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Nuclear Spectrcosopy in Exotic Nuclei

Nuclear Spectrcosopy in Exotic Nuclei. Paddy Regan Dept,. Of Physics University of Surrey, Guildford, UK p.regan@surrey.ac.uk. Outline. PART 1: Binary Collisions at Coulomb-barrier Energies: The valence N p .N n maximum…..approaching 170 Dy 104

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Nuclear Spectrcosopy in Exotic Nuclei

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  1. Nuclear Spectrcosopy in Exotic Nuclei Paddy Regan Dept,. Of Physics University of Surrey, Guildford, UK p.regan@surrey.ac.uk

  2. Outline • PART 1: Binary Collisions at Coulomb-barrier Energies: • The valence Np.Nn maximum…..approaching 170Dy104 • PART 2: Projectile fragmentation decay studies • The ‘magical journey south’ at N=126: 205Au126; 204Pt126 • In between…. a new subshell closure for Z=74: 190,2W

  3. Part 1: The Search for 170Dy

  4. It is well established that low-lying (quadrupole) collectivity depends on the PRODUCT of the number of VALENCE protons AND neutrons = Np.Nn. e.g., 172Hf Z=72; N=106; Np=(82-72)=10 ; Nn=(100-82)=18 →Np.Nn=180

  5. 170Dy104 Np.Nn=352 → (82-66)=16 X (126-104)=22 R(E(4+) / E(2+)) Systematics plot from Burcu Cakirli

  6. 170Dy, double mid-shell, ‘purest’ K-isomer ? (see Regan, Oi, Walker, Stevenson and Rath, Phys. Rev. C65 (2002) 037302) Kp=6+state favoured Max at 170Dy

  7. 170Dy (Z=66; N=104)The most ‘anti-magic’ nucleus… • Np.Nn =352 = maximum value for any nucleus for A<200. • Does it have max collectivity ? • It should arguably be the ‘best case’ of an axially symmetric, quadrupole deformed nucleus. • Experimental Problem • Neutron-rich, can’t get to with (stable beam) fusion-evaporation. • Solution • Can make with DIC at Coulomb barrier energies…if we know something to start with…

  8. Aim? To perform high-spin physics in stable and neutron rich nuclei. Problem: Fusion makes proton-rich nuclei. Solutions? (a)fragmentation (b) binary collisions/multi-nucleon transfer

  9. z x q1 q2 f1 f2 y

  10. PRISMA spectrometer gives (A<100) beam-like binary fragment ID (A,Z) using ToF, E and DE; Coincident gamma-rays measured using CLARA clover germanium array.

  11. P.A. Soderstroem, J. Nyberg, P.H. Regan et al., submitted to Phys. Rev. C (2009) • 82Se beam on a 170Er target; Total Z=34 + 68 = 102 ; A = 170 + 82 = 252 • Eb~20% above Coulomb barrier for DIC • Use PRISMA spectrometer to define direction and nature of (lighter) BLFs and • assume simple 2-body (elastic) kinematics for Doppler corrections • Assume no proton evaporation from (neutron-rich) residues…Ztot=conserved

  12. P.A. Soderstroem et al., submitted to Phys. Rev. C 84-Krypton ion selects Dy TLF binary partners Selecting on 84Kr fragments in PRISMA (co)-selects Dy isotopes with Amax =168; Doppler correct for heavy TLFs to observe 4+→2+ yrast transitions in 168,166,164,162Dy. Seems to work: 0n → 6n neutron total evaporation observed in coincidence with defined BLF.

  13. Can get extra TLF channel selection on ‘Q value’ in the reaction by gating on ‘short time of flight’ (i.e. max KE) fragments – Preferentially selects 0n channel. Example of 168Dy selection by gating on 84Kr fragments in PRISMA. 84Kr ions (162-8Dy) 84Kr ions plus shortest ToF (fastest ions). 168Dy selected.

  14. Yrast cascade in N=102 binary partner of 84Kr (i.e., 168Dy) identified; Gammas are in mutual coincidence

  15. Evidence for (the 4+ → 2+ transition in) 170Dy ? • Gated on 82Kr • + ToF • b) Gated on 82Kr + • ToF + • 777 keV gamma

  16. 168Dy (current valence max nucleus) yrast sequence observed up to spin Ip=10+ • Binary gating technique established and further developed using high-ToF gating. • Candidate for 4+→2+ transition in 170Dy determined (163 keV). Energy systematics suggest return to max collectivity • ….also establishes a line which to gate on uses a higher-statistics (e.g., thick target GAMMASPHERE or INGA etc. run).

  17. Part 2: Decay Gamma-ray Spectroscopy using Projectile Fragmentation Reactions: Triaxiality in Neutron-Rich W Nuclei.

  18. Accelerator facility at GSI-Darmstadt • The Accelerators: • UNILAC(injector) E=11.4 MeV/n • SIS 18Tmcorr. U 1 GeV/n • Beam Currents: • 238U - 108 pps • some medium mass nuclei- 109 pps • (A~130) • FRS provides secondary radioactive ion beams: • fragmentation or fission of primary beams • high secondary beam energies: 100 – 700 MeV/u • fully stripped ions

  19. Ion-by-ion identification with the FRS TOF E Primary beam energies of ~ 0.5 →1 GeV per nucleon (i.e. ~200 GeV) Cocktail of secondary, exotic fragments with ~ x00 MeV/u thru. FRS. Separate and identify event-by-event. Chemically independent.

  20. see S. Pietri et al., Nucl. Inst. Meth. Phys. Res. B261 (2007) 1079

  21. exp. pronounced shell gap shell structure quenched Physics Aside….is there N=126 shell quenching ? r-process abundances mass number A Assumption of a N=126 shell quenching leads to a considerable improvement in the global abundance fit in r-process calculations

  22. Information gathered from Passive Stopper RISING Stopped Beam (A~200) Within red line: nuclei populated measured using FRS + RISING with 1 GeV/u 208Pb beam. 205Au S.J. Steer et al., IJMP E18 (2009) 1002 204Pt ? N=126

  23. 204Pt126

  24. S. Steer et al., Phys. Rev. C78 (2008) 061302(R)

  25. S.J. Steer et al., Int. Jour. Mod. Phys. E18 (2009) 1002 ….isomer spectroscopy ‘down’ the N=126 line…first ID in such nuclei

  26. The Principle of the Active Stopper e- Focal plane implantation detector sensitive to electron emission Si Strips The waiting time between particle implantation and b-particle (or i.c. electron) emissionis a measure of the decay half-life. Gamma rays emitted following these decays are detected by the RISING array.

  27. RISING Active Stopper Measurements Passive Stopper: g ray from isomer cascades with T1/2 ~ 10 ns  1 ms. Active Stopper measurements: b-particles, internal conversionelectrons. T1/2 up to ~ minutes; associated with delayed g-rays. 5 cm x 5 cm DSSSD (16 strips x 16 strips = 256 pixels) x 3 = 758 total pixels. See P.H. Regan et al., Int. Jour. Mod. Phys. E17 (2008) 8 ; R. Kumar NIM A598 (2009) 754

  28. R. Kumar NIM A598 (2009) 754 log. pre-amp to measure electrons (0.1 MeV) & heavy ions (GeV) in same detector.

  29. Passive Stopper measurements: g-rays from isomer with T1/2 for 10 ns  1 ms. Active Stopper measurements: b -particles, i.c. electrons, T1/2 ms →mins

  30. Podolyak et al., Phys. Lett. B672 (2009) 116

  31. submitted to Phys. Rev. C (2009)

  32. 188Ta →188W 190Ta →190W 192Ta →192W

  33. 1/E(2+) New data points on R(4/2) for 190W and 1/E(2+) point to new sub-shell closure signature around Z~74 in neutron-rich nuclei (N >114).

  34. Summary • Approaching 170Dy using DICs – studying the best nuclear rotors. • Deep proton-hole excitations in N=126 isotones using fragmentation and • Isomeric gamma-ray spectroscopy (205Au, 204Pt, 203Ir) and • Isomeric Internal conversion spectroscopy (205Au) • First spectroscopy of very neutron-rich W isotopes using beta-delayed • fragmentation spectroscopy. • Evidence for a sub-shell closure around Z=74. • Future: • More RISING followed by ‘PRESPEC’ to see more N=126; N=82 isomers • Then ‘DESPEC’ project at FAIR. • More DIC at Legnaro using AGATA demonstrator. • High-statistics runs with large array for DIC. Use isomer ‘tags’ to performed full • spectroscopy of all such neutron-rich isotopes (see RVFJ talk) etc.

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