# Isotopes of rutherfordium

Rutherfordium (Rf) is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was either 259Rf in 1966 or 257Rf in 1969. There are 15 known radioisotopes from 253Rf to 268Rf (2 of which, 266Rf and 268Rf, are unconfirmed) and 4 isomers. The longest-lived isotope is 267Rf with an estimated half-life of 5 hours. The longest directly measured half-life is 263Rf at 11 minutes, and the longest-lived isomer is 261mRf with a half-life of 81 seconds.

## Table

nuclide
symbol
Z(p) N(n)
isotopic mass (u)

half-life decay
mode(s)[1][n 1]
daughter
isotope(s)
nuclear
spin
excitation energy
253Rf 104 149 253.10044(44)# 13(5) ms SF (51%) (various) (7/2)(+#)
α (49%) 249No
253mRf 200(150)# keV 52(14) µs
[48(+17-10) µs]
(1/2)(-#)
254Rf 104 150 254.10005(30)# 23(3) µs SF (99.7%) (various) 0+
α (.3%) 250No
255Rf 104 151 255.10127(12)# 1.64(11) s SF (52%) (various) (9/2-)#
α (48%) 251No
256Rf 104 152 256.101152(19) 6.45(14) ms SF (96%) (various) 0+
α (6%) 252No
257Rf 104 153 257.102918(12)# 4.7(3) s α (79%) 253No (1/2+)
β+ (18%) 257Lr
SF (2.4%) (various)
257mRf 114(17) keV 3.9(4) s (11/2-)
258Rf 104 154 258.10343(3) 12(2) ms SF (87%) (various) 0+
α (13%) 254No
259Rf 104 155 259.10560(8)# 2.8(4) s α (93%) 255No 7/2+#
SF (7%) (various)
β+ (.3%) 259Lr
260Rf 104 156 260.10644(22)# 21(1) ms SF (98%) (various) 0+
α (2%) 256No
261Rf 104 157 261.10877(5) 5.5(25) s α (76%) 257No 3/2+#
β+ (14%) 261Lr
SF (10%) (various)
261mRf 70(100)# keV 81(9) s β+ 261Lr 9/2+#
α (rare) 257No
262Rf 104 158 262.10993(24)# 2.3(4) s SF (99.2%) (various) 0+
α (.8%) 258No
262mRf 600(400)# keV 47(5) ms SF (various) high
263Rf 104 159 263.1125(2)# 11(3) min SF (70%) (various) 3/2+#
α (30%) 259No
265Rf[n 2] 104 161 265.11668(39)# 2.5 min SF (various)
266Rf[n 3][n 4] 104 162 266.11817(50)# 10# h 0+
267Rf[n 5] 104 163 267.12179(62)# 5# h
[2.3(+980-17) h]
SF (various)
268Rf[n 3][n 6] 104 164 268.12397(77)# 1# h 0+
1. ^ Abbreviations:
EC: Electron capture
SF: Spontaneous fission
2. ^ Not directly synthesized, occurs in decay chain of 285Fl
3. ^ a b Discovery of this isotope is unconfirmed
4. ^ Not directly synthesized, occurs in decay chain of 282Uut
5. ^ Not directly synthesized, occurs in decay chain of 287Fl
6. ^ Not directly synthesized, occurs in decay chain of 288Uup

### Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

## History of synthesis of isotopes by cold fusion

This section deals with the synthesis of nuclei of rutherfordium 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(50Ti,xn)258−xRf (x=1,2,3)

This reaction was first studied in 1974 by the team at Dubna. They measured a spontaneous fission activity assigned to 256Rf. [2] The reaction was further studied in 1985 by the GSI team who measured the decay properties of the isotopes 257Rf and 256Rf. The team were able to determine some initial spectroscopic properties of 257Rf and found that the alpha decay pattern was very complicated.[3]

After an upgrade of their facilities, they repeated the reaction in 1994 with much higher sensitivity and detected some 1100 atoms of 257Rf and 1900 atoms of 256Rf along with 255Rf in the measurement of the 1n,2n and 3n excitation functions. The large amount of decay data for 257Rf allowed the detection of an isomeric level and the construction of a partial decay level structure which confirmed the very complicated alpha decay pattern. They also found evidence for an isomeric level in 255Rf. [4] The GSI team continued in 2001 with the measurement of the 3n excitation function. In 2002, scientists at the Argonne University in Illinois began their first studies of translawrencium elements with the synthesis and alpha-gamma spectroscopy of 257Rf.[5] In 2004, the GSI began their spectroscopic studies of the 257Rf isotope. In 2007, the Lawrence Berkeley National Laboratory (LBNL) studied the 2n product, 256Rf, in a search for K-isomers and discovered three such isomers.[6]

Currently suggested decay level scheme for 255Rf from the study reported in 2007 by Hessberger et al. at GSI[7]
207Pb(50Ti,xn)257−xRf (x=2)

This reaction was first studied in 1974 by the team at Dubna. They measured a spontaneous fission activity assigned to 255Rf. The reaction was further studied in 1985 by the GSI team who measured the decay properties of the isotope 255Rf. A further spectroscopic study was reported in 2000 which led to a first decay level scheme for the isotope.[8] The isomeric level proposed in 1994 was not found. In 2006, the spectroscopy was continued and the decay scheme was confirmed and improved. [9]

206Pb(50Ti,xn)256−xRf (x=1,2)

The team at GSI first studied this reaction in 1994 in an effort to study neutron deficient isotopes of rutherfordium. They were able to detect 255Rf and 144 atoms of the new isotope 254Rf, which decayed by spontaneous fission.[4]

204Pb(50Ti,xn)254−xRf (x=1)

The team at GSI first studied this reaction in 1994 in an effort to study neutron deficient isotopes of rutherfordium. They were able to detect 14 atoms of the new isotope 253Rf, which decayed by spontaneous fission.[4]

208Pb(48Ti,xn)256−xRf (x=1)

In 2006, as part of a program looking at the effect of isospin on the mechanism of cold fusion, the team at LBNL studied this reaction. They measured the 1n excitation function and determined that the change of a Ti-50 projectile to a Ti-48 one significantly reduced the yield, in agreement with predictions.[10]

124Sn(136Xe,xn)260-xRf

In an important study, in May 2004, the team at GSI attempted the symmetric synthesis of rutherfordium by attempting to fuse two fission fragments. Theory suggests that there may be an enhancement of the yield. No product atoms were detected and an upper limit of 1000 pb was estimated for the yield of this reaction.[11]

## History of synthesis of isotopes by hot fusion

This section deals with the synthesis of nuclei of rutherfordium 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.

238U(26Mg,xn)264−xRf (x=3,4,5,6)

The hot fusion reaction using a uranium target was first reported in 2000 by Yuri Lazarev and the team at the Flerov Laboratory of Nuclear Reactions (FLNR). They were able to observe decays from 260Rf and 259Rf in the 4n and 5n channels.[12] They measured yields of 240 pb in the 4n channel and 1.1 nb in the 5n channel. In 2006, as part of their program on the study of uranium targets in hot fusion reactions, the team at LBNL measured the 4n,5n and 6n excitation functions for this reaction and observed 261Rf in the 3n exit channel.[13][14]

244Pu(22Ne,xn)266−xRf (x=4,5)

This reaction was reported in 1996 at LBNL in an attempt to study the fission characteristics of 262Rf. The team were able to detect the spontaneous fission (SF) of 262Rf and determine its half-life as 2.1 s, in contrast to earlier reports of a 47 ms activity. It was suggested that the two half-lives might be related to different isomeric states.[15] The reaction was further studied in 2000 by Yuri Lazarev and the team at Dubna. They were able to observe 69 alpha decays from 261Rf and spontaneous fission of 262Rf.[16] Later work on hassium has allowed a reassignment of the 5n product to 261mRf.

242Pu(22Ne,xn)264−xRf (x=3,4?,5?)

The synthesis of element 104 was first attempted in 1964 by the team at Dubna using this reaction. The first study produced evidence for a 0.3 s SF activity tentatively assigned to 260104 or 259104 and an unidentified 8 s SF activity. The former activity was later retracted and the latter activity associated with the now-known 259104.[17] In 1966, in their discovery experiment, the team repeated the reaction using a chemical study of volatile chloride products. The group was able to identify a volatile chloride decaying by short spontaneous fission with eka-hafnium properties. This gave strong evidence for the formation of [104]Cl4 and the team suggested the name kurchatovium. Although a half-life was not accurately measured, later evidence suggested that the product was most likely 259104.[17] In 1968, the team searched for alpha decay from 260104 but were unable to detect such activity. In 1970, the team repeated the reaction once again and confirmed the ~0.2 s SF activity. They also repeated the chemistry experiment and obtained identical results to their 1966 experiment and calculated a likely half-life of ~0.5 seconds for the SF activity. In 1971, the reaction was repeated again and 0.1 s and 4.5 s SF activities were found. The 4.5 s activity was correctly assigned to 259104.[17] A chemistry experiment in the same year reaffirmed the formation of a 0.3 SF activity for an eka-hafnium product.[17] Later, the 0.1−0.3 s SF activity was retracted as belonging to a kurchatovium isotope but the observation of eka-hafnium reactivity remained and was the basis of their successful claim to discovery.[17] The reaction was further studied in 2000 by Yuri Lazarev at Dubna. They were able to observe 261Rf in the 3n channel, later reassigned to 261mRf.

242Pu(20Ne,xn)262−xRf

This reaction was first studied in 1964 to assist in the assignments using the analogous reaction with a Ne-22 beam. The Dubna team were unable to detect any 0.3 s spontaneous fission activities.[17] The reaction was later studied in 2003 at the Paul Scherrer Institute (PSI) in Bern, Switzerland. They detected some spontaneous fission activities but were unable to confirm the formation of 259Rf.[18]

248Cm(22Ne,αxn)266−xRf (x=3?)

This reaction was studied in 1999 at the University of Bern, Switzerland in order to search for the new isotope 263Rf. A rutherfordium fraction was separated and several SF events with long lifetimes and alpha decays with energy 7.8 MeV and 7.9 MeV were observed. A second experiment using a study of the fluoride of rutherfordium products also produced 7.9 MeV alpha decays.[19]

248Cm(18O,xn)266−xRf (x=3?,5)

This reaction was first studied in 1970 by Albert Ghiorso at the LBNL. The team identified 261Rf in the 5n channel using the method of correlation of genetic parent-daughter decays. A half-life of 65 s was determined. [20] A repeat later that year using cation exchange chromatography indicated that the product did not form a +2 or +3 cation and behaved as eka-hafnium. A study of the properties of rutherfordium isotopes was performed in 1981 at the LBNL. In a series of reactions, a 1.5 s SF activity was identified and assigned to a fermium descendant although later evidence indicates a possible assignment to 262Rf. In contrast, in a subsequent review of isotope properties by Somerville et al. at LBNL in 1985, a 47 ms SF activity was assigned to 262Rf. This assignment has not been verified. [21] The reaction was further studied in 1991 by Czerwinski et al. at the LBNL. In this experiment, spontaneous fission activities with long lifetimes were observed in rutherfordium fractions and tentatively assigned to 263Rf. In 1996, chemical studies on the chloride of rutherfordium were published by the LBNL. In this experiment, the half-life was improved to 78 s. A repeat of the experiment in 2000 assessing the volatility of the bromide further refined the half-life to 75 s.

248Cm(16O,xn)264−xRf (x=4)

This reaction was studied in 1969 by Albert Ghiorso at the University of California. The aim was to detect the 0.1–0.3 s SF activity reported at Dubna, assigned to 260104. They were unable to do so, only observing a 10–30 ms SF activity, correctly assigned to 260104. The failure to observe the 0.3 s SF activity identified by Dubna gave the Americans the incentive to name this element rutherfordium.[17]

246Cm(18O,xn)264−xRf

In an attempt to unravel the properties of spontaneous fission activities in the formation of rutherfordium isotopes, this reaction was performed in 1976 by the FLNR. They observed an 80 ms SF activity. Subsequent work led to the complete retraction of the 0.3s - 0.1s - 80 ms SF activities observed by the Dubna team and associated with background signals.[17]

249Bk(15N,xn)264−xRf (x=4)

This reaction was studied in 1977 by the team in Dubna. They were able to confirm the detection of a 76 ms SF activity. The assignment to rutherfordium isotopes was later retracted. The LBNL re-studied the reaction in 1980 and in 1981 they reported that they were unable to confirm the ~80 ms SF activity. The Dubna team were able to measure a 28 ms SF activity in 1985 and assigned the isotope correctly to 260104.[17]

249Cf(13C,xn)262−xRf (x=4)

This use of californium-249 as a target was first studied by Albert Ghiorso and the team at the University of California in 1969. They were able to observe an 11 ms SF activity which they correctly assigned to 258104.[22]

Currently suggested decay level scheme for 257Rfg,m from the study performed in 2004 by Hessberger et al. at GSI[citation needed]
249Cf(12C,xn)261−xRf (x=3,4)

In their 1969 discovery experiments, the team at University of California also used carbon-12 beam to irradiate a californium-249 target. They were able to confirm the 11 ms SF activity found with a carbon-13 beam and again correctly assigned to 258104. The actual discovery experiment was the observation of alpha decays genetically linked to 253102 and therefore positively identified as 257104.[22] In 1973, Bemis and his team at Oak Ridge confirmed the discovery by measuring coincident X-rays from the daughter 253102.[23]

## Synthesis of isotopes as decay products

Isotopes of rutherfordium have also been identified in the decay of heavier elements. Observations to date are summarized in the table below. EC refers to electron capture.

Evaporation residue Observed Rf isotope
288Uup 268Rf (possible EC of 268Db)
291Lv, 287Fl, 283Cn 267Rf
282Uut 266Rf (EC of 266Db)
269Sg 265Rf
271Hs 263gRf
263Db 263mRf (EC of 263Db)
266Sg (possibly 266mSg) 262Rf (possibly 262mRf)
277Cn, 273Ds, 269Hs, 265Sg 261mRf, 261Rf
271Ds, 267Hs, 263Sg 259Rf
269Ds, 265Hs, 261Sg 257Rf
264Hs, 260Sg 256Rf
259Sg 255Rf

## Chronology of isotope discovery

Isotope Discovered Reaction
253Rf 1994 204Pb(50Ti,n) [4]
254Rf 1994 206Pb(50Ti,2n) [4]
255Rf 1974? 1985 207Pb(50Ti,2n)
256Rfg 1974? 1985 208Pb(50Ti,2n)
256Rfm1 2007 208Pb(50Ti,2n)
256Rfm2 2007 208Pb(50Ti,2n)
256Rfm3 2007 208Pb(50Ti,2n)
257Rfg,m 1969 249Cf(12C,4n) [22]
258Rf 1969 249Cf(13C,4n) [22]
259Rf 1969 249Cf(13C,3n) [22]
260Rf 1969 248Cm(16O,4n)
261Rfa 1970 248Cm(18O,5n) [20]
261Rfb 1996 208Pb(70Zn,n) [24]
262Rf 1996 244Pu(22Ne,4n) [15]
263Rfa 1990? 248Cm(18O,3n)
263Rfb 2004 248Cm(26Mg,3n) [25]
264Rf unknown
265Rf 2010 242Pu(48Ca,5n)
266Rf? 2006 237Np(48Ca,3n) [26]
267Rf 2003/2004 238U(48Ca,3n) [24]
268Rf? 2003 243Am(48Ca,3n) [27]

## Isomerism in rutherfordium nuclides

### 263Rf

Initial work on the synthesis of rutherfordium isotopes by hot fusion pathways focused on the synthesis of 263Rf. Several studies have indicated that this nuclide decays primarily by spontaneous fission with a long half-life of 10–20 minutes. Alpha particles with energy 7.8-7.9 MeV have also been associated with this nucleus. More recently, a study of hassium isotopes allowed the synthesis of an atom of 263Rf decaying by spontaneous fission with a short half-life of 8 seconds. These two different decay modes must be associated with two isomeric states. Specific assignments are difficult due to the low number of observed events. It is reasonable to tentatively assign the long life to a meta-stable state, namely 263mRf, and the shorter life to the ground state, namely 263gRf. Further studies are required to allow a definite assignment.

### 261Rf

Early research on the synthesis of rutherfordium isotopes utilised 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 by the GSI team on the synthesis of copernicium and hassium isotopes produced conflicting data. In this case, 261Rf was found to undergo 8.52 MeV alpha decay with a short half-life of 4 seconds. Later results indicated a predominant fission branch. These contradictions led to some doubt on the discovery of copernicium. However, it is now understood that the first nucleus belongs to an isomeric meta-stable state, namely 261mRf and the latter to the ground state isomer, namely 261gRf.[28] The discovery and confirmation of 261gRf provided proof for the discovery of copernicium in 1996.

### 257Rf

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 very complicated spectrum with as many as 15 alpha lines. A level structure diagram was calculated for both isomers.

## Spectroscopic decay level schemes for rutherfordium isotopes

### 257Rf

This is the currently suggested decay level scheme for 257Rfg,m from the study performed in 2004 by Hessberger et al. at GSI

[4]

### 255Rf

This is the currently suggested decay level scheme for 255Rf from the study reported in 2007 by Hessberger et al. at GSI

## Unconfirmed isotopes

### 268Rf

In the synthesis of ununpentium, the isotope 288115 has been observed to decay to 268Db which undergoes spontaneous fission with a half-life of 29 hours. Given that the electron capture of 268Db cannot be detected, these SF events may in fact be due to the SF of 268Rf, in which case the half-life of this isotope cannot be extracted. [27]

### 266Rf

In the synthesis of ununtrium, the isotope 282113 has been observed to decay to 266Db which undergoes spontaneous fission with a half-life of 22 minutes. Given that the electron capture of 266Db cannot be detected, these SF events may in fact be due to the SF of 266Rf, in which case the half-life of this isotope cannot be extracted.[26]

## Retracted isotopes

### 265Rf

In 1999, American scientists at the University of California, Berkeley, announced that they has succeeded in synthesizing three atoms of 293118. These parent nuclei successively emitted seven alpha particles to form 265Rf nuclei. Their claim was retracted in 2001. This isotope was finally created in 2010.

### 255mRf

A detailed spectroscopic study of the production of 255Rf nuclei using the reaction 206Pb(50Ti,n)255Rf allowed the tentative identification of an isomeric level in 255Rf. A more detailed study later confirmed that this was not the case.

## 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.

Projectile Target CN 1n 2n 3n
50Ti 208Pb 258Rf 38.0 nb, 17.0 MeV 12.3 nb, 21.5 MeV 660 pb, 29.0 MeV
50Ti 207Pb 257Rf 4.8 nb
50Ti 206Pb 256Rf 800 pb, 21.5 MeV 2.4 nb, 21.5 MeV
50Ti 204Pb 254Rf 190 pb, 15.6 MeV
48Ti 208Pb 256Rf 380 pb, 17.0 MeV

### 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.

Projectile Target CN 3n 4n 5n
26Mg 238U 264Rf 240 pb 1.1 nb
22Ne 244Pu 266Rf + 4.0 nb
18O 248Cm 266Rf + 13.0 nb

## 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:

$\,^{238}_{92}\mathrm{U}\ + \,^{24}_{12}\mathrm{Mg}\to \,^{259}_{104}\mathrm{Rf}\ + 3 \,^{1}_{0}\mathrm{n}$

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22. "Positive Identification of Two Alpha-Particle-Emitting Isotopes of Element 104", Ghiorso et al.., Phys. Rev. Lett. 22, 1317-1320 (1969). Retrieved on 2008-03-04
23. ^ "X-Ray Identification of Element 104", Bemis et al., Phys. Rev. Lett. 31, 647-650 (1973). Retrieved on 2008-03-04
24. ^ a b see copernicium
25. ^ see hassium
26. ^ a b see ununtrium
27. ^ a b see ununpentium
28. ^ "EVIDENCE FOR ISOMERIC STATES IN 261Rf", Dressler et al., PSI Annual Report 2001. Retrieved on 2008-01-29