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: [email protected]
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.