Neutron Capture Cross Section Measurements Research Articles

Nuclear applications of inorganic mass spectrometry

There are several basic characteristics of mass spectrometry that are not always fully appreciated by the science community. These characteristics include the distinction between relative and absolute isotope abundances, and the influence of isotope fractionation on the accuracy of isotopic measurements. These characteristics can be illustrated in the field of nuclear physics with reference to the measurement of nuclear parameters, which involve the use of enriched isotopes, and to test models of s-, r-, and p-process nucleosynthesis. The power of isotope-dilution mass spectrometry (IDMS) to measure trace elements in primitive meteorites to produce accurate Solar System abundances has been essential to the development of nuclear astrophysics. The variety of mass spectrometric instrumentation used to measure the isotopic composition of elements has sometimes been accompanied by a lack of implementation of basic mass spectrometric protocols which are applicable to all instruments. These metrological protocols are especially important in atomic weight determinations, but must also be carefully observed in cases where the anomalies might be very small, such as in studies of the daughter products of extinct radionuclides to decipher events in the early history of the Solar System. There are occasions in which misleading conclusions have been drawn from isotopic data derived from mass spectrometers where such protocols have been ignored. It is important to choose the mass spectrometer instrument most appropriate to the proposed experiment. The importance of the integrative nature of mass spectrometric measurements has been demonstrated by experiments in which long, double beta decay and geochronological decay half-lives have been measured as an alternative to costly radioactive-counting experiments. This characteristic is also illustrated in the measurement of spontaneous fission yields, which have accumulated over long periods of time. Mass spectrometry is also a valuable tool in the determination of neutron capture cross-section measurements and the application of such determinations in Planetary Science.

Production of a 173Lu target for neutron capture cross section measurements

Lutetium has been used as a radiochemistry detector to measure neutron fluence in NTS tests. A measure of the neutron capture cross sections on 173Lu is needed to improve the interpretation value of the Lu radiochemistry isotopic ratios. A natural hafnium target was irradiated with protons to produce neutron poor lutetium radioisotopes. The short lived species were allowed to decay prior to chemical processing resulting in predominantly 173Lu with a small amount of 174Lu. This material was deposited on a titanium foil for use in the neutron capture cross section measurement.

Neutron Capture Cross Sections for the Weak s Process

AbstractIn past decades a lot of progress has been made towards understanding the main s-process component that takes place in thermally pulsing Asymptotic Giant Branch (AGB) stars. During this process about half of the heavy elements, mainly between 90 ≤ A ≤ 209 are synthesized. Improvements were made in stellar modeling as well as in measuring relevant nuclear data for a better description of the main s process. The weak s process, which contributes to the production of lighter nuclei in the mass range 56 ≤ A ≤ 90 operates in massive stars (M ≥ 8 M⊙) and is much less understood. A better characterization of the weak s component would help disentangle the various contributions to element production in this region. For this purpose, a series of measurements of neutron-capture cross sections have been performed on medium-mass nuclei at the 3.7-MV Van de Graaff accelerator at FZK using the activation method. Also, neutron captures on abundant light elements with A < 56 play an important role for s-process nucleosynthesis, since they act as neutron poisons and affect the stellar neutron balance. New results are presented for the (n, γ) cross sections of 41K and 45Sc, and revisions are reported for a number of cross sections based on improved spectroscopic information.

112Cd(n, γ)113mCd反応の実効中性子捕獲断面積測定

112Cd(n, γ)113mCd反応の実効中性子捕獲断面積測定

Neutron capture cross-section measurement for the 186W( n, γ) 187W reaction at 0.0536 eV energy

The thermal neutron-induced activation cross section for the 186W( n, γ) 187W reaction was measured at 0.0536 eV neutron energy using TRIGA Mark-II research reactor, Atomic Energy Research Establishment, Savar, Dhaka, Bangladesh. The 197Au( n, γ) 198Au monitor reaction induced in a high-purity gold foil was used to determine the effective neutron beam intensity. The activities induced in sample and monitor foils were measured nondestructively by a high-resolution HPGe γ-ray detector. The present experimental cross-section value is the first one at 0.0536 eV. The obtained new cross section that amounts to 26.6±1.6 b is 2% higher than the recently reported data in ENDF/B-VII and 5% lower than that of JENDL-3.3.

中性子捕獲断面積高精度化のための核準位構築法の開発２ －1次元法の15N, 27Mgへの適用－

中性子捕獲断面積高精度化のための核準位構築法の開発２ －1次元法の15N, 27Mgへの適用－

S -Process Implications from Osmium Isotope Anomalies in Chondrites

Correlated isotopic anomalies in osmium (186Os, 188Os, 190Os, measured with respect to 189Os) extracted from primitive carbonaceous chondrites tightly constrain the ?n(190Os)/?n(188Os) ratio to be 0.859 ? 0.042 (?5%). A recent measurement of the Maxwellian-averaged neutron-capture cross sections (MACSs) for 186, 187, 188Os lowered the ?n(188Os) by 27% but did not measure ?n(190Os). From the ?n(190Os)/?n(188Os) ratio, we infer ?n(190Os) = 249 ? 18 mbarns for internal consistency with the new MACSs for the other Os isotopes. This approach is applicable to other isotopic anomalies in r-process/s-process ratios derived from meteorites for nuclei that do not have branching points between them. Branching at 185W and 186Re makes the 186Os/188Os ratio a neutron dosimeter for the s-process that, with the new cross sections, yields an average neutron density, nn ~ 3 ? 108 cm-3. This low neutron density is consistent with previous results from Sr, Zr, Mo, and Ba isotopes that indicated a minor contribution from the 22Ne(?, n)25Mg neutron source relative to the 13C(?, n)16O neutron source.

Measurement of Neutron Capture Cross-Sections of 105Pd with keV-Neutrons Produced by Using a Pulsed Proton Beam

Measurement of Neutron Capture Cross-Sections of 105Pd with keV-Neutrons Produced by Using a Pulsed Proton Beam

The use of C 6D 6 detectors for neutron induced capture cross-section measurements in the resonance region

Hydrogen-free deuterated benzene C 6D 6 liquid scintillators are widely used for high resolution neutron capture cross-section measurements at time-of-flight facilities, using the total energy detection principle in combination with the Pulse Height Weighting Technique (PHWT). The quality of the data deduced from such measurements depends on the accuracy of the detector response that is used in the calculation of the weighting function and on the normalization procedure. In addition, for nuclei with small capture to scattering ratios, i.e. light and near neutron magic nuclei, a correction for the sensitivity of the capture detector to the scattered neutrons is required. The MCNP code was used to simulate both the γ-ray and neutron transport in a C 6D 6 detection system including its surroundings. The weighting functions and neutron sensitivity were then deduced from the simulations and validated by experiments. The simulations have also been used to identify the sources of uncertainties in performing capture cross-section measurements with C 6D 6 detectors.

Neutron Capture and Total Cross-Section Measurements and Resonance Parameters of Gadolinium

Neutron capture and transmission measurements were performed by the time-of-flight technique at the Rensselaer Polytechnic Institute linac facility using metallic and liquid Gd samples. The liquid samples were isotopically enriched in either 155Gd or 157Gd. The capture measurements were made at the 25-m flight station with a multiplicity-type capture detector, and the transmission measurements were performed at 15- and 25-m flight stations with 6Li glass scintillation detectors. The multilevel R-matrix Bayesian code SAMMY was used to extract resonance parameters.Among the significant findings are the following. The neutron width of the largest resonance in Gd, at 0.032 eV in 157Gd, has been measured to be (9 ± 1)% smaller than that given in ENDF/B-VI updated through release 8. The thermal (2200 m/s) capture cross section of 157Gd has been measured to be 11% smaller than that calculated from ENDF. The other major thermal resonance, at 0.025 eV in 155Gd, did not display a significant deviation from the thermal capture cross section given by ENDF.In the epithermal region, the analysis provided here represents the most extensive to date. Twenty-eight new resonances are proposed, and other resonances previously identified in the literature have been revisited. The assignment of resonances within regions of complicated structure incorporated the observations of other researchers, particularly on the six occasions where ENDF resonances are recommended to be removed. The poor match of the ENDF parameters to the current data is significant, and substantial improvement to the understanding of gadolinium cross sections is presented, particularly above 180 eV where the ENDF resolved region for 155Gd ends.

Pulse shape analysis of signals from BaF 2 and CeF 3 scintillators for neutron capture experiments

The scope of this work is to study the characteristics of BaF 2 and CeF 3 signals using fast digitizers, which allow the sampling of the signal at very high frequencies and the application of the fitting method for analysis of the recorded pulses. By this procedure particle identification and the reconstruction of pile-up events can be improved, while maintaining the energy and time-of-flight resolution as compared to traditional methods. The reliability of the technique and problems connected with data acquisition are discussed with respect to accurate measurements of neutron capture cross-sections.

High-speed data acquisition system for neutron time-of-flight experiments

A high-speed data acquisition system comprising two parallel metering channels has been developed for neutron time-of-flight measurements. Each metering channel is equipped with a flash-ADC. The first channel is intended for measuring fast neutrons, whose energy ranges from tens of eV up to several keV, and uses a 40 MHz sampling rate. The second channel is intended for measuring slow neutrons, whose energy ranges down to several tenths of eV, and uses a 4 MHz sampling rate. Laboratory tests using the developed system have been performed and the system's efficiency dependence on the incoming counting rate has been obtained. As a result of this new design, the performance of the data acquisition system has increased by about an order of magnitude in comparison with existing systems. The designed data acquisition system is now used for the measurement of neutron capture cross-sections in a wide neutron energy range by a time-of-flight method.

Neutron capture cross section measurements for nuclear astrophysics at CERN n_TOF

A series of neutron capture cross section measurements of interest to nuclear astrophysics have been recently performed at n_TOF, the neutron spallation source operating at CERN. The low repetition frequency of the proton beam driver, the extremely high instantaneous neutron flux, and the low background conditions in the experimental area are optimal for capture cross section measurements on low-mass or radioactive samples. An overview of the measurements performed during the two experimental campaigns in 2002 and 2003 is presented with special emphasis on the measurement of the capture cross sections of the Os isotopes relevant for the cosmochronology based on the Re/Os clock.

Nuclear waste management: Status, prospects and hopes

The Back-n beam line of the China Spallation Neutron Source is the first white neutron facility in China. One of its goals is to perform radiative neutron capture cross-section (σnγ) measurements. To study the experimental backgrounds of this new facility, a measurement of the σnγ of 197Au was carried out with a C6D6 detector system at Back-n. Time-dependent backgrounds induced by the scattered neutrons and in-beam gamma-rays were measured and analyzed with natC and natPb samples. The black resonance filter technique was employed to evaluate the shape of the backgrounds and perform the normalization. Besides, Monte Carlo simulations of these backgrounds were carried out with the Geant4 toolkit. The simulated results were compared with the experimental ones and the good agreement found allowed us to validate the background evaluation method.

A low-mass neutron flux monitor for the n _TOF facility at CERN

A small-mass system has been developed for monitoring the flux of neutrons with energy up to 1 MeV at the new time-of-flight facility at CERN, n_TOF. The monitor is based on a thin Mylar foil with a 6Li deposit, placed in the neutron beam, and an array of Silicon detectors, placed outside the beam, for detecting the products of the 6Li(n, alpha)³H reaction. The small amount of material on the beam ensures a minimal perturbation of the flux and minimizes the background related to scattered neutrons. Moreover, a further reduction of the gamma-ray background has been obtained by constructing the scattering chamber hosting the device in carbon fibre. A detailed description of the flux monitor is here presented, together with the characteristics of the device, in terms of efficiency, resolution and induced background. The use of the monitor in the measurement of neutron capture cross-sections at n_TOF is discussed

Neutron Capture Cross-Section Measurement of 99Tc by Linac Time-of-Flight Method and the Resonance Analysis

The neutron capture cross section of 99Tc has been measured relative to the 10B(n,αγ) standard cross section by the neutron time-of-flight (TOF) method in the energy range of 0.005 eV to 47 keV using a detection assembly of Bi4Ge3O12 scintillators and a 46-MeV electron linear accelerator at the Kyoto University, Research Reactor Institute. The relative measurement has been normalized at 0.0253 eV to the reference value (22.9 ± 1.3 b) measured by Harada et al. The energy-dependent experimental data and the evaluated data in ENDF/B-VI, JENDL-3.2, JENDL-3.3, and JEF-2.2 are in general agreement with the current measurement. In particular, the JENDL-3.3 data, which have been released recently, show better agreement with the measurement in the lower-energy region.The resonance parameters at 5.6 and 20.3 eV have been analyzed by the KALMAN system using the current TOF data. The resonance integral calculated with the parameters obtained is derived to be 330 ± 19 b, which is close to the data obtained from JENDL-3.3 and evaluated by Mughabghab, although the resonance integrals from JENDL-3.2, ENDF/B-VI, and JEF-2.2 are smaller by ~6 to 8% than the current value. The resonance integral data measured by Harada et al. is larger by ~20%.

A low background neutron flux monitor for the n_TOF facility at CERN

A small-mass system has been developed for monitoring the flux of neutrons with energy up to 1 MeV at the new time-of-flight facility at CERN, n_TOF. The monitor is based on a thin Mylar foil with a 6Li deposit, placed in the neutron beam, and an array of silicon detectors, placed outside the beam, for detecting the products of the 6Li( n, α) 3H reaction. The small amount of material on the beam ensures a minimal perturbation of the flux and minimizes the background related to scattered neutrons. Moreover, a further reduction of the γ-ray background has been obtained by constructing the scattering chamber hosting the device in carbon fibre. A detailed description of the flux monitor is here presented, together with the characteristics of the device, in terms of efficiency, resolution and induced background. The use of the monitor in the measurement of neutron capture cross-sections at n_TOF is discussed.

A BGO detector system for studies of neutron capture by radioactive nuclides

A 16-section BGO scintillation detector having a total volume of 8.54 ℓ and an associated 40 MHz flash-ADC-based data taking system were developed. This detector system is intended for measurements of neutron capture cross-sections of radioactive nuclei by the time-of-flight method. The detector response function and efficiency, gamma-ray and neutron energy resolution, and backgrounds were evaluated experimentally and by calculation. Neutron capture cross-section measurements with 10 B , 197 Au and 237 Np samples were carried out in the energy range of 1– 1000 eV to demonstrate the performance of the system.

Combining Pulsed Neutron Spectroscopy and LWD Resistivity for Analysis Behind Casing in Small Holes

Summary Formation analysis from behind casing is gaining more relevance in assessing reservoirs, not only in aging fields but also in new wells completed either without the full complement of openhole petrophysical measurements or with openhole measurements of inadequate quality. Before this technology was available, formation evaluation in freshly drilled holes was dependent on the availability of openhole log measurements. This paper discusses field examples in which reservoirs were evaluated without a full suite of openhole log measurements. Examples from two sidetracks in a mature North Sea field supported by water injection are presented. The small hole size limited openhole data acquisition to resistivity and gamma ray (GR) measurements, which were acquired using logging-while-drilling (LWD) technology. In both cases, pulsed neutron (PN) measurements were recorded through casing to augment the existing data. Porosity and formation sigma (macroscopic thermal neutron capture cross section) measurements were obtained. Fluid saturations were computed using this porosity and the LWD resistivity. Saturations also were computed from the carbon/oxygen measurement, and these agreed with the resistivity-based answers. Lithology analysis was performed using elemental spectral capture yields and was reliable and consistent fieldwide. The results show that, in a mature oil field, PN measurements can provide a complete lithology and saturation analysis in circumstances in which insufficient openhole petrophysical measurements are available.

Measurements of (n,γ) neutron capture cross-sections with liquid scintillator detectors

A method for measuring (n,γ) neutron capture cross-sections using liquid scintillator detectors has been investigated. If the response function of the detector is known, and the efficiency as a function of energy is low and approximately constant, then gamma cascades can be counted via a method that is independent of the cascade path provided the detector response is manipulated to allow detection efficiency to be proportional to emitted gamma-ray energy. In this paper, we demonstrate the measurement of efficiency and response functions for a C 6D 6 liquid scintillator using gamma-ray sources and (p,γ) reactions on light nuclei. Methods to reproduce the detector response and efficiency data successfully using simulations are presented and discussed. An entire response matrix for the detector has been constructed using a new interpolation technique, allowing weighting functions that force the detector efficiency to be proportional to gamma-ray energy to be calculated. An analysis of the sources of error involved in making (n,γ) cross-section measurements with this method has been undertaken using Monte-Carlo simulation techniques.