Dissipative effects in spallation-induced fission ofPb208at high excitation energies

Abstract

Spallation reactions on fissile nuclei represent an appropriate tool to investigate dissipative effects in nuclear fission. In this work, we have studied transient and dissipative effects in proton-and deuteron-induced fission on 208 Pb at 500A MeV. A dedicated experimental setup optimized for inverse kinematics measurements made it possible to identify in atomic number both fission fragments with high resolution, and reconstruct the charge of the fissioning system. We could then determine the width of the fission fragments charge distribution as well as partial fission cross sections, both as a function of the charge of the fissioning system. These two complementary observables permitted us to investigate the dynamics of the process at small deformation, i.e., from the ground state to the saddle point. The description of these observables with advanced model calculations reveals the influence of the nuclear dissipation in the fission process at high excitation energy and in particular its manifestation through transient time effects.

Improvement of one-nucleon removal and total reaction cross sections in the Liège intranuclear-cascade model using Hartree-Fock-Bogoliubov calculations

“…The coupling of these model calculations has been benchmarked in several works about nucleon-induced reactions on different nuclei between carbon and uranium at incident energies from a few MeVs to 3 GeV, providing a satisfactory description of many observables such as isotopic distributions of evaporation and fission residues [20,21,41,[54][55][56][57][58][59], doubledifferential cross sections [2,21,41,42,60], total reaction and fission cross sections [2,41,42,[61][62][63], and pion production [2,44,64,65].…”

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.…”

Presaddle and postsaddle dissipative effects in fission using complete kinematics measurements

“…The combined use of the inverse kinematics technique and an efficient detection setup made it possible to identify fission reactions and determine the atomic number of the final fission fragments very accurately. The sum of the charges of both fission fragments was used as a measurement of the initial excitation energy while partial fission cross sections and the width of the charge distribution of the fission fragments were shown to be sensitive to presaddle dynamical effects [18,20,23,24]. 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].…”

“…The description of the range of compound nuclei produced in spallation reactions, whose characteristics cannot be accessed experimentally, has been validated describing the isotopic composition of final residual nuclei in reactions with nonfissile nuclei [33]. Moreover, the ground-to-saddle dynamics was constrained by measuring the atomic numbers of both fission fragments [16,18,23,30] or the isotopic composition of one of the two fragments [24,44,45]. We can then conclude that these calculations provide a reasonable description of the fissioning nuclei at the saddle-point deformation.…”