R. Dean Taylor

The molar volume and expansion coefficient of liquid He4

Abstract The density of liquid He 4 under its own vapor pressure has been measured over the range 0.5–2.8°K with a precision of about 0.006%. A direct pycnometric method was used; the cell was immersed in a liquid He 3 bath. Detailed measurements in the neighborhood of the lambda point temperature showed the density maximum to occur 0.006°K above T λ . Analysis of these data and other available data has yielded analytical equations from which the coefficient of expansion has been determined. Below 2.14°K the Landau formulations of He II was found to represent the data. In the neighborhood of T λ the density changes logarithmically in | T − T λ |. Above 2.2°K all available data are fitted as a function of the He 4 vapor pressure. The behavior near T λ is analyzed in terms of the theory of Buckingham and Fairbank.

Some Mössbauer effect considerations in gamma-ray laser development

Abstract The development of gamma-ray lasers (grasers) will, in all probability, depend on the use of recoillessly emitted gamma rays from nuclear transitions, i.e., the Mossbauer Effect (ME). Past research in Mossbauer spectroscopy which relates to grasers is briefly reviewed. The nuclear lifetimes, required for practical considerations, may have to be on the order of seconds, if storage and transfer processes prove infeasible. It is not clear that the ME has been observed in such long-lived states. Even if such long-lived states are not needed directly for grasers, successful observation of the ME in such a system will answer important questions of line broadening due to field inhomogeneities in single crystals. We propose some novel experimental schemes to observe the ME in 109Ag whose relevant lifetime is about 40 sec. The techniques proposed are: coincidence Mossbauer spectroscopy, conversion-electron Mossbauer spectroscopy, gravitational line sweeping, and gamma-ray self-absorption. Preliminary results using the later technique are presented. Possible future, relevant ME research is discussed.

Evidence for direct impact damage in metamict titanite CaTiSiO5

We have measured the dose dependence of the degree of amorphization of titanite, CaTiSiO(5). Titanite is an often metamict mineral which has been considered as a matrix for the encapsulation of radiogenic waste, such as Pu. The amorphous fraction p of geologically irradiated samples (ages between 0.3 and 1 Ga) follows p = 1 - exp(-B(a)D) where D is the total dose and the characteristic amorphization mass is B(a) = 2.7(3) × 10(-19) g. Amorphization follows the direct impact mechanism where each α-decay leads to a recoil of the radiogenic atoms (mostly Th and U), which then, in turn, displaces some 5000 atoms of the titanite matrix. The amorphization behaviour is almost identical with that of zircon, ZrSiO(4), which has a similar molecular mass. While the recrystallization mechanism and elastic behaviour of the two minerals are very different, we do not find significant differences for the amorphization mechanism. Our samples have undergone little reheating over their geological history, since heating over 800 K would lead to rapid recrystallization for which we have found no evidence.

Mössbauer Spectroscopy of Mg0.9Fe0.1 SiO3 perovskite

Ambient pressure Mossbauer spectra of Mg0.957Fe0.1SiO3 perovskite synthesized at pressure-temperature conditions of ∼50 GPa and 1700 K show that the iron is entirely high-spin Fe2+ and appears to be primarily located in the octahedral site within the crystal structure. We observe broad Mossbauer lines, suggesting a distribution of electric-field gradients caused by disorder associated with the Fe ions. Also, the perovskite exhibits magnetic ordering at temperatures < 5 K, implying that there is a magnetic contribution to the absolute (“third-law”) entropy of this phase.

Mossbauer Effect Experiments Applicable To Grasers

Gamma-ray experiments involving the Mossbauer Effect and multibeam Borrmann Effect in single crystal silver are reported. Calculations for multibeam radiation modes for electronic scattering in a (111)-oriented crystal of iron are detailed.

Mössbauer spectroscopy at very high pressures

The generation of Very High Pressure (VHP) with Diamond Anvil Cells (DAC) and its applications to Mössbauer Spectroscopy (MS) are described. We emphasize the technical aspects of DACs relevant to VHP-MS such as collimation. generation ofhydrostatic pressures, pressure calibration, and development of special cells. Examples are given showing the potential of VHP-MS in studies of molecular polymerization, order disorder phase transitions of molecular crystals, configuration transformations, and magnetic properties.

High pressure129I Mössbauer studies of GeI4 molecular crystals

The Mössbauer effect in129I in conjunction with Diamond-Anvil-Cell high pressure techniques was applied to investigate the high pressure phase(s) of the molecular crystal GeI4. The129IQuadrupole Interaction was the main probe for characterizing theintermolecular structural transformation with pressure. With increasing pressure, at about 15 GPa, the onset of a partial molecular-association phase (HP1) is first observed. In HP1 two out of the four iodines strongly overlap to form linear chains of GeI4. The HP1 phase coexists with the low pressure (LP) molecular phase, but its population increases with increasing pressure. AtP=20 GPa a second high pressure phase (HP2) is identified where all four iodines strongly overlap to form a three dimensional, fully molecular-associated structure. With increasing pressure and atP>20 GPa, HP2 is the only phase up toP=34 GPa, the highest pressure used. A significant hysteresis of the relative abundances with pressure is observed. The isomer shift of the HP2 and HP1 structures is considerably larger than that of the LP one.

High-pressure Mössbauer spectroscopy, a unique tool for unraveling the nature of electron correlations in Mott insulators

Combining the methods of Mössbauer spectroscopy, synchrotron XRD, and resistivity and using diamond-anvil cells enables the discovery and studies of new phenomena in magnetism and electronic correlation at high density. It is shown that Hunds rule concerning the high-spin state in TM-compounds does not hold in this regime resulting in spin-crossover and collapse of the magnetic state for even-valence TM ions and for the decline of magnetic exchange in the odd-valence species. This mechanism competes with the breakdown of the d–d electron correlation (Mott transition) in transforming the Mott insulators into normal metals. The experimental issues are described and examples of magnetic studies at very high-pressures are portrayed.

Temperature dependence of the Mössbauer effect on Sn-119 doped metallic lead

After perfecting a technique to manufacture extremely pure, homogeneous, smallconcentration Sn in Pb samples, we have measured the Debye-Waller factor over the temperature range of 4 to 300 K. Our data show some evidence of a line broadening between 4 and 50 K which may be accounted for by the spin-density wave model of lead proposed by A. Overhauser. [1]. Between 77 K and room temperature, we find a temperature dependence to the recoilless fraction consistent with a Debye model (Debye temperature = 107(2) K). Moreover, we find no evidence of a sudden precipitous falloff of the recoilless fraction, as was reported by Schechter et al. in 1989. The origin of this difference is discussed.

The Mössbauer effect in109Ag revisited

The 40-sec, 88-keV, first-excited state of109Ag provides a difficult case for the observation of the Mössbauer effect. The major difficulty is associated with the long lifetime which corresponds to a natural linewidth of only 10−17 eV. Our results indicate a 0.2% Mössbauer effect.