Author: M. Gupta |  Citation: ENSDF |  Cutoff date: 1-Aug-2015 

Authors: M. Gupta, Thomas W. Burrows |  Citation: ENSDF |  Cutoff date: 31-Dec-2008 

Author: A.Artna-cohen |  Citation: Nucl. Data Sheets 88, 277 (1999) |  Cutoff date: 31-Jul-1999 

 Full ENSDF file | Adopted Levels (PDF version) 

Q(β-)=-2928 keV SYS(n)= 5958 keV SYS(p)= 4244 keV SYQ(α)= 8646 keV 50
  A  265Sg α decay (14.4 S)  B  265Sg α decay (8.5 S)

General Comments:

1970Gh01: direct synthesis through the reaction 248Cm(18O,5n) at beam energies of 90-100 MeV. The 18O beam was provided by the Berkeley heavy ion accelerator (HILAC). The yield of 261Rf corresponded to a cross-section of 5 nb. The excitation function for the reaction was determined approximately. The bombarding energies were close to the calculated peak at 97 MeV. Recoils of interest were deposited via a He gas jet on a rotating wheel with a cycle rate of 48 s. α spectroscopy was performed using two stationery and two rotating Si-Au surface barrier detectors. Target impurities resulted in a host of contaminants in the observed spectra. The main α particle group was determined to be at 8.28 ± 0.02 MeV. The joint mother-daughter activity was 70 s ± 10. The combined half-life was 7% longer than of the mother activity alone resulting in the corrected T1/2 of 65 s ± 10 for 261Rf. No SF branch could be determined although an upper limit of 10% was estimated.

1994Lo27, 1994La22: produced as daughter nuclide in the reaction 248Cm(22Ne,5n) E=116, 121 MeV using the U400 cyclotron at Dubna and the H-gas filled separator DGFRS. See 265Sg adopted data set for experimental details. Four α-α-(α) correlations at 121 MeV were attributed to parent 265Sg with Eα=8.71-8.91 MeV at a production cross-section of 260 pb accurate to within an approximate factor of three. The Eα=8.16 to 8.17 MeV α-α correlation was attributed to the decay of 261Rf to 257No. The experimenters note that except for a single triple α correlation, 261Rf could not be distinguished from 257No with certainty due to the similarity in α energies. Experimenters suggest T1/2=65 s, Eα≈8.29 MeV. Evaluators note that 4 α-SF events attributed to 266Sg and 262Rf are reassigned by 2006Dv01 to 265Sg and 261Rfb (3 s state) which was unknown at the time. 1997Sc48 note that the half-lives of the Sg isotopes could not be measured due to the lack of an initial implantation signal in the Si detectors.

1996Ka66: direct synthesis through the reaction 248Cm(18O5-,5n) at LBL, E(lab)=117 MeV. Reaction cross-section was 5 nb. The purpose of this chemistry experiment was to study the relative volatility of tetrachlorides of the group 4 elements Zr, Hf and Rf using gas phase isothermal chromatography. Reaction products were transported through the heavy element volatility instrument (HEVI) described in 1992Ka57. Total transportation yield was 60% to 80%. Species leaving the chromatography column were transported to the merry-go-round (Mg) detector system (1980Ho25) where 6 pairs of PIPS detectors were used to measure the kinetic energies of coincident fission fragments and α particles. The step time was 1 minute and the detection efficiency was about 60%. α spectra from all chemistry measurements in the energy window 8.15 to 8.38 MeV were analysed. α total of 170 α-α correlations were observed close to the expected value of 169. The suggested α decay half-life was T1/2=78 s +11-6 (68% c.i.) using the maximum likelihood method. The experimenters point out that a short component appears to exist in the observed SF events providing an upper limit of 11% corresponding to T1/2(SF)>709 s assuming no other decay modes. The evaluators note that estimates for randomness are not provided for the α-α correlations making it difficult to distinguish true mother-daughter correlations from random events. However due to the improved ability of HEVI to seperate out non-volatile interfering activities prior to counting, the half-life arrived at in this work is expected to be better than the earlier value from 1970Gh01.

1996Ho13: descendent of 277112 produced by 208Pb(70Zn,n), E=343.8 MeV (E*=10.1 MeV; σ=1.0 pb +18-4) at GSI/SHIP. Two EVR-α1-α2-α3-α4-α5-α6 events observed. One event retracted by 2002Ho11; σ revised to 0.4 pb +9-3. See 277112 Adopted Levels in 2005Gu33 for details. One energy-time correlation recorded with one escaped particle for this nucleus with a time interval of 32.7 s (subsequently retracted by 2002Ho11) and one α decay with Eα=8.52 MeV ± 20 after a time interval of 4.7 s.

1996La12: descendant of 273Ds using the hot fusion reaction 244Pu(34S,5n) E=190 MeV at Dubna using DGFRS in collaboration with LLNL. The cross-section was about 0.4 pb for 273Ds. See 273Ds Adopted Levels in 2005Gu33 for details. The energy window was set at 8.15 to 8.45 MeV for 261Rf and 257No with a time window of 1100 s following the emission of the first α. Suggested properties from the most likely single event in beam off conditions are Eα=8.20 MeV with a lifetime of 117 s corresponding to T1/2=79 s. The assignment took into account the presence of a small ΔE signal from preliminary results, which was attributed at the time to a conversion electron accompanying the decay of the 8.63 MeV α (from 265Sg) to 261Rf and 257No. 1996La12 reason that this is consistent with odd-A 265Sg decaying preferentially to an excited state in daughter 261Rf.

1997Sc48, 1997Sc49: daughter of 265Sg produced in the hot fusion reaction 248Cm(22Ne,5n) E=121 MeV at GSI using the UNILAC. See 265Sg adopted levels for details. Two chemical separation techniques were used in these studies intended to study the chemical properties of Sg and α spectroscopy was done using a rotating wheel. The decay of 265Sg was not seen although the presence of 261Rf and 257No as daughter products indicates the production of the parent. Three events were observed which could be attributed to 261Rf and daughter 257No: Eα=8.24, 8.26 and 8.52 MeV with |t=33.5, 22.7 and 142.0 s respectively (the last in ’parent-daughter’ mode). Energy resolution was ± 60 keV (FWHM) for α particles between 5 and 12 MeV in the PIPS detectors with a detection efficiency of 33% (for α’s) and 66% (SF fragment). The overall efficiency for an α-α correlation was 11% and for an α-SF correlation, 22%. α detailed analysis for random events is provided. The experimenters conclude that they have no evidence for the direct observation of 265Sg α decay. 1997Sc49 report results from gas chromatography using the OLGA III (1996Tu05) setup. Three decay events attributable to 265Sg were unambigiously measured. The following properties were attributed to 261Rf: Eα=8.36, 8.15 and 8.34 MeV ± 6 (FWHM) corresponding to |t=31.0, 53.3 and 48.4 s respectively. Average Eα=8.28 MeV ± 6 with T1/2=31 s +42-11 (evaluators). The authors note that with about 90% probability, the isotopes of Rf and No were formed between the time taken for chemical separation and their measurement about 28 s later given the experimental set-up used. α random analysis is not provided. The evaluators note that ambiguities exist in both these experiments and the data from 1997Sc48, 1997Sc49 were not considered in the reanalysis by 2008Du09. See also 1998Sc50 and 1999Tu04 for chemistry.

1998Tu01: produced as the daughter of 265Sg by the hot fusion reaction 248Cm(22Ne,5n) E=121, 123 MeV at the GSI UNILAC; see 265Sg adopted levels for experimental details. See also 266Sg Adopted Levels in 2005Gu33 for details. α total of 13 α-decay chains were attributed to 265Sg (and daughter 261Rf) and an additional three events attributed to 266Sg (and daughter 262Rf). Three more decay chains (events 14 to 16) with a higher degree of randomness were also suggested as belonging to the decay of 265Sg.

Correlated decay chains with Eα(mother)=8.57-9.07 MeV (265Sg) and Eα(daughter(s))=8.05-8.45 MeV (261Rf, 257No), and Eα(mother)=8.41-8.85 MeV (266Sg) and ESF(daughter) |> 20 MeV (262Rf) within 6.3 s are shown below: see also 265Sg adopted levels for details.

Note that Events 14-16 (assignments in parentheses) have a high degree of randomness and were not included in the re-evaluation by 2008Du09. Also, the three events (17 to 19) which were originally assigned to 266Sg (262Rg) through this work were reassigned to 265Sg (261Rf) via 2006Dv01 and considered as such in 2008Du09.

2000La34: direct synthesis through 244Pu(22Ne,5n)261Rf at beam energies of 114 and 120 MeV with an estimated cross-section of 4 nb at both energies. 69 α-α correlations linking decays of 261Rf and 257No were observed. Experiments were conducted using the Dubna U400 cyclotron. Recoil products were separated by DGFRS described in 1993LaZS. At a beam energy of 120 MeV, 17 correlated α-α pairs were detected with the following properties: Eα1=8.22-8.41 MeV, δt=0.4-181 s and Eα2=8.07-8.37 MeV. At 114 MeV, 25 pairs of α events were observed with Eα1=8.25-8.36 MeV, δt=0.2-129 s and Eα2=8.21-8.40 MeV. The number of α-α correlations from genetically unrelated events was estimated to be less than 0.1 within the time window of 200 s over the total of 42 linked events observed. An α energy spectrum was measured for the first time centered around Eα=8.3 MeV with standard deviation σ=33 keV. α single α energy peak was observed at Eα=8.30 MeV ± 0.06 for 261Rf without any contribution from α decays originating from the daughter. An estimate of the half-life is not given. No α decays corresponding to the 8.52 MeV events ascribed to 261Rf in 1996Ho13 or 1997Sc49 were seen.

2000Sy01: direct synthesis of 261Rf in the reaction 248Cm(18O,5n) at a beam energy on target of 99 MeV, to study the relative volatility of group 4 tetrabromides. The chromatographic system HEVI (heavy element volatility instrument) with an improved temperature profile (1992Ka57) was used in which less volatile species are retained. Upon exiting the chromatography column, molecules of interest were transported to the Mg wheel system (1980Ho25). α spectroscopy was performed by PIPS detectors allowing a solid angle detection efficiency of ≈60% for α and SF decays. The step time was set at 1 min since the experiment was designed to measure the known 78 s state in this nucleus. α decays were measured in energy window 8.12 - 8.38 MeV and T1/2=74 s +7-6 (68% c.i.) was suggested using a MLDS fit. The weighted average halflife when combined with 1996Ka66 was T1/2=75.5 s |+6.6. The recommended value was T1/2=75 s ± 7. The experimenters note that the half-life fit assumed the presence of the daughter 257No and 211Pom. The energy resolution, reaction cross-section and estimates of randomness are not provided. In the absence of a random analysis, evaluators note that it is difficult to distinguish between true mother-daughter correlations and random events. However as with the work of 1996Ka66, due to the improved ability of HEVI to seperate out non-volatile interfering activities prior to counting, the half-life reported in this work is expected to be better than the earlier value from 1970Gh01. The experiment was not sensitive to detecting a shorter half-life state. The volatility temperature of 261RfBr4 was 175o C and the adsorption enthalpy given as -87 ± 7 kJ/mol-1.

2001Hu22:produced as daughter in the chemical study of seaborgium oxide hydroxide with the reaction 248Cm(22Ne,4n)266Sg at a beam energy of 119 MeV provided by UNILAC, GSI. See 265Sg adopted levels for details. The results of this experiment were reassigned to 265Sg and 261Rf in the analysis by 2008Du09 and are relevant here. Two α-SF events were measured in the energy window of 8.61 to 8.75 MeV attributed to 266Sg and 262Rf: Eα1=8.66 MeV, |t1=84.9 s and |t(SF)=7.0 s; Eα2=8.70 MeV, |t2=4.8 s, |t(SF)=3.7 s. From both events, probabilities of randomness were 0.82 (no random), 0.16 (one random) and 2% (both random). Evaluators note that the average Eα=8.680 MeV ± 28 corresponding to T1/2=31 s +57-12 (Sg). The two-event SF T1/2(SF)=4 s +7-2 (Rf).

2002Ho11: descendant of 277112. The experiment of 1996Ho13 was redone at GSI/SHIP. The 208Pb(70Zn,n) reaction was again used at projectile energies of 346.1 MeV (E*=12.0 MeV; σ=0.5 pb +11-4; 1 event) and 343.8 MeV (E*=10.1 MeV; σ<2.6 pb; 0 events). One EVR-α1-α2-α3-α4-SF event observed. Retracted one event observed by 1996Ho13. See 277112 Adopted Levels in 2005Gu33 for details. Two events were measured for this nucleus: Eα=8.52 MeV and T1/2=4.7 s and a SF event with a lifetime of 14.5 s. The experimenters conclude that two states exist in 261Rf with half-lives of 78 s +11-6 and 4.2 s +3.4-1.3. The first state decays by the emission of an α particle with Eα=8.28 MeV. The second state decays either by an α particle with Eα=8.52 MeV or by SF with a branching of about 40%. It is suggested that the level with T1/2=4.2 s and Eα=8.52 MeV offers a better agreement with ground state systematics. The state decaying with T1/2=78 s is suggested as being an isomeric state. The possibility that 261Rf may have two states was put forward here for the first time.

2003Tu05, 2003Du27, 2002Du21: descendent of 269Hs in a chemical study. 269,270Hs produced by 248Cm(26Mg,xn) at E=143.7-146.8 MeV (σ(269Hs)≈6 pb, σ(270Hs)≈4 pb) at GSI using UNILAC with In situ Volatilization and Online detection (IVO) as part of a large international collaboration. Three decay chains were attributed to 269Hs. Chemically separated Hs atoms were identified by observing genetically linked decay chains. See 269,270Hs Adopted Levels in 2005Gu33 for details.

The above properties are from 2003Tu05. Note that the life-times of the parent could not be estimated with the applied thermochromatography technique since deposition times are not measured. 2003Tu05 state that the first event with Eα=8.5 MeV +7-3, Δt=2.4 s is in good agreement with 261Rf observed in the first decay chain in 2002Ho11. Taking into account all three events listed above, the experimenters suggest Eα=8.5 MeV +7-3 (events 1 and 3 are energy redundant) and T1/2=4.2 s +34-13 (SF branch of 40%) for the ground state in 261Rf. The previously known 78 s state is proposed as an isomer, in agreement with 2002Ho11. 2003Du27 suggest a randomness probability of |<7×10-5 for decay sequences originating from 269Hs. Two more possible candidates for 269Hs are shown in 2002Du21 but not listed here since the parent (in the first event) and daughter (in the second event) were not observed. Tentative assignments of two decay chains initially made to 270Hs in 2003Tu05 and 2002Du21 are reassigned to 269Hs via 2006Dv01. All six events (three chains above, two tentative decays and one candidate for either 269Hs or 270Hs from 2002Du21) are taken into account by 2008Du09 in their reanalysis of 261Rf data.

2004Vo24: descendent of 269,270Hs produced by 248Cm(26Mg,xn), E=142-150 MeV at GSI using the UNILAC with Continuously Working Arrangement for Clusterless Transport of In-Situ Produced Volatile Oxides (CALLISTO). See 269Hs Adopted Levels in 2005Gu33 for details. One correlated α-α-α chain and five α-SF chains observed; the last α-SF chain is a likely candidate for a random correlation.

α-α-α chain:

α-SF events:

Events 3 and 4 were included in the reevaluation by 2008Du09 and taken as originating from 269Hs (undetected) decaying to 265Sg and ending by SF from the 261Rfb state.

2006Dv01: grand daughter of 269Hs produced by 248Cm(26Mg,5n) at GSI/UNILAC. This was the first campaign, with beam energies of 193 MeV and 185 MeV corresponding to E=145, 136 MeV at the center of target for the expected 5n and 4n channels respectively. Three arc-shaped segmented targets (95.8% 248Cm, 4.2% 246Cm); one segment contained 2% by weight of 152Gd (30% enrichment) for simultaneous production of α-decaying Os isotopes, a chemical analog to Hs. See 2002Ki25, 2002Du21, and 2002Du22 for description of rapid chemical separation and on-line detection method used. Detection system consisted of a linear array of 2×32 PIPS detectors in two Invar profiles forming a vacuum tight gas channel. Energy resolution=50 keV (FWHM). The energy region of interest at Eα=8.0-9.5 MeV contained 145 events in the first 25 detectors. α decay chain was defined as an 8.0-9.5 MeV α-decay followed within 300S in the same or neighboring detector pair by an α-decay in the same energy window or by an SF. The search was repeated within the chain until no α-decay was registered. Data analysis revealed 15 correlated decay chains in the first 25 detector pairs; eight at E=145 MeV and seven at E=136 MeV. Based on the count rates, 3×10-5 random sequences can be for α-α-α-α, 5×10-3 for α-α-SF, and 1 for α-SF. Cross sections of ≈3 pb were measured for 270Hs at 136 MeV and ≈7 pb for 269Hs at 145 MeV within an estimated accuracy of a factor of ≈3.

Of the 7 events pertaining to the decay of 265Sg and 261Rf, one event with Eα=8.29 MeV and T1/2=22 s was assigned to 261Rfa while the other 6 events were assigned to the SF branch of 261Rfb (states labelled in accordance with 2008Du09).

Evaluators Note: While the results from this experiment are contradictory to earlier reports, they also differ from the properties of 266Sg and 270Hs adopted in 2005Gu33. 2006Dv01 suggest that data pertaining to these nuclides from earlier studies are more consistent with decays from 265Sg and 269Hs and were erroneously assigned to 266Sg and 270Hs whose properties stand revised via their work. The quantities adopted in 2005Gu33 were in consonance with claims made by the various experimental groups, some of which cited ’unambigious’ results for these nuclides. It is worthwhile to note that many of the early studies were designed primarily to measure chemical properties such as the greatly increased volatility of chlorides of trans-actinides (i.e. elements above Z=103). Hence, the experimental focus was mainly on the transmission of SF activity through a suitable detection system indicative of the presence of a specific nuclide and chemical species of interest (or its absence), rather than on the accurate measurement of a half-life. Additionally, the chemistry experiments employed hot fusion reactions with actinide targets which are known to lead to a relatively large SF background when compared to Pb-target based cold fusion reactions. The non-negligible SF background through most of the chemistry experiments may have interfered with the direct identification of 261Rfb as pointed out by 2008Du09. Finally, the presence of 212Po (Eα=8.52 MeV, Iα=2.05%) from transfer reactions caused by the projectile on Pb impurites in the target, could also have hindered the detection of the ’second’ shorter lived state, 261Rfb with Eα=8.505 MeV, even if the energy gate had been adequate. Subsequent independent studies providing collateral information and improved characterisation and assignment properties, through both physical and chemical means were necessary in aiding more conclusive assignments and an accurate determination of physical properties.

2006Ni10: in a study of the reaction 238U(30Si,xn)268Sg at GSI using the UNILAC and SHIP. The SHIP setup was similar to earlier work. See 265Sg adopted levels for details. One event (number 8) was attributed to 265Sg and daughter 261Rf. The assignment was made taking into account an ’average’ half-life for 265Sg from 2003Tu05 (3 events) and 2002Ho11 (2 events). An SF event following an escaped α particle with a lifetime of 11.4 s was assigned to 261Rf. The SF fragments were attributed to the presumed 100% SF mode as a result of α-decay from 265Sg, populating the 3.6 s +23-11 state in daughter 261Rf. The TKE was not provided as one of the SF fragments was not measured. α single γ emission with energy 1172 keV was measured in coincidence with the SF decay in the clover detector. The experimenters suggest that the gamma ray originated from a 100% SF branch of the 3.2 s level in 261Rf fed by the 15.2 s parent state with half life 10.5 s in 265Sg. If confirmed, this may represent the proposed 20% branch from 265Sga to 261Rfb (evaluators).

2007Mo09, 2005MoZT, 2005MoZQ: descendent of 27712. Confirmatory experiments performed by the Japanese group at RIKEN with 208Pb(70Zn,n) E=345.9 MeV (σ=0.44 pb +59-29). Two EVR-α1-α2-α3-α4-SF events observed. Confirmed results of 2002Ho11. See 277112 Adopted Levels in 2005Gu33 for details. Randomness factors are provided in 2007Mo09. The revised half-life for 261Rf from 4 events (including the two from GSI) is suggested as 5.3 s +53-18. 2007Mo09 note that while Eα of grand-parent 269Hs from 2006Dv01 is somewhat low when compared to their results, the α energies for 265Sg and decay properties of 261Rf are consistent. 2007Mo09 suggest an α:SF branching of 1:3 from the fifth event chain.

2008Dv02: grand daughter of 269Hs produced by 248Cm(26Mg,5n) at GSI/UNILAC. This was the second campaign with beam energies of 197 MeV, 189 and 181 MeV corresponding to E=150, 140 and 130 MeV at the target center. The experimental set-up was the same as used in 2006Dv01. Excitation functions were measured for the 248Cm(26Mg,3-5n)269-271Hs reaction in this work. Five decay chains attributed to 269Hs were observed:

Revised decay properties were deduced from all events including those from 2006Dv01:

269Hs: α decay, Eα=9.13 MeV 5 with T1/2=4 s (T1/2 calculated using 2005Pa72) and 8.95 MeV 5; 270Hs: α decay, T1/2=23 s (calculated using 2005Pa72 and Q(α) deduced from their experimental data), Eα=8.88 MeV 5; 265Sg: α decay, T1/2=15 s +7-4, Eα=8.69 MeV 5; and 266Sg: SF decay, T1/2=360 ms +250-100; 261Rfa: α decay, T1/2=20 s +110-10, Eα=8.29 MeV 5; and 261Rfb: α decay, T1/2=3 s +1-1, Eα=8.51 MeV 5 with α:SF ratio 0.09:0.91.

2008Du09: reexamine all earlier data for 265Sg and 261Rf following reassignments via 2006Dv01. α comprehensive reevaluation of all data relevant to the production of 261Rf in the reaction 248Cm(22Ne,xn)270-xSg was undertaken in an attempt to address existing ambiguities in deduced properties due to the varying quality of older data. Decay properties previously assigned to 266Sg in 1994La22, 1996La12, 1998Tu01, 2001Hu22, 2003Tu05 and 2004Vo24 are reexamined together with new data from 2006Dv01 and 2008Dv02. Properties of 261Rf from 1996Ka66 and 1996Ho13 are also examined. α total of 60 events were considered for 265Sg and 261Rf following reassignments. Strong indications for two states (a and b) in 261Rf are suggested although it is not possible to conclusively deduce which (if any) is the ground state due to the poor quality of data.

Decay properties attributed to this nucleus are:

261Rfa: Eα=8.30 MeV; T1/2=68 ± 3 s; bSF<0.11

261Rfb: Eα=8.51 MeV; T1/2=3 ± 1 s; bSF=0.91.

From available data on the direct production of 261Rf the isomeric population ratio σ(261Rfa)/σ(261Rfb) > 2 is deduced. In the case of the direct production of the grand-parent 269Hs, the decays proceed preferentially to through ’b’ states.

2008Ga08: 261Rf produced as an EVR in 238U(26Mg6+,3n) reaction at the lowest excitation energy of 35.3 MeV corresponding to a centre of target beam energy of 121.8 MeV. Excitation functions are shown for the 4n, 5n and 6n channels and an analysis provided. Noting that 2008Dv02 have observed a 3n channel from the 248Cm+26Mg reaction of comparable magnitude to the 4n channel in this work, a dedicated irradiation was performed to examine this. Experimenters used a 238UF4 rotating target at the 88-Inch cyclotron facility at LBNL and with the Berkeley gas-filled recoil separator (BGS) filled with He gas. α pulsed beam was used for portions of the run to mimimise random correlations. α fast shut-off mode was used during the pulsed portion of this experiment, following the detection of an EVR during the beam pulse and the observed α within 300 s and ± 3.5 mm of the EVR. This enabled the detection of the 25 s daughter 257No in nearly background free conditions. 280 such beam shut-offs lasting 100 s each were triggered by a potential EVR-α event. 8 α particles were measured in the energy range 8.0-8.7 MeV. Expected random EVR-α-α sequences numbered 7 x 10-3 during the entire irradiation. The probability of obtaining a random EVR-SF sequence was within five half-lives of 261Rfb (estimated to be 6%). α single correlated EVR-α-α event with properties Eα=8.34 MeV 5 attributed to 261Rfa decay and |t=103.2 s, followed by Eα=8.30 MeV 5, |t=12.2 s from 257No decay was observed corresponding to a cross-section of 28 pb +92-26. One EVR-SF event with SF energy of 173.3 MeV and lifetime of 9.4 s could belong to either state of Rf. Given that only a 6% chance of a random EVR-SF event exists within 5 half-lives of 261Rfb, if confirmed, the authors claim that this is the first observation of the "isomeric " state produced directly as an EVR rather than as the daughter of 265Sg. The evaluators note that the assignment to the 261Rfb state is in agreement with 2008Du09 although any conclusion regarding whether this is an isomeric or ground state requires confirmation. The cross-section of the reaction in the 3n channel will increase by 14% to 32 pb +93-26 including this event. Note that the evaluators adopt a nomenclature for states ’a’ and ’b’ consistent with 2008Du09.

2011Ha13: Direct synthesis via the reaction 248Cm(18O,5n)261Rf at RIKEN. Though both states were populated, the purpose of the experiment was a detailed study of the isomeric state 261Rfb to determine its decay properties. Through this experiment, the decay properties of this state were directly measured for the first time. The 18O beam was accelerated to energies of 95.1 and 93.1 MeV (middle of target) by the RIKEN Linear Accelerator (RILAC). Eight arc-shaped targets of 230 μg/cm2 thick 248Cm2O3 enriched to 96.638% with 0.91 mg/cm2 Ti backing foil rotated on a 100-mm diameter wheel at 1000 rpm. EVRs were separated in-flight by GARIS and extracted by a gas jet system. The thermalised 261Rf ions were then attached to KCl aerosols and delivered to a shielded rotating wheel apparatus MANON for α-spectrometry. The gas jet transport efficiency was 52 ± 12% for 261Rfa. In MANON, the aerosol particles were deposited on Mylar foils, 40 of which were set on the periphery of the rotating wheel. The wheel was stepped to position the foils between 7 pairs of Si PIN photodiodes with 38% counting efficiency. Step intervals were set to 30.5 s and 2.0 s to enable the study of both the the 68 s 261Rfa and the 3 s 261Rfb respectively. Detector signals for α particles were set at 1-20 MeV and for SF fragments 5-150 MeV. FWHM for α detection was 50 keV in the top detectors and about 100 keV in the bottom ones. In the step interval of 30.5 s, 126 and 100 α events were registered with energy 8.0 to 8.41 MeV while 174 and 23 events were observed at a step interval of 2.0 s at beam energies of 95.1 and 93.1 MeV respectively. From a total of 120 α-α correlations, 113 decayed with an α energy of 8.01-8.37 MeV. The average energy was Eα=8.28 ± 0.05 MeV with a half life deduced to be 24 ± 3 s. These decay properties were consistent with the parent-daughter pair 261Rfa-{+257No. 6 α-α correlations were observed for the α group Eα=8.41-8.61 MeV at the 2.0 s time step. The average Eα=8.52|+0.05 MeV from the top detectors, with an α-decay half life of 2.4 ± 0.8 s consistent with 261Rfb. The number of random correlations was estimated to be 0.58 events. In addition to α decay, 5 and 8 SF events were measured respectively at 95.1 MeV and 93.1 MeV, 30.5 s time step, 77 and 9 SF events occured at 95.1 MeV and 93.1 MeV, 2.0 s time step. Assuming that all SF activity for the 30.5 s time step is from 261Rfa, a bSF<13% (68% c.i.) is deduced. From 86 fission events at the 2.0 s time step, a SF halflife of 1.8 ± 0.4 s was deduced (Fig 3) and assigned to 261Rfb. Refitted data taking into account the sum of 8.5 MeV a-decays (9 events) and SF activity (77 and 9 events at beam energies of 95.1 MeV and 93.1 MeV respectively) yielded a half life of 1.9 ± 0.4 sec for 261Rfb. Data are insufficient for a conclusive determination of the ground state. Experimenters suggest that the SF half life of 2.1 ± 0.2 s attributed to 262Rf by 1996La11, could possibly belong to 261Rfb. SF branch of bSF=73 ± 6% is derived. An exceptional α-α correlation at the 2.0 s time step with parent Eα=8.91 MeV, |t=4.05 s was tentatively proposed as belonging to 259Rf. The deduced cross-section (sum of both states) was estimated to be 23 ± 4 nb. Cross-section for 261Rfb=11|+2 nb at 95.1 MeV by comparing with 261Rfa=12|+3 nb as reported by 2002Na37. Evaluators note that cross-sections reported by 2002Na37 for the same reaction are: 8|+2 nb (Elab=91 MeV), 13|+3 nb (Elab=94 MeV) and 8|+2 nb (Elab=99 MeV). The ratio of cross-section in the current work is provided as 261Rfa/261Rfb=1.1 ± 0.2.

2012Ha05: Daughter of 265Sg produced using the reaction 248Cm(22Ne,5n)265Sg at RIKEN. The 22Ne projectiles were delivered on target by RILAC with Ebeam=117.8 MeV at the centre of target. For experimental details see Adopted Levels for 265Sg. α total of 18 and 24 observed events were attributed to parents 265Sga and 265Sgb respectively. Their corresponding daughters were respectively populated by 16 and 4 α decays to 261Rfa and 2 and 20 decays to 261Rfb. The decay branching was calculated to be 91% (265Sga to 261Rfa, 16 of 18 α decays) and 9% (265Sgb to 261Rfb, 4 of 24 α decays). α total of 25 decays from 18 SF events and 7 α-decays, were attributed to 261Rfb. Eα=8.51 ± 0.06 MeV from the 7 α decays. The corresponding α decay half-life is 10 s +6-3 (evaluators). α SF half life of 2.6 s +7-5 was deduced from the 22 fission events. Decay branching SF:α was bSF=0.82|+0.09 : bα=0.18|+0.09. In addition, a total of 15 lifetimes for 261Rfa were measured from triple α correlations from 265Sga or b to 261Rfa to 257No from which a half life of 59 ± 42 s was derived using an exponential decay curve analysis. Evaluators note the large uncertainty in this value. The average α energy was found to be 8.27 ± 0.06 MeV which agrees with the literature value.

2013Mu08: Direct synthesis using the reaction 248Cm(18O,5n)261Rf at RIKEN using the RILAC facility to investigate the short lived SF components. Beam energies in the laboratory frame were 88.2, 90.2, 94.8 and 101.3 MeV at centre of target. EVRs were separated by GARIS and transported to a Si detector array at the focal plane. Decay events were detected in beam off conditions. To distinguish between short-lived SF activity and the decay of 262Rf beam on-off periods were set to 6 s, 6 s, 0.1 s and 0.1 s. Following irradiations at each energy, background events were measured for several hours to estimate the contribution of long lived background SF events. Most of the events pertaining to 261Rfa and its α decay daughter 257No were observed at a beam energy of 94.8 MeV as expected going by the calculations of 2004Ni10. 26 α-α correlations were observed with the following properties: 25 events at a beam energy of 94.8 MeV with Eα (average)=8.28 ± 0.04 MeV (68% c.i.) consistent with the α-decay of 261Rfa; 1 event at a beam energy of 90.2 MeV with Eα=8.48|+0.04 MeV. The single event was attributed to 261Rfb noting that the lifetime of the decay daughter was 11.8 s consistent with literature values for 257No. Estimated total number of random events in the α- energy region 8.00 to 8.62 MeV was 0.14 suggesting true α-α correlations. Random α-SF sequences were estimated to be 0.09. Production cross-sections for 261Rfa were derived from the total events within the α-energy range 8.00 to 8.62 MeV at each beam energy: 0.47 ± 0.20 nb (Eb=88.92 MeV), 1.8 ± 0.6 nb (Eb=90.2 MeV), 12 ± 3 nb (Eb=94.8 MeV) and 4.1 ± 1.1 nb (Eb=101.3 MeV). Cross-sections for the SF nuclide 261Rfb were derived from the yields of 261Rfa although the SF event was observed only at a beam energy of 94.8 MeV, as follows:1.5 nb +18-15 (Eb=88.92 MeV), 3.7 ± 3 nb (Eb=90.2 MeV), 12 ± 4 nb (Eb=94.8 MeV) and 0.9 nb +26-9 (Eb=101.3 MeV). Total cross-sections for both states 261Rf(a+b) were 2.0 ± 1.8 nb (Eb=88.92 MeV), 5.5 ± 3.1 nb (Eb=90.2 MeV), 24 ± 5 nb (Eb=94.8 MeV) and 5.0 ± 2.9 nb (Eb=101.3 MeV). The experimenters concluded that the ratio of cross-sections 261Rfa:261Rfb depends on reaction employed.

2013Su04: Descendent of 277Cn produced in the cold fusion reaction 208Pb(70Zn,n)277Cn at RIKEN. Three beam energies were used: 347.5, 351.5 and 355.5 MeV. α single event was observed at 351.5 MeV. The fifth decay in the sequence with E(SF)=176 MeV and a decay time of 3.73 s was attributed to this nucleus. The corresponding half life of 4.7 s +36-14 is in agreement within uncertainties with known values for 261Rfb. Experimenters suggest that when 265Sg and 261Rf are produced as α-decay daughters, they form only one of the two isomeric states.

Other: 2002Na37, 2006Sc02, 2007Ha29, 2009Ha49. See also 1996La11 wherein properties assigned to 262Rf may belong to 261Rf.

Theory: see Nuclear Science References.

Assignment: reassessment of all data from 248Cm+22Ne reaction (2008Du09); chem, 248Cm(18O,5n)261Rf, Ebeam=95.1, 93.1 MeV (2011Ha13); 248Cm(18O,5n)261Rf, Elab=88.2, 90.2, 94.8 and 101.3 MeV (2013Mu08).

Levels: Current data do not allow a conclusive assignment of any level to the g.s.

Levels: 2012Au07 assign the longer lived state to the isomer.

Levels: The labeling of states is in accordance with 2008Du09.

Levels: Based on the re-examination of earlier data, 2008Du09 note that both states in 261Rf are fed with similar intensity when parent 265Sg is produced as an EVR.

Levels: In the case of the direct production of grand-parent 269Hs, the decays proceed preferentially through ’b’ states (2008Du09, 2012Ha05).

Levels: Cross-feeding of both states does occur. Relative intensities are from 2012Ha05.

XREF T1/2(level)
      XAB 68 s +3-3 
% α = 100
  234+X 57 AB 1.9 s 4 
% α = 27 6
% SF = 73 6

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Additional Level Data and Comments:

      X 68 s +3-3 
% α = 100
This level was labelled 261Rfa by 2008Du09 as possible g.s.
E(level): This level was labelled 261Rfa by 2008Du09 as possible g.s.
  234+X 1.9 s 4 
% α = 27 6
% SF = 73 6
This level was labelled 261Rfb by 2008Du09 as possible isomer.
E(level): This level was labelled 261Rfb by 2008Du09 as possible isomer.

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