Particle beams with uniform transverse distribution

A successfully tested method is described which achieves a more uniform illumination of an extended flat target by the charged particle beam from an accelerator, by proper use of a combination of quadrupole and octupole magneto-optical elements

High-energy neutron irradiation of superconducting compounds

The effect of high-energy neutron irradiation (E greater than 1 MeV) at ambient reactor temperatures on the superconducting properties of a variety of superconducting compounds is reported. The materials studied include the A-15 compounds Nb$sub 3$Sn, Nb$sub 3$Al, Nb$sub 3$Ga, Nb$sub 3$Ge and V$sub 3$Si, the C-15 Laves phase HfV$sub 2$, the ternary molybdenum sulfide Mo$sub 3$Pb$sub 0$.$sub 5$S$sub 4$ and the layered dichalcogenide NbSe$sub 2$. The superconducting transition temperature has been measured for all of the above materials for neutron fluences up to 5 x 10$sup 19$ n/cm$sup 2$. The critical current for multifilamentary Nb$sub 3$Sn has also been determined for fields up to 16 T and fluences between 3 x 10$sup 17$ n/cm$sup 2$ and 1.1 x 10$sup 19$ n/ cm$sup 2$. (auth

Weak Localization Effect in Superconductors by Radiation Damage

Large reductions of the superconducting transition temperature $T_{c}$ and the accompanying loss of the thermal electrical resistivity (electron-phonon interaction) due to radiation damage have been observed for several A15 compounds, Chevrel phase and Ternary superconductors, and $\rm{NbSe_{2}}$ in the high fluence regime. We examine these behaviors based on the recent theory of weak localization effect in superconductors. We find a good fitting to the experimental data. In particular, weak localization correction to the phonon-mediated interaction is derived from the density correlation function. It is shown that weak localization has a strong influence on both the phonon-mediated interaction and the electron-phonon interaction, which leads to the universal correlation of $T_{c}$ and resistance ratio.Comment: 16 pages plus 3 figures, revtex, 76 references, For more information, Plesse see http://www.fen.bilkent.edu.tr/~yjki

Proceedings of the TMS symposium on radiation facilities and defect studies

Intent of the symposium is to highlight the various means of producing and characterizing irradition-induced defects in materials of interest in nuclear applications. Viewgraphs are presented for 18 papers. Separate abstracts were prepared for the data base

The Brookhaven Radiation Effects Facility

The Neutral Particle Beam (NPB) Radiation Effects Facility (REF), funded by the Strategic Defense Initiative Office (SDIO) through the Defense Nuclear Agency (DNA) and the Air Force Weapons Laboratory (AFWL), has been constructed at Brookhaven National Laboratory (BNL). Operation started in October 1986. The facility is capable of delivering pulsed H{sup -}, H{sup o}, and H{sup +} beams of 100 to 200 MeV energy up to 30 mA peak current. Pulses can be adjusted from 5 {mu}s to 500 {mu}s length at a repetition rate of 5 pps. The beam spot on target is adjustable from 3 to 100 cm diameter (2 {sigma}) resulting in a maximum dose of about 10 MRads (Si) per pulse (small beam spot). Experimental use of the REF is being primarily supported by the SDI lethality (LTH-4) program. The program has addressed ionization effects in electronics, both dose rate and total dose dependence, radiation-sensitive components, and dE/dx effects in energetic materials including propellants and high explosives (HE). This paper describes the facility, its capabilities and potential, and the experiments that have been carried out to date or are being planned. 2 refs., 10 figs

INTERNAL FRICTION DUE TO DOMAIN-WALL MOTION IN MARTENSITICALLY TRANSFORMED A15 COMPOUNDS

A lattice instability in A15 materials in some cases leads to a cubic- to-tetragonal martensitic transformation at low temperatures. The transformed material orients in lamellae with c axes alternately aligned along the directions producing domain walls between the lamellae. An internal-friction (δ) feature below Tm is attributed to stress-induced domain-wall motion. The magnitude of the friction increases as temperature is lowered below Tm as (1-c/a) increases, and behaves as (1-c/a)2 from Tm down to the superconducting critical temperature where the increasing tetragonality is inhibited. The effect of strain in the lattice is to decrease the domain-wall internal friction, but not affect Tm. Neutron-induced disorder and the addition of some third-elements in alloying decrease both δ and Tm, with some elements reducing only the former. Less than 1 at . % H is seen to completely suppress both δ and Tm. Martensitically transformed V2Zr demonstrates lowtemperature internal-friction and modulus behavior consistent with easy β/m wall motion relative to the easy m/m motion of the A15's. For the V2Zr , a peak in δ is observed, qualitatively in agreement with expected β/m wall motion

ANOMALOUS ELASTIC SOFTENING OF POLYCRYSTALLINE Nb3Sn BELOW THE MARTENSITIC TRANSFORMATION TEMPERATURE

Nb3Sn exhibits an elastic shear-mode softening associated with a structural transformation from cubic to tetragonal symmetry at a temperature of Tm∼ 45K. Ultrasonic propagation measurements in single crystals indicate that the shear modulus ½(C11-C12) softens dramatically as the temperature is decreased towards 45K, but increases rapidly below Tm. We have measured Young's modulus and internal friction of polycrystalline Nb3Sn between 300 and 4.2K. Our results indicate that transforming polycrystals soften monotonically down to 18K with no discontinuity in the slope at Tm. These results are found to be incompatible with polycrystalline averages of single crystal elastic constants, even when the possibility of the presence of a fraction of non-transforming material is taken into account. A tentative explanation of this anomalous softening below Tm is given in terms of stress-induced domain wall motion below Tm

Internal friction due to domain-wall motion in martensitically transformed A15 compounds

A lattice instability in A15 materials in some cases leads to a cubic-to-tetragonal martensitic transformation at low temperatures. The transformed material orients in lamellae with c axes alternately aligned along the <100> directions producing domain walls between the lamellae. An internal-friction (delta) feature below T/sub m/ is attributed to stress-induced domain-wall motion. The magnitude of the friction increases as temperature is lowered below T/sub m/ as (1-c/a) increases, and behaves as (1-c/a)/sup 2/ from T/sub m/ down to the superconducting critical temperature where the increasing tetragonality is inhibited. The effect of strain in the lattice is to decrease the domain-wall internal friction, but not affect T/sub m/. Neutron-induced disorder and the addition of some third-elements in alloying decrease both delta and T/sub m/, with some elements reducing only the former. Less than 1 at. % H is seen to completely suppress both delta and T/sub m. Martensitically transformed V/sub 2/Zr demonstrates low-temperature internal-friction and modulus behavior consists with easy ..beta../m wall motion relative to the easy m/m motion of the A15's. For the V/sub 2/Zr, a peak in delta is observed, qualitatively in agreement with expected ..beta../m wall motion

RELAXATION-TIME SPECTRUM FOR A HYDROGEN RELAXATION PEAK IN A Nb-50-at. %-V ALLOY

Internal friction has been measured vs. temperature from 15-330 K for equiatomic Nb-V alloys with up to 28 at. % H. A broad peak at roughly 125 K shows characteristics which strongly suggest that it is a reorientation peak. Detailed analysis using a newly developed method indicates the activation energy spectrum to be bell shaped and to be over the range 0.10 to 0.30 eV. A pre-exponential factor of 5x10-13 is determined

INTERNAL FRICTION AND YOUNG'S MODULUS OF Nb3Sn BETWEEN 6 AND 300 K

Young's modulus and internal friction were measured in polycrystalline NB3Sn between 6 and 300K. Below the martensitic transformation temperature a large rise in internal friction is observed, in agreement with recent observations. In this temperature range, the internal friction is found to be nearly independent of frequency, a behavior consistent with stress-induced wall motion between domains of the tetragonal phase as the main source of relaxation. Internal friction peaks observed at 140 and 260K in Nb/Nb3Sn composite tapes are tentatively attributed respectively to an α peak in Nb and hydrogen in the Nb3Sn layer. Annealing at 750 °C for 1/2h is found to decrease the transformation temperature from 49 to 40K, and to modify the softening of Young's modulus in this temperature range. Young's modulus at 6K is 46% of room temperature value and is unchanged by the annealing treatment