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

Outline

Overall Physics aim,

Are nuclei (with Z=40-50) Rotators or Vibrators ?

* New high-spin data on 101,102Ru from WNSL

* 100Mo at high-spins using CHICO

* Signature of vibrator to rotor structural evolution.

Nuclear Rotations and Vibrations

• What are the signatures (in even-even nuclei) ?– (extreme) theoretical limits

2 (4 ) 4(5) 20

( 1), 3.332 (2 ) 2(

(4 ) 2 = 2.00

3)

( 1

6

2 )N

J

EE N

EE J J

E

E

Alignment (rotational picture at least) driven by Coriolis interaction on high-j, low- orbitals (ie. ones with large jx on collective rotation axis.

Vcor = -jx.

eg.

h11/2 [550]1/2 ‘intruder’

FS for N~57, 2~0.15->0.2

jx

Alignments and rotational motion in ‘vibrational’ 106Cd (Z=48, N=58),

PHR et al. Nucl. Phys. A586 (1995) p351

Can subtract off a reference (core) aligned angular momentum to see effect of quasi-particle alignments as a function of frequency.

2( 1)X

x X ref

ref X ref

I I I K

i I I

I I

ix=5h

CSM ref. Bengtsson Frauendorf and May, At. Data. Nuc. Data. Tab. 35 (1986) p15

Experimental Details

96Zr (9Be,4n)101Ru, pace~900mb

Enriched (85%) 670g/cm2 96Zr foil on 5mg/cm2 natPb support.

Ebeam=44 MeV, lmax~25 h

YRASTBALL array at WNSL

6 clover germaniums @ 90o

5 co-axial detectors @ 50o + 126o

3 co-axial detectors @ 160o

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 100Ru and neighbours....butmid-shell (Z=40-50) natureis consistent with largest (but still small) collectivity in theregion.

Q.Are these nuclei deformed or vibrational ?RuMoZr Pd Cd Sn

(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 101Ru.

Decay scheme for 101Ru Rotational bands observed from 11/2-, 5/2+ and 7/2+ ‘bandheads’.

Backbending observed in positiveparity bands (1 and 2), but not innegative parity band (band 3).

Pauli blocking argumentssuggest aligning particles are therefore of h11/2 neutron nature.

h11/2 band (3) does however show some alignment at higher frequency...what causes this?

Yamamoto et al. Phys. Rev. C in press

TRS calculations for 101Ru by Furong Xu (Bejing) for differentparity (and signature) configs.

2

=0.2MeV

=0.4MeV

=0.3MeV

=0.6MeV

Quasi-particle alignments and kinematic moments of inertia

ix=10 h11/2

band

h11/2

band

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

Q. Are backbends necessarily due to rotational alignment ?

A. NO ! Can be vibrational – rotational structure change!!

Structural change from vibrator to rotator appears to be a feature of this region.

Rotation stabilized by population of aligned pair of h11/2 neutrons.

New type of crossing, Vibrator to Rotor !!!

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

lab (measured in CHICO)

Tim

e of

Flig

ht

60o

beam-likefragments target-like

fragments

elastics

(a) Fold > 10

(a) All

grazing angle

Tim

e of

Flig

ht

angle in CHICO

Fold > 10

All tim

e of

flig

ht

many thanks to......

• Arata Yamamoto (Surrey/Yale student).

• 101-102Ru Expt. Con Beausang (+ Yalies)

• 100Mo Rochester, Manchester, 88”, Surrey

• Vibrator-Rotator plots, long discussions with Con B. Rick Casten, Victor Zamfir and Jing-Ye Zhang

Summary and Future Look• 101,102Ru (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.