Evaporation residue cross-sections as a probe for nuclear dissipation in the fission channel of a hot rotating nucleus

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A statistical model calculation for the decay of a compound nucleus is presented where the compound nuclear spin dependence of the Kramers modified fission width is included. Specifically, the spin dependences of the frequencies of the harmonic oscillator potentials osculating the rotating liquid-drop model potential at equilibrium and saddle regions are considered. Results for the 16 O+ 208 Pb system show that the energy dependence of the dissipation strength extracted from fitting experimental data is substantially reduced when the spin dependence of the frequencies is properly taken into account.During the last two decades, experimental and theoretical investigations of heavy ion induced fusion-fission reactions at beam energies above Coulomb barriers have made significant contributions to the understanding of the nuclear bulk dynamics at high excitation energies. Specifically, careful analyses of experimental values of multiplicities of pre-scission light particles (mainly neutrons and γ 's) [1-8], evaporation residue cross sections [9-11] and mass and kinetic energy distributions of fission fragments [1][2][3] have established that the fission dynamics of a hot compound nucleus is dissipative in nature. The theoretical analyses are usually performed either by employing the Langevin equation in a dynamical model of nuclear fission [12][13][14] or by using the statistical model where the fission width includes the effects of dissipation [15]. The later approach is used more frequently [6-9,11] since it is rather straightforward to implement it in a standard statistical model code for the decay of a compound nucleus.Considering fission as a diffusive process of a Brownian particle across the fission barrier in a viscous medium, Kramers solved the corresponding Fokker-Planck equation with a few simplifying approximations which finally yielded the so-called Kramers modified Bohr-Wheeler expression for fission width as [15,16] whereIn the above, BW is the fission width due to Bohr and Wheeler [17] and β is the strength of the reduced dissipation coefficient. ω gs and ω sad are the local frequencies of the harmonic oscillator potentials which osculate the liquid drop model nuclear potential at the ground state and the saddle configurations, respectively, while m gs and m sad are the corresponding inertia parameters. T is the nuclear temperature. The dimensionless quantity η = β/2ω sad is often used as a free parameter in order to fit experimental data. * Corresponding author: santanu@veccal.ernet.in A number of assumptions are usually made while applying Eq. (1) in statistical model calculations. A constant value for the parameter η is usually assumed for all spin values of the compound nucleus (CN). The centrifugal barrier however changes the potential profile at higher values of spin of a CN, which consequently results in a spin dependence of the frequencies ω gs and ω sad of the osculating harmonic oscillator potentials [18]. Figure 1 shows the frequencies as a function of spin for the compound nucleus 224 Th. ...

mentioning

confidence: 99%

Spin dependence of the modified Kramers width of nuclear fission

Sadhukhan

Pal

2008

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“…In our dynamical calculations we use {c,h,α} [43] parametrization of the compound nucleus shape. Since only symmetrical fission is considered, the parameter describing the asymmetry of the shape is set to α = 0 [11,24]. The deformation coordinate q is obtained by the relation q(c, 44], where c and h correspond to the elongation and neck degrees of the freedom of the nucleus, respectively.…”

Section: Theoretical Modelmentioning

confidence: 99%

“…To extract reliable and precise information about β, it is important to explore those experimental signals sensitive to presaddle dissipation effects only. While several observables (e.g., evaporation residue cross section [23,24] and its spin distribution [25,26]) were proposed, these evaporationchannel-related quantities are an indirect indicator of presaddle dissipation as compared to those provided via fission channels. As the most direct consequence of dissipation, fission is hindered; that is, fission probability (or fission cross section) is reduced.…”

Section: Introductionmentioning

confidence: 99%

Spallation reaction versus heavy-ion fusion: Fission excitation functions as a fundamental probe of presaddle nuclear dissipation

Tian

2015

Due to nuclear dissipation, fission cross sections drop with respect to standard statistical model predictions.Using the stochastic dynamical model of fission, we calculate the drop of fission cross sections, σ drop f , as a function of presaddle dissipation strength (β) under two contrasting initial conditions for produced 224 Th compound nuclei: (i) high excitation energy but low angular momentum (available in spallation reactions) and (ii) low excitation energy but high angular momentum (available in heavy-ion fusion). We find that the former type of conditions not only significantly increases the influence of friction on fission cross sections but also substantially enhances the sensitivity of σ drop f to β. Our findings suggest that on the experimental side, to accurately obtain information of presaddle friction strength with fission excitation function, it is optimal to choose the spallation reaction approach induced by energetic protons as a way to populate excited nuclear systems.

“…In our dynamical calculations we use a {c, h, α} [63] parametrization of the compound nucleus shape. Since only symmetrical fission is considered, the parameter describing the asymmetry of the shape is set to α = 0 [6,64]. The deformation coordinate q is obtained by the relation 65], where c and h correspond to the elongation and neck degrees of the freedom of the nucleus, respectively.…”

Section: The Modelmentioning

confidence: 99%

Significant role of deformation in probing postsaddle nuclear dissipation with light particle emission

2010

Using a one-dimensional Langevin model, we study the effects of deformation on the multiplicities of postsaddle neutrons, protons, α particles, and giant dipole resonance (GDR) γ rays of a heavy fissioning system 240 Cf as probes of postsaddle nuclear dissipation (β). It is shown that postsaddle dissipation effects on these light particles have a significant deformation dependence. Furthermore, we find that the role of deformation depends on the type of the particle. It reduces the sensitive influence of β on protons, α particles, and GDR γ rays substantially, whereas it enhances the sensitivity of neutrons to β. The results suggest that to accurately extract the postsaddle friction strength by comparing measured prescission particle multiplicities of heavy nuclei with calculations based on statistical models or stochastic equations like Langevin equations, it is important to take into account the deformation effects. The influence of model dimensionality is discussed.