707 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
Design and commissioning of a timestamp-based data acquisition system for the DRAGON recoil mass separator
The DRAGON recoil mass separator at TRIUMF exists to study radiative proton
and alpha capture reactions, which are important in a variety of astrophysical
scenarios. DRAGON experiments require a data acquisition system that can be
triggered on either reaction product ( ray or heavy ion), with the
additional requirement of being able to promptly recognize coincidence events
in an online environment. To this end, we have designed and implemented a new
data acquisition system for DRAGON which consists of two independently
triggered readouts. Events from both systems are recorded with timestamps from
a MHz clock that are used to tag coincidences in the earliest possible
stage of the data analysis. Here we report on the design, implementation, and
commissioning of the new DRAGON data acquisition system, including the
hardware, trigger logic, coincidence reconstruction algorithm, and live time
considerations. We also discuss the results of an experiment commissioning the
new system, which measured the strength of the
keV resonance in the NeNa radiative proton
capture reaction.Comment: 11 pages, 7 figures, accepted for publication in EPJ A "tools for
experiment and theory
Comparison of optical model results from a microscopic Schr\"odinger approach to nucleon-nucleus elastic scattering with those from a global Dirac phenomenology
Comparisons are made between results of calculations for intermediate energy
nucleon-nucleus scattering for 12C, 16O, 40Ca, 90Zr, and 208Pb, using optical
potentials obtained from global Dirac phenomenology and from a microscopic
Schr\"odinger model. Differential cross sections and spin observables for
scattering from the set of five nuclei at 65 MeV and 200 MeV have been studied
to assess the relative merits of each approach. Total reaction cross sections
from proton-nucleus and total cross sections from neutron-nucleus scattering
have been evaluated and compared with data for those five targets in the energy
range 20 MeV to 800 MeV. The methods of analyses give results that compare well
with experimental data in those energy regimes for which the procedures are
suited.Comment: 22 pages, 12 figure
Martian rocks, minerals, and mantles
The variable nature of Mars was first observed almost 400 years ago and modern observations began almost 40 years ago, culminating with the flotilla of spacecraft now at or heading for Mars. We now know that the atmosphere, which produced the visible variation of Mars, has also covered it with a mantle that makes difficult any detailed investigation of the rocks and minerals of Mars
Inhibition of E. coli F1-ATPase: insertion of epsilon's C-terminal domain and the transition to the MgADP-inhibited state are competing processes at the catalytic dwell step
Guidelines, guidelines and more guidelines: And we still do not know how to follow-up patients with breast cancer
BACKGROUND: A major challenge facing us is the provision of health care and appropriate allocation of available resources for the treatment of patients with breast cancer. This is of particular concern in the provision of follow-up care. With the increasing incidence of breast cancer and the improvements in survival which have resulted in up to 75% of patients surviving for more than five years, an increasing resource is required. However, there is controversy as to the most appropriate schedule for follow-up of these patients. This brief review has focused on the evidence-base and guidelines that currently exist in the United Kingdom for the follow-up of patients who have been treated for breast cancer. METHODS: A review of the current guidelines published in the United Kingdom for the follow-up of patients with breast cancer (National Institute for Clinical Excellence, Scottish Intercollegiate Guidelines Network, British Association of Surgical Oncology) and the randomised controlled trials evaluating the follow-up of patients with breast cancer was undertaken. RESULTS: The results have demonstrated the different follow-up protocols currently indicated in these guidelines within the same country. Furthermore, the lack of well designed, randomised controlled trials on which to base a follow-up protocol for patients with breast cancer is apparent. CONCLUSION: The evidence-base on which these guidelines have been developed is lacking. It is apparent that well designed randomised controlled trials are needed urgently if we are to understand the most appropriate and effective ways of following up patients with breast cancer
- …