1. 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

2. 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) (R)
 more data on the lighter Tl isotopes to search for better chiral candidates

3. 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)

4.  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

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

6. 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

7. 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 / and h9/2  i13 /2-3

8.  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-

9. E < 110 keV
Emin = 37 keV

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

11. 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

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

13. 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

14. 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

15. 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

16. 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.

17. 28+
Examples for DSAM analysis in 194Tl  I = 28, Band 2
483 keV
931 keV

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

19. 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?

20. 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

21. 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)
I. Kuti et al., Phys. Rev. Lett. 113 (2014)

22. 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

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

24. 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

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

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

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

28. 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.