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β β-DECAY HALF LIVES OF NUCLEI APPROACHING

• DECAY HALF LIVES OF NUCLEI APPROACHING

THE R-PROCESS PATH NEAR THE THE R-PROCESS PATH NEAR THE N=126 SHELL CLOSURE N=126 SHELL CLOSURE

Ana Isabel Morales López Universidade de Santiago de Compostela For the RISING colaboration

• A. I. Morales, EURORIB'10

MOTIVATION

One of the most relevant questions of physics in the new century is the origin of heavy elements in the Universe [1]. The r-process is thought to be one on the main factors in the production of heavy elements in stellar nucleosynthesis Nuclei around the r-process waiting point A=195 are completely unexplored

• A. I. Morales, EURORIB'10

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The β-decay half lives of nuclei in the N~126 region represent a key issue to understand the r-process near the A=195 waiting-point, because they determine: The matter flow through the N=126 bottleneck, which fixes the abundance pattern The velocity of synthesis of the heavier r-process nuclei

[1] E. Haseltine, Discover Magazine,23:2, 2002

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MOTIVATION Our goals:

• Investigate the production of heavy neutron-rich nuclei
• Measure their structural properties by means of:
• β -delayed gamma ray spectroscopy
• Isomeric spectroscopy
• Measure their β-decay

half-lives

• A. I. Morales, EURORIB'10

➢ Half lives modify the abundance curve ➢

The predictions of theoretical calculations in the N~126 region are rather discordant

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[1] P. Möller, B. Pfeiffer and K.-L. Kraft, Phys. Rev. C, 67:0055802, 2003 [2] I.N. Borzov and S. Goriely, PEPAN 34, 2003

MOTIVATION Our goals:

• Investigate the production of heavy neutron-rich nuclei
• Measure their structural properties by means of:
• β -delayed gamma ray spectroscopy
• Isomeric spectroscopy
• Measure their β-decay

half-lives

• A. I. Morales, EURORIB'10

➢ Half lives modify the abundance curve ➢

The predictions of theoretical calculations in the N~126 region are rather discordant

➢

2

[1] P. Möller, B. Pfeiffer and K.-L. Kraft, Phys. Rev. C, 67:0055802, 2003 [2] I.N. Borzov and S. Goriely, PEPAN 34, 2003

PRODUCTION OF HEAVY NEUTRON RICH NUCLEI

• Mainly abrasion of protons
• Low excitation energy

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

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

Cold-fragmentation channels:

• A. I. Morales, EURORIB'10

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Charge states in heavy neutron rich nuclei represent a challenge. To overcome this issue, a pulsed beam structure is provided by the SIS-18 synchrotron

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EXPERIMENTAL TECHNIQUE

Active Stopper: ion-β time-position correlations

• n an event by event basis

RISING γ-ray spectrometer: β-γ prompt-time correlations

• A. I. Morales, EURORIB'10

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MEASUREMENT OF HALF LIVES: CHALLENGES

Modulated background in the time-correlated spectra due to:

• Pulsed time structure of the primary beam
• High implantation rates (Several nuclei

implanted at the same time)

• Long β-decay half lives

Background sources

δ-electrons beam pause β-decay electrons from other nuclides γ-labelled β-decays “false” β-decays Induced by γ-ray background false pixel ID(x,y) due to multiplicity in the DSSSD's Multiple Implantation events

1/fimplant << τ

Multiple Implantation Events (MIE) Measured tβ Neighbour

Pixels

t (s) True tβ

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“false” implantations

{

Multiple Implantation Events (MIE)

• A. I. Morales, EURORIB'10

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10 20 30

ANALYSIS TECHNIQUE: NUMERICAL FIT

Due to the modulated time structure of these particular data, performing a data analysis based on exponential fits to the time-correlated spectra becomes a difficult matter

Determination of the background

Forward-time ion-β correlations:

ƒβ + ƒbackground

Backward-time ion-β correlations:

ƒbackground ➢ ➢ ➢

• A. I. Morales, EURORIB'10

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202Ir 204Pt

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ANALYSIS TECHNIQUE: NUMERICAL FIT

• A. I. Morales, EURORIB'10

Monte Carlo simulation:

Input Parameters: Free Parameter:

• Spill sequence • Lifetime τ
• Implantation rate
• γ-labelled β-decay rate (pause)
• γ-probability
• ε

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χ2 fits to two independent time distributions:

• Experimental ion-β time-correlated spectra
• Calculated time distribution obtained from Monte-Carlo simulations

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Ideal case: fI → 0 Tpause → ∞

ANALYSIS TECHNIQUE: NUMERICAL FIT

• A. I. Morales, EURORIB'10

We use as fitting function the ratio between the forward- and backward- time distributions (less sensitive to statistical fluctuations) We perform sets of simulations, each with a different value of the free parameter τ, and calculate the χ2 from the measured and simulated ratios We perform a least squares fit to the projection of χ2 into the lifetime coordinate

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In order to obtain a plausible value of the half live, it is essential to define a figure of merit F

F = N true

Nback

= 3

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Limits of the method

For a given value of F, there is an upper limit of the half live T1/2 measurable!!

Analytical fit of 202Ir: benchmark of the method

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T1/2 (202Ir) = 15 ± 3 s , F = 30

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EXAMPLES OF THE FITTING PROCEDURE

• A. I. Morales, EURORIB'10

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202Ir 204Pt

T1/2 = 15 ± 3 s T1/2 = 16 s

+6

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RESULTS

• A. I. Morales, EURORIB'10

Previously reported half lives: T1/2 (204Au) [1] = 39.8  0.9 s T1/2 (202Ir) [2] = 11  3 s

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[1] D.A. Craig et al.,

• J. Phys. G: Nucl. Phys,

10:1133, 1984 [2] T. Kurtukián-Nieto et al.,

• Nucl. Instr. and Meth. A,

589:472, 2008

COMPARISON WITH THEORETICAL PREDICTIONS

• A. I. Morales, EURORIB'10

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The T1/2 values calculated with the FRDM + QRPA approach overestimate the measured half lives by one order of magnitude The calculations of the new DF3 + QRPA model, which includes GT + FF trasitions on an equal microscopic footing, agree with the measured half lives within a factor of two, except for 201Ir

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SUMMARY AND CONCLUSIONS

• A. I. Morales, EURORIB'10

The β-decay half lives of six nuclei approaching the r-process path near the waiting-point A~195 have been measured with an analysis very efficient numerical method The novel technique deals with complex background structures in the time-correlated spectra The T1/2 of 202Ir has been measured with both analytical and numerical techniques, representing the benchmark of the new method The succesful description of our measurements with the new DF3 + QRPA model may indicate the importance of FF transitions in the β-decay process

• f heavy neutron-rich nuclei

If the tendency of the aforementioned predictions is kept towards A~195, the r-process matter flow through the N=126 bottleneck may be faster than expected by the current r-process standard calculations, thus increasing the velocity of synthesis of the heaviest nuclei Further expectatives to investigate the r-process path near the waiting-point A~195 rely on the new generation of in-flight secondary beam facilities, like RIKEN or FAIR, where higher beam intensities will be available

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S312-Experiment Collaboration

A.I. Morales, J. Benlliure , E. Casarejos, M. E. Estévez, S. Verma Universidad de Santiago de Compostela, Spain

• P. H. Regan, Zs. Podolyak, W. Gelletly, P.M.Walker, N. Alkhomashi, N. Al-Dahan, I. J.Cullen, G.

Farrely, A. B. Garnsworthy, S. B. Pietri, S. J. Steer, E. B. Suckling, P. D. Stevenson, Department of Physics, University of Surrey, Guildford

• J. Gerl, K.-H. Smidth, M. Górska, H. J. Wollersheim, P. Boutachkov, S. Tashenov, I. Kojouharov, H.

Shaffner, R. Kumar, N. Kurz Gesellschaft für Schwerionenforschung mbH, Darmstadt, Germany B.Rubio, A. Algora, A. Agramunt, F. Molina Instituto de Física Corpuscular, Universidad de Valencia, Spain

• J. Grebosz

The Henrik Niewodniczanski Institute of Nuclear Physics, Krakow, Poland

• G. Benzoni

INFN, Universitá degli studi di Milano, Milano, Italy

• D. Mücher

IKP, Universität zu Köln, Köln, Germany

• A. M. Bruce, A. Denis, S. Lalkovski

School of Environment and Technology, University of Brighton, Brighton, UK

• Y. Fujita, A. Tamii

Research Center for Nuclear Physics, Osaka University, Osaka, Japan

• R. Hoischen

Department of Physics, Lund University, Lund, Sweden

• Z. Liu, P. J. Woods

Department of Physics and Astronomy, University of Edimburgh, Edimburg, UK

• C. Mihai

National Institute for Physics and Nuclear Engineering, Bucharest, Romania

• J. J. Valiente-Dobón

INFN-Laboratori Nazionali di Legnaro, Italy