Geel Linear Accelerator Research Articles

A new measurement on 56Fe(n,inl) using GAINS@GELINA

The extended dataset of 56Fe(n,n’γ) cross sections measured by our group more than a decade ago at GELINA (Geel Linear Accelerator) was used in many recent evaluations like ENDF, JEFF and CIELO. Despite the special measures we took to ensure reliability and accuracy, concerns were raised by various groups with regard to several features of this dataset (absolute normalization and/or shape) and therefore the 56Fe(n,inl) cross section is still under the evaluation by the International Nuclear Data Evaluation Network (INDEN). Consequently, a new experiment is now under preparation aiming to take advantage of the numerous experimental improvements of the GAINS (Gamma Array for Inelastic Neutron Scattering) setup implemented over the years. While γ spectroscopy combined with the time-of-flight method will remain the main technique involved, several other experimental details will differ substantially.

140,142Ce Neutron Cross Section Resolved Resonance Region Evaluation

A resolved resonance region evaluation of 140,142Ce was conducted by Oak Ridge National Laboratory. Requested by the US Nuclear Criticality Safety Program, this evaluation is based on recent high-resolution transmission and capture high-resolution measurements of natCe and 142Ce conducted at JRC-Geel at the Geel Linear Accelerator facility. It is also based on recently measured thermal constants available from the EXFOR database [1]. Starting from the resonance parameters from the ENDF/B-VIII.0 library [2] and following a preliminary R-matrix analysis [3], an updated set of resonance parameters and corresponding covariance information was derived by the fit of these experimental datasets using the Reich–Moore approximation of the R-matrix theory, as implemented in the SAMMY code system [4]. The resolved resonance region upper energy limit for 140Ce was kept at 200 keV, whereas the 142Ce resonance region was extended from 13 to 26 keV. This new evaluation was found to be in good agreement not only with several integral quantities of interest to the reactor physics community, but also with the stellar Maxwellian-averaged cross section [5].

(n, xn) cross sections on56,57Fe

The Gamma Array for Inelastic Neutron Scattering (GAINS) operated at the Geel Linear Accelerator (GELINA) neutron source was used to measure ( n ,x n ) cross sections on 56,57 Fe reaching a level of uncertainty of the order of 5%. Serious difficulties arise in case of the 57 Fe isotope from the fact that the first excited level has an energy of only 14 keV and its decay could not be observed. Therefore a delicate combination of experimental and theoretical approaches have to be used to infer the inelastic cross section. The particularities of the two measurements are presented concentrating on the approaches allowing us to overcome specific problems in each case.

Neutron inelastic cross-section measurements forMg24

The $\ensuremath{\gamma}$ production cross sections from neutron inelastic scattering on ${}^{24}\mathrm{Mg}$ were measured for neutron energies up to 18 MeV at GELINA (the Geel Linear Accelerator), the neutron source operated by EC-JRC-IRMM, Belgium. The level cross section and the total inelastic cross section were determined. We used the GAINS (Gamma Array for Inelastic Neutron Scattering) spectrometer with seven large-volume high-purity germanium (HPGe) detectors placed at ${110}^{\ensuremath{\circ}}$ and ${150}^{\ensuremath{\circ}}$ with respect to the beam direction. The neutron flux was determined with a $^{235}\mathrm{U}$ fission chamber. The results are compared with calculations performed with the talys 1.6 code using the default settings.

Cross-section measurements for theFe56(n,xnγ) reactions

A measurement was performed at the white neutron source Geel Linear Accelerator (GELINA) of the Institute for Reference Materials and Measurements to determine the cross sections for the ($n$, ${n}^{\ensuremath{'}}\ensuremath{\gamma}$) and ($n$, $2{n}^{\ensuremath{'}}\ensuremath{\gamma}$) reactions on $^{56}\mathrm{Fe}$. The Gamma Array for Inelastic Neutron Scattering (GAINS) was used. The results are scaled to the neutron-induced fission cross section of ${}^{235}\mathrm{U}$. The paper emphasizes the multiple checks performed to assure the consistency of the results. $\ensuremath{\gamma}$ production cross sections, total inelastic cross sections, and level production cross sections were determined. A good agreement exists with previous measurements. Theoretical calculations were performed with the talys 1.6 reaction code using default parameters, but also using a microscopic approach. These calculations are discussed in comparison to the experimental data.

The Limits of the GAINS Spectrometer

The Gamma Array for Neutron Inelastic Scattering (GAINS) is currently operating at the GELINA (Geel Linear Accelerator) neutron source producing highly precise neutron inelastic scattering data. A series of recent investigations explored the limits of the experimental setup and technique. Measurements on 12C and 57Fe attempted the determination of very high and very low energy gamma production cross sections, respectively. An extended analysis was directed towards the generation of covariance matrices specifically for this experimental approach.

Evaluation of Tungsten Neutron Cross Sections in the Resolved Resonance Region

We generated a preliminary set of resonance parameters for 182,183,184,186W in the neutron energy range of thermal up to several keV. The evaluation methodology uses the Reich-Moore approximation to fit with the R-matrix code SAMMY, the high-resolution measurements performed in 2010 and 2012 at the Geel linear accelerator facility. For 183W, the transmission data and capture cross sections calculated with the set of resonance parameters are compared with the experimental values, and some of the average properties of the resonance parameters are discussed. In the analyzed energy range, this work almost doubles the existing resolved resonance evaluations in the ENDF/B-VII.1 library. A preliminary analysis of the performance of the calculated cross sections based on Lead slowing-down benchmarks is presented and briefly discussed.

Development of an experimental set-up for the measurement of neutron-induced fission and capture cross sections of highly radioactive fissile nuclei

The measurement of neutron-capture cross sections of many actinides is complicated by the difficulty in separating capture γ-rays from the large fission-fragment prompt γ-ray background. For example, current estimates of the capture cross section of 233U show large discrepancies, with differences of more than 20%. To improve the accuracy of data, a new experimental set-up for the simultaneous measurement of the neutron-induced capture and fission cross sections was designed, assembled and optimized. The measurements will be performed at the GEel LINear Accelerator (GELINA) neutron time-of-flight facility in Belgium, where neutron cross sections can be measured over a wide energy range with high energy resolution. The fission detector consists of a dedicated multi-plate high-efficiency ionization chamber (IC). The γ-rays produced in capture reaction are detected by an array of C6D6 scintillators. Fission γ–rays are distinguished from capture γ–rays by the anticoincidence signals from the IC and the C6D6 detectors. For the undetected fission events a correction has to be applied based on the efficiency of the IC that should be high and known with a high accuracy. The performance of the IC during dedicated test experiments is presented, focusing on the determination of the detection efficiency.

Development of an experimental set-up for the measurement of neutron-induced fission and capture cross sections of radioactive fissile nuclei

A new experimental set-up for a simultaneous measurement of neutron induced capture and fission cross sections was designed, assembled and optimized. The measurements will be performed at GEel LINear Accelerator (GELINA) neutron time- of-flight facility in Belgium, where neutron cross sections can be measured over a wide energy range with high energy resolution. The fission events detector consists of a dedicated multi-plate high efficiency fission ionization chamber (IC). The -rays produced in capture reaction are detected by an efficient array of C6D6 scintillators. Fission -rays events are distinguished from capture events by the anticoincidence signals from the IC and the C6D6 detectors. For the undetected fission events a correction has to be applied with respect to the efficiency of the IC that should be high and known with a high precision. Another important issue is the good separation between fission-fragment (FF) and the high alpha pile-up. The performances of the IC during test experiments are presented, focusing in particular on the detection efficiency.

Neutron Total and Capture Cross Section of Tungsten Isotopes

A new set of measurements for the total and capture cross section determination of W isotopes was done using GELINA (GEel LINear Accelerator), a neutron Time-Of-Flight facility at the Institute for Reference Materials and Measurements (IRMM). Measuring stations at different flight path lengths were used in order to cover a broad neutron energy range with high resolution demands. Experimental techniques adopted for both transmission and capture measurements are well established using a (6)Li glass detector and C(6)D(6) scintillation arrays as detections systems respectively., As target samples highly enriched (182,183,184,186)W metallic discs were used.

The gamma efficiency of the GAINS spectrometer

The GAINS (Germanium Array for Inelastic Neutron Scattering) setup was recently installed at the GELINA (Geel Linear Accelerator) neutron source of EC-JRC-IRMM. In order to produce reliable neutron inelastic cross-section data the setup requires a precise efficiency calibration for each sample under investigation. These samples represent extended sources in non-standard orientations and with significant self-attenuation of gamma rays. We present in detail the efficiency calibration procedure performed with two Monte Carlo simulations using MCNP and GEANT4. A good agreement is found between the two independent approaches. The general features of the simulations are discussed together with an in-depth description of the various factors influencing the efficiency.

R-Matrix Analysis of 238U High-Resolution Neutron Transmissions and Capture Cross Sections in the Energy Range 0 to 20 keV

The neutron resonance parameters of 238U were obtained from a SAMMY analysis of high-resolution neutron transmission measurements and high-resolution capture cross-section measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA) in the years 1970–1990 and from more recent transmission and capture cross-section measurements performed at the Geel Linear Accelerator. Compared with previous evaluations, the energy range for this resonance analysis was extended from 10 to 20 keV, taking advantage of the high resolution of the most recent ORELA transmission measurements. The experimental database and the method of analysis are described in this paper. The neutron transmissions and the capture cross sections calculated with the resonance parameters are compared with the experimental data. A description is given of the statistical properties of the resonance parameters and of the recommended values of the average parameters. The new evaluation results in a slight decrease of the effective capture resonance integral and improves the prediction of integral thermal benchmarks by 70 to 200 pcm.

R-Matrix Analysis of 232Th Neutron Transmissions and Capture Cross Sections in the Energy Range Thermal to 4 keV

Neutron resonance parameters of 232Th were obtained from the Reich-Moore SAMMY analysis of high-resolution neutron transmission measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA) by Olsen in 1981, along with the high-resolution neutron capture measurements performed in 2005 at the Geel Linear Accelerator (GELINA, Belgium) by Schillebeeckx and at the n-TOF facility (CERN, Switzerland) by Aerts. The ORELA data were analyzed previously by Olsen with the Breit-Wigner multilevel code SIOB, and the results were used in the ENDF/B-VI evaluation. In the new analysis of the Olsen neutron transmissions by the modern computer code SAMMY, better accuracy is obtained for the resonance parameters by including in the experimental database the recent experimental neutron capture data. The experimental database and the method of analysis are described in the report. The neutron transmissions and the capture cross sections calculated with the resonance parameters are compared to the experimental values. A description is given of the statistical properties of the resonance parameters. The new evaluation results in a decrease in the capture resonance integral.

The 10B(n,α0)/10B(n,α1γ) Branching Ratio

The 10B(n,α0)/10B(n,α1γ) branching ratio has been measured at the Geel linear accelerator based time-of-flight spectrometer in the incident neutron energy range from 0.1 keV up to 2 MeV. A twin Frisch-grid ionization chamber has been used with two very thin 10B samples mounted back-to-back on the common cathode. This type of ionization chamber made it possible to measure both the energy and the angular distribution of the emitted reaction products (alpha particles and 7Li nuclei) with a clear separation of both reaction channels: emission to the ground state (α0) and first excited state (α1). The branching ratio 10B(n,α0)/10B(n,α1γ) was found to be in good agreement with the ENDF/B-VI evaluation up to ˜1 MeV incident neutron energy. At higher energies (>1 MeV), a clear deviation is observed. The present branching ratio data have been entered into the ongoing International Atomic Energy Agency Coordinated Research Project on “Improvement of the Standard Cross Sections for Light Elements.” A preliminary R-matrix calculation reproduces the measured branching ratio in the whole energy range up to ˜2 MeV.

Resistive Plate Chambers with Gd-coated electrodes as thermal neutron detectors

Resistive Plate Chambers (RPCs) are wide spread, cheap, easy-to-build and large size detectors, used mainly to reveal ionising particles in high-energy physics experiments. Here a technique, consisting in coating the inner surface of the bakelite electrodes with a mixture of linseed oil and Gd2O3 is reported. This allows to make RPCs sensitive also to thermal neutrons, making them suitable to be employed for industrial, medical or de-mining applications. Thermal neutron-sensitive RPCs can be operated at atmospheric pressure, are lightweighted, have low γ-ray sensitivity and are easy to handle even when large areas have to be covered. This paper reports the results of the first test of this detector, performed at the Geel Linear Accelerator (GELINA) in Belgium.

Reinvestigation of 85Kr Branching in the Light of New 82, 84, 86Kr Neutron Capture Cross Section Measurements

High resolution neutron capture cross section measurements of 3 stable krypton isotopes, 82Kr, 84Kr and 86Kr, have been performed at the Geel Linear Accelerator (GELINA) on both resolved and unresolved resonance region. In addition, a total cross section measurement on 84Kr has also been performed in the energy range from 1 eV up to 120 kev. Accurate values of the Maxwellian–averaged capture cross section have been derived for all isotopes for stellar temperatures ranging from kT=5 up to 100 keV. A re–investigation of the s–process in the mass region around the prominent branching at 85Kr has been undertaken in the light of our new results.

Resonance neutron capture in 60Ni below 450 keV

High-resolution neutron capture cross-section measurements on 60Ni have been performed at the Geel Linear Accelerator in the energy range from 1 to 450 keV. An experimentally determined weighting function, obtained by a total energy detection set-up, has been applied to the measured capture spectra. The parameters of 275 resonances have been determined in a recent reanalysis using the FANAC R-matrix shape fitting code. Accurate values of the maxwellian-averaged capture cross section for stellar temperatures ranging from kT=5 to 100 keV, corresponding to different scenarios of s-process stellar nucleosynthesis, have been calculated. The distributions of partial radiative widths for s- and p-wave resonances have been derived. A correlation of 0.64 between capture and reduced neutron widths is compatible with the presence of nonstatistical effects in the capture of 60Ni.

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.