Neutron cross sections of 239Pu in the energy range 0.01–6 MeV

doi:10.1016/S0306-4549(96)00104-1

Annals of Nuclear Energy

Volume 24, Issue 14, September 1997, Pages 1127-1140

https://doi.org/10.1016/S0306-4549(96)00104-1 Get rights and content

Abstract

Complete, selfconsistent calculations of neutron cross sections for ²³⁹Pu in the energy range 0.01–6 MeV are presented. The direct calculations are made within the coupled channel method using a deformed optical potential defined by regional parameters. For the compound nucleus calculation a new HRTW version of the statistical model extended by us to describe fission at subbarrier energies is used. The parameters adopted for the fundamental fission barrier and the excitation energies at the saddle points of the discrete transition states are deduced from analyses of subbarrier fission probabilities. To describe the continuous part of the transition state spectrum analytical expressions have been established. The calculated neutron cross sections are in good agreement with the experimental data.

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Cited by (11)

Nuclear Data Sheets for A = 239
2014, Nuclear Data Sheets
Spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented here for all nuclei with mass number A=239.
In general, a relatively small amount of new data on this mass chain has been reported since the previous evaluation in 2003. However, special evaluations, such as “Database of prompt gamma rays from slow neutron–capture from elemental analysis” (2007ChZX), have provided additional precise data for levels in ²³⁹U. Also, new Coulomb excitation measurements in ²³⁹Pu have extended the knowledge of the 1/2[631] rotational band up to $J π = (55 / 2 +)$ , and that of the octupole vibrational band up to $J π = (53 / 2 -)$ .
For historical knowledge it is worth mentioning the report on the “Discovery of isotopes of the transuranium elements with 93 <= Z <= 98” (2013Fr02), where the information for elements Np, Pu, Am, and Cf with mass number A=239 is presented.
The alpha hindrance factors (HF) presented in this evaluation were calculated using values of the radius parameter (r₀) interpolated from those for even–even adjacent nuclei given by 1998Ak04.
Prompt fission neutron emission calculations and description of sub-barrier fission cross section resonances for 234U(n, f)
2013, Nuclear Physics A
Citation Excerpt :
This fact confirms the findings already observed and discussed in the case of another fertile nucleus 238U [8]. In the pre-scission stage the strong sub-barrier resonances below 1 MeV in the experimental fission cross section of 234U are described by nuclear reaction model calculations including the refined statistical model for fission with sub-barrier effects of Vladuca et al. (for details see Refs. [8–16]). The dependencies on En of prompt emission quantities such as the prompt neutron multiplicity as a function of fragment mass and the average multiplicity as a function of TKE are determined, too.
Prompt emission quantities of ²³⁴U( $n, f$ ) (average quantities as a function of fragment mass, average quantities as a function of total kinetic energy (TKE) and total average quantities) have been calculated within the Point-by-Point model. The recently measured fission fragment distributions at incident energies (En) ranging from 0.2 MeV to 5 MeV have been used.
The insensibility of the fragment pair multiplicity $ν_{pair}$ to the partition of total excitation energy (TXE) between complementary fully accelerated fission fragments has been confirmed. This gives confidence in the predicted total average prompt emission data as a function of En and of average quantities as a function of TKE at different incident energies of ²³⁴U( $n, f$ ).
The systematic behavior of experimental ratios $ν_{H} / ν_{pair}$ as a function of heavy fragment mass number $A_{H}$ together with the TXE partition based on modeling at scission allow the parameterization of the excitation energy ratio $E_{H}^{⁎} / TXE$ . This opens up the possibility to provide $ν (A)$ of ²³⁴U in the absence of any experimental data.
For the first time the calculation of $〈 ν 〉 (TKE)$ at many En revealed two interesting aspects: the slope $d TKE / d ν$ does not vary with En and the flattening of $〈 ν 〉$ at low TKE values is more pronounced at low incident energies.
The correlation between the sub-barrier resonant behavior of the fission cross section of fertile actinides (characterizing the pre-scission stage) and the visible fluctuations of their fission fragment and prompt neutron emission data (characterizing the post-scission stage), already discussed in the case of ²³⁸U( $n, f$ ) is outlined and quantitatively supported by ²³⁴U( $n, f$ ), too.
The pronounced vibrational resonances in the experimental fission cross section of ²³⁴U placed below 1 MeV can be described by the statistical model for reaction cross section calculation including the optical model for fission.
Properties of the reaction 238U(n, f) at the vibrational resonances
2009, Nuclear Physics A
Recent fission cross-section calculations for the reaction ²³⁸U $(n, f)$ , based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by $Δ A = 1.5$ and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy $E_{n} = 0.9 MeV$ . This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on ²³⁸U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from $E_{n} = 2.0 MeV$ down to 0.9 MeV, where the fission characteristics at the vibrational resonance at $E_{n} = 0.9 MeV$ were investigated for the first time. The previously reported distinct structure in the angular anisotropy around $E_{n} = 1.2$ and 1.6 MeV was observed at $E_{n} = 0.9 MeV$ as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at $E_{n} = 1.8 MeV$ . At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.
Inner barrier shape symmetries in 237Np neutron data evaluation up to 40 MeV incident energy
2006, Nuclear Physics A
The ²³⁷Np neutron induced cross-section evaluation up to 40 MeV incident energy is made in consistent way, based on models with all parameters in agreement with experimental data, theoretical and statistical considerations. The resulted cross-sections describe very well the existing experimental data without any re-normalization and adjustments. The shape symmetry change at the inner fission barrier for the Np isotopes is determined on the basis of an original systematic, the fission level density and the fission barrier parameters being obtained according with these symmetries. The prompt fission neutron multiplicity and spectra of n + ²³⁷Np are calculated with a refined model that takes into account the entire fission fragment range with a point-by-point treatment. In the incident energy range where multiple fission chances are involved the average values of the model parameters are determined by point-by-point treatment. The prompt fission neutron multiplicity and spectra, as well as the average prompt gamma-ray energy are obtained in very good agreement with the existing experimental data. The systematic behaviours of the model parameters as function of quantities in connection with the respective Np fissioning nuclei are also obtained in the frame of this work.
Nuclear data sheets for A = 235, 239
2003, Nuclear Data Sheets
The evaluator presents in this publication spectroscopic data and level schemes from radioactive decay and nuclear reactions studies for all nuclei with mass numbers A=235,239.
A relatively small amount of new data on the level structure of nuclei with A=235,239 has been reported since the previous evaluation of this mass chain. The data presented here are mostly that from 1993Sc22 and 1992Sc18 but updated and modified as needed.
A new isotope of americium, ²³⁵Am (9.9 min), which was chemically and mass separated, was identified using a He-jet recoil transport technique (1996Gu11,1997Sh21,1999Ya13), and has been included in this evaluation.
A new isotope of protactinium, ²³⁹Pa (1.8 hr.) has been included in this evaluation. This nuclide was produced by bombarding uranium with ¹⁸O projectiles; it was chemically separated and its genetic relationship to its daughter nuclide ²³⁹U was well established (1995Yu01,1996Yu06).
The alpha hindrance factors (HF) presented in this evaluation were calculated using values of the radius parameter (r₀) interpolated from those for even-even adjacent nuclei given by 1998Ak04.
Fission cross-section evaluations in the frame of the multi-modal fission model for 237Np(n, f)
2002, Nuclear Physics A
The statistical model for fission cross-section evaluation is extended by including the concept of multi-modality of the fission process. The three most dominant fission modes, the two asymmetric standard I (S1) and standard II (S2) modes and the symmetric superlong (SL) mode are taken into account. Based on calculations of the nuclear surface in the frame of the multi-modal random neck-rupture model, separate outer fission barriers are considered for each mode, while the inner barriers and isomeric wells are assumed to be the same. Deconvoluted fission cross sections for S1, S2 and SL modes for ²³⁷Np(n, f), based on experimental branching ratios, were calculated for the first time in the incident energy range 0.01 to 5.5 MeV providing good agreement with the experimental fission cross-section data.
Including the concept of multi-modal fission into nuclear reaction models could lead to a better understanding and provides a more consistent description of the fission process.

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Annals of Nuclear Energy

Abstract

References (36)

Ann. Nucl. Energy

Comp. Phys. Com.

Phys. Rev. C

Phys. Rev. C

Specifications for an international code intercomparison for fission cross section calculations

NEA

Results of an international code intercomparison for fission cross section calculations

NEA/P&T

Can. J. Phys.

Nucl. Sci. Eng.

Phys. Rev.

Ann. Phys. (NY)

Z. Phys. A

Statistical properties of excited atomic nuclei

Izv. Akad. Nauk. SSSR. Ser. Fiz.

Yad. Fiz.

Phys. Rev. C

JAERI-M 6996

Phys. Rev. C

Results of coupled channels calculations for neutron cross sections of a set of actinide nuclei

NEANDC 228

Cited by (11)

Nuclear Data Sheets for A = 239

Prompt fission neutron emission calculations and description of sub-barrier fission cross section resonances for <sup>234</sup>U(n, f)

Properties of the reaction <sup>238</sup>U(n, f) at the vibrational resonances

Inner barrier shape symmetries in <sup>237</sup>Np neutron data evaluation up to 40 MeV incident energy

Nuclear data sheets for A = 235, 239

Fission cross-section evaluations in the frame of the multi-modal fission model for <sup>237</sup>Np(n, f)