Significant role of level-density parameters in probing nuclear dissipation with light-ion-induced fission excitation functions

Presaddle and postsaddle dissipative effects in fission using complete kinematics measurements

“…The measurements obtained for many different fissioning systems over a broad range in fissility and temperature were compatible with a constant value of the reduced dissipation parameter at small deformations of β = 4.5 × 10 21 s −1 , corresponding to transient times (τ trans ) between 1.0 and 3.3 × 10 −21 s. These results are compatible with the ones obtained from the investigation of some fusion reactions [16,25]. Moreover, the overall good description of the data over a broad range of excitation energies also validated previous conclusions on the temperature independence of the dissipation parameter [11,12] and references therein.Postsaddle dissipative effects have been mostly investigated by measuring prescission particles and γ rays emitted in fusion-fission reactions [15,[26][27][28]. In some cases, fission or evaporation-residue cross sections were also measured, constraining presaddle effects.…”

“…Although the first explanation of nuclear fission was provided by Bohr and Wheeler [3] using the transition-state model, new experimental observations revealed the complexity of nuclear fission and provided new challenges for theory. In particular, the investigation of pre-and postscission neutron multiplicities [4,5], giant dipole resonance (GDR) γ -ray emission [6,7], multiplicities of charged particles [8,9], and fission and evaporation cross sections [10][11][12] established that the dynamical evolution of the fissioning system cannot be explained just in terms of the statistical model of Bohr and Wheeler [3]. These results suggested that the understanding of the fission process requires a dynamical approach, describing the coupling of intrinsic and collective excitations of the nuclear constituents.…”

Proton-induced fission of181Ta at high excitation energies

“…In all fission models, a critical parameter contributing to the fission width is the ratio of the level-density parameter used in the fission channel to that used in neutron evaporation, a f /a n [51][52][53][54][55][56]. Several parametrizations based on measurements of the resonances produced in neutron capture and proton or α elastic scattering reactions have been used to determine these level-density parameters.…”

Light charged particles emitted in fission reactions induced by protons onPb208

“…The observation of an excess of prescission neutron [21][22][23]27] and γ -ray [21][22][23]25] multiplicities with respect to the predictions of the statistical model, for nuclei in a wide range of fissility, has been the most direct confirmation of dynamical effects in fission. This conclusion was also supported by the observation of a reduction of the fission cross sections [9,[12][13][14] and the widths of the charge distributions of the fission fragments [10,15,16,19] with respect to the statistical model. However, different attempts to constrain the magnitude of the dissipation parameter used to describe the fission dynamics with transport equations yield quite different results [1,2], leading to transient times between 1 × 10 −21 and 30 × 10 −21 s [7,8,14,19,24,28,29].…”

“…This transient time or fission delay affects many experimental observables, such as the total and partial fission cross sections [7][8][9][10][11][12][13][14][15], the charge and mass distributions of the fission fragments [10,11,[15][16][17][18][19], the neutron multiplicities [1,[20][21][22][23], and the multiplicity of prescission light charged particles [24] and γ rays [25,26]. The observation of an excess of prescission neutron [21][22][23]27] and γ -ray [21][22][23]25] multiplicities with respect to the predictions of the statistical model, for nuclei in a wide range of fissility, has been the most direct confirmation of dynamical effects in fission.…”