ASRC International Workshop Perspectives in Nuclear Fission

Transcription

1 Fit Fritz Peter Heßberger GSI Helmholtzzentrum für Schwerionenforschung mbh, D Darmstadt, Germany Helmholtz Institut Mainz, D Mainz, Germany ASRC International Workshop Perspectives in Nuclear Fission JAEA, Tokai, Japan March 2012 Sunrise Tokai Beach, March 13, 5:52 a.m. Version

2 Outline of the talk Physical Motivation Experimental set-up SHIP at GSI GSI experiments on spontaneous fission investigations spontaneous fission half-lives SF hindrance for odd mass nuclei SF of K isomers EC delayed fission TKE and mass distributions Superheavy elements Conclusions Collaborations

3 Spontaneous Fission General Considerations U Surface energy s Spontaneous 100 fission is 15 an essential decay 75 liquid drop mode of net effect many (x5) nuclides 10 in the + shell effects transuranium region. 5 B (LD) B f It was 0 and still is f believed to terminate t liquid the drop 0-25 upper end of the charts of nuclei. E / MeV E / MeV -5 Coulomb energy ,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1-20 U gs U s = shell effect at saddle point U gs = shell effect at ground state B f (LD) = liquid drop fission barrier B f =B(LD) f + U gs (+ U s ) 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 F.P.Heßberger spalt2 F.P.Heßberger

4 Shell Effects in Heaviest Nuclei Prediction of SHE - nuclei Macroscopic Microscopic Calculations (164) (164) H. Meldner Arkiv fys. 36,593 (1967) Pr roton num ber Proto on number 120 Sobiczewski R. Smolanczuk 118 & Smolanczuk & A. Sobiczewski P. Möller Möller et al. et al. ADNDT 59, 1845 (1995) Neutron number F.P.Heßberger /konferenzen/danf11/she/she_schaleneffekte

5 Shell Effects in Heaviest Nuclei Skyrme-Hartree-Fock (SHF) and Relativistic Mean Field (RMF) Approaches SkP predicts proton shell Z=126, other SHF param. rather Z=120, SkI4 shows also strong shell effects at Z=114; SHF predict neutron shell essentially at N=184, partly strong shell effects at N=172 RMF predict proton shell at Z=120 and strong neutron shell rather at N=172. M. Bender et al. PL B515, 42 (2001)

6 Velocity separator SHIP SHIP Separation time: 1 2 μs Transmission: % Background: Hz Det. E. resolution: kev Det. Pos. resolution: 150 μm Dead time: 25 μs Mastertitelformat bearbeiten

7 Spontaneous Fission Activities Investigated at SHIP b sf > 0.01 b sf = sf finvestigated t at tship Z = 114 Z = 120 Pr roton nu umber Z 112 N = Z = N = N = Neutron number N hess/konferenzen/tokai_0312/region

8 26 SF halflife systematics of ee - nuclei U N = 152 T sf / s N = 162 Pu 10 6 Cm 10 1 Cf 10-4 Fm Hs No Rf Sg Features strong enhancement of fission half-lives at the deformed neutron shell N = 152 for No, Fm, Cf, (Cm) enhancement at N = 152 vanishes at Z = 104 (Rf) steep decrease of fission halflives on the neutron deficient side and partly at the neutron rich side rather flat behavior of fission at N = for Rf and Sg increase of fission halflives towards the deformed neutron shell indicated for Rf, Sg, Hs, (No) Neutron number SHIP Data (neitron deficient side): J. Khuyagbaater et al. EPJA 37, 177 (2008), EPJA 46, 59 (2010), F.P. Heßberger et al. Z. Phys. A 359 (1997)

9 SF halflife systematics of ee - nuclei Influence of N = 152 shell on SF halflives disappeared at Z = 104 2nd barrier drops below gs (Oganessian et al. 1975, Randrup et al. 1973); effect ect visible in change of barrier curvature ħω f (Heßberger 1985) Calculations of Smolanczuk et al. (1995) reproduce well SF known in 1995, deviations for SF discovered after ( ) h f / MeV 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 U Pu Cm Cf Fm No Rf Sg T sf /s 10 4 Rutherfordium ( Z=104) Seaborgium (Z=106) Hassium (Z=108) Neutron number 0,76 0,78 0,80 0,82 0,84 0,86 0,88 0,90 0,92 Status: Calculations: R. Smolanczuk et al. PRC 52, 1871(1995) x F.P.Heßberger SHIP-Data: tsf_hqomsys85 G.Münzenberg et al. ZPA 332,227 (1985), ZPA 324,489 (1986), F.P. Heßberger, GSI-Report GSI (1985) F.P.Heßberger et al. ZPA 359, 417 (1997), S.Hofann et al. EPJA 10, 57 (2001) K.Nishio et al. EPJA 29, 561 (2006), D.Ackermann et al. To be published

10 SF hindrance of odd-mass nuclei Due to angular momentum conservation fission of nuclei with odd Z, odd cannot follow most energetic favourable path effective enhancement of fission barrier ( specalisation energy ) enhancement of fission life-times J. Randrup et al. NPA 217, 221 (1973) ) T sf / s 1E13 1E odd A ee odd A ee odd A ee Fm No Rf High gs spins for Fm (7/2+,9/2+) ( ) 0,01 R. Vandenbosch, J.R. Huizenga, Nuclear Fission (1973) 1E Neutron number

11 SF hindrance of odd-mass nuclei Hindrance Factor 261 Db Fm 1/2+6[31] 257 Rf 1/2 2+[620] 259 Sg 1/2 2+[620] 261 Sg 3/2 2+[622] 243 Fm 7/ /2+[743] 251 No 7/2 2+[624] 255 Rf 9/2 2-[734] 47m Md 1/2 2-[514] 53m Lr 1/2 2-[521] 57m Db 1/2 2-[521] 53g Lr 7/2- -[514] b 9/2+[734]?, 1/2-[521]? 257 7m Db 1/2 2-[521] Even Z Nuclei Odd Z Nuclei

12 Spontaneous fission of K - isomers K isomers are multi qp configurations and thus SF is expected to be strongly hindered 254m1 No Assigned as 2qp state with K π = 8 - b sf =(2.0±1.2) x 10-4 T sf 1400 s T sf,calc 1s HF 1400 T sf (iso) / T sf (gs) m2 No Assigned as 4qp state with K π = 16 - or 16 + b 13 sf 1.3 x 10-4 T sf s T sf,calc 0.5 μs HF > 3x10 6 T sf (gs) s F.P.Heßberger et al. ZPA 43,55 (2010) Previously known: 256m Fm E* = kev, K π = 7 -, T 1/2 = 70 ns b sf = 2x10-5 T sf = 3.5 s T sf = s T sf (iso) / T sf (gs)

13 EC delayed fission Decay scheme of 246 Md EC delayed fission in 246 Md Q EC = 6.16 (±0.33) MeV B f ( 246 Fm) = 6.49 MeV (Kowal et al.) B f ( 246 Fm) = 6.13 MeV (Möller et al.) b sf ( 246 Fm) 0.12 (prod. by EC of 246 Md) b sf ( 246 Fm) = ± 008(di 0.08 (direct prod. by 208 Pb( 40 Ar,2n) 246 Fm (M. Venhart et al. EPJA 47:20 (2011)) Excess EC delayed fission of 246 Md; P ECDF = 0.13 ± 0.03 However: two states in 246 Md T 1/2 ( 246 Fm,sf) 7.6 s (ER-sf corr.) T 1/2 ( 246 Fm,α) 6.2 s (ER-α corr.) EC mainly from 246m2 Md; probably also Small contribution from 246m1 Md P ECDF ( 246m2 Md) > 0.1 S. Antalic et al. EPJA 43, 35 (2010) highest probability for EC delayed SF in transuranium region

14 Implation Depth of ER < range of SF products p(sf1+sf2) 40 %. Large PHD due to high h ionisationi density Si Detector TKE - Distributions nts Eve No Experiment Monte-Carlo-Simulation <TKE> = 203 MeV range of sf products ca. 22 m SF1 Even nts Rf Experiment Monte-Carlo-Simulation <TKE> = 207 ± 13 MeV Advanced simulations using GEANT 4 started 5 SF2 ER Ev vents Rf Experiment Monte-Carlo-Simulation <TKE> = 220 ± 15MeV 3 Impl. Depth ER ca m F.P. Heßberger PHD GSI E kin,sf / MeV

15 TKE Distributions (preliminary results) Events / 5 MeV 30 sf ( 252 No) (b 25 sf =36%) E = 162 MeV (uncorr.) E = 49 MeV (FWHM) 20 (Ix0.05) sf ( 255 Rf) (b sf =50%) E = 165 MeV (uncorr.) E = 36 MeV (FWHM) E / MeV (Stop detector) 259 Sg (254 ms), sf(235 ms) ca. 92 (411 ms) 0 (9610, 9035 kev) (9540,... kev) (94 %) 1/2+[620] sf (6 %) 11/2-[725] 16 Event ts / 5 MeV sf ( 256 Rf) (b sf = 98%) E = 184 MeV (uncorr.) E = 41 MeV (FWHM)) sf ( 260 Sg Rf) (b sf ( 260 Sg) = 71 %) E = 185 MeV (uncorr.) E = 53 MeV (FWHM) Eve ents / 5 MeV sf ( 255 Rf) (b sf =50%) E = 165 MeV (uncorrected) E = 36 MeV (FWHM) sf( 259 Sg) (b sf = 6%) E = 180 MeV (uncorr.) E = 13 MeV (FWHM) sf( 258 Sg) (b sf = 100%) E m = 182 MeV (uncorr.) E / MeV (Stop detector) E / MeV (Stop detector)

16 TKE - Distributions 215 MeV ( 259 Sg) (preliminary) 243 Fm 198 MeV ( 255 Rf) (preliminary) 241 Fm D.C. Hoffman, M.R. Lane, RCA 70/71, 125 (1995) 244 Fm Published Results: (J. Khuyagbaatar et al. EPJ A 37, 177 (2008)) 244 Fm: 193 ± 12 MeV (194 MeV*) 243 Fm: 198 ± 15 MeV 241 Fm: 196 ± 10 MeV *Literature

17 Transition from asymmetric to symmetric fission N = 150 ( 252 No) N = 151 ( 255 Rf) N = 152 ( 254 No) N = 152 ( 256 Rf) N = 152 ( 258 Sg)? N = 153 ( 259 Sg) N = 154 ( 256 No) N = 154 ( 258 Rf) N = 154 ( 260 Sg)? D.C. Hoffman, M.R. Lane, RCA 70/71, 125 (1995) No Rf Sg Literature SHIP Data

18 TKE of Superheavy Elements 279 Ds Estimation of TKE on basis of few events is dangerous!! Nevertheless: data indicate that fission energies of 283,284 Cn and 279 Ds follow the trend 284 Cn of predictions by Schmitt and Mosel and Viola systematics 283 Cn Asymmetric mass split 279 Ds 283 Cn 284 Cn < TKE > / MeV pre-neutron post-neutron Unik Viola Symmetric mass split Z 2 /A 1/3 Konferenzen/Tokai0312/UnikViola H.W. Schmitt, U. Mosel, NP A186, 1 (1972)

19 Fission barriers of Superheavy Nuclei Strong Variation of fission barrier values; extremely high values for SLy6 (p-shell at Z = 120; for Z = 120 agreement of Kowal (MM), Smolanczuk (MM), NLZ-2, NL3* within 1 MeV, Möller values typically MeV higher. 12 B sf / MeV B sf / MeV Möller et al. Kowal et al. Bürvenich et al. (NL3) Bürvenich et al. (SkP) Bürvenich et al. (SLy6) Bürvenich et al. (SkI4) Smolanczuk (B f (dyn)) 48 Ca Cm Möller et al. Kowal et al. Bürvenich et al. (SLy6) Bürvenich et al. (NLZ-2) Smolanczk (B f (dyn)) Abusara et al. (NL3*) A (Z = 116) 50 Ti Cf 54 Cr Cm A (Z = 120) DANF11/SHE/Spaltbarrieren_Z116_Z120B,

20 Fission Halflives of SHE No fission observed for , 290, , : upper limits <10 3 x T sf (theo) T sf of , 282,284 Cn 1-2 orders of magnitude lower than T sf (theo) Presently experiments going on at GSI on synthesis of elements 120 by 248 Cm( 54 Cr,xn) 302-x 120 and Predicted (MM) T sf (T α ) for 249 Cf( 50 Ti,xn) 299-x 120 and being prepared for el. 119 Element 120 isotopes: by 249 Bk( 50 Ti,xn) and will start on April 26, : 28 ms (1-10 μs) t sf / s calculations: R. Smolanczuk et al. PRC 52, 1871 (1995) R. Smolanczuk PRC 56, 812 (1997) Cf( 50 Ti,4n) Cm( 54 Cr,4n) Z=118 Z= Neutron number Z=112 Z=114 Z=116 konferenzen/danf11/she/tsfshe F.P.Heßberger : odd-mass nucleus 27 ms(ee) x HF HF 1000 (42 μs) : 11 ms ( 7 μs) ) Taking differences of expected values and calculations and variances of fission barrier predictions still can be expected: Element 120 isotopes decay by α - emission

21 Summary and Outlook SF properties of 237,238 Cf confirmed, SF branching of 240 Cf experimentally determined, new sf activities or sf branches ( 241,243 Fm, 251 No, 264 Sg, 266 Hs, 283 Cn) SF of 259 Sg confirmed; spin and parity of fissioning level assigned for the first time; strong evidence of symmetric fission of 259 Sg EC delayed fission of 246m2 Md identified; highest EC delayed fission probability in transuranium region SF branching in K isomers 254m1 No and upper limit for 254m2 No measured; high stability of K isomers against SF demonstrated Investigation of SF properties nuclei Z > 106 (TKE, mass split, ) Spin dependence of hindrance factors SF probabilities of K isomers 250 Fm, 252 No (high gs sf branch), 253 No EC delayed fission shape dependence of fission properties; so far (Z 92) : prolate deformation; approaching spherical shells oblate shapes spherical nuclei approaching spherical shells oblate shapes, spherical nuclei new techniques to overcome disadvantages of implantation methods for TKE and mass distrubtion measurements gas cells, ion traps

22 TRAPSPEC Experiment in 2011 Reaction: 150 Sm( 50 Ti,5n) 195 Po Study of separation of 195m Po (T 1/2 = 1.9 s) and 195g Po (T 1/2 =4.6s)onthe basis of trapping times L.-L. Andersson GSI Scientific Report 2011

23 Collaborations GSI, Darmstadt D.Ackermann, M.Block, S.Heinz, F.P.H., S.Hofmann, B.Kindler, I.Kojouharov, J.Khuyagbaatar, B.Lommel, R.Mann, B.Sulignano (former members) Helmholtz Institut Mainz L.-L.Andersson Comenius University Bratislava, Slovakia S. Antalic, Z.Kalininova, S.Saro, M.Venhart, B.Streicher (former members) University y Jyväskylä, y Finland M.Leino, J.Uusitalo JAEA Tokai, Japan K.Nishio FLNR JINR Dubna, Russia A.G.Popeko, A.Yeremin

24 Jadambaa Khuyagbaatar PHDs * S. Antalic Comenius University Bratislava (2004 * B. Streicher Comenius University Bratislava (2006) * B. Sulignano Joh. Gutenberg Univ. Mainz (2007) * J. Khuyagbaatar, St. Petersburg State University (2009) Barbara Sulignano Stanislav Antalic Branislav Streicher