Nuclear Structure of Superheavy Nuclei Investigated at GSI

Transcription

1 Nuclear Structure of Superheavy Nuclei Investigated at GSI Fritz Peter Heßberger GSI Helmholtzzentrum für Schwerionenforschung mbh, D Darmstadt, Germany Helmholtz Institut Mainz, D-5599 Mainz, Germany 16th ASRC International Workshop Nuclear Fission and Structure of Exotic Nuclei Tokai (Japan), March 18 2, 214 Version

2 The Strong Force One of the four basic interactions, playing among others an essential role for Quark Gluon - Plasma Formation and development of stars Development of the universe Synthesis of the chemical elements Structure of the atomic nuclei

3 The Strong Force 6 experiment 1/2-[521] 2n / MeV M.Hempel PHIUZ 1/214 2, exp. values SLy4 - prediction 1,5 1, Shell strength parameter 2n of nobelium isotopes ( Z = 12) E* / kev E* / kev x3 253+x2 21+x1 +x1 +x2 +x3 7/2-[514] 3/2-[521] 7/2+[633] HFB - SLy4 (Chatillon et al. EPJ A3, 397 (26)) 3/2-[521] 7/2-[514],5 7/2+[633] Neutron shell N = 152, Neutron Number exp: E.Minaya Ramirez et al. Science 337, 127 (212) SLy4: J.Dobaczewski et al, arxiv:nucl-th/4477vl (24) 1/2-[521] 243 Es 249 Es 247 Es 245 Es 251 Es 253 Es

4 Proton number 125 (Chemical) Elements: Group of nuclides ( isotopes ) with same proton but different neutron numbers, 12 >6 having equal chemical properties, 4-6 nearly identical atomic properties, ( Coulomb 115 force ) Z = but different nuclear structure ( strong force ): 1-2 not 11 a proper concept Z = 18 <1 for nuclear physics Shell effects -E sh /MeV R.Smolanczuk, A. Sobiczewski Proc. EPS Conf. Low Energy Nucl. Dyn., St. Petersburg, Russia, 1995 & priv. comm. Karlsruher Nuklidkarte, 8. Auflage 212 N = 162 Superheavy Nuclei N (SHN) = 184 Stability limit of a droplet against prompt fission Neutron number

5 Predictions of Superheavy Nuclei Tin region: All parametrizations predict the same shell closures, Z=5, N=5 and N=82 High shell effects in narrow regions around the shell closures SHN region: Shell closures are strongly dependent on the parametrization, Z = 114, 12 or 126, N = 172 or 184 Wide regions of high shell effects are predicted From: M.Bender et al. Phys. Lett. B 515, 42 (21)

6 Predictions of Superheavy Nuclei shell gaps are very sensitive to treatment of spin-orbit coupling; shell gap at Z = 114 or Z = 12, essentially defined by the energy differences of 2f7/2- and 2f5/2- - states states of low j with low (2j+1) degeneracy 2f 5/2, 3p 3/2, 3p 1/2 for protons at Z = s 1/2, 3d 3/2, 3d 5/2 for neutrons at N = lead to an extended region of high shell effects strong mutual influence of proton and neutron shell closures From: E.Litvinova Contr. to FUSHE 212 RMF + BCS +QVC

7 Predictions of Superheavy Nuclei Are superheavy nuclei different? Yes! Competition between short-range nuclear forces and long-range electrostatic repulsion results in the Coulomb frustration effects # results in large number of low-energy configurations # results in non-pertubative shifts in single particle levels # results in exotic topologies of nuclear density like bubbles or toroids Electromagnetic interaction is highly non pertubative - gives rise to huge self-consistent polarization effects / rearrangement Is the concept of magicity useful in superheavy nuclei? Probably not! Because of very high level density and the Coulomb frustration effects (from: W. Nazarewicz, Contribution to FUSHE 212)

8 Physics Motivation for Synthesis and Nuclear Structure Investigations of SHN Understanding nuclear structure of SHN is essential for understanding their properties and stability i.e. the strong force and thus the limits of our world Why synthesis and nuclear structure investigations? Atomic nucleus is quantum mechanical ensemble of nucleons (p, n) Properties determined by fundamental interactions - nucleon nucleon interaction - Coulomb interaction - spin orbit interaction -. Topic Questions Understanding decay properties and structure of nuclei is thus essential for understanding fundamental interactions Are there proton and neutron shells at all? Superheavy nuclei (SHN) are a specific class of exotic nuclei ensembles of extremely large numbers of protons and neutrons How strong are they? despite of high density of nuclear levels gaps between single particle states occur Where at certain numbers are of they Z and N located indicating shell? closures no macroscopic ( collective ) fission barrier any more, stability against prompt disruption due to shell effects ; B f depends on single particle levels shell structure determines nuclear mass excess determines Q-values for α- and β- decay

9 proton number Expected Decay Modes and Halflives of SHN proton number proton number Z= Z=114 Z= Z=18 1 SF SF -decay -decay -decay -decay expected decay modes and halflives N=162 <1-4 <1 <1-4 <1 4 <1-8 <1-4 <1 >1 4 <1 4 <1 <1 <1 4 <1-4 < < > 1 9 N=184 Smolanczuk et al. [1995] Smolanczuk [1997] theoretical -halflives expected decay modes and halflives <1-4 Muntian et al. [23] Poenaru et al. [198] < theoretical sf-halflives / s theoretical -halflives Karpov et al. [212] > Nuclear <1 Structure > 5 <1 4 <1-4 N=162 <1 4 neutron number > 1 9 Synthesis <1 <1 >1 4 <1 N=184 StrukturSWK/Abbildungen/SHE_Zerfall F.P.Heßberger,

10 T /s E / MeV Comparison of expected Q α - values and α-halflives of Element 12 - isotopes Cwiok et al., HFB-SLy4 Cm( 54 Cr,4n) : E α (expected) = MeV T α = 5.1 μs 1 1 1E-4 Myers, Swiatecki et al. Smolanczuk, Sobiczewski Typel, Brown, HFB-SKX Litvinova, RMF Litvinova, RMF+VC 249 Cf( 5 Ti,3n) : E α (expected) = MeV T α = 1.5 μs 249 Cf( 5 Ti,4n) : E α (expected) = MeV T α = 1.3 μs Attempts to synthesize element Z = 12 at GSI:,1 64 Ni U 32 12,1 * : events, 12 days irrad. time, σ < 9 fb (SHIP) 54 Cr Cm * : events, 4 days irrad. time, σ < 56 fb (SHIP) 1E-3 5 Ti Cf * : events, 4 days irrad. Time, σ <2 fb (TASCA) Separation times 1E-5 SHIP: ca. 2.1 s Production of 1E-6 an element 12 isotope would deliver a direct conclusion TASCA: ca..7 s about location 1E-7 of the proton shell; nuclear structure investigation will test models at 1E-8 lower proton numbers and stimulate improvements of models Neutron number

11 Velocity separator SHIP SHIP Separation time: 1 2 μs Transmission: 2 5 % Background: 1 5 Hz Det. E. resolution: kev Det. Pos. resolution: 15 μm Dead time: 25 μs

12 Decay Spectroscopy of SHN Nuclear structure investigations require a large amount of events, but production rates of SHN are low (ca. 24 /d/nb) Examples 255 Rf 259 Sg 257 Rf Nuclear structure of odd-a even Z nuclei is similar along isotone line Nuclear structure of odd-a even Z nuclei is similar along isotope line 253 No EC 253 Md 255 No Study of systematics of (low lying) Nilsson levels in odd A nuclei Study of K - isomers 251 Fm EC 253 Fm 249 Cf N = 151 N = 153

13 E / kev Counts 935 kev 392 ms 924 kev 296 ms 9545 kev 254 ms 961 kev 411 ms 972 kev 255 ms Decay Study of 259 Sg Production by 26 Pb( 54 Cr,n) 259 Sg; σ 1 nb; α-decay from two states 259g Sg (T 1/2 41 ms) and 259m Sg (T 1/2 25 ms) observed stop stop + box E / kev S. Antalic et al. to be published

14 Counts Counts Decay Study of 259 Sg counts To identify an isomeric state in the daughter nucleus 255 Rf delayed coincidences between α-particles and CE were searched CE not in coincidence with 961 kev and 935 kev lines!! CE in coincidence with 9545 kev; no gammas or K X-rays observed in del. coincidence with α particles; first decay scheme of 259 Sg based on α-, α-γ-, α-ce measurements and systematics for N=153 isotones T 1/2 4 μs t/ms (594) 11/2-[725] (462) (1/2+[62]?) (15)(4 s) 5/2+[622] CE 9/2-[734] 255 Rf 254 ms 411 ms (92) 935, 392 ms, HF=4 924 kev, 196 ms, (HF=3) 9545 kev, 254 ms, HF=4 972 kev, 255 ms, HF= Sg 961 kev, 411 ms, HF=26 1/2+[62] 11/2-[725] E α / kev S. Antalic et al. to be published

15 199.9 (M2+E3) 167(M2+E3) (E2) (E2) (M1) (M1) Decay schemes of the N=153 Isotones 255 No, 257 Rf, 259 Sg 255 No 257 Rf 259 Sg (1/2+[62]) (11/2-[725]) (1/2+[62]) 4.9 s 5.7 s (1/2+[62]) (11/2-[725]) 254 ms 411 ms (11/2-[725]) (1/2+[62]) (3/2+?) (1/2+[631]) (7/2+[624]) (5/2+[622]) 21 s (9/2-[734]) 353.8(E1) (M1) 251 Fm (35+x) 7742 (19%) ( %) 793 (11 %) 7941 (.4 %) 895 (58 %) (8255 (3 %) (829 (5 %) (1/2+[62])? (3/2+) (3/2+,5/2+,7/2+) (5/2+[622]) 24 s (11/2-) (9/2-[734]) 91(M1) 283 (E2) 253 No (845) (851) (868) 8778 (8962) (895) 921 (462) (1/2+[62])? ca.15 (5/2+[622]) 4 s (9/2-[734]) 255 Rf F.P. Heßberger et al. EPJ A29,165 (26) F.P. Heßberger et al. Z.Phys. A 359,415 (1997) B. Streicher et al. EPJ A 45, 275 (21)

16 counts EC decay study of No Energy levels in Fm K (Fm) counts/kev K (Md) K (Md) 15 ( 253 Fm) 188 ( 253 Fm) 394 ( 253 Md) 451 ( 253 Md) 12 coinc. K -X-rays (Fm) Production: 253 No EC ( 5%) 253 Md EC ( 99%) 253 Fm 8 4 No mass separation Requires X-ray (Fm) γ coincidences for identification 12 8 coinc. E =15.2 kev Also requires coincidences with CE for background suppression (from environment) Coinc. K -X-rays of Md 12 coinc. E =187.5 kev E / kev E / kev

17 E / kev counts EC decay study of No Energy levels in Fm /2-[725] isomeric state identified; decay pattern established on the basis of systemtics ( 251 Cf) No connections to low energy level studied by Asai et al. via -decay of 257 No established CE energy suggests 7/2+[613] at 14 kev strong X-ray X-ray coincidences suggest also EC-decay in a so far not identified level T 1/2 =.5±.3μs =.56 ±.6 s t( (188 kev) - CE) / rel. units Md? 7/2-[514] ±.3 s x+398 9/2+[615] x /2-[725] x /2+ x+61 9/ ? x ~ /2+[613] /2+ 3/2+[622] Fm253_TAC_E t(ce - gamma) / rel. units Fm S. Antalic et al. EPJA 47:62 (211) / /2+ 1/2+[62]

18 E * / kev E / kev E * / kev E / kev Nilsson-Levels in N=151 and N=153 Isotones Theory: A.Parkhomenko, A.Sobiczewski, Acta. Phys. Pol. B36, 3115 (25) 1/2 + [62] Theory: A.Parkhomenko, A.Sobiczewski, Acta. Phys. Pol. B36, 3115 (25) 11/2- [725] /2 + [622] 7/2 + [624] 9/2 - [734] 245 Pu 247 Cm 249 Cf 251 Fm 253 No 255 Rf 257 Sg 2 1 9/2- [734] 7/2+[613] 3/2+[622] 1/2+[62] 247 Pu 249 Cm 251 Cf 253 Fm 255 No 257 Rf 259 Sg 261 Hs Experiment 3/2 + [622] 1/2 + [631] 1/2 + [62] 7/2 + [613] 5/2 + [622] 25 s 7/2 + [624] 9/2 - [734] 245 Pu 247 Cm? 45 s 249 Cf (? = assignment uncertain) 11/2 - [725]?? 21 s 24 s 4 s 251 Fm 253 No 255 Rf /2+[622] 9/2- [734] 11/2- [725] 19 ns 1.3 s 3/2+[622] 7/2+[613] 1/2+[62] 249 Cm 251 Cf.5 s 253 Fm Experiment 4.9 s.24 s.41 s 255 No 257 Rf 259 Sg Niv153_21118, Theorie = Parkhomenko + Sobiczewski

19 Systematics of Nilsson-Levels in Es - Nuclei Studied by α-γ Coincidence Measurements of Md-Isotopes Counts counts Counts 222 kev 28 kev 34 kev 353 kev Ar + 29 Bi, E = 4.66 AMeV γ-rays in coincidence with -decays of 247 Md E / kev Ca + 27 Pb, E = ( ) AMeV -rays coinc. E + E = ( ) MeV coinc. 'escape' -particles from 253 No ( = 9 nb) 253 Md ( eff = 2 nb) E / kev E / kev konferenzen/tan97/md253alga F.P.Heßberger

20 Decay schemes of odd-mass Md-isotopes (simplified) 247 Md 7/2 - [514] 249 Md 7/2 [514] 251 Md 7/2 [514] 253 Md 7/2 [514] Md 7/2 [514] 8422 HF~1 826 HF~1 754 HF~3 71 HF~ / /2 [514] (E1) 243 Es 7/2 [514] 9/ /2 [633] (E1) - 7/2 [514] (E1) - 7/2 [514] (E1?) + 9/ /2 + 7/2 + [633] + + 7/2 [633] 247 Es (3/2 [521]?) 249 Es 251 Es 9/2 9/2 7/2 [633] /2 [514] (E1) 45.2 (E1) 7/2 [633] (3/2 [521]?) (3/2 [521]?) 3/2 [521] 245 Es graph/demo/nivmend F.P.Heßberger

21 E / kev Nilsson levels in odd-mass Es isotopes Nilsson single proton levels 5,3 45 theory: P.Möller et al. ADND59, 185 (1995), , E((7/2-[514])-(7/2+[633])) 2 4,225,2,75 f 5/2 7/2-[514] Energy difference between 2f7/2- and f 7/2 2f5/2- defines if proton shell 1/2-[521] is at i 13/2 Z = 114 or 1 at Z = 12 in selfconsistent models h 9/2 96 3/2-[521] 7/2+[633] 5, E((9/2+)-(7/2+)), Mass Number, Mass Number StrukturSWK/Abbildungen/Es_Isotope_deformation

22 E* / kev E* / kev E* / kev E* / kev E* / kev Nilsson states in odd-mass Es - isotopes StrukturSWK/Abbildungen/Niv_Md_Vergleich_neu, experiment (f 5/2 ) x1 +x1 243 Es 253+x2 +x2 245 Es 294+x3 +x3 247 Es 249 Es 1/2-[521] 251 Es 7/2-[514] 3/2-[521] 7/2+[633] 253 Es (h 9/2 ) (f 7/2 ) (i 13/2 ) 6 HFB - SLy4 (Chatillon et al. EPJ A3, 397 (26)) 6 cranked shell model (Zhang et al. PRC85,14324 (212)) 4 3/2-[521] 7/2-[514] 4 1/2-[521] 2 7/2+[633] 1/2-[521] 243 Es 245 Es 247 Es 249 Es 251 Es 253 Es 2 7/2-[514] 3/2-[521] 7/2+[633] 245 Es 247 Es 249 Es 251 Es 253 Es macros.-micros. (Parkhomenko & Sobiczewski, Act. Phys. pol. B35,2447 (24)) 6 7/2-[514] 6 macros. - micros (+TCSM) Adamian et al. PRC82,5434 (21) 9/2+[624] 4 1/2-[521] 4 7/2-[514] 5/2-[523] 2 3/2-[521] 2 7/2+[633] 243 Es 245 Es 247 Es 249 Es 251 Es 253 Es 255 Es 3/2-[521] 7/2+[633] 243 Es 245 Es 247 Es 249 Es 251 Es 253 Es 255 Es

23 Counts Decay of a K-Isomeric state in 252 No 17 (4+ --> 2+)(line dublett) K (+ 123?) (6+ --> 4+) 224 (8+ --> 6+) Pb( 48 Ca,2n) 252m No 252 No (25) 1 ms 1254 kev (8 _ ) (7-) 1148 (6-) 173 (5-) 115 (4-) 966 (3-) 929 (2-) No252iso F.P.Heßberger E / kev No252_KIsomer_1486 F.P.Heßberger ,5-,8-9/2+ 7/2-1/2-7/2+ 3/2-1/ /2+ 1/2+ 7/2+ 11/2-9/2-7/2+ 5/2+ Z N B. Sulignano et al. EPJ A 33, 327 (27) 8-

24 E* / kev K-isomers in N=15 isotones 2 2-quasi p 2-quasi n 3- (1/2-,5/2+) 6+ (5/2+,7/2+) 8+ (7/2-,9/2-) (1/2+,7/2+) 5- (3/2-,7/2+) 4+ (1/2-,7/2-) 7- (7/2+,7/2-) K isomers 7- (5/2+,9/2-) in N = (1/2-,3/2-) e-e isotones interpreted as 16 2-quasi neutron states of the configuration /2+[624] 9/2-[734] 4- (3/2-,5/2+) 8- (7/2+,9/2-) gs exp. 8- in N = 149 odd T 1/2 =? A, even Z isotones gs in 1.8 N s = 151 odd A, even Z 4- (1/2-,7/2+) isotones 246 Cm 248 Cf 25 Fm 4+ (1/2-,7/2-).1 s 252 No calc. J.-P. Delaroche et al. Nucl. Phys. A 771, 13 (26)

25 Summary and Conclusions Macroscopic microscopic models did a great job in predicting a region of relative high nuclear stability above the actinides ( superheavy elements ) at Z = 114 and N = 184 Selfconsistent models using effective NN interactions put the old shell predictions in question, Z = 12, 126 and also N = 172 appear as possible candidates for proton and neutron shells From experimental side, it seems that the region itself has been reached, but so far not its center around N = 184 Presently it seems not possible to reach the center directly by complete fusionevaporation reactions; advanced radioactive beam facilities providing neutron rich projectiles with high intensity required Detailed nuclear spectroscopy is a suited method to obtain detailed information about the nuclear structure and the underlying nuclear force and to test the predictive power of models with respect to location and strengths of nuclear shells (even without reaching the center ) On long-term view a general (or common) parametrization of the strong force to decribe commonly evaluation of the universe and the stars as well as the atomic nuclei is desired; investigation of superheavy nuclei are hopefully a valuable tool to reach that aim.

26 (Present) SHIP SHN Spectroscopy Collaboration (Spokesman: F.P. Heßberger) GSI, Darmstadt D.Ackermann, M.Block, S.Heinz, F.P.H., B.Kindler, I.Kojouharov, J.Khuyagbaatar, B.Lommel Helmholtz Institut Mainz L.-L.Andersson, E. Minaya, M. Laatiaoui, C. Doese (also EMA Univ. Greifswald, Germany) Comenius University Bratislava, Slovakia S. Antalic, Z. Kalininova, B. Andel University Jyväskylä, Finland M.Leino, J.Uusitalo JAEA Tokai, Japan K. Nishio