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Ununquadium ( ) is the temporary name of a radioactive chemical element with the temporary symbol Uuq and atomic number 114.

About 80 decays of atoms of ununquadium have been observed to date, 50 directly and 30 from the decay of the heavier elements ununhexium and ununoctium. All decays have been assigned to the four neighbouring isotopes with mass numbers 286-289. The longest-lived isotope currently known is 289114 with a half-life of ~2.6 s, although there is evidence for an isomer, 289b114, with a half-life of ~66 s, that would be one of the longest-lived nuclei in the superheavy element region.

Recent chemistry experiments have strongly indicated that element 114 possesses non-'eka'-lead properties and appears to behave as the first superheavy element that portrays noble-gas-like properties due to relativistic effects.

History

De facto discovery

In December 1998, scientists at Dubna (Joint Institute for Nuclear Researchmarker) in Russia bombarded a Pu-244 target with Ca-48 ions. A single atom of element 114, decaying by 9.67 MeV alpha-emission with a lifetime of 30 s, was produced and assigned to 289114. This observation was subsequently published in January 1999. However, the decay chain observed has not been repeated and the exact identity of this activity is unknown, although it is possible that it is due to a meta-stable isomer, namely 289m114.

In March 1999, the same team replaced the Pu-244 target with a Pu-242 one in order to produce other isotopes. This time two atoms of element 114 were produced, decaying by 10.29 MeV alpha-emission with a half-life of 5.5 s. They were assigned as 287114. Once again, this activity has not been seen again and it is unclear what nucleus was produced. It is possible that it was a meta-stable isomer, namely 287m114.

The now-confirmed discovery of element 114 was made in June 1999 when the Dubna team repeated the Pu-244 reaction. This time, two atoms of element 114 were produced decaying by emission of 9.82 MeV alpha particles with a half life of 2.6 s.

This activity was initially assigned to 288114 in error, due to the confusion regarding the above observations. Further work in Dec 2002 has allowed a positive reassignment to 289114.

+ → → + 3


In May 2009, the Joint Working Party (JWP) of IUPAC published a report on the discovery of element 112 ununbium in which they acknowledged the discovery of the isotope 283112. This therefore implies the de facto discovery of element 114, from the acknowledgment of the data for the synthesis of 287114 and 291116 (see below), relating to 283112, although this may not be determined as the first synthesis of the element. An impending report by the JWP will discuss these issues.

The discovery of element 114, as 287114 and 286114, was confirmed in January 2009 at Berkeley. This was followed by confirmation of 288114 and 289114 in July 2009 at the GSI (see section 2.1.3).

Naming

Ununquadium (Uuq) is a temporary IUPAC systematic element name. Research scientists usually refer to the element simply as element 114.

According to IUPAC recommendations, the discoverer(s)of a new element has the right to suggest a name. No naming suggestions have yet been given by the (claimant) discoverers.

Current experiments

In April 2009, the collaboration of Paul Scherrer Institutemarker (PSI) and Flerov Laboratory of Nuclear Reactions (FLNR) of JINRmarker carried out another study of the chemistry of element 114. Results are not yet available.

Future experiments

The team at RIKEN have indicated plans to study the cold fusion reaction:

+ → → ?


The Transactinide Separator and Chemistry Apparatus (TASCA) collaboration based at the Gesellschaft für Schwerionenforschungmarker (GSI) will perform their first chemistry experiments on E114 starting in August 2009, following their successful production of the element in April 2009.

The FLNR have future plans to study light isotopes of element 114, formed in the reaction between 239Pu and 48Ca.

Isotopes and nuclear properties

Nucleosynthesis

Target-Projectile combinations leading to Z=114 compound nuclei

The below table contains various combinations of targets and projectiles which could be used to form compound nuclei with Z=114.

Target Projectile CN Attempt result
208Pb 76Ge 284114
232Th 54Cr 286114
238U 50Ti 288114
244Pu 48Ca 292114
242Pu 48Ca 290114
239Pu 48Ca 287114
248Cm 40Ar 288114
249Cf 36S 285114


Cold fusion

This section deals with the synthesis of nuclei of ununquadium 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.

208Pb(76Ge,xn)284−x114
The first attempt to synthesise element 114 in cold fusion reactions was performed at Grand accélérateur national d'ions lourds (GANIL), France in 2003. No atoms were detected providing a yield limit of 1.2 pb.

Hot fusion

This section deals with the synthesis of nuclei of ununquadium 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. The excited nucleus then decays to the ground state via the emission of 3-5 neutrons. Fusion reactions utilizing 48Ca nuclei usually produce compound nuclei with intermediate excitation energies (~30-35 MeV) and are sometimes referred to as "warm" fusion reactions. This leads, in part, to relatively high yields from these reactions.

244Pu(48Ca,xn)292−x114 (x=3,4,5)
The first experiments on the synthesis of element 114 were performed by the team in Dubna in November 1998. They were able to detect a single, long decay chain, assigned to 289114. The reaction was repeated in 1999 and a further 2 atoms of element 114 were detected. The products were assigned to 288114. The team further studied the reaction in 2002. During the measurement of the 3n, 4n, and 5n neutron evaporation excitation functions they were able to detect 3 atoms of 289114, 12 atoms of the new isotope 288114, and 1 atom of the new isotope 287114. Based on these results, the first atom to be detected was tentatively reassigned to 290114 or 289m114, whilst the two subsequent atoms were reassigned to 289114 and therefore belong to the unofficial discovery experiment. In an attempt to study the chemistry of element 112 as the isotope 285112, this reaction was repeated in April 2007. Surprisingly, a PSI-FLNR directly detected 2 atoms of 288114 forming the basis for the first chemical studies of element 114.

In June 2008, the experiment was repeated in order to further assess the chemistry of the element using the 289114 isotope. A single atom was detected seeming to confirm the noble-gas-like properties of the element.

During May-July 2009, the team at GSI studied this reaction for the first time, as a first step towards the synthesis of element 117. The team were able to confirm the synthesis and decay data for 288114 and 289114.

242Pu(48Ca,xn)290−x114 (x=2,3,4)
The team at Dubna first studied this reaction in March-April 1999 and detected two atoms of element 114, assigned to 287114. The reaction was repeated in September 2003 in order to attempt to confirm the decay data for 287114 and 283112 since conflicting data for 283112 had been collected (see ununbium). The Russian scientists were able to measure decay data for 288114,287114 and the new isotope 286114 from the measurement of the 2n, 3n, and 4n excitation functions.

"Measurements of cross sections and decay properties of the isotopes of elements 112, 114, and 116 produced in the fusion reactions 233,238U , 242Pu , and 248Cm+48Ca", Oganessian et al., JINR preprints, 2004. Retrieved on 2008-03-03
In April 2006, a PSI-FLNR collaboration used the reaction to determine the first chemical properties of element 112 by producing 283112 as an overshoot product. In a confirmatory experiment in April 2007, the team were able to detect 287114 directly and therefore measure some initial data on the atomic chemical properties of element 114.

The team at Berkeley, using the Berkeley gas-filled separator (BGS), continued their studies using newly acquired 242Pu targets by attempting the synthesis of element 114 in January 2009 using the above reaction. In September 2009, they reported that they had succeeded in detecting 2 atoms of E114, as 287114 and 286114, confirming the decay properties reported at the FLNR, although the measured cross sections were slightly lower; however the statistics were of lower quality.

As a decay product

The isotopes of ununquadium have also been observed in the decay of elements 116 and 118 (see ununoctium for decay chain).

Evaporation residue Observed Uuq isotope
293116 289114
292116 288114
291116 287114
294118, 290116 286114


Retracted isotopes

285114
In the claimed synthesis of 293118 in 1999, the isotope 285114 was identified as decaying by 11.35 MeV alpha emission with a half-life of 0.58 ms. The claim was retracted in 2001 and hence this ununquadium isotope is currently unknown or unconfirmed.

Chronology of isotope discovery

Isotope Year discovered Discovery reaction
286Uuq 2002 249Cf(48Ca,3n)
287aUuq 2002 244Pu(48Ca,5n)
287bUuq ?? 1999 242Pu(48Ca,3n)
288Uuq 2002 244Pu(48Ca,4n)
289aUuq 1999 244Pu(48Ca,3n)
289bUuq ? 1998 244Pu(48Ca,3n)


Fission of compound nuclei with Z=114

Several experiments have been performed between 2000-2004 at the Flerov Laboratory of Nuclear Reactions in Dubna studying the fission characteristics of the compound nucleus 292114. The nuclear reaction used is 244Pu+48Ca. The results have revealed how nuclei such as this fission predominantly by expelling closed shell nuclei such as 132Sn (Z=50, N=82). It was also found that the yield for the fusion-fission pathway was similar between 48Ca and 58Fe projectiles, indicating a possible future use of 58Fe projectiles in superheavy element formation.

Nuclear isomerism

289114

In the first claimed synthesis of element 114, an isotope assigned as 289114 decayed by emitting a 9.71 MeV alpha particle with a lifetime of 30 seconds. This activity was not observed in repetitions of the direct synthesis of this isotope. However, in a single case from the synthesis of 293116, a decay chain was measured starting with the emission of a 9.63 MeV alpha particle with a lifetime of 2.7 minutes. All subsequent decays were very similar to that observed from 289114, presuming that the parent decay was missed. This strongly suggests that the activity should be assigned to an isomeric level. The absence of the activity in recent experiments indicates that the yield of the isomer is ~20% compared to the supposed ground state and that the observation in the first experiment was a fortunate (or not as the case history indicates). Further research is required to resolve these issues.

287114

In a manner similar to those for 289114, first experiments with a 242Pu target identified an isotope 287114 decaying by emission of a 10.29 MeV alpha particle with a lifetime of 5.5 seconds. The daughter spontaneously fissioned with a lifetime in accord with the previous synthesis of 283112. Both these acitivities have not been observed since (see ununbium). However, the correlation suggests that the results are not random and are possible due to the formation of isomers whose yield is obviously dependent on production methods. Further research is required to unravel these discrepancies.

Yields of isotopes

The tables below provide cross-sections and excitation energies for fusion reactions producing ununquadium isotopes directly. Data in bold represent maxima derived from excitation function measurements. + represents an observed exit channel.

Cold fusion

Projectile Target CN 1n 2n 3n
76Ge 208Pb 284Uuq <1.2 pb


Hot fusion

Projectile Target CN 2n 3n 4n 5n
48Ca 242Pu 290Uuq 0.5 pb, 32.5 MeV 3.6 pb, 40.0 MeV 4.5 pb, 40.0 MeV <1.4 pb, 45.0 MeV
48Ca 244Pu 292Uuq 1.7 pb, 40.0 MeV 5.3 pb, 40.0 MeV 1.1 pb, 52.0 MeV


Theoretical calculations

Evaporation residue cross sections

The below table contains various targets-projectile combinations for which calculations have provided estimates for cross section yields from various neutron evaporation channels. The channel with the highest expected yield is given.

MD = multi-dimentional; DNS = Dinuclear system; σ = cross section

Target Projectile CN Channel (product) σmax Model Ref
208Pb 76Ge 284114 1n (283114) 60 fb DNS
208Pb 73Ge 281114 1n (280114) 0.2 pb DNS
238U 50Ti 288114 2n (286114) 60 fb DNS
244Pu 48Ca 292114 4n (288114) 4 pb MD
242Pu 48Ca 290114 3n (287114) 3 pb MD


Decay characteristics

Theoretical estimation of the alpha decay half-lives of the isotopes of the element 114 supports the experimental data.The fission-survived isotope 298114 is predicted to have alpha decay half life around 17 days.

In search for the island of stability: 298114

According to macroscopic-microscopic (MM) theory , Z=114 is the next spherical magic number. This means that such nuclei are spherical in their ground state and should have high, wide fission barriers to deformation and hence long SF partial half-lives.

In the region of Z=114, MM theory indicates that N=184 is the next spherical neutron magic number and puts forward the nucleus 298114 as a strong candidate for the next spherical doubly magic nucleus, after 208Pb (Z=82, N=126). 298114 is taken to be at the centre of a hypothetical ‘island of stability’. However, other calculations using relativistic mean field (RMF) theory propose Z=120, 122, and 126 as alternative proton magic numbers depending upon the chosen set of parameters. It is possible that rather than a peak at a specific proton shell, there exists a plateau of proton shell effects from Z=114–126.

It should be noted that calculations suggest that the minimum of the shell-correction energy and hence the highest fission barrier exists for 297115, caused by pairing effects. Due to the expected high fission barriers, any nucleus within this island of stability will exclusively decay by alpha-particle emission and as such the nucleus with the longest half-life is predicted to be 298114. The expected half-life is unlikely to reach values higher than about 10 minutes, unless the N=184 neutron shell proves to be more stabilising than predicted, for which there exists some evidence. In addition, 297114 may have an even-longer half-life due to the effect of the odd neutron, creating transitions between similar Nilsson levels with lower Qalpha values.

In either case, an island of stability does not represent nuclei with the longest half-lives but those which are significantly stabilized against fission by closed-shell effects.

Evidence for Z=114 closed proton shell

Whilst evidence for closed neutron shells can be deemed directly from the systematic variation of Qalpha values for ground-state to ground-state transitions, evidence for closed proton shells comes from (partial) spontaneous fission half-lives. Such data can sometimes be difficult to extract due to low production rates and weak SF branching. In the case of Z=114, evidence for the effect of this proposed closed shell comes from the comparison between the nuclei pairings 282112 (TSF1/2 = 0.8 ms) and 286114 (TSF1/2 = 130 ms), and 284112 (TSF = 97 ms) and 288114 (TSF >800 ms). Further evidence would come from the measurement of partial SF half-lives of nuclei with Z>114, such as 290116 and 292118 (both N=174 isotones). The extraction of Z=114 effects is complicated by the presence of a dominating N=184 effect in this region.

Difficulty of synthesis of 298114

The direct synthesis of the nucleus 298114 by a fusion-evaporation pathway is impossible since no known combination of target and projectile can provide 184 neutrons in the compound nucleus.

It has been suggested that such a neutron-rich isotope can be formed by the quasifission (partial fusion followed by fission) of a massive nucleus. Such nuclei tend to fission with the formation of isotopes close to the closed shells Z=20/N=20 (40Ca), Z=50/N=82 (132Sn) or Z=82/N=126 (208Pb/209Bi). If Z=114 does represent a closed shell, then the hypothetical reaction below may represent a method of synthesis:

+ → + + 2


Recently it has been shown that the multi-nucleon transfer reactions in collisions of actinide nuclei (such as U+Cm) might be used to synthesize the neutron rich superheavy nuclei located at the island of stability.

It is also possible that 298114 can be synthesized by the alpha decay of a massive nucleus. Such a method would depend highly on the SF stability of such nuclei, since the alpha half-lives are expected to be very short. The yields for such reactions will also most likely be extremely small. One such reaction is:

( , 2n) → → → + 10


Chemical properties

Extrapolated chemical properties

Oxidation states

Element 114 is projected to be the second member of the 7p series of non-metals and the heaviest member of group 14 (IVA) in the Periodic Table, below lead. Each of the members of this group show the group oxidation state of +IV and the latter members have an increasing +II chemistry due to the onset of the inert pair effect. Tin represents the point at which the stability of the +II and +IV states are similar. Lead, the heaviest member, portrays a switch from the +IV state to the +II state. Element 114 should therefore follow this trend and a possess an oxidising +IV state and a stable +II state.

Chemistry

Element 114 should portray eka-lead chemical properties and should therefore form a monoxide, UuqO, and dihalides, UuqF2, UuqCl2, UuqBr2, and UuqI2. If the +IV state is accessible, it is likely that it is only possible in the oxide, UuqO2, and fluoride, UuqF4. It may also show a mixed oxide, Uuq3O4, analogous to Pb3O4.

Some studies also suggest that the chemical behaviour of element 114 might in fact be closer to that of the noble gas radon, than to that of lead.

Experimental chemistry

Atomic gas phase

Two experiments were performed in April–May 2007 in a joint FLNR-PSI collaboration aiming to study the chemistry of element 112. The first experiment involved the reaction 242Pu(48Ca,3n)287114 and the second the reaction 244Pu(48Ca,4n)288114. The adsorption properties of the resultant atoms on a gold surface were compared with those of radon. The first experiment allowed detection of 3 atoms of 283112 (see ununbium) but also seemingly detected 1 atom of 287114. This result was a surprise given the transport time of the product atoms is ~2 s, so element 114 atoms should decay before adsorption. In the second reaction, 2 atoms of 288114 and possibly 1 atom of 289114 were detected. Two of the three atoms portrayed adsorption characteristics associated with a volatile, noble-gas-like element, which has been suggested but is not predicted by more recent calculations. These experiments did however provide independent confirmation for the discovery of elements 112, 114, and 116 via comparison with published decay data. Further experiments were performed in 2008 to confirm this important result and a single atom of 289114 was detected which gave data in agreement with previous data in support of element 114 having a noble-gas-like interaction with gold.

See also



References

External links




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