Dubnium

Dubnium is a chemical element with symbol Db and atomic number 105. It is named after the town of Dubna in Russia, where it was first produced. It is a synthetic element (an element that can be created in a laboratory but is not found in nature) and radioactive; the most stable known isotope, dubnium-268, has a half-life of approximately 28 hours.

In the periodic table of the elements, it is a d-block element and in the transactinide elements. It is a member of the 7th period and belongs to Group 5. Chemistry experiments have confirmed that dubnium behaves as the heavier homologue to tantalum in group 5. The chemical properties of dubnium are characterized only partly. They are similar to those of other group 5 elements.

In the 1960s and 1970s, microscopic amounts of dubnium were produced in laboratories in the former Soviet Union and in California. The priority of the discovery and therefore the naming of the element was disputed between Soviet and American scientists, and it was not until 1997 that IUPAC established "dubnium" as the official name for the element.

§History

§Discovery

Dubnium was reportedly first discovered in 1968 at the Joint Institute for Nuclear Research at Dubna (then in the Soviet Union). Researchers there bombarded an americium-243 target with neon-22 ions. They reported a 9.40 MeV and a 9.70 MeV alpha-activity and assigned the decays to the isotope ²⁶⁰Db or ²⁶¹Db:

243
95Am + 22
10Ne â†' 265âˆ'x
105Db + x n

Two years later the Dubna team separated their reaction products by thermal gradient chromatography after conversion to chlorides by interaction with NbCl₅. The team identified a 2.2 second spontaneous fission activity contained within a volatile chloride portraying eka-tantalum properties, likely dubnium-261 pentachloride, ²⁶¹DbCl₅.

In the same year, a team led by Albert Ghiorso working at the University of California, Berkeley conclusively synthesized the element by bombarding a californium-249 target with nitrogen-15 ions. The team published a convincing synthesis of ²⁶⁰Db in the reaction between californium-249 target and nitrogen-15 ions and measured the alpha decay of ²⁶⁰Db with a half-life of 1.6 seconds and a decay energy of 9.10 MeV, correlated with the daughter decay of lawrencium-256:

249
98Cf + 15
7N â†' 260
105Db + 4 n

These results by the Berkeley scientists did not confirm the Soviet findings regarding the 9.40 MeV or 9.70 MeV alpha-decay of dubnium-260, leaving only dubnium-261 as possible produced isotope. In 1971, the Dubna team repeated their reaction using an improved set-up and were able to confirm the decay data for ²⁶⁰Db using the reaction:

243
95Am + 22
10Ne â†' 260
105Db + 5 n

In 1976, the Dubna team continued their study of the reaction using thermal gradient chromatography and were able to identify the product as dubnium-260 pentabromide, ²⁶⁰DbBr₅.

In 1992 the IUPAC/IUPAP Transfermium Working Group assessed the claims of the two groups and concluded that confidence in the discovery grew from results from both laboratories and the claim of discovery should be shared.

§Naming controversy

The Soviet, later Russian, team proposed the name nielsbohrium (Ns) in honor of the Danish nuclear physicist Niels Bohr. The American team proposed that the new element should be named hahnium (Ha), in honor of the late German chemist Otto Hahn. Consequently hahnium was the name that most American and Western European scientists used and appears in many papers published at the time, and nielsbohrium was used in the Soviet Union and Eastern Bloc countries.

An element naming controversy erupted between the two groups. The International Union of Pure and Applied Chemistry (IUPAC) thus adopted unnilpentium (Unp) as a temporary, systematic element name. Attempting to resolve the issue, in 1994, the IUPAC proposed the name joliotium (Jl), after the French physicist Frédéric Joliot-Curie, which was originally proposed by Soviet team for element 102, later named nobelium. The two principal claimants still disagreed about the names of elements 104-106. However, in 1997 they resolved the dispute and adopted the current name, dubnium (Db), after the Russian town of Dubna, the location of the Joint Institute for Nuclear Research. It was argued by IUPAC that the Berkeley laboratory had already been recognized several times in the naming of elements (i.e., berkelium, californium, americium) and that the acceptance of the names rutherfordium and seaborgium for elements 104 and 106 should be offset by recognizing the Russian team's contributions to the discovery of elements 104, 105 and 106.

§Chemical properties

§Extrapolated properties

Element 105 is projected to be the second member of the 6d series of transition metals and the heaviest member of group V in the Periodic Table, below vanadium, niobium and tantalum. Because it is positioned right below tantalum, it may also be called eka-tantalum. All the members of the group readily portray their oxidation state of +5 and the state becomes more stable as the group is descended. Thus dubnium is expected to form a stable +5 state. For this group, +4 and +3 states are also known for the heavier members and dubnium may also form these reducing oxidation states.

In an extrapolation of the chemistries from niobium and tantalum, dubnium should react with oxygen to form an inert pentoxide, Db₂O₅. In alkali, the formation of an orthodubnate complex, DbO3âˆ'
4, is expected. Reaction with the halogens should readily form the pentahalides, DbX₅. The pentachlorides of niobium and tantalum exist as volatile solids or monomeric trigonal bipyramidal molecules in the vapour phase. Thus, DbCl₅ is expected to be a volatile solid. Similarly, the pentafluoride, DbF₅, should be even more volatile. Hydrolysis of the halides is known to readily form the oxyhalides, MOX₃. Thus the halides DbX₅ should react with water to form DbOX₃. The reaction with fluoride ion is also well known for the lighter homologues and dubnium is expected to form a range of fluoro-complexes. In particular, reaction of the pentafluoride with HF should form a hexafluorodubnate ion, DbFâˆ'
6. Excess fluoride should lead to DbF2âˆ'
7 and DbOF2âˆ'
5. If eka-tantalum properties are portrayed, higher concentrations of fluoride should ultimately form DbF3âˆ'
8 since NbF3âˆ'
8 is not known.

§Experimental chemistry

The chemistry of dubnium has been studied for several years using gas thermochromatography. The experiments have studied the relative adsorption characteristics of isotopes of niobium, tantalum and dubnium radioisotopes. The results have indicated the formation of typical group 5 halides and oxyhalides, namely DbCl₅, DbBr₅, DbOCl₃ and DbOBr₃. Reports on these early experiments usually refer to dubnium as hahnium.

§Nucleosynthesis history

§Cold fusion

This section deals with the synthesis of nuclei of dubnium by so-called "cold" fusion reactions. These are processes which create compound nuclei at low excitation energy (~10-20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only.

²⁰⁹Bi(⁵⁰Ti,xn)^259-xDb (x=1,2,3)

The first attempts to synthesise dubnium using cold fusion reactions were performed in 1976 by the team at FLNR, Dubna using the above reaction. They were able to detect a 5 s spontaneous fission (SF) activity which they assigned to ²⁵⁷Db. This assignment was later corrected to ²⁵⁸Db. In 1981, the team at GSI studied this reaction using the improved technique of correlation of genetic parent-daughter decays. They were able to positively identify ²⁵⁸Db, the product from the 1n neutron evaporation channel. In 1983, the team at Dubna revisited the reaction using the method of identification of a descendant using chemical separation. They succeeded in measuring alpha decays from known descendants of the decay chain beginning with ²⁵⁸Db. This was taken as providing some evidence for the formation of dubnium nuclei. The team at GSI revisited the reaction in 1985 and were able to detect 10 atoms of ²⁵⁷Db. After a significant upgrade of their facilities in 1993, in 2000 the team measured 120 decays of ²⁵⁷Db, 16 decays of ²⁵⁶Db and decay of ²⁵⁸Db in the measurement of the 1n, 2n and 3n excitation functions. The data gathered for ²⁵⁷Db allowed a first spectroscopic study of this isotope and identified an isomer, ^257mDb, and a first determination of a decay level structure for ²⁵⁷Db. The reaction was used in spectroscopic studies of isotopes of mendelevium and einsteinium in 2003â€"2004.

²⁰⁹Bi(⁴⁹Ti,xn)^258-xDb (x=2?)

This reaction was studied by Yuri Oganessian and the team at Dubna in 1983. They observed a 2.6 s SF activity tentatively assigned to ²⁵⁶Db. Later results suggest a possible reassignment to ²⁵⁶Rf, resulting from the ~30% EC branch in ²⁵⁶Db.

²⁰⁹Bi(⁴⁸Ti,xn)^257-xDb (x=1?)

This reaction was studied by Yuri Oganessian and the team at Dubna in 1983. They observed a 1.6 s activity with a ~80% alpha branch with a ~20% SF branch. The activity was tentatively assigned to ²⁵⁵Db. Later results suggest a reassignment to ²⁵⁶Db.

²⁰⁸Pb(⁵¹V,xn)^259-xDb (x=1,2)

The team at Dubna also studied this reaction in 1976 and were again able to detect the 5 s SF activity, first tentatively assigned to ²⁵⁷Db and later to ²⁵⁸Db. In 2006, the team at LBNL reinvestigated this reaction as part of their odd-Z projectile program. They were able to detect ²⁵⁸Db and ²⁵⁷Db in their measurement of the 1n and 2n neutron evaporation channels.

²⁰⁷Pb(⁵¹V,xn)^258-xDb

The team at Dubna also studied this reaction in 1976 but this time they were unable to detect the 5 s SF activity, first tentatively assigned to ²⁵⁷Db and later to ²⁵⁸Db. Instead, they were able to measure a 1.5 s SF activity, tentatively assigned to ²⁵⁵Db.

²⁰⁵Tl(⁵⁴Cr,xn)^259-xDb (x=1?)

The team at Dubna also studied this reaction in 1976 and were again able to detect the 5 s SF activity, first tentatively assigned to ²⁵⁷Db and later to ²⁵⁸Db.

§Hot fusion

This section deals with the synthesis of nuclei of dubnium by so-called "hot" fusion reactions. These are processes which create compound nuclei at high excitation energy (~40-50 MeV, hence "hot"), leading to a reduced probability of survival from fission and quasi-fission. The excited nucleus then decays to the ground state via the emission of 3-5 neutrons.

²³²Th(³¹P,xn)^263-xDb (x=5)

There are very limited reports that this rare reaction using a P-31 beam was studied in 1989 by Andreyev et al. at the FLNR. One source suggests that no atoms were detected whilst a better source from the Russians themselves indicates that ²⁵⁸Db was synthesised in the 5n channel with a yield of 120 pb.

²³⁸U(²⁷Al,xn)^265-xDb (x=4,5)

In 2006, as part of their study of the use of uranium targets in superheavy element synthesis, the LBNL team led by Ken Gregorich studied the excitation functions for the 4n and 5n channels in this new reaction.

²³⁶U(²⁷Al,xn)^263-xDb (x=5,6)

This reaction was first studied by Andreyev et al. at the FLNR, Dubna in 1992. They were able to observe ²⁵⁸Db and ²⁵⁷Db in the 5n and 6n exit channels with yields of 450 pb and 75 pb, respectively.

²⁴³Am(²²Ne,xn)^265-xDb (x=5)

The first attempts to synthesise dubnium were performed in 1968 by the team at the Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna, Russia. They observed two alpha lines which they tentatively assigned to ²⁶¹Db and ²⁶⁰Db. They repeated their experiment in 1970 looking for spontaneous fission. They found a 2.2 s SF activity which they assigned to ²⁶¹Db. In 1970, the Dubna team began work on using gradient thermochromatography in order to detect dubnium in chemical experiments as a volatile chloride. In their first run they detected a volatile SF activity with similar adsorption properties to NbCl₅ and unlike HfCl₄. This was taken to indicate the formation of nuclei of dvi-niobium as DbCl₅. In 1971, they repeated the chemistry experiment using higher sensitivity and observed alpha decays from an dvi-niobium component, taken to confirm the formation of ²⁶⁰Db. The method was repeated in 1976 using the formation of bromides and obtained almost identical results, indicating the formation of a volatile, dvi-niobium-like DbBr₅.

²⁴¹Am(²²Ne,xn)^263-xDb (x=4,5)

In 2000, Chinese scientists at the Institute of Modern Physics (IMP), Lanzhou, announced the discovery of the previously unknown isotope ²⁵⁹Db formed in the 4n neutron evaporation channel. They were also able to confirm the decay properties for ²⁵⁸Db.

²⁴⁸Cm(¹⁹F,xn)^267-xDb (x=4,5)

This reaction was first studied in 1999 at the Paul Scherrer Institute (PSI) in order to produce ²⁶²Db for chemical studies. Just 4 atoms were detected with a cross section of 260 pb. Japanese scientists at JAERI studied the reaction further in 2002 and determined yields for the isotope ²⁶²Db during their efforts to study the aqueous chemistry of dubnium.

²⁴⁹Bk(¹⁸O,xn)^267-xDb (x=4,5)

Following from the discovery of ²⁶⁰Db by Albert Ghiorso in 1970 at the University of California (UC), the same team continued in 1971 with the discovery of the new isotope ²⁶²Db. They also observed an unassigned 25 s SF activity, probably associated with the now-known SF branch of ²⁶³Db. In 1990, a team led by Kratz at LBNL definitively discovered the new isotope ²⁶³Db in the 4n neutron evaporation channel. This reaction has been used by the same team on several occasions in order to attempt to confirm an electron capture (EC) branch in ²⁶³Db leading to long-lived ²⁶³Rf (see rutherfordium).

²⁴⁹Bk(¹⁶O,xn)^265-xDb (x=4)

Following from the discovery of ²⁶⁰Db by Albert Ghiorso in 1970 at the University of California (UC), the same team continued in 1971 with the discovery of the new isotope ²⁶¹Db.

²⁵⁰Cf(¹⁵N,xn)^265-xDb (x=4)

Following from the discovery of ²⁶⁰Db by Ghiorso in 1970 at LBNL, the same team continued in 1971 with the discovery of the new isotope ²⁶¹Db.

²⁴⁹Cf(¹⁵N,xn)^264-xDb (x=4)

In 1970, the team at the Lawrence Berkeley National Laboratory (LBNL) studied this reaction and identified the isotope ²⁶⁰Db in their discovery experiment. They used the modern technique of correlation of genetic parent-daughter decays to confirm their assignment. In 1977, the team at Oak Ridge repeated the experiment and were able to confirm the discovery by the identification of K X-rays from the daughter lawrencium.

²⁵⁴Es(¹³C,xn)^267-xDb

In 1988, scientists as the Lawrence Livermore National Laboratory (LLNL) used the asymmetric hot fusion reaction with an einsteinium-254 target to search for the new nuclides ²⁶⁴Db and ²⁶³Db. Due to the low sensitivity of the experiment caused by the small Es-254 target,they were unable to detect any evaporation residues (ER).

§Decay of heavier nuclides

Isotopes of dubnium have also been identified in the decay of heavier elements. Observations to date are summarised in the table below:

§Isotopes

§Isomerism

²⁶⁰Db

Recent data on the decay of ²⁷²Rg has revealed that some decay chains continue through ²⁶⁰Db with extraordinary longer life-times than expected. These decays have been linked to an isomeric level decaying by alpha decay with a half-life of ~19 s. Further research is required to allow a definite assignment.

²⁵⁸Db

Evidence for an isomeric state in ²⁵⁸Db has been gathered from the study of the decay of ²⁶⁶Mt and ²⁶²Bh. It has been noted that those decays assigned to an electron capture (EC) branch has a significantly different half-life to those decaying by alpha emission. This has been taken to suggest the existence of an isomeric state decaying by EC with a half-life of ~20 s. Further experiments are required to confirm this assignment.

²⁵⁷Db

A study of the formation and decay of ²⁵⁷Db has proved the existence of an isomeric state. Initially, ²⁵⁷Db was taken to decay by alpha emission with energies 9.16,9.07 and 8.97 MeV. A measurement of the correlations of these decays with those of ²⁵³Lr have shown that the 9.16 MeV decay belongs to a separate isomer. Analysis of the data in conjunction with theory have assigned this activity to a meta stable state, ^257mDb. The ground state decays by alpha emission with energies 9.07 and 8.97 MeV. Spontaneous fission of ^257m,gDb was not confirmed in recent experiments.

§Spectroscopic decay level schemes

²⁵⁷Db

§Retracted isotopes

²⁵⁵Db

In 1983, scientists at Dubna carried out a series of supportive experiments in their quest for the discovery of bohrium. In two such experiments, they claimed they had detected a ~1.5 s spontaneous fission activity from the reactions ²⁰⁷Pb(⁵¹V,xn) and ²⁰⁹Bi(⁴⁸Ti,xn). The activity was assigned to ²⁵⁵Db. Later research suggested that the assignment should be changed to ²⁵⁶Db. As such, the isotope ²⁵⁵Db is currently not recognised on the chart of radionuclides and further research is required to confirm this isotope.

§References

§External links

• Dubnium at The Periodic Table of Videos (University of Nottingham)
• WebElements.com â€" Dubnium

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