DSAM lifetime measurements in 194Tl Elena Lawrie iThemba LABS, South Africa P.L. Massiteng, A.A. Pasternak, O. Shirinda, J.J. Lawrie R.A. Bark, S.P. Bvumbi, B.G. Carlsson, R. Lindsay, F. Komati, J. Kau, N.Y. Kheswa, E.O. Lider, R. Lieder, T.E. Madiba, Maine, S.M. Maliage, I. Matamba, S.M. Mullins, S.H.T. Murray, K.P. Mutshena, J. Ndayishimye, S.S. Ntshangase, P. Papka, I. Ragnarsson, T.M. Ramashidzha, D.G. Roux, J.F. Sharpey-Schafer, P.A. Vymers

Chirality in the Tl isotopes suitable configuration – h9/2 proton with particle nature, i13/2 neutrons with hole nature the Tl isotopes may have triaxial shape first candidate chiral pair in the odd-odd 198Tl E.A. Lawrie et al. Phys. Rev. C 78 (2008) 021305(R)  more data on the lighter Tl isotopes to search for better chiral candidates

AFRODITE array at iThemba LABS, South Africa 9 HpGe clover detectors (7 cm x 5 cm), Compton suppressed with BGO shields efficiency of 1.8% at 1.3 MeV 8 HpGe LEPS detectors ( 1 cm x  6 cm) Experiment A – thin target 181Ta(18O,5n)194Tl at beam energy E(18O)=92 MeV Two weekends of beam time about 90 hours at 5 kHz rate of  -  coincidences and 25 kHz per clover Target was thin, ~1mg/cm2 recoiling nuclei with v/c ~ 0.8 % AFRODITE array 8 HpGe clover detectors, Compton suppressed with BGO shields arranged: 4 detectors at 900 and 4 detectors at 1350 (angular distribution and polarization measurements)

 preliminary analysis : Data analysis  preliminary analysis : gain-matching (3 channels per keV), Doppler shift correction for v/c=0.8%, energy and efficiency calibrations  for -coincidence analysis and building the level scheme -  matrix – 3 x 109 counts  for angular distribution ratios analysis, angular distribution matrices – 900 vs all; and 1350 vs all RAD=I(135)/I(90) gated on “all” RAD ~ 0.85 for stretched dipole RAD ~ 1.35 for stretched quadrupole or unstretched dipole  to determine electric or magnetic nature with linear polarization analysis linear polarization matrices V vs all; H vs all V,H – Compton scatted -rays inside one clover. V(H) – Compton scattering between two crystals which are perpendicular (parallel) to the beam direction Ap = (NV - NH)/ (NV + NH), =1 Ap > 0 for stretched electric transitions Ap < 0 for stretched magnetic  ’  ’ V H

extended with more than 130 new transitions Level scheme of 194Tl extended with more than 130 new transitions -> h9/2  i13 /2-1 and h9/2  i13 /2-3

Nucleon orbitals near the Fermi surface of 194Tl Expected bands configurations Proton configurations  Z = 81  Hg core + 1 proton quadrupole deformation ~ 0.15 Hg core Configuration  s1/2 h9/2 ,  = 9/2 Proton at the bottom of the h9/2 shell, i.e. particle nature Odd Tl isotopes: -> near ground state – s1/2 -> higher spins - h9/2

Nucleon orbitals near the Fermi surface of 194Tl Expected bands configurations Neutron configurations  N = 113 quadrupole deformation ~ 0.15 Configuration  i13/2, j -> j = p3/2, f5/2 i13/2 ,  = 5/2, but aligns with i=13/2 Neutron near the bottom of the i13/2 shell, i.e. hole nature i13 /2 i13 /23 Odd 193Hg isotone: -> i13 /2, i  6 ћ -> i13 /23, i  16 ћ -> i13 /22j 194Tl configurations, suitable for chiral symmetry: -> h9/2  i13 /2-1 and h9/2  i13 /2-3

 the only pair that is observed across its band crossing region Chiral pair in 194Tl  the only pair that is observed across its band crossing region  excellent near-degeneracy above the band crossings h9/2  i13 /2-3 band head 18- h9/2  i13 /2-3 h9/2  i13 /2-1 h9/2  i13 /2-1 band head 8-

E < 110 keV Emin = 37 keV

Energy near-degeneracy in 194Tl compared with other chiral pairs 128Cs 135Nd 104Rh

Near-degeneracy in the 4-qp pair in 194Tl compared with other chiral pairs The chiral pair with the best near-degeneracy? P.L. Masiteng et al, Phys. Lett. B 719 (2013) 83

The negative parity bands in 194Tl -> h9/2  i13 /2-1 and h9/2  i13 /2-3

Configuration of the third negative parity band  large alignment ~ 16, need i13 /2-3  negative parity, need h9/2  h9/2  i13 /2-3 good alignment additivity h9/2  i13 /2-3 i13 /2-3 h9/2  i13 /2-1 i13 /2-1 h9/2 P.L. Masiteng et al, Eur. Phys. J A 50 (2014) 119

Summarize the experimental data (experiment A):  three negative parity bands  observed below and above their band crossings 2 qp bands -> h9/2  i13 /2-1 configuration 4 qp bands -> h9/2  i13 /2-3 configuration Three 4-qp bands include:  the chiral pair with excellent near-degeneracy and maximum alignment  a third band with lower energy and bit lower alignment This third band is quite interesting  should it be associated with a h9/2  i13 /2-3 configuration with lower alignment, it should be non-yrast! Open questions:  what are the B(M1) and B(E2) of all three bands – to test chiral symmetry  what is the nature of the third band (i) could it be part of another chiral system? (ii) does it correspond to a different, perhaps axially symmetric shape?  theoretical calculations

Experiment B - Doppler Shift Attenuation Method lifetime measurements in 194Tl AFRODITE array 9 HpGe clover detectors, Compton suppressed with BGO shields arranged: 4 detectors at 450 and 4 detectors at 1350 6 LEPS detectors Trigger – 3 coincidences, at least 2  rays in the clovers 181Ta(18O,5n)194Tl at beam energy E(18O)=91 MeV Target had backing, 181Ta foil of 1mg/cm2 onto thick backing of Bi initial recoil velocity of v/c ~ 0.8 % Note the difficulty to measure lifetimes in heavy nuclei due to low v/c and small Doppler broadening Three weekends of beam time

Data analysis Preliminary – gain matching - 2 channels per keV, Asymmetric matrices – 1350 vs all; and 450 vs all background subtracted gated (on “all”) spectra at 450 and 1350 were analysed for Doppler broadening DSAM analysis – using the programs COMPA, GAMMA and SHAPE (analysis headed by Prof. A. Pasternak) Monte-Carlo methods to simulate the entry states in 194Tl and the decay (statistical decay, superdoformed bands, stretched M1 bands, known discrete levels) The lifetimes are extracted step by step starting with the highest-energy level of a band.

28+ Examples for DSAM analysis in 194Tl  I = 28, Band 2 483 keV 931 keV

Examples for DSAM analysis in 194Tl  I = 22, Band 1

Extracted B(M1)s and B(E2)s for the three negative parity bands  red and blue – chiral pair  black – third band The experimental transition probabilities in the 4-qp chiral pair  the same! Thus this pair shows the best known near-degeneracy! Third band – similar transition probabilities, consistent with the same configuration and the same (triaxial) nuclear shape. Multiple chiral systems?

Nuclear shape for the h9/2  i13 /2-n configuration in 194Tl Cranked Nilsson-Strutinsky calculations Deformation with 2 = 0.15,  = -400  -450 rotation predominantly around the intermediate axis  supports chiral symmetry No minimum with axially symmetric shape

Can 194Tl have two chiral systems?  why no fourth band is observed?  what multiple chiral systems look like? Expectation for two chiral systems Experimentally observed chiral systems 103Rh yrast chiral pair yrare chiral pair I. Hamamoto, Phys. Rev. C 88 (2013) 024327 I. Kuti et al., Phys. Rev. Lett. 113 (2014) 032501

Multi-particle Rotor Model of Carlsson and Ragnarsson  to establish the properties of multiple chiral systems  to understand the nature of the three negative parity bands single particles -> Nilsson potential with standard parameters -> h9/2  i13 /2-3 configuration is described as 1 proton in the h9/2 shell and 11 neutrons in the i13/2 shell core -> deformation 2 = 0.15 and  = 400; -> irrotational moment of inertia g-factors -> gR = 0.3; gs = 0.7 gs,free

Multi-particle Rotor Model calculations for the h9/2  i13 /2-3 bands (C,D) yrare chiral pair (A,B) yrast chiral pair

The MPR calculations suggest :  good energy near-degeneracy in the yrare chiral pair  larger energy discrepancy in the yrast chiral pair  the side partner of the yrast chiral pair lies at similar energy as the yrare pair  similar B(M1) and B(E2) in all partners of the chiral multiplet Excellent agreement between the MPR calculations and the experimental data The calculations suggest that the observed negative-parity bands may exhibit multiple chiral systems built on the same configuration

Testing the calculated bands for chiral geometry Projections of the total angular momentum

Testing the calculated bands for chiral geometry Expectation values of the angles between angular momenta of the proton, neutrons and collective rotation

Testing the calculated bands for chiral geometry Projections of the individual angular momenta

Summary  Chiral pair is observed below and above a band crossing in 194Tl.  The 4qp pair shows excellent near-degeneracy.  DSAM lifetime measurements: the near-degeneracy in the measured B(M1)s and B(E2)s is very good. Third band with negative parity – this band perhaps indicates that another chiral system is built on the same h9/2  i13 /2-3 configuration.  The calculations find that the yrare chiral system shows better near-degeneracy, than the yrast chiral system.