Collective Excitations in Exotic Nuclei

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1 Collective Excitations in Exotic Nuclei David Radford (ORNL) RIA Summer School, August 2002 I Nuclear Excitations: Single particle motion vs. Collective motion Collective Modes: Rotations and Vibrations II Experimental Techniques and Examples of Results Gamma-Ray Spectroscopy - High angular momentum Band crossing and mixing Band termination Wobbling mode in triaxial SDBs - Towards the limits of stability Giant Resonances (high-frequency vibrations) - Photoabsorption - Hot, rotating nuclei - Pygmy resonance - Nuclear resonance fluorescence Intermediate-Energy Coulomb Excitation - Fragmentation beams, light (sd shell) nuclei III Towards RIA: Experiments with n-rich beams at the HRIBF

2 Beam Nucleus Target nucleus Fusion In-Beam Gamma Spectroscopy: Fusion-Evaporation Reactions - Large cross section; ~1 barn - Ideal for populating states with very high angular momentum (as high as ~70 h) - Large gamma-ray multiplicity need high-granularity high-efficiency high-resolution gamma detector array - No Coulomb barrier for neutrons tends towards proton-rich nuclei, hard to make neutron-rich sec sec sec 10-9 sec Fast Fission Compound Formation hω ~0.75 MeV ~2x10 20 Hz Rotation I x n p I x Groundstate n γ n

3 Gammasphere 110 Compton-suppressed Ge detectors, each with 70% efficiency. Total efficiency = 9% at 1.3 MeV.

4 Euroball

5 Improving Peak-to-Background - gated spectra Total projection gates (8) One gate gates (28) 500 Two gates gates (56) Counts Three gates gates (70) 60 Four gates gates (56) Five gates 50 6-gates (28)

6 Fold Sensitivity limit as a function of fold 10-2 Gammasphere: different reactions v/c=2.5%; M =25; SE =60 kev min = (Sensitivity) v/c=1.5%; M =15; SE =80 kev v/c=0; M =15; SE =80 kev

7 How close are we now to complete spectroscopy? Selected examples of most-complete level schemes - From RadWare level-scheme data base - Sorted by number of gammas Nuclide Bands Levels Gammas /level 174 Hf Er Dy Tm Hf I Lu Ta W Lu Hf

8 174 Hf D.J. Hartley et al. Private Communication

9 163 Er G.B. Hagemann et al. Nucl. Phys. A618 (1997) 199

10 156 Dy F.G. Kondev et al. J. Phys. G25 (1999) 897

11 150 Gd Suderdef Bands S. Erturk et al. Private Communication

12 Number of N-fold coincidences Tm Number of coincidences calculated from the present level scheme for different -ray fold N = 7 N = 6 N = 5 N = 4 N = 3 N = 2 N = 1 1.4G 237M 31M 3.0M 223k 11k Intensity Threshold

13 Number of N-fold coincidences Number of coincidences calculated from the present level scheme for different -ray fold 2s 162 Tm 119 I 179 W 25min Calculation Time (s) Fold (N)

14 G.B. Hagemann et al., Nucl. Phys. A618 (1997) Er

15 163 Er G.B. Hagemann et al., Nucl. Phys. A618 (1997) 199 Circles show band crossings where E2 cross-band transitions are observed. These are used to accurately determine the inter-band interaction strength.

16 Smooth Band Termination At medium spin, the nucleus is a prolate collective rotor. As spin is increased, it changes smoothly to an oblate non-collective shape. The nucleus changes the mechanism by which it generates angular momentum, from collective rotation perpendicular to the symmetry axis, to singleparticle alignment. The rotational band terminates at an angular momentum that exhausts the total aligned spin of the valence particles.

17 Wobbling Mode of Triaxial Superdeformed Nuclei S.W. Ødegård et al., Phys. Rev. Lett. 86 (2001) 5866 Cranking: I+1 E2 I I-1 Wobbling: -Requires Triaxialty!

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19 Example of Recoil-Decay Tagging: 109 I C.-H. Yu et al., Phys. Rev. C59 (1999) R1836

20 Gamma Spectroscopy of 254 No P. Reiter et al., Phys. Rev. Lett. 84 (2000) 3542 Observe discrete transitions to spin 20 Measured entry distribution in (E X,I) Provides new data on formation mechanism and fission barrier.

21 The Giant Dipole Resonance protons neutrons The GDR can be considered as an oscillation of the protons against the neutrons. This results in a large oscillating electric dipole moment. Since the protons and neutrons are moving differently, the isospin T = 1; this is the Isovector GDR. E Γ 14 MeV ~ 4.2 MeV ~ 3.5 Strongly damped.

22 S. Kamerdzhiev and J. Speth, Nucl. Phys. A599 (1996) 373c (E1) Pb (M1) 0 (M1) 1 (E2) 0 (E2) 1 Giant resonance photoabsorption cross section in 208 Pb, decomposed into mulitpoles. Subscripts show isospin.

23 Giant Resonances Bohr and Mottelson, vol. 2, p. 475 Γ = 4.2 MeV E Res ~ 14 MeV

24 In the simple geometric picture, the M1 scissors mode is the magnetic analog of the Giant Dipole Resonance. p n p n M1 - Scissors E1 - GDR

25 Two-Phonon GDR in 136 Xe R. Schmidt et al., Phys. Rev. Lett. 70 (1993) 1767

26 Giant Dipole Resonance in Hot, Rotating Nuclei Populated in fusion-evaporation Detect GDR γ rays above tail of high-energy statistical γ rays E X Compound Nucleus γ x + α i GDR n i n n γ Spin

27 M.P. Kelley et al., Nucl. Phys. A649 (1999) 123c 100 Mo + 18 O ; E = MeV Gammas detected in 3 large NaI detectors Colder Hotter Γ 1 Γ 2

28 Y. Alhassid, Nucl. Phys. A649 (1999) 107c

29 The Pygmy Resonance:

30 D. Vretenar, Conference on Frontiers of Nuclear Structure, Berkeley CA, 2002 RRPA isovector dipole strength distributions in oxygen isotopes. 16 O 22 O 0.4 R[e 2 fm 2 ] 0.2 R[e 2 fm 2 ] O 28 O E[MeV] E[MeV]

31 D. Vretenar, Conference on Frontiers of Nuclear Structure, Berkeley CA, 2002 RRPA isovector dipole strength distributions in Sn isotopes. R[e 2 fm 2 ] Sn 114 Sn Sn 132 Sn R[e 2 fm 2 ] E[MeV] E[MeV]

32 X(γ,γ ) - Nuclear Resonance Fluorescence Ideal for studying low-lying dipole collectivity Gamma sources: - Bremsstrahlung - HIGS facility at Duke University free electron laser HIGS: Eγ tunable 5-8 MeV (or more?) FWHM Eγ/Eγ ~ 2-4% ~ 10 7 γ/s

33 Looking at the Target 5.0(2) MeV γ ray beam collimator 11B Target Pb beam monitor Ge Count Rate (a.u.) me x 3 40K First NRF at HIGS 5/15/01 Th Gamma Energy (MeV) 5.02 MeV 11B 3/2 1/2 3/2 Counts per 2 kev h beam 11B 3/2 1/2 DEP 3/2 gs SEP Gamma Energy (MeV) N.Pietralla et al., Nucl.Instrum.Methods A483, 556 (2002).

34 Parity Measurements with a Polarized Photon Beam 1 π Azimuthal Intensity distribution E1 Target M1 polarization plane 0+ θ = 90o Σ = I( ) - I( ) I( ) + I( ) = +1 for 1 + { TUNL/HIGS polarimeter: Ge φ Ge Ge Ge θ = 90 o

35 Proof of Principle 8.1(3) MeV γ ray beam collimator 32S Target Pb beam monitor Ge 8.12 MeV in plane SEP known 1+ M1 Counts per 2 kev out of plane DEP τ=0.23fs 32S( γ, γ ) E in = 8.1(3) MeV 5 h beam on target 3.7 g/cm Energy (kev) 32S Yale/TUNL polarimeter ~ 80% 0+ Asymmetry N.Pietralla et al., Nucl.Instrum.Methods A483, 556 (2002).

36 Polarimetry at HIgS M1 E1 N.Pietralla et al., Phys.Rev.Lett.88, (2002).

37 Intermediate-Energy Coulomb Excitation Ideally suited for use with fragmentation beams E Beam 30 MeV/u Large cross sections ~ 100 mb Can use thick targets ~ 100 mg/cm 2 Target γ Radioactive Beam Gamma Detector Virtual Photon Scattering

38 Coulomb excitation in the π(sd) shell ν(sd) ν(f 7/2 ) ν(pf) N=20 N=28 Z=20 Ca40 Ca42 Ca44 Ca46 Ca48 Ca58 Ar32 Ar36 Ar38 Ar40 Ar42 Ar44 Ar46 Ar54 S32 S34 S36 S38 S40 S42 S44 S50 Si26 Si28 Si30 Si32 Si34 Si36 Si38 Si44 Mg24 Mg26 Mg28 Mg30 Mg32 Mg34 Mg40 O16 Ne20 Ne22 Ne24 Ne26 Ne28 O18 O20 O22 O24 Z=8 Ne30 Ne32 Neutron drip line Experiment / Möller-Nix 1988 stable E(2+) and B(E2) known E(2+) and B(E2) measured at MSU nucleus known nucleus unknown References at

39 Intermediate energy Coulomb excitation - Ideally suited for beam-fragmentation products S b min bmin = a 0 cot ( θ max /2) / γ a 0 = Z S Z Au e2 m 0 c 2 β 2 Au θ max Zero degree detector E beam 40 MeV/nucl. β 0.3, γ 1.05 b min 20 fm touching spheres 1.2(A 1/3 S +A 1/3 Au )= 11 fm σ ~ 100 mb target ~ 100 mg/cm 2 K. Alder et al., Rev. Mod. Phys. 28, 432 (1956). A. Winther and K. Alder, Nucl. Phys. A 319, 518 (1979). C.A. Bertulani and G. Baur, Phys. Rep. 163, 300 (1988). T. Glasmacher, Ann. Rev. Nucl. Part. Sci. 48 (1998), 1.

40 Energy spectra in target and projectile frames for 40 S+ 197 Au Counts / (10 kev) /2+ 3/ Au γ 40 S Energy (kev) H. Scheit et al. Phys. Rev. Lett. 77 (1996) 3967 γ kev 0 kev 891 kev 0 kev β = 0 β = 0.27

41 Deformation parameters β 2 and excitation energies E(2 + ): 30 S 44 S β Mass A Experiment Shell model Relativistic Mean Field Hartree-Fock E(2 + ) (MeV) Sulfur Neutron number N N=28 shell gap weakened

42 NaI(Tl): Former APEX NaI trigger barrel and Ge detectors at the NSCL APEX trigger detector 24 position-sensitive NaI(Tl) crystals 20% photo-peak efficiency 15% energy resolution Ge: fold segmented Ge detectors ~0.1% - 6% photo-peak efficiency <3 kev energy resolution

43 32-fold segmented germanium detector at Michigan State University All 18 Detectors received Manufactured by Eurisys Mesures N-type germanium crystal 8 cm long, 7 cm diameter (75%) 32 segments, 1 central contact, all fully instrumented with analog electronics Warm FETs

44 Conclusions A whirlwind tour of a (very) few selected topics in the study of collective excitations Great diversity! - There is much to be done - An exciting challenge: extending these types of studies to RIA