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February 12-15,2003 PROCON 2003, Legnaro-Padova, Italy Jean Charles THOMAS University of Leuven / IKS, Belgium University of Bordeaux I / CENBG, France Spectroscopic studies of neutron deficient light nuclei decay properties of 21 Mg, 25 Si and 26 P nuclei
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Decay properties of neutron deficient light nuclei Selection rules: Fermi: T= J=0 ; f = i Gamow-Teller: T=0±1; J=0±1 ; f = i + (C.E.) emission Reduced transition probability: Global properties Short half-lives ( ms) High Q values Low S p values -delayed charged particle emission
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21 Mg, 25 Si and 26 P nuclei 14 13 12 stable nuclei emitters emitters p emitters p emitters 15 16 T z = -3/2T z = -2 26 P 21 Mg 25 Si 22 Al 20 Na 27 S 20 Mg 22 Si 23 Na 26 Mg 27 Al 30 Si 31 P 32 S 11
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target ion source degrader ( 9 Be) magnetic dipoles velocity filter Detection set-up Production of neutron deficient nuclei at GANIL (fragmentation) Production target Accelerator 36 Ar 18+ @ 95 MeV/u Fragment separation
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Detection set-up detection Identification time of flight: E1D6, E2 energy loss: E1D6, E1, E2, E3 Spectroscopic study -(2)p spectrum: E3 coincidence: E4 spectrum: germanium detector Acquisition trigger implantation: E1D6 radioactivity: E2, E3, E4 coincidence -(2)p radioactivity Implantation BEAM
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Identification and counting rate : 26 P example Identification matrix ( E,T.o.F) Counting rates Implantation: 65 ions/s ( 26 P) 300 ions/s ( 21 Mg, 25 Si) Contamination: 10 % (for 26 P) < 1 % (for 21 Mg, 25 Si)
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Analysis of -delayed proton spectra coincidence in E4 Counts Energy (keV) energy deposit in E3 Counts coincidence with E4 E 3 (keV)
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Identification of transitions 26 P -decay scheme Need for a good detection efficiency Use of in-beam experimental results spectrum in 26 P decay Counts Energy (keV)
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21 Mg decay scheme Experiment Theory
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25 Si decay scheme Experiment Theory
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26 P decay scheme ExperimentTheory
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- : n → p + e - + np ft - + : p → n + e + + E.C. : p + e - → n + np ft + Mirror asymmetry principle Charge independence hypothesis of nuclear interactions: symmetry of analog transitions Isospin symmetry breaking: asymmetry in mirror -decays
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Systematics of experimental values (A 40) Average asymmetry : 11 (1) % in the 1p shell (A<17) 0 (1) % in the (2s,1d) shell (17<A<40) = 4.8 (4) % Allowed Gamow-Teller transitions (log(ft)<6) 17 couples of nuclei 46 mirror transitions
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Mirror asymmetry in the decay of A=21 & A=25 nuclei Experiment Theoretical calculations N. A. Smirnova & C. Volpe INC + HOIC + WSINC + WS +1.8 -2.7 -0.6 10 ± 70 20 ± 30 0 ± 40 0 ± 20 30 ± 40 -2.7 -1.4 -4.1 +1.1 -2.1 -1.1 -8.5 -2.7-11.1 -7.1 -3.3-10.1 (%)
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Spectroscopic studies of neutron deficient nuclei Suitability of the fragmentation production method associated with the spectroscopic study of neutron deficient light nuclei good agreement between experimental results and shell model calculations (nuclear structure and decay strength) good selectivity and production rates access to decay properties of exotic nuclei ( 21 Mg, 25 Si, 26 P, 22 Al, 27 S) Perspectives complementarity with in-beam studies Rare decay modes: study of the 2p radioactivity Fundamental symmetries: study of the mirror asymmetry phenomenon evaluation of the Coulomb correction in super-allowed Fermi decays Need for high precision experiments
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L. Achouri, LPC Caen - France J. Äystö, P. Dendoveen, J. Honkanen, J. Jokinen, University of Jyväskylä - Finland R. Béraud, A. Ensallem, IPN Lyon - France A. Laird, University of Edinburgh – United Kingdom M. Lewitowicz, F. de Oliveira-Santos, M. Stanoiu, GANIL Caen – France B. Blank, G. Canchel, S. Czajkowski, J. Giovinazzo, CENBG Bordeaux - France C. Longour, IReS Strasbourg – France Collaboration
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21 Mg, 25 Si and 26 P nuclei (MeV) Z N 26 P 25 Si 21 Mg
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Half-life of 26 P Time correlation procedure implantation radioactivity Correlation intervals T rad (ms) Counts E 4 > 0 Energy (keV) counts
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Analysing procedure N impl : number of implanted ions E ff , E ff p : detection efficiencies detection: radioactive sources p detection: simulations, nuclei implantation depth C and C p : corrections on N and N p detection: acquisition triggering p detection: fitting procedure + coincidence condition N and N p : number of counts in spectra detector calibration ( sources, known - and -p transitions) B.R. Measurement of absolute transition intensities: Transition assignment (energy):
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Identification of -(2)p transitions (2)p transition identification via total decay energy in coincidence with (2)p emitted from the I.A.S. Counts Energy (keV) Counts Energy (keV)
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Mirror asymmetry sources Origin and consequences of the isospin non-conservation in nuclear interactions Nature: Coulomb effects second class currents in weak interaction calculation of -decay transition probabilities Effects: nucleon-nucleon interaction description shell structure of nuclear states beyond the V-A model of decay theory
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Coulomb effects Mirror asymmetry in allowed Gamow-Teller transitions: with isospin configuration mixing radial overlap of nucleon wave functions: “binding energy effects”
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Binding energy effects The last proton of the + emitting nucleus is less bound than the last neutron of the - emitting nucleus : S p + < S n - radial overlap mismatch of wave functions in the + decay: + 0
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Systematic approach of binding energy effects How to see the binding energy effects on the radial overlap of the nucleon wave functions? increasing of with R - /R + where: R - = S n - - S p - + E - * R + = S p + - S n + + E + * i.e. R - /R + - / + decreasing of as J i is increasing i.e. as the centrifugal barrier is increasing Behaviour of with the total angular momentum of the emitting nucleus Behaviour of with the binding energy difference of the initial and final nucleons
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