Neutrons from the fission of excited nuclei

Eismont, V.P.

doi:10.1007/bf01126415

Cited by 21 publications

(5 citation statements)

References 12 publications

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“…The available literature data on thermal neutron induced fission of 235 U are given in Table 1. Evaluations of the "scission" neutron yields [28][29][30][31][32][33][34][35][36][37][38][39][40] made in the framework of different theoretical models are in a quantitative agreement with the experimental evaluations given above. At the same time, there exists a sufficiently strong dependence of the obtained "scission" neutron yield on the parameters used for model calculations.…”

Section: Introductionsupporting

confidence: 65%

Experimental Investigation of the Properties of Scission Neutrons In Thermal-Neutron Induced Fission of 233U and 235U

Vorobyev,

Shcherbakov

2020

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The properties of “scission” neutrons from thermal neutron induced fission of 233U and 235U were obtained by comparing experimental angular and energy distributions of the prompt fission neutrons measured recently at PNPI with model distributions calculated under the assumption that all prompt fission neutrons are emitted from fully accelerated fragments. To obtain model distributions, it is assumed to use the spectra of prompt fission neutrons measured at small angles relative to the preferential direction of movement of light and heavy fragments because it is expected that just for these angles the contribution of non-primary mechanism is minimal while a contribution of neutrons emitted by complementary fragment can be taken into account correctly. It is also very important that in this approach it is possible to obtain the model distributions practically unlimited in low-energy range.

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Section: Introductionsupporting

confidence: 65%

Experimental Investigation of the Properties of Scission Neutrons In Thermal-Neutron Induced Fission of 233U and 235U

Vorobyev,

Shcherbakov

2020

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“…Neutron emission during fragment acceleration tends to lower the resulting values of MI' ' and give increased values for Mi'". Using the formalism of Eismont [30], it is seen that the fragment acceleration time is of the order of 5.5 x 10~i s for 24sCf and 244Cm fissioning systems and the neutron evaporation time, calculated with fragment excitation energies as determined from the parameters of postscission components, is estimated to be about 3 x 10 is s. This gives an estimate of neutrons evaporated during acceleration of the order of 0.02, which is rather small compared to the total number of excess neutrons. The excess prescission neutrons were used to estimate the dynamical fusion-fission delays.…”

Section: Introductionmentioning

confidence: 99%

Entrance channel effects in the fusion-fission time scales from studies of prescission neutron multiplicities

et al. 1994

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Prescission neutron multiplicities in fusion-fission reactions of B+ Np, B+ Th, C+ Th, and 0+ Th, lying on either side of the Businaro-Gallone mass asymmetry (uso), have been measured. The present data along with those available in literature for compound systems spanning the fissility range from 0.70 to 0.84 were analyzed in a consistent manner to deduce fusion-fission time scales for all systems. From the systematic behavior of all the data, the three components of total dynamical fusion-fission delay, namely, transient delay, saddle-to-scission delay, and formation delay, have been deduced. It is found that the formation delay depends on the entrance channel mass asymmetry relative to Businaro-Gallone point. The variations of the fusionfission time scales with fissility, ratio of fission barrier to temperature, and entrance channel mass asymmetry have been studied.PACS number(s): 25.70.Jj

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“…For systems with higher mass symmetry, neutrons (ν exp pre ) can also be emitted in the comparatively longer formation stage (ν pre-eq pre + ν NCNF pre ) of the CN [76,119,120] and/or during neck rupture (ν sc pre ) [121][122][123][124][125][126][127][128][129] and/or from the acceleration phase of fission fragments (ν acc pre ) [60,[130][131][132], which are not included in the present work. These are the most probable reasons for the mismatch in the experimental and the calculated ν pre for the symmetric combinations.…”

Section: Discussionmentioning

confidence: 99%

Detailed statistical model analysis of observables from fusion-fission reactions

2019

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Background:Despite remarkable success of the statistical model (SM) in describing decay of excited compound nuclei (CN), reproduction of the observables from heavy ion-induced fusion-fission reactions is often quite challenging. Ambiguities in choosing the input parameters, lack of clarity about inclusion of various physical effects in the model, and contradictory requirements of input parameters while describing different observables from similar reactions are among the major difficulties of modeling decay of fissile CN. Purpose: This work attempts to overcome the existing inconsistencies by inclusion of important physical effects in the model and through a systematic analysis of a large set of data over a wide range of CN mass (A CN ). Method: The model includes the shell effect in the level density (LD) parameter, shell correction in the fission barrier (B f ), the effect of the orientation degree of freedom of the CN spin (K or ), collective enhancement of level density (CELD), and dissipation in fission. Input parameters are not tuned to reproduce observables from specific reaction(s) and the reduced dissipation coefficient (β) is treated as the only adjustable parameter. Calculated evaporation residue (ER) cross sections (σ ER ), fission cross sections (σ fiss ), and particle, i.e., neutron, proton, and α particle, multiplicities are compared with data covering A CN = 156-248. Results: The model produces reasonable fits to ER and fission excitation functions for all the reactions considered in this work. Pre-scission neutron multiplicities (ν pre ) are underestimated by the calculation beyond A CN ∼ 200. An increasingly higher value of β, in the range of 2-4 × 10 21 s −1 , is required to reproduce the data with increasing A CN . Proton and α-particle multiplicities, measured in coincidence with both ERs and fission fragments, are in qualitative agreement with model predictions. Conclusions:The present work mitigates the existing inconsistencies in modeling statistical decay of the fissile CN to a large extent. Contradictory requirements of fission enhancement-obtained by scaling down the fission barrier to reproduce σ ER or σ fiss and fission suppression realized by introducing dissipation in the fission channel to reproduce ν pre , for similar reactions-have now become redundant. There are scopes for further refinement of the model, as is evident from the mismatch between measured and calculated particle multiplicities in a few cases.

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Neutrons from the fission of excited nuclei

Cited by 21 publications

References 12 publications

Experimental Investigation of the Properties of Scission Neutrons In Thermal-Neutron Induced Fission of 233U and 235U

Experimental Investigation of the Properties of Scission Neutrons In Thermal-Neutron Induced Fission of 233U and 235U

Entrance channel effects in the fusion-fission time scales from studies of prescission neutron multiplicities

Detailed statistical model analysis of observables from fusion-fission reactions

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