RESUMO
In 2002, an innovative neutron time-of-flight facility started operation at CERN: n_TOF. The main characteristics that make the new facility unique are the high instantaneous neutron flux, high resolution and wide energy range. Combined with state-of-the-art detectors and data acquisition system, these features have allowed to collect high accuracy neutron cross-section data on a variety of isotopes, many of which radioactive, of interest for Nuclear Astrophysics and for applications to advanced reactor technologies. A review of the most important results on capture and fission reactions obtained so far at n_TOF is presented, together with plans for new measurements related to nuclear industry.
Assuntos
Terapia por Captura de Nêutron/instrumentação , Terapia por Captura de Nêutron/métodos , Reatores Nucleares , Desenho de Equipamento , Análise de Falha de Equipamento , Nêutrons , Reprodutibilidade dos Testes , Sensibilidade e EspecificidadeRESUMO
By means of neutron interferometry the s-wave neutron scattering length of the (3)He nucleus was re-measured at the Institut Laue-Langevin (ILL). Using a skew symmetrical perfect crystal Si-interferometer and a linear twin chamber cell, false phase shifts due to sample misalignment were reduced to a negligible level. Simulation calculations suggest an asymmetrically alternating measuring sequence in order to compensate for systematic errors caused by thermal phase drifts. There is evidence in the experiment's data that this procedure is indeed effective. The neutron refractive index in terms of Sears' exact expression for the scattering amplitude has been analyzed in order to evaluate the measured phase shifts. The result of our measurement, b' c = (6.000 ± 0.009) fm, shows a deviation towards a greater value compared to the presently accepted value of b' c = (5.74 ± 0.07) fm, confirming the observation of the partner experiment at NIST. On the other hand, the results of both precision measurements at NIST and ILL exhibit a serious 12σ (12 standard uncertainties) deviation, the reason for which is not clear yet.
RESUMO
The151Sm(n,gamma)152Sm cross section has been measured at the spallation neutron facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The new facility combines excellent resolution in neutron time-of-flight, low repetition rates, and an unsurpassed instantaneous luminosity, resulting in rather favorable signal/background ratios. The 151Sm cross section is of importance for characterizing neutron capture nucleosynthesis in asymptotic giant branch stars. At a thermal energy of kT=30 keV the Maxwellian averaged cross section of this unstable isotope (t(1/2)=93 yr) was determined to be 3100+/-160 mb, significantly larger than theoretical predictions.
RESUMO
Off-diagonal geometric phases acquired by an evolution of a 1/2-spin system have been observed by means of a polarized neutron interferometer. We have successfully measured the off-diagonal phase for noncyclic evolutions even when the diagonal geometric phase is undefined. Our data confirm theoretical predictions and the results illustrate the significance of the off-diagonal phase.
