A TEM study of the stability of intermetallic precipitates in Zircaloy nuclear reactor components

Transmission electron microscopy (TEM) has been used to characterise the radiation-induced changes in Zr(Cr,Fe)₂ intermetallic precipitates present in Zircaloy-2 and -4 nuclear reactor components. The results show that the precipitates become completely amorphous at low fluences (<1 × 10²⁴ n·m⁻²) during low temperature neutron irradiation (about 330 K) with no associated chemical composition change. At higher temperatures (about 573 K), a duplex amorphous-crystalline structure is produced. The precipitates retain a crystalline core surrounded by a peripheral amorphous layer that advances inwards with increasing fluence. The amorphous outer layer is coincident with a depletion of Fe that is dispersed into the surrounding hcp -phase matrix. Subsequent post-irradiation heat-treatment below the amorphous phase recrystallisation temperature results in the back-diffusion of Fe into the precipitates.     

New low-energy measurement of the 18F(p,α)15O reaction

The ¹⁸F, produced during nova explosions, is the main responsible for the 511 keV γ-ray emitted during the outburst that could be detected with satellite missions or future γ-ray telescopes. In spite of many experimental efforts, the amount of ¹⁸F synthesized still suffers from large uncertainties concerning mainly the ¹⁸F(p,α)¹⁵O reaction. We report here on a new direct measurement of the ¹⁸F(p,α)¹⁵O cross section recently carried out at the RIB facility at Louvain-la-Neuve. Cross section data down to a center-of-mass energy of 400 keV have been obtained using an isobarically pure ¹⁸F beam (averaged intensity of the order of 10⁶ pps) and a thin polyethylene target. The reaction products were detected using the multi-strip detector array LEDA. The cross section data are analysed in the framework of the R-matrix model. Our main aim is to determine the interference sign between three 3/2⁺ resonances above threshold that can significantly affect the extrapolation of the ¹⁸F(p,α)¹⁵O cross section to the energies relevant for novae, and thus, the modeling of nova explosions.     

Pulsed optical parametric oscillators with intracavity optical parametric amplification: a critical study

It is known that the idler conversion efficiency of optical parametric oscillators (OPOs) can be increased by adding a second nonlinear crystal in the cavity. This crystal is pumped by the signal and acts as an optical parametric amplifier (OPA) for the idler. However, this technique unavoidably increases the oscillation threshold because of additional losses and increased build-up time due to cavity lengthening. In this paper, we investigate both theoretically and experimentally the benefits and drawbacks of this so called OPO–OPA configuration versus the singly resonant OPO (SRO) configuration. Calculations are found to be in agreement with an experimental study of a SRO and an OPO–OPA operating near 3.4 μm both pumped by a 90-mJ 27-ns Nd:YAG laser. Our study reveals that the OPO–OPA needs to be driven at least two times above threshold to produce more idler energy than the SRO. In addition, near 3 μm the OPO–OPA is particularly efficient given that the difference frequency wave generated in the second crystal is also output coupled. PACS 42.65.Yj; 42.65.Sf     

Specific architectures for optical parametric oscillators

Optical parametric oscillators are coherent light sources showing properties that are not encountered with usual laser sources. This article deals with optical architectures that take advantage of the specific properties of the three-wave mixing parametric interaction. We show that optical cavities can be specifically designed to increase either the optical conversion of the parametric process or to reduce dramatically the emitted line width. To cite this article: A. Berrou et al., C. R. Physique 8 (2007).     

Emittance measurements of a laser-wakefield-accelerated electron beam

The transverse emittance of a relativistic electron beam generated by the interaction of a high-intensity laser with an underdense plasma has been measured with the "pepper-pot" method. For parameters pertaining to the forced laser wakefield regime, we have measured an emittance as low as (2.7+/-0.9) pi mm mrad for (55+/-2) MeV electrons. These measurements are consistent with 3D particle-in-cell simulations of the experiment, which additionally show the existence of a relatively large halo around the beam core.     

Fragmentation of 95 MeV/u 12C and 75 MeV/u 13C. Application to secondary-beam production

The fragmentation of 95 MeV/u ¹²C and 75 MeV/u ¹³C projectiles, interacting with natural targets of different elements (⁹Be, Cu and ¹⁹⁷Au for ¹²C, and ⁹Be, Ni, and ¹⁸¹Ta for ¹³C) and of various thicknesses has been investigated at GANIL, using the doubly achromatic magnetic spectrometer LISE. The projectile-like fragments transmitted at 0 have been analysed using a silicon -E telescope and the -time-of-flight method. The results obtained with the thinnest targets are discussed in the scope of radioactive nuclear beam production. The fragment momentum distributions can be fitted by Gaussian peaks associated with exponential tails towards low momentum values. The experimental production yields are compared with the results of the simulation code LISE. It appears that this code reproduces satisfactorily the yields of nuclei close to the stability line, but strongly overestimates the production of very neutron-deficient nuclei.Received: 17 September 2002, Revised: 25 July 2003, Published online: 18 December 2003PACS: 25.70.Mn Projectile and target fragmentation - 29.90. + r Other topics in elementary-particle and nuclear physics experimental methods and instrumentation     

NMR spectroscopy of proteins encapsulated in a positively charged surfactant

Traditionally, large proteins, aggregation-prone proteins, and membrane proteins have been difficult to examine by modern multinuclear and multidimensional solution NMR spectroscopy. A major limitation presented by these protein systems is that their slow molecular reorientation compromises many aspects of the more powerful solution NMR methods. Several approaches have emerged to deal with the various spectroscopic difficulties arising from slow molecular reorientation. One of these takes the approach of actively seeking to increase the effective rate of molecular reorientation by encapsulating the protein of interest within the protective shell of a reverse micelle and dissolving the resulting particle in a low viscosity fluid. Since the encapsulation is largely driven by electrostatic interactions, the preparation of samples of acidic proteins suitable for NMR spectroscopy has been problematic owing to the paucity of suitable cationic surfactants. Here, it is shown that the cationic surfactant CTAB may be used to prepare samples of encapsulated anionic proteins dissolved in low viscosity solvents. In a more subtle application, it is further shown that this surfactant can be employed to encapsulate a highly basic protein, which is completely denatured upon encapsulation using an anionic surfactant.     

Mott transition, antiferromagnetism, and unconventional superconductivity in layered organic superconductors

The phase diagram of the organic superconductor kappa-(ET)2Cu[N(CN)2]Cl has been accurately measured from 1H NMR and ac susceptibility techniques under helium gas pressure. The domains of stability of antiferromagnetic and superconducting orders in the pressure vs temperature plane have been determined. Both phases overlap through a first-order boundary that separates two regions of inhomogeneous phase coexistence. The boundary curve merges with the first-order line of the metal-insulator transition which ends with a critical point at higher temperature. The whole phase diagram features a point-like region where metallic, insulating, antiferromagnetic, and non-s-wave superconducting phases all meet.     

Homomorphic cameo filter for pattern recognition that is invariant with changes in illumination

We introduce a new matched filter method that yields correlation peaks that are invariant with changes in the illumination of any targets. A scene containing objects at unknown locations is first subjected to a logarithmic transformation that changes the multiplicative constant to an additive background, which is then discriminated against by means of a composite filter containing a cameo of the true target and a term with which to discriminate against other false targets. Experimental results are shown.