Isotopes of rutherfordium

Rutherfordium (₁₀₄Rf) is a synthetic element and thus has no stable isotopes. A standard atomic weight cannot be given. The first isotope to be synthesized was either ²⁵⁹Rf in 1966 or ²⁵⁷Rf in 1969. There are 16 known radioisotopes from ²⁵³Rf to ²⁷⁰Rf (3 of which, ²⁶⁶Rf, ²⁶⁸Rf, and ²⁷⁰Rf are unconfirmed) and 4 isomers. The longest-lived isotope is ²⁶⁷Rf with a half-life of 2.5 hours, and the longest-lived isomer is ^261mRf with a half-life of 81 seconds.

List of isotopes[]

Nucleosynthesis[]

Super-heavy elements such as rutherfordium are produced by bombarding lighter elements in particle accelerators that induces fusion reactions. Whereas most of the isotopes of rutherfordium can be synthesized directly this way, some heavier ones have only been observed as decay products of elements with higher atomic numbers.^[10]

Depending on the energies involved, the former are separated into "hot" and "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets (actinides), giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons.^[10] In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products.^[11] The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).^[12]

Hot fusion studies[]

The synthesis of rutherfordium was first attempted in 1964 by the team at Dubna using the hot fusion reaction of neon-22 projectiles with plutonium-242 targets:

²⁴²
₉₄Pu
+ ²²
₁₀Ne
→ ^264−x
₁₀₄Rf
+ 3 or 5
n
.

The first study produced evidence for a spontaneous fission with a 0.3 second half-life and another one at 8 seconds. While the former observation was eventually retracted, the latter eventually became associated with the ²⁵⁹Rf isotope.^[13] In 1966, the Soviet team repeated the experiment using a chemical study of volatile chloride products. They identified a volatile chloride with eka-hafnium properties that decayed fast through spontaneous fission. This gave strong evidence for the formation of RfCl₄, and although a half-life was not accurately measured, later evidence suggested that the product was most likely ²⁵⁹Rf. The team repeated the experiment several times over the next few years, and in 1971, they revised the spontaneous fission half-life for the isotope at 4.5 seconds.^[13]

In 1969, researchers at the University of California led by Albert Ghiorso, tried to confirm the original results reported at Dubna. In a reaction of curium-248 with oxygen-16, they were unable to confirm the result of the Soviet team, but managed to observe the spontaneous fission of ²⁶⁰Rf with a very short half-life of 10–30 ms:

²⁴⁸
₉₆Cm
+ ¹⁶
₈O
→ ²⁶⁰
₁₀₄Rf
+ 4
n
.

In 1970, the American team also studied the same reaction with oxygen-18 and identified ²⁶¹Rf with a half-life of 65 seconds (later refined to 75 seconds).^[14][15] Later experiments at the Lawrence Berkeley National Laboratory in California also revealed the formation of a short-lived isomer of ²⁶²Rf (which undergoes spontaneous fission with a half-life of 47 ms),^[16] and spontaneous fission activities with long lifetimes tentatively assigned to ²⁶³Rf.^[17]

Diagram of the experimental set-up used in the discovery of isotopes ²⁵⁷Rf and ²⁵⁹Rf

The reaction of californium-249 with carbon-13 was also investigated by the Ghiorso team, which indicated the formation of the short-lived ²⁵⁸Rf (which undergoes spontaneous fission in 11 ms):^[18]

²⁴⁹
₉₈Cf
+ ¹³
₆C
→ ²⁵⁸
₁₀₄Rf
+ 4
n
.

In trying to confirm these results by using carbon-12 instead, they also observed the first alpha decays from ²⁵⁷Rf.^[18]

The reaction of berkelium-249 with nitrogen-14 was first studied in Dubna in 1977, and in 1985, researchers there confirmed the formation of the ²⁶⁰Rf isotope which quickly undergoes spontaneous fission in 28 ms:^[13]

²⁴⁹
₉₇Bk
+ ¹⁴
₇N
→ ²⁶⁰
₁₀₄Rf
+ 3
n
.

In 1996 the isotope ²⁶²Rf was observed in LBNL from the fusion of plutonium-244 with neon-22:

²⁴⁴
₉₄Pu
+ ²²
₁₀Ne
→ ^266−x
₁₀₄Rf
+ 4 or 5
n
.

The team determined a half-life of 2.1 seconds, in contrast to earlier reports of 47 ms and suggested that the two half-lives might be due to different isomeric states of ²⁶²Rf.^[19] Studies on the same reaction by a team at Dubna, lead to the observation in 2000 of alpha decays from ²⁶¹Rf and spontaneous fissions of ^261mRf.^[20]

The hot fusion reaction using a uranium target was first reported at Dubna in 2000:

²³⁸
₉₂U
+ ²⁶
₁₂Mg
→ ^264−x
₁₀₄Rf
+ x
n
(x = 3, 4, 5, 6).

They observed decays from ²⁶⁰Rf and ²⁵⁹Rf, and later for ²⁵⁹Rf. In 2006, as part of their program on the study of uranium targets in hot fusion reactions, the team at LBNL also observed ²⁶¹Rf.^[20][21][22]

Cold fusion studies[]

The first cold fusion experiments involving element 104 were done in 1974 at Dubna, by using light titanium-50 nuclei aimed at lead-208 isotope targets:

²⁰⁸
₈₂Pb
+ ⁵⁰
₂₂Ti
→ ^258−x
₁₀₄Rf
+ x
n
(x = 1, 2, or 3).

The measurement of a spontaneous fission activity was assigned to ²⁵⁶Rf,^[23] while later studies done at the Gesellschaft für Schwerionenforschung Institute (GSI), also measured decay properties for the isotopes ²⁵⁷Rf, and ²⁵⁵Rf.^[24][25]

In 1974 researchers at Dubna investigated the reaction of lead-207 with titanium-50 to produce the isotope ²⁵⁵Rf.^[26] In a 1994 study at GSI using the lead-206 isotope, ²⁵⁵Rf as well as ²⁵⁴Rf were detected. ²⁵³Rf was similarly detected that year when lead-204 was used instead.^[25]

Decay studies[]

Most isotopes with an atomic mass below 262 have also observed as decay products of elements with a higher atomic number, allowing for refinement of their previously measured properties. Heavier isotopes of rutherfordium have only been observed as decay products. For example, a few alpha decay events terminating in ²⁶⁷Rf were observed in the decay chain of darmstadtium-279 since 2004:

²⁷⁹
₁₁₀Ds
→ ²⁷⁵
₁₀₈Hs
+
α
→ ²⁷¹
₁₀₆Sg
+
α
→ ²⁶⁷
₁₀₄Rf
+
α
.

This further underwent spontaneous fission with a half-life of about 1.3 h.^[27][28][29]

Investigations on the synthesis of the dubnium-263 isotope in 1999 at the University of Bern revealed events consistent with electron capture to form ²⁶³Rf. A rutherfordium fraction was separated, and several spontaneous fission events with long half-lives of about 15 minutes were observed, as well as alpha decays with half-lives of about 10 minutes.^[17] Reports on the decay chain of flerovium-285 in 2010 showed five sequential alpha decays that terminate in ²⁶⁵Rf, which further undergoes spontaneous fission with a half-life of 152 seconds.^[30]

Some experimental evidence was obtained in 2004 for an even heavier isotope, ²⁶⁸Rf, in the decay chain of an isotope of moscovium:

²⁸⁸
₁₁₅Mc
→ ²⁸⁴
₁₁₃Nh
+
α
→ ²⁸⁰
₁₁₁Rg
+
α
→ ²⁷⁶
₁₀₉Mt
+
α
→ ²⁷²
₁₀₇Bh
+
α
→ ²⁶⁸
₁₀₅Db
+
α
 ? → ²⁶⁸
₁₀₄Rf
+
ν
_e.

However, the last step in this chain was uncertain. After observing the five alpha decay events that generate dubnium-268, spontaneous fission events were observed with a long half-life. It is unclear whether these events were due to direct spontaneous fission of ²⁶⁸Db, or ²⁶⁸Db produced electron capture events with long half-lives to generate ²⁶⁸Rf. If the latter is produced and decays with a short half-life, the two possibilities cannot be distinguished.^[31] Given that the electron capture of ²⁶⁸Db cannot be detected, these spontaneous fission events may be due to ²⁶⁸Rf, in which case the half-life of this isotope cannot be extracted.^[7][32] A similar mechanism is proposed for the formation of the even heavier isotope ²⁷⁰Rf as a short-lived daughter of ²⁷⁰Db (in the decay chain of ²⁹⁴Ts, first synthesized in 2010) which then undergoes spontaneous fission:^[8]

²⁹⁴
₁₁₇Ts
→ ²⁹⁰
₁₁₅Mc
+
α
→ ²⁸⁶
₁₁₃Nh
+
α
→ ²⁸²
₁₁₁Rg
+
α
→ ²⁷⁸
₁₀₉Mt
+
α
→ ²⁷⁴
₁₀₇Bh
+
α
→ ²⁷⁰
₁₀₅Db
+
α
 ? → ²⁷⁰
₁₀₄Rf
+
ν
_e.

According to a 2007 report on the synthesis of nihonium, the isotope ²⁸²Nh was twice observed to undergo a similar decay to form ²⁶⁶Db. In one case this underwent spontaneous fission with a half-life of 22 minutes. Given that the electron capture of ²⁶⁶Db cannot be detected, these spontaneous fission events may be due to ²⁶⁶Rf, in which case the half-life of this isotope cannot be extracted. In the other case, no spontaneous fission event was observed; it could have been missed, or ²⁶⁶Db might have underwent two more alpha decays to long-lived ²⁵⁸Md, with a half-life (51.5 d) longer than the total time of the experiment.^[5][33]

Nuclear isomerism[]

Currently suggested decay level scheme for ²⁵⁷Rf^g,m from the studies reported in 2007 by Hessberger et al. at GSI^[34]

Several early studies on the synthesis of ²⁶³Rf have indicated that this nuclide decays primarily by spontaneous fission with a half-life of 10–20 minutes. More recently, a study of hassium isotopes allowed the synthesis of atoms of ²⁶³Rf decaying with a shorter half-life of 8 seconds. These two different decay modes must be associated with two isomeric states, but specific assignments are difficult due to the low number of observed events.^[17]

During research on the synthesis of rutherfordium isotopes utilizing the ²⁴⁴Pu(²²Ne,5n)²⁶¹Rf reaction, the product was found to undergo exclusive 8.28 MeV alpha decay with a half-life of 78 seconds. Later studies at GSI on the synthesis of copernicium and hassium isotopes produced conflicting data, as ²⁶¹Rf produced in the decay chain was found to undergo 8.52 MeV alpha decay with a half-life of 4 seconds. Later results indicated a predominant fission branch. These contradictions led to some doubt on the discovery of copernicium. The first isomer is currently denoted ^261aRf (or simply ²⁶¹Rf) whilst the second is denoted ^261bRf (or ^261mRf). However, it is thought that the first nucleus belongs to a high-spin ground state and the latter to a low-spin metastable state.^[35] The discovery and confirmation of ^261bRf provided proof for the discovery of copernicium in 1996.^[36]

A detailed spectroscopic study of the production of ²⁵⁷Rf nuclei using the reaction ²⁰⁸Pb(⁵⁰Ti,n)²⁵⁷Rf allowed the identification of an isomeric level in ²⁵⁷Rf. The work confirmed that ^257gRf has a complex spectrum with 15 alpha lines. A level structure diagram was calculated for both isomers.^[37] Similar isomers were reported for ²⁵⁶Rf also.^[38]

Future experiments[]

The team at GSI are planning to perform first detailed spectroscopic studies on the isotope ²⁵⁹Rf. It will be produced in the new reaction:

${\displaystyle {\ce {{^{238}_{92}U}+{^{24}_{12}Mg}->{^{259}_{104}Rf}+3_{0}^{1}n}}}$

Chemical yields of isotopes[]

Cold fusion[]

The table below provides cross-sections and excitation energies for cold fusion reactions producing rutherfordium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

Hot fusion[]

The table below provides cross-sections and excitation energies for hot fusion reactions producing rutherfordium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

References[]

• Isotope masses from:
  • M. Wang; G. Audi; A. H. Wapstra; F. G. Kondev; M. MacCormick; X. Xu; et al. (2012). "The AME2012 atomic mass evaluation (II). Tables, graphs and references" (PDF). Chinese Physics C. 36 (12): 1603–2014. Bibcode:2012ChPhC..36....3M. doi:10.1088/1674-1137/36/12/003.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Isotopic compositions and standard atomic masses from:
  • de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.
  • Wieser, Michael E. (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. Lay summary. {{cite journal}}: Cite uses deprecated parameter |lay-url= (help)
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.

External links[]

• http://www.radiochemistry.org/periodictable/elements/isotopes_data/104.html
• http://www-phynu.cea.fr/science_en_ligne/carte_potentiels_microscopiques/choix/isotopes/z104_eng.html
• http://www.springermaterials.com/docs/info/978-3-540-70609-0_7730.html