Low-spin structure of 210 Bi

Transcription

1 Low-spin structure of 21 Bi investigated in cold-neutron capture reaction on 29 Bi Natalia Cieplicka, S. Leoni, B. Fornal INFN, Sezione di Milano 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

2 Physics motivation hy the 21 Bi nucleus is an ideal nucleus for testing the shell-model calculations? A rich ground for comparisons with theory, also states arising from coupling of proton and neutron particles excitations to the 3 first excited state of octupole character in doubly-magic 28 Pb are also expected. The unknown multipolarity of the main transition feeding the ground state introduced rather large uncertainties in measurements of the capture cross section to the ground state in 21 Bi. Z=82 N=126 The measured lifetimes may be compared to predictions of shell-model calculations. 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

3 Physics motivation hy the 21 Bi nucleus is an ideal nucleus for testing the shell-model calculations? A rich ground for comparisons with theory, also states arising from coupling of proton and neutron particles excitations to the 3 first excited state of octupole character in doubly-magic 28 Pb are also expected. Z=82 N=126 The unknown multipolarity of the main transition feeding the ground state introduced rather large uncertainties in measurements of the capture cross section to the ground state in 21 Bi. 28 Pb 21 Bi The measured lifetimes may be compared to predictions of shell-model calculations. 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

4 Physics motivation hy the 21 Bi nucleus is an ideal nucleus for testing the shell-model calculations? A rich ground for comparisons with theory, also states arising from coupling of proton and neutron particles excitations to the 3 first excited state of octupole character in doubly-magic 28 Pb are also expected. The unknown multipolarity of the main transition feeding the ground state introduced rather large uncertainties in measurements of the capture cross section to the ground state in 21 Bi. 28 Pb 21 Bi 4-, Z= T 1/ N=126 The measured lifetimes may be compared to predictions of shell-model calculations. 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

5 Experiment ILL Grenoble (PF1B line) 8 EXOGAM clovers 6 GASP detectors 2 ILL clovers n 29 Bi γ 21 Bi γ γ Cold neutron flux of /(ns cm 2 ) from the ILL reactor with energy < 5 mev 29 Bi solid target (3g) Population of capture state in 21 Bi at binding energy of 4.6 MeV 16 Ge detectors of EXILL array: 8 of EXOGAM, 6 of GASP, and 2 from ILL collaboration coincidence measurements of gamma rays 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

6 Experiment ILL Grenoble (PF1B line) 8 EXOGAM clovers 6 GASP detectors 2 ILL clovers 16 Ge detectors of EXILL array: 8 of EXOGAM, 6 of GASP, and 2 from ILL collaboration coincidence measurements of gamma rays 8 detectors of EXOGAM arranged into ring around the target at every 45 so angular correlation measurements could be performed Measurements of the lifetimes with FATIMA detectors 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

7 Experimental results: level scheme n 29 Bi γ 21 Bi γ γ γ 9/2 (4, 5 ) 4.6 MeV 29 Bi Capture state at neutron binding energy 1 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

8 gate Bi 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22,

9 gate Bi 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22,

10 Experimental results: level scheme 64 primary transitions (4 new) Population of neutron capture state at 465.2(1) kev excited states (31 new) 21 Bi 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

11 Angular correlations of g rays from 21 Bi The angular correlation function for a pair of coincident g rays connecting the nuclear states with spins J i J J f is usually expressed as: g (Θ) = 1 + A 2 P 2 (cos Θ) + A 4 P 4 (cos Θ) Θ the angle between the direction of emission of two g rays g P n (cos Θ) Legendre polynomials A n =q n A(1)A(2) the coefficients which depend on the attenuation factor q n as well as on the multipolarities of 1 and 2 g rays and the spins of involved nuclear states q 2 =.86(2) q 4 =.6(3) 674 kev degree 45 degree 9 degree Normalization: number of pairs of the detectors, efficiency (Θ) norm =.495(5) (4 combinations) norm45 = 2.2(12) (16 combinations) norm9 = 1 (8 combinations) 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

12 Spin-parity number of the capture state 4 (4, 5 ) J= J= (E1) A 2 =-.5(2), A 4 =.4(3) A 2 =.1(2), A 4 =.4(4) 3633 (E2) 4257 (E2) J= experiment 1337(E1) A 2 =-.8(3), A 4 =.4(5) theory (M1) (M1) 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, Bi

13 Multipolarity of the main transition leading to the ground state Not confirmed experimentally M1 multipolarity of 32-keV line A. Borella et al., Nucl. Phys. A 85, 1 (211) Thermal capture cross section for the 29 Bi(n,g) 21 Bi reaction to the ground state [mb] 1% M1 5% M1 +5 % E2 1% E2 25(.9) 19.3(.8) 17.2(.7) 4-, A. Borella et al., AIP Conf. Proc. 769, 648 (25) (M1) T1/2 = 3.4 x 1 6 y 1-5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

14 Multipolarity of the 32-keV line A 2 =.4(3), A 4 =.3(6).9 A 2 =.7(3), A 4 =-.1(6).9 A 2 =.5(3), A 4 =-.2(7).8 Pure E1-M1 A 2 =.6, A 4 =..8 Pure E1-M1 A 2 =.7, A 4 =..8 Pure E1-M1 A 2 =.7, A 4 = (6+) (5+) (4+) A 2 =.3(1), A 4 =-.1(2) Pure E1-M1 A 2 =.5, A 4 = (E1) (E1) 1175 (E1) (M1) (3+) 674 (E1) th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

15 A 2 =.1(5), A 4 =-.1(11).8.7 A 2 =.1(2), A 4 =.1(4) δ 32 =.4 δ 674 =.1 δ 113 =-.15(1) δ 113 =-.15(4) (4) 27 χ 2 = A 2_exp A 2 δ 1,δ 2 A 2_exp 2 + A 4_exp A 4 δ 1,δ 2 A 4_exp th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

16 δ 113 = , A 2 =.1(5), A 4 =-.1(11) A 2 =.1(2), A 4 =.1(4) δ 674 =.1(3) δ 32 =.4(8) A 2 =-.13(2), A 4 =-.2(5) A 2 =-.14(2), A 4 =-.3(3) δ 255 =.55 δ 674 =.15(29) δ 32 =.36(2) 4+ (5) (4) A 2 =.7(2), A 4 =.3(4) A 2 =.7(2), A 4 =-.1(5) δ 381 =-.37 δ 674 =.9(56) δ 32 =.5(5) th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

17 Multipolarity of 32-keV transition δ 674 =.1(3) δ 32 =.4(3) δ 2 = I L I L I(L) = 1 δ I(L ) = δ2 δ kev: 99.99% E1 +.1% M2 32 kev: 99.84% M1 +.16% E2 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

18 Decays to: 3+, 4-, 3- Spin-parity values: state at 1524 kev A.9 2 =.2(1), A 4 =-.2(2) A 2 =.1(1), A 4 =-.3(3) δ 53 =.6(2) A 2 =.4(3), A 4 =.3(6).8 δ 53 =.9(3) 4 + (4+) (E1) M1(+E2) E1(+M2) M1(+E2) δ 1175 =.3(5) 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

19 Decays to: (6-), 8-, 8-, 6-, 7-, 9- State at 1981 kev 4-, E A 2 =.11(3), A 4 =.4(5) Pure E2-E2 A 2 =.1, A 4 =.1 (7-) A 2 =-.19(3), A 4 =.(7) δ 143 =-.11(5) A 2 =-.1(3), A 4 =.2(7) δ 1398 =.5(5) E2 M1(+E2) 143 M1+E th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

20 Decays to: (6+), (5+), (4+), (4-), (6-), 6-, 4- State at 21 kev 4-, J= J= E1(+M2) (M1+E2) A 2 =-.14(2), A 4 =-.3(3) δ 255 =.4(8) (δ 255 =-.82(12)).8.7 A 2 =.24(3), A 4 =-.1(6) δ 393 =-.3(3) (5+) (5) M1(+E2) (E1+M2) J= J= E1(+M2) (M1+E2) A 2 =-.13(2), A 4 =-.2(5) δ 255 =.7(12) (δ 255 =-.88(19)) A 2 =-.11(2), A 4 =.3(4) δ 1596 =.1(3) M1(+E2) E1(+M2) 32 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

21 Decays to: 5+, 4+, 3-, 4-, 3+, 4-, 5-, 3-, 2- State at 27 kev A 2 =.1(5), A 4 =-.1(11) δ 113 =-.11(9) δ 113 =-.1(4) A 2 =-.6(5), A 4 =-.8(1) δ 1659 =.23(14).8.7 A 2 =.1(2), A 4 =.1(4) (4+) (4) E1+M2 (M1+E2) M1+E2 (E1+M2) M1(+E2) 993 E1(+M2) 32 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

22 Fed from: BE Decays to: 1-, 3-, 1- State at 1197 kev 4-, (M1) (E2) A 2 =-.5(4), A 4 =-.2(8).9 A 2 =.1(3), A 4 =.3(6).8.8 (δ 49 = ) δ 563 =.25(3) δ 563 =.1(8) (2) J= Theory J=1-(M1+E2) δ 563 =.18(16) A 2 =-.7(6), A 4 =-.5(11) A 2 =-.4 A 4 = M1+E

23 Fed from: BE, (6-), (6-), (6-) Decays to: 8-, 8-, 6- State at 1527 kev 4-, A 2 =.7(5), A 4 =.7(11).9.8 A 2 =.7(5), A 4 =.7(11).9.8 A 2 =.7(5), A 4 =.7(11) Theory J=2 (6 8) E2 J=2 (8 9) E2 A 2 =-.9 A 4 =. Theory J=2 (7 8) E2 J=2 (8 9) E2 A 2 =.1 A 4 =.14 Theory J=2 (6 8) E2 J=1 (8 9) M1 A 2 =-.7 A 4 =. (7-) 611 (M1) A 2 =.7(5), A 4 =.7(11) Theory J=1 (7 8) M1 J=1 (8 9) M1 A 2 =.5 A 4 = (M1)

24 Spin distribution from 29 Bi(n,g) 21 Bi reaction (experimental results: shell-model states) 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

25 Comparison with shell-model calculations Firmly known states used to fit TBME of p-n interaction E. K. arburton, B. A. Brown, Phys. Rev. C 43, 62 (1991) Experimental results (EXILL) Shell-model calculations

26 Comparison with shell-model calculations (3) 194 Firmly known states used to fit TBME of p-n interaction E. K. arburton, B. A. Brown, Phys. Rev. C 43, 62 (1991) (7-) 1527 Three states below 2 MeV without theoretical counterparts (1-,2) 1197 Experimental results (EXILL) Shell-model calculations Observed in other experiments

27 Comparison with shell-model calculations (2,3,4) (3,4) (4,5,6) (3,4,5) 287 (4,5,6) 273 (3,4,5) Pb 2.6 MeV (4,5) (πh 9/2 νg 9/2 ) , 3+, , 7+, 8+, (9+, 1+, 11+, 12+) (5) 4(+) Experimental results (EXILL) Shell-model calculations Observed in other experiments

28 Comparison with shell-model calculations Higher part of the level scheme: states arising from the valence particles excitations. Experimental results (EXILL) Shell-model calculations Observed in other experiments

29 Decay Feeder Measurements of the lifetimes of excited states in 21 Bi 32 kev LaBr 674 kev Clover LaBr Clover kev kev 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

30 Decay Feeder Measurements of the lifetimes of excited states in 21 Bi 32 kev LaBr 674 kev Clover 32 kev LaBr Clover LaBr Clover kev 32 LaBr gate 32 kev 1-5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

31 Decay Feeder Measurements of the lifetimes of excited states in 21 Bi 32 kev LaBr 674 kev Clover 32 kev LaBr Clover LaBr Clover kev 32 LaBr gate 32 kev 1-5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

32 Decay Feeder Measurements of the lifetimes of excited states in 21 Bi 1 2 C C compton C PRD ΔC = 22 ps ΔC compton = 16.8 ps PRD (E feeder E decay ) = ps π = τ = 2(9) ps τ lit = 7.5(14) ps Phys Rev. C 12, 1547(1975) kev kev 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215

33 Measurements of the lifetimes of excited states in 21 Bi 2, 32 kev τ = 2(9) ps 3+, 993 kev τ < 4 ps 5(+) kev kev , 1524 kev τ = 31(16) ps S. Lodetti, MS Thesis, Milano (215) 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, kev kev

34 Summary The level structure of 21 Bi investigated in cold neutron capture on 29 Bi was compared to shell model calculations some of the states must come from the core excitations. The analysis of angular correlations allowed to confirm almost pure M1 character of the main transition leading to the ground state. The results of present analysis of 21 Bi structure will serve as an excellent testing ground for the future calculataions. 5th orkshop on Nuclear Level Density and Gamma Strength, Oslo, May 18-22, 215