Beyond Band Termination: Spectroscopy at Ultra-High Spin

Beyond Band Termination: Spectroscopy at Ultra-High Spin

Sarah Rigby:: Nuclear Structure Seminar:: Beyond Band Termination: Ultra-High Spin Spectroscopy

Sarah Rigby

• What is meant by “Ultrahigh Spin”?• Why go beyond band termination?

– Very pure single-particle states– Keeps the theorists happy– Return to collectivity at higher spins– A further change of phase for the nucleus??

• First, a bit of history, then onto the recent experiment

Introduction


The 12 valence particles move in The 12 valence particles move in equatorial orbits, driving the equatorial orbits, driving the nucleus to an oblate shape!nucleus to an oblate shape!

Simpson et al., Phys. Rev. Lett. (1984)

Up to Band Termination: 158Er by Sharpey-Schafer et al


Valence nucleons relative to 146Gd core

Up to Band Termination: Rigid Rotor plot

Same information as on the previous spectrum represented on a rigid

rotor plot

Band terminating states are highly favoured i.e. much lower in energy on this plot

?What happens

up here?


82

30+

[(h9/2, f7/2)6 (i13/2)2]30+

Up to Band Termination: Tilted Fermi Surface Plot


Work from 1994, Simpson et al

(+,0) triples gate

Feeding 46+

Beyond Band Termination: 158Er


Very weak states observed to feed the 46+ state

Experiment at LBNL using GAMMASPHERE, 2004, by Paul, Simpson and Riley,analysed by A. Evans


• Gammasphere experiment @ LBNL: 114Cd(48Ca, xn)156,157,158,159Er• Discovery of states BEYOND band termination• 2 PRLs: A. O. Evans et al, PRL 92, 252502 (2004) and Paul et al., PRL, 98, 012501 (2008)• 2 PhD theses

Fig. adapted from A. O. Evans et al.




15-

[(h11/2)3(d3/2s1/2)1]15-

61/2+

[(h9/2, f7/2)4 (i13/2)3]61/2+

15-

[(h11/2)3(d3/2s1/2)1]15-

15-

[(h11/2)3(d3/2s1/2)1]15-

15-

[(h11/2)3(d3/2s1/2)1]15-

61/2+

[(h9/2, f7/2)4 (i13/2)3]61/2+

61/2+

[(h9/2, f7/2)4 (i13/2)3]61/2+

15-

[(h11/2)3(d3/2s1/2)1]15-

61/2+

[(h9/2, f7/2)4 (i13/2)3]61/2+

15-

[(h11/2)3(d3/2s1/2)1]15-

15-

[(h11/2)3(d3/2s1/2)1]15-

15-

[(h11/2)3(d3/2s1/2)1]15-

61/2+

[(h9/2, f7/2)4 (i13/2)3]61/2+

61/2+

[(h9/2, f7/2)4 (i13/2)3]61/2+

[(h11/2)3(d3/2s1/2)1]15-

[(h9/2, f7/2)4 (i13/2)3]61/2+

TFSCalcs

91/2- state feeding the 87/2- state via an E2 transition


And there is more!Clear bands seen in the 50-60 ħ spin regime in 158Er

2 such bands seen in 158Er and 2 in 157Er


Beyond Band Termination: Rigid Rotor plot

So, obvious question is what happens in the neighbouring Er nuclei?


Beyond Band Termination: TSD Bands

I = 69I = 69

Lund PES plot

TRS and PES plots predict a strong triaxial minimumConsistent with other CSM calcs - Ragnarsson

… and onto the present…(almost)

History lesson over….

Warning: Erbium can be intoxicating and poisoning is possible…

• Do we still see band termination as we increase neutron number?

• Terminating bands predicted in 160Er• What about the predicted stable triaxial

minima?• What happens beyond band

termination?

Other possibilities: Wobbling modes (D. J. Hartley et al, PLB, 608, 31,(2005)), TSDs (S. W. Odegaard et al, PRL 57, 811 (1986); A. Aguilar et al PRC 77 021302(R) (2008)), unpaired band crossings (M.A. Riley et al PRL 60 553 (1988))

Also, 2 other Gammasphere (@ LBNL) experiments studying TSDs in neighbouring nuclei: 161Tm and 160Yb (A. Aguilar et al)

New Experiment at Gammasphere: Motivation


Still questions to be answered:

The Experiment at Gammasphere

The experiment: •Fusion-evaporation reaction•116Cd(48Ca, xn)159,160,161Er•215 MeV beam from ATLAS accelerator•fold-7 trigger •coincidence ray data recorded from

Gammasphere•101 HPGe detectors used.

Offline analysis: • Data unfolded into 3 cubes and 4 hypercubes for

use with Radware for coincidence analysis – bulk of level scheme analysis

• Beyond the hypercube – using MTSort to produce 5, 6 and 7 spectra using the Peter Twin Method

• Produce cleaner spectraSarah Rigby:: Nuclear Structure Seminar:: Beyond Band Termination: Ultra-High Spin Spectroscopy


Only record the event if 7 Ge detectors fire simultaneously

Fusion evaporation reaction happens in the centre where the target wheel is mounted

Record energy and detector numbers in the data stream for offline analysis

Use Radware for complete spectroscopy and Prof Twin’s method for looking at very weak structures

Heavy metal collimators and absorbers in front of Ge dets

The Experiment at Gammasphere

Analysis: Going beyond the hypercube

6 analysis → must be in coincidence with at least 5 other ’s as detailed in a gate list.


2 New side bands observed at low spin, with linking transitions

Some evidence of band termination observed with fragmentation at the top of the ground state band

2 Floating bands found

Results: 160Er


1604


Results: TSD candidate band in 160Er



With thanks to James Ollier for the spectrum


A “Family” of erbium TSDs! The candidate band in 160Er compared to bands in 157,158Er.

The bump is associated with the i13/2 alignment

And compared to neighbouring bands in 152Dy (SD) and 163Lu (TSD)


TSD bands in the rare-earth region

159


Pattern of TSD type bands all the way to the Hf174 region

“Valley of triaxiality”

Rigid Rotor references (x) taken against 158Er:

156Er – 0.00715I(I+1)157Er – 0.00707I(I+1)158Er – 0.00700I(I+1)159Er – 0.00693I(I+1)160Er – 0.00685I(I+1)161Er – 0.00678I(I+1)

No signs of termination in 161Er

Terminating states in 157Er Termination observed in 159Er, X. Wang – analysis ongoing

“Classic” terminating band in 156Er

Termination observed in 158Er

Will we see termination in 160Er?

Results: 161Er


Results: 161Er


•161Er – difficult to resolve high energys•Ground state band still yrast at very high spin (>60ħ)•Unlikely to see other structures•Analysis ongoing

Summary of Results from latest data:• Low spin level schemes extended for 159,160Er – probably

vibrational bands• High spin level schemes extended for 159,160,161Er• Band termination observed in the ground state band of

159Er• Fragmentation seen in 160Er – still under analysis – is it

abrupt termination?• TSD candidate bands observed in 159,160Er, 3 new TSD-

type bands• 161Er appears to be a near-perfect rotor

Analysis and interpretation still ongoing: • Best possible spectra for weak bands being produced• Ingemar Ragnarrson currently working on the theory• Comparisons of the TSDs to neighbouring nuclei still ongoing• Search for feeding transitions to terminating state(s) still ongoing.

Summary

S. V. Rigby1, A. O. Evans1, J. Ollier2, A.Pipidis3, E. S. Paul1, M. A Riley3, J. Simpson2,

X. Wang3, A. Aguilar3, M. Akmali1, D. Appelbe2, M. P. Carpenter4, D.B.Campbell3, P.T.W.Choy1, R. M. Clark7, M. Cromaz7, I. Darby5, P. Fallon7, D. J. Hartley6, A.Gorgen7,

D.T.Joss2, F. Kondev4, T. Lauritsen4, I. Y. Lee7A. O. Machieavelli7,

P. J. Nolan1, M. Petri1, N. Rowley2, C. Teal3, J. Thomson1, C.Unsworth1, D.Ward7, S. Zhu4

and

I. Ragnarsson8, F. Saric8

1 University of Liverpool, Liverpool, L69 7ZE, UK

2 STFC Daresbury Laboratory, Warrington, UK

3 Florida State University, Tallahassee, Florida, USA

4 Argonne National Laboratory, Argonne, USA

5 U.S. Naval Academy, Annapolis, USA

6 University of Tennessee, Knoxville, USA

7 LBNL, USA

8 Lund University, Sweden

Acknowledgements

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Beyond Band Termination: Spectroscopy at Ultra-High Spin