H. van der Ploeg

VISCOSITY, FISSION TIME SCALE AND DEFORMATION OF 156DY

In the fusion-fission reaction Ar-40 + Cd-116 --> Dy-156*, fission, at E-b = 216 MeV and 238 MeV, gamma-rays were measured in coincidence with fission fragments. The interpretation of the gamma-ray spectra is done with the help of a modified version of the statistical-model code CASCADE. The spectra can be reproduced with nuclear viscosities in the range 0.01 <gamma <4. The extracted fission time scale is of the order of 10(-19) s. The CASCADE analysis seems to favor a prolate deformation with beta approximate to 0.45

Neutron decay of deep-hole states in 89Zr populated by the 90Zr(3He,α)89Zr reaction at 102 MeV

Abstract Neutron decay of excited hole states populated by the 90 Zr( 3 He,α) reaction at an incident energy of 102 MeV has been investigated. The α-particles were analysed in a magnetic spectrograph positioned at 1.4° and detected with a multi-wire drift chamber. Excitation energies in 89 Zr up to 26 MeV were studied. An array of neutron detectors was positioned around the target at angles ranging from 68° to 209°. The neutron data were analysed in terms of decay into final states, using statistical model calculations as a tool to extract non-statistical decay.

Study of the gamma emission probability accompanying the spontaneous fission of 252Cf

Abstract A study of the gamma emission accompanying the spontaneous fission process of 252 Cf has been performed. The photon emission probability between 3 and 70 MeV and its angular dependence with respect to the fission direction were measured. These measurements were performed as a function of the mass asymmetry of the fission fragments. A large anisotropy in the energy region between 8 and 12 MeV has been found, confirming the results of a previous performed experiment [1]. This phenomena up till now remains unexplained.

Neutron decay of deep hole states and isobaric analog states in Sn-115 populated by the (He-3,alpha) reaction at 102 MeV

Neutron decay of excited hole states and isobaric analog states (IAS) populated by the Sn-116(He-3,alpha) reaction at an energy of 102 MeV has been investigated. The alpha -particles were analysed in a magnetic spectrograph positioned at 1.4 degrees and detected with a multiwire drift chamber. Excitation energies in Sn-115 up to 23 MeV were studied. An array of neutron detectors was positioned around the target at angles ranging from 68 degrees to 209 degrees. The neutron data were analysed in terms of decay into final states, using statistical model calculations as a tool to extract lower limits of nonstatistical decay. Nonstatistical decay was observed from states up to an excitation energy of about 18 MeV. The neutron emission of the IAS is compatible with 100% statistical decay

Neutron decay of deep-hole states populated by the 208Pb(3He,α)207Pb reaction at 102 MeV

Abstract Neutron decay of excited hole states populated by the 208 Pb( 3 He,α) reaction at an energy of 102 MeV has been investigated. The α-particles were analysed in a magnetic spectrograph positioned at 1.4° and detected with a multi-wire drift chamber. Excitation energies in 207 Pb up to 25 MeV were studied. An array of neutron detectors was positioned around the target at angles ranging from 68° to 209°. The neutron data were analysed in terms of decay into final states, using statistical-model calculations as a tool to extract non-statistical decay.

Viscosity and fission time scale of Dy-156 at high excitation energies and large angular momenta

The reaction Ar-40 + Cd-116 --> Dy-156* --> fission as Studied at beam energies E(b) = 216 MeV and E(b) = 238 MeV, wherein gamma-rays were measured in coincidence with fission fragments. From these spectra the nuclear viscosity gamma and fission time scale were deduced by comparing to statistical model calculations yielding viscosities in the range 0.01 <gamma <4 and fission time scales of the order of 10(-19) s.

Dynamical fission life-times deduced from γ-ray emission observed in the fusion-fission reaction: 20Ne on 209Bi

Abstract The γ-ray emission spectra between 4 and 20 MeV have been measured for the fusion-fission reactions 20 Ne on 209 Bi → 229 Np ∗ at beam energies 150, 186 and 220 MeV. In addition for the latter experiment the angular dependence of the γ-ray emission with respect to the spin axis has been measured. From fits to the measured γ-ray yields, a dynamical fission life times of 4 ± 1, 1.0 ± 0.5 and 2 ± 1 × 10 −19 seconds were obtained for the three beam energies respectively.

Di-lepton yield from the decay of excited 28Si states

Abstract The first dilepton yield measurements from excited nuclear states obtained with a new Positron-Electron Pair Spectroscopic Instrument (PEPSI) are reported. Nuclear states in 28Si, with an initial excitation energy E ∗ = 50 MeV , were populated via the isospin T = 0 reaction 4He + 24Mg and the mixed-isospin 3He + 25Mg reaction. In both reactions the dilepton (e+e−) and photon decay yields were measured concurrently. An excess of counts in the e+e− spectrum, over the converted photon yield, is observed in the energy region above 15 MeV. An analyses is discussed whereby the observed excess counts are assumed to represent the isoscalar E0 strength in excited nuclear states.