22 research outputs found
The Solar Neutrino Problem - An Update
The B solar neutrino flux as measured by Super-Kamiokande is consistent
with the Ar production rate in Cl at Homestake. GALLEX and SAGE,
continue to observe Ge production rates in Ga that are consistent
with the minimal signal expected from the solar luminosity. The observed B
solar neutrino flux is in good agreement with that predicted by the standard
solar model of Dar and Shaviv with nuclear reaction rates that are supported by
recent measurements of nuclear fusion cross sections at low energies. The
measurements of Super-Kamiokande, SAGE and GALLEX suggest that the expected the
pep, Be and NO solar neutrino fluxes are strongly suppressed. This can be
explained by neutrino oscillations and the Mikheyev-Smirnov-Wolfenstein effect.
Since neither a flavor change, nor a terrestrial variation, nor a spectral
distortion of the B solar neutrino flux has been observed yet, the solar
neutrino problem does not provide conclusive evidence for neutrino properties
beyond the standard electroweak model. The deviations of the experimental
results from those predicted by the standard solar models may reflect the
approximate nature of of solar models and of our knowledge of nuclear reaction
rates, radiation transport and particle diffusion in dense stellar plasmas.
Only future observations of spectral distortions, or terrestrial modulation or
flavor change of solar neutrinos in solar neutrino experiments, such as
Super-Kamiokande, SNO, Borexino and HELLAZ will be able to establish that
neutrino properties beyond the minimal standard electroweak model are
responsible for the solar neutrino problem.Comment: To be published in Physics Reports, Proceedings of ``From Atomic
Nuclei to Stars and Galaxies'' Haifa, January 12-16, 1998. No enclosed
figure
Compilation and R-matrix analysis of Big Bang nuclear reaction rates
We use the R-matrix theory to fit low-energy data on nuclear reactions
involved in Big Bang nucleosynthesis. A special attention is paid to the rate
uncertainties which are evaluated on statistical grounds. We provide S factors
and reaction rates in tabular and graphical formats.Comment: 40 pages, accepted for publication at ADNDT, web site at
http://pntpm3.ulb.ac.be/bigban
Alpha scattering and capture reactions in the A = 7 system at low energies
Differential cross sections for He- scattering were measured in
the energy range up to 3 MeV. These data together with other available
experimental results for He and H scattering were
analyzed in the framework of the optical model using double-folded potentials.
The optical potentials obtained were used to calculate the astrophysical
S-factors of the capture reactions HeBe and
HLi, and the branching ratios for the transitions into
the two final Be and Li bound states, respectively. For
HeBe excellent agreement between calculated and
experimental data is obtained. For HLi a value
has been found which is a factor of about 1.5 larger than the adopted value.
For both capture reactions a similar branching ratio of has been obtained.Comment: submitted to Phys.Rev.C, 34 pages, figures available from one of the
authors, LaTeX with RevTeX, IK-TUW-Preprint 930540
Performance and application of an open source automated magnetic optical density meter for analyzing magnetotactic bacteria
We present a spectrophotometer (optical density meter) combined with
electromagnets dedicated to the analysis of magnetotactic bacteria. We have
ensured that our system, called MagOD, can be easily reproduced by providing
the source of the 3D prints for the housing, electronic designs, circuit board
layouts, and microcontroller software. We compare the performance of this novel
system to existing adapted commercial spectrophotometers. In addition, we
demonstrate its use by analyzing the absorbance of magnetotactic bacteria as a
function of their orientation with respect to the light path and their speed of
reorientation after the field has been rotated by 90o. We continuously
monitored the development of a culture of magnetotactic bacteria over a period
of five days, and measured the development of their velocity distribution over
a period of one hour. Even though this dedicated spectrophotometer is
relatively simple to construct and cost-effective, a range of magnetic
field-dependent parameters can be extracted from suspensions of magnetotactic
bacteria. Therefore, this instrument will help the magnetotactic research
community to understand and apply this intriguing micro-organism
Modern theories of low-energy astrophysical reactions
We summarize recent ab initio studies of low-energy electroweak reactions of
astrophysical interest, relevant for both big bang nucleosynthesis and solar
neutrino production. The calculational methods include direct integration for
np radiative and pp weak capture, correlated hyperspherical harmonics for
reactions of A=3,4 nuclei, and variational Monte Carlo for A=6,7 nuclei.
Realistic nucleon-nucleon and three-nucleon interactions and consistent current
operators are used as input.Comment: 29 pages, 4 figure
An open-source automated magnetic optical density meter for analysis of suspensions of magnetic cells and particles
We present a spectrophotometer (optical density meter) combined with electromagnets dedicated to the analysis of suspensions of magnetotactic bacteria. The instrument can also be applied to suspensions of other magnetic cells and magnetic particles. We have ensured that our system, called MagOD, can be easily reproduced by providing the source of the 3D prints for the housing, electronic designs, circuit board layouts, and microcontroller software. We compare the performance of our system to existing adapted commercial spectrophotometers. In addition, we demonstrate its use by analyzing the absorbance of magnetotactic bacteria as a function of their orientation with respect to the light path and their speed of reorientation after the field has been rotated by 90°. We continuously monitored the development of a culture of magnetotactic bacteria over a period of 5 days and measured the development of their velocity distribution over a period of one hour. Even though this dedicated spectrophotometer is relatively simple to construct and cost-effective, a range of magnetic field-dependent parameters can be extracted from suspensions of magnetotactic bacteria. Therefore, this instrument will help the magnetotactic research community to understand and apply this intriguing micro-organism
Primordial Nucleosynthesis for the New Cosmology: Determining Uncertainties and Examining Concordance
Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) have
a long history together in the standard cosmology. The general concordance
between the predicted and observed light element abundances provides a direct
probe of the universal baryon density. Recent CMB anisotropy measurements,
particularly the observations performed by the WMAP satellite, examine this
concordance by independently measuring the cosmic baryon density. Key to this
test of concordance is a quantitative understanding of the uncertainties in the
BBN light element abundance predictions. These uncertainties are dominated by
systematic errors in nuclear cross sections. We critically analyze the cross
section data, producing representations that describe this data and its
uncertainties, taking into account the correlations among data, and explicitly
treating the systematic errors between data sets. Using these updated nuclear
inputs, we compute the new BBN abundance predictions, and quantitatively
examine their concordance with observations. Depending on what deuterium
observations are adopted, one gets the following constraints on the baryon
density: OmegaBh^2=0.0229\pm0.0013 or OmegaBh^2 = 0.0216^{+0.0020}_{-0.0021} at
68% confidence, fixing N_{\nu,eff}=3.0. Concerns over systematics in helium and
lithium observations limit the confidence constraints based on this data
provide. With new nuclear cross section data, light element abundance
observations and the ever increasing resolution of the CMB anisotropy, tighter
constraints can be placed on nuclear and particle astrophysics. ABRIDGEDComment: 54 pages, 20 figures, 5 tables v2: reflects PRD version minor changes
to text and reference
Nieuw voegwerk voor de wijk. Een netwerkanalyse van de effecten van volkshuisvestingsregelgeving op besluitvormingsprocessen rondom wijkverbetering.
Veranderingen binnen de volkshuisvesting in Nederland, gecombineerd met de grondwettelijke verantwoordelijkheid van de overheid voor het volkshuisvestingsbeleid en de wil van de rijksoverheid om sturing te geven aan dat beleid, hebben tot een tweetal vragen geinspireerd. In hoeverre is de rijksoverheid binnen de huidige ordening, waarin het rollenspel tussen de actoren belangrijker is geworden en de rijksoverheid daar niet meer als vanzelfsprakend sturing aangeeft, nog in staat tot het beinvloeden van ontwikkelingen in de volkshuisvesting? En, als daar mogelijkheden voor zijn, welk type regelgeving kan de rijksoverheid daarvoor het beste inzetten? Deze vragen hebben geleid tot deze studie ...
Zie: Samenvatting