Progress in half lives of nuclei approaching the r-process path at N=126

José BenlliureUniversidad de Santiago de Compostela, Spain

INPC 2007

Motivation

One of the most relevant questions of physics in the new century is the origin of heavy elements in the Universe(Connecting quarks with the cosmos, USA NRC repport 2003)

The r-process is though to be one on the main actors in the production of heavy elements in stellar nucleosynthesis

Very little is known about the stellar sites where the r-process occurs, but still less about the nuclei involved in this process

Nuclei around the waiting point A=195 are completely unexplored

J. Benlliure INPC 2007

Motivation

Our present understanding of the r-process around A=195 is far from being complete:

- shell quenching at N=126

- nuclear deformation

J. Benlliure INPC 2007

Motivation

Our present understanding of the r-process around A=195 is far from being complete:

- shell quenching at N=126

- nuclear deformation

Our goal:

- investigate the production of heavy neutron-rich nuclei

- measured its -decay half-lives

J. Benlliure INPC 2007

Production of heavy neutron-rich nuclei

- Mainly abrasion of protons- Low excitation energy

Projectile fragmentation in peripheral heavy-ion collisions at relativistic energies:

Experimental technique:

- large fluctuation in the N/Z of the projectile residue - large fluctuations in the excitation energy induced in the collision - good isotopic identification (charge states rejection)

Cold-fragmentation channels:

J. Benlliure INPC 2007

INPC 2007

Production of heavy neutron-rich nuclei

Experiment:208Pb(1 A GeV)+Be (107 ions/s)

FRS at GSI

One of the challenges of

the experiment was the iden-

tification of charge states Beam energy (~ 700 A MeV)

FRS+energy degrader

J. Benlliure

Production of heavy neutron-rich nuclei

Results: 208Pb(1 A GeV)+Be

25 new heavy neutron-rich nuclei “south” of lead have been produced

Their production cross sections allow us to investigate the predictive power of different reaction models

EPAX

COFRA

J. Benlliure INPC 2007

Measurement of half lives

Experimental technique and setup:Ion-beta time-position correlations using an active stopper

Detectors:

ionization chamber (reaction rejection in the stopper)

2 scintillator detectors (veto)

4 double-side Si strip detectors: surface: 5x5 cm2 thickness 1 mm 2x16 3.125 mm strips

Position correlation Setting centerd on 198Irproduced implanted

J. Benlliure INPC 2007

Measurement of half lives

Time correlations under complex background conditions:pulsed beam + activation due to long-lived neighbouring nuclei

Spill length 3 s

Cycle 10 s

Implantation

decay

Time

Forward-time correlated :pausebackground

spillbackground fff

Backward-time correlated :pausebackground

spillbackground ff

J. Benlliure INPC 2007

Forward time

Backward time

Measurement of half lives

Fitting procedure:- fit of the measured ratio of forward/backward correlations with simulated ratios

- Fitting functions are obtained from Monte Carlo simulations including spill sequence, ion

implan- tation rate, -decay lifetime, e- background frequency, during spill and pause, and e- efficiency

-The only free parameters of the fit are the lifetime () and the detection efficiency ()

J. Benlliure INPC 2007

Results

Results for 195Re (analysis during beam pause):

T1/2 = 6+1-1 s

= 9 s

= 30%

J. Benlliure INPC 2007

Results

Results for 198Ir (benchmarcking of the method):

Previously measured value:

T1/2(198Ir) = 8+3-3 s

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

This work:

T1/2(198Ir) = 8+2-2 s

J. Benlliure INPC 2007

Results

Measured half lives:

half lives of 8 new neutron-rich nuclei and an upper limit for other 5 nuclei have been determined

J. Benlliure INPC 2007

Results

Bechmark of model calculations: RPA (Moller et al.) clearly overestimate

the measured half lives close to the

shell

CQRPA (I. Borzov et al.) assuming almost

spherical nuclei but with a realistic

description of first forbidden transitions

describes the data close to the shell

First forbidden transitions seem to play major role in decay close to the neutron shell N=126

Some half lives may also indicate the influence of deformationJ. Benlliure INPC 2007

NucleiExp. Data

Grosstheory[1]

RPA(GT)+f(Gr.Th)[2]

DF3+ CQRPA[3]

203Pt < 11 s 41.1 s 564.0 s 12.7 s

202Ir 11+3-3 s 8.5 s 68.4 s 9.8 s

201Ir < 42 s 18.5 s 130.0 s

200Ir < 30 s 13.3 s 124.1 s

199Ir 6+5-4 s 96.6 s 370.6 s 46.7 s

198Ir 8+2-2 s 377.1 s 19.1 s

200Os 6+4-3 s 16.0 s 187.1 s 6.9 s

199Os 5+4-2 s 17.2 s 106.8 s 6.6 s

196Re 3+1-2 s 5.1 s 3.6 s 1.4 s

195Re 6+1-1 s 10.3 s 3.3 s 8.5 s

194Re 1+0.5-0-5 s 16.1 s 70.8 s 2.1 s

190Ta < 6 s 8.5 s 1.4 s 1.5 s

189Ta < 14 s 34.7 s 96.8 s 10.0 s

[1] T.Tachibana, et al. Proc. ENAM95, Arles, 1995, p.763[2] P. Möller, et al. Phys. Rev. C 67, 055802 (2003)[3] I. N. Borzov Private Communication

We have demonstrated that it is possible to explore heavy neutron-rich nuclei Around 25 new neutron-rich nuclei close to N = 126 have been produced usingcold-fragmentation reactions

It is technically possible to implant those nuclei and performe 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

half lives (or upper limits) of 13 new heavy neutron-rich nuclei near N=126

have been measured.

Recent model calculations (CQRPA, I. Borzov) show an improved prediction power compared to standard RPA (Möller et al) and Gross Theory (Tachibana et al). First-forbidden transitions close to the shell and deformation far from the shell seems to strongly influence decay of heavy neutron-rich nuclei

New measurements including - coincidences are being performed with RISING New RNB facilities like RIKEN or FAIR will offer a tremendous potential to further investigate the r-process path around the waiting point A = 195

Summary and conclusions

INPC 2007

S227-Experiment Collaboration

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

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

F. Rejmund GANIL, Caen, France

B. Blank, J.Giovinazzo, B. Jurado CENBG Bourdeaux-Gradignan, France