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