48Ca+249Bk Fusion Reaction Leading to Element Z=117: Long-Lived -Decaying 270Db and Discovery of 266Lr Khuyagbaatar, J.; Yakushev, A.; Düllmann, Ch. E.; Ackermann, D.; Andersson, L.-L.; Asai, M.; Block, M.; Boll, R. A.; Brand, H.; Cox, D. M.; Dasgupta, M.; Derkx, X.; Di Nitto, A.; Eberhardt, K.; Even, J.; Evers, M.; Fahlander, Claes; Forsberg, Ulrika; Gates, J. M.; Gharibyan, N.; Golubev, Pavel; Gregorich, K. E.; Hamilton, J. H.; Hartmann, W.; Herzberg, R.-D.; Heßberger, F. P.; Hinde, D. J.; Hoffmann, J.; Hollinger, R.; Hübner, A.; Jäger, E.; Kindler, B.; Kratz, J. V.; Krier, J.; Kurz, N.; Laatiaoui, M.; Lahiri, S.; Lang, R.; Lommel, B.; Maiti, M.; Miernik, K.; Minami, S.; Mistry, A.; Mokry, C.; Nitsche, H.; Omtvedt, J. P.; Pang, G. K.; Papadakis, P.; Renisch, D.; Roberto, J. Published in: Physical Review Letters DOI: 10.1103/PhysRevLett.112.172501 Published: 2014-01-01
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Citation for published version (APA): Khuyagbaatar, J., Yakushev, A., Düllmann, C. E., Ackermann, D., Andersson, L-L., Asai, M., ... Yakusheva, V. (2014). 48Ca+249Bk Fusion Reaction Leading to Element Z=117: Long-Lived α-Decaying 270Db and Discovery of 266Lr. Physical Review Letters, 112(17), . DOI: 10.1103/PhysRevLett.112.172501
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PHYSICAL REVIEW LETTERS
PRL 112, 172501 (2014)
þ 249 Bk Fusion Reaction Leading to Element Z ¼ 117: Long-Lived α-Decaying 270 Db and Discovery of 266 Lr
J. Khuyagbaatar,1,2,* A. Yakushev,2 Ch. E. Düllmann,1,2,3 D. Ackermann,2 L.-L. Andersson,1 M. Asai,4 M. Block,2 R. A. Boll,5 H. Brand,2 D. M. Cox,6 M. Dasgupta,7 X. Derkx,1,3 A. Di Nitto,3 K. Eberhardt,1,3 J. Even,1 M. Evers,7 C. Fahlander,8 U. Forsberg,8 J. M. Gates,9 N. Gharibyan,10 P. Golubev,8 K. E. Gregorich,9 J. H. Hamilton,11 W. Hartmann,2 R.-D. Herzberg,6 F. P. Heßberger,1,2 D. J. Hinde,7 J. Hoffmann,2 R. Hollinger,2 A. Hübner,2 E. Jäger,2 B. Kindler,2 J. V. Kratz,3 J. Krier,2 N. Kurz,2 M. Laatiaoui,1 S. Lahiri,12 R. Lang,2 B. Lommel,2 M. Maiti,12,† K. Miernik,5 S. Minami,2 A. Mistry,6 C. Mokry,1,3 H. Nitsche,9 J. P. Omtvedt,13 G. K. Pang,9 P. Papadakis,6,14 D. Renisch,3 J. Roberto,5 D. Rudolph,8 J. Runke,2 K. P. Rykaczewski,5 L. G. Sarmiento,8 M. Schädel,2,4 B. Schausten,2 A. Semchenkov,13 D. A. Shaughnessy,10 P. Steinegger,15,16 J. Steiner,2 E. E. Tereshatov,10 P. Thörle-Pospiech,1,3 K. Tinschert,2 T. Torres De Heidenreich,2 N. Trautmann,3 A. Türler,15,16 J. Uusitalo,14 D. E. Ward,8 M. Wegrzecki,17 N. Wiehl,1,3 S. M. Van Cleve,5 and V. Yakusheva1 1
Helmholtz Institute Mainz, 55099 Mainz, Germany GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany 3 Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany 4 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 5 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 6 University of Liverpool, Liverpool L69 7ZE, United Kingdom 7 The Australian National University, Canberra, Australian Capital Territory 0200, Australia 8 Lund University, 22100 Lund, Sweden 9 Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 10 Lawrence Livermore National Laboratory, Livermore, California 94551, USA 11 Vanderbilt University, Nashville, Tennessee 37235, USA 12 Saha Institute of Nuclear Physics, Kolkata 700064, India 13 University of Oslo, 0315 Oslo, Norway 14 University of Jyväskylä, 40351 Jyväskylä, Finland 15 Paul Scherrer Institute, 5232 Villigen, Switzerland 16 University of Bern, 3012 Bern, Switzerland 17 Institute of Electron Technology, 02-668 Warsaw, Poland (Received 22 February 2014; published 1 May 2014) 2
The superheavy element with atomic number Z ¼ 117 was produced as an evaporation residue in the Ca þ 249 Bk fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-μs and a few days. Two decay chains comprising seven α decays and a spontaneous fission each were identified and are assigned to the isotope 294 117 and its decay products. A hitherto unknown α-decay branch in 270 Db (Z ¼ 105) was observed, which populated the new isotope 266 Lr (Z ¼ 103). The identification of the long-lived 270 (T 1=2 ¼ 1.0þ1.9 Db marks an important step towards the observation of even more −0.4 h) α-emitter long-lived nuclei of superheavy elements located on an “island of stability.”
PACS numbers: 27.90.+b, 23.60.+e, 25.70.Gh
The existence of superheavy elements (SHE) was predicted in the late 1960s as one of the first harvests of the macroscopic-microscopic theory of the atomic nucleus [1,2]. Shell effects were predicted to be pronounced at Z ¼ 114 and N ¼ 184. Modern theoretical approaches [3,4] confirm this concept. Very long half-lives for these nuclei forming an “island of stability” are expected. In the past 50 years, a wealth of both theoretical and experimental work has been devoted to SHE [5,6] (and references therein). To date, nuclei associated with the “island of stability” can be accessed preferentially in 48 Cainduced fusion reactions with actinide targets . 0031-9007=14=112(17)=172501(5)
Successful use of these reactions was pioneered at the Dubna Gas-Filled Recoil Separator (DGFRS) at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia . Synthesis of nuclei of SHE with Z up to 118 has been reported . Identification was achieved by detecting their subsequent α-decay chain, which always ended with spontaneous fission (SF) [6,7]. Independent experiments at other laboratories have agreed with findings at the DGFRS [8–16]. Discovery of elements with Z ¼ 114 and Z ¼ 116 was approved by the IUPAC-IUPAP Joint Working Party , and recently they were named flerovium (Fl) and livermorium (Lv), respectively .
© 2014 American Physical Society
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The identification and assignment of nuclear α-decay chains of the odd-Z elements 115 [7,16,19] and 117 [20–23] is more difficult compared to that of the even-Z ones. They include more complex α-decay patterns and often longer half-lives, T 1=2 . Moreover, odd nucleons significantly hinder the SF decay of these nuclei, essentially leading to long partial SF half-lives. Thus, the reliability of the assignment of members of such chains is often reduced, because the probability to observe random correlations of unwanted background present in the data set increases with increasing the correlation search time, Δt, between two genuine chain members [5,6]. The longest half-lives detected for α-decaying nuclei of SHE are about 2 min, for 269 Sg  and 271 Sg . A successful closer approach towards the center of the “island of stability” depends on a reliable identification of single long-lived nuclei of SHE. An attractive option is to advance setups suitable for registering various nuclear decay modes. Also, detection of their ions stored in, e.g., Penning traps, has been suggested [24,25]. Here, we report on results of the 48 Ca þ 249 Bk fusion reaction leading to the SHE with Z ¼ 117. First reports on the observation of two isotopes 293;294 117 produced in the 249 Bk (48 Ca, 3–4n) reactions came from a collaboration working at the DGFRS [20,21]. Recently, further data were reported, again from the DGFRS [22,23]. This reaction is unique due to the short half-life of 249 Bk [T 1=2 ¼ 330ð4Þ d, ], necessitating dedicated production at Oak Ridge National Laboratory  and safe handling of the highly radioactive material. This severely limits the number of independent experiments dedicated to the production of element Z ¼ 117. The experiment was performed at the gas-filled TransActinide Separator and Chemistry Apparatus (TASCA) at GSI . A pulsed (5-ms-long pulses, 50 s−1 repetition rate) 48 Ca10þ beam was accelerated to energies of 268.3, 270.2, and 274.1 MeV. Targets in 249 Bk2 O3 form were electrodeposited  on 2.20ð11Þ μm thick Ti backings, which faced the beam. Four targets with an area of 6 cm2 each were mounted on a wheel, which rotated synchronously with the beam macrostructure . The ≈0.48 mg=cm2 thick target consisted of 249 Bk (62%) and its decay product 249 Cf (38%) at the beginning of the experiment. The average beam intensity was 4.7 × 1012 s−1 . The beam energies in the center of the target were estimated to be Elab ¼ 252.1ð21Þ, 254.0(21), and 258.0(21) MeV . Beam doses of 2.3, 4.9, and 3.9 × 1018 were accumulated during 6.3, 10.5, and 10.4 d, respectively. At these beam energies, the evaporation of three or four neutrons is expected from the 297 117 compound nuclei at excitation energies of E ¼ 37.8–41.3, 39.4–42.9, and 42.7–46.2 MeV  populating the evaporation residues (ER) 294 117 and 293 117 [20–23], respectively. Compared to the experiments on 288;289 Fl performed in 2009 [11,13], TASCA was significantly upgraded for the current experiment . According to the predicted
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average charge state (≈6.8) of 293;294 117 ions moving in helium gas at 0.8 mbar pressure [33,34], the TASCA magnetic system was set to center ions with a magnetic rigidity (Bρ) of 2.20 Tm into the focal plane. The detection system of TASCA consisted of a multiwire proportional counter (MWPC) and a focal plane detector box (FPDB). The latter consisted of a doublesided silicon strip detector (DSSSD)-based implantation detector (hereafter: stop detector), with eight DSSSDs mounted perpendicular in the backward hemisphere of the stop detector to form a five-sided box configuration (box detectors). The FPDB detection efficiency for α particles emitted from implanted nuclei was estimated to be about 80% based on data from test experiments. The stop detector comprised 144 vertical (X) and 48 horizontal (Y) strips on the front and back sides, respectively. Two adjacent single-sided Si-strip detectors were mounted directly behind the stop detector to register particles passing through the stop detector (veto detector). The efficiency of TASCA to guide ER to the stop detector was estimated to be 47(5)% . The average total triggering counting rate was ≈ 700 s−1 , distributed over the almost 7000 pixels of the stop detector. All preamplifier signals from the Y strips of the stop detector were digitized by 60 MHz-sampling ADCs and stored in 50 μs-long traces. All other signals, i.e., those from the X strips of the stop detector, from box and veto detectors, and from the MWPC were processed with analog electronics and peak-sensing ADCs with a dead time of about 35 μs. The stop detector’s analog and digital branches, which independently initialized the data storage, allowed determining the energy of events in two independent ways. Energy calibrations of stop and box detectors were made using an external α source and α decays of implanted nuclei. Different position and time correlation analyses between ER-, α-, and SF-like events were carried out. Promising candidates for decay chains were selected from results of analyses searching for ER-α-α (ΔtER-α < 1 s, Δtα-α < 20 s) and ER-α-α-SF (ΔtER-α < 20 s, Δtα-α < 200 s, Δtα-SF < 500 s) correlations. The search conditions were (i) ER-like events: energy of 3–20 MeV, coincident with a signal from the MWPC; (ii) first and second α-like events: 8.5–12 and 8–11 MeV, respectively; (iii) SF-like events: energies > 50 MeV. Alpha and SF-like events were required to be without coincident MWPC signals. In total, four chains of two different kinds, two long and two shorter ones with significantly different decay properties, all terminated by SF, were identified. All observed chains will be presented in detail in . Here, we focus on the two long decay chains, which are shown in Fig. 1. The chains were detected at Elab ¼ 254.0 (chain 1) and Elab ¼ 258.0 MeV (chain 2) in strips X ¼ 103 and 111, respectively, which are on the right half (higher Bρ) of the stop detector. In this area, the implantation rate, which is
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FIG. 1 (color online). Decay chains assigned to 294 117. Experimental energies in MeV and Δt of all events together with their digitized traces are shown. Energies of reconstructed α particles are marked by an asterisk (). Boxes with black triangles are corresponding to events observed during beam-off periods. The measured fission fragment energies are from stop þ box detectors. Theoretically predicted  α-decay energies (in MeV), partial α, and unhindered SF half-lives are given on the left.
overall dominated by transfer products that have lower Bρ than the ER, is significantly lower than the average over the whole stop detector . Chain 1 was found in pixel X ¼ 103 and Y ¼ 41 (Fig. 1). Five α decays occurred during beam-off periods, where the counting rate of α-like events within the energy range of 6–12 MeV was 8 × 10−5 s−1 in this pixel. This number is based on seventy-four events detected in all beam-off periods during the complete 254.0 MeV run. A small stop detector signal of a fourth member (α3 ) of the chain was stored only by the digital branch: an energy of 0.6 MeV was registered in strip Y ¼ 41 with Δt ¼ 5.96 s after α2, in a beam-off period. A coincident signal was registered in a box detector and a full energy of 9.3(3) MeV was reconstructed. The counting rate for such events with reconstructed full energies of 6–12 MeV in strip Y ¼ 41 with missing X signal was 4 × 10−4 s−1 during beam-off periods. One member, α6 , of chain 1 was detected during a beam-on period, where the counting rate was 1 × 10−3 s−1 . Only one SF-like event was detected in pixel (103,41) during the entire 254.0 MeV run. The counting rate of ERlike events was 4 × 10−3 s−1 . A probability of 5 × 10−15 for the observation of random sequences ER-α1–7 -SF similar to chain 1 was calculated, according to .
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Chain 2 occurred in pixel X ¼ 111 and Y ¼ 19 (Fig. 1). In all beam-off periods during the 258.0 MeV run, fifty-nine α-like events within the energy range of 6–12 MeV were detected, resulting in a counting rate of 7 × 10−5 s−1 ; six of these are members of chain 2. Only one SF-like event was found in this pixel during the entire 258.0 MeV run; it occurred 29 h after the last α decay. A signal with an energy Estop ¼ 4.64 MeV was detected during a beam-off period in between the α2 and α4 members of the chain and attributed to the α3 member. It was likely emitted into the backward direction under a rather shallow angle, thus leaving a relatively large fraction of its energy in the stop detector, but missed detection in the box detector. In total, only twenty-nine events with energies from 3–6 MeV were registered during the beamoff periods in the entire 258.0 MeV run, supporting the nonrandom origin of this event. The probability to observe random sequences similar to chain 2 was estimated to be 5 × 10−16 . The decay properties of the corresponding members of both chains are compatible with one common origin . The high energies exceeding 10 MeV for the first and second α particles and their correlation times point to an origin from heavy nuclei; however, the observed sequence ER-α1 -α2 is not compatible with that of any known nucleus with Z ≤ 116 [7,26]. On the other hand, high energetic αlike events, if registered solely with analog electronics, can result from the summing of overlapping low-energy signals . The registered traces of all members of both chains are also presented in Fig. 1. All signals from α particles are recorded as single-peak traces, and the deduced energies from the peak amplitudes agree with those from the analog branch within the energy resolution. Our data allow excluding pile-up events as the source of any member of the two decay chains. Thus, these chains are assigned to originate from the radioactive decay of ER with Z > 116, produced in the fusion of 48 Ca either with 249 Bk or 249 Cf, namely, 293;294 117 or 293;294 118. We attribute these chains to 294 117, where the odd proton and neutron are responsible [3,38] for the longest ever measured nuclear α-decay chain originating from SHE [7–16,19]. As is visible in the trace of the ER of chain 1, a small signal with an energy of about 1.4 MeV was observed 1.5 μs after the ER implantation. The probability to observe a signal with an energy of 1–2 MeV presumably due to heavy charged particles like He-gas atoms or protons in the trace of an ER-like event is less than 1%. The absence of such a signal in the trace of the ER of chain 2, where the ER was implanted deeper into the stop detector, leads us to interpret this signal as not being due to a charged heavy particle from a true member of chain 1 (e.g., an escaped α particle). However, various scenarios leading to the observation of such a signal (e.g., an isomer decaying by cascades of conversion electrons) are being considered .
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The theoretical calculation of decay properties of odd-odd nuclei is complicated by the scarce information on their structure . However, reliable predictions of the α-decay properties of nuclei originating from 294 117 have been given in , which satisfyingly agreed with results from the DGFRS [20–23]. The predictions from  are included in Fig. 1. Best agreement between the experimental and theoretical α-decay properties is observed for 294 117, 290 115, 286 113, and 270 Db. The experimental T SF of 266 Lr is about 3 × 103 times longer than the theoretical prediction, which can be explained by the presence of odd nucleons. Using the same hindrance factor as a rough approximation, T SF for 282 Rg, 278 Mt, 274 Bh, and 270 Db can be estimated to 1.2 h, 1.5 min, 1.3 h, and 12 h, respectively. These T SF values are larger than the predicted T α , which points to the dominance of α decay, in accordance with experimental observations. A possible SF branch of ≈8% can be estimated for 270 Db based on the estimated value for T SF and the experimental T α . A summary of the present experimental data together with results reported from groups working at the DGFRS [20–23] is given in Table I. Comparing these data, there is good agreement in most cases. A new finding in our work is the identification of α decay in 270 Db and the new nucleus
TABLE I. Decay properties of the nuclei originating from 294 117 observed in this work, reported from DGFRS  and from the combined data sets. Half-life uncertainties corresponding to the 68% confidence level are given, according to . Eα (MeV) T 1=2 Nuclei
N α =N SF b T 1=2
11.05(4) 10.81–10.97 α 5=þ60 þ41 ms 50 ms 100% 51 51þ94 −20 −18 −16 ms 290 115 10.31(4) 9.78–10.28 α 5=þ0.28 þ0.61 1.3þ2.3 s 0.24 s 100% 0.75 −0.09 −0.23 s −0.5 286 113 9.3(3) 9.61–9.75 α 6=13þ12 100% 7.9þ5.5 2.9þ5.3 −1.1 s −4 s −2.3 s 282 Rg 9.05(3), 8.86(3) 9.01(5) α 6=þ1.0 3.1þ5.7 1.0−0.3 min 100% 2.1þ1.4 −1.2 min −0.6 min 278 Mt 9.45(3) 9.38–9.55 α 5=þ6.2 þ3.6 3.6þ6.5 s 5.2 s 100% 4.4 −1.4 −1.8 −1.4 s 274 Bh 8.84(3) 8.76(5) α 5=54þ65 100% 42þ34 30þ54 −12 s −19 s −13 s 270 Db 7.90(3) SF α=SFc 2=1c 1.0þ1.9 17þ15 83=17% 1.1þ1.5 −0.4 h −0.4 h −6 h 266 Lr SF (not observed) SF -=2 þ21 11þ21 h 100% 11 −5 −5 h Corresponding α and/or SF branching ratios are given. Number of α and SF events used for the calculations of T 1=2 . One SF event out of four reported in , see text for discussion.
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Lr. In total four decay chains assigned to 294 117 were reported from the DGFRS. All chains were assigned to be terminated by SF of 270 Db. The measured Δt were 33, 38, 24, and 1.2 h [20,22,23]. Respecting our data, which indicate the half-lives of 270 Db and 266 Lr to be 1.0þ1.9 −0.4 and 11þ21 h, respectively, it appears that the last one of the −5 DGFRS chains, where a significantly shorter Δt was recorded than in the three first ones, was indeed terminated by SF of 270 Db. In the other cases, the α decay of 270 Db may have remained unidentified (as considered in [20–22]), while the chains terminated by SF of 266 Lr. If so, a branching ratio for SF, of about 1=6 ¼ 17% is deduced for 270 Db. This is in agreement with the estimate given above within the uncertainty of low statistics . The partial SF half-life (6.5 h) of 270 Db is comparable to those of 267 Db (T 1=2 ≈ 1.4 h, SF)  and 268 Db (T 1=2 ≈ 25 h, SF) , which lie in the vicinity of the neutron shell closure at N ¼ 162. The half-life of the new nucleus 266 Lr with N ¼ 163 is comparable to those of the heavier isotones 267 Rf (T 1=2 ≈ 1.3 h, SF)  and 268 Db. This suggests the stabilizing influence of the N ¼ 162 shell closure to extend towards lower Z at least down to Z ¼ 103, which is also seen in the low Eα value of 270 Db. The cross sections for the production of 294 117 were þ2.0 evaluated to 0.7þ1.6 −0.6 and 0.9−0.7 pb at Elab ¼ 252.0–256.1 and 255.9–260.0 MeV, respectively. These are consistent with values reported from the DGFRS. At Elab ¼ 250.0– 254.1 MeV no events were observed, corresponding to an upper cross section limit of 2.9 pb . In conclusion, we observed the nuclear decay of two atoms of element Z ¼ 117 and its daughter products, synthesized in the 48 Ca þ 249 Bk reaction. The nuclei 294 117, 290 115, 286 113, 282 Rg, 278 Mt, 274 Bh, 270 Db were identified by their α decays and 266 Lr by its SF decay, which terminated the decay chains. Results of the present work confirm previously reported data [20–23] on the decay chains assigned to 294 117. In addition, we report a previously unknown α branch in 270 Db, which populated the new SF unstable nucleus 266 Lr. Our data confirm the perseverance of the N ¼ 162 shell closure towards lighter Z, at least down to Z ¼ 103. 270 Db is the most long-lived α-decaying nucleus above No (Z ¼ 102). The decay chain members from 290 115 to 266 Lr all decay with T 1=2 ≳ 1 s, which opens prospects for their chemical investigation and off-line studies. Our experimental data show the sensitivity of TASCA for the identification of the radioactive decay of single nuclei with half-lives from sub-μs up to about 1 d. Future studies of SHE designed to allow a closer approach towards the center of the “island of stability” may require the safe measurement of even longer half-lives. We are grateful for GSI’s ECR ion source and UNILAC staff, and the Experimental Electronics department for their continuous support of the experiment. This work was financially supported in part by the German BMBF (05P12UMFNE), the Helmholtz association
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(VH-NG-723), the Australian and Swedish Research Councils, the U.S. Department of Energy by LLNL (DEAC52-07NA27344), the Laboratory Directed Research and Development Program at LLNL (11-ERD-011) and the Helmholtz Institute Mainz. This work was co-sponsored by the Office of Science, U.S. Department of Energy, and supported under U.S. DOE Grant No. DE-AC0500OR22725, and at Vanderbilt DOE Grant No. DEFG05-88ER40407, and the UK Science and Technology Funding Council (STFC). J. M. G., K. E. G., and H. N. were supported by the U.S. Department of Energy, Office of Science, Nuclear Physics, Low Energy Physics Program, through the Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231.
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