We have completed a new set of total cross section measurements of 31 elements and isotopes spanning the periodic table from Aϭ1 to 238. We employed the same technique as in Finley et al. ͓Phys. Rev. C 47, 237 ͑1993͔͒ with refinements intended to allow measurements on separated isotopes and improved systematic error control. The goal of the new measurement was 1% statistical accuracy in 1% energy bins with systematic errors less than 1%. This was achieved for all but the thinnest samples. Stringent checks of systematic errors in this measurement resulted in a reassignment of systematic uncertainties to the neutron total cross sections reported in Finley et al. Microscopic optical model calculations were carried out to interpret the results of the experiment. Two specific types of optical models were employed. The Jeukenne-Lejeune-Mahaux model was used in the range of 5-160 MeV, and a model based on the empirical effective interaction of Kelly was used from 135 to 650 MeV. These models are shown to be useful for predicting both neutron total cross sections and proton reaction cross sections. They are particularly important for light nuclei, for which standard global phenomenological parametrizations of the optical potential are insufficiently accurate.

New high-precision measurements of the neutron total cross section for hydrogen and the deuteriumhydrogen cross section difference were performed for neutron energies between 7 and 600 MeV. The results are compared with state-of-the-art Faddeev calculations of the neutron-deuterium system up to 300 MeV. Above 100 MeV, this comparison reveals significant limitations of the nonrelativistic 3N Hamiltonian using nucleon-nucleon forces only. [S0031-9007(98)06526-0] PACS numbers: 25.10. + s, 21.30. -x, 21.45. + v, 25.40. -h Three-nucleon scattering based on modern NN forces has matured in recent years, and computationally accurate solutions of the three-nucleon ͑3N͒ Faddeev equation can be achieved [1]. This allows the comparison of theoretical predictions with a rich set of data in elastic nd scattering and the nd breakup process, including various spin observables. Most of these data lie in the energy range below 100 MeV projectile energy. The agreement of the theoretical predictions with nearly all experimental observables is very good in this energy regime, and little room is left for the action of 3N forces [1]. The next step in testing this approach to the threenucleon system is to carry out similar calculations at higher energies and compare their predictions with high precision data. It should be emphasized that calculations of the type discussed here use the most modern NN forces which describe the Nijmegen NN data base up to about 350 MeV with a x 2 per datum very close to 1. It turns out that the theoretical predictions for the three nucleon observables are very stable with respect to the choice of these specific NN potentials [1]. Although these new potentials are of various types (e.g., some are local while others are nonlocal), possible off-shell differences in the NN t matrices are hardly visible in the 3N observables.In this Letter we present new, fully converged calculations of the total nd cross section in the 10 -300 MeV energy range and compare them with experimental results derived from recent high-precision measurements of the nd-np total cross section difference, reported here for the first time. Previous comparisons with nd differential cross section data above 100 MeV [1] showed that the theory underpredicts those differential cross sections at large angles. Therefore we may also anticipate discrepancies in the nd total cross section. In the present work these discrepancies are exhibited by comparing the new high-precision measurements with a fully converged 3N Faddeev calculation, and are found to be significant above approximately 100 MeV.The new measurement of the difference of the neutron total cross sections for deuterium and hydrogen, s d-p , was undertaken as part of a new survey of total cross sections over a wide mass and energy range carried out at the LANSCE͞WNR spallation source as part of the Accelerator Production of Tritium project. The measurements used a slightly modified version of the experimental setup described by Finlay et al. [2]. The main variation from Ref.[2] was th...

The microstructural changes in high-energy electron-irradiated poly(vinylidene fluoridetrifluoroethylene) 68/32 mol % copolymer have been studied by X-ray diffraction, FTIR spectroscopy, and differential scanning calorimetry. The macroscopic polarization response in these materials was investigated by examining the dielectric and polarization behavior in a broad temperature and frequency range. It was found that besides reducing crystallinity in the copolymer film, irradiation produces significant changes in the ferroelectric-to-paraelectric phase transition behavior. The irradiation leads to a reduction in the polar domain size to below a critical size (a few nanometers), resulting in the instability of the macroscopic ferroelectric state and transforming the structure of the crystalline region in the copolymer from a polar all-trans ferroelectric to a nonpolar state represented by a trans-gauche conformation. However, a reentrant polarization hysteresis was observed in the copolymers irradiated with higher doses (>75 Mrad). Therefore, there is an optimized dose that generates a copolymer with a nonpolar structure but relatively high crystallinity whose electromechanical performance is the best. In the copolymers in this optimum dose range, FT-IR data revealed that there is not much change in the molecular conformation with temperature, even as the temperature passes through the dielectric peak, indicating that there is no symmetry breaking in both the macroscale and local level. Although the lattice spacing of the crystalline region along the molecular chain direction discontinuously changes between two special cases, the interchain spacing continuously changes with the irradiation dose, reflecting a strong intrachain coupling between the nonpolar and polar regions. On the other hand, the X-ray data reveal that the crystalline size perpendicular to the polymer chain does not change with irradiation until at doses exceeding 85 Mrad.

The dielectric behavior of the high-energy electron irradiated poly(vinylidene fluoride–trifluoroethylene) 68/32 mol % copolymer was characterized in the dose range from 0 to 175 Mrad. It was found that as the dose increases, the copolymer evolves from a normal ferroelectric to a relaxor ferroelectric and then to a simple relaxor. Correspondingly, the crystalline morphology changes from a two-phase coexistence between a polar and nonpolar phases at low doses (<50 Mrad) to a macroscopically uniform nonpolar phase for doses beyond 50 Mrad, as revealed by x-ray data. Interestingly, it was observed that in the whole dose range, the Vogel–Fulcher (V–F) relationship could describe the measured relationship between the frequency and the peak temperature of dielectric constant well. For the copolymers irradiated with dose near 75 Mrad, which show a typical relaxor ferroelectric response, the V–F behavior is a result of freezing of the relaxation time (into polar glass state). For the copolymers irradiated with doses lower that 50 Mrad where a polar phase and nonpolar phase coexist, the dielectric data at near the ferroelectric-paraelectric temperature show a dielectric peak with V–F type relaxation followed by a transition from a relaxor ferroelectric and normal ferroelectric as the temperature is lowered, suggesting a possible freezing process even in this dose range.

Enhanced electrocaloric effect in ferroelectric poly(vinylidene-fluoride/trifluoroethylene) 55/45 mol% copolymer at ferroelectric-paraelectric transition Appl. Phys. Lett. 98, 122906 (2011); 10.1063/1.3569953 Intrinsic electrocaloric effects in ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymers: Roles of order of phase transition and stresses Appl. Phys. Lett. 96, 102903 (2010); 10.1063/1.3353989 Electrocaloric effect of the relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer Appl. Phys. Lett.We present directly measured electrocaloric effect (ECE) from the poly(vinylidenefluoridetrifluoroethylene) 65/35 mol. % copolymer. The data reveal a large difference in the ECE between that measured in applying and in removing the electric field. The difference is significantly reduced by modifying the copolymer with 20 Mrad of high energy electron irradiation. Moreover, an isothermal entropy change DS int ¼ 160 J kg À1 K À1 and an adiabatic temperature change DT int ¼ 35 C can be induced in the irradiated copolymer. These results demonstrate the promise of achieving a significant ECE in ferroelectric polymers near first-order ferroelectric-paraelectric transition where multiple intermediate phases can exist. V C 2012 American Institute of Physics.