Beta-delayed fission: from neutrondeficient to neutron-rich nuclei

Transcription

1 Beta-delayed fission: from neutrondeficient to neutron-rich nuclei Andrei Andreyev University of York, UK 200,202,204Fr N/Z~ ,194,196At 186,188Bi 178,180,182Tl Z=82 Tl ms 1 10th ASRC Workshop. JAEA, 22rd March 2013

2 Collaboration A. Andreyev V.Truesdale R. Wadsworth D. Jenkins M. Vermeulen C. Barton M. Bentley CERN, Geneva JAEA Tandem K. Nishio S.. Mitsuoka I. Nishinaka H. Makii S. Chiba A.Iwamoto H.Koura Y.Utsuno LBNL(Berkeley): Jorgen Randrup P. Möller, A. Sierk Yukawa Institute (Kyoto, Japan) T. Ichikawa J. Elseviers C. Van Beveren L. Ghys M. Huyse D. Radulov E. Rapisarda P. Van Duppen U. Koster (ILL, Grenoble, France) R. Page, OLL, University of Liverpool N. Patronis, University of Ioannina, Greece S. Vermote, C. Wagemans (Gent, Belgium) M. Veselský (Slovakia) I.Tsekhanovich (CENBG, France) L. Popescu, D. Pauwels, SCK CEN, Mol, Belgium S. Antalic, Z. Kalaninova, Comenium University, Slovakia

3 Outlook Brief (experimental) review on low-energy fission Low-energy fission in new regions of the Nuclear Chart Beta Delayed Fission ( DF) what it is and why? DF 194,196 At, 202 Fr Further plans and ideas

4 Symmetric vs Asymmetric Fission Macroscopic only (like a liquid drop) Microscopic effects added (nuclear shells and pairing)

5 Experimental information on low-energy fission Nuclei with measured charge/mass split (RIPL-2 + GSI) Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Z= Ir 196 Au - particle induced x - e.m. induced E*~11 MeV K.-H. Schmidt et al.

6 Experimental information on low-energy fission Nuclei with measured charge/mass split (RIPL-2 + GSI) Region of our interest: fission of nuclei with A~ , N/Z~ : Hg,Pb,Po,Rn JAEA and ISOLDE(CERN) Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Z= Hg N/Z= Ir 196 Au - particle induced x - e.m. induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~ : predominantly symmetric, e.g. FRS(GSI)

7 Beta-Delayed Fission Discovery: 232,234 Am (1966, Dubna) 180 Tl(Z=81,N=99) I=(4,5) Q EC MeV + /EC N Low-energy fission! (E*~3-12 MeV, limited by Q EC ) Relatively low angular momentum of the state 12 cases known before our work (neutron-deficient Uranium region) DF DF branch B f ~ 10 MeV N DF P DF = N DF N 180 Hg(Z=80 N=100) deformation P DF depends strongly on: Q EC of the parent: the higher Q EC, the larger the P DF B f of the daughter: the lower B f, the larger the P DF Actually, Q EC -B f and -strength S are the most important parameters

8 Mass Separator ISOLDE Hot cells HRS Target 50 g/cm 2 GPS Target 50 g/cm 2 Proton beam 1.4 A, 1.4 GeV from PSB GPS Magnet Target RILIS Hot Cavity Extractor Ion Source HRS Magnets Mass separation Laser beams Control Room Protons Target material 60 kv Reaction products (neutral) Experiments Ions ISOLDE uses more than half of CERN s protons! Experiments 8

9 Detection system for DF studies at ISOLDE Annular Si Si MINIBALL Ge cluster pure 180 Tl kev from RILIS+ISOLDE C-foils 20 g/cm 2 Annular Si beam from ISOLDE ff Si ff C-foil Si detectors

10 Energy/Mass distribution of fission fragments from 180 Hg ASYMMETRIC energy split! Thus asymmetric mass split: M H =100(4) and M L = 80(4) E ff1 -E ff2 coincidences ~330 events Singles 1111 ff The most probable fission fragments are 100 Ru (N=56,Z=44) and 80 Kr (N=44,Z=36) 10

11 CLDM (P. MÖller et al., yet unpublished) 180Hg CLDM: Clay Liquid Drop Model (circa 2008) gs 180Hg Asymmetric valley PM: Look, it s trivial...

12 New Type of Asymmetric Fission in Proton-Rich Nuclei via DF of 180 Tl E Q EC ( 180 Tl)=E * max=10.44 MeV Q EC ( 180 Tl)-B f ( 180 Hg)=0.63 MeV P DF ( 180 Tl)=3.6(7) Kr 100 Ru 90 Zr+ 90 Zr Calculations according to 5D fission model (P. MÖller et al., Nature 409, 785 (2001))

13 Two types of asymmetry: what s the difference? 180 Hg 238 U

14 DF of 178 V. Liberati et al (submitted to PRC, 2013) counts Hg 180 Hg ~0.8 ions/s ~0.3 ff/h ~200 ions/s ~20 ff/h Q EC ( 178 Tl)=E* max ( 178 Hg)=11.14 MeV Q EC ( 178 Tl)-B f ( 178 Hg)=1.82 MeV At this level of statistics: also asymmetric fission of 178 Hg, with mass split similar to 180 Hg E* max ( 180 Hg)=10.44 MeV Fission Fragments Energy in Si detector [MeV]

15 Low-energy Electromagnetically-Induced Fission in-flight at FRS(GSI), K.-H. Schmidt et al. Primary beam 238 U at 1 AGeV 1 g/cm 2 primary target Separated RIBs from FRS Pb secondary target E*~11-12 MeV (GDR) Identified RIB from FRS e.g. 204 Rn Fission Active Fragments Target 204 Rn@FRS symmetric (E*~12 MeV) 15

16 New Region of Asymmetric Fission Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Z=82 178,180 Hg ISOLDE 187 Ir 196 Au - particle induced x - e.m. induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~ : predominantly symmetric, e.g. FRS(GSI)

17 New Region of Asymmetric Fission Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Lightest Hg isotopes with N/Z~1.25: asymmetric Z=82 178,180 Hg ISOLDE 187 Ir 196 Au - particle induced x - e.m. induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~ : predominantly symmetric, e.g. FRS(GSI)

18 From Asymmetry to Symmetry Heavy Actinides, N/Z~1.56: predominantly asymmetric; spontaneous fission, fission isomers Lightest Hg isotopes with N/Z~1.25: asymmetric Z=82 178,180 Hg ISOLDE 187 Ir 196 Au - particle induced x - e.m. induced E*~11 MeV Pre-actinides, light Ir-Th N/Z~ : predominantly symmetric, e.g. FRS(GSI)

19 Fission of Proton-rich nuclei with A~ Courtesy P. Moller (LANL) and J. Randrup (LBNL), 5 th ASRC workshop on Fission, Tokai 2012 CERN-ISOLDE EC -delayed Fission (Low Energy Fission) 200,202 Fr 194,196 At 186,188 Bi 180,178 Tl JAEA tandem Heavy-ion induced Fission Mercury chain of isotopes (High Excitation Energy) Courtesy K. Nishio

20 IS534, 9-14 May 2012: Mass Distributions Measurements of 194,196 Po via DF of 194,196 At Singles 1111 ff 180 Hg ~20 ff/h E ff1 -E ff2 coincidences ~330 events ~7 coincident ff/h

21 IS534, 9-14 May 2012: Mass Distributions Measurements of 194,196 Po via DF of 194,196 At Energy in Si2 (MeV) 196 At DF of 194 At: singles: 40 ff/hour Coincidences: ~10 ff/hour Energy in Si1 (MeV) 194 At Energy in Si2 (MeV) 180 Hg Energy in Si2 (MeV) Clear difference in energy (thus, mass) distribution Energy in Si1 between (MeV) fission of 180 Hg and 194,196 Po Energy in Si1 (MeV)

22 May and June 2012: Mass Distributions Measurements via DF of 194,196 At and 200,202 Fr Gradual transition from asymmetry in 180 Tl to a mixture of symmetric and asymmetric in 196 At and 202 Fr! Fission Fragment Mass

23 Recent Theory Efforts for the Hg s chain Several theory groups have initiated their calculations for the long chain of Hg isotopes Must account for asymmetry of 178,180 Hg (data by Andreyev et al.) Must account for apparent symmetry of 198 Hg (data by Itkis et al.) Need excitation-energy dependence (as higher-energy data start to become available JAEA experiments by Nishio, Andreyev et al.)

24 Some examples: Brownian Metropolis Shape Motion based on J. Randrup and P. Moller, PRL 106, (2011) Phys. Rev. C 85, (2012)

25 Some examples: Improved Scission-Point Model asymmetric Inter-fragment distance is not fixed and calculated. values of ~0.5-1 fm result (Wilkins fixed at 1.4 fm) Symmetric! Mass symmetry/asymmetry doesn t change as a function of E* (up to E*~60 MeV) good for future experiments Also, J.-L. Sida et al. private communication

26 Some examples: Mean-field HFB+Gogny D1S

27 Fusion- Fission Reactions at JAEA s tandem New experiment at JAEA (March-April 2012) 36 40Ar + 144Sm Hg 36 40Ar + 154Sm Hg 36 40Ar + 142Nd Pt 90Zr + 90Zr 180Hg Ebeam from 160 to 235 MeV New reaction chamber, larger MWPC new electronics (time-stamping) Analysis in progress

28 Mapping beta-delayed fission: from neutron-deficient to neutron-rich nuclei Invited review in Reviews of Modern Physics Submitted: Jan March 2013: got very positive referees reports

29 Known Beta-delayed fission nuclei

30 Beta-delayed fission in tne neutron-rich Fr nuclei

31 Mapping Terra Incognita in Low-Energy Fission To be shown in: A. N. Andreyev, M. Huyse, P. Van Duppen, Beta-delayed Fission, Review of Modern Physics (under refereeing now)

32 Thank you! Strong support from Reimei Program of ASRC (JAEA) is very much appreciated 32