-DECAY HALF LIVES OF NUCLEI APPROACHING -DECAY HALF LIVES OF - PowerPoint PPT Presentation

β -DECAY HALF LIVES OF NUCLEI APPROACHING β -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 ➢ 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 ➢ Nuclei around the r -process waiting point A=195 are completely unexplored [1] E. Haseltine, Discover Magazine,23:2, 2002 1 A. I. Morales, EURORIB'10

MOTIVATION ➢ Half lives modify the abundance curve ➢ The predictions of theoretical calculations in the N~126 region are rather discordant [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 ➢ 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 2 A. I. Morales, EURORIB'10

MOTIVATION ➢ Half lives modify the abundance curve ➢ The predictions of theoretical calculations in the N~126 region are rather discordant [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 ➢ 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 2 A. I. Morales, EURORIB'10

PRODUCTION OF HEAVY NEUTRON RICH NUCLEI ➢ 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 ➢ 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: - Mainly abrasion of protons - Low excitation energy 3 A. I. Morales, EURORIB'10

EXPERIMENTAL TECHNIQUE ➢ Active Stopper: ion- β time-position correlations on an event by event basis ➢ RISING γ -ray spectrometer: β - γ prompt-time correlations 4 A. I. Morales, EURORIB'10

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 0 10 20 30 Background sources ➢ δ -electrons beam pause ➢ β -decay electrons from other nuclides γ -labelled β -decays { Induced by γ -ray background ➢ “false” β -decays false pixel ID(x,y) due to multiplicity in the DSSSD's ➢ “false” implantations Multiple Implantation Events (MIE) Multiple Implantation events 1/fimplant << τ Multiple Implantation Events (MIE) Measured t β Neighbour Pixels t (s) True t β 5 A. I. Morales, EURORIB'10

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 202 Ir 204 Pt ƒ β + ƒ background ➢ Forward-time ion- β correlations: ƒ background ➢ Backward-time ion- β correlations: 6 A. I. Morales, EURORIB'10

ANALYSIS TECHNIQUE: NUMERICAL FIT ➢ χ 2 fits to two independent time distributions: • Experimental ion- β time-correlated spectra • Calculated time distribution obtained from Monte-Carlo simulations  Monte Carlo simulation: ➢  Input Parameters: Free Parameter:  • Spill sequence • Lifetime τ • Implantation rate • γ -labelled β -decay rate (pause) • γ -probability • ε Ideal case: f I → 0 T pause → ∞ 7 A. I. Morales, EURORIB'10

ANALYSIS TECHNIQUE: NUMERICAL FIT ➢ 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 χ 2 from the measured and simulated ratios ➢ We perform a least squares fit to the projection of χ 2 into the lifetime coordinate Limits of the method ➢ N true In order to obtain a plausible value of the half live, F = = 3   N back  it is essential to define a figure of merit F ➢ For a given value of F, there is an upper limit of the half live T 1/2 measurable!! T 1/2 ( 202 Ir) = 15 ± 3 s , F = 30 ➢ Analytical fit of 202 Ir: benchmark of the method 8 A. I. Morales, EURORIB'10

EXAMPLES OF THE FITTING PROCEDURE 202 Ir T 1/2 = 15 ± 3 s 204 Pt +6 T 1/2 = 16 s -5 9 A. I. Morales, EURORIB'10

RESULTS ➢ Previously reported half lives:  T 1/2 ( 204 Au) [1] = 39.8  0.9 s T 1/2 ( 202 Ir) [2] = 11  3 s  [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 10 A. I. Morales, EURORIB'10

COMPARISON WITH THEORETICAL PREDICTIONS ➢ The T 1/2 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 201 Ir 11 A. I. Morales, EURORIB'10

SUMMARY AND CONCLUSIONS ➢ 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 T 1/2 of 202 Ir 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 of 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 12 A. I. Morales, EURORIB'10

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