water or steel reflector but would still exceed the world production of this isotope.[95]
Berkelium-247 can maintain a 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).[95]
Little is known about the effects of berkelium on human body, and analogies with other elements may not be drawn because of different radiation products (electrons for berkelium andalpha particles,neutrons, or both for most other actinides). The low energy of electrons emitted from berkelium-249 (less than 126 keV) hinders its detection, due to signal interference with other decay processes, but also makes this isotope relatively harmless to humans as compared to other actinides. However, berkelium-249 transforms with a half-life of only 330 days to the strong alpha-emitter californium-249, which is rather dangerous and has to be handled in aglovebox in a dedicated laboratory.[96]
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,[Do rats really live for 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.[97] In all these organs berkelium might promote cancer, and in theskeleton, its radiation can damage red blood cells. The maximum permissible amount of berkelium-249 in the human skeleton is 0.4 nanograms.[6][98]
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