Microscopic theory of dissipation for slowly time-dependent mean field potentials

Aleshin, V.P.

doi:10.1016/j.nuclphysa.2005.06.004

Cited by 4 publications

(7 citation statements)

References 35 publications

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“…More generally, theoretical studies on surface motion of a cavity wall [18,187,188,189], among which multipole vibrations and fission are, suggest that the original wall damping mechanism [16] overestimates the dissipation rate in nuclear matter. That is understood by chaotic singleparticle motion and quantal effects, which violate the original assumption of full randomisation [190]. This deficiency in the early derivation of the one-body theory partly contributes to the variety of contradicting conclusions on nuclear dissipation [191].…”

Section: Discussionmentioning

confidence: 99%

Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission

Schmitt

Schmidt²,

Kelić³

et al. 2010

Phys. Rev. C

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38 pages, 15 figuresPeripheral collisions with radioactive heavy-ion beams at relativistic energies are discussed as an innovative approach for probing the transient regime experienced by fissile systems evolving towards quasi-equilibrium. A dedicated experiment using the advanced technical installations of GSI, Darmstadt, permitted to realize ideal conditions for the investigation of relaxation effects in the meta-stable well. Combined with a highly sensitive experimental signature, it provides a measure of the transient effects with respect to the flux over the fission barrier. Within a two-step reaction process, 45 proton-rich unstable spherical isotopes produced by projectile-fragmentation of a stable 238U beam have been used as secondary projectiles. The fragmentation of the radioactive projectiles on lead results in nearly spherical compound nuclei which span a wide range in excitation energy and fissility. The decay of these excited systems by fission is studied with a dedicated set-up which permits the detection of both fission products in coincidence and the determination of their atomic numbers with high resolution. The width of the fission-fragment nuclear charge distribution is shown to be specifically sensitive to pre-saddle transient effects and is used to establish a clock for the passage of the saddle point. The comparison of the experimental results with model calculations points to a fission delay of (3.3+/-0.7).10-21s for initially spherical compound nuclei, independent of excitation energy and fissility. This value suggests a nuclear dissipation strength at small deformation of (4.5+/-0.5).1021s-1. The very specific combination of the physics and technical equipment exploited in this work sheds light on previous controversial conclusions

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Section: Discussionmentioning

confidence: 99%

Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission

Schmitt

Schmidt²,

Kelić³

et al. 2010

Phys. Rev. C

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“…Let t 0 represent the collision time: t 0 ∼ r 0 /v F , where r 0 is the range of the effective force and v F is the Fermi velocity. Then, following [49,34], we postulate that for |t| > t 0…”

Section: Frictionmentioning

confidence: 99%

“…Turning to microscopic expressions for friction γ, we remark that for quite a long time [27,28,29,30,31,32,33,34] they were extracted from the linear response function of a thermally equilibrated Fermi gas confined by the external forces on the small variations of those forces. And only recently [35] it has been realized that in the bulk of such a gas, there will be no mass flow, j x , defined as the expectation of the momentum density operator…”

Section: Introductionmentioning

confidence: 99%

A self-confined Fermi-gas model for nuclear collective motion

Aleshin

2008

Preprint

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The well-known phenomenological dynamic equation for the nuclear shape parameter was derived from first principles with the aim of obtaining the microscopic or many-body expressions for inertia B, deformation force f , and friction coefficient γ of collective motion in hot nuclei.Nuclear evolution, viewed as a sequence of quasiequilibrium stages, is described with the aid of the density matrix ρ q constrained to given expectation values of nuclear Hamiltonian H, number operator N , and operators Q, P , and M of coordinate, momentum, and inertia of collective motion. Having chosen an explicit expression for Q in terms of the nucleon field operators ψ x , ψ *x , we construct the corresponding expressions for ρ q , P , and M, using a certain canonical transformation of ψ x , ψ *x , the equation of continuity and an assumption that the collective variables p n,t = tr (P n,t ρ q ) where P n,t are Heisenberg representations of P 1 = Q, P 2 = P , P 3 = M, change with time much slower than the number density and the momentum density, from which p n,t are built. The same assumption was used to get the closed-form solutions of the dynamic equations for p n,t , from which we extract the desired many-body expressions for B, f , and γ. After adaptation of those general expressions to a self-confined Fermi gas with the phenomenological effective force, they are used for critical analysis of the previous microscopic models of those quantities and for elucidating the distinctive features of dissipative collective motion in atomic nuclei.

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“…Thus, the hydrodynamic formula for M does not imply with necessity that the mean free path of the nucleon is small compared to the nuclear size. Turning to microscopic expressions for friction γ , we note that for more than three decades [31][32][33][34][35][36][37][38][39] they were extracted from the linear response function of thermally equilibrated Fermi gas enclosed in a cavity on slow variations of the shape of the cavity. And only recently [40] it has been realized that in the bulk of such a gas, there is no mass flow, j x , defined as the expectancy of the momentum density operator p x .…”

Section: Introductionmentioning

confidence: 99%

“…In[39] we use the t 0 − t 3 version of the Skyrme force to obtain the semiclassical expression for Γ 1 in finite spherical nuclei and compare it to the values of Γ 1 in infinite matter.The smallness of the collision time τ 0 compared to the mean free path time ∼ Γ −1 , allows to use the asymptotic form (151), (152) for all t in (150), givingγ = im 2 νλ |φ λν | 2 f λfν ∞ −∞ t dt e −η|t| d4 dt 4 e −Γ λν |t| e −iE λν t ,…”

mentioning

confidence: 99%