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Proposal P245

Proposal P245. Conversion Electron Study to Identify the Spherical 0 2 + State in 32 Mg via its E0 Decay.

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Proposal P245

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  1. Proposal P245 Conversion Electron Study to Identify the Spherical 02+ State in 32Mg via its E0 Decay. P.G. Thirolf1, V. Bildstein1, J. Cederkäll2,4, T. Faestermann1, L.M. Fraile5, R. Gernhäuser1, D. Habs1, P. Hoff6, U. Köster3, T. Kröll1, R. Krücken1, C. Lang1, H. Mach7, R. Lutter1, T. Morgan1, W. Schwerdtfeger1, M. Sewtz1, O. Tengblad5, K. Wimmer1 Munich1 – CERN2 – Grenoble3 - Lund4 – Madrid5 – Oslo6 – Uppsala7 - Collaboration Spokesperson: Peter G. Thirolf Contactperson: Alexander Herlert ISOLDE/ INTC-Meeting, May 19, 2008

  2. Motivation: ‘Island of Inversion’ shape coexistence in Mg:  search for deformed 02+ in 30Mg: IS414  search for spherical 02+ in 32Mg : P245 0+ levels in Mg isotopes: ISOLDE/ INTC-Meeting, May 19, 2008

  3. + + + = + - 2 0 a 0 1 a 0 g sph def + + + = - - + 2 0 1 a 0 a 0 exc sph def Nuclear structure information from E0 transitions determine the monopole strength: overlap between the wave functions from the deformed and spherical potential minima: depends on the mixing amplitude a : (simple 2-level mixing model) ISOLDE/ INTC-Meeting, May 19, 2008

  4. Experimental technique: E0 spectroscopy with Mini-Orange Spectrometer transmission function: 1/f = 1/g + 1/b Mini-Orange: magnetic filter and transport system cooled Si(Li) target • 8 permanent magnet wedges • B = 160 mT (toroidal) • central Pb absorber efficiency: ~ 6.5 % @ 1000 keV ISOLDE/ INTC-Meeting, May 19, 2008

  5. 0.2 mm scintillator 75 mm Kapton window catcher foil b beam e- Mini-Orange Si(Li) detector Experimental Setup b coincidence of E0 with b decay electrons: 32Na 32Mg • Ge detector for beam composition and • normalization • large Qb value (20.02 MeV): suppress Compton-scattered e- by Teflon-coating of target chamber  very sensitive technique for weak E0 transitions ISOLDE/ INTC-Meeting, May 19, 2008

  6. E (22+ 02+)def: 678 keV IS414: First Identification of 02+ in 30Mg • confirmation of 02+ assignment from fast-timing studies: (H. Mach et al.) • E0 measurement: r2(E0) = 5.7(12) * 10-3 W. Schwerdtfeger et al., to be submitted to PRL ISOLDE/ INTC-Meeting, May 19, 2008

  7. Mixing Amplitude in 30Mg correlation of mixing amplitude and E0 strength: b1 : 0 (spherical g.s.) 0.16 (BMFA, Egido et al.) 0.39 (from B(E2)=241 e2fm4) b2 : 0.59 (BMFA, Egido et al.; identical value from estimated B(E2) via Grodzins systematics) b1=0 b1=0.16 b1=0.39 a2 = 0.0065(35)  a = 0.08(2)  weak mixing between spherical and deformed potential minima ISOLDE/ INTC-Meeting, May 19, 2008

  8. medium spin: (>2+, ≠ 2-, 0+, 2+) strong b feeding negative parity (3-,2-) no 0+ ground state band Search for 02+ in 32Mg • disadvantage compared to 30Mg: • - no clear spectroscopic candidate for 02+ • - reduced beam intensity for 32Na • preliminary level scheme: 32Na  32Mg* (H. Mach et al.): - candidates for spherical 22+ intruder: 2550 keV, 2857 keV, 3121 keV ISOLDE/ INTC-Meeting, May 19, 2008

  9. spherical 21+ 01+ 22+ 02+ deformed 22+ 02+ 21+ 01+ Systematics of 2+ 0+ transitions in Mg • scenario: - underlying assumption: smooth variation of transitions with same deformation - 30Mg: E(22+ 02+) = 678 keV fits well in systematics • - 32Mg: - estimate E(22+ 02+) by extrapolation • from 28,30Mg: ~1480keV • - 2550 keV state = spherical 22+ intruder state  in this scenario: low-lying 02+ state in 32Mg: (2550-1480) keV ~ 1070 keV ISOLDE/ INTC-Meeting, May 19, 2008

  10. Proposed Search for 02+ (32Mg) - success of proposed search clearly depends on energy of 02+ state: a) high-lying 02+ ( > ~1.2 MeV): - competition of E2 transition 02+ 21+ too strong for E0 observation (given the limited intensity of 32Na beam) - lower limit on E(02+) could be extracted b) low-lying 02+ ( < ~ 1.2 MeV): - comparison with 30Mg (E(02+)= 1789 keV, E(21+)= 1482 keV): (i) reduced E2 competition (E5 scaling): ~ (DE(32Mg)/DE(30Mg))5 e.g.: for 32Mg (E(02+)= 1070 keV, E(21+)= 1480 keV): (185 keV/307 keV)5 ~ 12.5 (ii) improved E0 transmission efficiency of MO: ~ *2 (iii) negligible background from internal pair creation theoretical predictions: 1.4 MeV (Caurier et al., 2001) – 3 MeV (Otsuka et al, 2004) ISOLDE/ INTC-Meeting, May 19, 2008

  11. E0 measurement in 32Mg • for low-lying 02+: no competition by internal pair creation • lower E2 competition  longer lifetime for 02+ state: reduction of prompt background via delayed b-E0 coincidence gating  improvement of signal-to-background ratio we propose to explore the scenario of a low-lying 02+ state in 32Mg ISOLDE/ INTC-Meeting, May 19, 2008

  12. Rate Estimate and Beamtime Request • rate estimate: • based on 100/s 32Na beam: • - reduction of beam intensity (vs. 30Mg): 1/40 • - efficiency gain: * 2 • - reduced E2 competition: * 10 • - no competition by internal pair production • - scaling from 30Mg: 1.4 E0/hour expected in peak ≈ 1MeV •  100 counts in peak during 11 shifts • beamtime request for E0 study in 30Mg: 11 shifts for 32Na beam 1 shift for stable beam tuning 12 shifts totally ISOLDE/ INTC-Meeting, May 19, 2008

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