558 research outputs found
Systematics of the odd-even effect in the resonance ionization of Os and Ti
Measurements of the odd-even effect in the mass spectrometric analysis of Ti and
Os isotopes by resonance ionization mass spectrometry have been performed for ΔJ = + 1, 0 and -1 transitions. Under saturating conditions of the ionization and for ΔJ = + 1 transitions odd-even
effects are reduced below the 0.5% level. Depending on the polarization state of the laser large
odd isotope enrichments are observed for ΔJ = 0 and -1 transitions which can be reduced below
the 0.5% level by depolarization of the laser field
Laser-induced isotopic selectivity in the resonance ionization of Os
Isotope selective effects in resonance ionization mass spectrometry (RIMS) pose a potentially serious limitation to the application of this technique to the precise and reproducible measurement of isotope ratios. In order to identify some of the underlying causes of isotope selectivity in RIMS and to establish procedures for minimizing these effects, we investigated laser-induced isotope selectivity in the resonance ionization of Os. A single-color, one-photon resonant ionization scheme was used for several different transitions to produce Os photoions from a thermal atomization source. Variations in Os isotope ratios were studied as a function of laser parameters such as wavelength, bandwidth, power and polarization state. Isotope selectivity is strongly dependent on laser power and wavelength, even when the bandwidth of the laser radiation is much larger than the optical isotope shift. Variations in the ^(190)Os/^(188)Os ratio of ≈20% for a detuning of 0.8 cm^(−1) were observed on a transition with a small oscillator strength. Large even—odd isotope selectivity with a 13% depletion of ^(189)Os was observed on a ΔJ = +1 transition at low laser intensity; the odd mass Os isotopes are systematically depleted. For ΔJ = −1 and 0 transitions the isotope selectivity was reduced by polarization scrambling and for strongly saturating conditions. A technique employing the wavelength dependence of even—even isotope selectivity as an internal wavelength standard was developed to permit accurate and reproducible wavelength adjustment of the laser radiation. This technique provides control over laser-induced isotope selectivity for single-color ionization and enabled us to obtain reproducible measurements of ^(192)Os/^(188)Os and ^(189)Os/^(190)Os ratios in the saturation regime for a ΔJ = +1 transition with a precision of better than 0.5%. The application of this wavelength-tuning procedure should significantly improve the quality of RIMS isotope ratio data for many elements
Systematics of isotope ratio measurements with resonant laser photoionization sources
Sources of laser-induced even-even and odd-even isotopic selectivity in the resonance ionization mass
spectroscopy of Os and Ti have been investigated experimentally for various types of transitions. A set
of conditions with regard to laser bandwidth and frequency tuning, polarization state and intensity was
obtained for which isotopic selectivity is either absent or reduced below the 2 % level
Laser-induced isotopic effects in titanium resonance ionization
Titanium isotope ratios have been measured by resonance ionization mass spectrometry (RIMS) with special emphasis on the nature of laser-induced isotopic selectivity. A pronounced wavelength dependence of even mass isotope ratios is caused by large nuclear volume effects near the
magic neutron number 28 in ^(50)Ti . Optical isotope shifts, ranging from 0.07 to 0.21 cm^(-l), between ^(50)Ti and ^(46)Ti were measured for several transitions. The ^(50)Ti/^(46)Ti and ^(48)Ti/^(46)Ti ratios, nevertheless, exhibited only mass-dependent fractionation, in which the lighter Ti isotopes were enriched by ~2.5%/amu, when the laser operating parameters were properly controlled. Odd-even mass isotopic selectivity in the resonant ionization process was also examined for several transitions as a function of the laser polarization state and intensity. Under saturating conditions for a ΔJ= +1 transition and a high degree of laser depolarization for a ΔJ = 0 transition, the odd-even isotopic enhancement was reduced below the 2% level. The Ti isotope data agree with our previous results for Os and indicate that, by a careful choice of resonance transitions and laser operating parameters, isotope ratios can be measured accurately and reliably with RIMS
Do SiC grains in Orgueil differ from those in Murchison?
Studies of individual presolar SiC grains have shown that most are enriched in Si-29, Si-30, and C-13, and depleted in N-15, compared to solar-system abundances and that many have large excesses of Mg-26, most plausibly from in situ decay of Al-26. Stone et al., observed that Si from a family of platy SiC grains define a linear array on a 3-isotope plot that does not pass through normal solar-system Si. In contrast, Si-isotope data from over 100 3-4 micron SiC grains from Murchison from an elongate ellipse enclosing the Stone et al. linear array but also including 'normal' solar-system Si. To investigate whether this difference in Si isotopes indicates different populations of SiC in the two meteorites and to improve the characterization of Orgueil SiC, we used the PANURGE ion microprobe to measure Si, C, N, and Mg isotopes and Al and Na concentrations in a suite of 2-5 micron SiC grains from a new sample of Orgueil
A Zero-Gravity Instrument to Study Low Velocity Collisions of Fragile Particles at Low Temperatures
We discuss the design, operation, and performance of a vacuum setup
constructed for use in zero (or reduced) gravity conditions to initiate
collisions of fragile millimeter-sized particles at low velocity and
temperature. Such particles are typically found in many astronomical settings
and in regions of planet formation. The instrument has participated in four
parabolic flight campaigns to date, operating for a total of 2.4 hours in
reduced gravity conditions and successfully recording over 300 separate
collisions of loosely packed dust aggregates and ice samples. The imparted
particle velocities achieved range from 0.03-0.28 m s^-1 and a high-speed,
high-resolution camera captures the events at 107 frames per second from two
viewing angles separated by either 48.8 or 60.0 degrees. The particles can be
stored inside the experiment vacuum chamber at temperatures of 80-300 K for
several uninterrupted hours using a built-in thermal accumulation system. The
copper structure allows cooling down to cryogenic temperatures before
commencement of the experiments. Throughout the parabolic flight campaigns,
add-ons and modifications have been made, illustrating the instrument
flexibility in the study of small particle collisions.Comment: D. M. Salter, D. Hei{\ss}elmann, G. Chaparro, G. van der Wolk, P.
Rei{\ss}aus, A. G. Borst, R. W. Dawson, E. de Kuyper, G. Drinkwater, K.
Gebauer, M. Hutcheon, H. Linnartz, F. J. Molster, B. Stoll, P. C. van der
Tuijn, H. J. Fraser, and J. Blu
Cross section and analyzing power of pol{p}p -> pn pi+ near threshold
The cross section and analyzing power of the pol{p}p -> pn pi+ reaction near
threshold are estimated in terms of data obtained from the pol{p}p -> d pi+ and
pp -> pp pi0 reactions. A simple final state interaction theory is developed
which depends weakly upon the form of the pion-production operator and includes
some Coulomb corrections. Within the uncertainties of the model and the input
data, the approach reproduces well the measured energy dependence of the total
cross section and the proton analyzing power at a fixed pion c.m. angle of
90deg, from threshold to T_p = 330 MeV. The variation of the differential cross
section with pion angle is also very encouraging.Comment: 20 pages, Latex including 4 eps figure
An Experimental Study of Metallic Diffusion and Phase Equilibria in Fremdlinge
Fremdlinge are opaque assemblages within CAis that are mainly composed of NiFe metal, V-magnetite and µm-sized RuOs nuggets. The prevailing scenario for their origin includes condensation, aggregation and equilibration at low T (≾ 600°C); they are then thought to be introduced into CAis at high T, followed by rapid cooling, thereby preserving the textures and assemblages from the pre-CAl, low T histories (Armstrong et al., 1985; Armstrong et al., 1987). A constraint on cooling rates of Fremdlinge comes from sharp contacts observed between RuOs nuggets and NiFe
metals that enclose them. To determine the length of time that these contacts could have been held at high T, thin-film diffusion experiments were conducted with an
electroplated Ru film on Ni. Samples were annealed at 1400, 1200, 1000 and 800°C for 0.3-137 hours. Measured Ru profiles in Ni were consistent with the following Arrhenius expression: D(cm^2/sec) = 0.0050exp(-2.3 x 10^(12)/RT) (T in K, R in Cgs units). Based on these data, we calculate that cooling rates of ≳ 10^5C/hr are necessary to preserve sharp contacts between RuOs and NiFe metals if they experienced the T of CAl melting (~ 1400°C) (Stolper and Paque, 1986). We consider this rate unreasonable in light of cooling rates inferred from experimental studies of the silicate portions of CAis (10^(-1) to 10^2°C/hr) (Stolper and Paque, 1986)
Injection of Radioactivities into the Forming Solar System
Meteorite studies have revealed the presence of short-lived radioactivities
in the early solar system. The current data suggests that the origin of at
least some of the radioactivities requires contribution from recent
nucleosynthesis at a stellar site. This sets a strict time limit on the time
available for the formation of the solar system and argues for the theory of
the triggered origin of the solar system. According to this scenario, the
formation of our planetary system was initiated by the impact of an
interstellar shock wave on a molecular cloud core. The shock wave originated
from a nearby explosive stellar event and carried with it radioactivities
produced in the stellar source. In addition to triggering the collapse of the
molecular cloud core, the shock wave also deposited some of the freshly
synthesized radioactivities into the collapsing system. The radioactivities
were then incorporated into the first solar system solids, in this manner
leaving a record of the event in the meteoritic material. The viability of the
scenario can be investigated through numerical simulations studying the
processes involved in mixing shock wave material into the collapsing system.
The high-resolution calculations presented here show that injection occurs
through Rayleigh-Taylor instabilities, the injection efficiency is
approximately 10%, and temporal and spatial heterogeneities in the abundances
of the radioactivities existed at the time of their arrival in the forming
solar system.Comment: 13 pages, including 3 figures. Better-quality figures available at
http://www.public.asu.edu/~hvanhal/pubs
- …