Evidence for pygmy and giant dipole resonances in 130Sn and 132Sn.
The dipole strength distribution above the one-neutron separation energy was measured in the unstable 130Sn and the double-magic 132Sn isotopes. The results were deduced from Coulomb dissociation of secondary Sn beams with energies around 500 MeV/nucleon, produced by in-flight fission of a primary 238U beam. In addition to the giant dipole resonance, a resonancelike structure ("pygmy resonance") is observed at a lower excitation energy around 10 MeV exhausting a few percent of the isovector E1 energy-weighted sum rule. The results are discussed in the context of a predicted new dipole mode of excess neutrons oscillating out of phase with the core nucleons.
Adrich
P
P
Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany.
Klimkiewicz
A
A
Fallot
M
M
Boretzky
K
K
Aumann
T
T
Cortina-Gil
D
D
Pramanik
U Datta
UD
Elze
Th W
TW
Emling
H
H
Geissel
H
H
Hellström
M
M
Jones
K L
KL
Kratz
J V
JV
Kulessa
R
R
Leifels
Y
Y
Nociforo
C
C
Palit
R
R
Simon
H
H
Surówka
G
G
Sümmerer
K
K
Waluś
W
W