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The Highs and Lows of the A~100 Region Paddy Regan Dept. of Physics, University of Surrey, UK and WNSL, Yale University, New Haven, CT

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Presentation on theme: "The Highs and Lows of the A~100 Region Paddy Regan Dept. of Physics, University of Surrey, UK and WNSL, Yale University, New Haven, CT"— Presentation transcript:

1 The Highs and Lows of the A~100 Region Paddy Regan Dept. of Physics, University of Surrey, UK and WNSL, Yale University, New Haven, CT e-mail: p.regan@surrey.ac.uk

2 Outline Overall Physics aim, Are nuclei (with Z=40-50) Rotators or Vibrators ? * New high-spin data on 101,102 Ru from WNSL * 100 Mo at high-spins using CHICO * Signature of vibrator to rotor structural evolution.

3 Nuclear Rotations and Vibrations What are the signatures (in even-even nuclei) ? –(extreme) theoretical limits

4 Alignment (rotational picture at least) driven by Coriolis interaction on high-j, low-  orbitals (ie. ones with large j x on collective rotation axis. V cor = -j x.  eg. h 11/2 [550]1/2 ‘intruder’ FS for N~57,  2 ~0.15->0.2 jxjx

5 Alignments and rotational motion in ‘vibrational’ 106 Cd (Z=48, N=58), PHR et al. Nucl. Phys. A586 (1995) p351

6 Can subtract off a reference (core) aligned angular momentum to see effect of quasi-particle alignments as a function of frequency.  i x =5h CSM ref. Bengtsson Frauendorf and May, At. Data. Nuc. Data. Tab. 35 (1986) p15

7 Experimental Details 96 Zr ( 9 Be,4n) 101 Ru,  pace ~900mb Enriched (85%) 670  g/cm 2 96 Zr foil on 5mg/cm 2 nat Pb support. E beam =44 MeV, l max ~25 h YRASTBALL array at WNSL 6 clover germaniums @ 90 o 5 co-axial detectors @ 50 o + 126 o 3 co-axial detectors @ 160 o

8 Ru (Z=44) in the centre of the ‘deformed’ region for N=56-58 Anharmonic vibrator for the ground state ‘band’ is the usual explanation for 100 Ru and neighbours....but mid-shell (Z=40-50) nature is consistent with largest (but still small) collectivity in the region. Q.Are these nuclei deformed or vibrational ? RuMoZrPdCdSn

9 (a) gamma-gamma(b) triple coincidences Detailed spectroscopy allowed by investigating gamma-decay sequences from high-spin states. YRASTBALL allows triple coincidences to be routinely observed. Band-like structures are clearly observed in 101 Ru.

10 Decay scheme for 101 Ru Rotational bands observed from 11/2 -, 5/2 + and 7/2 + ‘bandheads’. Backbending observed in positive parity bands (1 and 2), but not in negative parity band (band 3). Pauli blocking arguments suggest aligning particles are therefore of h 11/2 neutron nature. h 11/2 band (3) does however show some alignment at higher frequency...what causes this? Yamamoto et al. Phys. Rev. C in press

11 TRS calculations for 101 Ru by Furong Xu (Bejing) for different parity (and signature) configs. 22   =0.2MeV  =0.4MeV  =0.3MeV  =0.6MeV

12 Quasi-particle alignments and kinematic moments of inertia  i x =10 h 11/2 band h 11/2 band

13 If we parameterize with (E  / J) vs. J Can see if rotor or vibrator by inspection

14

15 Q. Are backbends necessarily due to rotational alignment ? A. NO ! Can be vibrational – rotational structure change!!

16

17 Structural change from vibrator to rotator appears to be a feature of this region. Rotation stabilized by population of aligned pair of h 11/2 neutrons. New type of crossing, Vibrator to Rotor !!!

18 Is apparent backbending in region actually due to crossing between vibrational and rotational-like mechanisms for generating spin ?

19   lab (measured in CHICO) Time of Flight 60 o beam-like fragments target-like fragments elastics (a) Fold > 10 (a) All grazing angle Time of Flight

20 angle in CHICO Fold > 10 All time of flight

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23 many thanks to...... Arata Yamamoto (Surrey/Yale student). 101-102 Ru Expt. Con Beausang (+ Yalies) 100 Mo Rochester, Manchester, 88”, Surrey Vibrator-Rotator plots, long discussions with Con B. Rick Casten, Victor Zamfir and Jing- Ye Zhang

24 Summary and Future Look 101,102 Ru (and neighbours) look like  -soft, anharmonic vib. nuclei at low-spins (eg. E(4 + )/E(2 + )~2.3)..... BUT also have apparent rotational-like behaviour eg. band-crossing, alignments etc. Paradoxically, Coriolis (rotational) effects are largest in nuclei which have SMALL deformations (ie. require large energies/frequencies to rotate). A=100 may be the best tests of Coriolis induced effects in nuclei. Vibrational – Rotational ‘phase’ change around spin 10? Smooth evolution with crossing of anharmonic vibrational states and rotation-aligned configurations. Plot of E  /J verses J gives model independent crossing.

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