Prevalence of Fission and Evaporation in the Decay of Heavy Nuclei Excited up to 1000 MeV with Energetic Antiprotons

Jahnke, U.; Bohne, W.; Egidy, T. von; Фігуера, П.; Galin, J.; Goldenbaum, F.; Hilscher, D.; Jastrzȩbski, J.; Lott, B.; Morjean, M.; Pausch, G.; Péghaire, A.; Pieńkowski, L.; Polster, D.; Proschitzki, S.; Quednau, B.; Rossner, H.; Schmid, Sebastian; Schmid, W.

doi:10.1103/physrevlett.83.4959

Cited by 44 publications

(19 citation statements)

References 17 publications

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“…Some extra heating is generated by the pion cascades from thep annihilation which leads to considerable excitation energies already at much lowerp momenta. The reported fragment multiplicities, however, are significantly smaller for the less energetic antiproton beams even if identical bins of excitation energy are selected [18]. It is therefore an open question whether the excitation energy by itself is the only parameter that governs the decay properties and fragment production.…”

Section: Useful Reactionsmentioning

confidence: 90%

Hot fragmentation of nuclei

Trautmann¹

2001

Nuclear Physics A

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Today, we have a variety of reactions at hand that can be used to multi-fragment nuclei. In many of these reactions even several sources of fragments can be discerned and characterized.There is overwhelming evidence that these sources of fragments are hot. It is already less clear whether heat by itself is sufficient to initiate the fragment decay. What causes fragmentation, and when and how are the fragments (pre)formed? These questions have remained as much a challenge as the complementary class of questions to which they are related: What observations derive their significance from the liquid-gas phase behavior of extended nuclear matter? And, can we observe a phase transition in finite nuclei?Recent developments, largely coming from complex analyses of data sets measured in 4-π-type experiments as well as from calculations based on advanced theoretical concepts, will be discussed.

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Section: Useful Reactionsmentioning

confidence: 90%

Hot fragmentation of nuclei

Trautmann¹

2001

Nuclear Physics A

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“…Their ratio, the fission probability P f E f E = inel E , provides the best possible evidence for the presence of dissipative or transient effects in fission. The selected reactions, similar to antiproton [3,4] or peripheral relativistic heavy-ion reactions [5,6], are thought to deposit high thermal excitation with minimum ballast from collective excitations (angular momentum, shape distortions, or compression) which could have a detrimental influence on the decay.The maximum E reached in the present investigation is only about 4-5 MeV=nucleon or 1000 MeV, as will be shown below. This excitation is well below that reported for the onset of multifragmentation [7], the decay of a hot nucleus into multiple clusters and nucleons.…”

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confidence: 99%

“…For the level density parameters we assumed a n A=10 MeV ÿ1 and for their ratio at the transition state and at ground state deformation a f =a n 1:000, 1.017, and 1.022 for U, Bi, and Au, respectively. For U and Au these values are the same as in [3], while for Bi we have chosen an intermediate value. No extra transient time for fission is applied.…”

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confidence: 99%

Fast Decision in Favor of the Slow Fission Process

et al. 2005

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The fission probability P f of highly excited targetlike nuclei produced in reactions of 2.5 GeV protons on Au, Bi, and U was studied as a function of excitation energy E whereby E is deduced eventwise from the multiplicity of evaporated light particles. At the highest E of 1000 MeV P f amounts to 30% with all 3 target nuclei irrespective of the initial fissility. Statistical-model calculations satisfactorily reproduce the observed evolution of P f with E -provided that no extra transient delay is introduced. Fission thus is decided upon very fast and early in the long deexcitation chain towards scission which comprises as much as 80% of all evaporated alpha particles. DOI: 10.1103/PhysRevLett.95.162701 PACS numbers: 24.75.+i, 24.10.2i, 25.40.Sc, 24.60.Dr At low excitation energy nuclear fission is strongly influenced by nuclear shell effects, resulting, for instance, for U in the well known binary split into two mass asymmetric fission fragments. With increasing excitation these nuclear structure effects are washed out, and macroscopic properties such as nuclear dissipation dominate the collective flow of nuclear matter from the equilibrium deformation via the saddle towards scission into two fragments of about equal size. The overall time elapsed from the equilibrium deformation up to scission is found to be a few times 10 ÿ20 s. This time is quite long compared to other processes such as the emission of neutrons which is 10 to 100 times faster. This observation has led to the notion of fission being a slow process [1].Since the magnitude of nuclear dissipation is most likely deformation dependent [2], the characteristic time governing the flow over the saddle point, where the decision about fission is made, might be considerably shorter than the one close to the scission point. In particular, at high excitation energies, where the emission times of light charged particles are very short, a long transient delay for fission at the saddle point would strongly favor the emission of charged particles, thus reducing the fissility and, consequently, second-and higher-chance fission probabilities. Conversely, low dissipation at the saddle or a minimum transient delay tends to keep fission competitive with particle evaporation even at high excitation. In other words, although the entire fission process is slow, the decision to fission can be fast.We have chosen 2.5 GeV proton-induced reactions to excite three target nuclei, Au, Bi, and U with different fissilities Z 2 =A 31:7, 33.0, and 35.6, respectively, in order to study the evolution of the inelastic reaction, inel , and the fission cross section, f , as a function of excitation energy, E , and fissility. Their ratio, the fission probability P f E f E = inel E , provides the best possible evidence for the presence of dissipative or transient effects in fission. The selected reactions, similar to antiproton [3,4] or peripheral relativistic heavy-ion reactions [5,6], are thought to deposit high thermal excitation with minimum ballast from collective excitations ...

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“…In particular, the determination of presaddle dissipation strength is the focus of intense studies [21][22][23][24][25][26][27][28][29][30][31]. Although prescission particles have been suggested to be a principle observable of nuclear dissipation [19], they are a less direct signature of presaddle effects because of the interference of postsaddle emission.…”

Section: Introductionmentioning

confidence: 99%

“…To this end, recently two alternative approaches, i.e. proton- [25] and antiproton-induced [24,27] spallation reactions as well as peripheral relativistic heavy-ion collisions [21,22,26] have been used to yield compound systems, and the observables investigated are fission probabilities and widths of fission-fragment charge distributions. The compound nuclei (CN) populated in these reactions have very large excitation energies E * , up to 1 GeV, and low angular momenta, which is in sharp contrast with the situation of fusion reactions via which the formed CN have high spin ℓ c (∼ 75 [32]) but low excitation energy (< 250 MeV).…”

Section: Introductionmentioning

confidence: 99%

Excitation energy and nuclear dissipation probed with evaporation-residue cross sections

2011

Phys. Rev. C

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Using a Langevin equation coupled with a statistical decay model, we calculate the excess of evaporation residue cross sections over its standard statistical-model values as a function of nuclear dissipation strength for 200 Hg compound nuclei (CN) under two distinct types of initial conditions for the populated CN: (i) high excitation energy but low angular momentum (produced via protoninduced spallation reactions at a GeV energy regime and via peripheral heavy-ion collisions at relativistic energies) and (ii) high angular momentum but low excitation energy (produced through fusion mechanisms). We find that the conditions of case (ii) not only amplify the dissipation effects on the evaporation residues, but also increase the sensitivity of this excess to nuclear dissipation substantially. These results suggest that in experiments, to obtain accurate information of presaddle nuclear dissipation strength by measuring evaporation residue cross sections, it is optimal to choose heavy-ion induced fusion reaction approach to yield excited compound nuclei.

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Prevalence of Fission and Evaporation in the Decay of Heavy Nuclei Excited up to 1000 MeV with Energetic Antiprotons

Cited by 44 publications

References 17 publications

Hot fragmentation of nuclei

Hot fragmentation of nuclei

Fast Decision in Favor of the Slow Fission Process

Excitation energy and nuclear dissipation probed with evaporation-residue cross sections

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