Measurement Of Fission Cross Section Research Articles

Fission transient times from fission probabilities of neighboring isotopes

Abstract We present a new and straightforward method to estimate the fission transient time by utilizing the cumulative fission probabilities of neighboring isotopes. The fission probabilities were determined as the ratio of the measured fission cross sections to the Bass Model fusion cross sections. For five neighboring 185–189 Os compound nuclei produced in 3 He/ 4 He-induced reactions on separated isotope W targets, the transient time τ D is estimated to be −21 seconds for excitation energies less than 150 MeV, and the most likely value of τ D is ≈10×10 −21 seconds.

New High-Resolution Fission Cross-Section Measurements of 233U in the 0.4-eV to 700-keV Energy Range

In support of the U.S. Department of Energy need for new data for criticality safety applications, new high-resolution fission cross-section measurements of 233U have been made at the Oak Ridge Electron Linear Accelerator. The measurements were carried out over a 0.4-eV to 700-keV energy range at the 80-m flight station using a fission chamber. Corrections were made for experimental effects to obtain the average fission cross section in this energy range. Results are compared to previous measurements.

Measurements of Neutron-induced Fission Cross Section of Protactinium-231 from 0.1 eV to 10 keV with Lead Slowing-down Spectrometer and at 0.0253 eV with Thermal Neutron Facility

(1999). Measurements of Neutron-induced Fission Cross Section of Protactinium-231 from 0.1 eV to 10 keV with Lead Slowing-down Spectrometer and at 0.0253 eV with Thermal Neutron Facility. Journal of Nuclear Science and Technology: Vol. 36, No. 6, pp. 549-551.

Measurements of Neutron-induced Fission Cross Section of Americium-243 from Thermal Neutron Energy to 15 keV Using Lead Slowing-down Spectrometer and Thermal Neutron Facility

The neutron-induced fission cross section of 243Am was measured relative to that of 235U from thermal neutron energy to 15 keV making use of a double fission chamber with 243Am and 235U electrodeposited layers.The data above 0.05 eV were measured using a lead slowing-down spectrometer coupled to an electron linear accelerator. A relative measurement to the 10B(n, α) reaction was also made using a BF3 counter at energies below 1 keV, and normalized to the absolute value obtained by using the cross section of the 235U(n,/) reaction between 200eV and IkeV. The existing experimental data by Wisshak and Käppeler, and by Knitter and Budtz-Jørgensen are in general agreement with the current measurement in the relevant energy region. However, the data by Seeger are considerably higher. The evaluated nuclear data in JENDL-3.2, ENDF/B-VI and JEF-2.2, whose data were broadened by the energy resolution function of the spectrometer, have been compared with the measured result.The fission cross section for thermal neutrons was measured in a neutron field with a pure Maxwellian spectrum. The derived result at 0.0253 eV is 81.3±2.5 mb. The data by Wagemans et al. and in ENDF/B-VI are close to the current measurement. However, the values by Asghar et al., Gavrilov et al. and Mughabghab, and the evaluated data in JENDL-3.2 and JEF-2.2 are discrepant from the current result by a factor of 1.4 to 2.4.

Measurements of Neutron-induced Fission Cross Section of Americium-243 from Thermal Neutron Energy to 15 keV Using Lead Slowing-down Spectrometer and Thermal Neutron Facility.

The neutron-induced fission cross section of 243Am was measured relative to that of 235U from thermal neutron energy to 15 keV making use of a double fission chamber with 243Am and 235U electrodeposited layers. The data above 0.05 eV were measured using a lead slowing-down spectrometer coupled to an electron linear accelerator. A relative measurement to the 10B(n, α) reaction was also made using a BF3 counter at energies below 1 keV, and normalized to the absolute value obtained by using the cross section of the 235U(n,/) reaction between 200eV and IkeV. The existing experimental data by Wisshak and Käppeler, and by Knitter and Budtz-Jørgensen are in general agreement with the current measurement in the relevant energy region. However, the data by Seeger are considerably higher. The evaluated nuclear data in JENDL-3.2, ENDF/B-VI and JEF-2.2, whose data were broadened by the energy resolution function of the spectrometer, have been compared with the measured result. The fission cross section for thermal neutrons was measured in a neutron field with a pure Maxwellian spectrum. The derived result at 0.0253 eV is 81.3±2.5 mb. The data by Wagemans et al. and in ENDF/B-VI are close to the current measurement. However, the values by Asghar et al., Gavrilov et al. and Mughabghab, and the evaluated data in JENDL-3.2 and JEF-2.2 are discrepant from the current result by a factor of 1.4 to 2.4.

Measurements of Neutron-induced Fission Cross Section of Protactinium-231 from 0.1eV to 10keV with Lead Slowing-down Spectrometer and at 0.0253eV with Thermal Neutron Facility.

Measurements of Neutron-induced Fission Cross Section of Protactinium-231 from 0.1eV to 10keV with Lead Slowing-down Spectrometer and at 0.0253eV with Thermal Neutron Facility.

Measurement of fast neutron induced fission cross section of thorium using Lexan plastic track detector

Fission-track registration characteristics of Lexan solid state nuclear track detectors have been used to measure the fast neutron induced fission cross section of232Th. The fast neutrons (≅14.2MeV) were produced with the help of an AN-400 model Van-de-Graaff accelerator at Banaras Hindu University laboratory using3H(2H,n)4He reaction and were used to irradiate the fissile target deposited on the plastic detector. The track densityT, registered on the plastic detector is related to the fission cross sectionσ f, through the relationT=knσ fot wheren is the number of fissile atoms per cm2 in the deposit, o is the neutron flux,k is fission track registration efficiency andt is the time of irradiation. The fission cross sectionσ f of232Th, relative to the well measured fission cross section of238U, was found to be 0.36±0.04 barn.

R-Matrix Analysis of the 240Pu Neutron Cross Sections in the Thermal to 5700-eV Energy Range

A Bayesian R-matrix-fitting code called SAMMY is used in a new analysis of the 240Pu neutron cross sections in the 0.02- to 5700-eV energy range. This work aims to resolve the discrepancies revealed by the JEF2 validation studies. A set of experimental data, suitable for the analysis, is determined from preliminary SAMMY analyses of the available experimental data treated individually. Finally, the sequential analysis of the selected transmission, total, and fission cross-section measurements gives an accurate set of resonance parameters. For the 1.056-eV resonance, the analysis of a recent transmission measurement gives parameters very close to those adopted in ENDF/B-VI. In the energy range above 200 eV, the average capture cross section calculated from the present evaluation is in agreement with ENDF/B-VI but is 25% lower than the values of JEF-2 and JENDL-3. A rigorous study of the statistical properties of the resonance parameters is done during this work. Although 158 new resonances are identified by the careful examination of the experimental fission cross-section and transmission data, the value of the average spacing above 2750 eV (D0 = 16.10 eV) shows a 25% loss of resonances compared with the lower energy region (D0 = 12.06 eV). Finally, for the s-wave resonance parameters, this work recommends the following average values: resonance spacing, D0 = 12.06 ± 0.60 eV; strength function, S0 = (1.032 ± 0.071) × 10−4 eV; and capture width, .

Nuclear fission of neutron-deficient protactinium nuclides

Fragment velocity, kinetic energy, mass yield, and element yield distributions in the fission of neutron-deficient Pa isotopes produced in the reactions of ${}^{16}$O and ${}^{18}$O on ${}^{209}$Bi have been measured at incident beam energies near and above the Coulomb barriers by the time-of-flight and radiochemical methods. An asymmetric mass-division component has been observed. Measured fission cross sections were compared with the results of statistical model calculations which take into account two fission barrier heights for symmetric and asymmetric yields. The fission barrier height deduced for the asymmetric fission is found slightly lower than that for the symmetric one. The difference between the two barrier heights in the fission of the present protactinium nuclides $(N\ensuremath{\sim}135)$ is considerably smaller than that in the neutron-rich nuclide of ${}^{233}$Pa $(N\ensuremath{\sim}142)$, indicating that the difference sensitively depends on the neutron number of the fissioning nuclide.

Fission Cross-Section Measurements of 241Am between 0.1 eV and 10 keV with Lead Slowing-Down Spectrometer and at Thermal Neutron Energy

Making use of back-to-back type double fission chambers and a lead slowing-down spectrometer coupled to an electron linear accelerator, the cross section for the 241Am(n,f) reaction has been measured relative to that for the 235U(n,f) reaction in the energy range from 0.1 eV to 10 keV. To avoid the interference between the 241 Am and the 235U resonances, the fission cross section below 1 keV was measured relative to the 10B(n, α) reaction with a BF3 counter, and the result obtained was normalized to the absolute value by the 235U reference data between 200 eV and 1 keV The measured result has been compared with (a) the evaluated nuclear data contained in the ENDF/B-VI and JENDL-3.2 libraries and (b) the existing experimental data, with the evaluated and measured data being broadened using the energy resolution function of the spectrometer.There is general agreement between the evaluated data and this measurement, although some discrepancies are found in the energy region where the cross-section shapes show a pronounced structure. The JENDL-3.2 data are underestimated by a factor in the range 1.2 to 2.3 between 22 and 140 eV, while the more recently measured data by Dabbs, Johnson, and Bemis and the evaluated data in ENDF/B-VI are in good agreement with the measurement within the uncertainties. In the energy range from 1 to 10 keV, the current result is 15 to 18% higher than the evaluations and the data of Dabbs, Johnson, and Bemis. Some of the earlier experimental data that were measured over part of the relevant energy region are not always in agreement with the current measurement. The fission cross section for thermal neutrons was also measured in a pure Maxwellian neutron spectrum field with double fission chambers. The derived result at 0.0253 eV is 3.15 ± 0.097 b, which is obtained relative to the reference value of 586.2 b for the 235U(n,f) reaction. The ENDF/B-VI data are in good agreement with the current measurement, while the JENDL-3.2 value is lower by 4.2%. The ratios of the earlier experimental data to the current value are distributed between 0.89 and 1.02.

Barriers for pion-induced fission near A = 208

Measured fission cross sections for π+ and π− beams of 150 MeV on isotopically separated targets near A=208 show a smooth dependence of the fission probability on the target mass, without any indication of the increased fission barriers expected for fission at low excitation near doubly magic 208Pb. Extraction of fission barriers from these data is attempted for a range of possible excitation energies, finding that these barriers are near 8 MeV near A=208, without much sensitivity to the assumed excitation energy.

Fission Cross-Section Measurements of the Odd-Odd Isotopes 232Pa, 238Np, and 236Np

Transmutation of actinide waste into fission products could be enhanced by using resonance fission of odd-odd target materials; those of interest are 232Pa, 238Np, and 242Am. Fission cross-section measurements of two of these short-lived materials were performed at Los Alamos National Laboratory. Samples were produced by the (d,2n) reaction in the Los Alamos Ion Beam Facility followed by fast radiochemistry to separate the odd-odd target of interest. The fission cross section of the nanogram samples was measured in a high intensity pulsed neutron beam produced by 800-MeV proton spallation. Using this procedure, the fission cross sections of the 1.3-day 232Pa and 2.1-day 238Np were successfully measured in the energy range from 0.01 eV to 50 keV. The fission cross section of the relatively long-life isotope 236Np was also measured in the same system while the short half-life isotopes were being prepared. The results and resonance analysis are presented.

Relative and absolute neutron-induced fission cross sections of 208Pb, 209Bi, and 238U in the intermediate energy region.

Measurements of neutron-induced fission cross sections for $^{208}\mathrm{Pb}$, $^{209}\mathrm{Bi}$, and $^{238}\mathrm{U}$ have been performed in the 70--160 MeV energy region at the neutron beam facility at the The Svedberg Laboratory in Uppsala using the $^{7}\mathrm{Li}$(p,n) reaction as neutron source. The fission fragments were detected by thin-film breakdown counters. The neutron flux was measured relative to the n-p scattering cross section with a proton-recoil spectrometer. The results are compared with calculations and with earlier reported experimental data. \textcopyright{} 1996 The American Physical Society.

Electromagnetic fission of238U at 600 and 1000 MeV per nucleon

Electromagnetic fission of238U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsacker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.

Electromagnetic and nuclear fission of238U in the reaction of 100, 500, and 1000 A�MeV208Pb with238U

The folding- and azimuthal-angle and velocity distributions for the238U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV208Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the208Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (≈1.5 b) is approximately constant between 100 and 1000 A·MeV.

Normalization of the239Pu Fission Cross Section

Measurements have been performed at the Geel Linear Accelerator from 0.01 eV up to 1,000 eV in order to investigate the normalization of [sup 239]Pu fission cross-section measurements. Two different experiments were performed using surface barrier detectors and a double ionization chamber, respectively. In both cases, the [sup 10]B(n, [alpha]) reaction was used as a flux monitor. The results indicate that the Weston and Todd data should be renormalized by [approximately]3%, resulting in a satisfactory agreement with ENDF/B-VI.

Normalization and Minimum Values of the239Pu Fission Cross Section

Measurements were carried out to accurately determine the shape of the fission cross section of [sup 239]Pu down to 0.025 eV in order to determine a more accurate normalization for previously reported fission cross-section measurements from Oak Ridge National Laboratory at higher neutron energies. Also, experimental backgrounds were carefully studied to verify the cross section between resonances. Results indicate a 3.0% higher normalization should have been used previously, and the low cross sections between resonances reported earlier were correct. New differential cross-section data were obtained from 0.002 to 100 eV.

Measurement of Fission Cross Section of Neptunium-237 in Resonance Region with Electron Linac—Driven Lead Spectrometer

Measurement of Fission Cross Section of Neptunium-237 in Resonance Region with Electron Linac—Driven Lead Spectrometer

R-Matrix Analysis of239Pu Neutron Transmissions and Fission Cross Sections in Energy Range from 1.0 keV to 2.5 keV

Abstract The results of the 239Pu high resolution neutron transmission measurements of Harvey et al. and of the 239Pu high resolution fission cross section measurements of Weston & Todd performed at the Oak Ridge Electron Linear Accelerator (ORELA) were analysed in the energy range from 1 keV to 2. 5 keV by the Bayesian code SAMMY using the Reich-Moore approximation of the R-matrix theory. The results obtained in a previous analysis in the energy range from thermal to 1 keV were updated by taking into account the recent renormalization of the experimental fission data by Weston & Todd. The statistical properties of the parameters of the resonances identified in the energy range from thermal to 2. 5 keV were examined and improved values of the average parameters were obtained. The resonance parameters are given in an ENDF-6 format file available from JAERI Nuclear Data Center and from NEA Data Bank (OECD).

Determination of extra-push energies for fusion from differential fission cross-section measurements.

Apparent discrepancies between values of extra-push energies for fusion of two heavy nuclei derived through measurements of fusion evaporation residue cross sections and of differential fission cross sections have been reported by Keller et al. We show here that with the inclusion of the recently proposed preequilibrium fission decay channel in the analysis, there is no inconsistency between the two sets of data in terms of the deduced extra-push energies.