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Scaling laws, transient times and shell effects in helium induced nuclear fission  [PDF]
Th. Rubehn,K. X. Jing,L. G. Moretto,L. Phair,K. Tso,G. J. Wozniak
Physics , 1996,
Abstract: Fission excitation functions of He-3 and He-4 induced compound nuclei are shown to scale exactly according to the Bohr-Wheeler transition state prediction once the shell effects are accounted for. The presented method furthermore allows one to model-independently extract values for the shell effects which are in good agreement to those obtained from liquid-drop model calculations. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign an upper limit for the transient time of 10 zs.
3He- and 4He-induced nuclear fission - a test of the transition state method  [PDF]
Th. Rubehn,K. X. Jing,L. G. Moretto,L. Phair,K. Tso,G. J. Wozniak
Physics , 1997,
Abstract: Fission in 3He and 4He induced reactions at excitation energies between the fission barrier and 140 MeV has been investigated. Twentythree fission excitation functions of various compound nuclei in different mass regions are shown to scale exactly according to the transition state prediction once the shell effects are accounted for. New precise measurements of excitation functions in a mass region where shell effects are very strong, allow one to test the predictions with an even higher accuracy. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign limits for the fission transient time. The precise measurement of fission excitation functions of neighboring isotopes enables us to experimentally estimate the first chance fission probability. Even if only first chance fission is investigated, no evidence for fission transient times larger than 30 zs can be found.
Manifestation of transient effects in fission induced by relativistic heavy-ion collisions  [PDF]
B. Jurado,C. Schmitt,K. -H. Schmidt,J. Benlliure,A. R. Junghans
Physics , 2004, DOI: 10.1016/j.nuclphysa.2005.04.020
Abstract: We examine the manifestation of transient effects in fission by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Available total nuclear fission cross sections of 238U at 1 A GeV on gold and uranium targets are compared with a nuclear-reaction code, where transient effects in fission are modelled using different approximations to the numerical time-dependent fission-decay width: a new analytical description based on the solution of the Fokker-Planck equation and two widely used but less realistic descriptions, a step function and an exponential-like function. The experimental data are only reproduced when transient effects are considered. The deduced value of the dissipation strength depends strongly on the approximation applied for the time-dependent fission-decay width and is estimated to be of the order of 2x10**21 s**(-1). A careful analysis sheds severe doubts on the use of the exponential-like in-growth function largely used in the past. Finally, we discuss which should be the characteristics of experimental observables to be most sensitive to transient effects in fission
Transient effects in fission evidenced from new experimental signatures  [PDF]
B. Jurado,C. Schmitt,K. -H. Schmidt,J. Benlliure,T. Enqvist,A. R. Junghans,A. Kelic,F. Rejmund
Physics , 2004, DOI: 10.1103/PhysRevLett.93.072501
Abstract: A new experimental approach is introduced to investigate the relaxation of the nuclear deformation degrees of freedom. Highly excited fissioning systems with compact shapes and low angular momenta are produced in peripheral relativistic heavy-ion collisions. Both fission fragments are identified in atomic number. Fission cross sections and fission-fragment element distributions are determined as a function of the fissioning element. From the comparison of these new observables with a nuclear-reaction code a value for the transient time is deduced.
Limit laws for occupation times of stable processes  [PDF]
David Nualart,Fangjun Xu
Mathematics , 2013,
Abstract: We prove two limit laws for functionals of one dimensional symmetric 1-stable process using the method of moments, and give a remark on Rosen's paper \cite{Rosen}.
Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission  [PDF]
C. Schmitt,K. -H. Schmidt,A. Keli?,A. Heinz,B. Jurado,P. N. Nadtochy
Physics , 2010, DOI: 10.1103/PhysRevC.81.064602
Abstract: Peripheral 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.
Limit laws of transient excited random walks on integers  [PDF]
Elena Kosygina,Thomas Mountford
Mathematics , 2009, DOI: 10.1214/10-AIHP376
Abstract: We consider excited random walks (ERWs) on integers with a bounded number of i.i.d. cookies per site without the non-negativity assumption on the drifts induced by the cookies. Kosygina and Zerner [KZ08] have shown that when the total expected drift per site, delta, is larger than 1 then ERW is transient to the right and, moreover, for delta>4 under the averaged measure it obeys the Central Limit Theorem. We show that when delta in (2,4] the limiting behavior of an appropriately centered and scaled excited random walk under the averaged measure is described by a strictly stable law with parameter delta/2. Our method also extends the results obtained by Basdevant and Singh [BS08b] for delta in (1,2] under the non-negativity assumption to the setting which allows both positive and negative cookies.
Transient and quasi-stationary dissipative effects in the fission flux across the barrier in 1 A GeV 238U on deuterium reactions  [PDF]
J. Benlliure,E. Casarejos,J. Pereira,K. -H. Schmidt
Physics , 2005, DOI: 10.1103/PhysRevC.74.014609
Abstract: Isotopic cross sections of all projectile residues with Z above 23 produced in collisions induced by 238U at 1 A GeV on deuterium have been measured. The isotopic distributions reflect the role of evaporation and fission in the formation process of these nuclei. The comparison of the measured cross sections with Monte-Carlo de-excitation codes including an analytical description of the dynamics of fission shows the sensitivity of the data to nuclear dissipation. Moreover, the large excitation-energy range covered in this experiment together with the high accuracy of the measured cross sections allowed to clearly separate and quantify the role of transient and quasi-stationary dissipative effects in the fission-decay width.
The transient distributions of nuclear weapon-generated tritium and its decay product 3He in the Mediterranean Sea, 1952–2011, and their oceanographic potential  [PDF]
W. Roether,P. Jean-Baptiste,E. Fourré,J. Sültenfu?
Ocean Science Discussions (OSD) , 2013, DOI: 10.5194/osd-10-649-2013
Abstract: We present a comprehensive account of tritium and 3He in the Mediterranean Sea since the appearance of the tritium generated by the atmospheric nuclear-weapon testing in the 1950's and early 1960's, based on essentially all available observations. Tritium in surface waters rose to 20–30 TU in 1964 (TU = 1018 · [3H]/[H]), a factor of about 100 above the natural level, and thereafter declined 30-fold up to 2011. The decline was largely due to radioactive tritium decay, which produced significant amounts of its stable daughter 3He. We present the scheme by which we separate the tritiugenic part of 3He and the part due to release from the sea floor (terrigenic part). We show that the tritiugenic component can be quantified throughout the Mediterranean waters, typically to a ±0.15 TU equivalent, mostly because the terrigenic part is low in 3He. This fact makes the Mediterranean unique in offering a potential for the use of tritiugenic 3He as a tracer. The transient distributions of the two tracers are illustrated by a number of sections spanning the entire sea and relevant features of their distributions are noted. By 2011, the 3He concentrations in the top few hundred meters had become low, in response to the decreasing tritium concentrations combined with a flushing out by the general westward drift of these waters. Tritium-3He ages in Levantine Intermediate Water (LIW) were obtained repeated in time at different locations, defining transit times from the LIW source region east of Rhodes. The ages show an upward trend with the time elapsed since the surface-water tritium maximum, which arises because the repeated observations represent increasingly slower moving parts of the full transit time spectrum of LIW. The transit time dispersion found by this new application of tritium-3He dating is considerable. We find mean transit times of 12 ± 2 yr up to the Strait of Sicily, 18 ± 3 yr up to the Tyrrhenian Sea, and 22 ± 4 yr up into the Western Mediterranean. We furthermore present full Eastern Mediterranean sections of terrigenic 3He and tritium-3He age in 1987, the latter one similarly showing an effect of the transit time dispersion. We conclude that the available tritium and 3He data, in particular if combined with other tracer data, are useful for constraining the subsurface circulation and mixing of the Mediterranean Sea.
Universal scaling laws of chaotic escape in dissipative multistable systems subjected to autoresonant excitations  [PDF]
Ricardo Chacon
Physics , 2009, DOI: 10.1088/1751-8113/43/22/222002
Abstract: A theory concerning the emergence and control of chaotic escape from a potential well by means of autoresonant excitations is presented in the context of generic, dissipative, and multistable systems. Universal scaling laws relating both the onset and lifetime of transient chaos with the parameters of autoresonant excitations are derived theoretically using vibrational mechanics, Melnikov analysis, and energy-based autoresonance theory. Numerical experiments show that these scaling laws are robust against both the presence of noise and re-shaping.
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