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, Spain2 Gesellschaft für Schwerionenforschung mbH (GSI), D-64291 Darmstadt, Germany

3 Institut de Physique Nucléaire, F-91406 Orsay Cedex, France4 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

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

ExperimentSIS18/FRS at Darmstadt, Germany

Centered on 198Ir Produced Implanted

active stopper

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

thickness 1 mm 2x16 3.125 mm strips

2 scintillator detectors (veto)

Monoenergetic degrader

4

Implantation-decay correlations

Time

Implantationlike

Spill length ~ 2 s

Cycle 10 s

5

Numerical analysis method

Forward time

Backward time

conventional analysis tools, based on analytical time-distribution functions could not be applied

A new analysis procedure has been developed to extract the β-decay half-lives by using a numerical function

Background evaluation (uncorrelated events):

backward-time correlations

6

The time sequence of fragment implantation and β 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.

Monte-Carlo simulation

7

Fitting procedure: example

spill+pause only pause

8

Fitting procedure: example

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

spill+pause

9

Results for 195Re :

T1/2 = s

spill+pause only pause

1

16T1/2 = s 2

26

Fitting procedure: example

10

Applicability of the method

NF be the total number of implanted fragments

the beta detection efficiency

the background rate of beta-like signals

Nº. of 'true' beta decays detected during a time T1/2 after the implantation

2FN

The N. of beta-like background detected in the same time is

21TNF

From the experience we gained during the analysis we found that we can expect to obtain reliable results with our analysis method if

2142

TNN

FF

64

2

21FNT

11

Results

Nuclide T1/2 / s(experiment)

Gross Theory1 FRDM + RPA + Gross Theory2

202Ir125 11±3 8.5 68.4199Ir122 6 (+3 -5) 96.6 370.6198Ir121 8±2 377.1

200Os124 6 (+4 -3) 16 187.1199Os123 5 (+4 -2) 17.2 106.8196Re121 3 (+1 -2) 5.1 3.6195Re120 6±1 10.3 3.3194Re119 1±0.5 16.1 70.81 T.Tachibana, et al. Proc. ENAM95, Arles, 1995, p.763

2 P. Möller, et al. Phys. Rev. C 67, 055802 (2003) (incl. deformation and 1. forbidden)

12

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.)