Use of ultracold neutrons for condensed-matter studies

Ultracold neutrons have such low velocities that they are reflected by most materials at all incident angles and can be stored in material bottles for long periods of time during which their intrinsic properties can be studied in great detail. These features have been mainly used for fundamental-physics studies including the detection of a possible neutron electric dipole moment and the precise determination of neutron-decay properties. Ultracold neutrons can also play a role in condensed-matter studies with the help of high-resolution spectrometers that use gravity as a strongly dispersive medium for low-velocity neutrons. Such studies have so far been limited by the low intensity of existing ultracold-neutron sources but could be reconsidered with more intense sources, which are now envisaged. This report provides a broad survey of the properties of ultracold neutrons (including their reflectivity by different types of samples), of ultracold-neutron spectrometers that are compared with other high-resolution instruments, of results obtained in the field of condensed matter with these instruments, and of neutron microscopes. All these subjects are illustrated by numerous examples

High-resolution neutron-induced [gamma]-ray production cross sections for oxygen and beryllium for neutron energies from 4 to 200 MeV.

Measurements of discrete gamma-ray spectra from neutron-induced reactions on a Be0 sample have been performed using the broad-spectrum neutron spallation source of the WNR facility at LANSCE. Incident neutron energies were determined by the time-of-flight technique. Two high-purity Ge detectors were used to detect gamma rays in the energy range from 0.150 to 9.0 MeV. Spectra were measured at seven angles. Absolute gamma-ray production cross sections were delermined from the measured neutron fluence and detector efficiencies. A new evaluation of the reaction cross sections has been performed based on this data. GNASH model calculations were performed for 20 < E, < 150 MeV, and are compared with the data. KEY WOlWS: neutron-induced reactions, gamma-ray cross section, measurement, evaluation, oxygen, beryllium, germanium detecto

Characteristics of a Lead Slowing-Down Spectrometer Coupled to the LANSCE Accelerator

Abstract A description is given of a lead slowing-down spectrometer (LSDS) installed at the 800-MeV proton accelerator of the Los Alamos Neutron Science Center (LANSCE). The LSDS is designed to study neutron-induced fission on actinides that can only be obtained or used in small quantities. The characteristics of this LSDS (energy-time relation, energy resolution, neutron flux) are presented through simulations with MCNPX and measurements with several different methods. Results on neutron-induced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, additional MCNPX calculations have been performed to simulate the measurement of the cross-section for 235m Uðn; fÞ using different target quantities and different initial isomer-to-ground state compositions

First Measurements with a Lead Slowing-Down Spectrometer at LANSCE

Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described

First Measurements with a Lead Slowing-Down Spectrometer at LANSCE

Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described

First Measurements with a Lead Slowing-Down Spectrometer at LANSCE

Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described

Spallation-neutron sources

Of particular interest for neutron-physics studies are spallation-neutron sources (SNSs) using intense proton beams with energies in the GeV range. Some SNSs already provide average fluxes of thermal and cold neutrons comparable with those of high-flux reactors. Most SNSs are pulsed with high peak fluxes that can be used with the powerful time-of-flight (TOF) method. Also, SNSs could be developed to much higher performance

LA FISSION

Dans cet exposé, nous rappelons d'abord quelques généralités sur le phénomène de fission qui se déroule en trois phases : oscillations du noyau, franchissement du point-seuil, scission. Chaque phase est décrite en s'appuyant soit sur des calculs théoriques déduits du modèle de la goutte liquide, soit sur des résultats expérimentaux. L'effet des couches magiques est montré et discuté. Ensuite, nous traitons plus en détail la fission induite par neutrons en procédant d'abord à un rapide examen des sections efficaces de fission et en les expliquant par l'effet d'appariement des neutrons et par l'introduction du concept de voie de sortie du phénomène de fission. Une attention particulière est accordée à la fission induite par neutrons lents (ou neutrons de résonances) qui permettent d'exciter le noyau composé à une énergie voisine de la barrière de fission. Ces résonances sont des états purs de spin et de parité et se prêtent bien à la vérification de la théorie des états de transition de A. Bohr. Dans cette optique, nous examinons les grandeurs physiques suivantes : largeurs de fission (valeur moyenne et fluctuations), distribution en masse et énergie cinétique des fragments. Deux noyaux sont traités en détail, pour lesquels nous avons le plus d'information : 235U et 239Pu. Enfin nous donnons un aperçu du phénomène des quasi-résonances et de leur importance dans les sections efficaces des principaux noyaux fissiles.In this paper, some general properties of the fission process are reviewed by first examining the three phases into which it develops : oscillations of the nucleus, passing through the saddle point, scission. Each of these steps is described together with some theoretical calculations derived from the liquid drop model and with some experimental results. The effect of closed shells is shown and discussed. Then, fission induced by neutrons is treated in more detail, first by a brief examination of fission cross sections which can be explained by the pairing of neutrons and by the introduction of the concept of exit channels in fission. Special attention is then given to fission induced by slow neutrons (or resonance neutrons) which can excite the compound nucleus at an energy close to the fission barrier. Such resonances have purity of spin and parity and therefore, are well suited for the verification of A. Bohr's transition state fission theory. Having this theory in mind, the following parameters are examined : fission widths (average value and fluctuations), mass distribution and kinetic energy of the fission fragments. Two nuclei are studied in detail, for which we have the most complete information : U235 and Pu239. Finally, a glimpse is given on the quasi-resonance phenomenon and its importance in the fission cross sections of the main fissile elements

Détermination des fonctions densités du rhodium 103 pour des neutrons « s » et « p »

From the Rh103 total neutron cross section, measured up to 800 eV, we have determined values of " s " and " p " wave neutron strength functions : S0 = (0.46 ± 0.05) × 10-4 S1 = 5.5 × 10-4 Mean spacing of " s " resonances is 35 eV and about 16 eV for " p " resonances. The nuclear radius is : R = (6.3 ± 0.2) 10-13 cm.Nous déterminons, d'après des mesures de la section efficace neutronique totale de 103Rh jusqu'à 800 eV, les valeurs des fonctions densité neutronique « s » et « p ». S0 = (0,46 ± 0,05) × 10-4 S1 = 5,5 × 10-4. L'espacement moyen des résonances « s » est de 35 eV, celui des résonances « p » d'environ 16 eV. Le rayon du noyau est : R = (6,3 ± 0,2) × 10-13 cm

Contribution à la détermination des paramètres des résonances neutroniques de l'uranium 235

The 235U fission cross-section, measured by time of flight and already published, has been shape-analysed by the least-squares methods assuming that the resonances are of the Breit-Wigner one-level type broadened by Doppler effect. σof and Γ values are given for several resonances. In addition, the transmission of 235U has been measured between 8 and 22 eVwith a resolution of 0,01 μs/m. The complete set of parameters (spin excepted) is thus obtained for five resonances. A large dispersion of Γγ values is observed.La section efficace de fission de 235U mesurée par temps de vol et déjà publiée a fait l'objet d'une analyse de forme, par la méthode des moindres carrés, en admettant que les résonances sont du type Breit-Wigner à 1 niveau élargies par effet Doppler. Des valeurs de σof et Γ sont ainsi calculées pour plusieurs résonances. En complément la transmission de 235U a été mesurée par temps de vol entre 8 et 22 eV, avec une résolution de 0,01 μs/cm. On obtient ainsi la série complète de paramètres (spin excepté) pour cinq résonances. Une grande dispersion dans les valeurs de Γγ est observée