356 research outputs found
phi puzzle in heavy-ion collisions at 2 AGeV: how many K-from phi decays?
The preliminary experimental data on production in the reaction
Ni(1.93 AGeV) + Ni point to a puzzling high yield which can not be
reproduced with present transport codes. We survey the experimental situation
and present prospects for dedicated measurements of the multiplicities
with the and channels at HADES and FOPI.Comment: talk at Strange Quarks in Matter 2001, Frankfurt Sep. 24 - 29, 200
Feasibility of transurethral resection of the bladder (TURB) done by narrow-band imaging (NBI)
Coherent Pair Production by Photons in the 20-170 GeV Energy Range Incident on Crystals and Birefringence
The cross section for coherent pair production by linearly polarised photons
in the 20-170 GeV energy range was measured for photon aligned incidence on
ultra-high quality diamond and germanium crystals. The theoretical description
of coherent bremsstrahlung and coherent pair production phenomena is an area of
active theoretical debate and development. However, under our experimental
conditions, the theory predicted the combined cross section and polarisation
experimental observables very well indeed. In macroscopic terms, our experiment
measured a birefringence effect in pair production in a crystal. This study of
this effect also constituted a measurement of the energy dependent linear
polarisation of photons produced by coherent bremsstrahlung in aligned
crystals. New technologies for manipulating high energy photon beams can be
realised based on an improved understanding of QED phenomena at these energies.
In particular, this experiment demonstrates an efficient new polarimetry
technique. The pair production measurements were done using two independent
methods simultaneously. The more complex method using a magnet spectrometer
showed that the simpler method using a multiplicity detector was also viable.Comment: 10 pages, 13 figures, 1 table, REVTeX4 two column, Version for
publicatio
Linear to Circular Polarisation Conversion using Birefringent Properties of Aligned Crystals for Multi-GeV Photons
We present the first experimental results on the use of a thick aligned Si
crystal acting as a quarter wave plate to induce a degree of circular
polarisation in a high energy linearly polarised photon beam. The linearly
polarised photon beam is produced from coherent bremsstrahlung radiation by 178
GeV unpolarised electrons incident on an aligned Si crystal, acting as a
radiator. The linear polarisation of the photon beam is characterised by
measuring the asymmetry in electron-positron pair production in a Ge crystal,
for different crystal orientations. The Ge crystal therefore acts as an
analyser. The birefringence phenomenon, which converts the linear polarisation
to circular polarisation, is observed by letting the linearly polarised photons
beam pass through a thick Si quarter wave plate crystal, and then measuring the
asymmetry in electron-positron pair production again for a selection of
relative angles between the crystallographic planes of the radiator, analyser
and quarter wave plate. The systematics of the difference between the measured
asymmetries with and without the quarter wave plate are predicted by theory to
reveal an evolution in the Stokes parameters from which the appearance of a
circularly polarised component in the photon beam can be demonstrated. The
measured magnitude of the circularly polarised component was consistent with
the theoretical predictions, and therefore is in indication of the existence of
the birefringence effect.Comment: 12 pages, 12 figures, 1 table, REVTeX4 two column, Version for
publicatio
Results on the Coherent Interaction of High Energy Electrons and Photons in Oriented Single Crystals
The CERN-NA-59 experiment examined a wide range of electromagnetic processes
for multi-GeV electrons and photons interacting with oriented single crystals.
The various types of crystals and their orientations were used for producing
photon beams and for converting and measuring their polarisation.
The radiation emitted by 178 GeV unpolarised electrons incident on a 1.5 cm
thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the
String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised
photon beams.
A new crystal polarimetry technique was established for measuring the linear
polarisation of the photon beam. The polarimeter is based on the dependence of
the Coherent Pair Production (CPP) cross section in oriented single crystals on
the direction of the photon polarisation with respect to the crystal plane.
Both a 1 mm thick single crystal of Germanium and a 4 mm thick multi-tile set
of synthetic Diamond crystals were used as analyzers of the linear
polarisation.
A birefringence phenomenon, the conversion of the linear polarisation of the
photon beam into circular polarisation, was observed. This was achieved by
letting the linearly polarised photon beam pass through a 10 cm thick Silicon
single crystal that acted as a "quarter wave plate" (QWP) as suggested by N.
Cabibbo et al.Comment: Presented at International workshop "Relativistic Channeling and
Related Coherent Phenomena", Frascati (Rome) 23-26 March 200
Multidimensional performance of periurban horticulture:Assessing agroecological transition and soil health
This study aims to produce evidence on the performance of horticultural systems in the peri-urban area of Buenos Aires (Argentina), based on the 10 Elements of Agroecology with an emphasis on soil health. To this end, we performed a survey with the Tool for Agroecology Performance Evaluation (TAPE) Step 1, and we measured soil physical, chemical, and biological properties in family farms self-defined as agroecological or conventional. Results reflected overall low transition-to-agroecology indices and poor soil quality in the studied area. However, self-defined agroecological systems have made significant strides toward agroecological transition with higher scores mainly affecting elements describing management and innovation aspects of the agroecological transition but also those describing the social aspects and the enabling environment. Finally, self-defined agroecological farms exhibit better soil health scores for the indicators aggregates stability, soil organic carbon, and soil basal respiration, than conventional farms. Nevertheless, there is still potential to be exploited and constraints to be overcome to improve the transition to agroecology of horticultural systems in this area.</p
Catalytic Reactions in Heavy-ion Collisions
We discuss a new type of reactions of a phi meson production on hyperons, pi
Y--> phi Y and anti-kaons bar-K N --> phi Y. These reactions are not suppressed
according to Okubo-Zweig-Iizuka rule and can be a new efficient source of phi
mesons in a nucleus-nucleus collision. We discuss how these reactions can
affect the centrality dependence and the rapidity distributions of the phi
yield.Comment: talk at 6th International Workshop on Critical Point and Onset of
Deconfinment (CPOD2010), Dubna, August 22-28, 201
Multidimensional performance of periurban horticulture:Assessing agroecological transition and soil health
This study aims to produce evidence on the performance of horticultural systems in the peri-urban area of Buenos Aires (Argentina), based on the 10 Elements of Agroecology with an emphasis on soil health. To this end, we performed a survey with the Tool for Agroecology Performance Evaluation (TAPE) Step 1, and we measured soil physical, chemical, and biological properties in family farms self-defined as agroecological or conventional. Results reflected overall low transition-to-agroecology indices and poor soil quality in the studied area. However, self-defined agroecological systems have made significant strides toward agroecological transition with higher scores mainly affecting elements describing management and innovation aspects of the agroecological transition but also those describing the social aspects and the enabling environment. Finally, self-defined agroecological farms exhibit better soil health scores for the indicators aggregates stability, soil organic carbon, and soil basal respiration, than conventional farms. Nevertheless, there is still potential to be exploited and constraints to be overcome to improve the transition to agroecology of horticultural systems in this area.</p
Coherent bremsstrahlung, boherent pair production, birefringence and polarimetry in the 20-170 GeV energy range using aligned crystals
The processes of coherent bremsstrahlung (CB) and coherent pair production
(CPP) based on aligned crystal targets have been studied in the energy range
20-170 GeV. The experimental arrangement allowed for measurements of single
photon properties of these phenomena including their polarization dependences.
This is significant as the theoretical description of CB and CPP is an area of
active theoretical debate and development. With the theoretical approach used
in this paper both the measured cross sections and polarization observables are
predicted very well. This indicates a proper understanding of CB and CPP up to
energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to
determine the polarization parameters in our measurements. New technologies for
high energy photon beam optics including phase plates and polarimeters for
linear and circular polarization are demonstrated in this experiment. Coherent
bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger
enhancement for hard photons than CB for the channeling orientations of the
crystal. Our measurements and our calculations indicate low photon
polarizations for the high energy SOS photons.Comment: 23 pages, 27 figures, 2 tables, REVTeX4 two column
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