Production and beta decay lifetimes of heavy neutron-rich nuclei approaching the r-process path

Teresa Kurtukian-Nieto

Universidad de Santiago de Compostela (Spain)

Hirschegg 2006 Astrophysics and Nuclear Structure January 15 to 21, 2006

Robustness of the closed neutron shell N=126 with the neutron excess Evolution of collective structure and shapes

MotivationMotivation

Understanding the astrophysical r-process close to the waiting point

Reproduce the abundance pattern

The solar system r-process abundances as determined by Käppeler et al. (1989)

MotivationMotivation

J. Benlliure et al.,Nucl. Phys. A 660 (1999) 87

Production of heavy neutron-rich nuclei incold-fragmentation reactions (S227):

r-process nucleosynthesis

208Pb(1000 A MeV)+Be :

production cross sections of neutron-rich nuclei around N=126

measurements of half lives Knowing half-lives is important:

Half-lives determine the r-process time scale at the r-process “ladders” (N=50, 82, 126)where abundances peak

But also the others are important because affect abundances between peaks.

Experimental method

194W 2h 41 min

186Lu 10 h 45 min

A/Z

1. Identification and implantation of heavyneutron-rich nuclei (FRS-GSI):

30 new heavy neutron-rich nuclei

Production cross sections

EPAX

COFRA J. Benlliure et al.,Nucl. Phys. A 660 (1999) 87

Implantation technique

4 double-side strip detectors: surface: 5x5 cm2

thickness 1 mm 2x16 3.125 mm strips

Detectors:

ionization chamber (reaction rejection in the stopper)

2 scintillator detectors (veto)

Centered on 198Ir Produced Implanted

Experimental setup for β’s half-lives

Position correlation

Implantation-decay correlations with large background

Forward-time correlated :pausebackground

spillbackground fff

Backward-time correlated :pausebackground

spillbackground ff

Spill length 3 s

Cycle 10 s

Time

Implantation

Background evaluation: backward-time correlations

Forward time

Backward time

Half-live determination:Fit to an analytical function: exponential decay + const/Poisson-distributed background

Chi2 of the ratios in forward/backward time of the measured data with respect to a numerical functionMonte Carlo simulation of time correlation distributions with the experimental conditions found at the FRS and two free parameters: lifetime and efficiency

Forward/backward time ratios for 195Re

Previous experimental data:

T1/2(198Ir) = 8 ± 3 s

A. Szalay et al., Radiochem. Radioanal. Letters 14/2/136 (1973)

Our result:

= 12.5 s (T1/2 = 8.6 ± 1 s )

2D Chi2 map: versus efficiency for 198Ir77

Good agreement between our measurement and the previous one

NucleiExp. T1/ 2

(s)

Gr. Th.

T1/ 2 (s)

QRPA(GT)

Gr.Th.(ff )

T1/ 2 (s)203Pt 10.1±3.0 41.1 564.0198I r 8.6±1.0 377.1200Os 3.7±1.3 16 187.1199Os 15.2±3.

0

17.2 106.8194Re 7.0 ±1.0 16.10 70.8195Re 5.5 ±1.0 10.3 3.3196Re 4.2 ±1.0 5.1 3.6189Ta 3.8 ±1.0 34.7 96.8190Ta 2.9 ±1.0 8.5 1.4

In general theoretical predictions do not reproduce the measured half lives

But Gross Theory by Tachibana et al, seems to works better than QRPA(GT) + Gr. Th. (ff)

It would be nice to compare with other theoretical models like I. N. Borzov

ββ-decay time -decay time correlationscorrelations

QRPA (GT) + Gross Th. (ff) : P. Möller, B. Pfeiffer and K.-L. Kratz. Phys. Rev. C 67, 055802 (2003)

Gross Theory :T.Tachibana, M.Yamada, Proc. Int. Conf on exotic nuclei and atomic masses, Arles, 1995, p.763

We have demonstrated that is possible to explore the region of the heavy neutron-rich

nuclei

Cold-Fragmentation constitutes a suitable reaction mechanism to produce heavy

neutron-rich nuclei. We have a tremendous potential in future at GSI-FAIR

Around 30 new neutron-rich nuclei close to N = 126 have been identified

It is technically possible to implant those nuclei and to make position and time correlations

to determine β half-lives

In spite of the pulsed beam structure we have introduced a technique to evaluate

the β background.

Preliminary half-lives of some new heavy neutron-rich nuclei near N=126 have been

measured.

In general the existing theoretical models in this region (Möller et al and Tachibana et

al ) do not reproduce the experimental data.

New measurements coupling with ’s (RISING) will be performed next months

SummarySummary

S227-Experiment Collaborators

J. Benlliure , E. Casarejos, D. Cortina-Gil ,M.Fernandez-Ordoñez, J. Pereira Universidad de Santiago de Compostela, Spain

F.Becker , D. Henzlova, B. Jurado ,K.-H. Schmidt ,O. Yordanov Gesellschaft für Schwerionenforschung mbH, Darmstadt, Germany

L. Audouin F. Rejmund Institut de Physique Nucleaire, Orsay, France

B. Blank, J.GiovinazzoCENBG Bourdeaux-Gradignan, France