F. Gunsing

Pulse shape analysis of liquid scintillators for neutron studies

The acquisition of signals from liquid scintillators with Flash ADC of high sampling rate ð 1G S=sÞ has been investigated. The possibility to record the signal waveform is of great advantage in studies with g’s and neutrons in a high count-rate environment, as it allows to easily identify and separate pile-up events. The shapes of pulses produced by g-rays and neutrons have been studied for two different liquid scintillators, NE213 and C6D6: A 1-parameter fitting procedure is proposed, which allows to extract information on the particle type and energy. The performance of this method in terms of energy resolution and n=g discrimination is analyzed, together with the capability to identify and resolve pile-up events. r 2002 Elsevier Science B.V. All rights reserved.

Development and performance of Microbulk Micromegas detectors

A new Micromegas manufacturing technique, based on kapton etching technology, has been developed recently, resulting in further improvement of the characteristics of the detector, such as uniformity and stability. Excellent energy resolution has been obtained, reaching 11% FWHM for the 5.9 keV photon peak of the 55Fe X-ray source and 1.8% FWHM (with possible evidence of less than 1%) for the 5.5 MeV alpha peak of the 241Am source. The new Microbulk detector shows several advantages like flexible structure, low material and high radio-purity, opening thus new possibilities for both accelerator and low counting-rate experiments. The detector has already been used in CAST and n-TOF, while it is being tested for future neutrinoless double-beta decay experiments like NEXT. Details of the production of several types of Microbulk detectors will be described. First benchmark results will be presented, demonstrating the enhanced performance of Microbulk detectors.

Incineration of 241Am induced by thermal neutrons

Abstract An experimental study of the 241Am incineration in a high-intensity thermal neutron flux was carried out at the high-flux reactor of the Institut Laue-Langevin in Grenoble. The combination of nuclear γ -ray spectroscopy and off-line mass spectrometry methods made possible the measurement of several parameters of the transmutation chain and the first experimental determination of the unknown 242gsAm thermal neutron capture cross section, which plays an essential role in the 241Am incineration process. During a 19 days irradiation in a thermal neutron flux of 5.6×10 14 n /( s cm 2 ) , (46±5) % of the initial 241Am was transmuted by neutron capture of which (22±8) % was incinerated by nuclear fission. A value of the thermal neutron cross section of 242gsAm( n ,γ ) of (330±50) barns was obtained. We show that this keeps the option open to incinerate 241Am by high-intensity moderated neutron fluxes.

Observation of Large Scissors Resonance Strength in Actinides

The orbital M1 scissors resonance has been measured for the first time in the quasicontinuum of actinides. Particle-γ coincidences are recorded with deuteron and (3)He-induced reactions on (232)Th. The residual nuclei (231,232,233)Th and (232,233) Pa show an unexpectedly strong integrated strength of B(M1)=11-15μ(n)(2) in the E(γ)=1.0-3.5 MeV region. The increased γ-decay probability in actinides due to scissors resonance is important for cross-section calculations for future fuel cycles of fast nuclear reactors and may also have an impact on stellar nucleosynthesis.

Data reduction and uncertainty propagation of time-of-flight spectra with AGS

Results of neutron time-of-flight measurements are commonly used to parameterize neutron induced reaction cross sections in the resonance region based on the R-matrix reaction theory. Reaction yields or transmission as well as their covariance information are derived starting from measured counting spectra. They are then used in a least squares adjustment for obtaining model parameters. In this paper, a compact formalism is presented to propagate both the correlated and uncorrelated uncertainty components. Full information on the origin of each correlated component of the covariance matrix is maintained. This is particularly important in order to avoid a bias on the model parameters through a phenomenon known as Peelles Pertinent Puzzle (PPP). This compact formalism was implemented into the data reduction code AGS (Analysis of Geel Spectra).

Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis.

The 63Ni(n,γ) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from   kT=5-100  keV with uncertainties around 20%. Stellar model calculations for a 25M⊙ star show that the new data have a significant effect on the s-process production of 63Cu, 64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

GEANT4 simulation of the neutron background of the C6D6 set-up for capture studies at n_TOF

The neutron sensitivity of the C6D6 detector setup used at n TOF for capture measurements has been studied by means of detailed GEANT4 simulations. A realistic software replica of the entire n TOF experimental hall, including the neutron beam line, sample, detector supports and the walls of the experimental area has been implemented in the simulations. The simulations have been analyzed in the same manner as experimental data, in particular by applying the Pulse Height Weighting Technique. The simulations have been validated against a measurement of the neutron background performed with a nat C sample, showing an excellent agreement above 1 keV. At lower energies, an additional component in the measured nat C yield has been discovered, which prevents the use of nat C data for neutron background estimates at neutron energies below a few hundred eV. The origin and time structure of the neutron background have been derived from the simulations. Examples of the neutron background for two di erent samples are demonstrating the important role of accurate simulations of the neutron background in capture cross section measurements.

Constant-temperature level densities in the quasicontinuum of Th and U isotopes

Particle-gamma coincidences have been measured to obtain gamma-ray spectra as a function of excitation energy for 231-233Th and 237-239U. The level densities, which were extracted using the Oslo method, show a constant temperature behavior. The isotopes display very similar temperatures in the quasi-continuum, however, the even-odd isotopes reveal a constant entropy increase Delta S compared to their even-even neighbors. The entropy excess depends on available orbitals for the last unpaired valence neutron of the heated nuclear system. Also, experimental microcanonical temperature and heat capacity have been extracted. Several poles in the heat capacity curve support the idea that an almost continuous melting of Cooper pairs is responsible for the constant-temperature behavior.

Assessment and Propagation of the 237Np Nuclear Data Uncertainties in Integral Calculations by Monte Carlo Techniques

Abstract A new method to produce covariance or dispersion matrices for the resonance parameters of neutron cross sections was developed. The technique uses resonance shape analysis in association with Monte Carlo treatment of the uncertainties. The method was implemented in the error propagation tool MCFIT. This program provides a user-friendly textual interface for the shape analysis code REFIT. It was designed to take into account the main sources of uncertainties involved in time-of-flight measurements. Its capability is illustrated with the simultaneous analysis of 237Np capture and transmission data. The covariance matrix obtained in this work was used to interpret oscillation measurements of 237Np samples carried out in the Minerve reactor located at Cadarache.

Accuracy of the Pulse Height Weighting Technique for Capture Cross Section Measurements

The accuracy of the pulse height weighting technique for the measurement of neutron capture (n,γ) cross sections is investigated. Detailed Monte Carlo simulations of capture measurements are compared to experimental data. Several causes of systematic deviation are investigated and their effect is quantified.