A Stolk

PROTON PARTICLE-HOLE STATES AND COLLECTIVE EXCITATIONS IN PB-196

Abstract A collective band built on the Jπ = 0+ intruder state in 196Pb has been identified by measuring coincidences between γ-rays following the 198Hg(α,6n)196Pb reactions and conversion electrons corresponding to the E002+ → 01+ transition. The band-structure is disturbed by a Jπ = 8+ isomer (T 1 2 = 50 (15) ns ) . This newly found isomer and the Jπ = 11− isomer observed previously in this nucleus are interpreted as proton 2p-2h states too. Evidence for the proton 2p-2h nature of the Jπ = 11− isomer is obtained from the measured g-factor g = 0.96 (8).

Observation of strongly deformed shapes in 154-152Dy nuclei at medium temperatures☆

The gamma-decay of the giant dipole resonance (GDR) built on excited states of 154-152Dy nuclei is studied. The selection of GDR decay from high spin states leading to specific exit channels was made possible by triggering on high spin isomers. The deduced energy splitting of the GDR implies large deformations (\beta\ congruent-to 0.4-0.5). The resonance widths of the components are comparable to those of the GDR built on the ground state indicating small shape fluctuations.

The giant dipole resonance built on excited states in 113,114Sn and level densities in the vicinity of the neutron separation energy

Abstract The statistical decay of the excited state giant dipole resonance induced by capture of 24 and 26 MeV α-particles on 110Cd has been investigated exclusively for events feeding isomeric levels in the final nuclei 113Sn and 114Sn. This was done by measuring coincidences between high-energy γ-rays and delayed γ-rays depopulating the J π = 11− 2 and Jπ = 7− isomers in 113Sn and 114Sn, respectively. GDR and level-density parameters were deduced from fits of spectra obtained from statistical-model calculations to the data. The GDR energy (E = 15.2±0.5 MeV) and strength (S = 0.9±0.1) are close to the values of the ground state GDR. The increase of the width of the GDR (Γ = 7±1 MeV) is in agreement with previous measurements at higher excitation energies. The level-density parameters obtained from the fits can be well reconciled with the interpolated Dilg parameters.

The angular-momentum dependence of the giant dipole resonance in 154Dy∗

Abstract The statistical γ-ray decay of the compound nucleus 154 Dy ∗ formed at an excitation energy of 69 MeV is studied in three angular-momentum windows 〈 J 〉 = 31,42 and 50ℏ. The GDR strength function extracted from the data indicates a constant centroid energy for the resonance E GDR = 15.2 ± 0.5 MeV, a small deformation of the nuclear shape for all spin regions and an increase in the resonance width with increasing angular momentum. The latter might indicate larger shape fluctuations at higher angular momenta. A systematical study of the dependence of the extracted strength function with different methods of analysis is presented, and their discrepancies discussed.

GDR DISSIPATION AND NUCLEAR SHAPE IN HOT FAST-ROTATING DY NUCLEI

Abstract The statistical γ-ray decay of the GDR built on excited states in Dy nuclei has been investigated for selected domains of angular momentum up to about 70ħ and temperatures in the range 1–2 MeV. The GDR strength distribution extracted from the data indicate large average nuclear deformations ( β ∼ 0.35) at high angular momentum and average temperatures T ⩾ 1.5 MeV. The experimental observation is supported by results from calculations in which thermal shape fluctuations are taken into account around an oblate equilibrium deformation β eq . Although this equilibrium deformation increases with angular momentum, the calculations show rather large and constant average deformations 〈 β ∼0.35.

GDR γ-ray decay in 156Dy∗ from regions selected on temperature and angular momentum

The strength distribution of the GDR built on highly excited states in a restricted temperature domain in Dy-156 and Dy-155 nuclei has been deduced by subtraction of gamma-ray spectra obtained for the decay of Dy-154* and Dy-156* from regions selected on angular momentum. The resulting difference spectra have been analyzed within the statistical model. The results show a large deformation (\beta\ similar to 0.51 +/- 0.29 and 0.35 +/- 0.14) for the angular-momentum regions with [J] similar to 32 (

Exclusive measurement of the statistical γ-decay of the giant dipole resonance built on excited states in 114Sn

) over bar h at T approximate to 1.8 +/- 0.2 MeV and [J] similar to 46 (

SHAPE TRANSITION AT HIGH-SPIN MANIFESTED IN THE GAMMA-RAY DECAY OF THE GIANT-DIPOLE RESONANCE IN HOT DY NUCLEI

) over bar h at T approximate to 1.7 +/- 0.2 MeV respectively, in satisfactory agreement with calculations performed in the framework of Landau theory of shape transitions and statistical fluctuations. The deduced centroid energies are in agreement with the systematics of the GDR built on the ground state. The width of the GDR shows a systematic increase with increasing temperature.

ROTATIONAL SIDE BANDS IN DY-160

Abstract For the first time the statistical γ-decay of the giant dipole resonance built on excited states has been investigated exclusively in one nucleus. This was done by measuring coincidences between high energy γ-rays following α-capture in 110Cd and delayed γ-rays depopulating the Jπ=7− isomer in 114Sn.