Search Results (27 total)

The mass-symmetric reactions ⁵⁸Fe,⁵⁸Ni +⁵⁸Fe,⁵⁸Ni were studied at a beam energy of E_beam=30 MeV/nucleon in order to investigate the isospin dependence of fragment emission. Ratios of inclusive yields of isotopic fragments from hydrogen through nitrogen were extracted as a function of laboratory angle. A moving source analysis of the data indicates that at laboratory angles around 40° the yield of intermediate mass fragments (IMF’s) beyond Z=3 is predominantly from a midrapidity source. The angular dependence of the relative yields of isotopes beyond Z=3 indicates that the IMF’s at more central angles originate from a source which is more neutron deficient than the source responsible for fragments emitted at forward angles. The charge distributions and kinetic energy spectra of the IMF’s at various laboratory angles were well reproduced by calculations employing a quantum molecular-dynamics code followed by a statistical multifragmentation model for generating fragments. The calculations indicate that the measured IMF’s originate mainly from a single source. The isotopic composition of the emitted fragments is, however, not reproduced by the same calculation. The measured isotopic and isobaric ratios indicate an emitting source that is more neutron rich in comparison to the source predicted by model calculations.

Isotopically resolved intermediate-mass fragments and light charged particles have been detected from the reactions ⁴⁰Ar and ⁴⁰Ca with ⁵⁸Fe and ⁵⁸Ni at E_beam=33 and 45 MeV/nucleon. There is an angular dependence to the isotopic ratios. A moving source analysis shows that fragments emitted at Θ_lab=40° can be attributed primarily to a composite source while the fragments emitted at backward angles are primarily from a targetlike source. The results are compared to predictions of QMD, BUU, and GEMINI. QMD generally reproduces the charge distribution and energy spectra and has partial success with the isobaric ratios when the system is chemically equilibrated. All of the models have difficulty reproducing the isotopic ratios when the system is not chemically equilibrated.

Iron films grown on a two-monolayer Ni/W(110) substrate undergo a temperature-driven spin reorientation which can be followed experimentally from a low-temperature, perpendicularly magnetized monodomain state all the way to the high-temperature paramagnetic state without a significant change of the atomic structure. Measurements of the magnetic susceptibility, using the magneto-optic Kerr effect, give the entire temperature vs thickness phase diagram for the spin reorientation, including measurements close to the multicritical point where the reorientation and Curie temperatures are equal. The technique is particularly sensitive to the formation of the perpendicularly magnetized domains associated with the spin reorientation and reveals that films with a thickness in the neighborhood of the multicritical behavior also display a maximum in the temperature at which domain formation begins. Thinner films undergo the transition from ferromagnetic-to-paramagnetic behavior directly from the domain state, but no signature of the critical temperature itself is seen in the magnetic susceptibility.

The fcc to bcc structural change in the growth of ultrathin iron films is studied for films grown on a (111) fcc surface, in an attempt to mimic the geometry of the martensitic transition in bulk iron, where the interface is formed from the close-packed planes of the two structures. The use of a 2-ML Ni/W(110) substrate allows good wetting and lattice matching of the iron, without introducing either significant interdiffusion at the interface or a large amount of magnetic material. Low-energy electron diffraction and angle-resolved Auger electron spectroscopy show that the iron films grow as a slightly distorted (111) fcc surface for 3 ML, after which a surface cell intermediate to fcc and bcc appears in a specific Kurjumov-Sachs (KS) orientation. Thicker films show a simultaneous relaxation of the surface unit cell to a bcc structure and the movement of the layer stacking from the fcc position to the KS bcc position. The approximate layer-by-layer evolution of the structure is in marked contast to the complicated growth transition seen for iron films grown on (001) fcc surfaces. This promises to be a useful system to isolate the relation of the structural transition and magnetism, without important contributions from other growth processes.

We modify the method of Albergo et al. for determining the temperature of an excited nucleus from double ratios of isotope yields and present a statistical model which accounts for the population and decay of excited states of the emitted fragments. Nuclear temperatures are extracted using experimental ratios of isotopic yields of fragments from helium through carbon for the reactions ⁴⁰Ca + ⁵⁸Ni, ⁴⁰Ar + ⁵⁸Ni, ⁴⁰Ca + ⁵⁸Fe, and ⁴⁰Ar + ⁵⁸Fe at 33 MeV/nucleon projectile energy. Using the model we obtain consistent values for the temperature from various isotope combinations within the experimental error when accounting for the population and decay of the excited fragments.

The spectral distribution of light generated by a finite cavity with one moving mirror is compared with that produced by a fixed cavity containing a time-varying dielectric. In both cases a motion over a finite time interval is considered. Although the moving mirror is usually considered to be an idealization for the time-varying dielectric, there are qualitative differences in the spectra produced. The spectral distribution for the moving mirror case behaves as 1/n³, while that for the time-varying dielectric behaves on average as 1/n⁴ but is rapidly oscillating.

Spin-polarized low-energy-electron-diffraction (SPLEED) rotation curves have been measured for 2-ML ultrathin iron films grown on a tungsten (001) substrate. This LEED technique emphasizes intraplane multiple scattering and is found to be very sensitive to the magnetism and geometry of the film. The contributions of the exchange and spin-orbit polarization, as well as the intensity asymmetry upon magnetization reversal, are shown theoretically and experimentally to be separable using symmetry-based arguments. These symmetries confer particular advantages to SPLEED rotation curves for the study of ultrathin magnetic films: they provide many independent checks of the internal consistency of the data, allow a direct evaluation of the effects of instrumental asymmetries, allow a means (symmetry averaging) for correcting for small misalignments, and permit qualitative conclusions to be drawn directly from the data. For example, the remanent axis of magnetization for Fe/W(001) is shown to be in-plane along [10]. Large magnetic asymmetries, A_u, of 2% are observed for this system, showing that it is experimentally feasible to perform SPLEED with neither a spin-polarized electron source nor a polarization detector.

The heat capacity of a vacuum-annealed sample of tantalum has been measured in the normal and superconducting state from 1.3 to 25°K. In the normal state the heat capacity C_N can be represented by the expression C_N=0.00136T+464.4(T / θ)³, where θ varies from approximately 255 degrees at the lowest temperature to 220 degrees at the highest. The zero-field transition temperature, T_c, and the critical field H_c were determined using a calorimetric method. T_c was found to be 4.39°K. The width of the transition was approximately 0.03°C. The measured critical field was within experimental error comparable to that calculated from the heat-capacity data. The tantalum sample therefore exhibited the properties of an ideal superconductor characteristic of the so called soft superconductors. The critical field at the absolute zero, from an extrapolation of the critical field data, was found to be 780 gauss.

The heat capacity of a vacuum-annealed sample of niobium has been determined in the temperature range 1.5 to 30°K. The heat capacity measurements in the normal state below the transition temperature were carried out in magnetic fields up to 4130 gauss. The effect of vacuum annealing on the heat capacity, transition temperature, and critical field was determined in an effort to obtain a so-called ideal superconductor. After the initial annealing, within experimental error, the heat capacity in the superconducting state was found to be independent of further treatment. In the normal state the heat capacity (C_n) can be represented by the relation C_n=0.0018T+464.4(T / θ)³, where θ varies from 256 to 320 degrees depending on the temperature for the best annealed sample. No simple relationship holds for the heat capacity in the superconducting state. The heat capacity data could not be fitted to any existing corresponding state theory for superconductors.

The zero-field transition temperature (T_c) and the critical field (H_c) were found to depend on the extent of annealing. For T_c=9.07°K, (dH_c / dT)_T=Tc=1148 gauss deg^-1 whereas for T_c=9.17°K, the highest transition temperature measured, (dH_c / dT)_T=Tc=734 gauss deg^-1. From the heat capacity data (dH_c / dT)T=T_c=415 gauss deg^-1 and H₀, the critical field at absolute zero=1944 gauss. It was not possible by vacuum annealing even at 2100°C to obtain an "ideal" superconductor.

In experiments on the thermal conductivity of liquid helium II, an anomalous heat conduction has been found in the vicinity of the heat source. The magnitude of this effect is given as a function of temperature and is shown to be independent of the heat flux. The effect has been ascribed to a thin layer of liquid helium in the vicinity of the energy source having a poor heat conduction. The existence of this layer is probably a consequence of the finite rate of conversion from superfluid to normal particles.

Experiments have been carried out to measure the "frozen in" magnetic moments produced in superconducting spheres of tin, through demagnetization from fields sufficient to completely or partially destroy their superconductivity. Moments were determined by measuring periods of a torsion pendulum made from the spheres. Both solid tin spheres and spheres filled with non-superconducting material were examined.

Small permanent moments, fixed in magnitude and direction, and probably associated with traces of impurity, were observed for the solid spheres while considerably larger fixed moments were observed for the hollow spheres. The observed permanent moments, which amounted to one to three percent of the induced moments calculated for the critical field depended to a slight extent on the magnitude of the fields used to produce them and were also dependent on the size and geometry of the spheres and on the temperature.

Entry of the spheres into the intermediate state was evidenced by a sudden gain in the total magnetic moment when the measuring field attained a value which was close to 2/3 of the critical field, H_c. This gain was best observed by measurements of this coefficient for eddy current damping. We interpret this effect as due to the formation of regions of normal metal in the intermediate state. We also interpret the permanent moments, which can only be changed by bringing the specimens into fields in excess of 2 / 3 H_c, as due to isolated regions of normal metal. The amounts of normal metal, in either the superconducting or the intermediate regions, can be evaluated either from measurements of magnetic moment or from measurements of coefficients of eddy current damping. Results obtained by the two methods are in agreement.

The results can be explained in terms of Landau's theory of the intermediate state that postulates the presence of threads or plates of normal metal. Our results indicate that these regions of normal metal must have diameters or thicknesses which are quite small compared to their lengths and that the magnetic moments which they represent can be trapped in a particular direction for a considerable length of time.

The densities of carefully purified crystals of sodium chloride have been determined by the method of "crystal flotation" in pure ethylene dibromide. The results yield d_27.634°C=2.16165±0.00002g / ml, which reduces to d_20°=2.16366±0.00003g / ml.

It is found that six successive precipitations of NaCl in the manner employed by Richards and Wells in their determination of the atomic weights of sodium and of chlorine are required to effect purification to constant density (± about 4×10^-6 g/ml), and also that exposure to air produces surface contamination sufficient to cause erratic changes in apparent density that may amount to as much as 5×10^-4 g/ml within a few minutes. Combination of our value for d_NaCl with that of C. A. Hutchison and H. L. Johnston for d_LiF and with Straumanis, Ievins, and Karlsons' value for the lattice constant of LiF relative to the Siegbahn value for NaCl yields 0.443640±0.000025 for the ratio of the molecular weights of LiF and NaCl, respectively. With the adoption of 22.997 (International Atomic Weight Committee) for the atomic weight of sodium, this ratio yields 18.994±0.001 for the atomic weight of F. With the adoption of 22.994 (Birge) for sodium, the F atomic weight comes out 18.992. Either of these figures is in reasonable agreement with the value 22.995±0.002, based on densities and lattice constants of fluorite and calcite, and with the gas density determinations for compounds of F, but are somewhat lower than the mass spectrograph value of 18.999±0.001 for F¹⁹. It appears that the determination of relative molecular weights by combination of x-ray and density data are as reliable, in favorable cases, as by other standard atomic weight methods. In calcite, fluorite, and rock salt crystals, used to obtain the data underlying these computations, there is no evidence of any appreciable influence of "crystal mosaic" patterns which Zwicky thought might influence crystal densities by as much as 1 percent.

By the methods of photographic photometry, the alternating intensities in emission of the Δv=0 sequence of the Fulcher bands of deuterium have been investigated in order to determine the nuclear spin. The alternation of intensities was determined for the P, Q and R branches of each of the 5 bands studied. The average value of g_s / g_a obtained from 4 plates is 1.97±0.03. This value agrees best with the theoretical ratio of 2 and a consequent nuclear spin of 1. Since the even rotational levels are more intense than the odd levels, the nucleus obeys Bose-Einstein statistics.

The large electronic isotope effect in the spin coupling of ²II-terms, reported by Johnston and Dawson for boron monoxide and for the heads of the OH² bands is confirmed by measurements of the fine line structure in the isotopic OH bands. Both for BO and for OH the observed increase in the doublet spacings, in the molecule with the heavier isotope, agrees closely with the results to be expected from Hill and Van Vleck's theoretical treatment of spin coupling energy, although their equations do not so well reproduce the separate doublets. This isotope effect is zero at the band origins, increases to a maximum (which amounts to 20 cm^-1 in the 0″ vibrational level of OH) and approaches zero asymptotically at high rotational quantum numbers. A large electronic isotope effect in Λ-doubling is also observed for OH. This effect, which is zero at the band origin and increases with increasing rotation, amounts to 17 cm^-1 at K=25. It is not correctly accounted for by Van Vleck's equations for Λ-doubling although ratios of the Λ-doublets in OH¹ and OH² are correctly given, for a case b molecule. No noticeable effect was observed on the spacings of the ²Σ-doublets in the excited state of OH.

The investigations begun in the neighborhood of the (0′,1″) band at λ3428 (cf. reference 4) have been extended to include the bands of the (0,0) and (1′,0″) sequences. Approximately 400 lines were observed in this region of the spectrum, of which 120 are reported for the first time. All but 18 of these have been successfully assigned. The assignments include the identification of a new (2,2) band with its principal head at λ3185; the identification of ^RRR₂₁ and of ^PPP₁₂ satellite branches of the (1,1) band; complete assignments of the regular R-branches of the (1,1) and (2′,1″) bands; and extensions of nearly all the remaining branches of the bands which appear in this region. The new data are also utilized to obtain Λ-doublets and spin doublets to K=35 for the 0″ level and to somewhat smaller values of K for the 1″ and 2″ levels. The experimental results are compared with equations in the literature. Molecular constants derived from the new (2,2) band are in good agreement with those derived from the (1′,2″) band.

Although the ultraviolet "water vapor" bands were among the earliest molecular spectra to undergo extensive investigation, heretofore only six bands have been reported. These involve transitions between the (0, 1, 2) vibrational levels of an excited ²Σ state and the (0, 1) vibrational levels of the normal ²Π state. With improved conditions for exciting the spectrum we have obtained 91 lines, heretofore unreported, in the neighborhood of the (0′, 1″) band at λ3428. 44 of these lines prove to be extensions of λ3428; 54 lines (including 11 in common with λ3428) form a new band with a head at λ3484; and 4 lines remain unidentified. Analysis of the new band identifies it as the (1′, 2″) band of the water vapor system. Rotational constants for the normal ²Π electronic state of OH are: B_e^′′=19.025, α^′′=0.724, D_e^′′=-1.97×10^-3, β^′′=5.0×10^-5, and r_e^′′=0.964×10^-8 cm. Vibrational constants are: ω_e^′′=3734.9 and x_e^′′ω_e^′′=82.6. Rotational and vibrational constants for the excited ²Σ state are also evaluated and are tabulated.

The wave-lengths of 134 absorption bands of ClO₂ have been measured with the Hilger E-1 and E-185 quartz instruments. The bands have been arranged in progressions and the energy levels deduced. Five vibrational levels of the normal electronic state have been found; the vibrational levels of the excited electronic state can be described with the use of two vibrational quantum numbers. Formulae for these bands and their isotope separations are given. The shapes of the molecule in the normal and excited electronic states have been calculated from Bjerrum's formulae for the vibrations assuming valency forces and all but one solution for each electronic state is excluded by the isotope effect or the intensity distribution of the bands. The central force formulae give no possible solutions. The normal modes of vibration for the molecule have been calculated. The intensity distribution is consistent with the extension of the Franck-Condon principle to polyatomic molecules. The predissociation limit agrees with a very approximate extrapolated value for the energy of dissociation through one mode of vibration of the excited electronic state into a ClO molecule and an O atom. Dissociation through a second mode of vibration of the excited electronic state gives only a very rough value. A discontinuity in the ΔE:v₂^′ curve noted by Goodeve and Stein appears at about the energy required for an oscillation of the molecule through a straight line position and it is proposed that this is the cause of the discontinuity.