 -delayed fission of neutron-deficient Fr and At isotopes Lars Ghys 1,2 1 Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven 2 Belgian Nuclear Research Centre, SCKCEN, Mol, Belgium BriX workshop Liège / 05

The nuclear fission process 2 time Fision- fragment masses Discovered in 1939 by Hahn & Strassmann Applications in nuclear power plants/ bombs True microscopic description still lacking today counts mass counts mass

Low-energy fission throughout the nuclear chart A.N. Andreyev, M. Huyse and P. Van Duppen, Rev. Mod. Phys. 85, 1541 (2013) asymmetry transition symmetry asymmetry ??? Introduction set up results conclusion 3

 - delayed fission (  DF) QQ BfBf      DF 1)  decay excitation energy < Q value Spin/ parity sensitivity 2)fission U and Pb region (isospin dependence) Mass/ energy distributions  DF probability and partial half life QβQβ Introduction setup results conclusion 4

1.4 GeV protons HRS mass separator GPS mass separator GPS target Windmill position HRS target ISOLDE facility - CERN S.Rothe et al./ Nature Comm 4 (2013) µA of protons on UC x target isotopic (isomeric) pure beam beam energy of keV Introduction setup results conclusion 5

Experimental set-up: “Windmill system” Si spectrum e - /e +  particles  -  summing Fission fragments Introduction setup results conclusion 6 Si front Si back C foil ISOLDE beam FF C-foils Si detecors 241 Am

~2 FF/h 2 isomers ~8 FF/h ~43 FF/h 2 isomers ~27 FF/h Reference Introduction setup results conclusion 7 L. Ghys et al., Phys. Rev. C 90, (R) (2014)

Fission at low excitation energies: systematics A.N. Andreyev, M. Huyse and P. Van Duppen, Rev. Mod. Phys. 85, 1541 (2013) Introduction set-up results conclusion 8

Finite Range Liquid Drop Model (P. Möller & J. Randrup) Introduction setup results conclusion 9 Experimental triple-humped vs calculated double-humped transition L. Ghys et al., Phys. Rev. C, (R) (2014)

HFB theory with Gogny D1S nuclear force (M. Warda et al.) Broad and flat plateau in potential- energy surface Introduction setup results conclusion 10

11 Partial βDF half life T βDF QQ BfBf      DF Elongation Energy

Theoretical considerations on T βDF 12 β- strength function S β (E * )= constant Fermi function F(Q β – E * ) ~ (Q β – E * ) a Ratio of decay widths Γ f / Γ tot (E * ) ~ exp[-b(B f -E * )] Introduction set-up results conclusion

T βDF versus (Q β – B f ) Introduction set-up results conclusion 13 L. Ghys et al., Phys. Rev. C 91, (2015)

Conclusions & outlook Conclusions o  df of 200,202 Fr and 194,196 At confirmed and studied at ISOLDE →Triple- humped mass distributions →Comparison with theoretical calculations (P. Möller et al., M. Warda et al.) o Semi-empirical description of T  df o Long measurement time  parasitic  decay information Outlook o Separation of isomers in 202 Fr and 194 At (CRIS) Introduction set-up results conclusion 14

Thank you! A.N. Andreyev D. Jenkins V. Truesdale X. Derkx J. Lane V. Liberati H. de Witte J. Elseviers L. Ghys S. Sels M. Huyse D. Radulov M. Rajabali C. Van Beveren P. Van den Bergh P. Van Duppen T. E. Cocolios E. Rapisarda ISOLDE & RILIS L. Ghys L. Popescu D. Pauwels Comenius University, Bratislava B. Andel S. Antalic Z. Kalaninova K. Nishio, Y. Nagame, JAEA, Tokai, Japan F.P. Hessberger, SHIP (GSI), Darmstadt, Germany U. Koster, ILL, Grenoble, France R. Page, OLL, University of Liverpool P. Möller, Los Alamos National Laboratory, US M. Warda, M. Curie-Sklodowskiej, Poland 15

Back-up 16

Pulse height defect (PHD) Why use Si detectors? o intrinsic efficiency ≈100 % o good energy resolution : FWHM ~ 1.5 MeV o compact size Disadvantage : Pulse height defect o Energy loss in detector dead layer o Electron – hole recombination o Nuclear collisions Schmitt calibration [1] : E = aX + b → E = (a+a’M)X+ b + b’M [1] : H. W. Schmitt et al. / Phys. Rev. 137 (1965) B837 – B Lowering of recorded pulse height

Calibration measurements – Lohengrin, ILL 18/21 Fission framents from 235 U(n,f) Energy/q and mass/q selection Energy accuracy of ~100 keV 5 detectors tested : 2x PIPS; 3x surface barrier (1x annular) Mass range : 48 – 149 amu Energy range : 12 – 100 MeV

Calibration method – coincident fission events 19/21 Measurement : pulse height detector 1 (X 1 ) and detector 2 (X 2 ) calibration E 1 = (a 1 + a’ 1 M 1 )X 1 + b 1 + b’ 1 M 1 energy calibration det 1 (Lohengrin – ILL) E 2 = (a 2 + a’ 2 M 2 )X 2 + b 2 + b’ 2 M 2 energy calibration det 2 (Lohengrin – ILL) M 1 E 1 = M 2 E 2 Conservation of linear momentum M 1 + M 2 = A Conservation of mass  Extraction of M 1, M 2, E 1 & E 2 Additional corrections Si back Si front ~900 Ǻ Si back Si front Energy loss in carbon foils (MeV)0.3(1)1.1(2) Energy losses in dead layer (MeV)0.3(3) Systematic calibration error (MeV)0.5(5) Prompt neutron emission : ~0.7 MeV/ neutron

‘Parasitic’ information on decay : 202 Fr  -  plot : 202 Fr 202 Fr 198 At Single  ’s M. Huyse et al./ PRC 46 (1992)