New technique to determine beta half-lives in complex background conditions

1
New technique to determine beta half-lives in
complex background conditions

• T. Kurtukian-Nieto1, J. Benlliure1, K.-H.
  Schmidt2, L. Audouin3, F. Becker2, B.
  Blank4, E. Casarejos1, M. Fernandez-Ordoñez1, J.
  Giovinazzo4, D. Henzlova2, B. Jurado4, J.
  Pereira1, F. Rejmund5, and O. Yordanov2
• 1 Universidad de Santiago de Compostela, E-15782
  Santiago de Compostela, Spain
• 2 Gesellschaft für Schwerionenforschung mbH
  (GSI), D-64291 Darmstadt, Germany
• 3 Institut de Physique Nucléaire, F-91406 Orsay
  Cedex, France
• 4 CENBG, F-33175 Gradignan, France
• 5 GANIL, Bd Henri Becquerel, BP 55027, 14076 Caen
  Cedex 5, France

2
Layout

• Introduction
• Experiment
• Numerical analysis method
• Results
• Conclusion

3
Experiment
208Pb(1 A GeV)Be (107 ions/s)
4
Implantation-decay correlations
Spill length 2 s Cycle 10 s
5
Numerical analysis method

• conventional analysis tools, based on analytical
  time-distribution functions could not be applied
• A new analysis procedure has been developed to
  extract the ß-decay half-lives by using a
  numerical function

Background evaluation (uncorrelated events)
backward-time correlations
6
Monte-Carlo simulation

• The time sequence of fragment implantation and ß
  detection are simulated according to the
  experimental conditions observed in the FRS
• spill sequence
• fragment implantation rate
• background rate during spill and pause
• leaving two free parameters the lifetimes and
  the efficiency
• The code produces time-correlation spectra in
  forward- and backward-time direction.

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Fitting procedure example
spillpause
only pause
8
Fitting procedure example
spillpause

• The ratios typical behaviour
• Starts with a value larger than one.
• It decreases and crosses the value of one at
  about 2?.
• At still larger times, the ratio decreases
  further due to the 'shielding' of background
  correlations in forward time direction
• We learn that it is necessary to make
• time correlations covering several spills in
  order to distinguish between different lifetimes.

only pause
9
Fitting procedure example

• Results for 195Re

only pause
spillpause
T1/2 s
T1/2 s
10
Applicability of the method
NF be the total number of implanted fragments
nb the background rate of beta-like signals
e the beta detection efficiency
Nº. of 'true' beta decays detected during a time
T1/2 after the implantation
The N. of beta-like background detected in the
same time is
From the experience we gained during the analysis
we found that we can expect to obtain reliable
results with our analysis method if
11
Results
Nuclide T1/2 / s(experiment) Gross Theory1 FRDM RPA Gross Theory2
202Ir125 113 8.5 68.4
199Ir122 6 (3 -5) 96.6 370.6
198Ir121 82 377.1
200Os124 6 (4 -3) 16 187.1
199Os123 5 (4 -2) 17.2 106.8
196Re121 3 (1 -2) 5.1 3.6
195Re120 61 10.3 3.3
194Re119 10.5 16.1 70.8
1 T.Tachibana, et al. Proc. ENAM95, Arles, 1995,
p.7632 P. Möller, et al. Phys. Rev. C 67, 055802
(2003) (incl. deformation and 1. forbidden)
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Conclusions

• We have developed a new numerical method for
  extracting the half-life information from
  delayed-coincidence experiments in the case of
  complex background conditions.
• The method is also applicable for any other kind
  of complex time-dependent background.
• With this method, we have provided a tool which
  considerably extends the possibilities for
  exploiting the manifold advantages of the
  in-flight technique in determining half-lives of
  exotic nuclei.
• Beta half lives have been measured for 8 new
  neutron-rich isotopes.
• Many measured values deviate strongly from the
  Gross Theory and from the FRDM RPA Gross
  Theory. (T1/2 values are smaller than expected.
  r-process is faster.)