Structure of neutron-rich Mg isotopes explored by beta-decay of spin-polarized Na isotopes

Transcription

1 Structure of neutron-rich Mg isotopes explored by beta-decay of spin-polarized Na isotopes K. Tajiri, T. Shimoda, K. Kura, M. Kazato, M. Suga, A. Takashima, T. Masue, T. Hori, T. Suzuki, T. Fukuchi, A. Odahara, Y. Hirayama A, N. Imai A, H. Miyatake A, C.D.P. Levy B, M. Pearson B, K.P. Jackson B Osaka Univ., KEK A,TRIUMF B polarized spin-parity assignments polarization β-γ coincidence β-γ-γ coincidence EFES-NSCL workshop at NSCL, Feb. 4-6, 2010

2 Systematic measurements for Mg isotopes 28 Mg, 29 Mg, 30 Mg, 31 Mg, 32 Mg N = 16, 17, 18, 19, 20 β-decay of 28,29,30,31,32 Na (polarized) spin-parity assignments of the levels in 28,29,30,31,32 Mg structure of 28,29,30,31,32 Mg

3 Table of Contents Principle of Measurement Experiments at TRIUMF Results with 28 Na Beam: 28 Mg Structure Results with 29 Na Beam: 29 Mg Structure Comparison with Shell Model Calculations Summary

4 Principle of Measurement Experiments at TRIUMF Results with 28 Na Beam: 28 Mg Structure Results with 29 Na Beam: 29 Mg Structure Comparison with Shell Model Calculations Summary use of spin-polarized radioactive beam

5 β-decay from a spin-polarized nucleus β-decay angular distribution A=1 P=1 θ decay intensity A: asymmetry parameter of allowed β-decay P: polarization of the parent nucleus initial final ~0 A takes very different values depending on the final state spin.

6 xn 0 xn 0 xn 0 xn 0 polarization ε: detection efficiency spin reversed β-ray detection free from instrumental asymmetry P can be evaluated from AP value for a transition to the known spin state. A spin assignment

7 In the case of cascade feeding Deduced A from β-γ coincidence is affected by the feeding from upper levels. measured from β-γ 1 coincidence known unknown

8 required statistics in AP measurement : β-γ total yields Large polarization is important Experimental AP value fluctuates statistically. expected frequency to measure AP value

9 Monte Carlo simulation Beta-rays were generated according to P=0.05 Y=100 P=0.4 Y=100 P=0.8 Y=100 P=0.05 Y=40000 P=0.4 Y=400

10 where to perform the experiment? TRIUMF ISAC in Canada polarized radioactive beam world-highest polarization

11 Principle of Measurement Experiments at TRIUMF Results with 28 Na Beam: 28 Mg Structure Results with 29 Na Beam: 29 Mg Structure Comparison with Shell Model Calculations Summary

12 Isotope Separator / ACcelerator TRIUMF ISAC radioactive nuclear beams produced in target fragmentation induced by a 500 MeV 100 µa proton beam commissioned in Aug. 2001

13 TRIUMF ISAC Polarized Beam Line neutralizer polarized A Na 0 Kiefl 8 Li: transverse β-nmr condensed matter physics re-ionizer Shimoda A Na: transverse 11 Li: transverse decay spectroscopy unpolarized A Na kev 1.9 m B 10Gauss polarized A Na +1 two laser beams Kiefl 8 Li: longitudinal β-nmr condensed matter physics pumping within 2.6µs beam velocity tuning C.D.P. Levy et al. Nucl. Instr. and Meth. B204 (2003) 689

14 pumping the two ground-state hyperfine levels in order to achieve high polarization atom 1/2 nucleus 3/2 905 MHz D1 673 nm laser freq. ν

15 Achieved polarization Phil 8 Li: 80%, 9 Li: 56%, 11 LI: 55%, 20 Na: 57%, 21 Na: 56%, 26 Na: 55%, 27 Na: 51%, 28 Na: 45%, Corrected for spin-relaxation K. Minamisonno et al., Nucl. Phys. A746(2004)673c 28 Na: 28%, 29 Na: 36% Uncorrected for spin-relaxation Preset work Pumping for 11 Be + beam is in progress.

16 9 HPGe detectors + plastic scintillator telescopes 28,29,30,31,32 Na decay at TRIUMF L: β asymmetry, 60% LEPS polarization Na beam 30.4 kev 30% R: β asymmetry, 60% β-asymmetry: β γ, β γ γ, γ γ 50% 60% Pt stopper 40% B~85mT 60% 45% total efficiency plastic scintillators (1.5 mm) β- and γ-rays Ge β energy threshold: eliminates Al contaminants from trigger β energy : assigns β-decay branch

17 Principle of Measurement Experiments at TRIUMF Results with 28 Na Beam: 28 Mg Structure Results with 29 Na Beam: 29 Mg Structure Comparison with Shell Model Calculations Summary

18 system check with polarized 28 Na beam and search for something new 800 particles per sec

19 New transition? 28 Na 28 Mg B.G kev kev Single escape B.G γ-ray spectrum with all Ge detectors

20 New level in 28 Mg 1473 kev gated by 2907 kev γ-ray new 7461keV 4557 kev 5269 kev 3083 kev 28 Mg 1473 kev g.s kev gated by 5269 kev γ-ray

21 polarization of 28 Na β-ray energy spectrum AP = (5) Select ground-state transition : A = -1.0 P = 0.283(5) uncorrected for spin-relaxation

22 2389 kev γ-ray peaks ( ) coincident with β-rays R-detector Pol.+ L-detector Pol.+ spin assignments of the levels in 28 Mg R-detector Pol.- L-detector Pol.- AP = -0.25±0.01 A = -0.89±0.05 I π = successful assignments

23 spin assignment of new level at MeV 5269 I β? I γ 7.461(1) MeV MeV 2192 kev γ-rays was too weak to determine A is affected by the β-decay Asymmetry to the MeV level 5269 I γ g.s Mg +0.5 or -0.5 or -1.0 γ MeV level 2+

24 Revised Decay Scheme of 28 Na and New Levels in 28 Mg

25 Principle of Measurement Experiments at TRIUMF Results with 28 Na Beam: 28 Mg Structure Results with 29 Na Beam: 29 Mg Structure Comparison with Shell Model Calculations Summary

26 29 Na decay spin-parity assignments of 29 Mg levels 200 particles per sec

27 first observation of β-decay to the 1 st excited state ( MeV ) β ray energy spectrum gate energy loss was estimated by using GEANT4 excluded γ rays coincidence with the β ray 54.6 kev

28 Polarization of 29 Na The ground state of 29 Mg (3/2+) is the only spin-known state. It is not possible to exclude the transition to the 55-keV level by setting a gate on β-ray energy. β ray energy spectrum 1 st + gs

29 Ratio of AP values A A γ 1 γ 2 P P = A A γ 1 γ 2

30 Spins and parities of the MeV & MeV levels Select the β rays in coincidence with each γ rays N R + N L + N R - N L - A γ N + R N + L N - R N - L P = 0.161± A γ P = ± γ γ A P A = γ = 2.19 ± γ A P A

31 In allowed transition γ γ A P A = γ = 2.19 ± 0.59 γ A P A A ratio takes 3 3 patterns 3/2 + 5/2 + Exp. result

32 Polarization of 29 Na A γ 1586 = -0.4 (I f π : 3/2 + ), A γ 2560 = +0.6 (I f π : 1/2+) P P = = A A γ 1586 γ γ A A γ P 2560 P = 0.40 ± 0.11 = 0.35 ± 0.02 P = 0.36 ± 0.11 Polarization of 29 Na 36±11 %

33 Spin-parity of MeV Level 33

34 Spin-parity of MeV Level (3/2, 5/2)+ counts = I A doublet P A P A P I + I I I Energy [ kev ] + I A = ( I + I ) A A doublet I A = 0.27 ± I π = 3/2 + : A = -0.4 Similarly A = 1.03 ± 1.89 I MeV 34

35 Relative intensity of γ-rays from MeV and MeV levels (3/2, 5/2) + 3/ < not observed not observed /2 + 5/ / /2 + 3/ / (5/2 + ) / (1/2 + ) 3/ /2 + 1/ I π E I π E Exp. USD MeV level 5/2+

36 Revised Decay Scheme of 29 Na and Spin-Parity Assignments of 29 Mg Levels I????

37 Finding of the1583 kev γ-ray for the first time Relative intensity is the clue to spin assignment. 5.8 (13) 0.26 (10) /

38 Comparison with Shell Model ( NuShell ) 5.8 (13) 0.26 (10) / / / /2+ 0 3/2+ 1/ /2+ 1/ /2+ 3/ Exp. USD USDA USDB Exp. Weisskopf Estimate USD USDA USDB 1583 kev 5/2+ 1/ kev 5/2+ 3/ MeV and MeV levels are suggested to be 1/2+ and 5/2+, respectively.

39 Revised Decay Scheme of 29 Na and Spin-Parity Assignments of 29 Mg Levels II??

40 5 7/2 + Systematics (Exp.) Z=12 P. Baumann et al., Phys. Rev. C39, (1989) etc /2-3/2-3/2 + 5/2 + 3/2 + 7/2 + 1/2 + 7/2-3/2-5/2 + (3/2,5/2) + (5/2,9/2) + 3/2 + (3/2,5/2) + 1/2 + (3/2,7/2) + (5/2,7/2) + (3/2,5/2) + 3/2 + 1/2 + (1/2to5/2) (+) (1/2to5/2 + ) (1/2to5/2 + ) 2 5/2 + 5/ /2 + 3/2 + 5/2 + 3/2 + (7/2 - ) (3/2 - ) (1/2to7/2 + ) (1/2to7/2 + ) 0 25 Mg 1/2 + 5/ Mg 1/ Mg 3/2 + (7/2 - ) (3/2 - ) 31 Mg (1/2to5/2 + ) (3/2 + ) 1/2 (+) 33 Mg (1/2to7/2) (+) (5/2 + ) (3/2 + )

41 Systematics (Exp.) N=17 3/ P. Baumann et al., Phys. Rev. C39, (1989) etc. (3/2,5/2) + (7/2,9/2) + 3/2 + (1/2to5/2) + 1/2 + (1/2to5/2) + (1/2to5/2) + 3 (3/2,5/2) + 3/2 + 1/2 + 3/2-7/2-5/2 +,(3/2) + 3/2-7/2-5/2 + (3/2,5/2) + (3/2,5/2) + 3/2 + 3/2 + 2 (7/2 - ) 5/2 + 5/2 + (3/2,5/2) + 1 (3/2 - ) 1/2 + 1/2 + 1/ Ne (3/2 + ) 29 Mg 3/ Si 3/ S 3/ Ar 3/2 +

42 1.431 MeV & MeV levels (3) 1794 < 0.1 5/2+ 3/2+ possible transition type kev 1794 kev E1 M2 M2 E /2+ (5/2+) 1 7/2-3/2-7/2-3/2-2 Weisskopf Estimate 2129 kev 1794 kev E1 : 10 7 hindrance M2 : 10 1 hindrance (1/2+) 0 3/2+ 29 Mg Either one reproduces the experimental intensity ratio, if the above hindrance factors are assumed.

43 Revised Decay Scheme of 29 Na and Spin-Parity Assignments of 29 Mg Levels III (3/2-, 7/2-) (3/2-, 7/2-)

44 Principle of Measurement Experiments at TRIUMF Results with 28 Na Beam: 28 Mg Structure Results with 29 Na Beam: 29 Mg Structure Comparison with Shell Model Calculations Summary

45 28 Mg NuShell calculations B.A. Brown et al., Phys. Rev. C74, (2006). 1.4 (2) 4.6 (1) 0.5(1) 5.2 (2) 2 + (0, 1, 2) (1) 5.8 (2) (2) (1) 5.8 (1) <0.1 > (4) 5.2 (1) 0.4 (1) 5.8 (1) (1) 6.0 (1) 4.0 (6) 5.0 (1) 1.9 (4) 5.2 (1) (3) 4.3 (1) (0, 1, 2) (59) 5.5 (4) (7) 4.6 (1) I b log ft 0 + Exp Mg Calc I b log ft

46 Comparison with Shell Model Calculation 29 Mg Not predicted by USD interaction exp USD USDA USDB Code : NuShell B.A. Brown et al., Phys. Rev. C74, (2006).

47 29 Mg (1/2+) log-ft (5/2+) Comparison with USD calculation Log ft 3/2+ 1/2+ 3/2+ (5/2)+ Energy Level [ MeV ] Delta-log ft (calc-exp) (5/2+) 1/2+ 3/2+ (5/2)+

48 Comparison with Shell Model Calculation 2 (Monte Carlo Shell Model by Utsuno et al.) exp MCSM by Y.Utsuno

49 Summary The decay spectroscopy with spin-polarized 28,29 Na was successfully carried out at TRIUMF. The decay schemes of 28,29 Na were revised drastically. In 28 Mg, 13 γ rays and 9 levels were newly found in the β decay of 28 Na. Spins and parities of the 4 levels were newly proposed. In 29 Mg, the 336 kev, 1793 kev, and 1583 kev γ rays were newly found in the β decay of 29 Na. Spins and parities of the 5 levels were newly proposed. The level structures of 28,29 Mg were discussed by comparing with the shell model calculation (NuShell). The level energies and logft values of the levels in 28,29 Mg were reasonably reproduced by the shell model calculation using USD interaction. For the 29 Mg, the MeV and MeV levels, the small β transition suggests negative parity of these levels, being in good agreement with the MCSM calculation.

50 Summary The decay spectroscopy with spin-polarized 28,29 Na was successfully carried out at TRIUMF. The decay schemes of 28,29 Na were revised drastically. In 28 Mg, 13 γ rays and 9 levels were newly found in the β decay of 28 Na. Spins and parities of the 4 levels were newly proposed. In 29 Mg, the 336 kev, 1793 kev, and 1583 kev γ rays were newly found in the β decay of 29 Na. Spins and parities of the 5 levels were newly proposed. The level structures of 28,29 Mg were discussed by comparing with the shell model calculation (NuShell). The level energies and logft values of the levels in 28,29 Mg were reasonably reproduced by the shell model calculation using USD interaction. For the 29 Mg, the MeV and MeV levels, the small β transition suggests negative parity of these levels, being in good agreement with the MCSM calculation.

51 Summary The decay spectroscopy with spin-polarized 28,29 Na was successfully carried out at TRIUMF. The decay schemes of 28,29 Na were revised drastically. In 28 Mg, 13 γ rays and 9 levels were newly found in the β decay of 28 Na. Spins and parities of the 4 levels were newly proposed. In 29 Mg, the 336 kev, 1793 kev, and 1583 kev γ rays were newly found in the β decay of 29 Na. Spins and parities of the 5 levels were newly proposed. The level structures of 28,29 Mg were discussed by comparing with the shell model calculation (NuShell). The level energies and logft values of the levels in 28,29 Mg were reasonably reproduced by the shell model calculation using USD interaction. For the 29 Mg, the MeV and MeV levels, the small β transition suggests negative parity of these levels, being in good agreement with the MCSM calculation.