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Partition of the total excitation energy between complementary fragments

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 Added by Claudia Morariu
 Publication date 2011
  fields
and research's language is English
 Authors C. Manailescu




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Two methods of the total excitation energy (TXE) partition between complementary fission fragments (FF) are compared: one based on the classical hypothesis of prompt neutron emission from fully accelerated FF with both fragments having the same residual nuclear temperature distribution,the second one on the systematic behavior of the experimental multiplicity ratio { u}H/({ u}L+{ u}H) as a function of the heavy fragment mass number AH,the complementary FF having different residual temperature distributions.These methods were applied on six fissioning systems: 233,235U(nth,f), 239Pu(nth,f), 237Np(n5.5MeV,f), 252Cf(SF), 248Cm(SF) and fragment excitation energies,level density parameters,fragment and fragment pair temperatures were compared.Limitations of the classical TXE partition method are shown.Residual temperature ratios RT=TL/TH versus AH,local and global parameterizations of RT(AH) for the neutron induced fissioning systems are obtained.Average values of quantities characterizing prompt neutron emission are discussed.A linear decrease of <RT> with the mass number of the fissioning nucleus and a linear decrease of the average C parameter with the fissility parameter is obtained.Point by Point (PbP) model calculations validate the RT(AH) parameterizations.The multi-parametric matrix { u}(A,TKE) as well as prompt neutron and gamma-ray emission quantities as a function of fragment mass,total average prompt neutron multiplicity and spectrum and prompt neutron multiplicity distribution P({ u}) were calculated.The global RT(AH) parameterization extends the use of the PbP model to predict prompt neutron emission quantities for fissioning systems without experimental data.An explanation of the less pronounced sawtooth shape of { u}(A) and the increase of { u}(A) with incident neutron energy only for heavy fragments is given and exemplified by quantitative results of the PbP model.



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We study how the excitation energy of the fully accelerated fission fragments is built up. It is stressed that only the intrinsic excitation energy available before scission can be exchanged between the fission fragments to achieve thermal equilibrium. This is in contradiction with most models used to calculate prompt neutron emission where it is assumed that the total excitation energy of the final fragments is shared between the fragments by the condition of equal temperatures. We also study the intrinsic excitation-energy partition according to a level density description with a transition from a constant-temperature regime to a Fermi-gas regime. Complete or partial excitation-energy sorting is found at energies well above the transition energy.
172 - R. Yanez , L. Yao , J. King 2014
The total kinetic energy release in the neutron induced fission of $^{235}$U was measured (using white spectrum neutrons from LANSCE) for neutron energies from E$_{n}$ = 3.2 to 50 MeV. In this energy range the average post-neutron total kinetic energy release drops from 167.4 $pm$ 0.7 to 162.1 $pm$ 0.8 MeV, exhibiting a local dip near the second chance fission threshold. The values and the slope of the TKE vs. E$_{n}$ agree with previous measurements but do disagree (in magnitude) with systematics. The variances of the TKE distributions are larger than expected and apart from structure near the second chance fission threshold, are invariant for the neutron energy range from 11 to 50 MeV. We also report the dependence of the total excitation energy in fission, TXE, on neutron energy.
The isospin properties of primary and secondary fragments produced in multifragmentation of Fe + Ni and Fe + Fe systems with respect to Ni + Ni system are analyzed within the statistical multifragmentation model framework. The reduced neutron and proton densities show an asymmetry in the primary fragments, that is lessened after secondary decay. with increasing isospin (N/Z) this effect increases, while the sensitivity of fragment isospin towards excitation energy and N/Z of the primary fragments remains unchanged.
74 - Fei Niu , Chun-Wang Ma 2018
The ratio of pairing-energy coefficient to temperature ($a_{p}/T$) of neutron-rich fragments produced in spallation reactions has been investigated by adopting an isobaric yield ratio method deduced in the framework of a modified Fisher model. A series of spallation reactions, 0.5$A$ and 1$A$ GeV $^{208}$Pb + $p$, 1$A$ GeV $^{238}$U + $p$, 0.5$A$ GeV $^{136}$Xe + $d$, 0.2$A$, 0.5$A$ and 1$A$ GeV $^{136}$Xe + $p$, and $^{56}$Fe + $p$ with incident energy ranging from 0.3$A$ to 1.5$A$ GeV, has been analysed. An obvious odd-even staggering is shown in the fragments with small neutron excess ($Iequiv N - Z$), and in the relatively small-$A$ fragments which have large $I$. The values of $a_{p}/T$ for the fragments, with $I$ from 0 to 36, have been found to be in a range from -4 to 4, and most values of $a_{p}/T$ fall in the range from -1 to 1. It is suggested that a small pairing-energy coefficient should be considered in predicting the cross sections of fragments in spallation reactions. It is also concluded that the method proposed in this article is not good for fragments with $A/A_{s} >$ 85% (where $A_{s}$ is the mass number of the spallation system).
Relations between the total beta+ Gamow-Teller (GT+) strength and the E2 strength are further examined. It is found that in shell-model calculations for N=Z nuclei, in which changes in deformation are induced by varying the single-particle energies, the total GT+ or GT- strength decreases monotonically with increasing values of the B(E2) from the ground state to the first excited J=2+ state. Similar trends are also seen for the double GT transition amplitude (with some exceptions) and for the spin part of the total M1 strength as a function of B(E2).
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