Nuclear Spectrcosopy in Exotic Nuclei Paddy Regan Dept,. Of Physics University of Surrey, Guildford, UK

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Nuclear Spectrcosopy in Exotic Nuclei Paddy Regan Dept,. Of Physics University of Surrey, Guildford, UK

Outline PART 1: Binary Collisions at Coulomb-barrier Energies: –The valence N .N maximum…..approaching 170 Dy 104 PART 2: Projectile fragmentation decay studies –The ‘magical journey south’ at N=126: 205 Au 126 ; 204 Pt 126 – In between…. a new subshell closure for Z=74: 190,2 W

Part 1: The Search for 170 Dy

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

R(E(4 + ) / E(2 + )) Systematics plot from Burcu Cakirli 170 Dy 104 N .N =352 → (82-66)=16 X ( )=22

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

170 Dy (Z=66; N=104) The most ‘anti-magic’ nucleus… N .N =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…

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

z x y    

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.

P.A. Soderstroem, J. Nyberg, P.H. Regan et al., submitted to Phys. Rev. C (2009) 82 Se beam on a 170 Er target; Total Z= = 102 ; A = = 252 E b ~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…Z tot =conserved

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

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 168 Dy selection by gating on 84 Kr fragments in PRISMA. 84 Kr ions ( Dy) 84 Kr ions plus shortest ToF (fastest ions). 168 Dy selected.

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

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

168 Dy (current valence max nucleus) yrast sequence observed up to spin I  =10 + Binary gating technique established and further developed using high-ToF gating. Candidate for 4 + → 2 + transition in 170 Dy 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).

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

Accelerator facility at GSI-Darmstadt The Accelerators: UNILAC (injector) E=11.4 MeV/n SIS 18Tm corr. U 1 GeV/n Beam Currents: 238 U pps some medium mass nuclei 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

Ion-by-ion identification with the FRS TOF E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.

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

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

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

204 Pt 126

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

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

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

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

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

Passive Stopper measurements:  -rays from isomer with T 1/2 for 10 ns  1 ms. Active Stopper measurements:  particles, i.c. electrons, T 1/2 ms → mins

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

submitted to Phys. Rev. C (2009)

188 Ta → 188 W 190 Ta → 190 W 192 Ta → 192 W

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

Summary Approaching 170 Dy using DICs – studying the best nuclear rotors. Deep proton-hole excitations in N=126 isotones using fragmentation and 1.Isomeric gamma-ray spectroscopy ( 205 Au, 204 Pt, 203 Ir) and 2.Isomeric Internal conversion spectroscopy ( 205 Au) 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.