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Measuring the 235 U α values at a thermal point
... New measurements of TNC, which can be used in the standards evaluation, have been available since Axton's evaluation. The most important one is the microscopic measurement of the ratio of capture to fission cross section (alpha) for 235 U at 0.0253 eV by Adamchuk et al. [17] with 2.1% uncertainty; additionally, there are three measurements of ratio 233 U, 239 Pu and 241 Pu to 235 U at 0.0253 eV done at LANL and CERN n TOF with estimated uncertainties between 2 and 3%. ...
... Another potential problem in Axton fit is the absence of capture cross section data or measured alpha value at 0.0253 eV for fissile targets. Adamchuk et al. [17] measured an alpha value of 0.1690 ± 0.0035 at 0.0253 eV using a neutron beam with a mechanical selector (chopper), but this set was published in 1988 well after the Axton report [10]. However, Axton used data from postirradiation experiments (PIE) by Lounsbury [18] (one year irradiation of samples) and later re-analyzed using Monte Carlo simulations by Beer et al [19]. ...
... However, Axton used data from postirradiation experiments (PIE) by Lounsbury [18] (one year irradiation of samples) and later re-analyzed using Monte Carlo simulations by Beer et al [19]. It is remarkable that Beer et al. 235 U alpha value of 0.1697 ± 0.0029 is in good agreement with Adamchuk et al. data of Ref. [17]. Such agreement is surprising considering that g-factors for fission and capture cross sections evaluated by Westcott [20] are not currently considered to be very accurate despite the very low quoted uncertainties at the time (0.1% for 235 U fission and 0.3% for 239 Pu fission). ...
An IAEA project to update the Neutron Standards is near completion. Traditionally, the Thermal Neutron Constants (TNC) evaluated data by Axton for thermal-neutron scattering, capture and fission on four fissile nuclei and the total nu-bar of 252Cf(sf) are used as input in the combined least-square fit with neutron cross section standards. The evaluation by Axton (1986) was based on a least-square fit of both thermal-spectrum averaged cross sections (Maxwellian data) and microscopic cross sections at 2200 m/s. There is a second Axton evaluation based exclusively on measured microscopic cross sections at 2200 m/s (excluding Maxwellian data). Both evaluations disagree within quoted uncertainties for fission and capture cross sections and total multiplicities of uranium isotopes. There are two factors, which may lead to such difference: Westcott g-factors with estimated 0.2% uncertainties used in the Axton's fit, and deviation of the thermal spectra from Maxwellian shape. To exclude or mitigate the impact of these factors, a new combined GMA fit of standards was undertaken with Axton's TNC evaluation based on 2200 m/s data used as a prior. New microscopic data at the thermal point, available since 1986, were added to the combined fit. Additionally, an independent evaluation of TNC was undertaken using CONRAD code. Both GMA and CONRAD results are consistent within quoted uncertainties. New evaluation shows a small increase of fission and capture thermal cross sections, and a corresponding decrease in evaluated thermal nubar for uranium isotopes and 239Pu.
... There are different experimental techniques to tackle this problem. One of them is the γ -ray multiplicity methodology developed in the 1980s [16,17], recently applied by Danon et al. [8]. Another is the fission tagging technique [18], used by Jandel et al. [7] and in this work. ...
... 17. Average 235 U capture (x = γ ) and fission (x = f ) cross sections σ x = E 2 E 1 σ x (E )dE/(E 2 − E 1 ) obtainedin this work together with those in the JEFF-3.3, ...
We measured the neutron capture-to-fission cross-section ratio (α ratio) and the capture cross section of U235 between 0.2 and 200 eV at the n_TOF facility at CERN. The simultaneous measurement of neutron-induced capture and fission rates was performed by means of the n_TOF BaF2 Total Absorption Calorimeter (TAC), used for detection of γ rays, in combination with a set of micromegas detectors used as fission tagging detectors. The energy dependence of the capture cross section was obtained with help of the Li6(n,t) standard reaction determining the n_TOF neutron fluence; the well-known integral of the U235(n,f) cross section between 7.8 and 11 eV was then used for its absolute normalization. The α ratio, obtained with slightly higher statistical fluctuations, was determined directly, without need for any reference cross section. To perform the analysis of this measurement we developed a new methodology to correct the experimentally observed effect that the probabilities of detecting a fission reaction in the TAC and the micromegas detectors are not independent. The results of this work have been used in a new evaluation of U235 performed within the scope of the Collaborative International Evaluated Library Organisation (CIELO) Project, and are consistent with the ENDF/B-VIII.0 and JEFF-3.3 capture cross sections below 4 eV and above 100 eV. However, the measured capture cross section is on average 10% larger between 4 and 100 eV.
... This experiment had an important impact by lowering the uncertainties for capture for 233 U, 235 U and 239 Pu. The agreement of Beer's estimated 235 U thermal α value of 0.1697 ± 0.0029 with the only existing microscopic measurement of 0.1690 ± 0.0035 by Adamchuk [79] is excellent, which inspires confidence in using the unique Beer α data for 233 U and 239 Pu fissile targets. ...
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... This experiment had an important impact by lowering the uncertainties for capture for 233 U, 235 U and 239 Pu. The agreement of Beer's estimated 235 U thermal α value of 0.1697 ± 0.0029 with the only existing microscopic measurement of 0.1690 ± 0.0035 by Adamchuk [79] is excellent, which inspires confidence in using the unique Beer α data for 233 U and 239 Pu fissile targets. ...
With the need for improving existing nuclear data evaluations, (e.g., ENDF/B-VIII.0 and JEFF-3.3 releases) the first step was to evaluate the standards for use in such a library. This new standards evaluation made use of improved experimental data and some developments in the methodology of analysis and evaluation. In addition to the work on the traditional standards, this work produced the extension of some energy ranges and includes new reactions that are called reference cross sections. Since the effort extends beyond the traditional standards, it is called the neutron data standards evaluation. This international effort has produced new evaluations of the following cross section standards: the H(n,n), ⁶Li(n,t), ¹⁰B(n,α), ¹⁰B(n,α1γ), natC(n,n), Au(n,γ), ²³⁵U(n,f) and ²³⁸U(n,f). Also in the evaluation process the ²³⁸U(n,γ) and ²³⁹Pu(n,f) cross sections that are not standards were evaluated. Evaluations were also obtained for data that are not traditional standards: the Maxwellian spectrum averaged cross section for the Au(n,γ) cross section at 30 keV; reference cross sections for prompt γ-ray production in fast neutron-induced reactions; reference cross sections for very high energy fission cross sections; the ²⁵²Cf spontaneous fission neutron spectrum and the ²³⁵U prompt fission neutron spectrum induced by thermal incident neutrons; and the thermal neutron constants. The data and covariance matrices of the uncertainties were obtained directly from the evaluation procedure.
The present state of knowledge concerning the cross sections, etc., of the isotopes ²³âµU, ²³â¸U, ²³â¹Pu, ²â´Â°Pu, and ²â´Â¹Pu is discussed. Most cross sections important to the design of U--Pu-fueled light-water reactors with current fuel--water ratios are known with reasonable accuracy. The three most serious discrepancies remaining are the following: the difference between integral measurements and differential calculations of ²³â¸U resonance capture, the discrepancy between irradiation experiments and other measurements of the ratio of capture to fission in ²³âµU, and the uncertainty in the parameters of the 1 eV resonance of ²â´Â°Pu. 2 figures, 2 tables. (RWR)
The multiplicity spectrometry technique of secondary emission of nuclei excited by thermal neutrons for the neutron cross sections measurements is developed. A multisectional NaI(Tl) scintillation detector for gamma-ray detection with high efficiency and 4pi-geometry is used. The experiments are performed on a stationary 2 MW research reactor by the time-of-flight method. Such a method and apparatus are also to be used to measure the important neutron constant ``alpha'' (capture-to-fission ratio) for 235U.
Remarks on the 2200 m/sec and 20 ~ Maxwellian neutron data for 23~U, =35U, and 241Pu
- H Lemmel
H. Lemmel, "Remarks on the 2200 m/sec and 20 ~ Maxwellian neutron data for 23~U, =35U, and 241Pu," in: Proc. Int. Specialists Symp. on Neutron Standards and Applications, Gaithersburg, Maryland, USA, 1977, NBS-493 (1977), pp. 170-173.
Measuring the value of ? for235U resonances
- Yu V Adamchuk
- M A Voskanyan
- G V Muradyan
- Yu. V. Adamchuk
Yu. V. Adamchuk, M. A. Voskanyan, G. V. Muradyan, et al., "Heasuring the value of for 23SU resonances," At. Energ., 57, No. 4, 251-257 (1984).
World Request List for Nuclear Data, INDC (SEC)-88
- Wrenda
An analysis of experiments on critical assemblies to verify constants for thermal reactor calculations
- M S Yudkevich
- M. S. Yudkevich
Measuring Neutron Cross Sections and the Quantity ? with High Precision Using the Spectrometry Multiplicity Method
- G V Muradyan
- Yu V Adamchuk
- Yu G Shchepkin
- M A Voskanyan
Measuring α values for239Pu resonances,” ibid
- V Yu
- M A Adamchuk
- G V Voskanyan
- Yu G Muradyan
- Shchepkin